Early Mesoproterozoic inliers in the Chiapas Massif Complex of southern Mexico: Implications on Oaxaquia-Amazonia-Baltica configuration

Abstract

In this contribution, zircon U-Pb geochronology by LA-ICPMS and Sm-Nd isotope systematics by TIMS are reported on metaigneous rocks from the central Chiapas Massif Complex (CMC), southern Mexico. The data reveal the existence of a Paleo- to early Mesoproterozoic crust that differs significantly from surrounding units and from typical Precambrian basement of Oaxaquia. On the basis of Sm-Nd depleted mantle model ages (TDM(Nd)) between 2.0 and 1.8 Ga a distinct crustal slice within the CMC is defined and named the Catarina Unit. It comprises banded felsic and mafic orthogneiss (amphibolite) yielding Calymmian U-Pb zircon ages, corresponding to igneous protolith crystallization between ∼1.58 and ∼1.50 Ga. Inherited zircon cores with 207Pb/206Pb apparent ages between ∼1.85 and ∼1.70 Ga and whole rock ɛNd(1.55Ga) values between + 1.2 and − 0.7 in granitic protoliths further suggest reworking of an older crustal component. Similarly, amphibolite has initial ɛNd(1.50Ga) ranging from + 2.8 to − 0.5, indicating only minor depleted mantle additions during the formation the mafic protoliths. Zircon overgrowths document a major Tonian thermotectonic event at ∼0.96–0.94 Ga that caused anatexis of Calymmian protoliths and intrusion of granitic melts (now biotite-amphibole quartz-feldspathic gneiss). Finally, all rocks underwent Permian-Triassic metamorphism and ductile deformation, commonly observed in the CMC. These results are interpreted in terms of an accretionary active margin established along the border of Amazonia in Calymmian times, where Catarina Unit protoliths formed by mixing of mantle-derived magmas with reworked cratonic crust. In Stenian times (1.2–1.1 Ga), the juvenile Oaxaquia arc accreted to the cratonic margin, followed by the collision between Amazonia and Baltica during final Rodinia amalgamation at ∼0.99 Ga. Subsequently, the newly formed orogen entered into a period of uplift, gravitational collapse, and relaxation similar to the Dalane phase in Baltica, leading to mantle upwelling and anatexis of the Catarina Unit at 0.96–0.94 Ga.