|
Yong-Fei Zheng, University of Science and Technology of China (China)
Zi-Fu Zhao, University of Science and Technology of China (China)
Qiong-Xia Xia, University of Science and Technology of China (China)
|
|
UHP metamorphic rocks commonly evolve under nominally anhydrous conditions and have been examined mainly in terms of phase relationships among crystalline minerals, but the important possibility of generation of hydrous melt/supercritical fluid phase by dehydration melting of hydrous minerals is increasingly being examined. Demonstrating that partial melting took place in UHP rocks is not easy because of extensive retrograde reaction and reequilibration of mineral assemglages during exhumation. In this contribution, we present petrographic identification of, with geochemical support to, partial melting has been obtained for the first time from UHP metamorphic rocks in the Dabie-Sulu orogenic belt, China. Breakdown of phengitic muscovite during the initial exhumation is considered to be responsible for the melting, with resultant variations in lithochemistry.
Two continuous core segments of about 3 m length from the Chinese Continental Scientific Drilling (CCSD) main hole in the Sulu orogen show large variations in the abundance of such elements as SiO2, LREE and LILE at the contact between high-T/UHP eclogite and granitic gneiss. This provides insight into melt-induced element mobility between different slab components. The presence of partial melts is corroborated by petrographic observations: (1) occurrence of biotite + plagioclase coronas around phengite in some samples of eclogite and gneiss, indicating melt generation by phengite breakdown; (2) aggregate of granitic minerals such as quartz, feldspar and biotite in some gneisses, suggesting the presence of hydrous granitic melt; (3) occurrence of felsic vein in some eclogites and gneisses, which may represent melt channelway.
Some low-T/UHP granitic gneiss in the Dabie orogen exhibits extremely low contents of FeO + MgO + TiO2 (1.04 to 2.08 wt%), high SiO2 contents of 75.33 to 78.23 wt%, and high total alkali (Na2O + K2O) contents (7.52 to 8.92 wt%). They are comparable with compositions predicted from partial melting of felsic rocks by experimental studies. Petrographic observations also provide evidence for the presence of partial melts: (1) breakdown of phengitic muscovite to biotite plus melt quartz, and (2) felsic veins of fine-grain minerals occurs locally between coarse-grain minerals. The local accumulation of felsic minerals suggests microscale transport of granitic melts relative to the host rocks. Nevertheless, the partial melts did not escape from the host gneiss, resulting in a kind of metatexite migmatites.
Partial melting of UHP metamorphic rocks containing hydrous minerals can not only play a crucial role in the initiation of rapid exhumation of UHP slabs, butalso provide an efficient way to enhance element mobility within slabs. Thus, understanding the extent, conditions and timing of partial melting in UHP metamorphic rocks in as many individual terranes as possible will provide important constraints on tectonic models for exhumation of UHP rocks.
|
