The geochemical variations of the upper cenozoic volcanism along the Calama-Olacapato-El Toro transversal fault system in central Andes (similar to 24 degrees S): petrogenetic and geodynamic implications

摘要

In this paper, we present new geochemical and Sr-Nd isotopic data for several Upper Miocene volcanic centres aligned along one of the most extensive transcurrent lineament in the Central Andes, the Calama-Olacapato-El Toro (COT). The transversal volcanic belt along COT is constituted by large composite volcanoes and a caldera structure; they are, from NW to SE, Puntas Negras, Rincon, Tul Tul, Del Medio and Pocitos (TUMEPO), Quevar Aguas Calientes and Tastil. In order to compare chemical data from the different centres along the COT transect, differentiation effects were minimised by using data extrapolated at 60% SiO2 with least-square regression method. In the western sector of the COT, the volcanic products of Puntas Negras and Rincon show relatively high K2O and Sr-87/Sr-86 and low Rb/Cs, Ta/Th, La/Yb, Nd-143/Nd-144. To the east, the TUMEPO products have high Sr and Nd-143/Nd-144, La/Yb and Ba/Rb and low Y, Sr-87/Sr-86. In the easternmost COT sector, Quevar, Aguas Calientes and Tastil volcanic complexes exhibit low La/Yb, high Sr-87/Sr-86 and low Nd-143/Nd-144. On the basis of these data, we propose a petrogenetic and geodynamical model for Central Andes at 24circleS, In correspondence of Miocene-Quaternary volcanic arc (Puntas Negras and Rincon), the magmas inherited a calcalkaline signature partly modified by upper crustal and/or sediment assimilation. In the central eastern sector, melting, assimilation, storage and homogenisation (MASH) processes occurred at the base of a thickened crust. In this COT sector, TUMEPO products show an evident lower crust signature and could be considered representative for MASH derived magmas. In the easternmost sector, Quevar, Aguas Calientes and Tastil products could represent magmas generated by partial melting of underthrusted Brasilian shield and mixed with magmas derived by MASH processes. (C) 2002 Elsevier Science B.V All rights reserved. [References: 37]