MAGMATIC ARC METAMORPHISM - PETROLOGY AND TEMPERATURE HISTORY OF METABASIC ROCKS IN THE COASTAL CORDILLERA OF NORTHERN CHILE

We investigated the metamorphic cooling history of underplated magmati c rocks at midcrustal depth. Granulites and amphibolites occur within the Jurassic magmatic belt of the Coast Range south of Antofagasta in northern Chile between 23 degrees 25' and 24 degrees 20' S. The protol iths of the metamorphic rocks are basic intrusions of Early Mesozoic a ge. They are part of the magmatically formed crust, and the essentiall y dry magmas were emplaced in an extensional regime. The granulites (c linopyroxene-orthopyroxene-plagioclase) show all stages of fabric deve lopment from magmatic to granoblastic fabrics. Pyroxene compositions w ere reset at temperatures around 800 degrees C independent of the stag e of textural equilibration. The granulites were partially amphiboliti zed at upper amphibolite facies temperatures of 600-700 degrees C. Fol lowing cooling, a possible reheating to greenschist facies temperature s around 500 degrees C is indicated by prograde zoning in magnetite-il menite pairs. Mineral assemblages are not suitable for barometry, but a conservative estimation of the garnet-in reaction at given whole-roc k compositions suggests maximum pressures in the granulite facies of a round 5 kbar, and similar pressures are indicated by phengite barometr y for the greenschist facies. The P-T path of granulite-amphibolite me tamorphism is one of slow cooling from magmatic temperatures with hete rogeneous deformation. The thinning of the pre-Andean (Precambrian-Tri assic) crust was apparently compensated by the magmatic underplating a nd this special tectonomagmatic setting caused the prolonged residence of the accreted rocks at midcrustal levels.