Tectonic evolution of the East Junggar terrane: Evidence from the Taheir tectonic window, Xinjiang, China

The Taheir tectonic window is located at the southern part of the Yemaquan magmatic arc in East Junggar. This paper reports detailed petrology, mineral chemistry, whole rock and Sr–Nd isotope geochemistry, hornblende 40Ar/39Ar dating, in-situ zircon U–Pb dating and Hf isotope analyses from igneous and meta-igneous rocks from this region. Our new results show that the Taheir tectonic window consists of metamorphic and deformed Ordovician volcanic rocks and granitic porphyries, Ordovician–Silurian granites and undeformed Silurian–Devonian granitic diorite, diorites and rhyolitic porphyries. The Ordovician volcanic rocks and granitic porphyries and Ordovician–Silurian granites in the Taheir tectonic window exhibit distinct features of Andean-type continental arc, such as enrichment in Pb, K and U, depletion in Nb, P and Ti, negative Eu anomalies, high La/Yb, Th/Yb and Ta/Yb values, a high proportion of dacite, rhyolite and andesite of the calc-alkaline series, massive contemporary granitic intrusions, mixtures of the juvenile material and > 2.5 Ga crust, and extensive crystallization differentiation. These Ordovician volcanic rocks witnessed a series of tectonic events, including burial associated with the intrusion of 454–449 Ma granitic porphyries, underthrusting and subsidence to a depth in the middle crust associated with the intrusion of 443–432 Ma granites. The formation of albite–hornblende schists, hornblende–albite–quartz leptynites and amphibolites, the transformation from continental to continental island arc at approximately 432 Ma, the exhumation associated with the intrusion of 416–406 Ma diorites with geochemical signatures of continental island arc, and exhumation and erosion between 398 Ma and 390 Ma are also identified. The arc types that are associated with the Taheir tectonic window and its host, the Yemaquan magmatic arc, changed from Andean-type continental arc to continental island arc after the intra-arc rifting that began at 432 Ma. On the basis of the new evidence, the tectonic regime of the East Junggar terrane is redefined and a new model is proposed. It is suggested that the East Junggar terrane is related to the southward subduction of the Paleo-Asian ocean plate beneath the Junggar continent in the early Paleozoic and later shift to intra-oceanic subduction.