Felsic veins within UHP eclogite at xitieshan in North Qaidam, NW China: Partial melting during exhumation
Felsic veins within the Xitieshan UHP eclogite in North Qaidam, NW China vary from 1cm or less to several decimetres in width, exhibiting coarse granular in texture and heterogeneous distribution in mineral assemblages. They encircle or cut the deformed sub-lentoid eclogite body. Some fine veins ext...
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| Published in: | Lithos 2012-04, Vol.136-139, p.187-200 |
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| Main Authors: | , , , , , , |
| Format: | Article |
| Language: | eng |
| Subjects: | |
| Online Access: | Get full text |
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| Summary: | Felsic veins within the Xitieshan UHP eclogite in North Qaidam, NW China vary from 1cm or less to several decimetres in width, exhibiting coarse granular in texture and heterogeneous distribution in mineral assemblages. They encircle or cut the deformed sub-lentoid eclogite body. Some fine veins extended into and ended in the eclogite, without visible link to their country rocks. They consist mainly of feldspar and quartz with minor amounts of muscovite, biotite and amphibole. To constrain their formation age, the conditions and source of vein-forming fluid/melt, and the relationship between the veins and host UHP eclogite, a combined study of zircon U–Pb dating and mineral inclusions was carried out on the felsic veins and their host eclogite. Zircon grains from the felsic veins are perfectly euhedral with zonations, characterized by very low Th/U ratios and steep HREE patterns without Eu anomalies. These zircons contain micro-inclusions of plagioclase, muscovite, garnet and quartz, which are consistent with mineral assemblage of the veins except for garnet captured from the host eclogite, indicating that these zircons crystallized during the veining. LA-ICPMS U–Pb dating yielded a weighted mean 206Pb/238U age of 428±2Ma, representing the formation age of veins. Zircon grains from the host eclogite are oval to round with homogeneous and/or cloudy internal-structure; residual core can be observed locally. They contain eclogite–facies micro-inclusions of garnet, omphacite, rutile and kyanite in metamorphic domains. LA-ICPMS U–Pb dating gave a metamorphic age of 458±2Ma. Geochemical analyses show that the felsic veins are tonalitic with high Na/K ratios, high Sr contents but low Y and Yb contents, and LREE-enriched patterns, consistent with partial melting of omphacite in eclogite at elevated pressures. Therefore, the felsic veins were most likely derived from partial melting of the host UHP eclogite. There is a considerable difference of about 30 Myr in zircon U–Pb age between the UHP eclogite–facies metamorphism and the felsic veining, indicating that the partial melting took place in the exhumation stage of the UHP terrain.
► Felsic veins occur within UHP eclogite in North Qaidam and show tonalitic composition. ► Zircon U–Pb dating yields an age of 428±2Ma for felsic veining and 458±2Ma for eclogitization. ► The felsic veins are derived from partial melting of the UHP eclogite during exhumation. |
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| ISSN: | 0024-4937 1872-6143 |