Late Triassic to Early Jurassic anatectic magmatism in the Chiapas Massif Complex, southern Mexico: New evidence for the onset and mode of crustal extension in western equatorial Pangea

Abstract

During the Triassic, continental breakup started in western equatorial Pangea, while active continental margins developed along North and South America’s Pacific borders. Besides many uncertainties about how peri-Gondwanan terrains of Mexico evolved during and after Pangea’s amalgamation, the apparent absence of Late Triassic magmatism has been an enigmatic feature of Mexico’s geology. Here, we present the first documented occurrences of Late Triassic igneous rocks and a coeval high-grade metamorphic event in southern Mexico, as well as evidence for subsequent Early Jurassic ductile deformation and magmatism. Samples come from the Chiapas Massif Complex (CMC), a crystalline basement of the southern Maya Block. Field evidence suggests that garnet-bearing leucogranitic stocks intruded a former structural boundary between different Precambrian-Paleozoic basement units. High-precision Sm-Nd garnet-whole-rock ages (ID-TIMS partial dissolution techniques) and U-Pb geochronology on zircon rims (LA-ICPMS) revealed Norian igneous crystallization ages for the leucogranites (∼221–213 Ma). Isotope and geochemical data indicate that these granites originated from crustal reworking by anatexis. A garnet-bearing leucosome from a migmatite yielded a slightly younger Sm-Nd age (∼204 Ma), indicating cooling soon after the peak conditions of the anatectic event. Additionally, monazite from a mylonitized leucogranite yielded an Early Jurassic U-Pb age (∼179 Ma), indicating ductile deformation. Nearly coeval zircon ages from an undeformed pink granite and a nearby volcaniclastic sequence (∼184–189 Ma) further imply contrasting erosion levels during the Early Jurassic. The new data support models for regional stretching of eastern and southern Mexico, where crustal extension during the gravitational collapse of a collisional orogen in paleo-northern Mexico led to continental rifting and the breakup of Pangea. In the CMC, crustal extension is reflected by Norian anatectic crustal melts and migmatites, as well as Early Jurassic plutonic and extrusive magmatism that are localized in certain fault-bounded blocks with different erosion levels compared to adjacent blocks.