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Podiform Chromitites in the Luobusa Ophiolite (Southern Tibet): Implications for Melt-Rock Interaction and Chromite Segregation in the Upper Mantle |
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| Authors: | ZHOU MEI-FU; ROBINSON PAUL T; MALPAS JOHN; LI ZIJIN |
| Institution: | 1CENTRE FOR MARINE GEOLOGY, DALHOUSIE UNIVERSITY HALIFAX, N.S., CANADA B3H 3J5 2DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF HONG KONG HONG KONG 3DEPARTMENT OF MINERAL RESOURCES, CHINA UNIVERSITY OF GEOSCIENCES WUHAN, 430074, PEOPLE'S REPUBLIC OF CHINA |
| Abstract: | The Luobusa ophiolite in the Indus—Yarlung Zangbo sutureof southern Tibet hosts the largest known chromite deposit inChina. The podiform chromitites occur in a well-preserved mantlesequence consisting of harzburgite with abundant lenses of dunite.The harzburgites have relatively uniform bulk-rock compositionswith mg-numbers 100 Mg/(Mg + Fe)] ranging from 89 to 91 andshow flat, unfractionated, chondrite-normalized platinum groupelement (PGE) patterns. Their accessory chromite varies widelyin cr-number 100Cr/(Cr + Al)] (18–66). These rocks areessentially residua left after extraction of mid-ocean ridgebasalt (MORB)-type magmas. The podiform chromitites displaynodular, massive, disseminated and banded textures and typicallyhave dunite envelopes that grade into the surrounding harzburgiteand diopsidic harzburgite with increasing pyroxene contents.They consist of relatively uniform chromite with high cr-numbers(74–82), have strongly fractionated, chondrite-normalizedPGE patterns with enrichment in Os, Ir and Ru relative to Rh,Pt and Pt, and are believed to have formed from a boniniticmagma produced by a second stage of melting. Dunites containaccessory chromite intermediate in composition between thoseof harzburgite and chromitite and are believed to be the productsof reaction between new boninitic magmas and old MORB-type peridotites.The melt-rock reaction removed pyroxene from the peridotitesand precipitated oli-vine, forming dunite envelopes around thechromitite pods. The melts thus became more boninitic in compositionand chromite saturated, leading to precipitation of chromitealone. The interplay of melt-rock interaction, chromite fractionationand magma mixing should lead to many fluctuations in melt composition,producing both massive and disseminated chromitites and phaselayering within individual podiform bodies observed in the Luobusaophiolite. KEY WORDS: boninitic magmas; dunite envelope; melt—rock interaction; MORB peridotities; podiform chromitites *Corresponding author. Present address: Department of Geology, Laurentian University, Sudbury, Ont, Canada P3E 2C6. |
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