PROVENANCE AND EVOLUTION OF LOWER PALEOZOIC ROCKS FROM YUCATAN: NEW ISOTOPIC (HF, ND) AND AGE EVIDENCE

The Yucatan peninsula, which forms part of the Maya block of Central America, contains lower Paleozoic rocks in the Maya Mountains of Belize. The pre-Mesozoic geologic history of the Maya block is related to the evolution of the mid-Proterozoic basement in Mexico (Oaxaquia), other Paleozoic peri-Gondwanan terranes, and the Pan-African-Brasiliano type basement of Florida. Detrital zircon grains from Lower Paleozoic sedimentary rocks in the Yucatan Peninsula have an age distribution characterized by major probability peaks at ~1.0, ~1.2, and ~1.5 Ga. Here, we present new Lu-Hf data (solution MC-ICPMS) paired with U-Pb ages (ID-TIMS), which yields precise information about the time and geochemical environment of zircon growth.

The ¹⁷⁶Hf/¹⁷⁷Hf_(i) of ~1.0 Ga detrital zircon grains lie on a crustal evolution trajectory similar to that defined by older, ~1.2 to ~1.5 Ga grains. This trajectory is consistent with those that would be produced by crustal reservoirs that separated from the depleted mantle between 1.70 and 2.05 Ga. However, some grains have less radiogenic ¹⁷⁶Hf/¹⁷⁷Hf_(i) indicating influence from older cratonic crust. Zircon grains from granitoids that intruded the Early Paleozoic sedimentary rocks yielded crystallization ages from ~415 to ~400 Ma. More radiogenic Hf isotope ratios indicate anatexis of a crustal reservoir that is distinct from that of the sedimentary rocks. The Sm-Nd systematics of whole rock samples further support the results from the Hf isotopes in zircon grains.

The data suggest a more continental provenance for the sedimentary rocks from Yucatan as compared to typical Oaxaquia outcrops in southern and central Mexico. The results indicate that sediments were shed either from mid-Proterozoic complexes of NW Amazonia or from similar continental sequences that were thrust over Oaxaquia during the Grenville orogeny and subsequently eroded in the early Paleozoic. Integrating the data into a new model, the southern Maya block formed during the opening of the Rheic Ocean along the western margin of Amazonia adjacent to Oaxaquia. The paucity of Ediacaran (Pan African-Brasiliano) signatures implies that before the Silurian, the southern Maya block evolved geographically separated from NW Yucatan and Florida, where Pan African-Brasiliano crystalline rocks have been reported.