Sm-Nd, Rb-Sr and U-Pb geochronology of a Grenville terrane in southern Mexico:: origin and geologic history of the Guichicovi complex

The Guichicovi complex forms the metamorphic basement near the western edge of the Maya terrane in southern Mexico. Petrographic field studies show that the Guichicovi complex, like other high grade metamorphic terranes in southern Mexico, consists of paragneisses and a igneous suite of rocks metamorphosed under granulite facies conditions. The Sm-Nd isotopic results yield depleted mantle model ages of 1.4-1.6 Ga for the metaigneous rocks and of 1.5-2.0 Ga for the metasediments. On a Sm-Nd isochron diagram the isotopic results of the metaigneous suite of rocks define an isochron whose slope corresponds to an age of similar to 1.44 Ga. This age can be interpreted as the time of a major crust forming event. Discordant U-Pb zircon data from a felsic orthogneiss plot on a chord intersecting the concordia at 1231+/-43 and at 975+/-35 Ma. The upper intercept age is interpreted as the time of zircon crystallization, hence the age of intrusion of the granitic precursor. The lower intercept age reflects the time of peak granulite facies metamorphism. Concordant U-Pb isotope ratios of a zircon fraction from a charnockite (991 +/-4 Ma) gives the time of crystallisation of the charnockite. Garnet/whole rock ages (Sm-Nd) between similar to 911 and similar to 933 Ma and biotite/whole rock ages (Rb-Sr) between similar to 866 and similar to 882 Ma document the cooling history of the Guichicovi complex. Age and petrography of the Guichicovi complex confirm its connection with other granulite exposures in Mexico, which comprise the Oaxaquia microcontinent. The results support the hypothesis that the Precambrian crust in Mexico together with the Colombian Andes and the Adirondacks originated during the mid-Proterozoic. The Grenville orogenic event depressed these terranes into the roots of the orogen. Our data indicate that the Guichicovi complex stayed for >70 Ma at a deep crustal level before uplift. Subsequent breakup of Rodinia left Oaxaquia including the Guichicovi complex together with Gondwana. Further collisions of Gondwana and Laurentia and repeated transpressional interactions in the Paleozoic and early Mesozoic led to the present day position of Grenville fragments in Mexico. (C) 1999 Elsevier Science B.V. All rights reserved.