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REE and PGE Geochemical Constraints on the Formation of Dunites in the Luobusa Ophiolite, Southern Tibet

REE and PGE Geochemical Constraints on the Formation of Dunites in the Luobusa Ophiolite,... The Luobusa ophiolite, Southern Tibet, lies in the Indus–Yarlung Zangbo suture zone that separates Eurasia to the north from the Indian continent to the south. The ophiolite contains a well-preserved mantle sequence consisting of harzburgite, clinopyroxene (cpx)-bearing harzburgite and dunite. The harzburgite contains abundant 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 the mantle–crust transition zones in other ophiolites. All of the dunites in the ophiolite have a similar mineralogy, comprising mainly olivine with minor orthopyroxene and chromite and traces of clinopyroxene. They also display similar chemical compositions, including U-shaped chondrite-normalized REE patterns. Mantle-normalized PGE patterns show variable negative Pt anomalies. Detailed analysis of a chromite-bearing dunite dyke, which grades into the host cpx-bearing harzburgite, indicates that LREE and Ir decrease, whereas HREE, Pd and Pt increase away from the dunite. These features are consistent with formation of the dunite dykes by interaction of MORB peridotites with boninitic melts from which the chromitites were formed. Because the transition-zone dunites are mineralogically and chemically identical to those formed by such melt–rock reaction, we infer that they are of similar origin. The Luobusa ultramafic rocks originally formed as MORB-source upper mantle, which was subsequently trapped as part of a mantle wedge above a subduction zone. Hydrous melts generated under the influence of the subducted slab at depth migrated upward and reacted with the cpx-bearing harzburgites to form the dunite dykes. The modified melts ponded in small pockets higher in the section, where they produced podiform chromitites with dunite envelopes. At the top of the mantle section, pervasive reaction between melts and harzburgite produced the transition-zone dunites. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Petrology Oxford University Press

REE and PGE Geochemical Constraints on the Formation of Dunites in the Luobusa Ophiolite, Southern Tibet

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References (62)

Publisher
Oxford University Press
Copyright
© The Author (2004). Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oupjournals.org
ISSN
0022-3530
eISSN
1460-2415
DOI
10.1093/petrology/egh091
Publisher site
See Article on Publisher Site

Abstract

The Luobusa ophiolite, Southern Tibet, lies in the Indus–Yarlung Zangbo suture zone that separates Eurasia to the north from the Indian continent to the south. The ophiolite contains a well-preserved mantle sequence consisting of harzburgite, clinopyroxene (cpx)-bearing harzburgite and dunite. The harzburgite contains abundant 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 the mantle–crust transition zones in other ophiolites. All of the dunites in the ophiolite have a similar mineralogy, comprising mainly olivine with minor orthopyroxene and chromite and traces of clinopyroxene. They also display similar chemical compositions, including U-shaped chondrite-normalized REE patterns. Mantle-normalized PGE patterns show variable negative Pt anomalies. Detailed analysis of a chromite-bearing dunite dyke, which grades into the host cpx-bearing harzburgite, indicates that LREE and Ir decrease, whereas HREE, Pd and Pt increase away from the dunite. These features are consistent with formation of the dunite dykes by interaction of MORB peridotites with boninitic melts from which the chromitites were formed. Because the transition-zone dunites are mineralogically and chemically identical to those formed by such melt–rock reaction, we infer that they are of similar origin. The Luobusa ultramafic rocks originally formed as MORB-source upper mantle, which was subsequently trapped as part of a mantle wedge above a subduction zone. Hydrous melts generated under the influence of the subducted slab at depth migrated upward and reacted with the cpx-bearing harzburgites to form the dunite dykes. The modified melts ponded in small pockets higher in the section, where they produced podiform chromitites with dunite envelopes. At the top of the mantle section, pervasive reaction between melts and harzburgite produced the transition-zone dunites.

Journal

Journal of Petrology – Oxford University Press

Published: Mar 3, 2005

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