American Mineralogist, Volume 71, pages 916-923, 1986

Ba-rich adularia from the Ouachita Mountains, Arkansas: Implications for apostcollisional hydrothermal system

KBvrN L. SHnlroN, JonN M. Ruonn * Lours M. Ross, Gnoncn W. Vrrr,nDepartment of Geology, University of Missouri-Columbia, Columbia, Missouri 6521I

Dlvro E. Snronvu,NNDepartment of Geology, Brooklyn College, Brooklyn, New York 11210

AssrRAcr

Jet-black Ba-rich adularia occurs as twinned, euhedral crystals within the Womble Shale(Lower Ordovician) on the shore of Lake Ouachita, Arkansas, in the area of the Bentonuplift. The adularia crystals are localized along organic-rich seams in silty micrite, inintimate association with postfolding quartz veinlets. They display two distinct habits.Earlier adularia of the Felsdbanya habit are Ba zoned with typical core compositions nearIte.r6qBqo.rsoy{11r.,u,1Si1, ,oqO, and edges near Ilo rroBqo.oroy{llr orn;Si1, nssyOs. Later, blocky ad-ularia crystals are not Ba zoned and have compositions similar to those in the edges ofcrystals with the earlier habit. X-ray diffraction analysis indicates that both adularia habitsare dominantly monoclinic with a low degree of triclinicity. K-Ar analysis yields a date of262 + l0 Ma, consistent with a Permian age for adularia mineralization. Fluid-inclusion6[1a srrggest that adularia was deposited from a hydrothermal fluid with a salinity of 3wto/o NaCl equivalent at a maximum temperature of 300{ and a maximum p;essure of1.9 kbar. This pressure corresponds to a maximum lithostatic depthof 7.6 km, considerablyless than that postulated for rocks overlying the Womble Shale (> 15 km). The age, com-position, and depth of formation of the Ba-rich adularia place severe constraints on thepostcollisional evolution of the Ouachita Mountains and may suggest a genetic tie to theformation of Mississippi Valley-type Pb-7-n-Ba deposits.

INrnooucrroN

Jet-black, Ba-rich adularia occurs as twinned, euhedralcrystals within thinly bedded micrite of the lowest mem-ber of the Womble Shale (Lower Ordovician), in outcropon the shore of Lake Ouachita, Arkansas, in the area ofthe Benton uplift. The Womble Shale is a black, carbo-naceous, graptolitic, illitic shale, intercalated with siltymicrite, silty sandstone, and phosphatic conglomerate beds(Sterling et al., 1966). The adularia crystals appear to belocalized along organic-rich seams in the micrite, fre-quently in intimate association with postfolding quartzveinlets. Adularia is frequently the only mineral that canbe used to date events within the Ouachita Mountains.Thus, detailed geochemical studies were undertaken todetermine the composition and age of the adularia, andtheir relationship to the structural evolution of the Oua-chita Mountains.

AouI,.cnrA. MINERAr,ocy AND cHEMrsrRy

Pehography

Micrite samples from the Womble Shale were collectedat the tip of a small peninsula [459850 m E, 3828460 mN, Universal Transverse Mercator (uTM) grid zone l5l

+ Present address: Shell Western Exploration Production, Inc.,Mid-Continent Division, P.O. Box 991, Houston, Texas 77001.

0003{04xl86/0708-o916$02.00 916

on the shore of Lake Ouachita, Arkansas. Examinationof thin sections of micrite containing adularia crystals andquartz veinlets indicates synchronous deposition. Al-though the adularia crystals are generally larger (up to 2mm diameter) than the quartz veinlets (less than 0.2 mmwide), they occur within the veinlets, cluster tightly againstthe veinlets, and float in the micrite matrix as much asl0 cm from the veinlets (Figs. la, lb, lc). Adularia oc-crurences distal to the quartz veinlets tend to be restrictedto organic-rich seams and stylolites within the micrite.This suggests to us that movoment of hydrothermal fluidsoccurred along stylottes during the adularia-forming eventand that incorporation ofthe abundant associated organicmatter may be responsible for the jet-black appearance ofthe adularia.

Previously reported adularia crystals from the same re-gion are restricted to quartz veins. They occur especiallyat vein margins adjacent to shaly rocks or as cement orcoats on shale fragments included in the quartz veins (Eng-el, I 95 l), or they line irregular vugs at the centers oflargerquartz veins (Bass and Ferrara, 1969).

In thin section the adularia crystals display two dis-tinctive cross sections (Fig. l). The first is a diamond-shaped cross section (Felstlbanya habit of Kalb, 1924);many such crystals display sector zoning and locally com-plex extinction patterns (Figs. la, lb). Many larger adu-

SHELTON ET AL.: Ba-RICH ADULARIA

*"dtrf,

Fig. l. (a) and (b) Photomicrographs of Ba-zoned Felstibanya habit of adularia crystals, contained within micrite, displayingoptical discontinuity between solid-inclusion-rich cores and inclusion-poor edges. Note sector zoning of diamond-shaped crosssection. Cross-polarized transmitted light. (c) Larger, blocky habit of adularia in micrite. Cross-polarized transmitted light. (d) and(e) Scanning electron photomicrographs ofthe Ba-zoned Felsiibanya habit ofadularia crystals displaying penetration twinning. (f)Larger, blocky habit of adularia displaying multiple penetration twinning in a cyclical fashion. All scale bars are 0.5 mm. Repre-sentative scanning electron photomicrographs (d), (e), and (f) are positioned opposite their thin-section counterparts (a), (b), and (c),respectively.

9t7

9 1 8 SHELTON ET AL.: BA-RICH ADULARIA

Table l. Typical chemical compositions of zoned Ba-richadularia crystals

w t % Edge

5

4

3

H,Fig. 2. Catibrated traverses of Ba-zoned adularia (crystals 9

and I 5) showing systematic decreases of mole percent BaO fromcenters to edges. The interval between analyses is 40 pm.

in their inclusion-rich cores, whereas the more inclusion-free edges were found to be Ba poor. The larger, blockyhabits were also found to be Ba poor, resembling the edgesof the earlier habit. This suggests to us that the large,blocky crystals were likely formed later than the Ba-zoned,narrow crystals. Though the time frame over which thechange from Ba-rich to Ba-poor crystallization took place

cannot be directly determined, it is likely less than 100 000yr. This is a generally accepted upper limit for small hy-drothermal systems (Norton and Cathles, 1979; Cathles,l 98 l ) .

Elechon-probe microanalysis

An enr- eux electron microprobe with four wavelength-dispersive spectrometers was used to determine the chem-ical compositions of individual adularia crystals in pol-ished sections. An operating voltage of I 5 kV and a beamcurrent of 20 nA were used. Standards used were ortho-clase for K, Al, and Si and barite for Ba. The data werereduced following the correction methods of Bence andAlbee (1968) and Albee and Ray (1970). Results of anal-yses are shown in Table I (gross zoning) and Figure 2(details).

Calibrated traverses were performed, normal to the ccrystallographic axes of the long, narrow (Felstibanya) habitof adularia crystals in the polished sections. Great carewas taken to avoid solid inclusions. Results of the tra-verses indicate that Ba and Al substitute to a large degreefor K and Si in the cores of crystals. This substitutiondecreases systematically outward from the crystal coresto edges (Fig. 2). The adularia analyses yield a typical corecomposition of K,o ruorBa.o ,royAllr ,.,ySi1z srsrOs and a typicaledge composition of \o nrnBqo oroy{l1r orn1Si1, ,r1Or.

Similar analyses ofadularia with the larger, blocky habitrevealed no systematic Ba zonation and are nearly iden-ticalto those ofthe Ba-poor edges ofthe Ba-zoned elongateadularia crystals. This supports our qualitative observa-tion based on sEM-EDS study that the larger blocky habitofadularia may be paragenetically later than the elongatehabit.

K2oBa0At -0 .

s i 0 2

Total

BaA1s i

, ( R l + * R 2 * )

1 2 , 3 3 1 5 . 8 1

9 . 5 0 I . 8 82 0 . 4 0 I 9 . 3 9

58.99 63 .78

t01 .22 100.86

0 . 7 6 0 0 . 9 3 50 . 1 8 0 0 . 0 3 41 . t 6 l 1 . 0 5 92.849 2 ,955

0,940 0 .969

o 23 i

oo

f sE 4

321

A t o n i c c o n t e n t s c a l c u l a t e d o n t h e b a s i sof I oxygens

laria crystals of this type (Fig. lb), viewed in the planenormal to the c axis, display cores with extinction parallelto (010) cleavage and edges with slightly inclined extinc-tion (Chaisson, 1950). Optic axial planes have variableorientations, and values of2 Z(negative) are variable (from31" to 43'). Commonly the cores of these adularia crystalscontain numerous solid inclusions (most less than l0 pmin diameter), whereas the edges tend to be freer of inclu-sions (Figs. I a, I b). The solid inclusions are quartz, calcite,sodium feldspar, pyrite, iron oxide, and an unidentifiedFe-Mg silicate (see next section).

The second distinctive cross section of adularia crystalsin thin section exhibits larger, blocky, irregular outlinesconsisting of multiple crystals (Fig. lc). They frequentlydisplay undulatory extinction and have variable values of2V (negative). Many of these crystals contain the sameassortment of solid inclusions as found in the diamond-shaped crystal cross sections.

Cathodoluminescent analysis did not indicate the pres-ence of detrital K-feldspar cores.

Scanning electron microscopy (srvr) andenergy-dispersive spectrometry ( nos)

Samples of the Womble Shale micrite were dissolvedin glacial acetic acid. Insoluble residues included blackadularia crystals, one conodont element, fragments of thin-bedded shale, qvartz veinlets, and abundant black organicmatter. Representative samples ofthe two distinctive hab-its of adularia were examined with an Amray 1600T-Kevex 7000 sEM-EDS system. Individual crystals werephotographed directly; others were mounted in epoxy andpolished.

Representative scanning electron photomicrographs arepositioned opposite their thin-section counterparts in Fig-ure I (ld, le, 1f). Penetration twins ofthe long, narrowFelsbbanya habit are shown in Figures ld and le; multiplecyclic penetration twins of the larger, blocky habit areshown in Figure lf.

Elemental X-ray information on the polished crystalsprovided qualitative analyses ofthe adularia and helpedto identify the solid inclusions (see Petrography section).The Fels0banya habit revealed significant amounts of Ba

H4 O P m

SHELTON ET AL.: Ba-RICH ADULARIA 9t9

Adularia typically has low contents of minor and traceelements: NarO < 1.5, RbrO < 0.37, BaO < 1.7 (rare-ly, < 3.2), and SrO < 0.25 wto/o (Weibel, 1957, l96l; Ry-bach and Nissen, 1967; Gubser and Laves, 1967; Smith,1974b; Dimitriadis and Soldatos, 1978; Cernj'and Chap-man, 1984). The adularia crystals examined here containno detectable Na. However, they have large Ba contents,up to 9.5 wt0/o oxide (Fig.2, Table l).

The high contents of Ba suggest the hydrothermal leach-ing and remobilization of Ba from rocks at depth withoutreprecipitation of Ba in the form of crystallochemicallymore accommodating species (e.g., Ba phosphates, well-site, or barite).

X-nly DTFFRACTToN sruDyA study of the adularia structural state was performed

using a Phillips X-ray diffractometer. X-ray scans wererun at lolmin between 4o and 46o 20 across powderedsamples of adularia using CuKa radiation. Further scanswere nrn at t/q"/min between 20' and 36" 2|,Ihis being thecritical range for determining feldspar structure (Steiner,1970). The reflections were identified using the powderpatterns of Borg and Smith (1969).

All the adularia crystals have (201) reflections at 20 x2l'. This suggests a very pure (low-Na) K-feldspar (Bowenand Tuttle, 1950). The (l3l) reflection at approximately20 : 29.6, in some samples (larger, blocky adularia crys-tals) has an adjacent, smaller (l3l) peak developed at asmall A20 angle (less than 0.15") to the (131) peak, sug-gesting a small proportion of triclinic material (Smith,1974a). The elongate Ba-zoned adularia crystals do notyield discernible (l 3 l) peaks, though their ( I 3 l) peaks areof variable width. The A2d values for (040){200) at ap-proximately 20 :27.5 fit monoclinic adularia or mono-clinic orthoclase (Borg and Smith, 1969). The adulariacrystals examined plot within the orthoclase region of theb-c quadrilateral of Stewart and Wright (1974).

The adularia crystals are therefore dominantly mono-clinic, though some may have a low degree of triclinicity.Afonina and Shmankin (1970) claimed that Ba substitu-tion inhibits ordering in K-feldspar, on the basis of com-paring weight percent Ba to the (l3l) triclinic index ofspecimens. It is therefore not surprising that the Ba-richadularia crystals of our study are dominantly monoclinic.

D,Lrn op MTNERALIZATIoN

K-Ar data were obtained for Ba-rich adularia. Ar ex-traction was made using a conventional extraction systemwith an induction furnace. The Ar-isotope compositionwas determined using a 6-in.-radius, Nier-type mass spec-trometer. A spike of 38Ar added before each analysis en-abled calculation ofthe quantity ofo0Ar released from thesamples. K analyses were made using standard atomicabsorption techniques. The results are presented in Table2. The adularia grains yielded a date of 262 + l0 Ma,consistent with a Permian age of mineralization.

The adularia was chosen for dating in this study becauseit is the only phase present in the rock that can be dated

Table 2. K-Ar data of adularia

S a m p l e

D e s c r i p t i o n % K

Rad iogen ic4oA.

rcclg STP Atmospheric Date'

x to -6 4o [ r% n . ! , + za

B a - r i c h 1 0 . 6 3 I 1 4 . 4adu lar ia 10 .22crys ta I sfronrmi c r i te

4 . 5 2 6 2 + l 0

r - 1 1 - 1 - 1 1 - 1' K , = 4 . 9 6 2 x l 0 " y ' , K . = 5 . 8 1 x l 0 " y ' ,' ' B

.

40r / r = o .o t toz (s te iqer and Jeser , 1977)

by K-Ar methods. K-Ar behavior of adularia is not wellknown, and it is not within the scope of our study tosystematically investigate it through application of aoAr/3eAr age-spectrum dating. We do not intend for a singleK-Ar date for an adularia to stand alone as an indicatorof the age of formation of the hydrothermal quartz veins.We do argue that concordancy of this K-Ar date with Rb-Sr dates for presumably cogenetic adularias elsewhere inthe Ouachita Mountains is an indicator that these datesare geologically meaningful.

We believe that the K-Ar date of 262 + l0 Ma obtainedin our study represents the age offormation ofthe adulariaand associated quartz veins. It seems unlikely to us thatthe adularia crystals would have experienced significantAr loss due to either their structural state or the thermalhistory of the host rock. The adularia crystals are almostcompletely monoclinic; X-ray diffraction analysis suggestsvery low triclinicity. Previous studies (Halliday andMitchell, 1976)have suggested that an inverse correlationexists between triclinicity of adularia and K-Ar age, whichhas been interpreted as representing Ar loss related to thestructural state. Monoclinic adularia crystals (low-triclin-icity crystals such as ours) are least likely to lose Ar unlessthey have experienced a significant high-temperature,postdepositional reheating (hydrothermal) event. Such ahigh-temperature event is unlikely: the adularia crystalsshow no evidence of significant recrystallization; a con-odont geothermometer suggests a maximum rock tem-perature of approximately 300t; and fluid inclusions showno evidence of a postdepositional, high-temperature hy-drothermal event. Therefore, we suggest that the K-Ardate of 262 + l0 Ma represents the age of formation ofthe adularia and associated quartz veins. This date is inagreement with Rb-Sr ages of adularias that are from else-where in the Ouachita Mountains but that are assumedto have had similar geologic histories (Bass and Ferrara,1969; Denison et al., 1977).

The concordancy of K-Ar and Rb-Sr dates has beenused as a test of the reliability of dates for minerals thatare not routinely dated. For example, the reliability ofglauconite dates, which may be subject to inaccuraciesfrom sources similar to those that may afect adularia (i.e.,the presence of a detrital component or Ar loss), is rou-tinely tested in this manner (see, e.g., Grant et al., 1984).Bass and Ferrara (1969) obtained ages for cogenetic ad-

920 SHELTON ET AL.: BA-RICH ADULARIA

Tn "c

Fig. 3. Histograms of homogenization temperatures (Z) offluid inclusions in adularia and quartz. Note the similarity offluid-inclusion homogenization temperatures in both minerals.B : primary inclusions in the larger, blocky habit of adularia,p = primary inclusions in the Felsdbanya habit of adularia, P :primary inclusions in quartz, PS: pseudosecondary inclusionsm quartz.

ularia in quartz veins in the Ouachita Mountains, GarlandCounty, Arkansas. Their results by the K-Ar method were190 + 5, 205 + 6, and2l4 + 6 Ma-apparent Triassicages. They ascribed these ages to Ar loss from their ad-ularia crystals of variable triclinicity. Rb-Sr data for the205 Ma sample yielded an age between 279 and 287 Ma(Bass and Ferrara, 1969); (I : 1.39 x l0-" yr-'). Thisrange of ages is 263 to 2TlMaif a47-b.y. halfJife of 87Rb

is used (Denison et al.,1977). The difference in age (263or 27 | Ma) depends on the initial 875r/865r ratio chosen-0.704 for the older age or 0.709 for the younger age. Den-ison et al. (1977), from their recalculated ages, concludedthat the age of q\araz + adularia mineralization in theOuachita Mountains was Permian, using any reasonableinitial Sr ratio (0.700 to 0.709). A Permian age is in agree-ment with the K-Ar data of our study.

Fr,uro-rNcr,usroN sruDY

Fluid inclusions (74 primary and pseudosecondary;Roedder, 1984) were examined in thin, doubly polishedplates of adularia and associated quartz veins. Micro-thermometric measurements were made usinga FluidInc.gas-flow fluid-inclusion stage. Replicate measurements ofthe homogenization temperatures of standard synthetic

Table 3. Fluid-inclusion data ofadularia and quartz

S a l i n i t ye q . w t %

liaCl

Fig.4. Histograms "r;;:* ";;uid

inclusions in adulariaand quartz (weight percent NaCl equivalent). Note the similarityof fluid-inclusion salinities in both minerals. Abbreviations as inFig. 3.

fluid inclusions in quartz (Shelton and Orville, 1980)showed a reproducibility within + O.2f at temperaturesup to 350'C. Replicate measurements of the melting tem-peratures of pure-water synthetic fluid-inclusion stan-dards and COr-rich inclusions from Camperio, Italy,showed a reproducibility within +0.1"C. The results ofheating and freezing experiments on fluid inclusions arepresented in Table 3 and Figures 3 and 4. The salinitydataare based on freezing-point depression in the systemHrO-NaCl (Potter et al., 1978).

Adularia crystals were found generally to be ill-suitedfor fluid-inclusion study owing to their relative opacity,a result of significant inclusion of organic matter. Only 22suitable fluid inclusions were found within 20 crystalsstudied. A larger number of suitable fluid inclusions (52)was found in associated quartz veins. Although the tem-perature and salinity data presented are dominantly frominclusions in quartz, the gr€at similarities of data fromquartz and adularia suggest that both phases are products

of the same hydrothermal system.All fluid inclusions observed in adularia and quartz are

water-rich, low-salinity inclusions consisting of liquid anda small bubble that comprises approximately 10 to 15volo/o of each inclusion. The inclusions are nearly sphericaland range in size from l0 to 40 pm. No daughter mineralswere observed. Crushing tests suggest that CO, and otherdissolved gases are not present in significant concentra-tions in the inclusion fluids. All of the inclusions homog-enize to the liquid by vapor disappearance.

Primary fluidinclusionsin adularia homogenize at tem-peratures between 134 and l5l'C and have salinities be-tween l.l and3.4 wto/o NaCl equivalent. No correlationsexist between location within the Ba-zoned adularia crys-tals and either fluid-homogenization temperature or fluidsalinity.

Primary andpseudosecondary fluid inclusions in quartz

homogenize at temperatures between 131 and 157'C andhave salinities between 0.9 and 3.7 wto/o NaCl equivalent.The nearly identical ranges of homogenization tempera-ture and salinity data from adularia and quartz suggestthat both phases are likely products of the same hydro-thermal system (Table 2, Figs. 3 and 4)' Our fluid-inclu-sion studies revealed no evidence (i.e., presence of sec-

Samp les

Inc I us ionTypeP P S r o r

Hro o .T

" . 'me l t

Felsi jbanyah a b i ta d u l a r i ac r y s t a l s

b l o c k ya d u l a r i acrysta' ls

v e t n -f i l l i n g

1 3 0

9 0

l . t t o 3 . 2

1 . 3 t o 3 . 4

0 . 9 t o 3 . 7

' 1 3 4 t o l 5 l - 0 . 6 t o - 1 . 8

'136 to 148 -0 .7 to -2 .0

l3 l to 157 -0 .4 to -2 ,236 16

58 l5

P = pr inary f lu id inc lus ions , Ps = pseudosecondary inc lus ions

ondary inclusions) of a postdepositional hydrothermalsystem.

It is not possible to use the fluid-inclusion data of thisstudy to determine the formation temperature of the ad-ularia and associated quartz veins because an independentestimate ofpressure is not possible. It is possible, however,to estimate pressure (and by inference, depth) in the hy-drothermal system by interpreting the fluid-inclusion ho-mogenization temperatures and salinities within the con-text of another geothermometer-conodonts. A conodontfrom the adularia sample locality has a CAI (color alter-ation index) of 4 (R. L. Ethington, University of Missouri-Columbia, pers. comm.). A CAI of 4 indicates that theconodont sample has experienced a temperature between190 and 300'C (Epstein et al., 1977). Which end of thetemperature range the CAI datum indicates is dependentupon the length of time the sample remains at a giventemperature. For durations of less than 100 000 yr (themaximum life of most small hydrothermal systems: Nor-ton and Cathles, 1979; Cathles, 1981) a temperature be-tween 250 and 300"C is indicated.

Assuming a maximum formation temperature of 300.C,and usingan average homogenizationtemperature of 145.Cand salinity of 2.8 wto/o NaCl equivalent, the calculatedmaximum pressure on the hydrothermal system at thetime of adularia and quartz formation was approximately1.9 kbar (Potter, 1977;Haas, l97l; Lemmlein and Klev-stov, 196l). This estimate of maximum pressure corre-sponds to a maximum lithostatic depth of approximately7.6 km. For a formation temperature of 250oC, the cal-culated pressure is approximately l. I kbar, correspondingto a lithostatic depth of approximately 4.4 km. Thesedepth estimates are considerably less than that postulatedfor rocks overlying the Womble Shale (greater than 15km: Viele, 1974; Sharp, 1978). This suggests either (1)there was a significant hydrostatic component to the pres-sure due to an interconnected network ofopen fracturesabove the site of adularia deposition or (2) there was sig-nificant erosion in this portion of the Ouachita Mountainsprior to Permian hydrothermal activity.

DrscussroN oF REsuLTs AND THErR IMpLrcATroNS

The age, composition, temperature, and depth of for-mation of the Ba-rich adularia crystals place constraintson the postcollisional evolution of the Ouachita Moun-tains.

The date of 262 t l0 Ma is consistent with a Permianage for adularia and associated qtartz deposition. Thisage is in agreement with several investigators who havesuggested that the Ouachita orogeny extended into Perm-ian time (Waterschoot van der Gracht, l93l; Bass andFerrara, 1969; Denison et al., 1977; Desborough et al.,1985). Early Permian deformation is known in the Mar-athon region of Texas and in northeastern Mexico, andone might therefore infer that deformation and associatedhydrothermal activity persisted into Permian time in theOuachita Mountains where absence of Upper Pennsyl-

SHELTON ET AL.: Ba-RICH ADULARIA 92r

vanian and Permian rocks does not allow setting an upperstratigraphic limit on deformation.

Within the Arkoma basin, the youngest rocks exhibitingdeformation are mid-Desmoinesian (Dane et al., 1938)although some workers (e.9., Melton, 1930) have sug-gested thatjoint patterns in the Permian strata ofcentraland eastern Oklahoma were formed during the waningphases of the Ouachita orogeny. Uplift of the region ofthe Benton uplift, however, probably started somewhatearlier. Houseknecht (1981) reported the presence offrag-ments of slate, possibly from the Stanley or Jackfork For-mations ofMississippian and Morrowan age, respectively,in the Desmoinesian Hartshorne Sandstone of the Ar-koma basin. Moreover, conglomerates composed of chertpebbles have been reported in mid-Desmoinesian strataof the Ardmore basin of Oklahoma (Tomlinson andMcBee, 1959). If the chert pebbles are from the ArkansasNovaculite or older siliceous formations, as seems mostprobable, some part of the lower Paleozoic rocks of theuplift was exposed to erosion before the Middle Penn-sylvanian. The earliest hint of deformation of Ouachitarocks comes from a poorly understood series of radio-metric dates of Devonian age obtained from exotic blocksin the Carboniferous strata of the Marathon region ofwestern Texas and from micas from the lower Paleozoicstrata of the Broken Bow uplift (Denison et al., 1977).Recent work ([Jnderwood and Viele, 1985) suggests thatthe thrust sheets of the Ouachitas were pushed onto thesouthern margin of the North American craton by mid-Atokan time. Although the thickness of the Atokan strataacross the Benton uplift is not known, it seems probablethat uplift ofthis region may have started by Atokan time.The deformation front continued to migrate northward,finally involving rocks of mid-Desmoinesian or youngerage in the northern part of the Arkoma basin.

A Permian hydrothermal event recorded in the adulariaof our study may also coincide with a 255 to 265 + 10Ma widespread thermal event in the southern Appala-chians (Kulp and Eckelmann, 1961). The similarity of theages of these events may suggest that crustal orogenicevents proceed from north to south along the Appalachianfoldbelt and then from east to west along the Ouachitafoldbelt.

It is tempting to interpret the origin of the Ba-rich ad-ularia within the context of the widespread occurrence ofPb-Zn-Ba mineralization of the midcontinent region. Thedate of 262 + l0 Ma (Permian) obtained for adularia ofour study is in agreement with ages suggested for Missis-sippi Valley-type Pb-Zn-Ba deposits of the Viburnumtrend, southeast Missouri. Wu and Beales (1981), on thebasis of dating of mineralization-related magnetic mate-rial by paleomagnetic methods, suggested a Late Penn-sylvanian-Early Permian age for the Pb-Zn-Ba mineral-ization.

Recently, Sharp (1978), Leach et al. (1984), and Rowanet al. (1984) have suggested that many Mississippi Valley-type Pb-Zn-Ba deposits in the central and eastern UnitedStates are related to rapid expulsion offluids from peri-

922 SHELTON ET AL.: BA-RICH ADULARIA

cratonic-foreland basins during the Appalachian-Oua-chita orogeny in late Paleozoic time. Their ideas are con-sistent with fluid-inclusion, geochemical, and geologicstudies suggesting that the Ouachita-Arkoma basin maybe the source of the ore-forming fluids responsible for thePb-Zn-Ba deposits of the Ozark region (Leach, 1973).Thick Carboniferous flysch sediments accumulated in theOuachita-Arkoma basin during the subduction phases ofthe Ouachita orogeny. During the collision phase, thesesediments were folded and carried northward in complexthrust sheets that overrode coeval shelf sediments on thecontinental margin (Viele, 1974;I-each et al., 1984).

The crustal collision is thought to have resulted in therapid migration of metal-bearing fluids onto the craton inresponse to sediment compaction and pressure from tec-tonic and burial metamorphic processes. Homogenizationtemperatures from fluid inclusions in dolomite from Mis-souri, Kansas, Arkansas, and Oklahoma provide evidencefor a broad-scale Late Pennsylvanian-Early Permian heat-ing of the Paleozoic section to temperatures of 90 to l60qC.Temperatures of 200 to 300"C have been documented forfluids expelled from the l2-18-km-thick Ouachita sec-tion. The observed temperatures may reflect the thermalgradient of a slowly cooling fluid moving northward outof a deep sedimentarybasin such as the Ouachita-Arkomabasin (Rowan et al., 1984; Leach and Rowan, 1985). Themaximum temperature (300'C) and maximum pressure(1.9 kbar) determined for the Ba-rich adularia crystals areconsistent with this Permian hydrothermal event.

Occurrences of Pb and Zn in western Oklahoma andTexas flank the continuation of the Ouachita foldbelt intoMexico (Leach et al., 1984), where an Early Permian de-formation is known. Bethke et al. (1984) have also sug-gested that uplift of the Pascola arch during the Ouachitaorogeny may have permitted Illinois basin water to flownorthward and deposit Pb-Zn-Ba ore in the Upper Mis-sissippi Valley.

The Pb-Zn-Ba deposits of the folded Appalachians, lo-cated in shelf carbonates, have a similar relationship todeep foreland basins and orogeny related to crustal col-lision. Thus, the occurrence of Pb-Zn-Ba deposits alsosuggests that crustal orogenic events, proceeding from northto south along the Appalachian foldbelt and then fromeast to west along the Ouachita foldbelt, may have resultedin a major period of Pb-Zn-Ba metallogeny in NorthAmerica (Irach et al., 1984).

In summary, the Permian Ba-Rich adularia crystals ofour study reflect the presence of postcollisional hydro-thermal activity in the Ouachita Mountains. The Ba-richadularia crystals and associated quartz veins are likely theresults of postfolding, Ouachita orogenic faulting thattapped a'vast reservoir of hot, excess-pressured fluids(Sharp, 1978), which may have resulted in Pb-Zn-Ba min-eralization in the northern Arkansas-southeastern Mis-souri area.

AcrNowr,nlcMENTS

The K-Ar analyses were performed in the laboratory of K. K.Turekian of the Department of Geology and Geophysics at Yale

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MeNuscnrpr REcETvED FrsR.uanv 22, 1985MeNuscnrrr AccEprED Me.ncn 18, 1986