Zircon U–Pb–Hf isotopes and bulk-rock geochemistry of gneissic granites in the northern Jiamusi Massif, Central Asian Orogenic Belt: implications for Middle Permian collisional orogeny and Mesoproterozoic crustal evolution.

We conducted bulk-rock geochemical and zircon U–Pb–Hf isotope analyses of gneissic granites from the northern Jiamusi Massif, in order to investigate their sources, petrogenesis, and geodynamic significance. Zircons contain fine-scale oscillatory zoning with high Th/U ratios (0.12–0.80), indicative of a magmatic origin. U–Pb dating indicates that they formed during the middle Permian (264 ± 4.2 Ma to 268.6 ± 0.81 Ma), and contain a 531 Ma inherited zircon core. The gneissic granites have high contents of K2O + Na2O and Al2O3, and low MgO and CaO. With A/CNK ratios of 1.01–1.11, they are classified as peraluminous calc-alkaline to alkaline S-type syn-collisional granites. Enriched in large ion lithophile elements such as K, Rb, Ba, and Th, they are depleted in high field strength elements such as Nb, Ta, and Ti. Bulk-rock geochemical analyses display moderately negative Eu anomalies (Eu/Eu*) of 0.36–0.52, and (La/Yb)Nratios of 13.24–29.51.In situHf analyses of zircon gave ϵHf(t) values of –0.1 to –3.2 and crustal average model ages () of 1.35–1.4 Ga. These data suggest that the gneissic granites formed by partial melting of Mesoproterozoic continental crust during a middle Permian collisional event. Hf isotopes indicate the substantial presence of Mesoproterozoic juvenile crust in the northern Jiamusi Massif, derived from depleted mantle that underwent crustal contamination at 1.4–1.35 Ga. The Jiamusi Massif provides a local record of the assimilation and subsequent rifting of the Rodinia–Gondwana supercontinent. [ABSTRACT FROM AUTHOR]