Late Paleozoic to Early Jurassic tectonic development of the high Andean Principal Cordillera, El Indio Region, Chile (29-30 degrees S)

Regional mapping (1:50,000) and U-Pb and K-Ar geochronology in the El Indio region refines the knowledge of the distribution, lithostratigraphy, and a ge of the sedimentary, volcanic, and intrusive rocks that comprise the regi onally extensive Pastes Blancos Group which is equivalent to the Choiyoi Gr oup of the Argentine Frontal Cordillera. The Pastes Blancos Group (which we elevate to Group status herein) includes at least two diachronous volcanic -sedimentary sequences: an older felsic volcanic and volcaniclastic unit, t he Guanaco Sonso sequence, that is Permian in age, and a younger bimodal vo lcanic and volcaniclastic unit, the Los Tiles sequence that is Middle Trias sic to Early Jurassic. Sedimentary rocks of the Los Tiles sequence are tran sitional upward into the overlying Early to Middle Jurassic shallow marine limestones of the Lautaro Formation. Intrusions that make up the regionally extensive Permian to Early Jurassic plutons of the Chollay and Elqui-Limari batholiths that were previously map ped as a single plutonic association, the Ingaguas Complex, include in the El Indio region at least three discrete intrusive units. These include: Ear ly Permian (280-270 Ma) biotite granites, Early to Middle Triassic (242-238 Ma) silica-rich leucocratic granites and rhyolitic porphyries that made up the bulk of the Chollay Batholith, and a younger Late Triassic-Early Juras sic unit (221-200 Ma) of mainly intrusive rhyolitic porphyries, extrusive d omes, and subordinate mafic intrusions and both felsic and mafic dikes, whi ch are coeval with volcanic rocks of the Los Tiles sequence. Our data show that latest Paleozoic to Early Jurassic intrusive, volcanic, and sedimentary rocks in the El Indio region of the High Andes of Chile bet ween 29-30 degrees S likely formed during extension driven processes after the cessation of Carboniferous-Early Permian subduction along the western e dge of Gondwana. These processes began by Late Permian time, but instead of recording a single and protracted magmatic event, as has been previously s uggested, rocks that belong to the Pastes Blancos Group and the Ingaguas In trusive Complex record at least three discrete periods of silicic to bimoda l magmatism which occurred during the Middle Permian to Early Jurassic inte rval. (C) 1999 Elsevier Science Ltd. All rights reserved.