PILLOW METABASALTS IN A MIDTERTIARY EXTENSIONAL BASIN ADJACENT TO THELIQUINE-OFQUI FAULT ZONE - THE ISLA-MAGDALENA AREA, AYSEN, CHILE

Pillow metabasalts and interbedded slates adjacent to the Liquide-Ofqu i fault zone (LOFZ) of the southern Chilean Andes have been studied us ing electron-microprobe mineral analysis, major and trace element whol e-rock geochemistry and Rb-Sr geochronology. The rocks show previously unrecognized mineralogical evidence of two metamorphic events in a lo w-pressure environment: an early greenschist-facies metamorphism, poss ibly during emplacement on the sea-door and a patchy amphibolite-facie s overprint, which could represent either continuing sea-floor metamor phism or contact metamorphism associated with Miocene plutons. The met a-igneous rocks are considered to have formed from mid-Tertiary basalt ic magmas with mixed within-plate/volcanic are characteristics as seen in present-day volcanoes in the area, but enriched in immobile elemen ts such as Ti, Zr, And Y. They were probably emplaced between 45 and 2 0 Ma, into a contemporaneous marine pull-apart or extensional basin wi th thin continental or oceanic door Many of the petrological and geoch emical features of an ophiolite suite may be recognized in the immedia te area. A chain of such basins developed immediately west of the LOFZ strike slip system, probably during an Eocene- Early Miocene period o f oblique subduction-convergence in southern Chile, and closed when th e approach angle became more orthogonal 25 Ma. Magmatic are plutonism was resumed at about 20 Ma and continued into the latest Cenozoic.