Lithospheric Architecture of the Lhasa Terrane and Its Control on Ore Deposits in the Himalayan-Tibetan Orogen

Abstract

Magmatic-hydrothermal ore deposits in collisional orogens are new targets for modern mineral exploration, yet it is unclear why they preferentially occur in some specific tectonic environments within these orogenic belts. We integrate geologic and geochemical data (especially zircon U-Pb dating and Lu-Hf isotope data) for Mesozoic-Cenozoic magmatic rocks and associated ore deposits in the Lhasa terrane, a highly endowed tectonic unit within the Himalayan-Tibetan orogen, and provide the first example in a continental collision terrane of the application of zircon Hf isotope data to image the lithospheric architecture and its relationship with ore deposits.Three crustal blocks are identified within the Lhasa terrane by the Hf isotope mapping method. They include a central long-lived Precambrian microcontinent with local reworking and two surrounding juvenile Phanerozoic crustal blocks with significant mantle contributions to constituent magmatic rocks. The three crustal blocks are bounded by two E-W-trending terrane-boundary faults, and each block is cut by two N-S-striking concealed faults. Isotopic signatures of zircons from the juvenile crustal blocks indicate that the Phanerozoic continental crust grew from several Mesozoic volcanic-plutonic arcs and by underplating of mantle-derived magmas generated during Mesozoic accretion and Cenozoic collision.Mesozoic subduction-related porphyry Cu-Au deposits and Cenozoic collision-related Cu-Mo deposits are exclusively located in regions with high eHf (>5) juvenile crust. Cu enrichment during differentiation of high fO 2 arc magmas is the key for the formation of Mesozoic subduction-related porphyry Cu-Au. By contrast, remelting of the lower crustal Cu sulfide-rich magmatic cumulates within the juvenile crust is interpreted to have played a key role in the formation of Cenozoic collision-related Cu-Mo deposits.Granite-related Pb-Zn deposits cluster in the oldest crustal regions or developed along the margin of the old crustal block bounded by lithospheric faults. The porphyry Mo deposits are localized along the reworked margins of the old crustal block. It is suggested that crustal reworking released Mo from the old crust to form porphyry Mo deposits, whereas leaching of Pb and Zn from the Paleozoic carbonate cover strata by felsic intrusion-driven fluids is critical to the formation of Pb-Zn ore deposits.Skarn Fe-Cu ore deposits are typically localized along a terrane boundary fault, i.e., lithospheric discontinuity, through which crust-derived felsic melt mixed with Cu-rich mantle-derived mafic magmas ascending upward. Associated granitoid rocks usually bear microgranular mafic enclaves and show a zircon Hf isotope array from negative to positive eHf values (-7.3 to +6.7), supporting mixing of juvenile mantle and evolved crustal sources.The Hf isotope maps show temporal-spatial relationships between crustal structure and the location of ore deposits, demonstrating that the structure, nature, and composition of the crust controlled the localization of ore deposits and ...

Slab Breakoff of the Neo‐Tethys Ocean in the Lhasa Terrane Inferred From Contemporaneous Melting of the Mantle and Crust

“…However, to understand the regional evolution, it is crucial to determine whether ancient lithosphere existed beneath the Lhasa Terrane. Previous studies suggested there was ancient lithosphere beneath the Lhasa Terrane, based on the Nd, Pb, and Hf isotopic model ages of postcollisional potassic and ultrapotassic rocks (Hou et al, ; Miller et al, ; Turner et al, ). Zhu et al () stated that the Lhasa Terrane has ancient basement rocks with Proterozoic and Archean ages in its center, with younger or juvenile Phanerozoic crust accreted to its northern and southern edges.…”

Early cretaceous topographic growth of the Lhasaplano, Tibetan plateau: Constraints from the Damxung conglomerate

“…The North Lhasa subterrane [including the central and north Lhasa subterranes of Zhu et al []] also documents long‐term Mesozoic–Cenozoic magmatism, associated with either southward subduction of Bangong‐Nujiang oceanic lithosphere in the north [e.g., Zhu et al, ] or northward subduction of the Neotethyan oceanic lithosphere in the south [e.g., P. Kapp et al , ]. Geochronological studies have revealed a magmatic flare up at 113 ± 5 Ma [ Hou et al , ]. Igneous rocks in the North Lhasa subterrane commonly yield enriched Hf isotopes indicating re‐melting of older crust [e.g., Chu et al , ; Chiu et al , ; Zhu et al , ].…”

Xenoliths in ultrapotassic volcanic rocks in the Lhasa block: direct evidence for crust–mantle mixing and metamorphism in the deep crust

“…1). Recently, Hf isotope mapping of the whole Lhasa terrane revealed three distinct crustal blocks: a Precambrian microcontinent in the center (central Lhasa subterrane) and reworked juvenile Mesozoic crustal blocks on the two sides (the northern and the southern Lhasa subterrane, Zhu et al 2011a andHou et al 2015). The Precambrian central Lhasa subterrane has Archean and Proterozoic crystalline basement, which rifted from the Gondwana margin in the Carboniferous-Middle Permian ).…”