Trace Elements of Multi‐stage Minerals and Titanite U‐Pb Dating for the Gneisses from Liansan Island, Sulu UHPM Belt

Abstract

Gneisses with anatectic characteristics from the Liansan Island in Sulu UHPM belt were studied for petrography, titanite U‐Pb dating and mineral geochemistry. Three origins of garnets are distinguished: metamorphic garnet, peritectic garnet and anatectic garnet, which are formed in the stages of the peak metamorphism, retrograded anatexis and melt crystallization, respectively. The euhedral titanite has high content of REE and high Th/U ratios, which is interpreted to be newly formed from an anatectic melt. The LA‐ICP‐MS titanite U‐Pb dating yields 214∼217 Ma ages for the titanite (melt) crystallization. The distribution of trace elements varies in response to the different host minerals at different stages. At the peak metamorphic stage, Y and HREE are mainly hosted by garnet, Ba and Rb are by phengite, Sr, Nb, Ta, Pb, Th, U and LREE are by allanite, and Y, U and HREE are by zircon. During the partial melting, Y, Pb, Th, U and REE are released into melt, which caused dramatic decline of these element content in the retrograded minerals. At last, titanite absorbs mostly of Nb, U, LREE and HREE from the melt. Therefore, different stages of metamorphism have different mineral assemblages which host different trace elements.