Identification of ca. 850 Ma high-temperature strongly peraluminous granitoids in southeastern Guizhou Province, South China: A result of early extension along the southern margin of the Yangtze Block

Strongly peraluminous granites are characterized by high A/CNK values (>1.1) and are important for understanding the regional tectonic regime. Here, we present integrated studies of zircon U-Pb ages, trace elements and Lu-Hf isotopes, whole-rock major and trace elements, and Nd isotope compositions for the newly discovered Nage granite porphyry in southeastern Guizhou Province. The porphyry has a crystallization age of 852 ± 5 Ma and shows wide ranges in SiO2 (64.5-72.51 wt%) and Al2O3 (13.08-19.34 wt%) contents, with low TiO2 (0.41-0.84 wt%) and P2O5 (<0.15 wt%) contents. All the samples are strongly peraluminous with A/CNK >1.1, and they have similar trace element patterns with depletion in high field strength elements (HFSE, e.g., Nb and Ta) and enrichment in large-ion lithophile elements (LILE, e.g., Rb, Th and U). Initial zircon Hf isotopes of the porphyry vary from εHf(t) = −7.6 to −0.6, whereas whole-rock Nd isotopes are relatively uniform with εNd(t) = −6.0 to −6.7. They record high zircon saturation temperatures (Tzr) (786-842 °C, mostly >810 °C), but low oxygen fugacity (log fO2 = −16 to −25, corresponding to ΔFMQ −3 to ΔFMQ −7) and emplacement pressure of <0.5 GPa. Major and trace elements, 10,000 Ga/Al ratios and Zr + Nb + Ce + Y values, show good correlations with Tzr, indicating that fractional crystallization may have played an important role. Although these granites show low P2O5 contents that decrease with increasing Rb, the high A/CNK (>1.1), K2O/Na2O (0.7-2.6; average 1.4) and CaO/Na2O (0.2-0.8; mostly >0.5) ratios suggest that clay-poor metasedimentary rock were dominant in the source. Combined with the Nd-Hf isotopes, we suggest that the Nage granite porphyry formed by anatexis of an immature arkose and greywacke source, which was a mixture of weathered juvenile igneous materials and minor recycled ancient crustal components. The ca. 850 Ma strongly peraluminous granites, along with coeval extension-related magmatism along the southwestern margin of the Yangtze Block, indicate an extensional environment started as early as ca. 850 Ma.