VARIATION IN LOWER CRETACEOUS SECONDARY MINERAL ASSEMBLAGES AND THERMAL-GRADIENTS ACROSS THE ANDES OF CENTRAL CHILE (30-35-DEGREES-S)

Stratigraphic sequences of Lower Cretaceous age occur in two belts alo ng central Chile. Between 30 degrees and 35 degrees S, the western bel t, situated in the Coast Range is composed of 7-14 km thick intra-are and forearc sequences dominated by shoshonitic to high-K calc-alkaline volcanic rocks, whereas the eastern belt in the High Cordillera consi sts of 2-5 km thick back-are sequences with a smaller propertion of vo lcanics which have a less alkalic affinity. The rocks in both belts ar e burial metamorphosed at greenschist, prehnite-pumpellyite and upper zeolite facies. However, their secondary mineral assemblages show diff erences which indicate that the thermal gradients were ca. 20 degrees C km(-1) in the Coast Range and 45-80 degrees C km(-1) in the High Cor dillera. This suggests that during the Lower Cretaceous, the crust was thinner in the east than in the west, i.e., contrary to the present s ituation.