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Second ISAG, Oxford (UK),21 -231911 993 509

THE ACHALA (CORDOBA, ARGENTINA) A COMPOSITE INTRUSION MADEOF INDEPENDANT MAGMATIC SUITES

(1) Centre de GBologie GBnBraleet MiniBre, Ecole des Mines, 35 rue Saint-Honore, 77305 Fontainebleau, FRANCE (2) Comisi6n .Nacional de Energia Athica, Avenidad del Libertador, 1871 , Buenos Aires, ARGENTINA

ABSTRACT : In the Sierra de Cordoba (Argentina), the Achala batholith consists in four independent peraluminous magmatic suites which shows contrasted major and trace elements spectra and variable metallogenic specialisation.

KEY WORDS : Sierras pampeanas, S-type granites, diff erenciation, melallbgenic specialisation

INTRODUCTION

The Achala batholith which outcrops over a surface of about 2500 is km2, a major element of the Sierra Grande de Cordoba, the most oriental of the Pampean ranges. Several deposits and showings of W, Cu, U, F, Li, Be, Nb and Ta, are spacially associated with the batholith. The recent papers dealing with its petrology and consideronly transversal cross-sections or limited sectors. The present study intends to take into account the whole batholith, since it is based on 84 reliable samples, carefuly selected out of the 450 available analyses, in order to to represent the different facies and the different sectorsof the intrusion. The Achala batholith is a post-tectonic intrusion emplaced in high grade rnetarnorphic rocks of the parnpean cycle (Upper Proterozoic).It is composite ; its main facies are synchronically emplaced in an environment of submeridian crustal shearing during alocal extensional stage.An horizontal shortening induces the formationof NW to NNW trending magmatic shear zones which host synkinematic intrusions. After its emplacement, the batholith was affected by large dextral wrench faults associated with mylonites, which are anteriorto the Upper Carboniferous, and by inverse faults related to the Andean cycle.

FOUR MAGMATIC SUITES Structurai, petrological and geochemical data allowone to divide the Achala batholithinto homogoneus sectors and to recognise at least four rnagmatic suites ; the limits between the different dornains, are not always obvious in the field, but are marked by strong geochemical discontinuities, particularly emphasizedby the radiometric data. Each suite regroups various petrographic facies : equigranular , porphyritic granodiorites and granites, coarse grained granites, equigranular granites, and . Al1 these different suites present a peraluminous character (S-type granites) but distinctions can be made using both traceand major elements :the Achala suite is richer in K, Rb, Sr, Zr, Th and U , the Champaqui suite richer in Na, Ba and Mg ; the Characato and Cumbrecita 510 Second ISAG, 0.xford (UK),21 -231911993

suites are intermediate between the former.The Characato suite is partieularly rieh in REE ;Y and Nb present here a different behaviour. Each suite seetms to have its proper metallogenic specialization.: the uranium and fluarite depssits and showings appear related to the Aehala suite , the W deposits and showings to the Characato suite and the pegmatites of metallogenic interest(Li, Be, Nb, Ta, P) to the Champaqui suite.

These difference that have been just mentionned, are sbserved even in the less differenciated members of the various suites : the parental magmas must tkus have been independent and these suites may be of very different ages.The publisked geoehronological studies carfled sut without taking car8 of the composite charasfer BI the bathslith, assing ts the Aehala bathslith an age varying frsm Siluflan Io Cadmniferous. : 399 +25 Ma for the Achalamite (probably errorehron) (Rapela &I al. 1982); 337k30 / 358+9 Ma for the Characato suite (Rapela & al. 1991). As long as new data are not available, one must thus be very cautious about any geotectonieal interpretationof the Achala batholith.

REFERENCES ALVAREZ, J. O., 1992. Beterminacidn petroldgiea de facies igneas regionales en el batolito de Achala y su psible evoluci6n. C.N.E.A. unpubl. report. BALDO, E. G. A., 1992. Estudio petrsl6gico y gesquimico de las rocas igneas y metamQrficas entre Pampa de Olaen y Characato, extremo nom de la Sierra Grande de CBrdoba. Cdrdsba, Repljblica Argentina. Thesis, Fat- Cs. Ex., Fis y Nat., Univ. Nac. de C&rdoba. CUNEY, M., LEROY, J., VALDIVIEZO, A., DAZIIANO, O., GAMBA, M., ZAWCO, J, MORELLO, O., NINCI, C. and MOLINA, P., 1987. Geschemistry of uranium mineralisedAehala granitie esmplex, Argentina : csmparissn wilh hercynian peraluminsus of Western Europe. In Metallogenesisof Uranium deposits. Inter. Afom. Enef. Agency, Vienne, p. 21 1-232. GAMBA, M., 1981. Difereneiaeiones petrogrhfieas y gecsquimicas en el Area de Copina, seetor' sudeste del batdito de Aehala, Sierra Grande de Cdrdoba. Distribuci6n del uranio en las distintas facies graniticas. C.N. €.A. unpubl. report. GORBILLO, C. E. and LENCINAS, A. N., 1979. Sierras Pampeanas de C6rddsba y San Luis. Se@ Simp. Geol. Reg. Ag., Acad- Mac. Cienc, Cdfdoba, 1, pn577-650- LIRA, R. , 1985. Tipslsgia y evolucidn de rocas graniticas en su relaeidn con el hemiciclo enddgeno de la geoquirnica del uranio. Aspectos rnetalogen6ticos. Sedor septentrional del batolito de Achala, prov. de CQrdsba. Thesis, Fae. Cs. Ex., Fis- y Nat., Umiv. N~c.dt? Cdrcioba. LIRA, R. and KIRSHBAUM, A., 1990. Geoehemical evolutions of the granites from the Achala batholith of the Sierras Pampeanas. Geological Society of America Special Paper, 241, p. 67-76. LOPEZ, L., 1991. Base de datos de litogeoquirniea. Batolitode Achala. C.N.E.A. unpwbl. report. MONSBERGER, G., 1990. Estudio geoldgieo y petrol6gico del granits de la Mes'a de la Mula Muerta y su entorno eneajonante. Pampa de Blaen, Depto. Punilla, C6rdoba. Fac Cs. €x., fis. y Nat., Umiv. Nac. de Cdrdoba wnpubl. report. PATIRO, M. G. de and PATIfi6 DOUCE, A. E., 19087. Petrologia y petrosg6nesis del batolito de Aehala, provincia de Cdrddsba, a la lum de la evideneia de camp. Amciacidm Geoldgica Argenfima, XLII (1/2),p. 201-205. RAPELA, C. W., 1982. Aspectos geoquirnicos y petroldgicosdel batolits de Achala, provincia de Cdrdoba. Asodacidn GeoldgicaAfgentina, XXXVII (3), p. 319-330. RAPELA, C. W., HEAMAN, L. M. and Mc NUW, R.J., 1982. Rb-Sr geochronslogy of granitoids rocks from the Pampean Ranges, Argentina.J. Geol., 90,p. 574-582. RAPELA, C. W., TOSELLI, A., HEAMAN, L. and SAAVEDRA, J., 1990. Granite plutonism of the Sierras Parnpeanas. An inner cordilieran paleornoie arc in the southern Andes. Geological Society of America Spedal Paper, 24 1, p. 1O 1 - 126. RAPELA, C. W., PANKHURST, R.J., MIWSCHBAUM,A. and BALDO, E. G. A:, 1991.faeies inttrusivas de edad carbonifera en el batolito de Achala : Evidencia de una anatexis regionalen las Sierras pamapeanas? Actas Congr. Geol. Chileno, p. 40-43. ZARCO, J. J.,1992. Prioritacl de indicios unraniferos,rnodels gen6ties basado en el cisallarniento magmatics seurrido durante el emplazamiento del batolito de Aehala. f2.N.E.A. unpubl. Second ISAG, Oxford (UK),21 -231911993 511

6.550

6.530

6.510

6.490

6.470

6.450 3. 580 3.600 3.620 3.640 3.620 3.600 3.580 uEA!&u Granodioritas I - E - IV - Dg aGranitos - granodioritas podiroides 111 A - B - C Granitos de grano grueso IV - A Ixl* * Granitos de grano grueso IV - B - C Granitos isogranulares de dos micas II -A Granitos granodioritas de grano fino I D - C/ 1 A - - - Q @- Granitos porfiroides 111 A - B Granitos de grano grueso IV -A Granitos equigranulares II - C Granitos de srano fino 1 - B

SUITE CHAMFAOU Granitos con dos micas

Granitos porfiroides - È m.y equlgranulares con dos micas

I11111 >P A >P 512 Second ISAG, Oxford (UK),21-231911993

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