Permian high pressure rocks - the basement of the Sierra de Limon Verde in Northern Chile

The gneisses and metabasites of the Sierra de Limon Verde were investigated by P-T-t determinations. The rocks are unique in the Central Andes because: of their high pressure metamorphic conditions with P approximate to 13 +/- 1 kbar at T approximate to 660 710 degrees C. Their age of metamorphism is approximate to 270 Ma, based on Sm-Nd mineral isochrons. Final uplift of the isolated basement block occurred in the Triassic with a K Ar age of biotite at ca 235 Ma. In our interpretation, the protolith of the Permian metamorphic rocks is the crust that formed and stabilized during Early Paleozoic. The Sierra de Limon Verde rocks give insight into the lowermost part of the crust in Early Mesozoic. Its Sm-Nd isotopic composition is indistinguishable from the composition of the crust that formed in the Early Paleozoic metamorphic-magmatic cycle (ca 500 Ma) in northern Chile and NW Argentina. The tectonic-geodynamic setting that triggered the high P (similar to 45 km depth) metamorphism and the locally restricted exhumation of the rocks remains speculative, Continental collision or a subduction related accretionary complex is unlikely considering the regional geological situation. Transpression-transtension in a strike slip system along the continental margin is suggested as a hypothesis for future investigations. (C) 1999 Elsevier Science Ltd. All rights reserved.