Issue 1: February 1996

Abstract

Podiform chromitites in the Luobusa ophiolite (Southern Tibet): implications for melt-rock interaction and chromite segregation in the upper mantle

M-F. Zhou, PT. Robinson¹, J. Malpas² and Z. Li³ 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, China and To whom correspondence should be addressed. Present address: Department of Geology, Laurentian University, Sudbury, Ont., Canada P3E 2C6
ABSTRACT
The Luobusa ophiolite in the Indus-Yarlung Zangbo suture of southern Tibet hosts the largest known chromite deposit in China. The podiform chromitites occur in a well-preserved mantle sequence consisting of harzburgite with abundant lenses of dunite. The harzburgites have relatively uniform bulk-rock compositions with mg-numbers [100 Mg/(Mg + Fe)] ranging from 89 to 91 and show flat, unfractionated, chondrite-normalized platinum group element (PGE) patterns. Their accessory chromite varies widely in cr-number [100Cr/(Cr + Al)] (18-66). These rocks are essentially residua left after extraction of mid-ocean ridge basalt (MORB)-type magmas. The podiform chromitites display nodular, massive, disseminated and banded textures and typically have dunite envelopes that grade into the surrounding harzburgite and diopsidic harzburgite with increasing pyroxene contents. They consist of relatively uniform chromite with cr-numbers (74-82), have strongly fractionated, chondrite-normalized PGE patterns with enrichment in Os, Ir and Ru relative to Rh, Pt and Pt, and are believed to have formed from a boninitic magma produced by a second stage of melting. Dunites contain accessory chromite intermediate in composition between those of harzburgite and chromitite and are believed to be the products of reaction between new boninitic magmas and old MORB-type peridotites. The melt-rock interaction, chromite fractionation and magma mixing should lead to many fluctuations in melt composition producing both massive and disseminated chromitites and phase layering within individual podiform bodies observed in the Luobusa ophiolite.

KEY WORDS: boninitic magmas; dunite envelopes; melt-rock interaction; MORB peridotites; podiform chromitites

Pages: 3 - 21

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