The zircon U–Pb and Hf isotope constraints on the basement nature and Paleozoic evolution in northern margin of Yili Block, NW China

Abstract

The northern margin of Yili Block in Xinjiang, China, consists of Precambrian basement, Early Paleozoic sedimentary rocks intersected with bimodal volcanic rocks, conglomerate and Late Paleozoic volcanic and sedimentary rocks. Five magmatic events are identified: ~2.5Ga, 1.5–1.4Ga, 1.1 to 0.9Ga, 480 to 450Ma and 410 to 350Ma based on SHRIMP zircon U–Pb ages of volcanic rocks and conglomerate in Tulasu Basin in west part of Northern Yili Block margin. The Precambrian ages record the evolution of the basement under Yili Block. The age population of 480 to 450Ma corresponds to the Early Paleozoic bimodal volcanic rock, that of 410 to 350Ma is documented by Late Paleozoic volcanic rocks.The εHf(t) value of ~2.5Ga zircon is 1.0, corresponding to TDM2 of 2.9Ga, indicating the old crust of Yili Block formed during 3.0 to 2.5Ga, which is also evidenced by TDM2 (3.2 to 2.6Ga) of 1.5–1.4Ga aged zircons. The old crust reworked at 1.5–1.4Ga, proved by εHf(t) (−16.3 to −1.3) value of the zircons. The 1.1 to 0.9Ga zircons show εHf(t) value of −7.6 to 2.7 and TDM2 of 2.3 to 1.7Ga, might be the remelting products of old crust with some contribution of mantle. The Precambrian age groups of Yili Block are coeval with Chinese Central Tianshan, Tarim Craton and Kyrgyz North Tianshan, suggesting that they may represent fragments of a single continent characterized by magmatic events at 1.5 to 1.4Ga and 1.1 to 0.9Ga.Bimodal volcanic rocks erupted at 480 to 450Ma, thus a rift-related tectonic setting was proposed in combination with reported geochemical characteristic. The Basaltic rocks with age of ~477Ma have εHf(t) value of 3.4 to 13.9, showing a magma source of depleted mantle with involvement of little crustal material. The felsic rocks with age of ~450Ma have εHf(t) value of −20.4 to 1.2, indicative of a continental crust source and little contribution of mantle magma. Their magma sources also show rift-related magma property. The volcanic rocks erupted during 410 to 350Ma were the products of subduction of the North Tianshan oceanic crust, which made contribution to crustal growth of northern Yili Block. The magma source of the Paleozoic volcanic rocks is depleted mantle contaminated by crust, constraint by their positive εHf(t) value (−0.2~12.8).