I-type granitoids in the eastern Yangtze Block: implications for the Early Paleozoic intracontinental orogeny in South China
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The Early Paleozoic intracontinental orogenic belt in the South China Block (SCB) is composed of massive granitoids and high-grade metamorphic rocks. Compared to the widespread distributions of early Paleozoic S-type granites in the eastern SCB, coeval I-type granitoids are rare and consequently receive much less attention. Two spatially associated granodiorite plutons in the northwestern rim of the orogen, namely the Banshanpu pluton and Hongxiaqiao pluton, have been investigated in order to determine how they fit into the geodynamic setting. The Hongxiaqiao pluton shares many lithological similarities with the Banshanpu pluton, except for the presence of abundant mafic microgranular enclaves (MMEs) in the Hongxiaqiao pluton. Zircon U–Pb dating has yielded weighted mean 206Pb/238U ages of 432 ± 3 Ma and 434 ± 3 Ma for the Banshanpu and Hongxiaqiao plutons, respectively, indicating that they were emplaced coevally in the early Silurian. Samples from the two plutons possess similar Nd–Sr isotope compositions (εNdT = − 8.32 to − 6.88; 87Sr/86Sri = 0.7109–0.7169), indicating that they were derived from a similar crustal source. Rocks from the Banshanpu pluton are intermediate- to high-K calc-alkaline and show strongly peraluminous (A/CNK > 1.1), adakite-like characteristics (Sr/Y ratios > 31; Yb < 0.91 ppm), consistent with an origin of partial melting of amphibolite in the garnet stability field. Samples from the Hongxiaqiao pluton contain lower SiO2 but considerably higher Fe2O3T, TiO2, P2O5 and highly incompatible elements (e.g. Rb, Cs, Th and U) than those of the Banshanpu pluton. The MMEs from the Hongxiaqiao pluton give ages similar to that of their host granite (429 ± 5 Ma), and their Nd–Sr isotope compositions (εNdT = − 7.45 to − 7.03; 87Sr/86Sri = 0.7115–0.7143) imply an origin from metasomatized lithospheric mantle. The Hongxiaqiao pluton was possibly produced by a magma mixing between a crustal melt and a lithospheric mantle-derived melt. Because the MMEs have relatively low Sr/Y (9–17) and (La/Yb)N (3–15) ratios, mixing of the lithospheric mantle-derived melt with an adakite-like melt would dilute the adakitic signature and make composition of the mixture deviate from adakitic characteristics. A comprehensive evaluation of geochronological data for magmatism and metamorphism in the orogeny reveals two phases of orogenesis: one before and one after ca. 440 Ma. The temporal and spatial developments of tectonomagmatism, as well as the increase in metamorphic grade, are interpreted to record the progress of the intracontinental orogeny, which started in the Wuyi-Yunkai domains of the Cathysia Block in the Ordovician and propagated westward into the Yangtze Block in the Silurian.
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