The zircon U-Pb age for the Neoproterozoic-Ordovician granites in the Kesang Rongdong region, southwestern Tianshan Mts., Xinjiang
西南天山科桑溶洞新元古代-早奥陶世花岗岩锆石U-Pb年代学研究

Neoproterozoic and Early Ordovician granite intruded into amphibolite has been identified in the Kesang Rongdong region, southwestern Tianshan Mts. Zircons in the gneissic muscovite granite have hydrothermal rims, which replaced their igneous mantles with detrital cores. SHRIMP analysis on these igneous zircon mantles(without mixture of hydrothermal rim and detrital core)gives an average U-Pb age of 752.3±5.1Ma(MSWD=0.95). This age represents the magma intrusion episode. Hydrothermal zircon rim formed in the Early Ordovician granite in some cases though most zircons do not show continuous hydrothermal rim. One euhedral zircon crystal consists of detrital core and hydrothermal rim, the latter has much younger U-Pb age of 419.5±5.7Ma comparing with most other measurements. In many other cases, however, zircon is composed with igneous rim and detrital core. The average U-Pb age, corresponding to igneous zircon rims(481.1±4.4Ma, MSWD=0.88), represents a magma intrusion episode in the Kesang Rongdong region. Thus, the gneissic muscovite granite and amphibolite have been intruded by the Early Ordovician granite. Afterwards, these different granites have undergone metamorphism as recorded by the hydrothermal zircon, which replaced igneous zircons.