Mineralogy and geochemistry of the Zedong Late Cretaceous (-94 Ma) biotite granodiorite in the Southern Lhasa Terrane: Implications for the tectonic setting and Cu-Au mineralization

Ore deposits in the Southern Lhasa Terrane that formed prior to the India-Eurasia collision are rare. Among them, the formation of the Kelu and Sangbujiala skarn Cu-Au deposits in the Zedong area is associated with a magmatic "flare-up" in the Late Cretaceous (-95 Ma). However, the tectonic setting, geochemistry, and petrogenesis of the mineralization-related magma of these deposits are not well understood. The biotite granodiorite is the dominant phase in the Kelu and Sangbujiala skarn Cu-Au deposits, and this study reports a set of biotite granodiorite veins (Zircon U-Pb age: 94.1 +/- 0.4 Ma) without skarn alteration intruding into the cumulated hornblendite of the Late Jurassic Zedong Terrane. The rocks are adakitic, and have primitive Sr-Nd-Pb isotope compositions ((87Sr/86Sr)i = 0.7040, epsilon Nd(t) = +3.1, (206Pb/204Pb)t = 18.457, (207Pb/204Pb)t = 15.564, (208Pb/204Pb)t = 38.215) and zircon epsilon Hf(t) values (+9.7 -+13.5). The trace elements of the rocks indicate that they are derived from the magma that originated from the partial melting of the mantle wedge strongly influenced by slab-derived fluids and sediments, and underwent the fractional crystallization of hornblende and plagioclase. The biotite granodiorite magma is rich in water and Cl with high oxygen fugacity (approximate to Delta NNO+2 -Delta NNO+4). It crystallizes at low pressure (approximate to 157-200 MPa) in a closed system, and the crystallization temperature is inferred to be about 708-733 degrees C and 588 degrees C in the early and late stages, respectively. Based on the analysis of zircon data from this study and previous papers, we find that the number of magmatic zircons in the Late Cretaceous Southern Lhasa Terrane began to increase intensively at 95 Ma, accompanied by a leap in epsilon Hf(t) values and crystallization temperatures, as well as a sharp decline of (Ce4+/Ce3+) ratios. Those zircons with high epsilon Hf(t) values and crystallization temperatures, and low (Ce4+/Ce3+) ratios disappeared at 89 Ma. Therefore, we suggest that the magmatic "flare-up" in the Late Cretaceous (95-89 Ma) Southern Lhasa was triggered by mid-ocean ridge subduction, while the flat subduction occurred during 109-95 Ma and after 89 Ma.