edades radimetricas del precuyano (schiuma y llambías, 2008)

644 Revista de la Asociacin Geolgica Argentina 63 (4): 644 - 652 (2008)

NEW AGES AND CHEMICAL ANALYSIS ON LOWER JURASSIC VOLCANISM CLOSE TO THE DORSAL DE HUINCUL, NEUQUN

Mario SCHIUMA1* and Eduardo Jorge LLAMBAS2

1 Apache Corporation Argentina, Buenos Aires.* Direccin actual: YPF S.A. Email: [email protected] Centro de Investigaciones Geolgicas, Universidad Nacional de La Plata-CONICET, La Plata, Buenos Aires.Email: [email protected]

ABSTRACT

New single zircon ages from hydrocarbon well cores in the A-1 Norte de la Dorsal and Anticlinal Campamento area of theNeuqun basin indicate that 199.0 1.5 Ma andesite lava flow and 203.75 0.26 Ma dacite breccia overlie a 286.5 2.3 Magranodiorite and 284.0 1.3 Ma andesite dike. The Lower Jurassic volcanics were deposited on a regional erosion surfaceaffecting the Permian rocks. In the studied area there is no record of Middle to Upper Triassic volcanics as in other areas ofthe basin. Exotic zircon crystals gave ages of Mesoproterozoic, Middle Cambian, Early Devonian and Early Carboniferous,suggesting a poliphasic basement. Chemical analyses of three selected samples show a calc-alkaline signature, supporting theexistence of a volcanic arc at the Early Jurassic as it has been proposed for the center of the basin.

Keywords: Neuqun basin, Precuyano cycle, Early Jurassic, Calc-alkaline volcanics.

RESUMEN: Nuevas edades del volcanismo Jursico Inferior de la cuenca Neuquina en la dorsal de Huincul. Se dan nuevas edades U/Pb en cristales nicos de circn de muestras de corona de pozos exploratorios en el rea petrolera A-1 Norte de la Dorsal y AnticlinalCampamento de la cuenca Neuquina. Estas edades permiten acotar un volcanismo Jursico Inferior apoyado directamentesobre un basamento gneo del Prmico Inferior. Una muestra de andesita arroj una edad de 199,0 1,5 Ma y una de dacita203,75 0,26 Ma. El basamento est constituido por una granodiorita de 286,5 2,3 Ma intruida por diques de andesita con284,0 1,3 Ma, ambas rocas estn cortadas por una superficie de erosin de carcter regional labrada durante el Trisico, posi-blemente Medio. En las perforaciones estudiadas no se han encontrado rocas trisicas. Los circones exticos hallados indicanla existencia de un complejo basamento con edades del Mesoproterozoico, Cmbrico Medio, Devnico Temprano yCarbonfero Temprano. Los anlisis qumicos muestras una filiacin calco-alcalina que apoya la hiptesis de la existencia deun arco volcnico Jursico Temprano en el centro de la cuenca Neuquina.

Palabras clave: Cuenca Neuquina, Ciclo Precuyano, Jursico Temprano, Volcnicas calco-alcalinas.

INTRODUCTION

The Neuqun Basin is part of an exten-sional system along the active margin ofSouth America. During the Triassic toEarly Jurassic times, many hemigrabenswere generated because the extensionalsystem described above. Those hemigra-bens, related to the rifting stage (Verganiet al. 1995), were filled by volcanic andsedimentary sequences (Franzese andSpalletti 2001, Pngaro et al. 2002, Fran-zese et al. 2007, Llambas et al. 2007) withvariable thickness, ranging from zero upto a few thousand meters. The riftingstage was followed by the Early Jurassictransgressive inundation of the CuyoGroup, thus changing from localized rif-

ting to a generalized subsidence (Lega-rreta and Uliana 1999).In a wide sense, these sequences weregrouped in the Precuyano cycle (Guli-sano et al. 1984) or in the syn-rift sequen-ces (Franzese et al. 2007). In most of thesections the Precuyano units overlie thevolcanic and plutonic complexes of Per-mian to Lower Triassic age, known as theChoiyoi Group (sensu Rolleri and CriadoRoque 1970). Occasionally the Precuya-no can be deposited over metamorphicrocks of Devonian to Carboniferous age(Franzese 1995). In spite of the geologi-cal and economic importance of thedeposits related to the Precuyano cycle,their ages are still unknown, althoughthey are stratigraphically constrained bet-

ween Late Triassic and Early Jurassic. Inthe studied area the volcanic rocks con-sist of highly altered andesites, dacitesand rhyolites, with minor basalts. Onaccount of the intense alteration datingmust be carried out in minerals resistantto changes, like zircon. In this researchwe dated the rocks by the U/Pb methodsin single zircon crystals.The results obtained in the study of coresamples of two wells Anticlinal Campa-mento and Cerro Guanaco, close to theDorsal de Huincul (Fig. 1), show theexistence of an active volcanism duringthe Early Jurassic. The chemical featuresof the analyzed samples show that theyare part of a volcanic association similarto a volcanic arc, as suggested by Ber-

645

mdez et al. (2002) and Llambas et al.(2007) for the center of the basin.

GEOLOGICALBACKGROUND

The basement of the Neuqun basinconsists of the paleozoic Colohuinculand Piedra Santa metamorphic forma-tions, the sedimentary Upper Carbonife-rous Andacollo Group and the Permianto Early Triassic volcano-plutonic assem-blages of the Choiyoi Group sensu Ro-lleri and Criado Roque (1970). This no-mination differs from the Choiyoi con-cept proposed by Groeber (1946), whenhe described the Upper Triassic volcanicsof the Cordillera del Viento, which hasbeen included in the Precuyano cycle byGulisano et al. (1984).In most of the Neuqun basin the Pre-cuyano volcano-sedimentary sequenceswere deposited over an erosion surfaceof regional extension, the Huarpic (=intra-Triassic) unconformity, carved onthe plutono-volcanic complexes of theChoiyoi Group (Llambas et al. 2007),which in many places constitutes the ba-sement of the Neuqun basin. In thewells studied this Group consists of gra-nodiorite and comagmatic andestic di-kes.The Precuyano units underlie the CuyoGroup (Pliensbachian to Bathonian)composed mainly by the Los Molles For-mation (Pliensbachian to Callovian) andthe Lajas Formation (Bajocian to Callo-vian). In the ireco area and northern ofDorsal de Huincul, the Los Molles For-mation usually starts with a silicified li-mestone, named the Chachil Limestone,it thickness ranges from a couple of me-ters to forty meters, and underlies a thicksequence of shales and sandstones depo-sited in a deep marine environment, attai-ning a thickness of more than 3000 m(Gulisano et al. 1984).

STRATIGRAPHICRELATIONS

During the exploration process in the A-

New ages and chemical analysis on Lower Jurassic volcanism

Figure 1: Location map ofthe samples studied in theA-1 norte de la Dorsal andAnticlinal Campamento,Neuqun province.

Figure 2: Columnar sections, not to scale, summarizing the stratigraphy of the studied samples. Inthe Anticlinal Campamento and Guanaco area the Lower Jurassic volcanics overlie the LowerPermian plutonic rocks. The dated Jurassic volcanics belong to a single eruptive event.

1, Norte de la Dorsal and AnticlinalCampamento areas (Fig. 1), it was plan-ned to study the volcanic formationsunderlying the Los Molles Formation.The exploration program included dri-lling of several wells and the analysis ofcore and cutting samples.Rotated samples and core samples wereobtained from several wells in order toadjust the stratigraphic and lithologicalinterpretation of this portion of theNeuqun basin. The samples were subse-quently analyzed and described in detail.Geochronological data were attemptedto obtain in several samples, and in all ofthem zircon crystals were separated ex-cept in YK-103. In addition, three che-mical analyses were carried out, includingtrace and rare elements.In the Anticlinal Campamento and Gua-naco wells (Figs. 1 and 2) the rocks un-derlying Los Molles Formation consist ofandesitic block and ash and lava flowdeposits, dacitic to rhyolitic ignimbritesand lavas and silicic ash fall deposits. It isalso important to point out the strongpropylitic alterations, mainly of hydro-thermal origin, overprinted the originalcomposition and textures of the rocks.The thickness of the volcanic sequence ishighly variable, ranging from zero up tomore than 450 meters. In some wells ofthe Anticlinal Campamento and Guana-co area there is no record of deposition,and the Cuyo Group overlies the pluto-nic rocks of the Choiyoi Group (sensuRolleri and Criado Roque 1970). Basedon the seismic data of this area, a totalthickness of over 1000 meters has beenestimated. The volcanic sequences,which may be correlated with the Pre-cuyano cycle, were deposited over a mas-sive granodiorite intruded by coeval an-desite dikes. The granodiorite has an alte-red cap of about 15 m thick. The EarlyPermian age of the granodiorite and theandesite dikes allow to include them inthe Choiyoi cycle.A possible stratigraphic correlation ofthe igneous and sedimentary depositsanalyzed in this paper is shown in Fig. 2.

GEOCHRONOLOGY

Five core samples from the A-1 Norte dela Dorsal and Anticlinal Campamentoareas (Fig. 1) were analyzed for geochro-nology. Dating was obtained using U-Pbmethod on single zircon crystals by Ac-tivation Laboratories Ltd. Analytical dataare shown in Table 1. Correlation betwe-en isotopical and stratigraphic ages wasmade according to the geologic timescale of Gradstein et al. (2004). No zir-con crystals were obtained in an andesitelava flow (YK-103).Three zircon crystals were selected fromeach sample (six crystals for YK 145).The outer zone of the crystals was remo-ved by abrasion. The ages obtained ineach of the crystals from the sample arenot homogenous and only the ageswhich plot in the concordia and are coh-erent with the stratigraphic location havebeen reproduced in Figs 3-4. The remai-ning ages, always older, are interpreted asexotic zircons coming from possiblecontamination.Two groups of ages have been obtainedwhich are compatible with the local stra-tigraphic column: 1) Permian ages, rela-ted to the Choiyoi cycle (sensu Rolleri andCriado Roque 1970) and 2) Early Jurassicages corresponding to the Precuyano cy-cle (sensu Gulisano et al. 1984).Choiyoi Cycle: Three zircon crystalswere analyzed from a granodiorite sam-ple (YK -1289) and other three from anandesite dike (YK-1290) intruding thegranodiorite (Table 1, Fig. 3). The grano-diorite consists of 50 % plagioclase(An12-25), 25 % of K-feldspar, 20 % ofquartz, and 5 % of biotite with scarcealteration to chlorite and sericite. Thebest age is the upper intercept of theconcordia at 286.5 2.3 Ma.The andesite is fine-grained and consistsof sericitized and kaolinitized plagiocla-se, fine-grained interstitial granular am-phibole, prismatic biotite, chlorite andfine-grained carbonate. Sericite (or illite),chlorite, carbonate and quartz are secon-dary minerals, partly replacing plagiocla-se and biotite. Two out of three zircon

crystals analyzed plot on the concordiaand the weighted mean 207Pb/206Pb age is284.0 1.3 Ma.The similar ages of andesite and grano-diorite suggest a cogenetic origin, a rela-tionship which is common in the Choiyoicycle of the Cordillera Frontal (Sato andLlambas 1993, Llambas and Sato 1995),conforming a plutono-volcanic associa-tion. The Lower Permian age of bothrocks correlates with the lower section ofthe Choiyoi Group. No exotic zirconshave been found in these samples.Precuyano Cycle: Two samples yieldedmeaningful results: YK-104, from Anti-clinal Campamento area and YK145from Guanaco area.YK-104 is a brecciated phenoandesitewith glomeroporphyritic texture, and ischemically classified as dacite (Table 2,Fig. 5). Phenocrysts (20 %) are of plagio-clase (An40) altered to sericite, kaoliniteand chlorite. The groundmass (80 %) hasintersertal texture composed of partiallyrecrystallized volcanic glass and plagio-clase needles with moderate orientationby magmatic flow. Submicroscopic, irre-gular vesicles are filled with quartz. Se-veral microfissures cross cut the sample,they are totally or partially filled withchlorite and/or quartz.The three zircon crystals dated fromsample YK-104 gave different ages (Ta-ble 1, Fig. 4a): 1140-1175 Ma, 517-522Ma and 199.0 1.5 Ma. We only consi-der the youngest one as the age of theandesite crystallization, which corres-ponds to the Sinemurian. The other twoages correspond to xenocrystals sugges-ting the presence of a basement withGrenvillian age and possible igneousbodies from the Middle Cambrian.The YK-145 sample is a greenish grayandesite with porphyritic texture. Thechemical analysis suggests an andesiticcomposition close to the field of dacites(Table 2, Fig.5) Prismatic and orientedplagioclase (An35) phenocrysts (25%)show an intense alteration to sericite, ka-olinite, epidote and calcite. The pilotaxictexture of the groundmass is composedof acicular plagioclase and interstitial

646 M. SCHIUMA AND E. J. LLAMBAS

647

glass with abundant iron oxide. Sub-microscope vesicles filled with chlorite,opaque minerals and calcite are common.The microfractures are filled with calcite,and in some rare cases with pyrite. Sixzircon crystals have been dated with dif-ferent ages (Table 1, Fig. 4b), suggestingthe presence of exotic zircons. Theyoungest age, 203.75 0.26 Ma, is inter-preted as the crystallyzation age of theandesite, in agreement with the stratigra-phic relations shown by the well analysis.It is comparable to the Hettangian strati-graphic age and does not differ from theandesite of the YK-104 sample, thus in-dicating the existence of an importantEarly Jurassic volcanism. Four crystalsout of the remaining five indicate ages of404.0 4.0Ma suggesting the existenceof possible igneous activity at the EarlyDevonian, on account that the grains areeuehedral (Table 1).An additional sample of andesite lava(YK-103) is located 85 m higher than the

YK-104 in the stratigraphic column.Unfortunately, no zircon crystals werefound, therefore it could not be dated. Itis an autobrecciated-andesite (Table 2,Figs. 2-5) with porphyritic to glomerulo-porphyritic texture and groundmasscomposed of plagioclasa needles immer-sed in glass. The plagioclase (An35) phe-nocrysts and the mafic minerals are in-tensely altered to chlorite, epidote andcalcite.

CHEMICALCHARACTERISTICS

Three chemical analyses of well coresamples from A-1, Norte de la Dorsaland Anticlinal Campamento areas havebeen carried out. All of them were per-formed at Activation Laboratories Li-mited. The results are shown in Table 2.Major elements and Sc. V and Cr wereanalyzed by sample fusion and measuredwith induction by argon plasma (ICP-

fusion). The remaining elements wereanalyzed by fusion and quantified in amass spectrometer (MS-fusion).

The analyzed samples have high valuesof LOI (loss on ignition) therefore nographs with major elements have beenused. We consider that the Winchesterand Floyd (1977) diagram based on im-mobile elements (Fig. 5) shows the classi-fication of alterated rocks more accura-ted than the TAS diagram. The samplesplot in the andesite and dacite field, in asimilar manner as the samples of the LaPrimavera (Pliensbachian) and Milla Mi-chico Formations (Upper Triassic-LowerJurassic) of Chacay Melehue area (Llam-bas et al. 2007).The Mg# value (Mg/Mg+Fe2+ assu-ming F3+/Fe2+ = 0.15) of the analyzedsamples is moderately high, ranging from43.4 to 51.9. This range coincides withthe samples from Chacay Melehue. Incontrast, the values of Cr, Co and Ni are


Sample Weight U Pb Pb(c) 206 Pb 208 Pb 206 Pb 207 Pb 207 Pb 206 Pb 207 Pb 207 Pb corr. fractions (g) (ppm) (ppm) (pg) 204 Pb 206 Pb 238 U err 235 U err 206 Pb err 238 U 235 U 206 Pb coef.

(a) (b) (c) (d) (e) (2%) (e) (2%) (e) (2%)YYKK--110044z1 1,80 114 23,2 1,6 1616,3 0,120 0,193538 0,20 2,11174 0,28 0,07914 0,18 1140.5+2.3 1152.6+3.2 1175.5+2.1 0,754z2 2,50 93 9,5 0,9 1364,7 0,359 0,083511 0,17 0,66536 0,35 0,05778 0,29 517.0+0.9 517.9+1.8 521.6+1.5 0,561z3 3,10 82 3,2 1,1 477,9 0,364 0,031353 0,75 0,21650 1,01 0,05008 0,64 199.0+1.5 199.0+2.0 198.8+1.3 0,777YYKK--114455z1 0,61 333 26,1 0,5 1864,6 0,360 0,064387 0,18 0,48655 0,30 0,05481 0,23 402.3+0.7 402.6+1.2 404.3+4.0 0,638z2 0,61 203 15,5 0,3 1621,0 0,323 0,064404 0,19 0,48650 0,36 0,05479 0,30 402.4+0.8 402.5+1.4 403.5+4.3 0,568z3 0,45 765 41,2 0,4 3304,9 0,149 0,052067 0,09 0,39012 0,14 0,05434 0,11 327.2+0.3 334.5+0.5 385.3+3.1 0,629z4 3,10 133 11,1 0,6 2819,7 0,441 0,065220 0,15 0,49614 0,23 0,05517 0,17 407.30.6 409.10.9 419.31.1 0,672z5 1,50 261 9,2 0,3 2915,6 0,217 0,032108 0,09 0,22231 0,17 0,05022 0,15 203.70.2 203.80.3 205.00.4 0,547z6 1,50 70 5,6 0,3 1706,4 0,376 0,065201 0,16 0,49325 0,37 0,05487 0,32 407.20.6 407.11.5 406.81.6 0,527YYKK--11228899z1 2,00 608 28,6 0,6 5753 0,153 0,045340 0,06 0,32525 0,12 0,05202 0,10 285.90.17 285.90.34 286.40.29 0,520z2 2,20 989 41,7 0,6 10438 0,112 0,042160 0,05 0,30202 0,09 0,05196 0,07 266.20.13 2680.24 283.50.20 0,590z3 1,90 810 37 0,8 5878 0,131 0,044950 0,07 0,32235 0,15 0,05201 0,13 283.40.20 283.70.42 285.90.37 0,500YYKK--11229900z1 3,70 711 32,8 0,9 8395,0 0,138 0,045010 0,05 0,32249 0,09 0,05196 0,07 283.80.14 283.80.26 283.70.20 0,630z2 2,80 405 18,9 1,8 1860,5 0,154 0,045080 0,13 0,32306 0,24 0,05198 0,20 284.20.37 284.30.68 284.50.57 0,590z3 1,80 518 24,7 0,7 3726,0 0,188 0,044700 0,09 0,32038 0,17 0,05199 0,14 281.90.25 282.20.48 284.90.40 0,570

Concentrations Ratios Age (Ma)

TABLE 1: Analytical data for zircons from A1, Norte dorsal, Anticlinal Campamento and Guanaco zones.

(a) Sample weights are estimated by using a video monitor and are known to within 40%. (b) Total common-Pb in analyses. (c) Measured ratiocorrected for spike and fractionation only. (d) Radiogenic Pb. (e) Corrected for fractionation, spike, blank, and initial common Pb. Mass fractiona-tion correction of 0.15%/amu 0.04%/amu (atomic mass unit) was applied to single-collector Daly analyses and 0.12%/amu 0.04% for dynamic Faraday-Daly analyses. Total procedural blank less than 0.5 pg for Pb and less than 0.1 pg for U. Blank isotopic composition: 206Pb/204Pb= 19.10 0.1, 207Pb/204Pb =15.71 0.1, 208Pb/204Pb = 38.65 0.1. Corr. coef. = correlation coefficient. Age calculations are based on thedecay constants of Steiger and Jger (1977). Common-Pb corrections were calculated by using the model of Stacey and Kramers (1975) and theinterpreted age of the sample.

low (Table 2), showing differences in thecomposition of the source, in the fusiongrade or in the subsequent differentiationprocesses.

Fig. 6 shows the expanded diagrams ofthe trace elements of the three analyzedsamples. There is little variation amongthem. Regarding the La Primavera andMilla Michico Formations, the analyzedsamples show a similar trend but withgreater enrichment.The behavior of rare earth elements (Fig.6b) is also typical of the calc-alkaline sui-tes, with a moderate slope in the lightrare earth elements and a weak horizon-tality in the heavy ones. The absence ofthe Eu depression could reflect a poordifferentiation process of the magmas inthe crust, probably because the rise ofthe magma through the crust was fast,without the emplacement of large mag-matic chambers. Regarding the Milla Mi-

chico and La Primavera Formations, on-ce again, a similar trend occurs but with amajor enrichment of the light rare earthelements.In the Harris et al. (1986) and Pearce et al.(1984) discrimination diagrams, the sam-ples plot in the field of the volcanic arcs(Figs 7a-7b) in agreement with the rareand trace elements trends.To sum up, the three analyzed sampleshave the signature of a volcanic arc. Theydo not differ from those samples fromthe Planicie Banderita well (Bermdez etal. 2002) and Chacay Melehue area (Lla-mbas et al. 2007), therefore it is probablythat this volcanic arc had a regional dis-tribution at the center of the Neuqunbasin.


YK-103 YK-104 YK-145

TABLE 2: Chemical analysis from Anti-clinal Campamento and Guanaco zones.

SiO2 53,13 69,10 49,90TiO2 0,67 0,31 1,17Al2O3 15,16 15,54 17,15Fe2O3 5,53 2,14 8,21MnO 0,10 0,03 0,11MgO 1,82 0,99 3,04CaO 8,29 0,92 3,95Na2O 3,66 4,77 2,80K2O 2,90 4,13 5,80P2O5 0,24 0,10 0,49LOI 8,55 2,07 7,40TOTAL 100,03 100,08 100,03

Trace elements, ppmCs 1,18 3,52 5,21Rb 58,80 130,58 198,26Ba 1040 2190 1060Th 4,16 12,35 10,62U 0,98 2,19 3,15Nb 3,99 6,81 9,01Sr 334,82 212,61 196,86Hf 3,85 5,26 7,79Zr 145,01 209,34 305,83Y 21,86 16,35 28,82V 106,88 20,25 170,62Cr

649

DISCUSSION ANDCONCLUSIONS

The ages indicated by the single zirconcrystals from core samples in the nor-thern section of the A-1 Norte de laDorsal and Anticlinal Campamentoshow the existence of an important mag-matic activity during the Hettangian andSinemurian, according to the geologic ti-me scale of Gradstein et al. (2004). Thenew Early Jurassic isotopic ages confirmthe Hettangian-Sinemurian stratigraphicages assumed by Groeber (1958), Stipa-nicic et al. (1968), Gulisano and Pando(1981) and Gulisano et al. (1984) for theSaic, Piedra del guila and Lapa For-mations, all of them included in the Pre-cuyano cycle. However, more studies areneeded to clarify the age of Lapa For-mation, which based on its flora contentwas related to Upper Triassic by Spallettiet al. (1991).This is the first time that the existence ofan intense magmatic activity during theEarly Jurassic in the Neuquen basin isdocumented by the U-Pb age in zirconsingle crystals. The volcanic rocks analy-zed are placed below the "intralisica" un-conformity that separates the beginningof the Cuyo transgressive inundationfrom the volcanic deposits related to therifting phase. This unconformity has aregional meaning because it representsthe transition of the rifting phase to agreater amplitude subsidence (Legarretaand Uliana 1999). The new ages suggestthat this unconformity is constrained tothe Sinemurian-Pliensbachian limit.Our research in the Anticlinal Campa-mento and Guanaco area shows that bet-ween the Choiyoi Group and the Precu-yano volcanics dated herein, there is amaximum hiatus of 82 Ma suggesting along period of erosion in this sector ofthe Dorsal de Hincul that favored the ex-humation of the plutons of the ChoiyoiGroup during the Triassic. The hiatus be-tween the Precuyano cycle and the firstsediments of the Cuyo cycle would beapproximately 4 Ma. The fact that theconglomerates at the base of the Los

Molles Formation include volcanic peb-bles similar to the rocks of the Precu-yano cycle suggests that they had a sub-air exposure at the beginning of the ma-rine inundation.In another localities of the Neuqun ba-

sin, the hettangian and sinemurian volca-nic activity continued along the Pliens-bachian and Toarcian, as it is shown atthe base of the Cuyo Group in the Cha-cay Melehue area by the volcano-sedi-mentary "Unnamed Unit" (Gulisano and


Figure 4: a) U/Pb concordia plot of single zircon crystal of andesite from South AnticlinalCampamento area. b) U/Pb concordia plot of single zircon crystal of andesite from Guanaco zone.

Gutirrez Pleimling 1995) -subsequentlycalled La Primavera Formation by Suarezand De La Cruz (1997)- dated by marinefossils as Pliensbachian to Toarcian by

Damborenea and Manceido (in Guli-sano and Gutirrez Pleimling 1995). Inaddition, the hyper-dense gravity flowsof andesitic composition (laharic depo-

sits) intercalated in the turbiditc blackshales and sandstones of the Los MollesFormation suggest the existence of an-desitic strato-volcanoes during that pe-riod (Llambas and Leanza 2005). Ano-ther indication that there was a conside-rable volcanic activity during the Pliens-bachian is the presence of abundantpiroclastic material on the base of theLos Molles Formation, as evidenced bythe Sierra de Chacaico Formation (Lean-za 1992). At the center of the Neuquenbasin, the volcanic activity diminishesdramatically during the Middle Jurassic,but along the west border of the basin ithad a remarkable development (Surez etal. 1988, Surez and Emparn 1997).In the studied area the basement of theLower Jurassic volcanics is formed by theChoiyoi Group (sensu Rolleri and CriadoRoque 1970). According to the age ofthe granodiorite, 286.5 2.3 Ma and theandesite dike, 284.0 1.3 Ma (Fig. 1),these rocks correlate with the lower sec-tion of the Choiyoi Group (Llambas etal. 1993).The ages determined in the exotic zirconcrystals reveal that the Choiyoi Groupevolved over a basement formed byrocks with zircons of grenvillian age ofuncertain provenance and devonian andcambrian igneous bodies. With theseages, we confirm the presence of a crys-talline basement formed by successiveigneous and metamorphic events, as sta-ted by Linares et al. (1988), Franzese(1995) and Varela et al. (2005).The chemical composition of the Juras-sic volcanics analyzed in this paper showsa magmatic arc signature, similar to thatproposed by Bermdez et al. (2002) forthe Planicie Banderita depocenter, in thenorthern portion of the Dorsal de Huin-cul, and by Llambas et al. (2007) for thesouthern part of the Cordillera delViento in northern Neuqun. Accordingwith the new ages at the Dorsal de Huin-cul, this volcanic arc was still active du-ring the Early Jurassic in the center ofthe Neuqun basin.


Figure 5: Classification of thevolcanic rocks according to theWinchester and Floyd diagram.For comparison are includedsamples from La Primavera andMilla Michic Formations (datafrom Llambas et al. 2007).

Figure 6: Abundance dia-grams of trace elements: a)and rare earth elements; b)normalized to primordialmantle and chondrite res-pectively, after Taylor andMcLennan (1985).

651

ACKNOWLEDGEMENTS

We express our gratefulness to Apachefor enabling us to publish the dating aswell as the geochemical studies carriedout. We appreciate the helpful commentsmade by Hctor Leanza, Marcelo Manas-sero and Ana Mara Sato on this manus-cript. The reviewers Alfonso Mosqueraand Juan Franzese are thanked for theircritical constructive comments that hel-ped to improve the manuscript.

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Recibido: 3 de marzo, 2008 Aceptado: 27 de junio, 2008

M. SCHIUMA AND E. J. LLAMBAS

edades radimetricas del precuyano (schiuma y llambías, 2008)

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