Lithostratigraphy, petrology and 40Ar-39Ar geochronology of the Crucero Supergroup, Puno Department, SE Peru
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The Cordillera de Carabaya region of the southern Peruvian Andes preserves a collage of Tertiary igneous suites, including extrusive and hypabyssal units, that are herein assigned to the Crucero Supergroup and Crucero Intrusive Supersuite, respectively. The supergroup consists of two petrologically and temporally distinct assemblages, the Picotani and the Quenamari Groups, both with silicic hypabyssal representatives. Stratigraphic, petrologic and 40Ar-39Ar geochronologic studies of these rocks demonstrate that they are exposed in four localities, the Quenamari, Antauta, Cayconi and Picotani volcanic fields. The Picotani Group incorporates a diverse assemblage of lavas and pyroclastic rocks, including lamprophyres (minette), medium- to high-K calc-alkaline basalts, shoshonites, and S-type rhyodacites and rhyolites, as well as commingled and mixed associations of these, constituting ten mappable formations and twelve delimited hypabyssal units. These were emplaced over a brief interval from ca. 22 -to- 26 Ma. The Quenamari Group, in contrast, comprises entirely silicic, strongly peraluminous suites including, biotite + sillimanite ± muscovite ± andalusite ± tourmaline-bearing pyroclastic units and cogenetic epizonal intrusions. These constitute three distinct volcanic formations and at least 8 intrusive bodies. Rocks of the Quenamari Group span a broad temporal interval, from ca. 6.5 to 17 Ma. The rock associations of the Crucero Supergroup are clearly distinct from time-equivalent igneous units of the calc-alkaline, Main Andean arc in southern Peru, thereby implying differing geodynamic conditions during their genesis.