Zircon U–Pb and muscovite 40Ar/39Ar geochronology of the gold-bearing Tianger mylonitized granite, Xinjiang, northwest China: Implications for radiometric dating of mylonitized magmatic rocks

This study focuses on zircon U–Pb and muscovite 40Ar/39Ar geochronology of gold-bearing mylonitized granite in the north Tianshan of China. All zircon samples have euhedral hydrothermal rims, which have replaced their igneous mantles. Igneous zircon mantles with narrow hydrothermal rims in sample TS06 give a weighted average U–Pb age of 403.7 ± 5.6 Ma (1 standard error, MSWD = 1.5, n = 18). Zircons in TS277 show a complex texture with core (detrital origin, 1276–2215 Ma) indicating igneous mantle and a hydrothermal rim. The igneous mantles give a weighted average U–Pb age of 443.4 ± 3.3 Ma (MSWD = 3.1, n = 10). Two younger ages (394 and 400 Ma) have been obtained on hydrothermal rims. The mantles plus rims of all zircons have positive εHf(t) values with an average of 3.99 ± 0.86 (MSWD = 2.8, n = 23). The positive εHf(t) values suggest that the granite magma is derived from melting of juvenile crustal rocks. The shearing deformation post-dated granite intrusion and introduced fluid, which replaced igneous zircons via dissolution and reprecipitation. However, the age data for zircon hydrothermal rims do not represent the time of the dissolution re-precipitation. The zircons crystallized in granite in the Early Silurian (~ 443 Ma) have been replaced on their rims during Middle Triassic deformation (248–238 Ma), which was followed by brittle deformation accompanied by gold deposition at ~ 220 Ma, based on 40Ar/39Ar dating of muscovite.