ZIRCON GEOCHRONOLOGY IMPLIES THE EXISTENCE OF PRE-DEVONIAN SEDIMENTARY ROCKS WITH GRENVILLIAN PROVENANCE IN THE MAYA MOUNTAINS OF BELIZE

Published ages and interpretation of geologic relations between rock units in the Maya Mountains of Belize, Central America, are not consistent. Whereas Carboniferous-Permian fossil-bearing strata have been taken to represent the depositional age of the bulk of sediments and metasediments in the Maya Mountains, U-Pb ages of zircon and monazite from granitic instrusives has yielded older, Silurian-Devonian ages. New field observations and U-Pb geochronological data were obtained by Laser Ablation–Multicollector ICP-MS on 395 individual detrital zircons extracted from sedimentary and metasedimentary rocks, and on 23 zircons separated from an interbedded tuff. Low-grade quartzites from the lowermost unit and from quartzite cobbles in discordantly overlying conglomerates yield age spectra with a large peak at ~1.02 Ga, medium peaks at ~1.21 Ga and at ~1.52 Ga, and few zircons in the 1.6-1.9 Ga and 520-600 Ma ranges. Zircons from a tuff interbedded with conglomerates covering basal quartzites yield a mean 206Pb/238U age of 407.1 ± 6.5 Ma (2σ). These results demonstrate that along with late Paleozoic sediments, the Maya Mountains contain a Silurian-Devonian vulcanoclastic unit coeval with magmatic intrustions, which rests unconformably over low-grade metamorphic rocks deposited in the early Paleozoic. A third clastic unit ranging between conglomerates and shales bears Pennsylvanian-Permian fauna. This succesion contains conglomerates with abundant granitic and volcanic pebbles, and graywackes whose detrital zircons were shed mostly from local igneous sources forming an almost single age population with a large peak at ~410 Ma. The vast abundance of middle Proterozoic zircons in early Paleozoic sedimentary units of the Maya Mountains of Belize implies a Grenville-orogenic-belt provenance for most of the early Paleozoic detritus in the central-eastern part of the Maya block. This is in contrast with mostly Trans-Amazonian and Pan-African ages of detrital zircons reported from Carboniferous-Permian sediments (Santa Rosa Group) of the southern part of the Maya block.