Geochronology and Geochemistry of the Francisco Gneiss: Triassic Continental Rift Tholeiites on the Mexican Margin of Pangea Metamorphosed and Exhumed in a Tertiary Core Complex

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 TDM 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/39 Ar 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.

[1]  J. Keppie,et al.  Terranes of Mexico Revisited: A 1.3 Billion year Odyssey , 2004 .

[2]  James G. Ogg,et al.  A new Geologic Time Scale, with special reference to Precambrian and Neogene , 2004 .

[3]  R. Hilst,et al.  Epeirogenic uplift above a detached slab in northern Central America , 2002 .

[4]  W. Dickinson.  The Basin and Range Province as a Composite Extensional Domain , 2002 .

[5]  M. Barton,et al.  A chemical and isotopic study of the Laramide granitic belt of northwestern Mexico: Identification of the southern edge of the North American Precambrian basement , 2001 .

[6]  A. Glazner,et al.  Geology and geochemistry of mafic to felsic plutonic rocks in the Cretaceous intrusive suite of Yosemite Valley, California , 2001 .

[7]  T. Lawton,et al.  Carboniferous to Cretaceous assembly and fragmentation of Mexico , 2001 .

[8]  J. Schumacher,et al.  Sphene (titanite): phase relations and role as a geochronometer , 2001 .

[9]  P. J. Patchett,et al.  Permo-Triassic continental arc in eastern Mexico: Tectonic implications for reconstructions of southern North America , 1999 .

[10]  C. Hodgson,et al.  Laser probe 40 Ar/ 39 Ar ages of early- and late-stage alteration assemblages, Rosario porphyry copper-molybdenum deposit, Collahuasi District, I Region, Chile , 1998 .

[11]  Elena Centeno García,et al.  Petrogenesis and tectonic evolution of central Mexico during Triassic-Jurassic time , 1997 .

[12]  J. Ayer,et al.  Neoarchean evolution of differing convergent margin assemblages in the Wabigoon Subprovince: geochemical and geochronological evidence from the Lake of the Woods greenstone belt, Superior Province, Northwestern Ontario , 1997 .

[13]  E. Centeno-Garcia,et al.  Oaxaquia, a Proterozoic microcontinent accreted to North America during the late Paleozoic , 1995 .

[14]  L. Silver,et al.  Tertiary metamorphic core complexes in Sonora, northwestern Mexico , 1994 .

[15]  S. Bowring,et al.  An empirical evaluation of the argon diffusion geometry in muscovite , 1994 .

[16]  P. Coney,et al.  Guerrero terrane of Mexico: Its role in the Southern, Cordillera from new geochemical data , 1993 .

[17]  F. Ortega-Gutiérrez,et al.  Tectonostratigraphic terranes and tectonic evolution of Mexico , 1993 .

[18]  James E. Wright,et al.  Early Mesozoic tectonic evolution of the western U.S. Cordillera , 1992 .

[19]  J. H. Stewart,et al.  Upper Triassic Barranca Group; Nonmarine and shallow-marine rift-basin deposits of northwestern Mexico , 1991 .

[20]  J. Aleinikoff,et al.  U-Pb geochronologic constraints on the origin of a unique monazite-xenotime gneiss, Hudson Highlands, New York , 1990 .

[21]  J. Severinghaus,et al.  Chapter 1: Cenozoic geometry and thermal state of the subducting slabs beneath western North America , 1990 .

[22]  W. McDonough,et al.  Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes , 1989, Geological Society, London, Special Publications.

[23]  C. Dupuy,et al.  Petrogenesis of the Natkusiak continental basalts, Victoria Island, Northwest Territories, Canada , 1986 .

[24]  I. Duncan,et al.  Diffusion of 40Ar in biotite: Temperature, pressure and compositional effects , 1985 .

[25]  T. Anderson,et al.  The evolution of Middle America and the Gulf of Mexico–Caribbean Sea region during Mesozoic time , 1983 .

[26]  M. F. Campa,et al.  Tectono-stratigraphic terranes and mineral resource distributions in Mexico , 1983 .

[27]  John W. Shervais,et al.  Ti-V plots and the petrogenesis of modern and ophiolitic lavas , 1982 .

[28]  H. Mullan.  Evolution of part of the Nevadan orogen in northwestern Mexico , 1978 .

[29]  D. McKenzie,et al.  Some remarks on the development of sedimentary basins , 1978 .

[30]  M. Bussell,et al.  The basic rock series in batholithic associations , 1977, Geological Magazine.

[31]  J. Winchester,et al.  Geochemical discrimination of different magma series and their differentiation products using immobile elements , 1977 .

[32]  D. Cowan,et al.  Early Mesozoic tectonic evolution of the western Sierra Nevada, California , 1975 .

[33]  J. Pearce.  Basalt geochemistry used to investigate past tectonic environments on Cyprus , 1975 .

[34]  A. Miyashiro.  Volcanic rock series in island arcs and active continental margins , 1974 .

[35]  Z. D. Cserna,et al.  Mapa geologico de reconocimiento y secciones estructurales de la region de San Blas y El Fuerte, estados de Sinaloa y Sonora , 1961 .

[36]  J. Buwalda.  The Cordilleran Section of the Geological Society of America , 1927, Science.