Geodynamics of Late Carboniferous–Early Permian forearc in north Chile (28°30′–29°30′S)


Creixell, C.
Oliveros, V.
Vásquez, P.
Navarro, J.
Vallejos, D.; Valin, X.; Godoy, E.; and Ducea, M. N.

Abstract: A large section of the Late Palaeozoic forearc is exposed along the coastal ranges of north–central Chile (28°–29° 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 Chañaral 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 40Ar/39Ar 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.

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Fig. 3. (a, b) Field features of the Llano del Chocolate Beds: (a) foliated conglomerate with rhyolitic volcanic clasts at the base of the sequence; (b) coarse breccia-like deposits, consisting of dominant quartz clasts of metamorphic origin, in a finegrained
sandstone matrix. (c, d) Field features of the Chañaral Epimetamorphic Complex: (c) decimetre-scale intercalation of slightly recrystallized metasandstones and metalutites near the contact with the Llano de Chocolate Beds; (d) well-foliated mélange facies with protomylonitic fabric.

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Journal of the Geological Society. doi:10.1144/jgs2016-010