Zircon U-Pb ages and Hf isotope compositions of the Mayuan migmatite complex, NW Fujian Province, Southeast China: Constraints on the timing and nature of a regional tectonothermal event associated with the Caledonian orogeny

摘要

U-Pb ages, trace elements, and Hf isotope compositions of zircons from the Mayuan migmatite complex in NW Fujian province have been determined to provide constraints on the source and genesis of anatexis and tectonothermal evolution related to the Caledonian orogeny in South China. The migmatites investigated consist of various amounts of mesosome, leucosome, and melansome. Zircons extracted from mesosome, leucosome, and granite samples are characterized by oscillatory overgrowths enclosing inherited cores or occur as newly grown grains. The ages of the inherited zircons from the leucosome and granite samples are consistent with those of adjacent basement paragneiss in the study area, suggesting that both leucosome and granite were generated by partial melting of the latter. A comparison of Hf isotopes between the newly-formed zircons and inherited cores indicates that the former resulted from the breakdown of preexisting inherited zircons and/or less Hf-rich minerals other than zircons at the source. One mesosome sample contains typical metamorphic zircons that yielded a weighted mean ~(206)Pb/~(238)U age of 453 ± 3 Ma. They show enrichments in heavy REEs (Lu_N/La_N up to 22,709), indicating their growth prior to garnet crystallization. The other mesosome sample, in contrast, contains both newly-formed metamorphic rims and grains that gave a weighted mean ~(206)Pb/~(238)U age of 442 ± 8 Ma. They are characterized by relatively low Th/U ratios, depletions in heavy REEs (Lu_N/La_N = 117-396), and low ~(176)Lu/~/(177)Hf ratios, suggesting their growth synchronous with garnet crystallization. The U-Pb ages of the mesosome samples are interpreted as recording the time of early (ca. 453 Ma) to peak (442 Ma) stages of a regional metamorphic event. Two leucosome and two granite samples yield consistent U-Pb ages of 438 ± 5 Ma to 442 ± 4 Ma, which provide constraints on the timing of subsequent anatexis and magmatism. The geochronological data reported here reveal a consecutive sequence of regional metamorphism, anatexis, and magmatism in NW Fujian province, lasting for at least 15 Myr, which was driven by the Caledonian orogeny that have affected a major part of the SCB.