Contact Metamorphism Of The Early Mesozoic Weiya Granitic Ring Complex From The Chinese Eastern Tianshan

The Chinese Tianshan in the southwestern part of the Central Asian Orogenic Belt(CAOB) is characterized by a variety of high-grade metamorphic rocks, which provide critical constraints for understanding the geodynamic evolution of the CAOB. Among these metamorphic rocks, the Weiya granulites have long been considered to represent a regional metamorphic orogenic event, which was interpreted to be related to an early Paleozoic plate subduction process or to display the Paleoproterozoic metamorphic basement of the Chinese Eastern Tianshan. The key point is that the metamorphic ages and metamorphic evolution of the Weiya granulites are quite controversial and lack of in-depth understanding. Based on predecessors’ research results, in this paper, we present a detailed petrology, metamorphic P–T conditions and zircon U–Pb geochronology of the Weiya granulite- and amphibolite-facies rocks in order to resolve these questions.The Weiya granulite-facies rocks which occur as lenses close to the west of the Weiya granitic ring complex, have a variety of felsic–pelitic and mafic granulites with different textural equilibrium mineral assemblages including garnet–cordierite–sillimanite-bearing granulites, garnet–sillimanite-bearing granulites, cordierite–sillimanite-bearing granulites, cordierite–orthopyroxene-bearing granulites and orthopyroxene–clinopyroxene-bearing granulites. Average P–T thermobarometric calculations and conventional geothermobarometry indicates that the Weiya granulites underwent early prograde metamorphism under conditions of 600–650 °C at 3.2–4.2 kbar and peak metamorphism of 750–840 °C at 2.9–6.3 kbar, indicating a rather high geothermal gradient of ca. 60 °C/km. Zircon U–Pb LA-ICP-MS dating revealed metamorphic ages between 244 ± 1 to 237 ± 3 Ma, which are in accordance with thecrystallization age of the Weiya granitic ring complex, showing that the Weiya granulites, formed at contact metamorphism of the surrounding rocks, caused by contemporaneous granitic ring complex intrusion. In addition, a Late Devonian metamorphic age of ca. 380 Ma was recorded in zircon mantle domains from two pelitic samples which is consistent with the metamorphic age of the Xingxingxia metamorphic complex in the Chinese Eastern Tianshan. This suggests that the Weiya granulites probably formed during the Late Devonian regional metamorphism, overprinted by the Early Triassic contact metamorphism. Moreover, far away from the Weiya granitic ring complex, there also exposed many lenses as product of the lower contact metamorphism, including amphibolite, sillimanite–cordierite schists and cordierite–anthophyllite schists, indicating that the contact metamorphic degree of the Weiya granitic ring complex gradually reduced outwards.