EARLY CRETACEOUS PROTOLITH AGES FOR METAVOLCANIC ROCKS FROM TAXCO AND TAXCO VIEJO IN SOUTHERN MEXICO

A metamorphosed vulcanosedimentary sequence from Taxco and Taxco Viejo outcrops near an isolated areas inside the Taxco synclinorium. This structural feature is part of the southern section of the Guerrero-Morelos platform. The protolith ages for the Taxco schist and Taxco Viejo greenstone were interpreted as Paleozoic and Triassic, respectively (Fries C, 1960). Using TIMS U-Pb zircon geochronology we analyzed three zircon fractions from a greenstone sample from Taxco and obtained a weighted average age (206Pb/238U) of 130 ± 2.6 Ma that we interpret as the crystallization age for the volcanic protolith. In addition, we analyzed another three zircon fractions from a porphytitic sample from Taxco Viejo. One of these zircon fractions is concordant with a weighted average age (206Pb/238U) at 131.7 ± 0.85 Ma that we interpret as a crystallization age for the volcanic protolith. The other two fractions are discordant, yielding Precambrian upper intercepts that we interpret as representing zircon inheritance from the nearby basement of the Oaxaca-Mixteca terranes (Campa, 2002). Within limits of analytical error, the protoliths for these two metavolcanic samples from Taxco and Taxco Viejo are the same age and perhaps similar to those arc volcanic deposits from the Guerrero terrane, Chiautla arc, and Juarez terrane. We interpret that the vulcanosedimentary sequences of the central section of the Guerrero-Morelos platform could have been deposited during the Early Cretaceous time in an environment typical of the Mexican Pacific arcs in the western margin of a Precambrian continent. In addition, preliminary Ar/Ar geochronology of metamorphosed volcanic rocks from Chapolapa-Izcuinatoyac formations at the southern end of the platform indicates the possibility that deformation and metamorphism recorded in the vulcanosedimentary sequence occurred during the Late Cretaceous Early Tertiary time.