Strontium and Oxygen Isotopic Data and Age for the Layered Gabbroic Dufek Intrusion, Antarctica

By R. W. Kistler1 , L D. White1 , and A. B. Ford 1

Open-File Report 00-133

2000

This report is preliminary and has not been reviewed for conformity with the U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY

U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025

TABLE OF CONTENTSAbstract 3Introduction 4Analytical Methods 6Table 1 7Strontium and oxygen isotopes 9Age of the Dufek intrusion 16Summary 18Table 2 19References Cited 20Table 3 24

Strontium and Oxygen Isotopic Data and Age for the Layered Gabbroic Dufek Intrusion, Antarctica

By R. W. Kistler, L D. White, and A. B. Ford

ABSTRACT

The Dufek intrusion is a mafic, differentiated stratiform igneous complex exposed in the northern Pensacola Mountains. Due to its similar age and geochemistry, it is considered to be part of the Ferrar Dolerites that are the hypabyssal phases of the Jurassic tholeiitic Ferrar group of Antarctica. The intrusion is composed principally of cumulate gabbros with lesser amounts of anorthositic, pyroxenitic, and magnetite cumulates that are capped conformably by a thick unit of granophyre of granodioritic composition. The Dufek intrusion was sampled from its exposed parts in two ranges in the Pensacola Mountains. The stratigraphically lower section is called the Dufek Massif and the upper one is called the Forrestal Range section. This report summarizes Rb and Sr abundances and 87Sr/86Sr isotopic values for 89 whole-rock and 52 mineral (plagioclase, pyroxene, apatite) samples, and d 180 values for 89 whole-rock and 54 mineral (plagioclase, pyroxene, magnetite) samples from the Dufek intrusion.

The calculated initial 87Sr/86Sr values for mafic whole-rock specimens generally increase upward within a range between 0.7083 and 0.7121 and indicate the Dufek intrusion crystallized from a mantle derived tholeiitic magma that was variably contaminated with crustal melts. Along with reversals in trends in mineral chemistry, a major reversal in the general upward increase of initial 87Sr/86Sr probably reflects an introduction of new magma in the Forrestal Range section about 1000 meters below the top of the intrusion. The initial 87Sr/86Sr of the capping granophyre of 0.7128, and values from 0.7154 to 0.7360 for six cross-cutting aplite dikes throughout the intrusion indicate all of these siliceous rocks are crystallized crustal melts. In several specimens initial 87Sr/86Sr values of pyroxene, and/or plagioclase and apatite are different and reflect a lack of isotopic equilibrium between cumulate and noncumulus phases in these rocks.

Whole-rock d 180 values from the Dufek Massif section are within the range (+5.0 to+6.9 per mil) reported for other layered

gabbroic intrusions, whereas d 180 values for rocks from the lower part of the Forrestal Range section are generally lower (+3.0 to+6.0 per mil) than those of the Dufek Massif. In the rocks from about 1000 meters below, to the top of the intrusion, d 180 ranges from 0.0 to +5.0 per mil. The low d 180 values along with elevated initial 87Sr/86Sr indicates rocks of the Forrestal Range section were derived from magma variably contaminated with 180 depleted, metamorphosed sedimentary rocks. Reversals of A 180 plagioclase- pyroxene in some rocks suggest that some low d 180 values resulted from subsolidus 180 depletion by interaction with a meteoric water- hydrothermal system.

Regressions of Rb-Sr whole-rock, mineral data for sixteen specimens yield ages that range from 148 Ma to 224 Ma. Many regressions have high MSWD due to the isotopic disequilibrium between cumulate and noncumulate phases in some rocks and ages derived from these regressions indicate only that these rocks are Mesozoic. Regressions with MSWD <1 in other rocks are interpreted to indicate the age of an event. An age of 184 ± 4 Ma is calculated from a precise Rb-Sr whole-rock isochron (MSWD=0.27) for six specimens from a 100 meter section of the capping granophyre. This age, along with a similar published U-Pb zircon age for the granophyre and incremental heating 40Ar/39Ar isochron ages for two plagioclase separates from gabbros indicate the Dufek intrusion is greater than 180 Ma; older than the main mass of Ferrar dolerites.

INTRODUCTION

The mostly gabbroic Dufek intrusion (82° 30' S 50° W) is more than 50,000 square kilometers in area (Behrendt and others, 1981) and probably 8-9 kilometers thick at its southern end (Ford 1976). The intrusion was mapped and sampled in detail by the U.S. Geological Survey in the austral summer 1965-66 as part of the final phase in completing mapping of the entire Pensacola Mountains (Schmidt and Ford, 1969). The intrusion is exposed in two of the northern ranges of the Pensacola Mountains near the southeastern edge of the Ronne Ice Shelf, Antarctica (fig. 1). The age of the Dufek intrusion is Permian or younger on the basis of field evidence (Ford, 1976). On the basis of radiometric dating, it is assigned to the hypabyssal dolerite phase of the Jurassic Ferrar Group tholeiitic igneous activity that occurred throughout the Transantarctic

Mountains (Ford and Kistler, 1980; Brewer and others, 1996; Fleming and others, 1997).

The layered igneous rocks of the Dufek intrusion formed by the accumulation of crystals by settling. Rocks of this origin are termed "cumulates" and their textures and structures reflect sedimentary processes. The cumulates are composed of cumulus phases that

80° S 40° W

50° W

81° S

82° S

Ron n*

mm m

Filchner '' i

ce shelf

9O* w

ANTARCTICA

nf

ce Shelf

\ S-Approxinate ice-shelf jf boundary

C ordin Peaks

1DD KM

83° S

83° S 84° S 50° W 48° W

Figure 1 . Location of Dufek intrusion in Antarctica and its approximate outline under ice (dashed line) as inferred from geophysical surveys (Behrendt and others, 1981). Major outcrop areas are shown in black.

settled and packed to form the framework of the rock and postcumulus material which formed in the place that it now occupies. Noncumulus rocks are those such as the capping granophyre of the Dufek intrusion. The stratigraphy of the layered Dufek intrusion is described in detail by Ford, (1970, 1976) and by Ford and Nelson (1972). Ten partial stratigraphic sections were measured and sampled in the area of the Dufek Massif and 11 in the Forrestal Range. These sections are intercorrelated within each range by using one or more key marker layers. A composite section was constructed with a 1800 meter thick lower part in the Dufek Massif and a 1700 meter thick upper part in the Forrestal Range (Table 1). A 2-3-km thick section of unexposed rock between the Dufek Massif and the Forrestal Range lies hidden beneath the Sallee Snowfield (fig. 1). A second hidden section 1.8-3.5 km thick, called the "basal section", is inferred to lie beneath the lowest exposed rocks of the Dufek massif on the basis of geophysical surveys (Behrendt and others, 1974). An unknown thickness of the capping granophyre has been eroded away.

Petrologic, mineralogic, and geochronologic studies of the Dufek intrusion by members of the U.S. Geological Survey are reported in Himmelberg and Ford, 1975, 1976, 1983; Drinkwater and others, 1985, 1986; Kistler and Ford, 1979; Ford and Kistler, 1980; Ford and others, 1983, 1986; Ford and Himmelberg, 1991. This report presents Rb and Sr concentration and Sr and oxygen isotopic data for whole-rocks and minerals of the Dufek intrusion. Preliminary results of this isotopic study were summarized in Ford and others (1986).

ANALYTICAL METHODS

The rubidium and strontium concentrations and Sr isotopic data reported in Table 3 were gathered by members of the Intrusive Geochronology Project in the Geologic Division, Sr isotope laboratory at the USGS in Menlo Park, California. Results are presented for whole-rock powders milled to less than 200 mesh, pyroxene separates -100+300 mesh, and plagioclase and apatite separates -170 mesh. Minerals were isolated by standard techniques of magnetic separation and by gravity in heavy liquids. Plagioclase separates with a prefix "A" have specific gravity <2.7 gm/cm3 whereas those with prefix "B" and all other plagioclase separates have specific gravity > 2.7 gm/cm3 (Table 3). Rubidium and strontium

abundances of whole-rocks and plagioclase separates were determined by energy dispersive X-ray fluorescence methods, whereas standard isotope dilution techniques were used to determined these elemental abundances in pyroxene and apatite. Concentrations of Rb and Sr by X-ray fluorescence are ± 3%, whereas they are ± 1% by isotope dilution. Strontium isotope ratios were determined using a MAT 261, 90° sector mass spectrometer, using the double rhenium filament mode of ionization. Strontium isotopic compositions are normalized to 86Sr/88Sr=0.1l94. Measurements of

Table 1. Sample Number, Rock Type and Elevation of Analyzed Specimensfrom the Exposed Dufek Massif and Forrestal Range Sections of the Dufekintrusion, AntarcticaDufek Massif SectionSample Number Rock TypeAnorthosite Zone192Fb193Fa195Fb

Elevation (meters)

2155232

Anorthosite (plag)AnorthositeAnorthosite

Lower Gabbro Zone38Fa leucogabbro 278 39Fa anorthosite layer 293 Middle Gabbro Zone46Fa mafic gabbro 506 49Fb mafic gabbro 585 277Fa mafic gabbro 643 52Fa mafic gabbro 698 54Fa mafic gabbro 827 117Fa mafic gabbro 936 57Fb felsic dike 986 58Fb mafic gabbro 1013 Upper Brown Zone252Fb pegmatite 1066 198Fi mafic gabbro 1067 226Fa mafic gabbro 1068 11 IFe mafic gabbro 1070 199Fa mafic gabbro 1097 Upper Layered Zone240Fc mafic gabbro 1173 230Fb mafic gabbro 1204 231Fd mafic gabbro 1213 237Fb mafic gabbro 1219 270Fa mafic gabbro 1493 266Fa mafic gabbro 1604 259Fa mafic gabbro 1809 259Fb leucogabbro 1809 257Fa mafic gabbro 1814

Hidden Section, Sallee Snowfield

Forrestal Range Section Sample Number Rock type Lower Layered zone

Elevation (meters)

79Fa78Fb77Fa76Fa75Fe74Fa99Fb99Ff99Fg97Fb306Fd306Fh94Fe94Fi87Fa87Fb86Fa85Fb84FaLower Inclusion83Fa83Fb300FbHenderson Bluff284Fa284Fb287Fe

anorthositegabbropyroxeniteanorthositeleucocumulategabbrogabbroleucocumulateaplitegabbroleucocumulategabbrogabbroleucocumulateapliteFe-rich gabbrogabbrogabbrogabbro

Zoneleucoinclusiongabbroaplite

pyroxenitegabbroleucoinclusion

205357103110114144145147221274275335340429430465491515

524525594

670672674

Upper Layered Zone311Fa310Fb310Fa184Fa309Fb309Fa185Fa188Fa189Fa297Fa-l297Fa-2lOlFa191Fb171Fa72Fa169Fb181Fa182Fc71R167FaUpper Inclusion166Fa157Fd

gabbrogabbrogabbroleucocumulategabbroleucocumulategabbroleucocumulateleucocumulategabbrocontact gabbroleucocumulategabbrogabbrogabbrogabbroleucocumulateleucocumulateinclusiongabbro

Zonegabbroinclusion

7358198229269339379421021105910701072109011161169118512311234125012531268

13101361

8

Forrestal Range Section (con't)Sample Number Rock type Elevation (meters)157Fgl pegmatite 1364157Fd inclusion 1361157Fgl pegmatite 1364157Fg2 pegmatite 1365 Upper Gabbro Zone158Fa gabbro 1405160Fa gabbro 1436161Fa gabbro 1435162Fe-2 gabbro 1482 Granophyre162Fc granophyre 1493163 Fa granophyre 1516164Fa grqnophyre 1542165Fa granophyre 1558313Fa granophyre 15753l5Na granophyre 1600317Na granophyre 163117Na gabbro 1620

NBS strontium carbonate standard SRM 987 yield a mean 87Sr/86Sr of 0.710239±0.000015 over this period. Analytical uncertainties in 87Sr/86Sr values are about ±0.008%. The initial 87Sr/86Sr values reported in Table 3 and whole-rock, mineral isochron ages in Table 2 were calculated using the decay constant for rubidium from Steiger and Jager (1977). and the ISOPLOT program of Ludwig (1988), respectively.

Oxygen was extracted from the samples by the BrF 5 method (Clayton and Mayeda ,1963) and converted to C02 by reaction with hot carbon. The C02 was analyzed on a MAT-250 isotope ratio mass spectrometer. Extraction and analysis of oxygen was by members of the USGS, Water Resources Division, stable isotope laboratory in Menlo Park, California. All of the d-values reported in Table 3 are given in per mil relative to the SMOW standard. All samples were analyzed in duplicate with reproducibility of d 180of ± 0.15 per mil or better.

STRONTIUM and OXYGEN ISOTOPES<

Whole-rock initial 87Sr/86Sr at 174 m.y., hereafter called Sri, versus elevation for a composite Dufek Masif-Forrestal Range section is shown figure 2. The variation with stratigraphic height is one of a general increase in Sri from 0.7083 to 0.713. Superimposed on this trend are (1) a reversal of Sri between the lower gabbro zone

9

and the upper gabbro zone in the Dufek Massif section (2) a reversal of Sri at the upper brown zone to a 1 km thick section called the upper layered zone of the Dufek Massif that shows little variation about a value of 0.709 and (3) a marked reversal in Sri from 0.711 to 0.709 about 200 meters above the lower inclusion zone in the Forrestal Range section and about 1000 meters below the top of the body. Reversals in an overall trend of iron enrichment, (Fe/(Fe+Mg) ratio, in cumulus pyroxenes occur near the same intervals in the Dufek intrusion (Himmelberg and Ford, 1976). The smaller variations from the general trend are associated with cyclic units in the intrusion and explained by convective overturn of the magma (Himmelberg and Ford, 1976). The strongest reversal in pyroxenes compositional trends associated with reversal in A^Os and V203 contents of ilmeno-magnetite occurs at the level of the lower

DUFEK INTRUSION

O3UU-

«?* 2500-CO *»*»»

*9c «vwt£ 2000-*w*

e .S i^nn

II 1000-

Kt\t\5OO-

0-

^>

4

,1

*

ijrf, ft'f^ *

_^tf '*

*,,,

r.. .. ;- '.--,.,

i,

P*

* <fb

.« #L>i

&

t£

0.707 0.709 0.711 (^Sr/^Srt

0.713

LEGEND

O Granophyre* Upper Gabbro Zone* Upper Inclusion Zone* Upper Layered ZoneD Henderson BluffX Lower Inclusion ZoneA Lower Layered Zone

* Upper Layered Zone

T Upper Brown Zone

A Middle Gabbro Zone* Lower Gabbro Zone

* Anorthosite Zone

Figure 2. Plot of initial 87Sr/86Sr versus elevation in meters for a composite of the Dufek Massif and Forrestal Range sections. The 2000 meter missing section beneath the Sallee snowfield is not accounted for on the diagram

inclusion zone of the Forrestal section (Himmelberg and Ford, 1977, fig. 2). A reversal of compositional trend of plagioclase occurs about 200 meters higher (Ford and Himmelberg, 1986). The reversal in trends of all cumulus minerals and the magnitude of the pyroxene

10

reversal strongly indicate an addition of new magma at this late stage of crystallization of the intrusion (Ford and Himmelberg, 1986). The strongest reversal of Sri in the Forrestal Range section begins just above the lower inclusion zone (fig. 2) and also is due to addition of new magma at this level of the intrusion.

Figures 3 and 4 show variations between Sri of minerals and whole-rocks at different elevations in the Dufek Massif and Forrestal Range sections, respectively. Sri of the pyroxenes from rocks in the Dufek Massif section and the lower layered zone in the Forrestal Range section are much less radiogenic than their whole- rock hosts and coexisting plagioclase and apatite separates. These pyroxenes had to have crystallized in a magma with Sri at least as low as 0.7076 and then settled into their present locations in the intrusion. Smaller differences in Sri between minerals and host rocks persist above the lower layered zone in the Forrestal Range

DUFEK MASSIF

zuuu:

1800i

1600:

g 1000:

"S 80(hJi : UJ 6001

40<h

20(h 0-

» Dt+ D

D D

* g] m m 4D J&3

tfD

D nD D ^

« n n , ,*^ 1 [

D

LEGEND

whole-rock

plagioclase

pyroxene

0.707 0.708 0.709 0.710 0.711

Figure 3. Variations of initial 87Sr/86Sr of plagioclase, pyroxene, and their host rocks at different elevations in the Dufek Massif section

section and in some cases pyroxene is the most radiogenic material in a rock (Figure 4). In spite of accumulating at magmatic temperatures, some minerals have not equilibrated isotopically during cooling.

11

Variation of d 180 with elevation is shown in figure 5 for whole-rocks in the Dufek intrusion. For the Dufek Massif section all mafic rocks have d 180 values between +5 and +7 per mil except for one sample with d 180 of +3.8 per mil. Two aplites that intrude the Dufek Massif have d 180 of +3.6 and +1.1 per mil. The trend of isotopic variation in d 180 in rocks of the Dufek Massif is remarkably similar to the strontium isotopic trend discussed above. Values of d 180 change from decreasing to increasing between the lower and middle gabbro zones, then abruptly decrease again to a constant value near +6 per mil in the 1 km thick upper layered zone. The values of d 180 for the rock specimens from the Forrestal Range section are much more variable and generally lower (0 to +6.0 per mil) than those from the Dufek Massif. In the Forrestal Range, rocks from the lower layered zone and the lower inclusion zone, excluding two aplites, have d 180 values that range from +3 to +6 per mil without any trend, and rocks from the upper layered, upper inclusion and upper gabbro

Forrestal Range Section

iouu-

1400^

^ 1200~

| 1000^

^ 800- O IS 600-

m 400-^

200^

0-

0.7

«

^

07 0.7

. Ji

AH

*

4M

d&"

AB^J^i

09 0.711 0.7

LEGEND plagioclasepyroxeneapatite whole-rock

Figure 4. Variations of initial 87Sr/86Sr of plagioclase, pyroxene, apatite, and their host rocks at diffferent elevations in the Forrestal Range section.

12

zones have d 180 values that range from 0 to +5 per mil also without any trend. However, as seen in the Sr isotope versus elevation trend , a major change occurs at the base of the upper layered zone. The overall oxygen isotope trend, however, is one of a general decrease in d 180 values. An aplite in the upper inclusion zone has d 180 of -2.3 per mil. Specimens of the capping granophyre have d 180 values that range from +4 to +6 per mil except for the specimen from the contact with the upper gabbro zone that has d 180 of +1 per mil.

Values of d 180 of plagioclase versus d 180 values of coexisting pyroxene or magnetite from rocks of the Dufek intrusion are plotted in figure 6 and figure 7, respectively. Lines of equal d 180 fractionations (A) between the minerals are labeled on each figure.

Figure 6 is modified from Schiffries and Rye (1989). At magmatic temperatures, plagioclase and pyroxene in equilibrium

Dufek Intrusion

-3-2-101234567 3500- ̂ -"- J-L"--"-" ""I"" "-"- MII|UU[IIM[IIM -3500

LU

-3-2-101234567

18 6 O (whole-rock, per mil)

Figure 5. Variation of d 180 (whole-rock, per mil) for the composite Dufekintrusion. Solid squares are Dufek Massif specimens, solid circles are the lower and solid triangles are the upper Forrestal Range specimens, respectively.

13

with a "normal" mafic magma have d 180 values of +6.0 and +5.5 per mil, respectively (point B, figure 6). If pyroxene and plagioclase are the dominant minerals in the rock, closed-system, equilibrium cooling will result in compositions that lie in triangle ABC. Subsolidus exchange between a normal basalt and externally derived aqueous fluids produces a steeply sloping disequiblibrium trend (long arrow) because plagioclase is more susceptible to isotopic exchange than pyroxene. The lines defined by A 180=1.5, and A 180=0.5, correspond to equilibration temperatures of approximately 550°C, and 1200°C, respectively (Schiffries and Rye, 1989). A regression of available emperical and experimental data of plagioclase-pyroxene d180 fractionation factors for magmatic temperatures gives (Dunn, 1986):

T(°C)=1320-239 (A 180 plag-opx).Using this equation, the apparent temperatures of equilibration for the minerals with A 180 Plag-pyx >= 0.0 in Fig 6 ranges from about 750°C to 1320°C. This equation is not pertinent to those specimens with A 180 plag-pyx < 0.0 because reversal of this partitioning resulted from subsolidus exchange with externally derived aqueous fluids. Dufek specimen 259Fa has d 180 values for plagioclase >2.7 gm/cm3 and <2.7gm/cm3 of +6.2 and +7.7 per mil, with A 180 plagioclase-pyx values of 0.9 and 2.4, respectively. Using the plagioclase-pyroxene d 180 partitions above, these Aplag-pyx values convert to temperatures of equilibration of about 1100°C and 750°C for the pyroxene and plagioclase >2.7 and <2.7gm/cm3, respectively. These apparent temperatures may not have much meaning because the Sr isotopic composition of pyroxene (0.70825) is not equilibrated with the two plagioclase species (0.70924 and 0.70926) in this gabbro. By analogy, the oxygen isotopes may not have equilibrated between plagioclase and pyroxene during cooling of this rock. However, in rock from the Dufek intrusion, compositions of coexisting cumulus Ca-rich (augite) and ca-poor pyroxenes projected on the Lindsley (1983) 1-atmosphere geothermometer indicate temperatures of crystallization from 1140°C to 1030°C and pairs of coexisting late-stage pyroxenes by the same geothermometer indicate formation of augite at 770°C and 650°C and of Ca-poor pyroxene at 800°C (Ford and Himmelberg, 1986). In spite of the

14

810 10

= 1.5

DUFEK INTRUSION Dufek Massif

Forrestal Range-2

8

d O (pyroxene)

FigureS.Plot modified from Schiffries and Rye (1989), of d180 plagioclaseversus d180 pyroxene for coexisting pairs in the Dufek intrusion. Vertical lines connect points for plagioclase "A" and "B" in three Forrestal Range rocks and one Dufek Massif rock.

possible lack of oxygen isotopic equilibration between some minerals, apparent crystallization temperatures derived from either oxygen isotope or pyroxene geothermoeters are similar in these rocks.

Figure 7 is a plot of d 180 of plagioclase versus d 180 of magnetite separates from Dufek intrusion rocks. Lines of equal fractionation factors, A 180 plagioclase-magnetite, 1.5, 2, and 3 are shown. Temperatures of plagioclase-magnetite equilibration for most specimens on this plot range from about 800°C to 1320°C (Anderson and others, 1971). However, like pyroxene and plagioclase, magnetite is a cumulate mineral in many of the rocks investigated, and the partitioning of d 180 plagiolase-magnetite may not represent equilibrium attained between these minerals during closed system cooling.

15

-4 -3 -2 -1

g'5>

a1

ogo

DUFEK INTRUSION

7 A =3

A =2

A =1.5

18d O per mil (magnetite)

Figure 7. Plot of d180 plagioclase versus d180 magnetite for Dufek intrusion rocks Lines of equal A 180 plagioclase-magnetite are shown.

AGE of the DUFEK INTRUSION

Samples from the Dufek intrusion dated by the US Geological Survey (Table 2) include conventional K-Ar ages of three plagioclase separates from cumulate anorthosites, a pyroxene separate from gabbro, and a whole rock fine-grained gabbro from widely spaced stratigraphic intervals in the Dufek Massif and Forrestal Range (Kistler and Ford, 1979). Total gas 40Ar/39Ar ages for two pyroxenes from the Dufek Massif and Forrestal Range were reported by Ford and Kistler (1980). The conventional K-Ar determinations showed considerable age discordancy between plagioclase, pyroxene, and whole-rock samples from different parts of the body, but showed concordancy between 3 plagioclase samples that average 171.6±4.3 Ma. The pyroxene K-Ar and 40Ar/39Ar ages ranged from 189 to 98 Ma. The difficulty of interpreting pyroxene ages due to the possibility of excess argon or to argon loss because of exsolution lamellae (Kistler and Dodge, 1966) gave little weight to these data and the

16

plagioclase average (171.6±4.3 Ma) was accepted as the best age for the intrusion (Ford and Kistler, 1980).

Brewer and others (1996) reported incremental heating 40Ar/39Ar isochron ages of 182.1 ±2.4 Ma and 182.9±2.5 Ma for plagioclase separates from the Dufek intrusion. Zircons from the intrusion yield U-Pb dates of 183.9±0.3 Ma for the capping granophyre and 182.7±0.4 Ma for a late cross-cutting dike (Minor and Mukasa, 1995).

Rb-Sr whole-rock, mineral ages calculated from regressions of data for two specimens from the Dufek Massif section and 14 specimens from the Forrestal Range section range between 224 and 147 Ma (Table 2). Values of MSWD greater than 1 for many of these regressions indicates scatter of data about the lines is due to factors other than analytical errors. The factor in these cases is most likely the Sri differences between cumulus and intercumulus minerals shown for some rocks in Figures 3 and 4. Due to this, the ages derived from regressions with MSWD greater than 1, even though they are all Mesozoic, do not represent crystalization ages of the Dufek intrusion. The two Dufek Massif, three of the Forrestal Range section, and the capping granophyre regressions all have MSWD less than 1. The error for these regressions is due only to analytical error, and the ages calculated for these specimens could represent crystallization ages.

The precise isochron for the capping granophyre (Table 2) yields an age of 183.9±7.1 Ma, the same as the zircon U-Pb age of 183.9±0.3 Ma (Minor and Musaka, 1995) for this unit of the Dufek intrusion. The age for this unit along with the two Dufek Massif gabbro Rb-Sr isochrons at 178.4±8.9 Ma and 182.0±17 Ma (Table 2) and the 40Ar/39Ar isochron ages of 182.1±2.4 Ma and 182.9±2.5 for two plagioclase separates (Brewer and others, 1996) from the Dufek intrusion strongly suggest the intrusion is greater than 180 Ma and most likely 184 Ma.

The three remaining precise whole-rock mineral isochrons are for two gabbros and a leucocumulate from the Forrestal Range section and yield ages that range from 176.1 ±5.6 Ma to 169.3±7.3Ma. These ages are the same within experimental error and their average age of 173±5 Ma overlaps with the average 171.6±4.3 Ma of the conventional K-Ar ages for three plagioclase separates (Kistler and Ford, 1979) from the intrusion. It is not likely it took 8 to 10 m.y. to

17

cool after emplacement of the Dufek intrusion. These dates probably reflect eqilibration of isotope systems in Dufek intrusion rocks during heating at the culminating 176.6+1.8 Ma time of emplacement of the Ferrar tholeiitic rocks elsewhere in Antarctica (Fleming and others, 1997).

SUMMARY

The extreme variation of strontium and oxygen isotope compositions of rocks from the Dufek layered mafic intrusion shows that it was formed from magma of tholeiitic composition, probably derived from the mantle, that was variably contaminated with siliceous melts derived from crustal rocks. The strontium isotopic composition of a cumulate pyroxene from the Dufek Massif indicates the primary tholeiitic magma had Sri at least as primitive as 0.7076. Compositional variation in the magmas was reinforced by fractional crystallization. In spite of accumulating at magmatic temperatures, not all minerals isotopically equilibrated as the intrusion cooled and crystallized. Oxygen isotope fractionation between plagioclase and pyroxene and magnetite generally indicate equilibration from about 1300°C to about 800°C. Some mafic rocks from the Forrestal Range with Sri values as radiogenic as 0.712 have values of d180 as low as +1 per mil. These same rocks have magmatic oxygen isotope temperatures of partition between plagioclase and pyroxene and require that the contaminating crustal melts were derived from rocks with 180 depleted by interaction with meteoritic hydrothermal systems prior to melting. The capping granophyre is derived from a crustal melt, and is not derived from the mafic Dufek magma by fractional crystallization. Rb-Sr whole-rock mineral ages from the Dufek Massif, a Rb-Sr whole-rock isochron for the capping granophyre, two 40Ar/39Ar plagioclase ages from the Dufek intrusion, and an U-Pb zircon age for the capping granophyre indicate the intrusion is at least 180 Ma and is probabably 184 Ma.

18

Table 2. Sample number, elevation, rock type, material dated and apparent Rb-Sr, K-Ar, or 40Ar/39Ar ages for samples of the Dufek Massif and Forrestal Range sections of the Dufek intrusion

Sample, Elevation Rock typemeters, (reference)DUFEK MASSIF192Fb, 21 (1)38Fa, 278(1)198Fa,1067 (1)237Fb, 1219

anorthosite leucogabbro mafic gabbro mafic gabbro

Material dated

plag., K-Ar pyx, 40Ar/39Ar

pyx, K-Ar wr, pyx, plag

259Fa, 1809

FORRESTAL RANGE 75Fe, 110(1) 75Fe, 110

74Fa, 114

99Ff, 145

94Fi,340 (1) 94Fi, 340

mafic gabbro wr, plag, plag

leucocumulate leucocumulate

gabbro

leucocumulate

leucocumulate leucocumulate

plag., K-Ar wr, plag, plag, apatite, pyx wr, plag, apatite, pyx wr, plag, plag, apatite pyx, 40Ar/39Ar

wr, plag, plag

84Fa, 83Fb, 515,525 gabbro, gabbro wr, apatite,wr, pyx, plag

310Fa, 310Fb, 822, 819 gabbro gabbro wr, wr, plag,pyx

184Fa, 926 leucocumulate wr,plag, plag,apatite, pyx

185Fa, 942 gabbro wr, plag, pyrx

297Fa, 1070(1)

101 Fa, 1090(1) 101 Fa, 1090

gabbro wr, K-Ar

leucocumulate plag, K-Ar leucocumulate wr, plag, plag

Apparent Age, MSWD, (87Sr/86Sr)fl

171.2±4.3 Ma 189.5±31.7Ma 97.5±2.4 Ma 178.4±8.9Ma, 0.28, 0.70919182.0±17Ma, 0.46, 0.70901

169.5±4.2 Ma189.1±30Ma, 43.10.71055146.7±17.7Ma, 11.5,

0.71057224.2±52 Ma, 5.62,0.70967148.4±7.3Ma201.1±50Ma, 7.3,0.71005176.1 ±5.6 Ma, 0.42,0.70978194.6±38 Ma, 4.3,0.70957165.5±31 Ma 18.2,0.70966174.0±6.6 Ma, 1.9,0.70939111.9±2.8Ma,106.3±4.2 Ma174.1 ±4.4 Ma173.7±22 Ma, 0.24,0.70949

19

Table 2. (con't) Sample number, elevation, rock type, material dated and apparent Rb-Sr, K-Ar, or 40Ar/39Ar ages for samples of the Dufek Massif and Forrestal Range sections of the Dufek intrusion

Sample, Elevation meters, (reference) 171 Fa, 1169

182Fc, 1250

158Fa, 1405

160Fa, 1436

162Fe-2, 1482

Materialdatedwr, pyx, plag

162Fc-315Na, 1493-1600, n=6 Reference(1) Ford and Kistler (1980), all others, this report

Rock type

gabbro

leucocumulate wr, plag, pyx, apatite plag, pyx, apatite wr, plag, pyx, apatite wr, plag, pyx, apatite 6 wr

gabbro

gabbro

gabbro

granophyre

Apparent Age, MSWD, (87Sr/86Sr){ 190.2±50Ma, 19, 0.70951161.5±24Ma, 11.2, 0.70996 156.8±5.3, 1.55, 0.71068162.5±23 Ma, 10.7, 0.71226169.3±7.3 Ma, 0.34, 0.71208183.9±7.1 Ma, 0.32, 0.71276

REFERENCES CITED

Behrendt, J.C., Drewery, D.J., Jankowski, E., and Grim, M.S., 1981, Aeromagnetic and radio echo ice-sounding measurements over the Dufek intrusion, Antarctica: Journal of Geophysical Research, v.86, p. 3014-3020.

Behrendt, J.C., Henderson, J.R., Meister, L and Rambo, W.L, 1974, Geophysical investigations of the Pensacola Mountains and adjacent glacerized areas of Antarctica: U.S. Geological Survey Professional Paper 844.

Brewer, T.S., Rex, D., P.G. Guise, and Hawkesworth, C.J., 1996,Geochronology of Mesozoic tholeiitic magmatism in Antarctica:

.implications for the development of the failed Weddell Sea rift

20

system: in Storey, B.C., King, E.G., and Livermore, R.A. (eds),Weddell Sea tectonics and Gondwana break-up, GeologicalSociety Special Publication No. 108, p. 45-61.

Clayton, R.N., and Mayeda, T.K., 1963, The use of brominepentafluoride in the extraction of oxygen from oxides andsilicates for isotopic analysis: Geochimica et CosmochimicaActa, v.27, p.43-52.

Drinkwater, J.L., Ford, A.B., Czamanske, G.K.,1985, Study of sulfidemineral distribution in the Dufek intrusion: Antarctic Journalof the United States, v. 20, no.5, p.50-51.

Drinkwater, J.L., Ford, A.B., Czamanske, G.K., 1986, Apatites of theDufek intrusion; a preliminary study: Antarctic Journal of theUnited States v.21, no.5, p.66-69.

Dunn, T., 1986, An investigation of the oxygen isotope geochemistryof the Stillwater complex: Journal of Petrology, v. 27, p. 987-997.

Fleming, T.H., Heimann, A., Poland, K.A., and Elliot, D.H., 1997,40Ar/39Ar geochronology of Ferrar Dolerite sills from theTransantarctic Mountains, Antarctica: Implications for the ageand origin of the Ferrar magmatic province: Geological Societyof America Bulletin, v. 109, no. 5, p. 533-546.

Ford, A.B., 1970, Development of the layered series and cappinggranophyre of the Dufek intrusion of Antarctica: in Symposiumon the Bushveld Igneouis Complex and other layered intrusions,eds. D.J.L. Visser and G. von Gruenewaldt, Geological Society ofSouth Africa Special Publication 1, p. 494-510.

Ford, A.B., 1976, Stratigraphy of the layered gabbroic Dufekintrusion, Antarctica: U.S. Geological Surtvey Bulletin 1405-D

Ford, A.B., and Himmelberg, G.R., 1991, Geology and crystallization ofthe Dufek intrusion: in The Geology of Antarctica, ed. R.J.Tingey, Oxford Monographs on Geology and Geophysics, 17,p.175-214.

Ford, A.B., and Kistler, R.W., 1980, K-Ar age, composition, and originof Mesozoic mafic rocks related to the Ferrar Group in thePensacola Mountains, Antarctica: New Zealand Journal ofGeology and Geophysics, v. 23, p. 371-390

Ford, A.B., Kistler, R. W., and White, L.D., 1986, Strontium and oxygenisotope study of the Dufek intrusion: Antarctic Journal of theUnited States, v.21, no.5, p. 63-66

21

Ford, A. B., and Nelson, S. W., 1972, Density of the stratiform Dufekintrusion, Pensacola Mountains, Antarctica: Antarctic Journalof the United States, v.7, no. 5, p. 147-149.

Ford, A.B., Mays, R.E., Haffty, J. and Fabbi, B.P., 1983, Reconnaissanceof minor metal abundances and possible resources of the Dufekintrusion, Pensacola Mountains: in Antarctic earth science, R.LOliver, J.R. James, and J.BJago,eds. p.433-436.

Ford, A. B., and Nelson, S. W., 1972, Density of the stratiform Dufekintrusion, Pensacola Mountains, Antarctica: Antarctic Journalof the United States, v.7, no. 5, p. 147-149

Himmelberg, G.R., and Ford, A.B., 1975, Petrologic studies of theDufek intrusion-iron-titanium oxides: Antarctic Journal of theUnited States, v.10, no.5, p.241-244.

Himmelberg, G.R., and Ford, A.B., 1976, Pyroxenes of the Dufekintrusion, Antarctica: Journal of Petrology, v.17, no.2, p.219-243.

Himmelberg, G.R., and Ford, A.B., 1977, Iron-titanium oxides of theDufek intrusion, Antarctica: American Mineralogist, v.62,p.623-633.

Himmelberg, G.R., and Ford, A.B., 1983, Composite inclusion ofolivine gabbro and calc-silicate rock in the Dufek intrusion, apossible fragment of concealed contact zone: Antarctic Journalof the United States, v. 18, no. 5, p. 1-4.

Kistler, R.W., and Dodge, F.C.W., 1966, Potassium-argon ages ofcoexisting minerals in pyroxene-bearing granitic rocks in theSierra Nevada, California: Journal of Geophysical Research,v.71, p.2157-2161.

Kistler, R.W., and Ford, A.B., 1979, Potassium-argon ages of Dufekintrusion and other Mesozoic mafic bodies in the PensacolaMountains: Antarctic Journal of the United States, v.14, no.5 ,p.8-9.

Lindsley, D.H., 1983, Pyroxene thermometry: American Mineralogist,v. 68, p. 477-493.

Ludwig, K.R.,1988, Isoplot version 2, A plotting and regressionprogram for isotope geochemists, for use with HP series200/300 computers: U. S. Geological Survey, open-file report87-601, 49 p.

Minor, D.R., and Mukasa, S.B., 1995, A new U-Pb crystallization ageand isotope geochemistry of the Dufek layered mafic intrusion:Implications for formation of the Ferrar volcanic province: Eos

22

(Transactions of the American Geophysical Union), v.76.p. 285-286.

Schiffries, C.M., and Rye, D.M., 1989, Stable isotopic systematics ofthe Bushveld complex: 1. Constraints of magmatic processes inlayered intrusions: American Journal of Science, v. 289, p.841-874.

Schmidt, D.L, and Ford, A.B., 1969, Geology of the Pensacola andThiel Mountains (scale 1:1 000 000): in Geologic maps ofAntarctica, V.C. Bushnell and Craddock, C, eds, Plate V.America Geographical Society, New York.

Steiger, R.H., and Jager, E., 1977, Subcommission Geochronology:Convention on the use of decay constants in geo-cosmochronology: Earth and Planetary Science Letters, v. 36,p.359-362.

23

Table

3.

Rubidium

, stro

ntiu

m

ab

un

da

nce

and

isoto

pic

data

, d

18O

values, and

ele

vatio

ns

for

rocks

and m

ine

rals

in th

e

Du

fek

La

yere

d

Intru

sion

A

nta

rctic

a.

Sam

ple N

um

be

r m

ate

rial

an

alyze

d

Du

fek

Massif

min

era

ls1 92Fb plagioclase38Fa pyroxene49F

b plagioclase1 1 1 Fa plagioclase1 1 1 Fa pyroxene237F

b pyroxene237F

b pyroxene2237F

b plagioclase237F

b magnetite

A259F

a plagioclaseB

259Fa plagioclase259F

a pyroxeneF

orre

stal

Range

min

era

lsA

75Fe plagioclase

B75Fe plagioclase

75Fe pyroxene75Fe apatite75Fe m

agnetite74Fa plagioclase74Fa pyroxene74Fa apatite74Fa m

agnetiteA

99Ff plagioclase

B99F

f plagioclase99F

f pyroxene99F

f apatite99F

f magnetite

Rb

(ppm

)

5.300.5

41.110.642.945.415.42na97.11.15.29

83.88.81.696.67na614.226.14na5717.916.096.38na

Sr

(ppm

)

216926228313.86.513.54296na1893099.2

23731630.2176.6na3159.36181.2na15433418.881 82.02na

Rb/S

r

0.0

25

0.060

.00

40.0350

.04

60.4520

.39

90

.01

8

0.5

14

0.0

04

0.575

0.3530

.02

80.0560

.03

8

0.0

19

1.520

.03

4

0.370

.05

40.6930.035

87Rb

/86S r

0.0710

.17

40

.01

20.010.13411.311.1540.053

1.4870.011.666

1.0230.0810

.16

20

.10

9

0.0

55

4.4010

.09

8

1.0710

.15

52.0070.101

87Sr/86S

r

me

asu

red

0.7

10

02

0.708410.709210

.70

89

90

.70

88

20.7

1083

0.7

12

12

0.70931

0.7

12

87

0.7

09

08

0.7

12

32

0.7

1327

0.7

1073

0.7

11

40.71041

0.7

1079

0.7

19

75

0.7

10

57

0.7

13

07

0.7

1028

0.7

13

66

0.7

09

86

(87Sr/86S

r)0

0.7

0985

0.7

0799

0.7

09

18

0.7

0897

0.7

08

49

0.7

0763

0.70930

.70

91

8

0.7

0924

0.7

0906

0.7

0825

0.7

1077

0.7

1053

0.7110

.71

01

4

0.7

1066

0.708810.7

1033

0.7

1046

0.7

099

0.7

0867

0.70961

d180

per

mil

6.9

6.27.14.74.6

3.87.76.25.3

4.43.23.2

1.13.74.0

0.95.43.54.1

2.1

ele

v.

mete

rs

21278585107010701219121912191219180918091809

110110110110110114114114

145145145145145

24

Table

3. (c

ont.)

Rubidium

, stro

ntiu

m

abundance and

isoto

pic

data

, d

180

valu

es,

and ele

vatio

ns

for

rocks

and m

inera

ls in

the

D

ufe

k L

aye

red

In

trusio

n

An

tarctica

.

Sam

ple N

um

be

r m

ate

rial

an

alyze

d

94 Fi pyroxeneA

94FJ plagioclase694FJ plagioclase94Fi m

agnetite84Fa apatite83F

b plagioclase83 Fb pyroxene83Fb m

agnetite310F

a pyroxene310F

a plagioclase3 1 0Fa m

agnetiteA1 84Fa plagioclaseB

184Fa plagioclase

184Fa pyroxene

1 84 Fa apatite1 84Fa m

agnetite185Fa pyroxene1 85Fa plagioclase1 85Fa m

agnetiteA

lOIF

a plagioclase

BIO

IFa

plagioclase1 01 Fa m

agnetite171 Fa plagioclase1 71 Fa pyroxene1 71 Fa m

agnetite1 82F

c plagioclase182F

c pyroxene1 82 Fc apatite1 82Fc m

agnetite

Rb

(ppm

)

4.840.45.5na6.2112.29.78na8.231.00na74.617.215.576.61na12.640na10587na9.311.66na3.516.042.12na

Sr

(ppm

)

11.5187313na238.930714.77na15.1307na21529920.42176.3na15.24287na222316na28312.11na33615.22

03

.4na

Rb/S

r

0.4170.2160.018

0.0260.040.662

0.5450.003

0.3470.0580.7620.037

0.82900.4730.275

0.0330.963

0.011.0550.01

87Rb

/86Sr

1.2

08

0.6

25

0.051

0.0

75

0.1

15

1.907

1.5780

.00

9

1.0040

.16

62.2070

.10

8

2.40201.3690

.79

7

0.0

95

2.7

9

0.033

.05

80.03

87Sr/86S

r

me

asu

red

0.7

113

0.7

1189

0.7

1027

0.7

09

94

0.710110

.71

45

8

0.7

1397

0.7

09

68

0.712110.7

0989

0.7

1486

0.7

10

2

0.7

1535

0.7

0944

0.7

12

88

0.7

11

48

0.709910.7

1707

0.7

1019

0.7

16

98

0.7

09

84

(87Sr/86S

r)0

0.7

0834

0.7

10

37

0.7

1015

0.7

09

75

0.7

09

83

0.7

09

82

0.7

10

12

0.7

09

66

0.7

09

66

0.7

0948

0.7

09

47

0.7

09

93

0.7

0948

0.7

0944

0.7

09

54

0.7

09

53

0.7

0968

0.7

10

26

0.7

1012

0.709510

.70

97

7

d180

per

mil

6.45.02.9

5.24.12.65.13.42.04.33.72.1

0.44.64

.41.72.80.31.365.63.34.84.4

2.2

ele

v.

mete

rs

34

03403403405155255255258228228229269269269269269429429421090109010901169116911691250125012501250

25

Table

3. (cont.)

Rubidium

, strontium

abundance

and isotopic

data, d180

values, and

elevations for

rocks and

minerals

in the

Dufek

Layered

Intrusion A

ntarctica.

Sam

ple N

um

be

r m

ate

rial

an

alyze

d

1 58Fa plagioclase1 58Fa pyroxene1 58Fa apatite1 58Fa m

agnetite1 60 Fa plagioclase160Fa pyroxene1 60Fa apatite1 60Fa m

agnetite1 62F

e plagioclase1 62F

e pyroxene1 62Fe apatite1 62 Fe m

agnetiteD

ufe

k M

assif

wh

ole

-rocks

193 Fa wr

195Fb w

r38F

a wr

39Fa w

r46F

a wr

49Fb w

r2 77 Fa w

r52Fa w

r54Fa w

r117Fa w

r57F

b wr

58Fb w

r252F

b wr

198Fi

wr

226Fa w

r1 1 1 Fe w

r

Rb

(ppm

)

0.211.65.17na5.826.365.44na7.813.837.79na3.30.95.16.244.50.36.83.62.32251.699.32.61.81.3

Sr

(ppm

)

37319.34240.7na49123.21312.7na43622.55265na176237230229163190199169160185186137128196178153

Rb/S

r

0.0

005

0.60.021

0.0

12

1.1360

.01

7

0.0

18

0.6130.029

0.0

19

0.0

04

0.0

22

0.0270.0250

.02

40.0020.0

40.0230.0121.210

.01

20.7760.0130.010

.00

8

87Rb

/86Sr

0.001 61

.73

60

.06

2

0.0

34

3.290.05

0.0

52

1.7760

.08

5

0.0

54

0.0110

.06

40

.07

80.0710

.06

90

.00

40

.11

60

.06

50

.03

63

.50

60

.03

42.2460

.03

80

.02

90

.02

5

87Sr/86S

r

me

asu

red

0.7

1073

0.7

14

55

0.7

10

77

0.7

12

53

0.7

19

89

0.7

123

0.712210.7

1633

0.7

12

25

0.7

10

07

0.7

0916

0.7

0845

0.7

087

0.7

089

0.7

0916

0.7

09

48

0.7

09

35

0.7

0936

0.7

0937

0.7

23

97

0.7

0942

0.7

1496

0.7

0963

0.7

0956

0.7091 7

(87Sr/86S

r)0

0.7

10

73

0.7

1024

0.7

10

62

0.7

1245

0.7

11

85

0.7

1218

0.7

1209

0.7

11

99

0.7

1204

0.7

0994

0.7

09

13

0.7

083

0.7

08

52

0.7

0873

0.7

0899

0.7

09

47

0.7

0907

0.7

092

0.7

0928

0.715410.7

0934

0.7

0948

0.7

0954

0.7

09

49

0.7091 1

d180

per

mil

4.34.3

1.82.23.8

0.8-0.12.4

-3.3

6.56.35.95.45.96.96.16.46.76.61.16.93.63.86.66

elev. m

ete

rs

140514051405140514361436143614361482148214821482

552322782935065856436

98

82793698610131066106710681070

elev. m

ete

rs

5523223229350658564369882793698610131066106710681070

26

Table

3.

(cont.)

Ru

bid

ium

-Stro

ntiu

m

abundance

and

isoto

pic

da

ta,

d180

va

lue

s, and

ele

vatio

ns

for

rocks

and m

inera

ls in

the

D

ufe

k Laye

red

Intru

sion

Anta

rctica.

Sam

ple N

umber

mate

rial

an

alyze

d

1 99Fa wr

240Fc w

r230F

b wr

231Fd w

r237F

b wr

270Fa w

r266F

a wr

259Fa w

r259F

b wr

257Fa w

rF

orre

stal

Range

79Fa w

r78F

b wr

77Fa w

r76F

a wr

75Fe w

r74F

a wr

99Fb w

r9

9F

f wr

99Fg w

r, aplite97F

b wr

306Fd w

r306FH

wr

94Fe w

r94F

i w

r87F

a wr, aplite

87Fb w

r86F

a wr

85Fb w

r

Rb

(ppm

)

02.64.93.65.36.32.812.515.110.2

wh

ole

-rocks25.324.821.91324.815.730.830.424118.919.617.228.621.931826.829.59.2

Sr

(pp

m)

163152145158124183140181246177

15118315217625320718325750.521925517316422553.3178186143

Rb/S

r

00.0

17

0.0340.0230.0430

.03

40.020

.06

90.0610

.05

8

0.1

68

0.1360

.14

40

.07

40

.09

80.0760

.16

80

.11

84.7720.0860

.07

70.0990

.17

40.0975.9660.1510

.15

90

.06

4

87Rb

/86Sr

00.050

.09

80.0660

.12

40.10

.05

80.20

.17

80.167

0.4

85

0.3920.4170

.21

40

.18

40.2190.4870.34213.880.250.2230

.28

80.5050.28217.370.4360.4590

.18

6

87Sr/86S

r

me

asu

red

0.7

09

44

0.708910.70910

.70

89

40.7

0953

0.7091 60.7

0929

0.7

09

47

0.7

09

40

.71

04

7

0.7

11

09

0.7

1022

0.7

1013

0.7

09

58

0.7

1115

0.7

11

14

0.7

10

96

0.7

10

80.7

6282

0.711510.711210

.71

14

40

.71

18

40

.71

07

40.7

6802

0.7

1173

0.7

1137

0.7

10

45

(87Sr/86S

r)0

0.7

0944

0.7

0879

0.7

08

86

0.7

0878

0.7

09

23

0.7

08

92

0.7

09

15

0.7

08

98

0.7

08

97

0.7

10

07

0.709910

.70

92

60.7

0912

0.7

09

06

0.7

10

46

0.710610

.70

97

70.7

0996

0.728910

.71

09

0.7

10

67

0.7

10

74

0.710610

.71

00

50.7

256

0.7

10

67

0.7

10

25

0.71

d180

per

mil

5.85.86.365.86.25.76.16.156.15.84.94.73.844.93.11.663.64.75.74.954.65.24.4

elev. m

ete

rs

1097117312041213121914931609180918101814

Forre

sta

l205357.21031101141441451472212

74

275335340429430465491

elev. m

ete

rs

1097117312041213121914931609180918101814

+D

ufe

k183418671871191719241928195819591961203520882089214921542243224422792305

27

Ta

ble

3.

(co

nt.)

Ru

bid

ium

-Stro

ntiu

m

abundance and

isoto

pic

data

, d

180

valu

es,

and e

leva

tion

s fo

r rocks

and m

inera

ls in

the

D

ufe

k Laye

red

Intru

sion

A

nta

rctica.

Sam

ple N

um

be

r m

ate

rial

an

alyze

d

84Fa w

r83F

a wr

83Fb w

r300F

b wr, aplite

284Fa w

r284F

b wr

287Fe w

r311 Fa w

r310F

b wr

310Fa w

r184F

a wr

309Fb w

r309F

a wr

185Fa w

r188F

a wr

189Fa w

r297F

a1 w

r297F

a2 wr

101 Fa wr

191Fb w

r171 Fa w

r72F

a wr

1 69Fb w

r181 Fa w

r182F

c wr

71 Fl w

r167F

a wr

166Fa w

r173F

c wr

Rb

(ppm

)

18.426.721.121323.817.288.416.215.912.735.919.115.111.22

0.4

26.213.715.271.118.92

3.4

26.732.232.73326.925.317.947.7

Sr

(pp

m)

16324218457.6167219268206206159217132220137217211164168229133144188273221226255166153271

Rb/S

r

0.1130.110

.11

53.70.1430

.07

90.330

.06

40

.07

70

.08

0.1

65

0.1450

.06

90.0

872

0.0

94

0.1

24

0.0

84

0.090.310.1420

.16

20.1420

.11

80

.14

80

.14

60

.10

50

.15

20

.11

70

.17

6

87Rb/86S

r

0.3270

.31

90.33210.740.4120

.22

70.9550.1860.2230.2310.4790.4190

.19

90.2370.2720.3590.2420.2620.8990.41 10.470.4110.3410

.42

80.4230.3050.4410.3390.51

87S r /86S r

me

asu

red

0.710610.710710.7

1058

0.7

4819

0.709810.71110

.71

28

30.7

1158

0.710210.7

1008

0.7

10

57

0.7

10

37

0.7

10

37

0.7

09

92

0.7

10

43

0.7

10

59

0.7

09

77

0.7

11

05

0.7

11

69

0.7

10

35

0.7

1058

0.7

11

60

.71

14

30.7

1087

0.7

1098

0.7

11

70.7

1118

0.7

11

02

0.7

1228

(87Sr/86S

r)0

0.709810.7

0993

0.7

0977

0.7

2195

0.7

08

80

.71

05

40.7

105

0.7

1113

0.7

0955

0.7

0952

0.7

094

0.7

0935

0.7

0988

0.7

09

34

0.7

0977

0.709710

.70

91

80.710610

.70

94

90.7

0935

0.7

0943

0.7

106

0.7

106

0.7

0982

0.7

09

95

0.7

10

95

0.71010.7

1019

0.7

11

04

d180

per

mil

54.44.90.84.65.46.24.21.83.92.33.73.34.91.51.22.62.00.12.96.03.91.01.94.20.75.05.22.5

elev. m

ete

rs

515524525594670672674735819822926937937942102110591070107210901116116911851231123412501253126813101322

elev. m

ete

rs

23292338233924082484248624882549263326362740275127512756283528732884288629042930298329993045304830643067308231243136

28

Table

3. (c

on

t.) R

ub

idiu

m-S

tron

tium

abundance

and iso

top

ic data

, d

180

valu

es,

and ele

vatio

ns

for

rocks

and m

ine

rals

in th

e

Du

fek

Laye

red

Intru

sion

Anta

rctica.

Sam

ple N

um

be

r m

ate

rial

an

alyze

d

1 57Fb w

r, aplite1 57F

d wr

1 57Fg1 w

r, gabbro pegmatite

157Fg2 w

r160F

a wr

161 Fa wr

162Fe2 w

r1 62 Fc w

r, granodiorite

163 Fa wr, granodiorite

1 64Fa wr, granodiorite

165Fa w

r, granodiorite313F

a wr, granodiorite

315Na w

r, granodiorite

17Na w

r, gabbro317F

a wr, granodiorite

Rb

(ppm

) S

r (p

pm

) R

b/S

r 87Rb

/86S r 87S

r/86Sr

measu

red

51556.626.431.937.247.949.398.613914415110014940.2147

77.1176144132199188248197152137142175131260141

6.680

.32

20.1830

.24

20

.18

70

.25

50

.19

90.5010

.91

41.0511.0630.5711.1370

.15

51.043

19.470

.93

20.5310.70.5410

.73

80

.57

51.4492.6493.0453.0811.6553.2950

.44

83.02

0.7

8359

0.7

25

83

0.7

1187

0.7

1246

0.7

1336

0.7

1317

0.7

1349

0.7

1656

0.7

1963

0.7

20

83

0.7

2076

0.7

1709

0.7

21

39

0.7

1347

0.7

2139

(87Sr/86S

r)0

0.7

3603

0.7

2355

0.7

1057

0.7

1075

0.7

12

04

0.7

1137

0.7

12

09

0.7

1276

0.7

12

76

0.7

12

76

0.7

12

76

0.7

12

76

0.7

12

76

0.7

12

38

0.71301

d180

elev.

pe

r m

il m

ete

rs

-2.3

2.03.73.91.70.80.915.23.94.65.95.84.74.3

136013621364136514361435148214931516154215581575160016201631

elev. m

ete

rs

317431763178317932503249329633073330335633723389341434343445

Notes

wr=

w

ho

le-ro

ckna=

not

analyzedP

lagioclase se

para

tes

with

p

refix

"A"

have sp

ecific

gra

vity <

2.7

gm

/cm3,

specimens

with

p

refix

"B"

and all

oth

er

specimens

have sp

ecific

gra

vity >

2.7

g

m/cm

3.

29