Regional metamorphism and origin of metasedimentary rocks of the Sierras Pampeanas of Tucumán and Catamarca, Argentina, along the margin of Gondwana

LARROVERE, MARIANO; TOSELLI, ALEJANDRO; ROSSI DE TOSELLI, JUANA

Mendoza - Argentina

Conferencia; Gondwana 12: Geological and Biological Heritage of Gondwana; 2005

The rocks of the sierras de Los Llanos, Potrerillo and Guayambá form part of the metamorphic basement of the Northwestern Sierras Pampeanas, as northern extensions of the Sierra de Ancasti. This basement is composed mainly of migmatites but also has biotite gneisses, quartz-feldspar gneisses, schists and concordant syntectonic tonalites. The migmatites represent the highest metamorphic grade, both homogeneous and heterogeneous varieties being recognized. The metatexites are mostly stromatitic migmatites with the mineral assemblage Qtz-Pl-Bt-(+-Kfs)-(+-Sil)-(+-Grt)-(+-Crd), less abundant diatexites have Qtz-Pl-Bt-Kfs-(+-Sil)-(+-Grt). This mineralogy is consistent with a low or medium P/T regional metamorphism in amphibolite to granulite facies conditions contemporaneous with migmatization. These events produced the regional foliation (S2) with a NNE–SSW strike and ENE dip. A relict older foliation (S1) is preserved within resisters. Geochemical data for metamorphic rocks from the study area were used to characterise the sedimentary protolith from which the migmatites and gneisses were derived. Data for samples of biotitic gneiss, schist (resister) and three migmatite mesosomes were plotted in diagrams for the classification of terrigenous sandstones and shales (Herron, 1988). The samples fall in the field of greywackes, indicating the original rocks that were later transformed during metamorphism. Metasedimentary rocks of the Pampean basement from Tucumán and Catamarca were considered to be higher-grade metamorphic equivalents of the Puncoviscana Formation (Aceñolaza and Toselli, 1981). Similar geochemical signatures for sedimentary and metamorphic rocks from different ranges in Northwest Argentina confirm this lithological continuity (Willner et al., 1990). Comparison of our samples with the 73 psammites and metapsammites samples studied by Willner et al., reinterpreted using Herron diagrams, shows a close correspondence, suggesting a genetic link to these greywackes as the sedimentary protolith. Chondrite-normalized REE concentrations show similar patterns to those of other sedimentary rocks such as NASC and ES, with La abundances of about 100 decreasing progressively through the other LREE to values of 10–20 for the HREE. The gneisses, schists, and mesosomes display very similar values, only subtly higher than those of NASC and average upper crust, independently of their metamorphic grade. REE concentrations normalized to upper crust show similar behaviour for the LREE (La, Ce, Pr, and Nd) and some MREE (Sm and Eu), but the HREE values are more varied, with Yb and Lu showing the most difference. The relative enrichment in REE, which tend to be concentrated in the silt fraction, together with the high relative abundances of Cr, Y and Ni, is coherent with the immobility of these elements during the sedimentary cycle and probably also during metamorphism. Low relative abundances of trace elements such as Sr and Ba, reflects the removal of feldspars in the sedimentary cycle. Overall, the geochemical data are compatible with rather immature sedimentary rocks poor in feldspars and a with detrital matrix, such as lithic greywackes. Nevertheless, it should not be discounted that the geochemical characteristics are inherited from the pre-sedimentary protolith itself, or have been affected by the regional metamorphism. Projections in the La-Th-Sc and Th-Sc-Zr/10 tectonic discrimination diagrams for greywackes (Bhatia and Crook, 1986) suggest deposition in sedimentary basins related to continental island arcs. A greywacke-rich sedimentary protolith would be consistent with metamorphism of the psammitic facies of the Puncoviscana Formation, corresponding to turbiditic deposition in active margins with crustal thickening (due to continental collision), which occurred during the Pampean Cycle (Late Precambrian–Mid Cambrian) in the Gondwana margin.