Nd and Sr isotope geochemistry of Grenvillian massif-type anorthosite from the Western Sierras Pampeanas (Argentina): magma sources and geotectonic implications

CASQUET, C., GALINDO, C., RAPELA, C., .PANKHURST, R., BALDO, E., DAHLQUIST, J., FANNING, C.M., GONZÁLEZ-CASADO, J.M. Y SAAVEDRA, J.

Punta de Este, Uruguay

Congreso; V South American Symposium on Isotope Geology; 2006

Facultad de Agronom¨ªa y Facultad de Ciencias, Universidad de la Rep¨²blica

Nd AND Sr ISOTOPE GEOCHEMISTRY OF GRENVILLIAN MASSIF-TYPE ANORTHOSITE FROM THE WESTERN SIERRAS PAMPEANAS (ARGENTINA): MAGMA SOURCES AND GEOTECTONIC IMPLICATIONS C. Casquet1, C.Galindo1, C. Rapela2, R.Pankhurst3, E. Baldo4, J. Dahlquist5, C.M. Fanning6, J.M. Gonz¨¢lez Casado7 y J. Saavedra8 1)       Dpto. de Petrolog¨ªa y Geoqu¨ªmica. Facultad de Ciencias Geol¨®gicas. Universidad Complutense de Madrid. 28040-Madrid. casquet@geo.ucm y cgalindo@geo.ucm.es 2)       Centro de Investigaciones Geol¨®gicas, Universidad Nacional de La Plata, Argentina. 3)       Visiting Research Associate, British Geological Survey, Keyworth, Nottingham, UK. 4)       Departamento de Geolog¨ªa, Universidad Nacional de C¨®rdoba, Argentina 5)       CRILAR-CONICET, La Rioja, Argentina 6)       Research School of Earth Sciences, The Australian National University, Canberra. 7)       Dpto. Qu¨ªmica Agr¨ªcola, Geolog¨ªa y Geoqu¨ªmica, Universidad Aut¨®noma, Madrid 8)       Consejo Superior de Investigaciones Cient¨ªficas, Instituto de Agrobiolog¨ªa y Recursos Naturales, Salamanca Keywords: Sierras Pampeanas, Anorthosite, Grenvillian Orogeny, Rodinia, Nd model age, INTRODUCTION Massif-type anorthosites are plutonic igneous complexes formed mainly of anorthosite, with smaller proportions of mafic differentiates rich in Fe and TiO2 (jotunite). These complexes are usually associated with Fe-Ti oxide mineralization (nelsonite) of economic significance (Aswhal, 1993). Although anorthosites are quite common rock types, these anorthosite complexes are rare: they are almost completely restricted in time to the Mesoproterozoic (Aswhal, 1993), where they are especially associated with the Grenvillian orogenic belt (1.0- 1.4 Ga). Thus they provide evidence of a singular tectonothermal event in Earth history. Moreover, correlation between anorthositic complexes in different continents can be used to refine palaeogeographical models for the Mesoproterozoic and the possible existence of Rodinia, the supercontinent supposedly formed at the end of the Grenville orogeny (Hoffman, 1991). The Sierras Pampeanas of Argentina are exposures of pre-Andean basement which show complex history of tectonothermal events and sedimentation lasting from Mesoproterozoic to Carboniferous times (e.g. Rapela et al., 2001). In two of the westernmost sierras (Maz , and Espinal, Fig. 1), two recently discovered anorthositic complexes consist of elongated bodies of very coarse-grained anorthosite (porphyritic facies) and leucogabbro, cut by dykes of mafic jotunite, and are physically located between Mesoproterozoic metasedimentary and meta-igneous host rocks that underwent Grenville age metamorphism at ca. 1.2 Ga (see Casquet el al., 2005b). The age of crystallization determined by U-Pb SHRIMP analysis of zircon is 1070 ¡À41 Ma (Casquet et al., 2005a). Fig.1. Sketch map showing location of massif anorthosites in the Sierras de Maz and Espinal. Inset shows location of the studied region in NW Argentina. Legend: 1) Metasediments, ortho-amphibolites and local orthogneisses; 2) Anorthosite massifs; 3) Famatinian granitoids; 4) Palaeozoic to Mesozoic sedimentary cover; 5) Cainozoic sedimentary cover. A) Famatinian shear zones; B) Post-Famatinian thrusts. The anorthosite is composed principally of plagioclase, variably recrystallised, amphibole and garnet, with minor biotite, titanite, ilmenite, apatite and even quartz. These rocks underwent regional metamorphism of almandine amphibolite facies of Ordovician age (431 ¡À30 Ma; U-Pb SHRIMP on zircon overgrowths). The metamorphism has partially modified the mineralogy, although original igneous textures are conserved. Chemically, they are notable for high contents of Sr (742-870 ppm), REE patterns slightly enriched in the light elements ( La/Ybn = 8¨C27) and a positive Eu anomaly (Eu/Eu* = 3-14). These are the only anorthosite complexes known in Argentina and the first of confirmed Grenville age so far recognised in South America. They are comparable to others in the Appalachian margin of Laurentia, Oaxaqua (Mexico) and the Natal-Namaqua belt (South Africa), with which they define a common anorthositic province of 1.0 ¨C 1.1 Ga, centred around the cratons of Amazonia, Kalahari and Laurentia-Australia (Casquet et al., 2005a). Massif-type anorthosites of probable Grenvillian age   have been recently reported as a part of a AMGC suite from the Arequipa-Antofalla craton in western Andean Per¨² (Martignole et al., 2005). ISOTOPE GEOCHEMISTRY Seven samples of anorthosite and one of jotunite from the sierras of Maz and Espinal were analysed for Sm-Nd y Rb-Sr (Table 1). The concentrations of Rb, Sr, Sm y Nd were determined by isotope dilution mass-spectrometry using 84Sr y 87Rb (Oak Ridge) spikes and a mixed 149Sm-150Nd spike. 87Sr/86Sr ratios were corrected for possible interference from 87Rb and normalised to 88Sr/86Sr = 0.1194. 144iNd/143Nd were similarly corrected for interference from 142Ce y 144Sm and normalised to 146Nd/144Nd = 0.7219. The results for NBS-987 and Johnson and Matthey standards were: 87Sr/86Sr = 0.710249 ¡À0.00002 (2¦Ò), and 143Nd/144Nd = 0.511809 ¡À 0.00002 (2¦Ò), respectively. Analytical uncertainties (2 ¦Ò) are 1% for 87Rb/86Sr, 0.1% for147Sm/144Nd, 0.01% for 87Sr/86Sr, and 0.006% for 143Nd/144Nd. The isotopic compositions of Nd y el Sr have been calculated for the age of crystallization (1070 Ma), and are also expressed in eSr y eNd notation. Nd model ages (TDM) were calculated. DISCUSSION AND CONCLUSIONS The values of eNd1070 vary between +3.4 y -1.2, mainly positive or slightly negative, which suggest a source impoverished in lithophile elements for the primary anorthosite parent magma, The corresponding eNd1070 for depleted mantle is + 5.6 so that a source intermediate in compositor between this and the chondrite uniform reservoir (eNdCHUR= 0) is indicated. The negative values point to the possible presence of crustal components in the anorthosite, whereas the range implies a mixture between depleted mantle and continental crustal material enriched in lithophile elements. These conclusions are supported by the isotopic composition of Sr (87Sr/86Sr 1070 = 0.7041 ¨C 0.7059) which are more radiogenic than that of Bulk Earth at the same age (= 0.7032). Nevertheless, we do not observe a close correlation between the isotopic compositions of Sr and Nd such as might be expected (Fig. 2). This could reflect isotopic heterogeneity in the depleted source and/or the crustal component, or even perhaps, post-crystallisation modification (e.g. during metamorphism). It is probably significant that the rocks with the highest K2O contents (see Table 1), are those that show the highest values of eSr1070. Thus we suggest that metamorphism was the main cause of the observed dispersion of isotopic compositions. Fig. 2. eNd vs 87Sr/86Srinitial plot of anorthosites and one jotunite from sierras of Maz and Espinal The model ages (TDM) vary between 1.18 and 1.51 Ga, which corresponds in a board sense to the time of the Grenville orogeny. Given that the enclosing metasedimentary rocks have values between 2.1 and 2.3 Ga (Porcher et al., 2004; Casquet et al., 2005b), the model age of the anorthosites must reflect the depleted mantle source from which they were derived, modified to some extent by the continental component The rock with the most positive eNd1070 value (MAZ-7216), and consequently the least contaminated by crustal material, has a TDM of 1.18 Ga, close to the age of crystallization, which suggests that that depleted source was formed at essentially the same time as the anorthosite magma. These conclusions are compatible with a model of an anorogenic mantle plume following the Grenville orogeny, with which the initial break-up of Rodinia began (Casquet et al., 2005b). The 1106¨C1122 Ma large igneous province (LIP), recognised by Hanson et al. (2004) between the Kalahari and Laurentia cratons, could be genetically related to the anorthosite event (Casquet et al., 2005a).   Acknowledgments. This work benefited from a PR1/05-13291 Complutense University grant and is a contribution to the 436 IGCP (Pacific Gondwana Margin). REFERENCIAS Ashwal, L.D. (1993): Anorthosites, Springer, Berl¨ªn, 422 pp. Casquet, C., Rapela, C.W., Pankhurst, R.J., Galindo, C., Dahlquist , J., Baldo, E.G., Saavedra, J., Gonz¨¢lez Casado, J.M., Fanning, M (2005a): Grenvillian massif-type anorthosites in the sierras Pampeanas. Journal of the Geological Society, London, 162, 9-12. Casquet, C., Pankhurst, R.J., Rapela, C.W., Fanning, C.M Galindo, C., Baldo, E.G., Dahlquist , J., Gonz¨¢lez Casado, J.M. Saavedra, J., (2005b): The Maz suspect terrane (Western Sierras Pampeanas). A new Proterozoic domain in the basement of tha Argentine Precordillera. Gondwana 12,  Abstract: 92. Hanson, R.E., Crowley, J.L, Bowring, S.A., Ramezani, J., Gose, W.A., Dalziel, I.W.D., Pancake, J.A., Seidel, E.K., Blenkinsopp, T.G. & Mukwakwami, J. 2004. 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(Eds) Cuyania: An exotic block to Gondwana. Gondwana Research, Sp. Vol. 7(4), 1009-10026. Rapela, C.W., Casquet, C., Baldo, E.G., Dahlquist, J.; Pankhurst, R.J., Galindo, C., Saavedra, J. (2001): Las Orog¨¦nesis del Paleozoico inferior en el margen proto-andino de Am¨¦rica del Sur, Sierras Pampeanas, Argentina. Journal of Iberian Geology, 27, 23-41. Sample K2O % Rb ppm Sr ppm Rb/Sr 87Rb/86Sr 87Sr/86Sr 87Sr/86Sr1070 eSr ESP-7056 0.45 3.97 742 0.0053 0.0155 0.704824 0.704587 20 MAZ-7207 1.49 35.00 818 0.0428 0.1238 0.707086 0.705191 28 MAZ-7208 0.42 1.00 862 0.0012 0.0034 0.704894 0.704843 23