Constraining the initiation of oceanic subduction of the Proto-Tethys Ocean beneath the Tarim block in northern Gondwana margin

The Proto-Tethys Ocean is principally defined as an ancient ocean distributed to the northern margin of the Gondwana landmasses, which initiated during the breakup of the Rodinia supercontinent and closed in the Early Paleozoic during the final assembly of Gondwana. Major continental blocks of China, including Tarim, Qaidam, South China and North China, were distributed in this ocean. Locally in the Altyn-Tagh UHP belt in the southeastern margin of the Tarim Craton, the ocean is referred to as the Altyn Ocean. The Kulamulake ophiolitic mélange occur within the South Altyn Terrane and was extensively sheared and deformed, with its southern and northern margins of the ophiolitic mélange delineated by a top to the northwest thrusting fault zone and a ductile shearing zone, respectively. The mélange thrust on to the latest Mesoproterozoic-Neoproterozoic Altyn group and Neoproterozoic-Paleozoic Bashikuergan group on its northern and southern margins, respectively. Stratigraphically from bottom to top it is composed of sheared serpentinite on the basal thrust, layered dunite-harzburgite, pyroxene peridotite, layered olivine pyroxenite and fine-grained meta-gabbro, along with exotic blocks of marble. Pillow basalt and plagio-granite have also been reported from within the mélange, which might be upper components of the ophiolite stratigraphy. All the exposed lithostratigraphic sequences occur as structural blocks. Therefore, overall lithologies and structures resembles those of ophiolitic mélanges. Meta-gabbro components of the mélange yield concordia ages of 518 ± 2 Ma, along with juvenile zircon Hf and whole rock isotopic signatures. The analyzed mafic-ultramafic samples display chemical characters that are comparable to E-MORB, but with some island-arc signatures, resembling those of SSZ type ophiolite. In addition, correlations of major and trace element compositions of all analyzed samples are indicative of fractional crystallization from a depleted mantle source. The overall lithological assemblages, isotopes and chemical compositions are consistent with a disrupted ophiolitic mélange during initial oceanic subduction environment. Therefore, we concluded the Kulamulake ophiolite recorded the initiation of oceanic subduction within the Paleo-Tethys Ocean in northern Gondwana margin. This research was supported by NSFC Projects (41730213 and 41190075) and Hong Kong RGC GRF (17307918 and 17301915).