U-Pb AGES OF KOMATIITIC ROCKS FROM SIERRA DE SAN LUIS, ARGENTINA

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1 240 U-Pb AGES OF KOMATIITIC ROCKS FROM SIERRA DE SAN LUIS, ARGENTINA Sato, A.M. 1, Gonzlález, P.D. 1, Basei, M.A.S. 2, Llambías, E.J. 1 1 Centro de Investigaciones Geológicas, UNLP. Calle 1 N 644, B1900TAC, La Plata, Argentina. sato@cig.museo.unlp.edu.ar gonzapab@cig.museo.unlp.edu.ar llambias@cig.museo.unlp.edu.ar 2 Instituto de Geociências, Universidade de São Paulo. Rua do Lago 562, CEP , São Paulo, Brazil. baseimas@usp.br Keywords: komatiites, Famatinian orogeny, Sierras Pampeanas. INTRODUCTION One of the distinguishing features of the western region of Sierra de San Luis in the Sierras Pampeanas of Argentina is the occurrence of a 45 km long belt of mafic-ultramafic extrusive rocks that appear as scattered outcrops sharing the same structural and metamorphic evolution as the hosting Nogolí Metamorphic Complex. Being still unclear the precise timing of the protolith evolution and the first orogenesis affecting them somewhere in the Proterozoic to the earliest Paleozoic, the final orogenesis took place through the Early Paleozoic Famatinian orogeny, with emplacement of magmatic arc in a region subjected to low to high-grade metamorphism in a collisional regime. In this contribution we present results of U-Pb isotopic study on the mafic-ultramafic rocks and discuss their possible interpretations. GEOLOGICAL BUCKGROUND The Nogolí Metamorpic Complex (Fig. 1) is composed of metamorphic rocks originated in volcanosedimentary protoliths, intruded by granitoids and pegmatitic veins. Micaschists, metaquartzites, paragneisses and migmatites are dominant rocks, while the mafic-ultramafic rocks are intercalated as part of their protolith. Main source for the following geological information are: González et al., 2002 and 2004; Sato et al., 2001 and Despite the two distinct structural and metamorphic events affecting the complex, which transformed the mafic-ultramafic rocks into strongly folded and boudinaged bodies and lenses of amphibolites, their komatiitic protoliths can be recognized in few areas because of the preservation of primary spinifex and ophitic to subophitic textures, their mineralogy and geochemistry. On the basis of their geochemistry, the rocks are classified into komatiites, komatiitic basalts and high-fe tholeiitic basalts. The first high-grade metamorphism, which in the metasedimentary rocks reached peak conditions of amphibolite facies at near 700 C, is only preserved as a relict paragenesis amp-cpx-pl?-qtz? within the maficultramafic rocks, together with NW-SE relict structures. The second high-grade Famatinian metamorphism is pervasively recorded in every rock type and the following assemblages are identified in the mafic-ultramafic rocks: (1) amp-pl-qtz-bt-ep-ttn-rt, (2) amp-pl-qtz-grt-bt-ep, (3) amp-ol-grt-cpx-qtz, (4) amp-plg-grt-cpx, and (5) amp-plcpx-qtz-opx. Thermobarometric data constrain their peak conditions of high-amphibolite to granulite facies at C and ~8 kb. Up to now, an age that best constrains the relict pre- Famatinian metamorphic and tectonic event is the conventional U-Pb age of 507 ± Ma (von Gosen et al., 2002) obtained from La Escalerilla Granite, which is one of the pre-orogenic granitoids intruding the Nogolí Metamorphic Complex already affected by the first orogenic event and both together are then deformed by the later Famatinian orogeny. Therefore, the pre- Famatinian tectonic event should have occurred prior to 507 Ma. The high-grade metamorphism of the Famatinian orogeny in the western Sierra de San Luis is constrained between 475 and 445 Ma on the basis of different isotopic data, while ages for the arc magmatism are between 507 and 445 Ma. From the mafic-ultramafic rocks, a Sm-Nd isochrone age of 1502 ± 95 Ma was regarded as a possible age of crystallization of the mafic-ultramafic volcanic protoliths. ISOTOPIC RESULTS Zircon crystals (Fig. 2) were separated from samples of metamorphosed komatiitc basalt (RQ-25-1) and spinifex komatiite (RQ-25-2), whose SiO 2 contents are 45.2% and 43.4% respectively. Zircon crystals are mainly subrounded, ovoid to multifaceted grains, especially in the sample RQ These morphologies are generally found in zircons of high-grade metamorphic rocks (Corfu et al., 2003). Only few crystals from sample RQ-15-2 have prismatic elongation of up to 2.5 to 1 axial ratio and pyramidal terminations, while more irregular morphology with resorbed contours are also observed in both samples.

2 Figure 1. Geology of the mafic-ultramafic rocks, western Sierra de San Luis.

3 Conventional U-Pb analyses of zircon fractions from both samples show low-u contents generally less than 200 ppm for subrounded ovoid grains, while in those fractions from sample RQ-25-2 containing prismatic crystals U contents are higher, between 320 and 540 ppm, with one fraction reaching the highest value of 2380 ppm. 2.5), while the highest U fraction shows a position that suggests recent Pb loss, probably due to the damage in the internal structure of crystals. Up to this point, the results suggest a better constrained age of 461 ± 5 Ma which can be related to the Famatinian metamorphism that might have originated the low-u metamorphic zircons as recrystallized, newly formed or overgrown crystals, and a certain suggestion of inherited age towards 1200 Ma, which might be related to magmatic crystallization, in agreement with previous Sm-Nd Mesoproterozoic age. Encouraged by the presence of possible inherited ages, we proceeded to apply SHRIMP U-Pb method on sample RQ From the five grains analysed by CL imaging, three show a single domain of diffuse dark CL with hardly seen internal structure, surrounded by a very narrow rim of bright CL. One multifaceted to stubby crystal shows a polygonal, oscillatory to fir-tree sector zoning with a less clear, bright and small inner domain (inherited core?). The remaining crystal, with a general bright CL, shows a wider spaced zoning domain that seems to truncate an inner zoned domain which could be interpreted as inherited core. Polygonal or oscillatory zoning is a typical structure found in zircons crystallized from melts, aqueous metamorphic fluids or in a fluid-assisted environment; on the other hand, the lack of internal structures associated with multifaceted to resorbed morphology is a feature of metamorphically grown or recrystallized zircons (Hoskin and Black, 2000; Rubato and Hermann, 2003; Hoskin and Schaltegger, 2003). Ten points were analyzed on these five crystals, and their 206 Pb/ 238 U ages vary between 517 and 428 Ma. Rejecting these two extremes, the remaining eight points ranging between 498 and 473 Ma provide a mean 206 Pb/ 238 U age of 483 ± 11 Ma with MSWD The three points with the highest U contents (3275 to 852 ppm) coincide with the 3 oldest ages (516 to 492 Ma) and highest Th/U ratios (2.44 to 1.45). Here, Pb loss controlled by metamictization can be interpreted. Remaining Th/U ratios vary between 0.3 and 0.8. Although on the basis of numerous previous studies, Hoskin and Schaltegger (2003) make a distinction between Th/U ratios from metamorphic (< 0.1) and magmatic zircons (>= 0.5), there are cases like that reported by Vavra et al. (1999), where they find that metagabbros show higher ratios when passing from amphibolite to granulite facies, starting from the magmatic level of 0.3 and reaching values higher than 1. Therefore, from the analysis of our SHRIMP results, we cannot conclusively state that the mean value of 483 ± 11 Ma can represent any age of (1) magmatic crystallization, (2) metamorphic crystallizationrecrystallization, or (3) magmatic crystallization disturbed by metamorphism and therefore, without Four fractions from sample RQ-25-1 yield a mean 206 Pb/ 238 U age of 461 ± 5 Ma, with MSWD 3.7. For sample RQ-25-2 the result is less clear. Two high-u fractions are arranged in an older age zone tending to an upper intercept of 1204 Ma. Three low-u fractions are aligned with a lower intercept of 444 ± 14 Ma (MSWD geological meaning, because the evidences are not clear when combining the morphological and crystal structural data with the analytical data. Besides, a magmatic age of 483 Ma would not agree with the previous geological information that requires a protolith age older than 507 Ma, and the age rather agrees better with an early stage of the Famatinian metamorphism, as constrained by conventional analyses from the sample RQ-25-1 (461 ± 5 Ma). However, and according to the evidence of Pb-loss in at least some of the analysed points, we can say that the oldest age of 516 Ma is a minimum age that constrains the primary crystallization. Therefore, and considering together with the conventional data, we suggest that it is still valid the metamorphism age at 461 ± 5 Ma, and a primary crystallization age older than 516 Ma, which can be as old as Mesoproterozoic time, as shown by inherited ages in the conventional data from sample RQ-25-2 and previous Sm-Nd isochrone data. REFERENCES Corfu, F., Hanchar, J.M., Hoskin. P.W.O. and Kinny, P., Atlas of zircon textures. In Hanchar, J.M. and Hoskin, P.W.O. (eds.) Zircon. Reviews in Mineralogy and Geochemistry 53, p Mineralogical Society of America. González, P.D., Sato, A.M. and Llambías, E.J., The komatiites and associated mafic to ultramafic metavolcanic rocks of western Sierra de San Luis. 15 Congreso Geológico Argentino, Actas 1:87-90, Calafate. González, P.D., Sato, A.M., Llambías, E.J., Basei, M.A.S. and Vlach, S.R.V., Early Paleozoic structural and metamorphic evolution of western Sierra de San Luis (Argentina), in relation to Cuyania accretion. Gondwana Research 7: Hoskin, P.W.O. and L.P. Black, Metamorphic zircon formation by solid-state recrystallization of protolith igneous zircon. Journal of Metamorphic Geology 18: Hoskin, P.W.O. and Schaltegger, U., The composition of zircon and igneous and metamorphic petrogenesis. In Hanchar, J.M. and Hoskin, P.W.O. (eds.) Zircon. Reviews in Mineralogy and Geochemistry 53, p Mineralogical Society of America. Rubatto, D. and Hermann, J., Zircon formation during fluid circulation in eclogites (Monviso, Western Alps): implications for Zr and Hf budget in subduction zones. Geochimica et Cosmochimica Acta 67: Sato, A.M., González, P.D. and Sato, K., First indication of Mesoproterozoic age from the western basement of Sierra de San Luis, Argentina. 3 SSAGI, CD volume p , Santiago. Sato, A.M., González, P. and Basei, M.A.S., Los ortogneises granodioríticos del Complejo Metamórfico Nogolí, Sierra de San Luis. 16 Congreso Geológico Argentino, Actas I: 33-40, La Plata. Vavra, G., Schmid, R. and Gebauer, D., Internal morphology, habit and U-Th-Pb microanalysis of amphibolite-to-granulite facies zircons: geochronology of the Ivrea Zone (Southern Alps). Contributions to Mineralogy and Petrology 134:

4 Figure 2. U-Pb diagrams and binocular microscope and CL images of zircon crystals.

5 Von Gosen, W., Loske, W. and Prozzi, C., New isotopic dating of intrusive rocks in the Sierra de San Luis (Argentina): implications for the geodynamic history of the Eastern Sierras Pampeanas. Journal of South American Earth Sciences 15: RESUMEN Al oeste de la Sierra de San Luis (Sierras Pampeanas, Argentina) afloran rocas komatiítcas formando parte del protolito volcano-sedimentario del Complejo Metamórfico Nogolí, el cual sufrió dos eventos tectónicos mayores asociados a metamorfismo de alto grado. El segundo de ellos es el mejor conocido y se debe a la orogenia Famatiniana del Paleozoico Temprano, mientras los datos existentes indican al primer evento como anterior a 507 Ma. Si bien las rocas máficas-ultramáficas son actualmente anfibolitas, preservan localmente su textura spinifex original, la cual junto con su geoquímica permiten clasificarlas en komatiitas, basaltos komatiíticos y basaltos de alto-fe. Se estudiaron mediante datación U-Pb convencional una muestra de basalto komatiítico (RQ-25-1) y otra de komatiita (RQ-25-2), realizándose en esta última datación SHRIMP adicional. Los circones de la muestra RQ-25-1 son crystales subredondeados, ovoides a multifacetados y pueden presentar bordes resorbidos, los cuales se habrían formado por metamorfismo. Las fracciones de estos cristales se alinean con una intercepción inferior de 461 ± 5 Ma en el diagrama de concordia. La muestra RQ presenta además escasos circones prismáticos con altos contenidos de U, que muestran pérdida reciente de Pb. El resultado de su datación convencional es menos claro, con dos fracciones que tienden a una edad vieja de 1204 Ma, y tres fracciones que se alinean con una intercepción inferior de 444 ± 14 Ma. Los diez puntos de datación SHRIMP sobre 5 cristales uno de ellos con zonación oscilatoria a poligonal dan edades 106 Pb/ 238 U de entre 516 y 428 Ma, con un valor medio de 483 ± 11 Ma. La información que ofrecen la morfología y estructura interna de los circones, los contenidos de U y relaciones de Th/U no son concluyentes para su origen magmático o metmórfico; y teniendo en cuenta que al menos parte de los cristales sufrieron pérdidas de Pb, consideramos que la edad de 461 ± 5 Ma es la que mejor representa la edad del metamorfismo Famatiniano, y que la cristalización primaria estaría relacionada a una edad mayor que 516 Ma, interpretación que podría estar de acuerdo con una isocrona Sm-Nd previa de 1502 Ma