Where did the lower Paleozoic rocks of Yucatan come from? A U-Pb, Lu-Hf, and Sm-Nd isotope study

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• 作者：Bodo Weber^a ; ^{bweber@cicese.mx} ; Erik E. Scherer^b ; Uwe K. Martens^c ; Klaus Mezger^d
• 关键词：Hafnium isotopes ; U&ndash ; Pb ages ; Zircon ; Paleozoic ; Gondwana ; Central America
• 刊名：Chemical Geology
• 出版年：2012
• 期刊代码：12_00092541
• 类别：ear
• 出版时间：18 June, 2012
• 卷：312-313
• 期：Complete
• 页码：1-17
• 文件大小：2227 K

摘要

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 (Martens et al., 2010). Here, we present new Lu-Hf data (MC-ICPMS) paired with U-Pb ages (ID-TIMS) for additional zircon grains from the same rocks. This analytical approach yields precise information about the time and geochemical environment of zircon growth, which in turn helps to distinguish between different crustal source regions that just happen to host zircon populations of similar age. In addition, single zircon grains from granitoids that intruded the sedimentary rocks were dated by U-Pb laser ablation MC-ICPMS and ID-TIMS, and their Hf-isotope compositions were determined by solution MC-ICPMS. The zircon data are complemented by Sm-Nd analyses of the sedimentary and igneous whole rocks.

The Yucatan Peninsula, which forms part of the Maya block of Central America, includes 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. The initial ¹⁷⁶Hf/¹⁷⁷Hf values 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 significantly less radiogenic ¹⁷⁶Hf/¹⁷⁷Hf_(t), indicating influence from even older cratonic crust. Zircon grains from granitoids that intruded the Early Paleozoic sedimentary rocks of the Yucatan Peninsula yielded Late Silurian to Early Devonian (~ 415-400 Ma) crystallization ages. More radiogenic Hf isotope ratios indicate anatexis of a crustal reservoir that is distinct from that of the Early Paleozoic 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 ca. 1.3 to 1.0 Ga 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 for early Paleozoic times, the southern Maya block is inferred to have 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.