LITHOSTRATIGRAPHY, PETROLOGY AND AR-40-AR-39 GEOCHRONOLOGY OF THE CRUCERO SUPERGROUP, PUNO DEPARTMENT, SE PERU

The Cordillera de Carabaya region of the southern Peruvian Andes prese rves a collage of Tertiary igneous suites, including extrusive and hyp abyssal 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 Ar-40-(3)9Ar geochronologic studies of these rocks demonstrate that they are exposed in four localities, the Quenamari, Antauta, Cayconi and Picotani volcanic fields. The Picotan i Group incorporates a diverse assemblage of lavas and pyroclastic roc ks, including lamprophyres (minette), medium-to high-K calc-alkaline b asalts, 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 co ntrast, comprises entirely silicic, strongly peraluminous suites inclu ding, biotite + sillimanite +/- muscovite +/- andalusite +/- tourmalin e-bearing pyroclastic units and cogenetic epizonal intrusions. These c onstitute three distinct volcanic formations and at least 8 intrusive bodies. Rocks of the Quenamari Group span a broad temporal interval, f rom ca. 6.5 to 17 Ma. The rock associations of the Crucero Supergroup are clearly distinct from time-equivalent igneous units of the calc al kaline, Main Andean are in southern Peru, thereby implying differing g eodynamic conditions during their genesis. (C) 1997 Elsevier Science L td.