PETROGENESIS OF THE ZHONGBA GRANITE, SOUTHERN TIBET: CONSTRAINTS ON THE SUBDUCTION DEPTH OF INDIAN CONTINENT

The Middle Eocene Zhongba pluton in the western part of the Gangdese magmatic belt, southern Tibet, had been reported by He et al. (2018). Geochemical and isotopic characteristics suggest mixing of different magma types, derived from partial melting of juvenile southern Lhasa crust and ancient Indian continental crust, to produce hybrid magmas of the Zhongba granitoids. We propose that asthenospheric upwelling caused by the breakoff of the northward subducting Neotethyan oceanic lithosphere provided the heat and material flux to trigger partial melting of the overlying Lhasa arc lithosphere and subducted sediments of the ancient Indian crust.

Zhongba granites offer magmatic evidence for that Indian crust probably had already reached a 100 km depth at late Eocene. The research on eclogites in the western Himalayas indicated that the Indian crust had already reached a 100 km depth equivalent at the beginning of collision, ca. 50-55 Ma ago. Meanwhile, the coesite-bearing UHP eclogites from Tso Morari Complex, India, and Kaghan,Pakistan, are evidence that the leading edge of the entire northwestern part of the Indian continental margin was subducted beneath the Kohistan–Ladakh arc to a minimum depth of 90 km or even at least 130 km. Zircon saturation temperature analysis suggests that the temperature of the hybrid magma parental to the Zhongba, Kailas and Zedong plutons (697-812^oC) is identical to the metasedimentary rocks of the Dora Maira Massif (700-800 ^oC), Western Alps, which were formed under metamorphic conditions at a depth of subduction of 100 km. Based on the experiment results, Xiong et al. (2006) proposed that the depth range for adakite production is ~50–85 km (1.5–2.5 GPa) under fluid-absent condition and ~50–100 km (1.5–3.0 GPa) under fluid present condition. All the samples from these three plutons without adakitic affinities might be form at a depth more than 100km, which provide the magmatic evidence for the conclusion that Indian crust had already reached a 100 km depth at late Eocene.