Geochronology and geochemistry of the Francisco Gneiss

Migmatized amphibolite-facies gneisses and amphibolites of the Francisco Gneiss exposed in the northern part of the Guerrero composite arc terrane have been interpreted as either Precambrian basement, a Triassic metamorphic complex, or a distinct terrane. Field observations suggest exposure in a metamorphic core complex of a protolith composed of interleaved bimodal igneous and sedimentary rocks. Geochemical data indicate that the amphibolites are within-plate, continental tholeiites. Recalculated T_DM ages for the rhyolites are consistent with partial melting of the Grenvillian basement of North America projected beneath the area. U-Pb isotopic analyses of zircon from two felsic rocks yielded concordant ages between 216 and 197 Ma due to a combination of inheritance and Pb-loss: the best estimate of protolith age is ∼206 Ma - i.e., Norian, Late Triassic. Concordant U-Pb titanite ages range from 112 to 98 Ma, whereas nearly concordant U-Pb xenotime ranges from 91 to 51 Ma. These are inferred to result from partial-complete resetting during the high-grade metamorphic event. ⁴⁰Ar/³⁹Ar analyses from the gneisses yielded plateau ages of 16.5 ± 1 Ma (muscovite) and 13 ± 1 Ma (biotite), which date cooling through ∼370°C and ∼300°C, respectively (early Middle Miocene). Biotite from the granitic sheet yielded a plateau age of 13 ± 2 Ma. These data are interpreted in terms of Miocene exhumation in a core complex of high-grade metamorphic rocks developed either over a slab window or as a result of tectonic burial during the Laramide orogeny. The extrusion of Upper Triassic, continental rift tholeiites is consistent with emplacement in a back-arc environment.