Proterozoic and Paleozoic Evolution 9F Atlantic Area of North-Patagonian Massif, Argentine

.. ..' DOC. 425

Proterozoic and paleozoic evolution 9f Atlantic area of North-Patagonian Massif, Argentine

Varela, R.l; Cingolani, C.l; Sato, A.l; Dalla Salda, L.l; Brito Neves, B. B. 2; Basei, M.A.S.2; Siga Jr., 0. 2 & Teixeira, W. 2 I ! / 1 UNLP-Argentina 2 USP-Brasi1

This contribution comprises partial results of the CNPq-CONICET International Cooperation Project, which studies the crustal evolution of the northern Patagonia during the Precambrian to Paleozoic times. It also provides information for the relationship between the so called South American Platform and Patagonian Platform, the former being consolidated at the end of the Precambrian, whilst the latter suffered recurrent tecto-thermal activities up to the Cenozoic . According to the scattered outcrops of the basement rocks, the study region can be divided into three areas (Fig . 1): Yaminue-Nahuel Niyeu area; Mina Gonzalito Sierra Pai.Leman area; and Sierra Grande-Punta Sierra area. At Yaminue-Nahuel Niyeu area the basement consists of two rock associations put together by thrusting1: - i) An infracrustal association (Yaminue Complex) of alternating schist-gneiss foliated granite. The intrusion of the granites occurred syn- kinematically respect to the main folding event. Parcially foliated and concordant porphyritic granodiorites were emplaced during the late stages of deformation . The structural pattern of the succession is characterized by NW verging overturned folds (with low-angle axial planes) and mylonitic belts2 . One Rb-Sr errochron made with different rock types yielded an age of 620 ± 45 Ma (IR: 0.7038 ± 0.0520), while an isochron of 680 ± 27 Ma (IR: 0.7018 ± 0.0010) was obtained for the late-kinematic granodiorite3 . Also for the same granodiorite a Sm-Nd TDM model age of 1.310 ± 0.043 Ga was obtained. Presently, the U-Pb zircon dating is being performed. - ii) A supracrustal association of low to very low grade metamorphism, the Nahuel Niyeu Formation, mainly composed of metagreywackes, slaty shales and phyllites, with minor metadiabase . The structure is the result of three phases of deformation, and the main phase determined the SW verging asymmetric and tight folding4. A reference isochron made upon phyllites5 yielded an age of 600 ± 25 Ma (IR: 0.70347) . At Mina Gonza1ito-Sierra de Paileman area the two successions described, Mina Gonzalito Gneis~ and EI Jagiielito Ectinite6 are respectively correlated with Yami.nue Complex and Nanuel Niyeu Formation. Again here the contact between them is of t ectonic char~cter (transcurrent faulting) . Quartz-biotite schists, biotite gneiss, muscovite and garnet bearing S-type foliated granites concordant to the metamorphic structures, amphibolites and marbles were recognized forMina Gonzalito Complex7. The Rb-Sr isochron obtained for this unit was of 850 ± 50 Ma (IR: 0.70734) 5, and its protolith was interpreted8 as a passive margin association, composed of dolomites and limestones, greywackes and shales, and also basic to acidic lavas, which underwent successive deformation and metamorphism. At present , additional Rb-Sr and u-pb datings are in process on gneisses and granites from this Complex. The EI JaqUelito Formation is mainly composed of quartz phyllite grading into metagreywacke, and quartz-biotite-clorite bearing phyllite. Neopaleozoic granites (Paileman Granite) are also important in this area , while Middle jurassic felsic volcanics (Marifil Formation) make up large plateaux.

326 South-American Symposium on Isotope Geology - Brazil, June 1997 The basement rocks of Sierra Grande-Punta Sierra area comprises on ly the l ow grade me t amorph i c s comparable to El JagLielito Formation . They are i ntruded by granitic to granodioritic plutons (Punt a Sierra Granite, Arroyo Salado Granodiorite), and both together are unconformably overlain by Silurian sedimentary rocks containing invertebrate fauna (Sierra Grande Formation) . This Silurian unit is folded and faulted, and the e xistence of some thermal contact effects led to the recognition. of post -Silurian intrusions. Fo rme r datings for these granitoids were a K-Ar age of 435 ± 20 Ma9 on Punta Sierra Granite, a nd Rb-Sr ages 1 0 of 453 ± 23, 50 5 ± 15 and 613 ± 68 Ma. In this contribution we present f ive new Rb-Sr i s oc h r on s for plutons of this area, with the following results (Fig .2): For outcrops of seashore area and its surroundings an isochron of 483 ± 22 Ma (I R: 0 . 7 0 43 ± 0.0013) was obtained for leucogranites of Punta Sierra, another one of 467 ± 1 6 Ma (IR:0. 7081 ± 0 . 0004 ) for granodiorites and gr ani t e s of Arroyo Salado Gr anod i or i t e, and an errorchron of 409 Ma (IR : 0 .7077 ± 0 .00 6 0) for leucogranites of Las Grutas . From the outcrops clos e to Hiparsa Mine (Fe), to the south of Sierra Grande, the ages obtained are 363 ± 57 Ma (IR:0 .7110 ± 0. 0 02 2) for a pink granite (Mina H'i.par s a Granite) , and 318 ± 28 Ma (IR:0.7070 ± 0 .0004) for Laguna Medina Granodiorite. The field relationships together with the obtained data are consistent with igneous a ctivities during Lower , Middle and Upper Paleozoic times. According to t he evidences exposed above , we can preliminarily i nterpret for the crustal evolution of the Atlantic area of t he North Patagonian Massif an o ldest tecto-thermal activity during the Proterozoic Brazilian Cycle, and later a c t i v i t i e s during Lower to Middle Paleozoic Famatinian Cycle, and also during Neopaleozoic to Eomesozoic Gondwanic Cycle. The main tectonic events for this crust should have occurred during t he Br a zilian Cycle, as evidenced by the infracrustal successions with overturned foldings and low angle mylonite and s hear zones t ypi c a l of napp e and thrust s t ruc tur e s. On the other hand, the c hemical studies of the supracrust al quartz greywackes11 point to felsic and intermediate volcanics of orogenic origin (Continental Island Arc of Active Continental Margin) as source of the detritus. Ba s ed on the above mentioned e l ements, we consider t his supracrustal unit as syn-orogenic flys ch-like deposits. The exhumat i on of this basement terrane i s considered to happen b e f o re t he d e posit i on of the Silurian sediments. In the regional scale, other remnants of Neoproterozoic basements are found to the north, at Sierras Australes of Buenos Aires (Ventania), while at Sierras Septentrionales (Tandilia) during the Brazilian Cycle sedimentary rocks in foreland situation cover unconformably the Transamazonian basement . The most complete Prot erozoi c evolution o f t h e region is found a t t he Uruguay - Rio Grande do SuI Sh i eld, whe re Don Feliciano (Cu c h i lla Dionisio ) Belt s urrounds t he Rio de l a P lata Crat on to t h e east12. Paleozoic plutonic activity, similar to those identified at North Pa t a gon i an Massif , can be also mentioned to the north of this area at the Chadileuvu Block, aeparat.ed by the Cretaceous Colorado Basin. There, low grade metamorphics of undetermined age are intruded by undeformed granitoids, one of which yielded a Rb -Sr i s ochron of Lower Dev on i an age13 , and older granitoids without p r e c i s e a ges are also pre s ent. We consider that the Neoproterozoic continental paleogeography t hat chara cterizes the Brazilian Cycle of Brasil and Uruguay can be traced to the s outh through t h e Atlantic region up to at least approximately 42°SL . At northern Patagonia the Brazilian tectomagmatic cycle was followed by other sedimentary, magmatic and structural pr ocesse s during the whole Paleozoic times . These Famatinian and Gondwanic acti vities are t oday recognized withou t much important superimposed me t amorphism, except those a ccompanying some my l on itic belts, b o th at North Patagonian Massif and Chadileuvu Block.

South-American Symposium on Isotope Geology - Brazil, June 1997 327 References

1. Carni nos , R . & Llarobi a s, E. IX Cong . Geol. Arg . Relatorio, 37-63 (1 9 84). 2 . Chernicoff, C. J. & Carninos, .R . Asoc . Geol. Arg., Revista 51, 107-108 (1 99 6) . 3. Carninos , R . , Chernicoff , C .J. & Varela, R . VIICong. Geeol . ChilenoII, 13 01-1305 (1994 ). 4 . Chernicoff,C.J . & Carninos, R . Asoc. Geol . Arg., Revista 51,201-212 (1 9 9 6). 5. Linares, E ., Ostera , H. & Parica, C . XI Cong o Geol . Arg. II, 251-253 (1 9 90). 6 . Rarnos, V .A. Asoc. Geol . Arg., Revista 3 D, 274-285 (1 97 5) . 7 . Giacosa, R .E . Zbl. Geol . Palaont . I (l/2 ), 89-100 (1 9 93) . 8 . Dalla SaIda, L. & Arag;on, E. VII Congo Geol. Chileno, II, 786-790 (1 9 94). 9 . Weber , E.I. Servo Geol. Nac . Bol. 196, 1 - 69 (1983) . 10. Hernandez, L. B ., Shimazu, M. & Kagarni, H. An . Meet. Min. Pet. Econ. Geol . Japan, Abs.(1989 ). 11. Cagnoni, M.C. ,Li n a r e s, E ., Ostera, H.A., Parica C.A. & Remesal, M. B. XII Congo Geol. Arg. I, 281-288 (1 993) . 12 . Fragoso Cesar, A.R.S., Machado, R ., Monteiro, R.L . & Sallet , R. IV Simp . Sul Brasileiro de Geologia, (1 98 9 ) . 13. Llambias, E .J., R. Melchor , H. Tickyj & Sato, A. XI I I Congo Geol . Arg. V, 417- 425 (1996) .

328 South-American symposium on Isotope Geo logy - Brazil, J une 1997 ...

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...... II ' 1.- Yami nue-Nahucl Niyeu area ,, - , r_~~< ~ 2.- Mina Gonzalito-Sierra : PailemAn area 3.- Sierra Grande-Punta ; '~ -- -,: 1'-_-- Sierra area ~ \ , " .. . , Nordr I ~ 4.- Chadileuvu Block ,'P-urrOftWl 1 ; 5.- Ventania " MIlDif' ~ \, Brazilian 6.- Tandilia

I 2 ~ Cycle 7.- Uruguay-Rio Grande do W7/7.I Transamazonian Sill Shield ~ Cycle Fig.1.- Regional s ketch of Neoproterozoic-Paleozoic outcrops of the Atlantic area. a) Neoproterozoic Inf racrustal association; b ) Neoproterozoic supracrustal association; c) Eopaleozoic granitoids; d ) Silurian sedimentary cover i a ) Middle t o Late Paleozoic granitoids ; f) Jurassic volcani c s . 0.76 0.76 t) ... '0 ""C/;) ee "C 0.75 ~ 0.75 ~ .....C/;) ....C/;) "" ee 0.74 0.74

0.73 PUNTA SIERRA 0.73 ARROYO SALADO Age: 483 ± 22 Ma Age: 467 ± 16 Ma 0.72 IR; 0.7043± 0.0013 0.72 IR: 0.7081 ± 0.0004 MSWD:3.29 MSWD:3.90 0.71 0.71 87 / 86 87Rb/ 86Sr Rb Sr 0.70 0.70 0 2 4 6 8 10 0 2 4 6 8 10 0.85 0.730 ts t5 '0 '0 "" "" 0.725 'i::: • MINA HlPARSA l;;: .....e.-J Age: 363 57 Ma ....e.-J ± ee "" IR: 0.7110 ± 0.0022 0.80 0.720 MSWD: 1.14

0.715 LAS GRUTAS + LAGUNA MEDINA 0.75 Age: 409 ± 30 Ma 0.710 Age: 318 ± 28 Ma IR: 0.7077 ± 0.0043 IR: 0.7070 ± 0.0004 MSWD : 12.16 MSWD: 5.61 0.705 87 ;86 Rb Sr 87Rbl86Sr 0.70 0.700 0 5 10 15 20 25 a 2 3 4 Fig. 2.- Rb.,.Sr di a g rams (Willi amson) f o r gran itoids of sier ra Grande -Punta Sierra area, North Patagonian Mass i f.

South -American Symp osium on Isotope Ge o logy - Br a z il , June 1 9 97 329