U/Pb geochronology of Devonian and older Paleozoic beds in the southeastern Maya block, Central America: Its affinity with peri-Gondwanan terranes

Abstract

Paleozoic rocks of the Maya (Yucatan) block of Central America include granitoids, rhyolitic-dacitic volcanic rocks, clastic sedimentary strata ranging from conglomerate to shale/phyllite, and minor limestone. To date, published isotopic ages, paleontologic data, and interpretations of field relations have been contradictory. For instance, U/Pb ages of zircon and monazite from granitoids yield Silurian-Devonian ages (ca. 420–405 Ma); the granitoids are bordered by metasedimentary contact aureoles, yet the bulk of their protolith has been assigned to the Pennsylvanian-Permian (Macal Formation or Santa Rosa Group). In this paper, we resolve the paradox by showing that pre–Lower Devonian beds are present in the Maya Mountains and constitute the wall rock for the intrusions. We dated 23 igneous zircons from a rhyolite interbedded with conglomerates obtaining a 406 +7 / −6 Ma (2s) median U/Pb age, which represents the time of eruption of the previously recognized volcaniclastic Bladen Formation. Furthermore, U/Pb geochronology of detrital zircon grains from a sandstone cobble, and outcrops of sandstone and quartzite yielded equivalent age spectra, indicating that they represent a separate geologic unit, which we herein define as the Baldy unit. Recognition of a distinct, pre-Devonian unit is justified because the sandstone cobble was collected from a conglomerate associated with the dated rhyolite, implying that the Baldy unit must have been deposited, lithified, exposed, and eroded before deposition of the Bladen Formation. The Baldy unit predates Silurian–Devonian magmatism because it does not contain interbedded volcanic rocks and is intruded by the dated granites. The five youngest detrital zircons of Baldy sedimentary rocks yielded ages in the 600–520 Ma range. These features imply that Baldy deposition took place some time between the Cambrian and Silurian. Baldy zircon age spectra have peaks at ca. 1.52 Ga, ca. 1.21 Ga, and ca. 1.02 Ga, consistent with provenance from Baltica or Amazonia, not Laurentia. Based on our new data and previously known geologic attributes of the Maya block, we propose that, during the Cambrian–Silurian, the block9s position was along the West Amazonia side of Gondwana9s periphery.