BASEMENT GEOLOGY AND GEOCHEMISTRY OF THE EL CRISTO AND SAN VERÁN HILLS, TANDILIA BELT, RIO DE LA PLATA CRATON

GRAFF, AILÍN AYELÉN; MARTINEZ JUAN CRUZ; DRISTAS, JORGE A.; MASSONNE, HANS -JOACHIM

Heidelberg

Congreso; The 23rd Latin American Colloquium on Earth Sciences; 2014

Universität Heidelberg

The southernmost exposures of the Rio de la Plata craton occur over more than 350 km in the NW-SE trending Tandilia belt of central-eastern Argentina. The basement of this belt is Paleoproterozic in age and composed of igneous and mainly high-grade metamorphic rocks with different degrees of deformation. The area of the El Cristo and San Verán hills, ca. 35 km west of Balcarce, well exposes 24 km2 of such rocks being representatives of the three major lithologies of the Tandilia belt. We present new field-survey results as well as petrographic and geochemical data of amphibole-biotite gneisses, garnet-biotite-bearing migmatites and metamorphosed granites and associated pegmatites. The gneisses exclusively crop out in the northern portion of the study area. Towards the south they change to migmatites. Pegmatite dykes are widespread in the entire area, whereas the main pegmatitic granite forms the structural antiform of the San Verán hill. The NE-SW trending main foliation (S1 + Smylonite), being subvertical (75-90º) was re-oriented to ENE-WSW by the granite. This granite shows K-feldspar porphyroclasts with core and mantle structure, a fine-grained recrystallized matrix (less than 40 vol.%), and quartz crystals with chessboard texture. A recrystallized matrix usually amounts to less than 20 vol.% in the migmatites and less than 10 vol.% in the gneisses. The gneisses are metaluminous to slightly peraluminous with variable contents (wt.%) of SiO2 (57-67), Fe2O3 (7.4-10.3), CaO (2.7-7.6), TiO2 (0.7-1.0), Na2O (1.7-2.9) and K2O (1.3-4.0), whereas mesosomes of migmatites are slightly peraluminous and richer in SiO2 (71-77), K2O (0.7-6) and Na2O (2.0-3.6), but poorer in Fe2O3 (1.5-6.0), CaO (0.9-2.9), TiO2 (0.1-0.4), MgO (0.1-0.60) and P2O5 (0.03-0.21). The gneisses are tonalitic to granodioritic. Mesosomes in migmatites are tonalitic, granodioritic, quartz-monzonitic and granitic. The granite also shows granodioritic variations and shares geochemical similarities with the mesosomes of the migmatites. The field survey reveals the occurrence of garnet and sillimanite in some varieties of the granite, indicative of an S-type granitoid. The REE distribution patterns of gneisses and migmatites prove analogous enrichment in LREE and negative Eu anomalies (Eu/Eu*= 0.4-0.64 and 0.24-0.59, respectively). In addition, the HREE patterns of the migmatites are more variable with depressed values of Yb and Lu. The granites show clearly negative Eu anomalies (Eu/Eu*= 0.22-0.38) and either positive or negative HREE slopes caused by fractionation of plagioclase and garnet, respectively. With the new geochemical characteristics and field relationships in mind, it is suggested that a progressive partial melting of the gneisses has led to the formation of small slightly peraluminous granitic bodies of pre- to syntectonic nature. The observed microstructures in all rocks indicate ductile deformation, overprinting the migmatization stage, having turned to brittle-deformation at the final stage when the rocks were exhumed.