Petrogenesis and tectonic implications of Triassic A-type granites in southeastern China: insights from zircon U-Pb-Hf isotopic and whole-rock geochemical compositions of the Luoguyan and Guiyantou granites in northwestern Fujian Province

Triassic A-type granites in eastern South China Block (SCB) are abundant in the Wuyi-Yunkai tectonic domain and provide an important opportunity to explore the early Mesozoic evolution of continental crust of the SE part of the SCB. We carried out U-Pb zircon dating, Lu-Hf isotope analyses of zircon, and whole-rock geochemical analyses for two granitic plutons, the Guiyantou (GYT) and Luoguyan (LGY) granites, from northwestern Fujian Province. LA-ICP-MS U-Pb zircon analyses yielded ages of 232 +/- 4 to 231 +/- 7 Ma and 221 +/- 5 Ma (Middle-Late Triassic) for the GYT and LGY granites. These two granites belong to metaluminous to weakly peraluminous high K calc-alkaline A-type granite that are enriched in K, Al, light rare earth element and Rb, Th, U, and Pb, and depleted in Nb, Ta, P, and Ti. Their rare earth element patterns are highly fractionated with (La/Yb)(N) ratios of 2-21 and strong negative Eu anomalies (Eu/Eu* = 0.02-0.31). In situ Hf isotopic analysis of zircon from the GYT and LGY granites yielded epsilon(Hf)(t) values ranging from -11.5 to -1.1, with corresponding two-stage Hf model ages from 1.98 to 1.33 Ga, from which it is inferred that the GYT and LGY magmas formed by partial melting of Proterozoic metasedimentary rock in the Cathaysia block. The two granites were emplaced at 232 and 221 Ma and together with Triassic A-type granites in coastal region of the SCB, which is in agreement with an extensional tectonic setting in the Middle-Late Triassic. We suggest that the Middle-Late Triassic A-type granites in eastern SCB were probably formed in an intracontinental, post-orogenic extensional regime that collision was between the SCB and an 'unknown block' or the eastern extension of Indochina block.