REE and PGE Geochemical Constraints on the Formation of Dunites in the Luobusa Ophiolite, Southern Tibet |
| |
Authors: | ZHOU MEI-FU; ROBINSON PAUL T; MALPAS JOHN; EDWARDS STEPHEN J; QI LIANG |
| |
Institution: | 1 DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF HONG KONG, POKFULAM ROAD, HONG KONG, CHINA
2 DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, UNIVERSITY OF GREENWICH AT MEDWAY, CHATHAM MARITIME ME4 4TB, UK |
| |
Abstract: | The Luobusa ophiolite, Southern Tibet, lies in the IndusYarlungZangbo suture zone that separates Eurasia to the north fromthe Indian continent to the south. The ophiolite contains awell-preserved mantle sequence consisting of harzburgite, clinopyroxene(cpx)-bearing harzburgite and dunite. The harzburgite containsabundant pods of chromitite, most of which have dunite envelopes,and the cpx-bearing harzburgites host numerous dunite dykes.Dunite also exists as a massive unit similar to those of themantlecrust transition zones in other ophiolites. Allof the dunites in the ophiolite have a similar mineralogy, comprisingmainly olivine with minor orthopyroxene and chromite and tracesof clinopyroxene. They also display similar chemical compositions,including U-shaped chondrite-normalized REE patterns. Mantle-normalizedPGE patterns show variable negative Pt anomalies. Detailed analysisof a chromite-bearing dunite dyke, which grades into the hostcpx-bearing harzburgite, indicates that LREE and Ir decrease,whereas HREE, Pd and Pt increase away from the dunite. Thesefeatures are consistent with formation of the dunite dykes byinteraction of MORB peridotites with boninitic melts from whichthe chromitites were formed. Because the transition-zone dunitesare mineralogically and chemically identical to those formedby such meltrock reaction, we infer that they are ofsimilar origin. The Luobusa ultramafic rocks originally formedas MORB-source upper mantle, which was subsequently trappedas part of a mantle wedge above a subduction zone. Hydrous meltsgenerated under the influence of the subducted slab at depthmigrated upward and reacted with the cpx-bearing harzburgitesto form the dunite dykes. The modified melts ponded in smallpockets higher in the section, where they produced podiformchromitites with dunite envelopes. At the top of the mantlesection, pervasive reaction between melts and harzburgite producedthe transition-zone dunites. KEY WORDS: meltrock interaction; REE; PGE; hydrous melt; mantle; ophiolite; Tibet |
| |
Keywords: | : melt rock interaction REE PGE hydrous melt mantle ophiolite Tibet |
本文献已被 Oxford 等数据库收录! |
|