Tectonic erosion and deep subduction in Central Tibet: Evidence from the discovery of retrograde eclogites in the Amdo microcontinent

Abstract

The Amdo microcontinent which separates the Qiangtang terrane to the north and the Lhasa terrane to the south is a key terrane for reconstructing the tectonic evolution of Central Tibet. We report the new finding of retrograde eclogites within the Amdo microcontinent in this study. The eclogites are characterized by peak metamorphic mineral assemblages of garnet, omphacite, rutile and quartz and underwent a four-stage metamorphic evolution, including a peak eclogite facies stage (M₁) at ~20–24 kbar and 580–620°C, followed by an HP granulite facies decompression stage (M₂) at ~13–15 kbar and 750–780°C, a subsequent MP-UHT granulite facies heating stage (M₃) at 8–10 kbar and >840°C and a final amphibolite facies retrogression (M₄) at 5.3–6.0 kbar and 560–580°C. The eclogites exhibit rare earth element distribution patterns and trace element abundances similar to those of N-MORB and arc-related volcanics, with depleted whole-rock ε_Nd(t) values of 3.4 to 4.2, and are inferred to have formed in a back-arc basin tectonic setting. Zircon and rutile U–Pb dating yields a protolith age of 226 ± 5 Ma, a peak eclogite facies metamorphic age of 190 ± 1 Ma, an HP granulite facies metamorphic age of 179 ± 1 Ma and an amphibolite facies retrograde age of 172 ± 1 Ma. The clockwise P–T–t paths and the oceanic protolith signature of retrograde eclogites suggest that part of the back arc basin was subducted to depths of ~80 km. Tectonic erosion associated with the subduction of the Amdo microcontinent beneath the Tethys Ocean accounts for the deep subduction of the back-arc basin and the absence of arc magmatic rocks in the northern Amdo microcontinent.