Ca. 800 Ma I-type granites from the Hong’an Terrane, central China: New constraints on the mid-Neoproterozoic tectonic transition from convergence to extension in the northern margin of the Yangtze Block

Abstract

The northern margin of the Yangtze Block was thought to have witnessed the tectonic shift from a compressional setting to an extensional setting at ca. 820 Ma, or to have remained as a subduction-related setting until ca. 700 Ma. Our new geochronological, isotopic, and geochemical data for the Daleishan granitic Pluton, which is the largest batholith in the Hong’an Terrane of the Qinling-Tongbai-Dabie Orogen, central China, provide new constraints on the Neoproterozoic magmatic and tectonic evolution of the Yangtze Block. Four representative samples from the Daleishan Pluton yield consistent zircon U–Pb ages within analytical errors of ca. 800 Ma. Samples from the pluton exhibit evolved compositions, including high SiO2 (71.53–77.05 wt%) and K2O (3.93–5.00 wt%) contents, low Zr/Hf (22–37) and Nb/Ta (7.9–16.4) ratios, enrichment in Rb, Th and U, and depletion in Ba, Sr, Eu, and Ti, with rare earth element tetrad effects in the chondrite-normalized element patterns. The studied samples have slightly elevated zircon δ18O values (+5.64 ‰ to + 7.11 ‰) relative to the mantle value but negative zircon εHf(t) (mostly of −12.0 to −5.5) and bulk rock εNd(t) (−9.6 to −7.9) values. Our new geochemical data classify the rocks as moderately to highly-fractionated I-type granite, rather than A-type granite as previously thought. These rocks may have been derived from the partial melting of mafic rocks in the region and the magma underwent fractional crystallization. Integrating the new and published data, we propose that the emplacement of the Daleishan Pluton was associated with the final stage of early Neoproterozoic oceanic-crust subduction along the northern margin of the Yangtze Block. Accordingly, a mid-Neoproterozoic tectonic transition from convergence to extension in response to the breakup of the Rodinia supercontinent may have occurred at 800–780 Ma.