ORIGIN AND EVOLUTION OF THE SW-MEXICAN CONTINENTAL CRUST: EVIDENCES FROM DETRITAL ZIRCON U-PB AGES AND HF ISOTOPIC EVOLUTION

Arc magmatism in Mexico has been a continuous phenomenon at least since the Lower Jurassic, but it remains unclear how much of the continental mass represents juvenile additions or reworking of pre-existent crust. In order to elucidate the tectono-magmatic evolution of the Mexican continental crust through time, here we report U-Pb geochronology and Hf isotopic data on modern detrital zircons from river-mouth sediments draining to the coastal plain; from San Blás, Nayarít to Zihuatanejo, Guerrero.

The detrital zircons record passive sedimentation and cyclic magmatism during the Mesozoic and Cenozoic. The oldest zircon populations (> 200 Ma) are sourced in the metasediments of the Arteaga complex, a passive basin that received Archaean, Grenvillian, Pan-African and Permo-Triassic detritus from western Pangea during the Upper Triassic (Centeno-García et al., 2011, GSA Bull). The first autochthonous magmatic pulse occurred in the Upper Jurassic (~158 Ma) and is characterized by highly variable ƐHf (-10 to +12), probably indicating reworking of Arteaga metasediments at the initial stages of eastward subduction during the progressive fragmentation of Pangea. An important magmatic pulse with highly juvenile ƐHf (+3 to +15) was established by the Lower Cretaceous (~112 Ma), in the context of tectonic extension and slab rollback that governed the evolution of the Guerrero Terrane (Martini et al., 2014, Geosphere). A third pulse occurred in the Upper Cretaceous (~83 Ma), and is distinguished by the bimodal compositions of the Puerto Vallarta (ƐHf = -5 to 5) and Manzanillo (ƐHf ~ 10) batholiths, generated as a response to gradual slab flattening and arc migration to the continental interior (Ferrari, et al., 2017, ESR). Later on, an Eocene magmatic pulse (~50 Ma) with variably positive ƐHf (+1 to +10) is identified to the SE, associated with two major left-lateral shear zones (Martini et al., 2009, GSA). The final magmatic stage recorded by zircons is the Oligocene-Miocene (~24 Ma) Sierra Madre Occidental (SMO). Zircons from the SMO display ƐHf (-2.5 to 6) that become more juvenile with time, presumably as a response to escalating crustal extension. Our study shows that juvenile additions are favored during periods of extension and rollback, whereas crustal reworking is enhanced during slab flattening and tectonic compression.