Bulk-rock geochemistry of the meta-igneous rocks from the Tandilia Range, Argentina

MARTÍNEZ JUAN C.; DRISTAS A. JORGE; MASSONNE, HANS -J; THEYE THOMAS

Göttingen

Congreso; 21th Colloquium on Latin American Geosciences.; 2009

Georg-August-Universität Göttingen

The Paleoproterozoic basement of the Tandilia Range, central-eastern Argentina, is the southernmost portion of the Río de la Plata Craton, which is mainly exposed in Uruguay. This basement is composed of igneous rocks and metamorphic rocks of the greenschist, amphibolite and granulite facies (Teruggi and Kilmurray, 1975). The metamorphic rocks are derived from sedimentary and igneous protoliths. The igneous rocks of the basement of the Tandilia Range are mainly acid plutonic or hypabysal rocks. However, basic undeformed dykes also occur. Polymetamorphic events and heterogeneous deformation are recorded in almost all basement rocks of the Tandilia Range. This basement, also known as the Buenos Aires Complex (Marchese and Di Paola, 1975), is overlaid by a marine Neoproterozoic – Early Paleozoic sedimentary succession. Our study presents new whole-rock chemistry (XRF) data and discrimination diagrams of major, minor and trace elements from petrographically studied igneous and metamorphic samples of the Tandilia Range in order to contribute to a better reconstruction of the geodynamic environment especially of this part of the Rio de la Plata Craton. Our samples are from four areas: (1) north of Barker and Sierra Alta de Vela, (2) Balcarce, (3) Azul and (4) Olavarría. The studied metamorphic rocks comprise migmatites, gneisses, calc-silicatic marbles, and amphibolites. Migmatites generally present a granoblastic texture and a dioritic to granitic composition according to the TAS chemical classification diagram (Cox et al., 1979). Invasive leucocratic material in migmatites indicates an open system precluding a chemical definition of its precise protolith. Such rocks together with gneisses, which are not abundant without migmatitic texture, and calc-silicate rocks (skarn mineralogy) characterize the stratified basement derived from sedimentary protoliths. Samples of quartz-dioritic to granitic composition (TAS classification), collected southeast of the town of Tandil, show granoblastic to lepidoblastic texture. The mineral assemblage in these rocks is quartz, plagioclase, K-feldspar, biotite occasionally with hornblende and garnet. An N-MORB normalized spider diagram displays enrichment in large ion lithophile elements (Cs, Ba, Rb and K) and a slightly depleted concentration in HREE. Rare earth element (REE) patterns normalized to chondrite (Taylor and McLennan, 1985) show a negative Eu anomaly with LREE enrichment. Data of the whole-rock chemistry of granoblastic to nematoblastic amphibolites plotted in the bivariant diagrams Zr/Ti - Ni and TiO2 - Ni (Winchester et al., 1980) suggest basic igneous protoliths for all such samples because of Zr/Ti < 1. These rocks consist of the main mineral assemblage hornblende and plagioclase occasionally with some quartz. The amphibolites form boudinaged bodies commonly less than 2 m wide and 20 m long. They are interlayered in granodioritic-tonalitic migmatites as it is, for instance, discernible in the northern Barker area. Classification of the amphibolites using the Zr/Ti2O*0.0001 - Nb/Y diagram (Winchester and Floyd, 1977) leads to an alkaline to subalkaline nature for these rocks. On the other hand, the ternary discrimination diagram 2Nb - Zr/4 - Y (Meschede, 1986) points to E-MORB type and within-plate tholeiite affinities. The data of all samples plot along a tholeiitic trend in the AFM diagram. REE diagrams normalized to chondrite (Taylor and McLennan, 1985) display an enrichment in LREE. The studied granitoids, which are emplaced into the stratified crystalline basement, range in composition from granites to diorites, mostly with calcic to calc-alkaline character, according to the classification diagrams based upon SiO2 – Na2O+K2O (Cox et al., 1979). In addition, most analysed granitoids are peraluminous with a slight tendency to a metaluminous character. The discrimination diagrams Rb - Y+Nb and Rb - Yb+Ta (Pearce et al., 1984) point to granites typical of an active continental margin (VAG) and within-plate (WPG) environment. Besides, syn-collisional and late to post-collisional granitoids were established by the ternary diagram Rb/30 - Hf - Ta*3 (Harris et al., 1986), which seems to be more appropriate for the derivation of a geodynamic environment using geochemical data. REE patterns normalized to chondrite (Taylor and Mc Lennan, 1985) show a gentle slope with LREE enrichment and (La/Yb)N » 3. Negative Eu (Eu/Eu*= 0.18 – 0.85) anomalies in most granitoids indicate feldspar fractionation in the corresponding magma sources. In addition, N-MORB normalized spider diagrams point to enrichment of large ion lithophile elements (Cs, Ba, Rb and K). It is noteworthy to mention the similar pattern of (preserved) granitoids and corresponding metamorphic rocks, migmatites and gneisses. This studied crystalline basement is reminiscent of a complex igneous and metamorphic evolution of the southernmost portion of the Río de la Plata Craton involving events of subduction and continent-continent collision.