· " i - i i i i i i abstract round mountain is a proainent rhyolite doae o£ late tertiary...
TRANSCRIPT
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THE GEOLOGY OF ROUND MOUNTAIN,
A BIMODAL VOLCANIC FIELD IN
NORTHWEST ARIZONA
by
MARK M. BUSH
A theaia aubaitted to the £aculty o£ the Gradaute School o£ the State
University o£ New York at Bu££alo in partial £~l£illaent o£ the requireaqpta £or the degree o£
Kaster o£ Arts
.June, 1986
" I -I
I
I
I I
I
ABSTRACT
Round Mountain is a proainent rhyolite doae o£ late
Tertiary age located in northwestern Arizona. The doae bee a
local relie£ o£ 268 • end ia approxiaetely 2.1 ka in
dieaeter at ita base.
Round Mountain developed through both endogenous end
ex9genous phases. Initial stages of doae devalopaent were
-characterized by endogenous growth end the £oraetion o£ -rhyolite breccia units which constitute approxiaately 30-40
percent o£ the total erupted rhyolite. Later volcanic
phases involved the eruption of 4 aeJor rhyolite flows that
cap the doae.
The Round Mountain volcanic £ield is atrongly biaodel. -conaisting of high-K rhyolite end elke1i olivine beaalt with
76.3 end 49.5 aean Si02 percentages respectively. The ·-· rhyolites are coaposed o£ approxiaately·go percent glass
with aoainant phenocrystelline phases o£ sanidine and
high-taapereture oligoclase.
Eapleceaent o£ Round Mountain is believed to have been
initiated by the intruaion into -th~ • upper ,crust of hot. -baaaltic aagaa that had aigrated preferentially along aaJor
cruat~l fracture zonea. Rhyolite aagaa waa generated by the
partial aelting o£ a granitic cruet and later eapleced along
aur£icial fracture zonea.
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:t ·~
·'- ~ ·- ~
ACKNOWLEDGEMENTS
I would like to aincerely thank Dr. John s. King, my
maJor advisor, £or hia greatly appreciated suggestions and
encourageaent concerning the preparation o£ this thesis.
His in£oraative discussions and coaments have lead not only
to the coapletion o£ this study, but also to a lasting
friendship.
I would alao like to thank Dr. John Fountain and Dr.
Charlea Clemency for their critical review of thia thesis.
Their coaaenta and suggestions have significantly improved
the content of this text.
My aost heartfelt appreciation goe~ to ay parents,
George and Pauline, for their supp?rt and encouragement. I
am grateful £or thier patience and ·.understanding during ay
never ending saga of 'higher education.
An expression of gratitude is also given to L. David
Nealey at the USGS, ~lagstaff for his willingness to aid ae
in ay reaeerch.
Laatly, I would like to thank Exxon for offering ae an
ideal JOb which provided a great~deal of incentive to
coaplete this atudy.
The aaJority of thia raaearch was funded under Netional
Aeronautic• end Space Adainiatration Planetary Grant NGR
330-151-08 awarded to Dr. John s. King. Minor funding waa
alao aade poaaible £roa a grent given by the Graduate
Student Aaaociation.
TABLE OF CONTENTS
PAGE Introduction.... . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . 1
Purpose o£ Study............................ .............. 1
Regional Setting .• . • .•. Structure .•••..•••.. Regional Geophysics. Stratigraphy •• Volcanics .....
Basalts o£ the Round Mountain General State~ent •••• Williaas Basalt <Twb).
Volcanic Field ••
Field Description ••• Petrographic Textures •.
Josep Basalt <TJb) •••••.. Field Description ••••.• Petrographic Textures ••
Vernon Basalt <Tvb) •• Field Description. Petrographic Textures.
Field Daa Basalt <T£db). Field Description •••• Petrographic Textures.
Basaltic Flows <undi££erentiated Field Description ••••••.•••• Petrographic Textures • ••
Bishop Place Basalt <Tbpb). Field Description ••••••••
Tb£)
Rhyolites o£ the Round Mountain Volcanic General Stateaent •. Field Description •. Petrographic Textures •••.•••••••••••
Lavender Gray Rhyolite <Tlgr> ••• ~. Vivian Rhyolite <Tvr>. Andrua Rhyolite <Tar>. Bu££ Rhyolite <Tbr) ..• Maaaive Gray Rhyolite <Tagr>. Perlite <Tpl> ••••• •••• Maaaive Purple Rhyolite Rhyolite Breccia <Trb> •• Puaiceoue Flow Rhyolite
<Tapr>
<Tp£r>.
Field.
Petrology o£ the Round General Stateaent •• Rhyolitea .•
Mountain Volcanica.
Baaal te ..•.•••••
. .
3 3 7 8
13
17 17 17 17 18 18 18 19 19 19 20 20 20 20 22 22 22 23 23
24 24 24 28 28 28 30 30 30 31 31 33 3~
34 34 34 41
--
EAplace•ent and Eruptive History o£ Round Mountain •••••••• General Statement ...•.....••••.•.•.•••••••••••••••••.••• Stage 1 ..... · ............................................ . Stage 2 . ............................................... . Stage 3 . ............................................... . Stage 4 . ............................................... .
Early Phase o£ Stage 4 ••••••.••••••• ~ ••••••••••••• ~ ••••
46 46 48 50 52 55 56
Late Phase of Stage 4 ....•...••.•.•...•.••• ~ ••••••.••• 57 Stage 5 •••••••••••••••••••••••••••• · ••••••••• -•••••••••••• Stage 6 ••.•........... ; .....
Concl uaione . ................................•.............
Reference&, Cited •......•••.•..•..••.•...
Append'icea • ••••••••••••••••••••••••••••• Appendix A . . Analog Investigation o£ Volcan--ic Dolle
Alignments o£ Northwestern Arizona to Various Lunar and ftartian Constructs~ ••••••
Introduction ••••••••••••••••••••••••••••••••••• ~ •••••• Volcanic Domes of Northwest Arizona ••••••••••••••••••• Volcanism on Mars and the Moon •••• w ••••••••••• ~ •••••••
General Stateaent •...•........•.••.••••..••......••. Volcanism . ....•.... .' ............•.. ~ •.......•....... Volcanic Construct Alignment •••••••••••••••••••••••• Tharsis Montes, Mars •••••••••••.••••••••••••••••.••• Tempe Fossae Region, Mars •••.••••••••••••••••••••••• Tharsis Tholus, Mars ••••••••••••••••••••••••••••.••• Lunar Construct and Crater Alignments ••••••••••••••.
Su••ary . ......................•..................
Appendix B Modal Analyses Results £or Sample Thin Sections . ................................... .
Appendix C . . Petrographic Mineral Descriptions •••••••.••••
Appendix D . . MaJor Element and N6rMativ~·Mineral Tables •••
Appendix E : Trace Element Tables ••••••• . . . . . . . . . . . . . . . . . . "LIST OF FIGURES
FIGURE
1. MaJor Fault Syste•a o£ Northern Arizona ••••••••••••••
2. CoMposite Hap o£ Faults o£ Northern Arizona ••••••••••
3. Aeroaagnetic Map with Fault Syste•a o£ Northern Arizona-~· ....•......••...•....•••..••••.••..•••.•..
4. Earthquake Epicenters and Faults SysteMs o£ Northern Arizona . ......................... · •.•..•..........••..
58 64
71
73
78
78 78 78 82 82 82 83 83 86 86 90 94
95
99
103
108
PAGE
5
9
10
5. Stratigraphic ColuMn of Northwest Arizona •••••••••••• 12
6. Photograph of Jo6ep Basalt Flow •••••••••••••••••••••• 21
7. Photoaicrograph o£ Pilotaxiti~ Fabric •••••••••••••••. 21
8. Photograph of Flow Banding in Vivian Rhyolite •••••••• 26
9. Photograph o£ Brecciation Texturee ••••••••••••••••.• : 26
10. Photogr~ph o£ Rhyolite Breccia •••••.•.••••••••••••••• 27
11. Photograph o£ Obsidian Lenses in Perlite Unit •••••••• 27
12. Photoaicrograph o£ Perlitic Fracturing ••••••••••••••• 29
13. Photoaicrograph o£ Spherulite o£ Cuartz and Feldspar . ........................................... .
14. Photoaicrigraph o£ Sanidine Phenocryst Detachaent ••••
15. Photoaicrograph o£ Brecciation Textures ••••••••••••.•
16. Variation Diagraae o£ MaJor Eleaent Versue Silica ••••
17. Andesite-Dacite-Rhyolite Classi£ication based on K20/Si02 . ................ . ..... · · · . · · · · · · · · · · · · · · · · · ·
18. Classification Diagraa o £ Non-Pota6sic Volcanic Rocks based on Total Alkalies Versus Silica ••••••••••••••••
19. Noraative Color Index Veraua Noraative Plagioclase Coaposition Diagraa . . .....•••••••••.•••••••••••••.•••
20. Variation Diagraaa o£ Trace Eleaents Veraue Silica •••
21a. Diagraa o£ Total Alkalies Versus Silica ••••••••••••••
21b. Diagraa of Na20 • K20 and CaO Ver~e Si02-···········
22. AFK Oiagraa ..•....•••. • ..•.••••••.•••••••••••••••••••
23. Volcanic Construct Location Map o£ the Round Mountain Area ••••••••••••••••••••••••••••••••••••••••
24. Stage 1 Diagraa- Eaplaceaent o£ Baaalt into Cruat •••
25. Stage 2 Diagraa - Partial Kelting o£ the Upper Cruat.
26. Stage 3 Diagraa - Initial Rhyolite Flow ••••••••.•••••
27. Stage 4 Diagrsa - Endogenous Phaae .••••••..•••••••.••
29
32
32
36
37
37
38
39
43
43
44
47
49
49
54
54
)
"-"-·--~ --I .- 28. Stege 5 Diagrea- Exogenous Phese •••••••••••••••••••• 59
"- !I ·- 29. Stage 6 Diagraa- Basaltic Activity Phase •••••••••••• 59
--I Photogreph a£
Photograph a£
30. Vivian Dome ••••••••••••••••• ~··········
31. Concentric Flo~s ••••••••••••.••••••••••
60 ...,{
60 '• -·- J Photograph o£ 32. Polygonal Weathering ••••••••••••••••••• 61
"""""' Photograph a£ 33. KeJor Rhyolite Flow •••••••••••••••••••• 61
-I ·- Photogreph of 34. Flow Interface .•••••...•.•....•••.••.•. 62
. .-I -
Photograph o£ 35. Andrus Flow Intrusion ••••••••••••••••.•
36. Photograph o£ Josep Baseltic Dike Crosscutting
62
·- Rhyolite, Flow .•......... ~ ..............•............. 65
1'\1!!1>. I 37. Photograph o£ Josep Dome •.•••••••••••••••••••••••.••• 65
·- 7 38. Geologic Map o£ the Round Mountain Dome ••••••••••.••• 68 .- ..._,
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39. Block Diagram o£ Scissor Fault ••••••••••••••••••••••• 69
'- ' .- 40. Photograph o£ Fault at North End o£ Round Mountain .••
41. Aerial Photog~aph a£ Roun~ Mounta~n ••••••••••••••••••
69
80 ,.-
I :- 42. Diagram o£ Lineaments in the.Round Mountain Area ••••. 81
·~ -I 43. Diegram a£ Tharsis Montes Alignment ••••••••••••.•.•••
44. Contoured Pit and Vent Densities o£ Arsia Mons •••.••• 87
85
/--' 45. Contoured· Pit and Vent Densities o£ Pavonis Mons ••••• 88
~- 46. Iaage o£ Slot-Like Vent in the Tempe Foasee Region ••• 89
,.- I .- 47. Viking Orbiter Iaage a£ Tharsis Tholus •••••••••••••.. 91
--·- I ·-48. Composite Map o£ the Tharsis Region ••••••••••••••.••• 92
49. Iaage a£ Lunar Construct AlignMent •••••• ~············ 93 ·-'""" '
LIST OF PLATES
:-·- ~ ·-1. Geologic Map of Round Mountain ••••••••••••••••••••• pocket
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I I I I I I •
INTRODUCTION
Round Mountain is c distinctive rhyolitic do•e - .
positioned on the Colorado Plateau in northwestern Arizona.
It ia located cpprox~actely 100 ka west of Flagstaff end 19
k• northeast o£ Seligaan~ Arizona. Round Mountain rises to a
aaxiaua elevation of 2163 a above sea level and has a local
relief of 268 aeters.
The strongly biaodcl Round Mountain volcanic field,
containing h~gh-K rhyolite& and alkali olivine baaalta~ has
received no previous investigative attention. The Mount
Floyd volcanic field iaaediately south o£ Round Mountain~
haa been intensi.vely studied by Nealey (1980).
Potassiu•-argon date £roa the volcanics in the northern
• sector o£ the Mount Floyd field indicate an age o£ 2.7 ~1-
0.7 a.y. <Nealey~ 1980). Based on volcanic stratigraphy,
the Round Mountain volcanics are considered to be aore
recent.
PURPOSE OF STU2Y
The purpose of thia atudy waa to ~nveatigate the
geology o£ Round Mountain. More specifically~ this research
waa aiaed at providing a detailed deacription of the
volcanic aorphology and procaaaea which were involved in the
£oraation of Round Mountain. Thia wca accoapliahed by
-(-,,.,.,
~ ·---·- developing o geologic aop o£ the etudy oreo in conJunction
·- with petrographic exaaination and geocheaicel analyais o£ - volcanic rock eaaplea collected during the aapping. An ·-- additional aspect o£ t~is study involved the investigation .-, -. o£ planetary analoga o£ Round Mountain to correlate possible
origins o£ siailar aorphologic £eatures o£ aartian ond lunar -·- I terroina.
--I The preparation o£ the geologic aop <plote 1> involved
£ield aapping o£ approxiaotely 21 ka2 on on enlarged 1973 --I ·-USGS Mount Floyd 7.5' topographic quadrangle. Contacts o£
the volcanic units were veri£ied using 1972 USGS aerial
:p photographs GS-VCZV 1-116, 1-118, and 1-127.
·-·- I Petrographic thin sections o£ 43 volcanic rock aoaplea
o£ the study area were prepared and exaained. Modal .-.
-I anolyaea ond petrographic descriptions were per£oraed, the
- results o£ which are presented in appendices B and C. _, -· Geocheaical data o£ 24 saaplea £roa the study area were
-I .. collected using whole rock x-ray £luorescence aethods. The
anolysea yielded both the •aJor and trace eleaent data • -·- I given in appendices 0 and E.
--' -·- t ---·----· - ? -
• • • • ' ' ' -' ' I ' ' --
REGIONAL SETTING
STRUCTURE
Northwestern Arizona ia a part of the southwestern
portion of the Colorado Plateau phy~iographic province •
This section of the Colorado Plateau is considered to be
coaposed of large structural blocks or regions whose
boundaries are delineated by north-northeast and northwest
trending faults <Lucchitta, 1974>. Theae structural blocks
have a low angle of dip to the northeast.
The aaJor north-northeast trending fault ayateaa of ,,
northwestern Arizona are~ froa east to west: the Oak Creek
Canyon ayatea: the Mesa Butte aystea: the Bright Angel
ayatea; and the Sinyala system <Figure 1>. The aaJor
northwest trending fault ayateaa are, from north to south:
the Moraon Ridges ayatea: the Kaibab systea; the Cataract
C~eek ayatea; ,and the Chino Va~ley syatea. These fa~lt ,.
systeas extend £or tena to hundreds o£ kiloaeters with
displaceaenta which range froa hundreds to thousands of .......
aetera <Lucchitta, 1974>. Moat of th·e faults are high angle
dip alip with their downthrown aidea to th~ west CFigure 2>.
The Round Mountain, Trinity Mountain, and Howard Hill
volcanic coaplexea lie along the southern proJection of the
Bright Angel fault aystaa, with the Mount Floyd volcanic
center lying directly on the fault trace aa ahown in figure
-
; • ' , ' , , 1 ' ' J ' , , ' ~ ' j ' J • ' ' ,_ ~ - ,_ ·- ._ ~ ,_ ._
114• . 113 1 17·~·---·--------- ... ......., ..... _________ _
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r' la•
~-. • .•••• , ............ ~;t~ -.......... ··\·:·s::~·- '''::~::::u r·::·a~::·::·;J.~~r:~·:~\~.:1~~n~~.· 1 ~· H• 11 ;··· .. .................... 112 · 111• tto•lo' , ..
Figure 1· • MaJor fault ayatema o! northwestern Arizona showing volcanic center locations. V~lcanic centers· are shown by circles: 1> Round Mtn.: 2> Mount Floyd: 3> Trinity Mtn.; 4> Howard Hill; S> 8111 Willia~a Htn.; G> Sitgreaves Mtn.; 7> Kendrick Peak; 8> Slate Mtn.; 9) Meaa Butte; 10> Shadow Mtn.: 11> Tuba Butta: 12> Wildcat Peak Cmodi!ie~ from Shoemaker and other&, 1974).
, , , ) ~ ~ .. .&..;....:.J J
0
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0 $0 I<ILOM£TERS .._L.......___._..__....__ ....... t
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figure· 2- Co~posite ~~p of.foults in northwestern Arizon~ the locotions of <R>-Round Mtn.: (T) -J'rintty Htn.; <Fl -Mount Floyd; and <SF> -San Francisco l'ttn. Normal faults are shown with~ dot on the downthrown side Ce!ter Shoe~eker snci others; 1974).
-·-.al· 110' II'
---!l -- )
-r --I 1. These three volcanic centera are bounded to the north and
- south by the Cataract Creek and Chino Valley fault systeas. -I - Locations o£ aany other volcanic centers located near
"" I -the aap area also correlate with aaJor fault systeas. The
Cenozoic volcanic centers of : Bill Williaaa Mountain, --I -Sitgreavea Mountain, Kendrick Peak- Slate Mountain_ Meaa
Butte, Shadow Mountain, Tuba Butte- and Wildcat Peak all lie
-I on, or along proJections of, the Mesa Butte fault aystea. - In northwestern Arizona, the Colorado Plateau Province ·- I - and the Baain and Range Province which adJoins it to the
·-·- ,. . south have distinctive structural_ geophysical, volcanic,
crustal_ and physiographic properties which distinguish
··-_._ I thea. The width of the phyaiographic boundary between these
'"""' two provinces variea, but is generally about 10 kiloaeters _._ I -- in northwestern Arizona. By contrast the geophysical
:I boundary in this region places constraints which liait the
width to approxiaately 80 kiloaeters (Shuey· et al, 1973).
··-·- I Lucchitta (1974> concluded that t~e parallelisa o£ certain
- str·uctural features which croas the boundary between the
·-I Colorado Plateau and the Basin and Range in northwestern .a., -:I
Arizona could only be the result o£ coaaon atreaa £ielda
baing applied to both provinces. The £act that the
:, structural de£oraation regiaea between the ·provinces ia
- di££erent ia a £unction o£ the crustal properties, with the
-I Colorado Plateau having a aore coapetent· and thick cruat.
·-_, -
The Colorado Plateau in this region ia characterized by
•<11111>,
I I I I I I I I I
I I I I I I
a 37-42 kiloaeter thick crust. There are nuaeroua
aoutheaat-northweat trending aonoclinea whoae steep liaba
dip to the northeast. Volcaniaa on the Pla~eau in this
region ia Plio-Pleistocene and aid-Tertiary with eruptive
locations that are closely associated with zones o£ weakness
in the lower crust <Eastwood, 1974>. Although the isolation
o£ volcaniaa aay be in part controlled by lower crustal or
upper aantle irregularitiesp it is the upper crustal
properties and weakness zones which deliait the locations
and atyle o£ these £aults. These upper crustal £aulta aay
aodi£y the £inal location o£ the volcanic centers.
REGIONAL GEOPHYSICS
The geophysical signatures o£ the Colorado Plateau and
the Basin and Range are aarkedly di££erent. Basin and Range
crustal thickneaa ia generally considered to be in the order
o£ 30 kiloaatera ,in contrast to that o£ the Colorado Plateau
which averages between 37-42
1979>. Baaed on aeiaaic datap
kiloaeter• <Keller et al.,
J'ulian <1970) proposed that
... under the Baain and Range the low verocity zone in the upper
aantle ia thick and shallow, in contrast to the Colorado
Plateau where it ia auch deeper and not aa well developed.
Thia correlate• well with the relatively high heat £low
values, ranging £roa 1 to 3 heat £low units, which are
characteristic o£ the Baain and Range (Roy and othera,
-n -l.!.l _) -I
1968).
Regional Bouguer gravity •ape which span the Colorado
Plateau Basin and Range phyaiographic boundary show a
definite change in gravity signatures. There is a decrease
fro• the higher values observed in the Basin and Range to
the relative~y low valuea in the Plateau region.
Aero•agnetic d~ta ahow an anoaaloua gradient across the
boundary between the two provincea with lowa over the Basin
and Range to the high aeen on the Colorado Plateau <Shuey et
elr 1973>. Residual aero•agnetic aapa <Figure 3> coapiled by
:r Sauck and Suaner (1971> and aodified by Lucchitta <1974>
exhibit ainor correlation with the in£erred locations o£
ancestral £ault systeaa and their proJections in northwest
Arizona. The Round Mountain- Mount Floyd, and Trinity _, - Mountain volcanic centers are located in an area yielding
-~-~.
reaidual •agnetic intensity values o£ leas than 0 gaaaaa.
Northwestern Arizona can be considered to be an area o£
active aeiaaicity. Figure 4 <Lucchittar 1974) ahowa the
plota o£ 26 recorded earthquakes in the•area in the period
-I £roa 1938 to 1973 and their relationship to the aaJor fault .....
o£ the region. By including the inferred
proJection& of theae £ault ayateaa_ there ia aolid evidence
that theae £ault ayateaa are currently active.
STRATIGRAPHY
•••• ,, ...... J. ____ _
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.... -to ..... u J
o to ... c...nu•l
llt4
Figure 3. - Residual aero~egnatic ~ap overlain by the traces of ~ncastrel fault ayste~s and related volcanic centers. The volcanic centers shown by gt~rs: 1> Trinity Mtn.; 2> Ro~nd "tn.t 3) Mount Floyd; 4) Howard Hill; 5) Bill Williams Mtn.; G> Sitgreaves Mtn.; 7> Kendrick Peak; 8). Slate Mtn.: ,, Mesa Butte: 10) Shadow Mtn.; 11> Tuba. Butte; 12> Wildcat Peak Cafter Sauck and Su~ner, 1971; Shoemaker, 1974>.
16'
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~~::::::{/;(i::.:}!:;·:.\i. ··::::;.:~\~·:.',\~·fi:,. ,.-·r:\!::l~·::~~!i1!.1W(««:}U~~if;i::-:·.·' ' .::·::-:::.;: .. :.;.·:·.:..:·:.:.:: ... :'/,:..:,:. o~~::::::. ,•:-';·:\~·;~·~•:::~\li tiW:·:-:·:~~t·:·:.:·""· ·
<( 1
1 o . l'>OWIL E s ... :.:,:;:;:·.>;;:;:,;:.:.~;.:·~:·· ':i:!f, ~~::'H:·:·;. ,. ::l~':':H:!:!~'€, \"~;q!lf:·:;:,\"~·11 t t i I t t ' • :((!{{:{:;.);/:' 'i.:m:~:?-'{i4Wt:~·.. . ..... :;.;!(i{i?ii:~:;.~··$..-;:;:;:;~~~~;::····" 1
0 ~0 I<ILO"£TERS .•:: ··:-·::·":·::: ··.·:· .•• 0 1•'•\'.•'••,•'·········1•!'••.··5 l ....... .. Cl I '0' ' ' ' . ..c•./ •':•:•·: • ,.,., .. _., ........ ,, .. ,,.,,.."'·'
<(
> w
2
•
AJ~[~i~~ 0
:sa•
• ~-
......... , ................. ,........... I I .... ,~. 1 • 11';~· ..... · . .-·.·;·.-.·.·.·:;::.--;, ........ ·.\·::•':·. 111 IS ·:·;··;··.;;; .... ·. ·.·.:.···.............. 111t , to•","o· ,, .. figure 4. - Earthquake epicenters (dots and open circles> showing their relationship to the
ma)or fault ayetema (after Shoemaker and othera, 1974). Volcanic centers are shown by nu~bered circles: 1> Round Mtn.; 2> Mount Floyd; 3> Trinity Mtn.; 4) Howard Hill; 5> Bill Williams Mtn.; &> Sitgreavea Mtn.; 7) Kendrick Peak; 8> Slate Mtn.; 9) Mesa Butte; 10) Shadow Mtn.: 11) Tuba Butte; 12> Wildcat Peak
' ,
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I
I The stratigraphic coluan £or northwest to north-central
Arizona is shown in figure 5. The rocks coaposing the .~
I Precaabrian baseaent consiat of the Vishnu Group, which are
mostly schists and ainor aaphibolitea, the Bass Doloaite,
I Hakatai Shale, Shinuao Quartzite, and the Dox Sandstone. The
I Precaabrian baseaent rocks are overlain by essentially
horizontal Paleozoic ·sediaentary units - o£ sandstone,
I liaestone, doloaite, and shale fro• various eolian, shallow
aarine, lacustrine, and £luvial environaents <Mckee,- 1974>.
I In the Chino Valley region iaaediately to the south of the
I Round Mountain and Mount Floyd volcanic coaplexes, the Chino
Valley Foraation, a sandatone and doloaitic limestone,
I overlies the Tapeata Sandatone <Hereford, 1975>. The Chino
Valley Foraation i-a not present in . the stratigraphic
I sequence in the Grand Canyon area. Overlying the Paleozoic
I rocks is the Moenkopi Foraation o£ Triassic age. This is
predoainantely a red ailty aandatone and is not aeen in the
I Grand Canyon stratigraphic aection £urther to the north. '.
' In aoae areaa o£ northwest to north-central Arizona,
I Cenozoic gravels, of early to aiddle Tertiary age, overlie ~
I the Mesozoic and Paleozoic aediaantary £oraations. These
gravels conaiat of reworked Keaozoic and Paleozoic
I aediaenta, along with a aigni£icant contribu.tion £roa
crystalline rocka of disputable origin <Young, 1982>. It ia
I generally accepted however that these gravels were deposited
acroaa the weatern Coconino Plateau and along the Mogollon
- ....
---""" _, ~
~
-~
~
" ~
·" --/'lilt.
/-/ ... ~
------~ -~-
------------------,_
-------·-
)
u 0 N 0 z &U
u
u 0 N 0 V)
w ~
u
PERMIAN
0 1----::::~~.;:,;:..::.::.·:..:..:..:,:~;.:;;;.-;;.:;,.:.:,;:
N 0 w __J
t:_
u 0 () w I
~ <(
CAMBRIAN
~2~~~;;;t> ~~:;*Y.
figure 5. - Stratigraphic column of the rocka of northYeatern Arizona (after Breed, 1974).
I· I I I I I I I I
-' ' -' I ---I
Ria £ro• aourcea to the aouth during early to middle
,Tertiary <Young, 1982).
Cenozoic volcanics overlie the Tertiary gravels and the
Mesozoic and Pal•ozoic ~ediaenta throughout aany parts o£
northwest and north-central Arizona. The observed aur£icial
volcanics are aainly' basaltic £lows, with aoae associated
interaediate to silicic volcanic centers o£ dacite,
andesite, and rhyolite. These volcanics are o£ Laramide,
middle Tertiary, and Plio-Pleistocene age <Eastwood, 1974>.
VOLCANICS
The southwestern United States was volcanically active
during three distinctive periods from late Mesozoic through
the Cenozoic. Daaon <1971) has deterained the teaporal
-
distributions £or the magmatic activity £rom radiometric -
data. The first episode is o£ Laramide age starting 80
m.y. ago and continuing £or about 35 a.y.: the second
episode / was aid-Tertiary in age and ranged £rom 40 to 10
m.y. ago, while the third maJor eruptive epi~ode occured in
the last 10 a.y., and is doainantely Plio-Pleistocene
<Damon, 1971; Eastwood , 1974>.
The late Cenozoic igneous suites of the southwestern
United States are predoainantely basaltic in nature. These -
lavas were erupted either as basaltic £ields, alkalic
igneous series derived from alkali basalt, or as bimodal
- 13 -
~ .• ~.
~· -··· -· --
-. ·- .l
::D ·~
-·
assemblages consisting o£ baaalt and ailica-rich rhyolite
<Christiansen and Lipasn, 1972>. Along the southern aargin
o£ the Colorado Plateau, Eastwoo~ <1974) classified the
mid-Tertiary volcanism as aainly calc-alkalic with alkaline
affinities. Baaed on petrographic and geochemical data, the
volcanic rocks o£ Plio-Pleistocene age along the southern
Colorado Plateau are predominantely alkaline with evidence
£or a aubalkal~ne affinity <Eastwood, 1974>. ~
These late Cenozoic volcanics are believed to have
derived by partial aelting froa a broad depth interval of
the mantle and have been modified by c~ystal fractionation
of olivine, plagioclase, and . clinopyroxene during their
ascent to the surface <Best and Brimhall, 1974>. DaJnon
<1971) commented that the mid-Tertiary and Plio-Pleistocene
magmas had fundamentally different source regions. Based on
data £ro• Laughlin and others <1972) and Leeman <1974>, the
strontium 87/86 ratios £or mid-Tertiary volcanic rocks are
higher relative to those of Plio-Pleistocene age. This aay
reflect a substantial degree of crustal contaaination for
the rocks of aid-Tertiary age, with little to no
contamination of the Plio-Pleistocene volcanics <Eastwood,
1974; Leeman, 1974).
The San Francisco volcanic field borders the Round
Mountain and Mount Floyd volcanic complexes to the east.
Situated on the aouthern edge of the Colorado Plateau, the
San Francisco volcanic field ia of late Tertiary, and
- 14 -
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Quaternary age <Moore and othera. 1976>. Rocks of the San
Francisco coaplex have been dated by Daaon and Leventhal
<1974>. and SaiLey <1958>. and show agea ,ranging £roa 10
m .. y. to 1000 years. The San Francisco volcanic field has
,an. areal extent o£ •ore than 5.000 square kiloaetera. It ia
composed of nuaeroua basaltic. interaediate. and silicic
volcanic centers which are genetically related <Moore and
others·, 1976>.
Baaed on geocheaical and petrographic data. the basic
volcanic rocks of the San Francisco field are considered to
be alkaline <Hunt. 1956; Eastwood. 1974; Moore and others.
1976) and the cheMically interaediate volcanic rocka are
found to have a aubalkaline affinity <Wenrich-Verbeek and
Thornton. 1974). Alkali-olivine basalt is by far the aoat
voluminous rock type in the volcanic field • The San
Francisco volcanic field contains alkali-olivine basalts.
high-alumina alkali basalts.. andeaitea. dacites, -rhyodacite&, and rhyolites <Hunt, 1956: Moore, 1973; Moore
and others. 1974; 1976).
Baaed on Sr 87/86 ratioa. the San Francisco volcanic&
ahow that they are aantle-derived with only ainor cruatal
contamination <Moore et al., 1974). By exaaining the
geochemical data <Moore et al., 1974> obtained froa the
volcanic rocks of the northern and eaatern areas of the San -
Francisco field. a full differentiation aeries froa basalt ).--.
to rhyolite haa been deaonstrated.
-- 15 -
-The Mount Floyd volcanic £ield adJoining Round Mountain
to the south is considered to be biaodal, with the
occurrence o£ alkalic baaalta and calc-alkalic rhyodacite&
<Nealey, 1980). The age o£ the Mount Floyd volcaniaa haa
been deterained, by potaaaiua-argon data, to be 2.7 +1- 0.7
a.y. <Nealey, 1980). The volcanic coaplex ia coaposed o£
baaaltic and ailicic lava £lows, doaea, cinder cones, and
various pyroclastic depoaita <Nealey, 1980). The location o£
the volcanic centers in the Mount Floyd £ield appears to be
in part controlled by the Bright Angel and Chino Valley
--t £ault syateas.
-J -- 11=\ -
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BASALTS OF THE ROUND MOUNTAIN VOLCANIC FIELD
General Stateaent
The basaltic rocks within and surrounding the Round
Mountain volcanic field have been classified in this study
on the baaia of color, texture, aineralogy, geoct;\eaistry,
and location of their source vents. They were categorized
into aap units and given inforaal naaes by the author. In
the following sections each basaltic aap unit ia briefly
deacribed both aacro and aicroscopically. A aore detailed
petrographic description of specific mineral characteristics
is given in Appendix c. Plagioclase coapoaitiona were
determined either by the Michel-Levy statistic~! method
<Heinrich, 1965> or the Microlite aethod <Heinrich, 1965>.
Olivine compositions were determined by optic signa and
variation in 2V angles <Poldervaart, 1950>. The aodal
analyses for 13 thin sections of the basalt units are
presented in Appendix B •
WILLIAMS BASALT (Twb>
Field Description
This basalt is light to aedium gray on a freah aurface
and dark reddish-black on weathered surfaces. A weathering
rind approxiaately 5 •• thick is coaaonly present. Visible
phenocrysts of olivine <1-3 aa> and quartz xenocrysts art
common. Gas vesicles ranging in aize fro• 1 to 10 •• aay
,_, ,-,.
,-,,
~
~
.~
~
.~
j'!II!J.
~
~
~.
--I occupy up to·so percent of any exposed aurfece. Thia unit
:for••. high pro:file dikes which coaaonly radiate :fro• a
central vent. Such dikea ere generally aaaaive end blocky
but aay exhibit aoae degree o£ :foliation due preauaably to
pulse eapleceaent.
Petrographic Textures
The Williaaa baaalt ia holocryatalline and contains
approxil•ately 60 percent phenocrysts of .pl.agioclaae <Ansa> • ··'
clinopyroxene <augite). olivine <Fo?s-aa>, end ainor
--1 Xenocryata of quartz are preaent and have opaques.
clinopyroxene reaction riaa. Plagioclase aicrolitea are
subparallel resulting in a pilotaxitic fabric. Plagioclase
generally ahows a aubophitic relationship with clinopyroxene
but aay alao :fora gloaeroporphyritic clots. Alteration is
alight with only ainor olivine alteration to produce
iddingsite and iron ataining.
JOSEP BASALT <TJb)
Field Description
Macroscopically thia basalt ia dark gray-black on a
weathered aurface and aaooth in appearance. On a fresh
surface it !a seen to contain epproxiaately 2 percent of
phenocryata of olivine <1-2 ••> that are altered to
iddingaite. In hand speciaen the Joaep baaalt ia very fine
grained and generally ahowa no veaiclea. Minor xenocrysts
o£ quartz ere present which have a visible reaction rind.
- 18 -
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Spheroidal weathering ia coaaon near the aource vent. Thia
unit £oras low profile £lowa and dikea. At £low £ronta thia
.baaalt o£ten exhibits a texture produced by the
incorporation o£ basaltic £ragaenta <figure~).
Petrographic Textures
Microscopically the Joaep basalt ia holocryatalline
with approxiaately 20 percent phenocrysts o£ olivine <Fo67>
and clinopyroxene <augite ?>. The aicrolitea o£ plagioclase
<Anso> which fora the· groundaass have a pilotaxitic fabric
'indicative o£ £low. Trace aaounts o£ quartz xenocrysts are
present co•aonly with a riaaing o£ clinopyroxene. Traces o£
secondary calcite occur within irregular voids.
_.,!'j.r
··~ ,(,~~~ ·'/('''
Clinopyroxene aay appear as radiating cluatera and coaaonly ~
contains opaques. Plagioclase and clinopyroxene are
subophitic and are occasionally eabayed by groundaasa.
VERNON BASALT <Tvb>
Field Description
Thia dark gray~ very fine grained basalt £oras low
profile dome& and flows. Very few gas veaiclea are present
in the aatrix in hand speciaen. Phenocrysts of olivine are
rarely visible and are slightly altered. Thia unit aay
exhibit saall scale flow £o.liation but is otherwise blocky
in structure. It is generally £reah in appearance with an
occasional weathering rind about 2 •• thick.
- 19 -
,_
--:I --- Petrographic Texture
.Ail"'
-I -The. Vernon is holocrystalline containing
approxiaately'20 percent phenocrysts .o£ plagioclase <An54>. -, ·-olivine (:Fo7s>, ·and .ainor c;:linopyroxene <augite>. The
g:r(>undaaaa o£ plagioclase,. clinopyroxene,. and opaques ·-_, exh.ibita a ·. ali.ght pilotaxitic fabric. There ia a ainor
rJ!IO,.
---; '
conc~ntratiori o£ opaq'uea in. ar_eaa with a aor.i well-developed
pilotaxi:tlc fabric. The intergranular apacea between
~ plagiochaae lath:li in the· g~roundaaasa are <;tccupied by discrete
·crystalS. of clinopyroxene and olivine .• ·'
Ophitic opaques and
=- clinopyroxene are present in the corroded corea o£
·--' plagioclase. Plagioclase and clinopyroxene are generally
subophitic. --- FIELD DAM BASALT <Tfdb> -=- Field Description
2t This very fine grained basalt ia light gray on
- weathered surfaces and aediua .gray on fresh surfaces. Minor
·~ aaounts o£ olivine phenocrysts are visible (about 1 aa in -·~ -
diameter>. This unit aay exhibit a massive,. blocky, platey,
or spheroidal appearance. The platey basalt is localized
A ·a, •
near the vent and is preauaed to be a product o£ cyclical
A e££uaion o£ aagma. This basalt generally £oras low profile
flows with an occasional high standing resistant ridge. ~
Goo,
~ I Petrographic Texture -~ --,......, - 20 -
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I· t qur·l' 6 Bo-;olt1c grovel <clast> 1nclusion w1thin o Josep bnsolL <1 JU) tlow £rurtL .
W'~slhPri nq ot thr> matt·ix - 1 '11'> t !\ -
Note the d1£ferentiol baoo 1 t. eurround in a th1•
Figur<" 7 , - Photomi crogroph <lOOx> sho wing the pilotaxitl c fobr l c of lhe plagioclaac microlilea in the Basalti c flow <Tbf) unil. The remaining groundmass is composed of c linopyroxen e and abundant opaques. Photo microg raph t ol<en with c r ossed polars.
2 1
Microacopically thia baaalt ia holocryatalline with
approxiaately 10 percent olivine <FoaG> and clinopyroxene
~I <augite)·phenocryata. The groundaaaa conaiata o£ aicrolitic 1---
plagioclase <AnsG>, clinopyroxene, olivine, and opaques.
The aicrolitic plagioclaae latha exhibit a aoderately well
developed pilotaxitic £abric. Grouridaaaa plagioclaae laths
£ora an intergranular texture with clinopyroxene and
olivine. Opaque crystals are coaaonly aubophitic with
olivine. Phenocryata o£ olivine aay ahow eabayaent along
open £ractures by groundaaaa.
~I -·· BASALTIC FLOWS <undi££erentiated) <Tb£)
Field Description
Macroscopically this basalt varies £roa light gray to
black on weathered aurfacea. Frequently there exiat& a buff
colored weathering rind with a thickness o£ 3 ••·
Phenocryst• of a visible size are not present. Thia dense
and reaiatant baaalt £oraa linear high standing dikes with
aaaociated ainor £lows. It aay exhibit a alight £low
. - layering which producea a alight platey regolith character •
Petrographic Texture
11i 1 -- Basalt o£ this group ia holocryatalline with 5 to 20
percent phenocrysts of plagioclase <An5o>, clinopyroxene
<augite ?>, and ainor ~livine. The groundaasa consists o£
plagioclaae, clinopyroxene~ olivine, and opaques which
typically £ora a well developed pilotaxitic £abric <£igure
- .,., -
I 7>. Calcite ia preaent aa anhedral growtha within veaiclea. ./·-
An intergranular texture ia exhibited between groundaaaa -
I c:ryatala of plagioclase, clinopyroxene, and olivine. Opaque --I
cryatala occaaionally occur in ophitic aaaociation with
corroded plagioclaae phenocryata.
I 8ISHOP PLACE 8ASALT <Tbpb)
--. Field Description
Thia baaalt ia brick-red and containa nuaeroua aaell
gee veaiclea leaa than 1 •• in die•eter. The color ia due
' to a high degree o£ alteration producing iron ataining.
Visible unaltered basaltic: stringers approxiaately 2 •• wide
and 10 •• in length are coaaon. Thia unit for~s a low
' profile flow which exhibits a clinker-like texture. It
erodes rapidly and contains large weathered vuga.
' Petrographic: data ia not available for thia unit.
I .ll...
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RHYOLITES OF THE ROUND MOUNTAIN VOLCANIC FIELD
General Stateaent
The rhyolite& o£ the Round Mountain volcanic £ield have
been claaai£ied in thia atudy on the baaia o£ color,
texture, £low unita, and geocheaiatry. Theae rhyolite• are
deaignated by aap unit naaea that are in£oraal and were
given by the author. In all there are 10 rhyolite unita
which exhibit aany aiailar aacroacopic and aicroacopic
texturea. Because o£ thia, a general £ield deacription ia
provided which ia applicable to all o£ the rhyolites.
Speci£ic re£erence to distinctive £eaturea £or the di££erent
£lows is noted. A brie£ petrographic description o£ each
unit is given in following aectiona. A aore detailed
description o£ the aineral optics and crystal habita ia
provided in Appendix c. Modal analytic reaulta o£ 30
rhyolite thin sections is given in Appendix B.
Field Description
These rhyolites exhibit a wide variety o£ colora
including bu££. gray, black, lavender, purple. as well as
co•binationa o£ these. A aaJority o£ the rhyolite units
show aany colora depending on the location o£ the outcrop.
Texturally these rhyolites vary widely, not only between
units, but alao within a single unit.
Flow banding and other related £low atructurea are
coMaon throughout aoat o£ the rhyolites. Flow banding
- ?4 -
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<figure 8> ia generally accented by variegated coloring of
individual £low loyera. Flow structures predoainote in the
Vivian rhyolite <Tvr>, Andrus rhyolite <Tar>, Gray rhyolite
<Tgr>, Perlite <Tpl>, and Massive purple rhyolite <Tapr>.
The intricate £low atructurea within theae unita enhance
differential weathering along flow linea to produce a platey
regolith on aoat alopea.
Brecciation textures (figure 9> ore preaent chiefly in
the Puaiceoua £low rhyolite <Tp£r>, Rhyolite breccia <Trb>,
and Lavender gray rhyolite <Tlgr> unita. Such brecciation
textures di££er in color, cloat aize, cloat ahape, claat
composition, and aatrix. The breccia units exhibit
spheroidal or bulboua- ahaped outcrops due to preferential
weathering o£ the interclast aatrix aaterial (figure 10).
The claata are predoainately angular to subangular and range
in aize £roa approxiaately 1.4 a to 1 ca. The claat aize
generally decreases with increasing distance £roa the source
vent. The claata are coapoaed o£ rhyolitic glaaa and
coaaonly exhibit £low banded textures. The Puaiceoua £low
rhyolite aay be claaai£ied aa an autobrecciated flow
rhyolite <Paraona, 1969> or aa a puaiceoua £low tu££.
Nuaeroua highly devitri£ied puaice claata are present.
Noticeable incluaiona o£ basalt are incorporated in thia
unit and the olivine present in the basalt coaaonly exhibits
a high degree o£ alteration.
The Perlite unit <Tpl> ia cheaically rhyolitic and
contains nuaeroua lenaea o£ pure black obsidian aet in a
- 25 -
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1-tqlJt" •• 8 Hond spec 1mvn ot V1v1t1n thyoltte exh~b1L1n ~1
. · x t ,~ n ~ 1 v e f 1 o \.1 band i n C) and v n r i e g F-1 t e rl co I o r ~ n g . No t »
th·~ 11 0 ..., c ontort.10n tPotut•' OL th~ r1ght r>nd o f t. t .. .
!'. lmpl•"·
F1gure 9 - Hand spec~mcn show1.ng the brecc1.ot~on texture of the Rhyolite brecria <Trb> unit. Note the flo\.1 banded and oba idion br~cc1o rlnets contained \oi1Lh 1n the highly d,..v1tr1f1ed mo rix.
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I· l • llll' •• 1 0 Rhyol.tte brCCCl..J Ullll <Tt'b) r~ :-:hl!ntlng CY.tenStVC
...... 1: hr·rllhl of Lh•· mal.r!.:.-: •>~•: · r•...,,tnd!.:'\ ':,1 !...,:-·' •.: •.:ta cle!>t.~ .
F i 'l ltt" 11 Elongate lenses < Tp I > .
of obsidian in perlite unil = '
)
' l -I I
• •
gray glassy •atrix (figure 11>. These lenses have an
approxiaate elongation o£ 25:1 due to vertical co•pression
or horizontal !low. Thia unit weathers rapidly to produce
an obsidian regolith paveaent.
The •oat pro•inent eli££ foraing unit is the Kaaaive
gray rhyolite <Tagr>. Thia rhyolite ia very dense, highly
resistant, and locally quite glaaay. It exhibits a
polygonal character due to the straight aaooth fractures
cauaed by froat wedging along Jointing.
Petrographic Textures
Lavender Gray Rhyolite <Tlgr)
This is a relatively hypocrystalline rhyolite wi th 17
percent phenocrysts of sanidine, plagioclase <An24 - high
oligoclase>, quartz, biotite, and opaques set in
aatrix. The resulting texture ia vitrophyric.
a glass
Perlitic
fracturing <figure 12> ia co•aon along with •icro!racturea
that follow phenocryst boundaries. Sanidine May be e•bayed
by plagioclase or contain ophitic plagioclase ~icrolites.
Minor exaolution of aanidine ia present but rare.
Vivian Rhyolite <Tvr>
A virtually holohyaline rhyolite containing
cryptocrystal line sanidine, quartz, plagioclase, opaques ,
and •inor clinopyroxene. Flow baneing is proainent with
•inute feldspar cryatale producing a hyalopilitic texture.
Spherulites, both single and coapound, co•posed of radiating
- 28 -
• l
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£ I
• • • • • • • •
F 1 q11 r·.• 1 2 Pho t.om 1 ct·oo t·a ph 11 1t;;t.ur1 ng tn ll 50mple <TLot >. PhotomlceograJ')h
<.i:,x> Ot
t.alcen
o r l'X nns1ve perl1L1c Lnv~nr!'!t' Gray rhyolltP
und•"t plOllP polArt:-t"'d
F1gutc 1 3 Photomtcrogroph <35x> of n 5pherulite compose~ ot radiati n g feldspar and quartz r-ryatal l itee. The ) photom i crograph of thle eamplc oi Maseive Purple rhyolite was taken with rroeeed poloro.
-
feldspar <sanidine ?> and quartz cryatals · are nuaeroua.
Perlitic fracturing o£ the aatrix ia ainor.
Andrus Rhyolite <Tar>
:I This rhyolite ia hypocryatalline with 3 to 20 percent
~I phenocryata of aanidine, -plagioclaae <An24>, and ainor
quartz, biotite, opaquea, and clinopyroxene. Gaa veaiclea
~~I ·coa11only contain secondary quartz cryatallization. A
-- pilotaxitic •anidine aicrolite texture ia frequently
present. Gloaeroporphyritic clots of plagioclaae, aanidine, ~·
and quartz are preaent. Eabayaent o£ sanidine phenocrysts
by matrix is rare.
1--Bu££ Rhyolite <Tbr>
This unit ia a hypocryatalline rhyolite containing 8 to
15 percent phenocrysts of plagioclaae <An24>, aanidine, and
quartz aet in a glaaa aatrix. Perlitic fracturing ia coaaon
::a along with a well-developed £low banding o£ the aatrix.
Glo•eroporphyritic clots o£ plagioclase and aanidine
phenocrysts exhibit a aubophitic relationship. Gaa veaicles
are occasionally nuaeroua and elongate in £ora.
' I
Massive Gray Rhyolite <Tmgr>
::t Thie unit ie eaeentially equivalent to the Buff
is rhyolite in Mineral habit and fabric. Flow banding not
~ well-developed. The occurrence o£ perlitic fracturing and
I""' r~
gaa veaiclea ia rare. The aatrix is coapoaed of 30 percent
• I
• • • • •
-I
I ~
I
I
gleaa end70 ~ercent cryptocrystalline £eldaper <aenicline ?)
end quartz. Quartz occasionally £oraa
discrete gloaeroporphyritic pods •
Grey Rhyolite <Tgr>
stringer& and
Thia hypocryatelline rhyolite ia quite aiailar in
teX,ture an~ aineralogy to the Maaaive grey rhyolite •
·.Exceptions , which distinguish theae uni ta include £ewer
phenocrysts in the Gray rhyolite wh~ch ere surrounded by
cryptocrystalline £eldaper and quartz.. Flow bending o£ the
aetrix may or aey not be present. Elongate gee veaiclea
commonly contain internally radiating ·growths o£ £1beroua
feldspar <?> and quartz. Glomeroporphyritic clota of
plagioclase end aanidine are present with minor biotite and
quar;-tz.
Perlite <Tpl>
Thia description ia applicable only to the ob&idian
lenaea within the Perlite unit. These lenses are
holohyaline with ainor £low banding end vein structure&
containing cryptocrystalline feldspar~ quartz~ and opaques.
Micro-spherulites <<.1 ••> of quartz and feldspar are
comaon. Pyroxene <?> trichitea radiating £rom opaque cores
are abundant.
Massive Purple Rhyolite <Tmpr>
This unit ia a predoMinantely holohyaline rhyolite with
-_,,..,. ... ~ ~~ :.-
----
• • • • (]
• ,. II
II
II
I
Fl'::n:r·! 14 PhoLomtcrograph <35x> of the dct.echmcnt. oi a cut. ed ra 1 ()henocryst of aon id i ne fr om o larger ean id i ne ,-,·y~t 11 ".•:!t in A glaon matrix <bl"'lc-1<> . Note t.hP 5uboph1t1c rclat~onshl.p of the s anldl.nc ond plogl.ocl o srld••·nn•-1 y·lLn. Tht•\ pholomp-rograph o( Rhyol1.te br .,-rt 1
<ltu> wag taken w1.Lh crosned polatu.
......... :-· . : J ~ " . .. .... \ " . . ~ < ':'\.
I " " ' , 'o ~,. l:,. \ ,,_ ',..: .... . ,.
· .. ... ~- -~-· ·~ · -;·~- \ ·< -~, ~ ' 1 ..... ' ( ....... .. ' .. .--,.. " -...• ~J .. ~ '.. ..-:: . ~ ~ ... \'-:. jf.; . .•·. :: ..
Ftqure 15 - Phot.om·crogroph <3Sx> of Rhyolite brecc1 a <Trb> unit. Pxhlbit.1ng email ecole brecc1et1on texluree. The c lastn have flow banding and ore completely h o l o hyal1ne. Th1s photomtcrogro ph was taken under plnnp polarized liqht.
32
I
- ainor cryptocrystalline £eld&par and quartz. Flow banding
is well developed with ainuous layering o£ feldspar and
I quartz crystallites. Spherulites (figure 13> consisting o£
radiating needle-like crystals o£ feldspar and quartz are
I Branching structures coMposed o£ feldspar and
I quartz are coaaon.
I Rhyolite Breccia <Trb>
This rhyolite is hypocryatalline to holohyaline with
approxiaately 15 percent phenocrysts o£ plagioclase
aanidine (figure 14>, and ainor quartz, biotite, and
opaques. Brecciation texture& (figure 15> predominate with
I angular claata o£ glaaa surrounded by a perlitic fractured
£low glass. The rhyolite contains occasional inclusions o£
highly altered olivine basalt. Overall granularity aay
appear hypidioaorphic due to the £ragaentation of
phenocrysts. Pyroxene <?> trichites are coMmon in the glass
aatrix.
I Pumiceous Flow Rhyolite <Tp£r>
I
I This ia a hypocrystalline rhyolite with 5 percent
phenocrysts o£ anhedral plagioclase, aanidine, end quartz.
It contains nuaeroua basalt inclusions of various textures
and aineral asaeablages. Brecciation textures predoainete
I I
and ainor · layering of glaaa shards produce ~ low tu££
I characteristics. Claata o£ puaice and £low banded glass are
J nuMerous.
I - 33 -
I ') I I I
I I (
I I
I
I
I
I
I
I
I
I
I
•
. :
PETROLOGY OF THE ROUND MOUNTAIN VOLCANICS
General Stateaent
Che~ical cnclyeee ueing whole rock x-ray fluorescence
aethode were per£oraed on 17 rhyolite aaaplea £roa the atudy
area. 0£ theee 17 eaaples, only the Lavender Gray rhyolite
and the Gray rhyolite units were not cheaically analyzed.
CheMical analyses o£ 7 baealtic apeciaens were also
COMpleted. All cheaical analyses were per£oraed by the
X-ray Aeaay Laboratories Inc., Don Mills, Ontario, Canada.
The analyzed unite include the Williaas basalt, Josep
basalt, Field Daa basalt, and Basalt Flows
<undi£ferenticted>. The Vernon and Bishop Place basalts were
not cheaically analyzed. The cheaical analyses yielded both
aaJor and trace eleaent data. The trace eleaents included
in the analyses are Cr, Rb, Sr, Y, Zr, Nb, and Ba. Resuits
o£ the aaJor cheaical analyses and CIPW norMative
calculations are provided in Appendix 0. Trace eleMent
reaults are given in Appendix E.
Rhyolites
All the rhyolites analyzed have virtually identical
•aJar cheaical contenta, with the exception o£ the Puaiceous
Flow rhyolite <Tp£r) or tu££. It should be noted that the
aaaple o£ Puaiceoua Flow rhyolite (27-1> exhibits a siailar
but diatinct cheaical expression due to its high percentage
·;{.""·~ . .,.,. '·
of volatiles <11.7 LOI> and nuaerous basaltic incluaiona.
Theae basal tic inelusiona lower the tot,al 5i02 content and
produce alight increaaea in Al203, FeO, Kgo,· and CaO
percentages. This saaple plots uniquely on all cheaical
diagraaa (figures 16, 19, 20, 21>.
MaJor elaaent cheaical data were plotted on Harker
variation diagraas <figure 1~> to express the hoaogeneity o£
these rhyolitea. The weight percentage o£ 5i02 varies froa
72.8 to 76.6 <LOI uncorrected) and 76.06 to 76.75 £or
noraalized reaulta. Soda <Na20> and K20 range froa 4.01 to
4.60 percent <LOI uncorrected>, and 4. 45 to 4.'87 percent
<noraalized>. Aluaina <Al203>.content £or the rhyolites ia
also tightly grouped and ranges from 13.01 to 13.40 percent
< noraal ized> •
These rocks have been classified as rhyolites <figure 'k
17> baaed on noraative color index <Ol~Opx+Cpx+Kt+Il+Ha> ·~.
versus noraative plagioclase coapoaition . (100
Ani<An+Ab+S/3Ne>> plota <Irvine and Baragar, 1971>. Cheaical
classification )(figure 17> o£ these rhyolites baaed on K20 ~.
and 5i02 abundances, groups thea all as high-K rhyolites
<Barker, 1981). Plots o£ total alkalies <Na20 + K20> and
5102 place these rhyolites in the aubalkaline field.
Trace eleaent variation diagrams <figure 20> for these
rhyolites show thea to be very aiailar in Cr, 5r, Be., and Zr ~
abundances. Baaed on the abundances~£ Rb, Y, and Nb the
rhyolite units have diatinctive expreaaiona. These
rhyolites exhibit variation in Rb, Y, and Nb aaounta ranging ~
- 35 -
j ' ' . i ., j I ) ' . J I ' J P . } } ) ) ) I J ) I l ) ., I ) ., I , ' I 1 . J ' ' j • ) J • } }
J8
~ .l' -..J
. . '· ..
,,~~· '_., . ./
, •• a • ,:1 II ~
·" .$1'
/'
t,
f
ROUND . sil
(;:) 8 Lt.! l( ·•1ft
'" .. . . ,,
:\ 1-1!\ I r-\ ~
I I?
J C,) If
', ~
II II Ill -. . . .
'<:1: I# \
1
l:l -:::- ~-~ ·;···:
, I .. "l 10~--~--~--~--~--~--~--~~ 0 ~- ( '<.:o
1P 21
"I , , I ---····-----
...... ... I ~ .., . 3
C,) 8 j ~ 2
, \.f) ;-';
~~ 1
.. ./.
~
~ ~
" ....
0~~~~~~--~--~--~--~---" 0 -
8r--------------------------------~
.8
·" .........
"'-
- (i
-! (;:) C\J. "l: "
-J <
2 -
. ~ .
0~~---~--~--~--~~--~--~ 0.ro 50 60 .10 50 60 ?0 80
5!02 SlOt?
t
10 80
(;:)
;,'\" .. .i', (~>~ •• ::~:;; ~-:: L. $];;,.-.
• c~~~~--~--~--~--~--~R--"
I,
8r-----------------------------~
6 -.. ,
t"v " :::.:: -
-*""'
~ .... .~. .. 1 ~1•-!..
.. .... . ~ ~ H ~ ft
5!02
Figure 16 - MaJor chemical variation diagro•s showing weight percent of Al203, FeO, MgO, CoO, No20, K20, P205, Ti02, and MnO plotted again$t Si02· Basalts and rhyolites of the Round Mountain volcanic field are plo~ted.
"'>'·'~~>,~
-~-
.:...;-. .;'"11.
I I I
I
I
I
I
I
I
I
I II
+
('< t liC H IGH - K
._. HIGH-K fHiYOL I T · z UJ DACITE u HIGH - K 0: w Q. ANDCSITC
rtHYOL IT t ._. :X: DAC I TE 0 UJ ANDESITE >
LOW-K f"!IIYOLI TE
LOW- ANOCSIT€ LOW - K DAC I T€
QL----L------~--~---------J--~----------~------GO 63
W(IG H T
65 75
PERCENT
F"1gL:r<: bas~d 0:1
a:! h ~9h - ~~
17 - Ch-?::~;c~l cl,:,s:>lflc~tlO:I of ~ndes1te, doc1te. ond ~eight pe~c~nt K~O and Si02· The Round Moun~ain rhyal1tes
:·hyo~'t};;.· <Aft~r e.a~keL 1981)
r--r-,--, -r 1 1 1 1 I I T
l(o -
I• -
... 12 -
<;" 0 10 -..... ~
.. 0 8 -..... "' z. r-
6 -
.- • -
1 -
I I I I 70
rhyolite <·> plot
Figure 18 - Classification of non-potossic volcanic rocks bosed on totol alkalies and Si02. The alkali olivine basalts <~> of Round Mountain plot as _)
basalte and borderline basaltic andesites. CA£ter Cox et al. , 1984>
- 17 -
' '· t·
(-
,-Alt.
,.,. ,-.....
"' "' I .., :-'*' I ~
·- -"""'
"' '
~ ,;<!IIi,
.- j
. .- ~ l
·"'""' ·"""' ~~
""""' ,:f~
,..._ ,"!ffllo.
'"""' !'*'
·-/- -~
,- I r" ,-. -·-11~
l -·-- I /!'J!Io,
-- ' ..-1a...
"" -·-·- II ·-
.. :X:
--~
r.
~ Q,
u
. Q,
0
;;
.X basalt w Cl z
X Tj 1:-
Tjl- X x; ~~db 0:: • Twb 0 ...1 1 Tuh 0 u
w Tbf 1
.?: 1-
andesite G: :::E 0:: 0 z dacite
0 ... D. ..
80 60 40 20 0
NORMATIVE PLAGIOCLASE COMPOSITION IOQAnltllb • An • 5t3 Nel
figure 19- Classification a£ the alkali olivine basalts <unit labeled lO and rhyolites <X) of the Round Mountain volcanic .field based on normative color index and normative plagioclase composition (Williams basalt Twb, Josep basalt- T]b, Field Dam basalt Tfdb, Basalt flowe- TbfJ. Plots are in catlon equivalents. (After Irvine and Baragar, 1971)
- ......
I
.J 0
.... ·- ...... .... ... .... ... - -- - - - - - -ROUND MOUNTAIN
100 10
J + 1 .1,----------~
I
~ ~ ~
• I
I 0
I ()()
:100
~
J()()
0
G()()
c;t' ()() (.j
('()()
I
OJrr
'--
80
. GO f ~ -+
t 10 . . . ~ . .
;>()
0
900 ; J 8001
JOO
GOO .... t 1 . r;J!OO
V)IOO . . 300
<'00
100 . -'\ ++ 0
100
80
GO
.... + + ::.....
10 . ;>() -
7U- ~ o)o I ~ll Cl7 ""'3V SIO~
. . -. . ..
. i
.
. +
.
. +
. . . . .
I I 60
I )o I so SIO~
30
~ ";>()
~ tO
0
I <'00
1000
BOJ ~
~001 100
<'00
0
250
<'00
ISO
~ 100
50
o,o
.. .
50 60
SIO~ 10
l
.. -..
Figure 20 - Troce elemen t variat1on dio!\9l'3ms showing Cr, Rb, Sr, Y, Zr, Nb, B.!!, Zr/B3, end Ba/Rb ebundenc~ <ppm> plotted against weight perc~n t 5102. Basa lts and rhyolites of the Round Moun~e1n volcenlc field are plot~md.
... - .· .. , - . .,
PO
. :~iii ·;, ·~~
.···:.l i
froa 210 to 360 ppa, 20 to 90 ppa, and 40 to 90 ppa
reapectively. A noticeable gap in the distribution o£ Rb
occura between 250 to 330 ppa. Thia hiatus in data provides
a clear distinction between the rhyoltiea aaapled in the
southern portion of the study area to those of Round
Mountain itael£. The southernaost rhyol~tes saapled have Rb
values of 210 to 230 ppa while thdae of Round Mountain are
330 to 360 ppa. Thia separation of trace eleaent abundances
,.,...
I ,;~ ;
with respect to saaple location occurs also for Y and Nb
without e visible break in the distribution of data. The
'"""" i 'I - southern rhyol~tes have values of 20 to 50 ppa and 40 to 60
,..., ppm for Y and Nb respectively. The rhyolites of Round
;fll>. , I
Mountain have Y and Nb abundances of 50 to 90 ppa end 70 to ;~ ; 90 ppm respectively. ·1~ I ,- The trace eleaent analyses have provided data to
indicate that the geologic aap <plate 1> prepared for the
study area aay have ainor discrepancies. The geologic map
~I ; '
was prepared on the baeia of field identification and
rfl"'
~~,I expression of the volcanic units~ along with aa]or chemical
and petrographic inforaation. Trace element analyses of the
rhyolite units indicate that the Mapped exposure of the
Andrus rhyolite.(Tar> and Rhyolite breccia <Trb>, to the
south of Round Mountain~ aay not be contiguous with the
exposures o£ these unite on Round Mountain. The Andrus
rhyolite aapped to the aouth of Round Mountain ia
equivalent, with re&pect to trace eleaent abundance&, to the
Vivian rhyolite <Tvr> unit. Conversely, . the Rhyolite
- 40 -(~,
I I breccia unit 'in the southern study area is not equival.ent.,
with respect to trace eleaent abundances, to the breccia
unit aapped on Round Mountain.
I Basalts
I The basaltic rocks in and around the Round Mountain
I volcanic field exhibit a fair aaount of variation in both
MaJor and trace eleaent abundances. It should be noted that
I one aaap~e, <• 29-6 Bas~lt Flows - undi££erentiated>, has
enoaalous trace eleaent and some aaJor eleaent values
relative to the rest o£ the baaal.tic suite o£ the area.
I This saaple is virtually out o£ the study area however and
exhibits no relationship to· any o£ the Round Mountain
I volcanic events.
I The rocks have been classi£ied (figure 18) as basal.ts
and borderline basaltic andesites based on total. al.kalie& Alil'.
I and Si02 abundances <Cox et al • ., 1984). These rocks pl.ot in ·""""'
the basaltic £1eld baaed on Irvine and Baragar's (1971> -I classification aystea using noraative color index and -
~
I normative plagioclase coaposition <£igure 19). More ~
apeci£ically, speciaena <4-8., T£db>,. . <27-2,. Twb>, and -..
I <28-2,28-3, TJb) are basalt with specimen <27-4,. Twb) being --I
a basaltic andesite. The clasai£ication o£ saaple (27-4> as
a 'basaltic andesite is believed to be caused by the -· -;1&.
occurrence o£ quartz xenocrysts aodi£ying the results o£ the. ) -chemical analyses. The inclusion o£ these xenocrysts aay ----
~\
- 41 - -
-'"""" -,.
) I
r- have increased the Si02 content au££iciently to place the !1111>,
,!1111>, speciaen in the basaltic andesite £ield.
I"' The 5i02 content in these basalts ranges £roa 47.02 to
('"" 52.49 percent (noraalized>. Excluding apeciaen <29-6, Tb£),
'" the total alkali content for these basalts range £roa 3.37
"""' to 4.26 percent <noraalized>. Baaed on MacDonald's <1968)
r-- classification <figure 21a> of alkaline and tholeiitic I"'""
1-suites using Na20 + K20 versus 5i02 plots, theae basalts are
1- alkaline with the exception of speciaen (27-4, Twb> lying in
'i' the tholeiitic or aubalkal,ine field. Classification of th~
i' i'
b~aalts using Irvine and Baragar's (1971) redefined
!; ..... alkaline-subalkaline discrimination boundary, designates
. ~--.. only the samples <28-2, 28-3, 28-5) of the Joaep basalt as
!'*" I
,'~ alkaline, excluding <29-6, Tb£). Based on AFK plots (figure
I
:!'*" 22> comparing the patterns of variation of tholeiitic,
"'"" ! alkaline, end calc-alkaline suites <Irvine and Baragar,
,..., 1971>, the basalts of Round Mountain are interpreted aa I
fA-I having an alkaline trend. This classification is subJective ,;-,
howe~er due to the insufficiency o£ available d~ta. There
;fll!' is a slight iron enrichment trend, but this aay be I
'i consistent with rocks of tholeiitic or alkaline suites.
f!M' I The argument for an alkaline character of the Joaep -'- J basalt samples (28-2, 28-3, 28-5) may be supported by the
'-! Yoder and Tilley <1962) Di-Fo-Ne-Cz tetrahedral
i J i
classification ayatea. The Joaep basalt speciaens lie in
.... J ,.,
the alkali olivine-basalt field on the Ne aide of the
.. critical plane of silica undersaturation .. , while ,the '1""
I I
!""' I
/"""' • - 42 -
•••.IIQ:II:IIIJIJI:IIJI·II ~ . ' 11<11 II Ill
ROUND MOUNTAIN ROUND t·WUNT ~IN
JP r-I
] l I I
I I
I I ...::~ ....
I .... I
I ~ ... : : .. : : I 1':: I I
I I
! I I
i ;>:~ I ~i.: . I
t/ ., :.: ;>: I
~ I )!:.
alkaline !/ ' I 8 +
~
I ~// CJ -...,J
~ ~ subaJ.kaline I I
I
·~-I I
I + I 1:<;:
41 :#d.,. . \.:::). -:1. (\J
~I ++ I
I I
~ -+'-I
~· +
I + I ., I
I I ..
I •i I I
I I I ·; .. I
I I
I
~I X I
I
It 0 l I I . I
40 50 50 lD 80 .:a 50 50 JO eo
5102 5102 (a) (b)
Figure 21- Plot of alkalies versus silico (a) and alkalies and Ca0 versus Si02 (b) for t~e elkali olivine basalts and rhyolites of the Round Mountain volcanic f1eld. Dashed line is ~acDonald's <19GB> dividing line for ~lkaline ond tholeiitic rocks. Solid line is Irvine end·Baragar's <1971) adJusted alkaline-tholeiite boundary.
-·
) ) ) ) ) ) } J ~ ) } ) ) ~ ) ) ~ ·~ -~ } } } ~- ~ )~: -~ .. ,.~~} ~}})})} ~-¥'\ }~ ~
)
/ /
/
/ /
AL------------------------IIo20 • K 20
ftgure 22 AFM diogrom c oli vine bosolts <unit lobeled volcanic field (Williams basalt Tfdb , Boso1t flo~s - Tbf). The and calc - alkaline co~position~
<After Irvine and Boragar, 1971 .
FeO • 0.8998 FczOJ
... - ........
" 1"'1> :-.
' TfJh -,.:~.
1\Nt. • " )1.
' '\ • Tjh '\
-----------------------------~M ngO
~r 1 ng the compositional trend of olkoli end rhyolites <·> of the Round 11ountoin
· ~b. Josep basalt - T]b, Field Dam basalt -~ shed line separotes t he tholeiitic (above) ~ter alkaline compositions were re~oved.
- 44 -
.. , . . . , • r;
.~ .
I I remaining basalt~ lie in the olivine th6leiite field. The
presence of noraative Ne in the three Joaep basalt aaaplea,
r averaging 2.7 percent, placea thea into. the alkali
I olivine-basalt field by neceaaity.
)
Nepheline ia not present
in thin sections o£ these basalts.
f Trace.element variation diagrams (figure 20) show theae
basalts to generally be grouped with other samples o£ the
f sa11.e unit. More specifically the Josep basalt aaaplea form
a fairly tight gr6uping, ~a d6 the Willia•a basalt aaaplea
£or moat trace eleaenta, with the exception ·o£ Cr
abundances. It is worth noting that the specimen <29-6,
Tb£> exhibits high Sr, Be, apd Zr values o£ 830 ppa, 1200 :~~:;~ f,·
ppm, and 210 ppm respectively. This specimen also haa low
Cr abundances o£ only 10 ppm. This enrichment in Sr end Ba _
along with low Cr abundances aay be consistent with a £ew
percent <<S> o£ fractional melting o£ a lherzolitic
<clinopyroxene, orthopyroxene, olivine, and spinel> llantle
composition <Gaat, 1968>.
-- 45 -
-I :-
~
:-~ l
·"'
·~"' :
r / ') l /
EMPLACEMENT AND ERUPTIVE HISTORY OF ROUND MOUNTAIN
General Stateaent
The Round Mountain volcanic field conaiata of various
rhyolite domes and flow units along with basaltic do•ea,
£lows, dikes, and cinder cones (figure 23>. Theae constructs
are the products o£ a coaplex volcanic history. Based on
field observations,. petrographic exa•inations, and
geocheaical data, a developmental model o£ 'the volcanic
complex has been constructed.
The MaJor rhyolitic doae o£ the volcanic field ia Round
Mountain. Field evidence suggests that Round Mountain
underwent both endogenous and exogenous phases in ita
development. Initial perioda o£ development were •ainly
endogenous and were followed by auccesaive exogenous stages
which modified the do•e"a physiographic expression.
Observation o£ the stratigraphy o£ the volcanic unite
associated with the dome reveal information on which to base
a relative aeaaure of the aaounta of volcanic material
erupted during each phase. The endogenous phase expressed
in the field represents approxiiRately 30 to 40 percent of
the total rhyolite erupted, with the relftaining 60 to 70
percent ox 1 the rhyolitic volcanics being o£ exogenous
nature.
The aaJor phases of eaplace•ent of the Round Mountain
volcanic complex have been grouped into aix stages. These
27'30"
Vernon dome
0 0
Josepn. dome~
Bishop
Round Mtn
0 Andrus dome
42'3o··
PQme 6 0 0 Field Dam
dome
~ Williams dome
0 .S Mile
2S'L-----------------------------------~L---------------~
, Figure 23 - Generalized mc.p o£ the Round Mountain area showing the locations of the volcanic constructs. The construe~ names are informal and given by the author.
- 47 -
L tL
~·i.L
,L
IL. ~. J.L,
:/L L ~
'.'_'{~~ ':'L ·.L .,\ .,,
,L.
.L 1.\ .~
Jr.. ,£,
,L
,L
A. ~
A A A
""·
-I
stages represent the aequence o£ volcanic developMent. aoae
of which aay have been synchronously evolving eventa.
Stage 1
Initiation of volcanic activity requirea the generation
of a driving heat source. This stage involved the '
emplacement of a baaaltic aagaa, acting sa the heat source,
into the upper cruat (figure 24>. The basaltic intrusion
uaed in this aodel is derived ·by partial Melting of the
aantle. Best and Brilll.hall <1974> suggest that source ,
regions for the late Cenozoic alkali basalts o£ the Grand.·
Canyon region are froa depths of 65 to 95 km. This estimate
ia based on the generation of alkali olivine basalt ·aagmaa
by incremental partial melting of a peridotite mantle at 20
to 30 kb. The upper cruat, having a granitic composition,
is believed to be the most likely site of emplacement £or
the subsequent formation o£ a rhyolitic Magma.
Crustal structure in the Round Mountain region was a
dominant control affecting the location of the initial
basaltic emplacement. The two maJor phases of structural
e.dJustaent in northern Arizona were (1) from latest
Cretaceous to late Eocene, and <2> fro• Miocene to present
<ShoeMaker et al.; 1974>. The later phase of adJustment
involved large displacement norMal faulting and thus led to
the development of MaJor structural blocks (figure 1, 2>
- .48 -
I I I
I
I
I
I
I
I
I
I
I
I
I I
• .. .. .. ·- ... Ill • • "' " • • • • • • • • • - • • • .. .. • • .. • c • • .. • • • • "1 ....................... , ................... . . . . . . . . . .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . .. . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . .. . . . "' . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . "" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . ·.•.·.·.·.·.· .. ·.·.·.·.·.·.·.·.·.·.·.·.·:, •••.·.•.·•·.·.·.•.·.·.·•·•·•·.·••.·.· .. . . .. . . . . . . . . . . .. .... . . . . . . . . . .. . . .. . ., . . . . . . ~---·.·.·.·.·.·.·.·.·.·.·.·.·.·.·.·.·.·.~
c "' c • Ill C M c .. M c c e M C e W C C J • • • • • • • • • • • • • • • • • • J . . . . . . .. . . . . . . . . . . . . d • • • • • • • • • • • • • • • • •
• • .. .. • .. • Ill .. .. • • • • .. • • • • • . . . . . . . _,. . . . . . . . . . . . ~ • C II c e M C c II c C .. • c tl • .,
• « • • • • .. • • • • • • • • .. • • .. .. • • • • • • • • • • • f . . . . . . ... . . . . . . . . .. . . .. . . .
l'. • • • • • • • • • • Ill • • • • • • • • • • • • • • • • • • • • • • • .. • • 1 •••••••• Ill ............................... - ..
•• ••••••••••••••••";••'••••"~•• •• ••••M ··•·••·••.••••••••••••'••••••• ••• ·•····I .•.. .. ·.·.·.-.·.·.·.··.·.-.-.· .. ·.·.·.· ·.·.·.·.·.·.·.·.·.·.·.·.·.·.· .. ·.·.·., . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . .. . .. . .. . ~--·.·.·.· .. ·.·.·.·.··.·.·.·.· .. · .. ·.·.··.·.·., • • • • • • • • • • • • • • • • • • • • '4
• Ill • ·"' • • • • • • ;II • • • • • • • .... ' . . - . . .. . . . . . . . . . . . . . . . .. .. . .. . . . . . . . . .. . .
• • • • <II • "' • W • If • IIIII • • • . . . . . . . . . .. . • II "' llf If W llf If If' It • . . .. . . . . . . . . . . . . . . . . . . . - . .
<STAGE 1J Emplacement F~gure 24 upper crust along zone o!
of basaltic magma weakness <fault or
into the fracture
eye.tem).
. -. ·- ... -.~ . • • • • • • • J . . . . . . . .. . . .. . .. . . . "' • • • a I ; • . '• .. . . . . . . . ... . . .. . . . . . - . . . . . . . . . .. . . . . . . . .. . . . . . .. . . . . . .
.. • .. • • • • • 'Ill • • • - • • • •
. . . . . . . .. . . . . . . . . .. . . . "" . . . .. . "" . .. . . . . . . . . . . "" . . . . .. . . . . . .. . . .. . .. . . . . . . .. . . . . . . . . .
• .. • • 4 • • • • ..
• "" • • • • • • 1111
. . . . .. . .. . . . . . .. .. . . . .. . . . . . . . . . . . . .
. - . ~ . ~ . . . . . . . . . . . .
. . . . . . . . . . . .. . ..... • • # .. • • • • • • .. • . . . ..
• • « • • • • • • • • • .. . . . . . .. . . . . . . . . ••••• c ••••••••••• , .. . .. . . . . . . . . . . . . . . . . - . . . .. . . .. . . - . . . . . . . . . . . .. " ............. "' . ' . . "'
Figure 25 <STAGE 2) 0£ the cruet Rhyolitic magma zone.
Rhyolite formation by partial melting caueed by the basaltic intrusion.
migrates preferentially along fracture
,_ ~ .
..• '
~--
.:.~~ .'i
~.-r
~-~~~
--·-''~~&
----
"""" ,.., .~
I /" ......, -- .delineated by the fault systeaa. These fault aysteaa are. -.~ I preauaed to extend to considerable depths in the crust and
.~ are thus largely responsible for controlling the sites of
·- I basaltic emplacement on a regional scale. The sites of ·-.- I ·-volcanic centers in northern and central Arizona (figure 1)
fora lineaments _parallel to these fault ayateaa and are
·~ -- believed to be controlled by them <Eastwood, 1974: Shoemaker
- et al., 1974>.
-- Stage 2
-- An upper crust of granitic composition is suggested as
- the most likely site of emplacement for the subsequent ~ - £ormation of a rhyolitic magma (figure 25)~ Eichelberger and ,-.
- Gooley <1977> suggested that since deep regions of the crust
·- have high temperatures, it follows that they are aost likely ,-to yield substantial amounts of rhyolitic magma due to the -- emplacem~nt of a hot basaltic intrusion. However probl~ms
- arise with causing small amounts-of ~hyolitic aagaa to - migrate to the surface without a basaltic heat source as a ·-- driving mechanisa. Therefore due to the very small Qmount
.- o£ rhyolite erupted in the Round Mountain volcanic field, - the upper crust was selected as the region for the -- generation of the rhyolitic magma. ,_ i The contact of the basaltic magma with the granitic -- host rock is believed to have caused partial aelting at
' """' granite miniaum temperatures to yield the rhyolitic aagaa. i'!!l!.. --- - 50 --~
I
I
I
I
I [
I
I
I
I
I
I
I
I
I
Based on theraal calculations. partial aelting o£ a granitic
crust due to the eaplaceaent o£ a'basaltic intrusion, is an
efficient process to generate rhyolitic '1\agaa <Lachenbruch
et al •• 1976).
After a sufficient aaount of rhyolitic magaa was
generated, aagaatic aigration began preferentially along a
fracture or fault zone. Fault zones provide an infinite
permeability £or a fluid mediua relative to the surrounding
country rock and thus would allow rapid upward aigration o£
the newly g~nerated rhyolitic aagaa.
The biaodal nature of the Round Mountain volcanic field.
is, in part, a consequence of fault or fracture zones being
present. Christiansen and Lipaan (19,72> have shown that a
change £roa interaediate to biaodal volcanism in the western
United States correlates well with the tiae when block
•• _,-:~4
,.~.
/ ·~
faulting began in the region. This reflects the importance ~.
that block faulting aay have on controlling the petr.ologic
characte~ o£ volcanic events. More speci£ic~lly, .it
indicates that block faulting aay greatly reduce any aixing
of basaltic and rhyolitic aagaaa by allowing a rapid upward
migration o£ the rhyolitic aagma. The short residence tiae
o£ the aagaaa at depth allows for little Modification by
fractionation proceaaes. -If aixing o£ basaltic and rhyolitic aag•as occurred. it
would result in the £orMation of hybrid aagaas which would
lie along a linear aixing line between the basaltic and -- 51 -
~ -- I -.? --- rhyolitic whole rock coapoaitiona <Eichelberger and Gooley.
/--
I - 1977>. The d~gr~e o£ hybridization depends on the residence
~ time o£ the two di££ering aagaaa, the geoaetry o£ the ,- I interface. and turburance o£ . the liquids <Yoder; 1973). --- I -Mechanical mixing coupled with the intiaate contact o£ a
basaltic liquid with a rhyolitic liquid~ results in the
.. ;,
_,..,. I - £ormation o£ various petrographic textures such as crenulate
-- ~
aargina o£ basic aasses, and wisp-like basic inclusions
1 ~ - • contained within a silicic aatrix <Blake et el., 1965>.
- Neither the textures related to aixing, nor rocks o£ hybrid
- compositions are present in the Round Mountain volcanic·
complex. This adds further support to the hypothesis that
fault or fracture zones controlled the location and rate o£
upward migration o£ the rhyolitic magma.
Stage 3
The upward aigration of rhyolitic magma along a fault
zone continued (figure 26> and ultimately the magma breached
the surface. The initial-eruptive event consisted o£ a
pumiceous £low <tuf£> unit. The total areal extent and
thickness o£ this basal unit are not specifically known. due
to the blanketing of this unit by latter eruptive events.
Based on field observations. this puaiceous flow <tu££) unit I
haa a aaxiaua estimated thickness o£ 15 meters. _This £low
unit is observed only in the south sector o£ the Round
Mountain volcanic coaplex. The unit extends a aini•u• o£
- c:;,-, -
I
I 2.4 ka to the south o£ ita presu•ed source vent. This
biased southward £low direction iaplies eruption £roa an
asyaaetric vent,. or an indication £or a aouthw.ard eloping
topography o£ the aurroundinga at the tiae o£ eruption.
Cheaically this puaiceoua £low ia rhyolite but it has
been designated aa a puaiceoua £low rhyolite <tu££) baaed on
ita texture. This unit contains aoae glaaa ahards
indicative o£ pyroclaatic activity. The highly devit~i£ied
matrix o£ thia unit contains aany aubangular £ragaents o£
pumice, glass,. and ainor basalt. The basaltic inclusions
are thought to have been accidentally incorporat~d into the
£low as it spread laterally over the surrounding plain. The
baaaltic fragments,. along with a £ew rounded inclusions o£ -various lithologies,. aay have been Tertiary gravels lying on
the surface at the tiae o£ eruption. Such gravels were
deposited on the Coconino Plateau £rom aourcea to the south
in early to aiddle Tertiary <Young,. 1982>.
Thia· puaiceoua £low rhyolite Ctu££> ia aiailar to an
initial per£oration breccia as described by Persona <1969> -baaed on the angular to aubangular texture o£ inclusions. f
However this unit haa been designated aa a rhyolite £low
<tu££> baaed on the preaence o£ aoae £low £eaturea exhibited
by the unit that are not preaent in per£oration breccias.
The abundance o£ puaice £ragaents and the light density.
o£ the £rothy glaaa aatrix in£era that the aagaa caae in -contact with water during eruption. The contact waa aoat
- 53 -
,-
,--
-~ I ,_
i "" ---,-a.,
I " I
I " .~
- I -" I -" (--
l -" -- I .-" '''\ ·- )
·- I --- ' ---' ,---' (~
~ I ---J -·-_,
i-·-r /-'
~,
--·'""" I --
·~~-.~.-,-.~--~.-y-.~.-,~.-r-·.~.---.-r-_,-_-.---~-~.-;.-.~-~.~---~ t. ~,~.~~-~=-~-~~-~-=-~-~-=-~-~~-~~-~-~~-~-~~-~-~~-~=--t r ... - • • • • • • • • ., • • • • • • • • • ~·-·.·.·.·.·.·.·.-.· .. ·.·.·.·.·.·.~---· =.·.·.·.·.·.·.·.·.·.·.·.·.·.·.·.·.·.·
• • • • • • • - • .. • - • • • • • - .. : • • • • • • • • • • .. Ill • ,. • • .. • . . . . . . . . .. .. "" . .. . .. . . .. . . .. . . . . .. .. . . . . - . . • • • • It • • • .. • . "' . . . . . . . . . . . . ..
·.·.·.•.·.·.·.·.••·.·.·.·•·.·.•.•••.· . . . . . . "" . . . . .. . . . . . . . . . . .. . . . . .. .. . . . . . . . . . . . • • .. "" • • • • • Ill • • • • • • .. •
' .. . . .. . . . ~- . . . .. . . . . ... _L
. . . . .. - . . . . . . . .~
: . . . . . . .. . . . - . . . . . . -. . . . . . . .. . .. . . . . . . . . . . . . . . . . . . - . . . . . . . . . . . . . . . . . .... : . . . . .. . . . . . "' . . . . .. . . . . . .. . . . .. . . . . . : . . . . . . . . . . . . . . .. . . . . . . . . . .. . .. . .. . . . . . . . . .. . . . . . . . "' . . . . . ..
Figure 26 <STAGE 3) Continued upward migration of rhyolitic magma breaching the surface. Initial £lows are pumiceous due to ~he contact o£- magma w1th water laden sedimentary rocks.
. . . . . . . . . . . . . . . . . -. .. . . .. . . . . . . . . . . .. . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .. . ..c • ... • • • - • • • If .. • • • . . . . .. . . . . . .. . . . . . . . - . . . .. . . . . ~ . . . . . . . . . . . . . . . . . . . . . . .. . . .. . .. . - . . . . .. . . . . .
.. . . . . . . . . . . . . .. .. . . .. . . . . . . .. - . . . . . . . . . . . . . - . - . . . . . . . . -. . . . . .. . . . . . . . . . . . . . . . . . . . . . . . • ..... -.....
: 'IIIli • • • • ....... -...... -· .. . • • • • • • • • • lll! • • • .. .. .. .. • . . . . . . . . . . . . . . . . . : . . . . . . . . . . . . . . . . . .
.. • c .. • • • • • • • • • • .. • • • .. ! • • • • • • • • • • - • • .. • • •
« • • • • • • • • • • • - • .. • • • • • • • • • • • • • • lll! • • • • . .................... . t ...... '111!1 .............. .
i·.·.·.·.·.·.·.·.·.·.·.·.·.·.·.·.· ..
l ................... . . . . . . . . . . . . . . . . . - . . . . . . . . . . . . . . . - . . . . ...................
Figure 27 <STAGE 4> MaJor growth exhibiting typical between
produce a endogenous dome development, ~ntimate contact the extruding magma and the crumble breccia maseive unit.
t:;l.
I I
likely aade as the rhyolitic aagaa per£orated the water
I charged aediaentary atrata o£ the Plateau. Thia would lead
I to the £oraation o£ puaice, glass, and possibly soae
pyroclastic activity which would have contributed tephra
I <Parsons, 1969).
I Stage 4
] This stage of doae developaent involved both early and
late eruptive phases. The existence o£ two separate but
I related phases is re£lected by both trace eleaent data and
£ield observations.
I Examination o£ the trace eleMent abundance·
distributions of the rhyolites of the Round "ountain
I volcanic £ield reveal the presence o£ 2 separate but related
I phases o£ rhyolitic aagaa generation. speci£icallyi More
Rb, Nb, and Y abundances exhibit a separation (figure 20)
I between those rhyolite units o£ the southern atudy area £roa
thoae o£ Round Mountain. Enrichaent-o£' Rb £roa 210 to 360
l ppa and Nb £roa 40 to 90 ppM is not easily explained by
I £ractionation processes within a single aagma chaabe~.
Since Rb and Nb £ollow potassium in crystal £ractionation
and sanidine is the aaJor phenocryst phase present in these
rhyolites, it is probable that a two aagaa chaaber source
may be responsible £or these trace eleaent abundance
variations. Using this in£oraation, it is auggeated that a
vent to the aouth o£ Round Mountain was the aource £or the
- 55 -
"!if!l!t'.
""""' ' """"' \
-;·{~
':)ii.li~ ~~
··~
---
rhyolite unita expoaed in the aouthern portion of the atudy
area. Likewiae the rhyolitea o£ the Round Mountain do111.e
were erupted froa a separate vent and aagaa chaaber.
The rhyolite units o£ the southern portion o£ the study
area erupted prior to those o£ Round Mountain. Evidence £or
this relative age relationship is expressed by the rhyolite
units iamediately to the south o£ Round Mountain. These
units dip aouth approxiJustely 1so_ away fro• Round Mountain.
The attitude o£ theae unita is presuaably due to the
tuaescence or upwarping caused by the initial endogenous
phase o£ Round Mountain.
Early Phase o£ Stage 4
Initial growth during this phase of development was
endogenous and wca ceuaed by the continued upwelling of
rhyolitic magma from a vent in the southern portion of the
aap arec. Continued expansion· o£ the do111.e caused the
cooled, solidified outer-aurface to creek and crumble ea its
atreching liait was exceeded <Macdonald, 1972>. Blocks
generated by the cracking then £ell fro• the top o£ the
expanding doae to fora e crumble breccia around the base.
Furtheraore whenever e_xpansion cracks on the dome opened,
hot viscou~ 1 aagaa ~as extruded resulting in massive breccia
units where the lava incorporated blocks o£ cruable breccia
<Macdonald, 1972>.
. '
I
I I J
I
Following the development of the ·breccia unit in the
southern portion of the study area, a minor phase of
exogenous activity took place. This involved the eruption
of thinly laainated rhyolite £l~s and the foraation of
Vivian dome.
Vivian doae (figure 30) ia located in the southern part
of the . Round Mountain volcanic field approximately one mile
due aouth of Andrus doae. The exogenous nature of Vivian
dome ia exhibited by successive flows that drape over the
sides of thia construct. The origin of theae flows can be
traced to a source vent located near the summit o£ the
dome.
Late Phase of Stage 4
Following the eruption of the Vivian rhyolite, the
volcanic activity ahifted northward to the Round Mountain
aite. This phase was also endogenous and similar to the
. ,.....,
i: ; .. \J. ,,;,;;... . ·~.
;..&.
~.'~
endogenous developaent of the early phaae at Vivian dome. A -·
rhyolite breccia (figure 27> virtually identical to the
breccia unit in the southern part o£ the aap area was foraed
during endogeous growth of Round Mountain. This endogenous
dome developaent is believed to have caused the earlier
£ormed rhyolite units adJacent to Round Mountain to dip
slightly to the south. The Round Mountain rhyolite breccia
unit is approxiaately 75-90 m thick and is present
- ~7 -
.-~
it . ---,-- ) .-..
- circumscribing the Round Mountain dome aa the lower slope /-former.
r"" (- Textures of both the aatrix and the clasts, show
' - extremehorizontal and'vertical variation throughout this -- unit. The brec:~ia clasts are generally angular to
~- subangular and highly devitri£ied. Sizes o£ the breccia - fragments generally decrease with increased distance £roa -- the source vent, ranging £roa approxiaately 1.5 meters to
·- leas than a centisneter.
·-(-- Stage 5 -- Further development of the Round Mountain dome involved .......
.--. change in the predominant eruptive character from
·- endogenous to exogenous. During this exogenous volcanic
phase <figure 28>, successive flows o£ rhyolite were erupted
from a vent located near the suamit o£ the dome <figure 31>.
Macdon•ld <1972> points out that a maJority, ~£ not all, o£
rhyolitic domes which exhibit late stage exogenous behavior
begin with an inital endogenous phase.
There were 3 snaJor rhyolite flows erupted during the
early development o£ this volcanic stage that spread
laterally over the doae, and thus saoothed the profile.
Baaed on textures and color,.the £lows have been aapped,
from younger to older, as <1> gray rhyolite, (2) aassive
gray rhyolite, and <3> bu££ rhyolite. The exposure o£ these
- 58 -
. . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . ~ . . . . .. . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . • • • • .. • • • • • • • • IIi • • • • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . .. . . . . . . . . . . . . . .. . . . . . . . .. . . . . . . . . . . . . . . . ...
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........... ·• ..... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............ · ..... . . . . . .. • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......... ·- ...... . . . . . . . . . . . . . . . . . . . .. -............. . . . . . . . . . . . . . . . . . .......... ~~· ...... . . . . . . . . .. . . . . . . . . .
E="igure <STAGE 5) Growth successive rhy61ite £lowe
28 style becomes that spread
exogenous with laterally. A
late Mtn.
stage dome develops on showing a pre£erential
o£ Round the southwest side southward £low direction.
Figure 29 <STAGE 6) Basaltic
... : .... . • • • • • • • ..i ............ . . . . . . . . . . . . . . . . . . . • • • • • • • • • • • ,r • • • • • • • • • • • • • • • • • ,:t • •• , •••• a a • & • • IIIII • •' a ,C • a a • a • . . . . . . . . . . . ......... . ....... . . . . . . . . . . . :. ...... . . . . . . . . . ~ . . . . . . . . . . . . . . . . . ~ . . . . . . . . . . . . . . . . . - . . . . . . . . . . . . . . . . . ) ....... .
~ . . . . . . . .
dikea intrude the £lows along the southeast. o£ Round Mtn •• £arming domes
southwest. and cinder
and rhyolite
north sides cones.
- 59 -
•• ~
J.. ~
,L,
,L
,L ·,L
L L ~-
ylll ,. ,L L ~
VJ.v Jsn ;:• t r:.+ I 1 0 .j U ~
i\p[H O;.<lmat.o::ly
-- - -...... ~ . . ,
dome { d tstnn t. cen 1: ~r > shO\.J 1ng a C• t.h1nly lsm1n 1•.:'! 1hy•.•l1t.~
km ~ c ro~~ ph~Loaraph.
smc-oth
flo·.;e.
tigure 31 - Concentric rhyolilP pulee £lo~e near e o urce vent e.t th .~ su mrn1 t of Round Mounta in.
60
FlQu~e 32 c t 1 • .c ysona w~u~nerLn9
( 0 r- m 1 n 1 "'., r, > l. v e 9 ["a y l" h y 0 l ~ te Round M·:•untu Ln.
c h o r .'3 •:: t. e r o 1
un.1.t iTmg~·>.
Lh~ Clltf
~;::,':\st s.1.de of
~1gure 33 MaJor cl1ff form1ng rhyolite flow unit <Tmgr> sho~ing changes in flo~ thickness on the east side of Round Moun ta.i n.
61
flgu~e 34 - Interface between the gray <Tgr> and mass~ve gr~y tTmgrl rhyolite flowa showing mixing and m1nor brecc1~lion textures . ~ast side of Round Hounta~n-
F1gure 35 - Small intrus1on of Andrus rhyolite <Tar> into e cavity on the upper flow eurface or the maaaive gray rhyolite <Tmgr> .
62
'· .
flows is confined to the Round Mountain, . doae. The liaited
lateral extent o£ these units infer high viscosity and low
volu~te. This is suppprted by the feet that the grey end
massive gray rhyolite <figure 32> flows ere thick, eli££
forming units (figure 33). Within ~heae flows there is
evidence for the presence o£ •ore fluid leva phe~es locally
which ere characterized by laairier flow structures on the
scale o£ a hand speci•en.
Relative tiaing of these flows can be defined by
examining the contacts between thea. Uaing this method, it
was deteratined that there·waa little or no elapsed time
between the grey rhyolite flow and the olde,r massive grey
rhyolite flow. This €an readily be demonstrated in the
field by evidence of viscous aixing at the inter·foce between
the two flows with only 1linor brecciation (figure 34>. The
relative tiae span between the eaplecement ·Of the aessive
grey rhyolite and the bu££ rhyolite was. however sufficiently
long to allow for cooling of the upper surface of the
massive grey rhyolite flow. This cooling period resulted in
.. :;_.,
~ )-
a very distinct, sharp contact between the succeaaive ~.
£lows.
The late phaae of thia' eruptive atage involved ainor
dome building on the auamit of the Round Mountain doae. Two -constructs were formed during thi~ phase: the £!rat was a
small rhyolite mound, and the second waa a secondary -
rhyolite doae o£ moderate size. The moderate sized dome, --.
- 63 -
,""'"' ;a.
;a.
,_ <) ,-~
·"" ---~
;a.
·""' ~ ------·-/~
·""""' -. .--·-I..., -~-·--,-·-·-----,,., A' , . ._
·--
designated Andrus doae,. exhibits an exogenous character.
This doae is lo6ated on the aouthweat end of Round Mountain
and .foras Round Mountain's highest po~nt of relief. ' . : .' . . .
The Andrus rhyolite·flows were relatively fluid <figure
35) COJil~ared to the JllasaiVe gray and gray rhyolite units and
exhibit a thinly laainated character. They are very siailar
in both texture and color to the flows £roa Vivian doae.
The greatest voluae of rhyolite £roa the Andrus doae was
directed to the south-southeast and was controlled by the
topography at the tiae of eruption.
Stage 6
The final stage of volcanic developMent of the Round
Mountain field involved the eaplacement of basalt resulting
in a variety of features including doaes, cinder cones,
radiating dikes, and soae flows <figure 29). All of these
late stage basaltic.constructs are located _in areas which
outline the rhyolitic doaes and £lows. Eichelberger and
Gooley <1977> observed that aa£ic bodies are typically
satellitic to silicic centers, in both plutonic and volcanic
complexes. Only the basaltic constructs of Joaep doae and
Williants doae 'exhibit direct petrologic contact with the
rhyolitic volcanics.
The source of these basalts is believed to be the aaJRe
basaltic aagaa intrusion which acted as a heat source £or
- 64 -
F1gure 36 Cross-cutting d1ke of Josep bosolt <TJb) through V1v1an rhyol1te <Tvr). south of Joaep dome.
Figure 37 - Josep dome showing o slight conical form minor late etege basal t eruption . Approximately across photograph.
- 65 -
due to .75 km
·· ~· , . ·, : ..
• ~ JL
~
(-, -I ,--.) (----/"(!0,
----------. .-, -------·--l---·-·-.----/a...
-·-/"""
-· -
the generation of the rhyolite. The eaplaceaent of these
late stage basaltic conatructa ia liaited both in tiae and
in apace. In the biaodal aaaeablagea o£ the Snake River
Plain and Yellowstone volcanic £ielda~ basaltic eruptions
occurre~ both prior to and after conatruct-£oraing rhyolite
eruptiona <Eaton et al.~ 1975>. In the Round Mountain
volcanic field,. both the northern and southern parts~
basaltic eruptions were liaited to a poat rhyolite stage.
The location• o£. theae basaltic constructs appears to
be controlled~ in part~ by fractures~ especially in the
southern hal£ of the field. The fracture control is
reflected by the linear character o£ the dikes radiating
£rom Williaaa doae,. and by the alignment of constructs
asaociated with the 3oaep basaltic eruptions. The exact
origin of theae lineaaenta ia not known,. but ia preauaed to
be the product of local atructural adJustment due to the
reaoval o£ rhyolitic aagaa £roa aubaurface chaabers.
The developaent o£ the fractures allowed basaltic
activity to develop as the .rhyolitic activity ceased.
Bailey and others <1976) have used the diatribution o£
basaltic vents to deteraine the position o£ the active
aubsur£a6e silicic body by assuaing that dense basaltic
liquid could not pass through the body.
Thena basaltic constructs and dikes can be seen
intruding various late stage rhyolite £lows in the southern
sector o£ the volcanic field. One o£ these basaltic
- 66 -
I'
I I
I
I I
I
I
I
I
I
I
I
I
features · 1& Josep doae (figure 37>. Josep doae risea
approxiaately 90 meters above the surrounding plain. It is
connected to two ainor constructs, to the north and
northwest, by £eeder dikea. These two smeller constructs
appear to have £oraed simultaneously as evidenced by
interconnecting £lows.
In figure 36, a dike of Josep basalt exhibits a
crosacuting relationship to a late stage Vivian x-hyolite·
£low. The emplacement of Joaep dome caused ainor upward
/ displacement o£ adJacent rhyolite units •. Thia displaceaent f
\ is probably due to pre-eruption tumescence of the iaaediete \
; surroundings as the basaltic magma pushed up through the t
/ rhyo1ite units. Josep dome development was involved in a
minor phase of cinder activity prior to the eruption of the
£inal flows. This cinder phase is recognized by areas of
cinder peveaent associated with the construct, and also by
the incorporation o£ cinders in some latter basaltic flows.
A distinctive basaltic feature in the southern sector
of the Round Mountain volcanic £ield is the Willieas doae.
Located JUst southeast o£ Round Mountain, Williams doae has
a local relief of epproxiaately 55 aeters. Williaas dome
consists priaarily o£ radiating dikes which trend roughly
north-south and northeast. These dikes radiate £rom a
central vent located at the auamit,o£ the construct. There
is a saell depression at the auaai t o£ Williaa·a doae which
is filled doainantly with basaltic cinders indicating a late
- 67 -
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__ ,"'!- __
-:.1-~
~
.Jif~~
-
-
-
,-
,...
---,... ('""""
~
~
,.-
.-.------------~ ---·---I '--I - ,>
I ; ·- ' ,-A
' -,. --f -r'll!l. -I -
~,
-' :- i ~
·.--fl-
. •
~~.:·~?1 Tbr ·";."
-....
'· .,... .
t. / /
:
-..,._ -~
0 .5 1lan .5 Mij
Figure 38- Geologic ·map of Round Mountain showing maJcr rotational £ault striking N70W. The relative motion is down to the eaet.
- 68 -
I
SOUTH
Figure 39 Block diagram represent~ng rotational or eciesor £ault relative movement with respect to the £aulting o£ Round Mountain. The downdropped side ie to the east with maJor displacement to the north and relatively zero to the south .
Figure 40 ' - Rotational £ault eouth> end o£ Round rhyolite £low unite to down to the le£t <east>.
expressed at the north <looking Mountain. Note truncation o£ the right. Relative motion is
69
,-,-
'--- ) -- stage pyroclastic eruptive phase. The depression i& -- probably due to withdrawal o£ basaltic aagaa as volcanic
All>, activity slowed and then ceased. - The :final phase o:f developMent o:f the Round !fountain -/.., volcanic field involved the developaent of late stage local
- :faults .. A aaJOr :fault etrikea approxiMately NlOE directly - through the eastern hal£ o£ Round Mountain (figure 38). Thia -- :fault scarp which ia traceable £or JUst under 2 kiloaeters ..
- The relative aotion is down to th~ eaat with relative - vertical diaplaceaents o£ 45-SS aetera to the north end o£ -- Round Mountain (:figure 40), and 3-6 aeters to the south.
i- These different aagnitudea o£ displaceMents indicate a - rotational, or scissor, :fault <figure 39) with ita vertex to ·-- the south o£ Round Mountain. Faulting ia typically
- associated with late stage volcanic developaent when
·- drainage o£ the underlying aagaa reaovee structural support -- <Macdonald, 1972). Since the location o£ aagaa chaabers
,.- associated with the Round Mountain volcanic activity ere - controlled by fault zonea, it is preauaed that the late -_, stage structural adJustments have also occurred along £eult I
,,-.,I zones.
~,
·-! --I • ,r@o.
-I . --r ·-· -r - 70 --
CONCLUSIONS.
Round Kountain ia a pro~inent rhyolite dome in
northwest Arizona. The dome is approximately 2.1 km in
dia~eter at ita baae and haa a local relief of 268 aeters.
Round Mountain is centered in a virtual sea o£ basalt and is
late Tertiary in age.
Round Kountain ia both endogenous and exogenous in
-nature. Initial· endogenous doae building involved the
I £ormation o£ rhyolite breccia units which constitute
approximately 30 to 40 percent o£ the total erupted
rhyolite. Later constructive volcanic phases, mainly
exogenous. were characterized by 4 maJor rhyolite £lows
which cap the dome.
The Round Mountain volcanic field exhibits strong .-
bimodal characteristics. consisting of high-K rhyolites and
alkali olivine basalts. The rhyolites are placed into 10
mappable units baaed on color and £ield textures. The
basalts are classed into map units on the basis o£ source
vents and £low morphology.
The rhyolites are composed o£ approximately 90 percent ~
glass with maJor phenocryatalline phases consisting o£
sanidine and high-temperature oligoclase.
Emplacement o£ Round Kountain began with basaltic magma
migrating to the upper crust preferentially along deep
crustal fracture zones. Rhyolitic magma was generated by
-
---~- ') ,.._
·- partial aelting of a granitic upper cruat and later eaplaced -- along aur£icial fracture zonea. The hot basaltic intruaiona
- were r~aponaible for the heat aource neceaaary to initiate
·"""" partial aelting. -( -· --/~
<~ ---'---· ·-·-/~
~ I ~ -I -'- I -~ I --' ·-'-,- ' ! ,- ' ·--' ("'
I~ a ('!"' - 72 -
REFERENCES CITED
B~iley~ R. A., DalryMple, G. B., and Lanphere, K. A., 1976, Volcanisa, Structure, and Geochronology o£ Long Valley Caldera, Mono County, Cali£iornia, Journal o£ Geophysical Research, vol. 81, p. 725-744.
Barker,F., 1981, Introduction to special issue on granites and rhyolties : A coaaentary £or the non-specialist, Journal o£ Geophysical Research, vol. 86, no. B11, p. 10131-10135.
Beat, K. G., and BriMhall, W. H., 1974, Late Cenozoic basaltic aagaas in the western Colorado Plateaus and· the Basin and Range transition zone~ U.S.A.~ and their bearing on aantle dynaaica, Geological Society o£ Aaerica Bulletin, vol. 85, p. 1677-1690.
Blake, D. H., Elwell, R. W. 0., Gibson, I. L., Skelborn, R. R., and Walker, G. P. L., 1965, Soae relationships resulting £roa the intimate association o£ acid and basic aagmaa, Quarterly Journal o£ the Geological Society o£ London, vol. 121, p. 31-50.
Breed, J. B., and Roat, E. C., 1974, Geology o£ the Grand Canyon, Museua of Northern Arizona, Flagstaff, Arizona, 185 p.
Carr, M.H., Saunders, R.S., Stroill, R.G., and.Wihelm.s, D.E., 1984, The geology of the terrestrial planets, NASA SP-469, p. 107-269.
Carr• M. H., 1980, The geology of Mara, American Scientist, vol. 68, p. 626-635.
Carr, M. H., and Scott, D. H., 1978, Geologic aap o£ Mara, Preliainary version, U.S. Geological Survey.
Carr, H. H., Greeley, R., Blasius, K. R., Guest, J. E., and Murray, ~- B., 1977, Soae Martian volcanic £eaturea as viewed £roa the Viking orbiters, Journal o£ Geophysical Research; vol. 82, no. 28, p.3985-4014.
Christiansen, R. L., and LipMan, P. W., 1972, Cenozoic volcanisa and plate tectonic evolution o£ the, western United States, pt. 2, Late Cenozoic, Royal Society o£ London Transactions, vol. 271, p. 249-28~.
Cox, K. G., Bell, J. D., and Pankhurst, R. J., 1984:, The Interpretation o£ Igneoua Rocke, Allen and Unwin publ., 450 p ••
,L
.L ~ ,L ,L
,L,
··:.L ~.L
~. ',L
··~.
---
c:-
~ ---------' -,4111., --------·---,-. -
,4!11>
-----.-; --I -_, --· -r --r--1 ('
~~I 1-;-J ·--I -
')
Deuaon,. P. E.,. 1971,. The relationship between Late Cenozoic volcaniaa and tectoniaa and orogenic-epirogenic periodicity,. in The late Cenozoic glacial agea,. Yale University Preaa,. p. 15-35.
DeMon, P. E.,. Sha£iqullah, K.,. and Leventhal,. J. S.,. K-Ar chronology £or the San Francisco volcanic £ield and rate of erosion of the Little Colorado River, in Geoiogy o£ northern Arizona, in Pt. 1 - Regional Studies, Geological Society of Aaer-ica Rocky Mountain Section.Meetinga, p. 221-235.
Eastwood, R. L., 1974, Cenozoic volcani&• end tectoni~on• of the southern Colorado Plateau, in Geology o£ northern Arizona, in Pt. 1 Regional Studies, Geological Society of Aaerica Rocky Mountain Section Meetings, p.236-2S6.
Eichelberger, silicic basaltic Monograph
J. c., and Gooley, R., 1977, Evolution of •agaa cha•bera end their relationship to
volcaniaa, Aaericen Geophysical Union
Gcst,. P. W., origin o£ Geocheaice 1057-1086.
20,. p. 57-77.
1968,. Trace eleaent fractionation tholeiitic and alkelihe •egae et Coaaocheaica Acta,. vol.
and the types,
32,. p.
Greeley, R.,. and Spudis, P., 1981,. Volcanism on Mars, Reviews of Geophysics and Space Physics, vol. 19, no. 1, p. 13-41.
Head, J.W.,. and Gifford, A.,. 1980,. Lunar acre doaes : origin, Moon Planets, vol. Classification end aodea o£
22, p. 235-258.
Heinrich,. E. W., 1965,. Microscopic Identification o£ Minerals, McGraw-Hill Book Co., New York, 414 p ••
Hereford, R., 1975, Chino northwestern Arizona, Bulletin, vol. 86,. p.
Valley £oraation (Caabrien ?> in Geological Society o£ Aaerica , 677-682.
Hodges,. C.A., 1979,. SoMe lesser volcanic provinces on Mars,. NASA Technical Meaorendua 80339, p. 247-249.
Hunt, C. B., 1956,. Cenozoic geology o£ the Colorado Plateau,. U.S. Geologiecl Survey Professional Paper 279, 99 p.
Irvine, T. N., end Baragar, W. R. A., 1971, A guide to the cheaical classification o£ the coa•on volcanic rocks,
- 74 -
I I i I
.. l , .
I
I
I ...
I
I I
Canadian Journal of Earth 523-548.
Sciences,. vol. 8,. p.
Julian, s: R.,. 1976, Regional variations in the structure in no~th America,. Transactions Geophysical Union,. vol. 51,. p. 359.
upper JRantle of AJRerican
Keller, G. R., Braile, L. W.,. and Schlue, J. W.,. 1979,.. Regional crustal structure o£ the Rio Grande rift £roa aurf'ace wavediapersion "'eaaureaents,. In: Rio Grenda Ri£t : Tectonics and Meg•atisa,. Aaerican Geophysical Union, Washington D.c.,. 438 p ••
Lachenbruch,. A • H. ,. Sea a,. J. H. ,. · T. H., 1976,. Geother•al Conduction Models £or the
Munroe, R. J.,. and Moses,.
o£ Geophysical Research,.
Setting end Siaple Heat Long Velley Caldera,. Journal
vol •. 81,. p. 769--784.
Laughlin,. A. W.,. Brookinai D. G.,. and Carden,. J. R., 1972, ratios o£ a single baselt
Science Letters,. vol. 14, Variations in the strontium £low, Earth and Planetary p. 79-82.
Leeman, w. P., 1974,. Late Cenezoic alkali-rich .baaalt fro• the western· Grand Canyon area,. Utah and Arizona,. isotopic coapoaition of atrontiua, Geological Society o£ Aaerica Bulletin,. vol. 85, p. 1658-1655.
Leonardi, P., 1976,. Volcanoes and Iapact Craters on the Moon and Mara, Elsevier Publ.,. p. 225-244.
Lucchitta, I., 1974, Structural evolution· a£ northwest Arizona and ita relation to adJacent Basin and Range province structures,. in Geology o£ northern Arizona,. in Pt. 1 - Regional Studies, Geological Society o£ Aaerica Rocky Mountain Section Meetings, p. 336-354.
Macdonald, G. A., 1972,. Volcanoes,. Prentice-Hall Publ., p. 510.
MacDonald, G. A.,. 1968, lavas,. Geological 477..:522.
Coaposition and origin of' Hawaiin Society o£ Aaerica Meaoir 116, ·P•
McKee,. E. 0, and McKee, E. H.,. 1972, Pliocene uplift o£ the Grand Canyon region, ti•e o£ drainage adJUStMent,. Geological Society o£ Aaerica Bulletin,. val. 83, p • 1923-1932.
Miyashiro, A., 1975, Volcanic rock setting,. Annual Reviews Earth and vol. 3, p. 251-269.
- 75 -
aeries and tectonic Planetary Sciences,
.:~
'.,:J. ~:·
i~f;-~
~~~-
--
-/,_
~
,_
.- ) ?
F""· ,_ ,_ ,-.
/8,
--..-..-..-,_
.-
..-/'*'
,,-,
-,_ ..--"""" ..--·""""' - '·<;~;:-· ·-.-,-~
. ..-. --~
~
~ ----
Moor.e,- R. B.,. Wolfe,. E. W.; and. Ulrich,. G. E.,. 1976,... Volcanic rocks of the eaatern and northern parts.of the Sa~ Francisc6 vqlcanic field,. Ari~ona: Jou~nal of' Research,. U. s. Geologi~al Survey,. vol. 4,. ·P· 549-560.
Moore,. R. B.,. Wolfe,. E. w.,. and Ulrich,. G. E.,. 1974,. Geology. o£ the eastern and northern parts of the San Francisco ·volcanic field,. Arizona: in Geology of Arizona,. Pt. II - Area studies and field guides: Geological S?ciety of Aaerica Rocky Mountain Section Meetings,. p. 465-494 •
Moore,. R. B.,. 1973,. Alkali-rich,. high-alu•ina basalt froa the San Francisco volcanic field,. Arizona,. Geological Society of Aaerica Abatracta with Prograaa, vol. 5, no. 7,. p. 744 ..
Nealey,. David L.,. 1980,. Geology of' Mount Floyd and vicintity Coconino county,. Arizona,. Northern Arizona
University,. M.S. thesis,. p. 144.
Parsons, W. H., 1969,. Criteria for the Recognition of of Volcanic Breccias: Review,. Geological Society of Aaerica Meaoir 115, p. 263-304.
Poldervaart,. A.,. 1950,. Correlat'ion · of physical properties and cheaical coaposition in the plagioclase,. olivine,. and orthopyroxene aeries,. Aaerican Mineralogist,. vol. 35,. p. 1067-1079.
Roy, R. F., Decker,. E. R.,. Blackwell,. D. D.,. and .Birch,. 1968,. Heat flow in the United States,. Journal Geophysical Research,. vol. 73,. p. 5207-5221.
F •" of
Sauck,. W. A.,. and Suaner,. J. S.,. 1971,. Residual aeromagnetic map o£ Arizij.pa,. Tuacon,. Arizona,. Un~rsity of Arizona. 11)
Sho~aaker,. E& M.,. Squires, R. L.,. and Abrams, M. J.,. 1974,. The Bright Angel and Mesa Butte fault sys~ems of northern Arizona,.. in Geology of northern Arizona, in Pt. 1 - Regional Studies,. Geological Society of America Rocky Mountain Section Meetings, p. 355-387 •
Shuey, R, T.,. Schellinger, D. K.,. Johnson,. E. H., and Alley, L. B.,. 1973,. Aero•agnetica and the transition between the Colorado Plateau and the Basin and Range provinces,. Geology,. vol. 1,. no. 3,. p. 107-110.
Smiley,. T. L.,. 1958,. The geology Crater,. Flagstaff, Arizona~ in Society Guidebook 9th Field
- 76 -
and dating New Mexico
Conference,
of Sunset Geological
Black Mesa
I I
Basin, northeastern Arizona, p. 186-190.
Toulain, P •• Baird, A. K., Clark, B. c .• Keil~ K., Rose, H. J., ChristLan, R. P., Evans, P. H., and Kelliher, W. C., 1977, Geocheaical and Mineralogical Interpretation o£ the Viking Inorganic Cheaical · Res~lts, Journal o£ Geophysical Research, vol. 82, p. 4625-4634.
Weririch-yerb~ek, K •• and Thornt6n, C. D., 1974, Preliminary geocheaical study of the lava £lows o£ the San Francisco Peaks, In: Geology o£ northern Arizona, in Pt. 2 Area Studies and Field Guide, Geological Society of America Rocky Mountain Section Meetings, p. 592-601.
Wise, D. U., Goloabek, M. P., and McGill, G. Tharais Province o£ Mara: Geologic Sequence, end a Deformation Mechanism~ ICARUS, vol. 45E>-472.
Yoder, H. S., 1973, Contemporaneous basaltic and magmas, American Mineralogist, vol. 58, p.
E., 1979, Geoaetry,
38, p.
rhyolitic 153-17.
Yoder, H.S., and Tilley, C. E., 1962, Origin of basaltic magmas: Ari experimental ~tudy of natural and synthetic rocks systems, Journal of Petrology, vol. 3,. p. 792-805.
Young, R. A., 1982, Paleogeoaorphic evidence for the structural h~story of the Colorado Margin in western Arizona, In: Mesozoic-Cenozoic tectonic evolution of the Colorado River region, Californa, Arizona, and Nevada, Frost, E. G., and Martin, D. L. eda., Cordilleran publ., San Diego, California, p. 29-39.
Young, R. A., McKee, E. H., 1978, Early and middle Cenozoic drainage and erosion in west-central Arizona, Geological Society of America Bulletin, vol. 89, p. 1745-1750.
- 77 -
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