Shallow subduction of Indian slab and tectono-magmatic control on post-collisional porphyry mineralization in southeastern Tibet

Abstract

The 1500-km long Gangdese belt in southern Tibet shows uneven distribution of giant to large porphyry deposits over 45 Mt Cu mainly in the middle-east segment between longitude 87–92°E. However, the western (80–86°E) and eastern end (92–96°E) is generally barren. The recent discovery of Demingding, Cuibaizi and Tangbula porphyry deposits in the east of 92°E has attracted great attention on their origin. Recent high-resolution seismic imaging shows the torn Indian lithosphere subducts with diverse angle. In contrast to the middle-east segment with moderate angle Indian plate subduction, the southeastern Tibet is characterized with a shallow dip of subduction (eastern of Comei rift), where Demingding, Cuibaizi and Tangbula deposits are located. To reveal the formation mechanism of these deposits as well as the Indian slab motion pattern beneath them, we compiled reported whole rock major and trace elements, Sr and Nd isotope signature, zircon trace element, U-Pb ages and Hf isotope data of Miocene ore-related adakitic rocks from Demingding and Tangbula to the north, and Oligocene ore-barren adakitic rocks from Linchi to the south. There is a clear negative correlation between zircon U-Pb ages of rocks and their distance to Indus-Yarlung Zangbo suture: Linchi is around 20 km away from the suture, with an average U-Pb age of 27.1 Ma, while Demingding and Tangbula rocks are 50 and 90 km away from the suture, with ages of 20.3 and 19.7 Ma respectively. Granitoids from Linchi have less compatible element Ni and Cr contents of 4.07 and 5.84 ppm on average respectively, while those of Demingding (20.1 and 43.4 ppm) and Tangbula (5.52 and 7.90 ppm) are much higher. Ni and Cr as compatible elements are concentrated in mafic minerals, and are thus proper indicators for mantle imprint indicators. In addition, as shown by Sr-Nd isotopes, Linchi granitoids have features of ancient crust [(87Sr/86Sr)i = 0.7059 to 0.7085, εNd(t) = − 6.20 to −2.90], while Demingding and Tangbula granitoids show less radiogenic features [(87Sr/86Sr)i = 0.7059 to 0.7068, εNd(t) = − 3.27 to −1.70], which are more similar to coexisting juvenile arc. Demingding and Tangbula granitoids also show signature of high fO2 and hydrous melt condition, as indicated by calculated fO2 of ΔFMQ + 2, and amphibole fractionation indicated by decreasing Dy/Yb with SiO2, since amphibole favors MREE (Dy) rather than HREE (Yb). Combining above, our work shows that the Indian slab constantly subducted shallowly beneath these deposits, given no mantle-like signature of near-suture Oligocene adakitic rocks. We suggest the cause to the formation of Demingding, Tangbula and Cuibaizi deposits is the reactivation of pre-enriched arc root materials, triggered by constant northward migration of Indian slab. This is supported by similar isotope signature between Miocene Demingding and Tangbula ore-related porphyries and coexisting arc, and absence of coeval pre-collisional porphyry deposits. The essential components for these porphyry deposits, high fO2 and water content, were also likely contributed from the pre-enriched juvenile arc root.