Geochronology, geochemistry, and isotope compositions of “Grenvillian” S-type granites in the North Qinling unit, central China: Petrogenesis and tectonic significance

Abstract

An integrated study of whole-rock elements, Sr–Nd isotopes, and zircon U–Pb ages and Hf-O isotopes was carried out for the “Grenvillian” granites from the North Qinling unit, central China. The results not only place clear constraints on the petrogenesis of these granites, but also give insights into the geodynamic evolution of the Qinling orogen during the assemblage of supercontinent Rodinia. LA-ICPMS zircon U–Pb dating of three representative granites from the Dehe, Fangzhuang, and Zhaigen outcrops yielded formation ages of ca. 940–900 Ma. These muscovite-bearing granites have high silica contents (72–79 wt%) and are weak to strong peraluminous (ASI = 1.01–1.15). They exhibit arc-like trace element distribution patterns with enrichment of LREE and LILE (e.g., Rb, K, Pb) but depletion in HFSE (e.g., Nb, Ta, Ti). Besides, they show variable initial 87Sr/86Sr ratios of 0.704 to 0.729 and relatively low εNd(t) values of −3.2 to −6.1, as well as variable zircon εHf(t) and δ18O values (−8.1–+2.6, 7.9‰–12.0‰). These results suggest their derivation from partial melting of a heterogeneous source that dominated by supracrustal components. Furthermore, a few inherited grains give negative εHf(t) values of −9.4 to −18.4 with two-stage Hf model ages up to 2.9 Ga, suggesting the involvement of minor Archean crustal components. Along with their peraluminous features, high zircon δ18O values, and enriched Sr–Nd isotope compositions, these granites were unambiguously derived from partial melting of metasedimentary rocks and thus are of S-type affinity. Geothermometric and compositional approaches suggest these S-type granites were likely generated by dehydration melting of metagraywacke- and/or metapelite-dominated sources. They might be formed in an accretionary orogen associated with the Rodinia-assembly during the early Neoproterozoic, through which the outboard North Qinling terrane was eventually adjacent to the South China Block.