UPb zircon geochronology of Tianshan eclogites in NW China: implication for the collision between the Yili and Tarim blocks of the southwestern Altaids

Abstract

Zircon grains separated from an eclogite boudin intercalated within blueschists in Tianshan (NW China) were investigated by Raman spectroscopy, cathodoluminescence imaging and secondary ion mass spectrometry (SIMS). These grains have a thin homo- geneous rim and an oscillatory inner zone domain with or without a relict inherited core. Omphacite, phengite and rutile inclusions were identified within the rim domain of zircon grains only, indicating that the rim had formed during peak eclogite-facies metamorphism. U-Pb zircon data record three distinct age populations: 2450 � 31 to 1956 � 26 Ma (for the inherited core), 486 � 8 to 428 � 6 Ma (oscillatory inner zone) as well as 319.5 � 2.9 Ma and 318.7 � 3.3 Ma (for the metamorphic rim). These new ages suggest that the metamorphism associated with the collision of the Yili and the Tarim blocks along the southwestern margin of the Altaids must have occurred during the Late Carboniferous (ca., 320 Ma).