Late Silurian plutons in Yucatan

Abstract

U-Pb measurements of zircons from two composite plutons in the Maya Mountains of the Yucatan Block (Belize) give Late Silurian ages. Zircons from one of the five compositional phases of the Mountain Pine Ridge pluton yield an age of 418±3.6 Ma. A second compositional phase gives a minimum age of 404 Ma, and zircons from a third phase, although plagued with high common Pb, yield ages consistent with the other two. Zircons from one compositional phase of the Hummingbird-Mullins River pluton indicate an age of about 410–420 Ma. These data demonstrate that two of the three Maya Mountains plutons residing among the strata of the Late Pennsylvanian through Permian Santa Rosa Group are older than that sedimentation. Although the third pluton was not dated, both the similarity of sedimentary facies patterns adjacent to it to those adjacent to one of the plutons dated as Late Silurian and a published single Rb-Sr age of 428 ± 41 Ma suggest this third pluton also was emergent during Santa Rosa deposition. Thus the new U/Pb dates and other data suggest that all three Maya Mountains plutons pre-date Late Carboniferous sedimentation and that none intrude the Santa Rosa Group. Although very uniform ages of about 230 Ma amongst all plutons, derived from abundant earlier dating by the K-Ar system, led to the conclusion that intrusion mostly had occurred in the Late Triassic, the U-Pb ages (obtained from the same sites as the K-Ar dates) demonstrate that the K-Ar ages do not derive from a Late Triassic intrusive episode. The K-Ar dates probably are a signature of the rifting associated with Pangean breakup and formation of the Gulf of Mexico. In a reconstructed Pangea, the position of the Maya Mountains Late Silurian plutons suggests that the Late Silurian Acadian-Caledonian orogen of eastern North America extended through the region of the future Gulf of Mexico. Finally, the U-Pb ages of the Maya Mountains plutons are the same as those of a group of shocked zircons found in the Chicxulub impact structure and its fallout layer. The presence of these ages in both locations suggests that the Maya Mountains exposures may be representative of the basement of the Yucatan Block, hence of the basement impacted by the Chicxulub object.