Diverse metavolcanic sequences in the Cambrian accretionary complex at the Pamir Syntax: Implications for tectonic evolution from Proto-Tethys to Paleo-Tethys

Abstract

The Pamir Plateau contains critical information on the tectonic evolution from the Proto- to Paleo-Tethys. In this contribution, we report geochronology, geochemistry and Sr-Nd isotopic compositions of diverse metavolcanic rocks in the Bulunkuole Group (BG) in the NE Pamir, to construct the architecture of the thick Cambrian volcanic-sedimentary sequence in the NE Pamir and to provide new insights into the tectonic evolution from Proto-Tethys to Paleo-Tethys. Our newly obtained zircon U-Pb ages combined with ages from literature reveal that these volcanic-sedimentary sequences were deposited during the Cambrian and metamorphosed during the Early Jurassic. The bimodal volcanic rocks (ca. 530 Ma) at the lowest BG show typical arc-like geochemical features, representing the initial arc produced by the southward subduction of the Proto-Tethys Ocean. Amphibolites (ca. 526 Ma) from the Maeryang area show an OIB-like affinity with high Nb/La ratios (0.75–1.01) and positive εNd(t) values (4.19–4.76), representing a seamount component in the Proto-Tethys Ocean. High-pressure mafic granulites to the north of Tashkorgan County exhibit E-MORB geochemical features. Combined with recent advances in the NE Pamir geology, we suggest that the southward subduction of the Proto-Tethys slab under the Mazar-Tianshuihai terrane initiated earlier than 530 Ma. Subsequent trench retreat led to the formation of the massive early Paleozoic accretionary wedge (BG) followed by the northward migration of arc magmatism to the BG. The Proto-Tethys Ocean at Pamir did not close in the early Paleozoic. Instead, it was finally closed during the early Mesozoic as revealed by the Early Jurassic amphibolite-facies metamorphism in the BG.