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)(N) ratios of 13.24-29.51. In situ Hf analyses of zircon gave E-Hf(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.