Pillow metabasalts in a mid-Tertiary extensional basin adjacent to the Liquiñe-Ofqui fault zone: the Isla Magdalena area, Aysén, Chile

Abstract

Pillow metabasalts and interbedded slates adjacent to the Liquiñe-Ofqui fault zone (LOFZ) of the southern Chilean Andes have been studied using electron-microprobe mineral analysis, major and trace element whole-rock geochemistry and RbSr geochronology. The rocks show previously unrecognized mineralogical evidence of two metamorphic events in a low-pressure environment: an early greenschist-facies metamorphism, possibly during emplacement on the sea-floor, and a patchy amphibolite-facies overprint, which could represent either continuing sea-floor metamorphism or contact metamorphism associated with Miocene plutons. The meta-igneous rocks are considered to have formed from mid-Tertiary basaltic magmas with mixed within-plate/volcanic arc characteristics as seen in present-day volcanoes in the area, but enriched in immobile elements such as Ti, Zr, And Y. They were probably emplaced between 45 and 20 Ma, into a contemporaneous marine pull-apart or extensional basin with thin continental or oceanic floor. Many of the petrological and geochemical features of an ophiolite suite may be recognized in the immediate area.A chain of such basins developed immediately west of the LOFZ strike slip system, probably during an Eocene- Early Miocene period of oblique subduction-convergence in southern Chile, and closed when the approach angle became more orthogonal 25 Ma. Magmatic arc plutonism was resumed at about 20 Ma and continued into the latest Cenozoic.