SHRIMP U–Pb dating, trace elements and the Lu–Hf isotope system of coesite-bearing zircon from amphibolite in the SW Sulu UHP terrane, eastern China

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• 作者：Fulai Liu ; Axel Gerdes ; Lingsen Zeng ; Huaimin Xue
• 关键词：Lamiaceae ; Salvia hydrangea ; Essential oil ; Camphor ; Antifungal ; Antibacterial ; Insecticidal ; Sitophilus granarius ; Tribolium confusum
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2008
• 出版时间：15 June 2008
• 年：2008
• 卷：72
• 期：12
• 页码：2973-3000
• 全文大小：2764 K

文摘

In this study, we link mineral inclusion data, trace element analyses, U–Pb age and Hf isotope composition obtained from distinct zircon domains of complex zircon to unravel the origin and multi-stage metamorphic evolution of amphibolites from the Sulu ultrahigh-pressure (UHP) terrane, eastern China. Zircon grains separated from amphibolites from the CCSD-MH drill hole (G12) and Niushan outcrop (G13) were subdivided into two main types based on cathodoluminescence (CL) and Laser Raman spectroscopy: big dusty zircons with inherited cores and UHP metamorphic rims and small clear zircons. Weakly zoned, grey-white luminescent inherited cores preserve mineral inclusions of Cpx + Pl + Ap ± Qtz indicative of a mafic igneous protolith. Dark grey luminescent overgrowth rims contain the coesite eclogite-facies mineral inclusion assemblage Coe + Grt + Omp + Phe + Ap, and formed at T = 732–839 °C and P = 3.0–4.0 GPa. In contrast, white luminescent small clear zircons preserve mineral inclusions formed during retrograde HP quartz eclogite to LP amphibolite-facies metamorphism (T = 612–698 °C and P = 0.70–1.05 GPa). Inherited zircons from both samples yield SHRIMP ²⁰⁶Pb/²³⁸U ages of 695–520 Ma with an upper intercept age of 800 ± 31 Ma. The UHP rims yield consistent Triassic ages around 236–225 and 239–225 Ma for G12 and G13 with weighted means of 229 ± 3 and 231 ± 3 Ma, respectively. Small clear zircons from both samples give ²⁰⁶Pb/²³⁸U ages around 219–210 Ma with a weighted mean of 214 ± 3 Ma, interpreted as the age of retrograde quartz eclogite-facies metamorphism. Matrix amphibole from both samples indicate Ar–Ar ages of 209 ± 0.7 and 207 ± 0.7 Ma, respectively, probably dating late amphibolite-facies retrogression. The data suggest subduction of Neoproterozoic mafic igneous rocks to UHP conditions in Middle Triassic (230 Ma) times and subsequent exhumation to an early HP (214 Ma) and a late LP stage (208 Ma) over a period of 16 and 6 Myr, respectively. Thus, early exhumation from a mantle depth of 120–100 km to about 60 km occurred at an average rate of 0.3 cm/y, while subsequent exhumation to a middle crustal level took place at approximately 0.54 cm/y. These exhumation rates are considerably slower than those obtained for UHP rocks in the Dora Maira and Kokchetav massifs (2–3 cm/y).