Late Cretaceous (ca. 90 Ma) adakitic intrusive rocks in the Kelu area, Gangdese Belt (southern Tibet): Slab melting and implications for Cu–Au mineralization
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The Gangdese Belt in southern Tibet (GBST) is a major Cu–Au–Mo mineralization zone that mostly formed after the India–Asia collision in association with the small-volume, though widespread, Miocene(18–10 Ma) adakitic porphyries. Cu–Au mineralization has scarcely been found in the regional Jurassic–Early Tertiary batholiths related to subduction of the Neo-Tethyan oceanic plate. Here, we report petrological, zircon geochronological and geochemical data for Late Cretaceous (90 Ma) intrusive rocks that contain Cu–Au mineralization from the Kelu area in the GBST. These rocks consist of quartz monzonites and diorites. The quartz monzonites, with SiO2 of 58–59 wt.% and Na2O/K2O of 1.1–1.2, are geochemically similar to slab-derived adakites characterized by apparent depletions in heavy rare earth elements (e.g., Yb = 1.4–1.5 ppm) and Y (16–18 ppm) contents, positive Sr but negative Nb and Ti anomalies on multi element variation diagrams. They have relatively low (87Sr/86Sr)i (0.7038–0.7039) ratios and high εNd(t) (+3.4 to +3.9) and in situ zircon εHf(t) (+9.3 to +15.8) values.The diorites exhibit high Mg-numbers (0.57–0.61) similar to those of magnesian andesites, and have (87Sr/86Sr)i (0.7040–0.7041) and εNd(t) (+3.0 to +4.4) values similar to those of the quartz monzonites. We suggest that the quartz monzonitic magmas were most likely generated by partial melting of the subducted Neo-Tethyan basaltic oceanic crust and minor associated oceanic sediments, with subsequent melt–mantle interaction, and the dioritic magmas were mainly derived by the interaction between slab melts and mantle wedge peridotites, with fractionation of apatite and hornblende. These slab-derived adakitic magmas have high oxygen fugacity that may have facilitated Cu–Au mineralization. The close association of the Late Cretaceous adakitic intrusive rocks and Cu–Au mineralization in the Kelu area suggests that the arc magmatic rocks in the GBST may have higher potential than previously thought for Cu–Au mineralization.
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