Petrogenesis of the Late Cretaceous–Early Paleocene High-Si Igneous Rocks in the Western Part of Southern Lhasa Terrane: Implications for the Tectonic Setting and Mineralization

Abstract

Many Late Cretaceous–Early Paleocene high-silica igneous rocks, associated with Pb-Zn-Ag deposits, were identified in western part of southern Lhasa terrane. The Dajiacuodong muscovite granite as one of the high-silica granites was dated by zircon U-Pb method and determined for whole-rock major and trace elements and zircon trace elements. Combined with previous data of other high-silica igneous rocks, petrogenesis, tectonic setting, and factors controlling Pb-Zn-Ag mineralization were studied. The muscovite granite emplaced at 68.2 ± 0.3 Ma and is peraluminous. All high-silica igneous rocks are S-type and whole-rock CaO, Na₂O, and Rb-Sr-Ba and zircon Hf isotopic compositions show that they were formed by partial melting of meta-sedimentary protoliths from the Lhasa terrane basement. Roll-back of the Neo-Tethys oceanic slab caused melting of the mantle wedge is the trigger for partial melting of basement. Zircon trace elements indicate that the high-silica igneous rocks are reduced and H₂O- and F-enriched. These geochemical features and magma sources of the high-silica igneous rocks are comparable with the W-Sn deposit related granites. Shallow emplacement of high-silica granites and coeval volcanic eruption induced rapid escape of fluids. This caused inefficiency of hydrothermal–magmatic interaction, which are reasons to form Pb-Zn-Ag rather than W-Sn deposits.

Keywords

Late Cretaceous–early Paleocene / high-silica igneous rocks / S-type / metal association / southern Lhasa terrane

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2025 Geological Society of China