LATE PERMIAN TO EARLY JURASSIC EVOLUTION OF THE CHIAPAS MASSIF COMPLEX, SE MEXICO - FROM A COLLISIONAL OROGEN TO CONTINENTAL RIFT

Permian- to Early Triassic granitoids are widespread in eastern and southern Mexico, reflecting diachronous collision of Laurentia and Gondwana. They intruded in two phases, (1) at the Gondwana continental margin before c. 275 Ma and (2) in a late- to post-collisional setting from c. 260-245 Ma. In the Chiapas Massif Complex (CMC) the second plutonic phase is coeval with regional metamorphism with differing metamorphic grade and P/T ratios. Metamorphic basement exposures in the CMC define two lithodemes: (1) El Triunfo Complex (TC), composed of c. 1Ga anorthosite and orthogneiss, covered by metasedimentary rocks, altogether metamorphosed and intruded by plutonic suites during the Ordovician and the Permian; and (2) the Catarina (CA) and Custepec (CU) units with no signs of an Ordovician event. CA is composed of c. 1.5-0.9 Ga orthogneiss with TDM_(Nd) of 2.0-1.8 Ga, while Permian mafic anatexite constitutes most of the CU with TDM_(Nd) of 1.3-1.0 Ga, indicative of reworked crustal domains with basaltic underplating. Recent fieldwork exploring the contact between both lithodemes revealed high P/T with kyanite-rutile-bearing assemblages at the TC side of the contact, suggestive of a Late Permian thrust boundary. Peraluminous, garnet-bearing, granitic stocks intruded the metamorphic basement, locally defining the strongly mylonitized contact between CA and TC, and into Permian plutonic rocks, close to other tectonic lineaments. Sm-Nd ID-TIMS dating of leached garnet and whole rock yielded ages at 217.8 ± 0.6 Ma and 215.5 ± 0.9 Ma (2σ), interpreted as the time of granite crystallization. Zircon is typically rare and inherited in these rocks, however, magmatic zircon rims yielded similar U-Pb LA-ICPMS ages. Monazite, instead, reflects ductile deformation at c. 179 Ma. At another locality, Permian granite is migmatized with garnet-bearing leucosome yielding a Sm-Nd garnet-WR age of 206 ± 6 Ma and an U-Pb age of zircon tips at 188 ± 2 Ma. The new findings suggest, for Late Permian times, an orogenic wedge, asthenospheric upwelling and batholithic intrusions. Uncommon Late Triassic crustal anatexis, followed by Early Jurassic ductile shear zones and probably bimodal magmatism confirms recent models calling for crustal extension of peninsular Mexico in a continental rift province.