Geodynamics of Late Carboniferous-Early Permian forearc in north Chile (28 degrees 30 '-29 degrees 30 ' S)

摘要

A large section of the Late Palaeozoic forearc is exposed along the coastal ranges of north-central Chile (28 degrees-29 degrees 30'S). This is characterized by three lithotectonic units: (1) the Punta de Choros Metamorphic Complex (basal accretion series), composed mostly of micaschists and metabasites; (2) the Chanaral Epimetamorphic Complex (frontal accretion series), formed by metaturbidites and metasediments; (3) the Llano del Chocolate Beds (forearc basin deposits), composed of a sedimentary sequence of clastic sedimentary rocks with minor limestones and acidic volcanic rocks. Within the basal accretion series, two distinctive blocks of garnet-bearing schists with amphibolite-facies metamorphism have been preserved, recording early stages of the subduction system. The stratigraphic record and the U-Pb dating of igneous (291-318 Ma) and detrital zircons (maximal deposition ages between 273 and 292 Ma) in the forearc basin deposits, coupled with Ar-40/Ar-39 ages for metamorphic rocks (319-280 Ma), indicate that forearc sedimentation was broadly contemporaneous with metamorphism and exhumation of the basal accretion series. The radiometric dating on garnet-bearing rocks indicates that the onset of the subduction system took place in the Mississippian, at a high thermal gradient. These conditions were responsible for generation of limited volumes of acidic melt, now recognized as acidic domes and tuffs within the forearc basin stratigraphy.