GEOCHEMICAL FEATURES OF LOWER CRETACEOUS BACK-ARC LAVAS IN THE ANDEANCORDILLERA, CENTRAL CHILE (31-34-DEGREES-S)

A back-are setting for the Lower Cretaceous volcanic rocks in the Ande an Cordillera of Central Chile (31-34 degrees S), previously proposed on paleogeographic grounds, is consistent with their geochemical featu res. The rocks are basic to intermediate lavas, predominantly basaltic andesites, of low initial Sr-87/Sr-86 ratio (0.7031-0.7039). They are characterized by relatively high contents of Ta, Nb, Zr and Hi compar ed with Y and Yb, consistent with an enriched mantle source, or a low degree of partial melting, or both. High contents of alkalis and Th re lative to HFS elements are interpreted as a crustal contribution. The Ba/La, Ba/Nb and V/Ti ratios are lower than in coeval island are lavas in the Coastal Cordillera. The Ce/Yb, Ce/Sm, Th/Yb and initial Sr-87/ Sr-86 ratios show a northward decrease within the studied sector consi stent with a thinner crust in the north during the Early Cretaceous, c ontrary to the present situation.