Late Cretaceous (ca. 90 Ma) adakitic intrusive rocks in the Kelu area, Gangdese Belt (southern Tibet): Slab melting and implications for Cu-Au mineralization

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 (similar to 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 (Sr-87/Sr-86)(i) (0.7038-0.7039) ratios and high epsilon(Nd)(t) (+3.4 to +3.9) and in situ zircon epsilon(Hf)(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 (Sr-87/Sr-86)(i) (0.7040-0.7041) and epsilon(Nd)(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. (C) 2012 Elsevier Ltd. All rights reserved.