Origin of chromitites in layered intrusions: evidence from chromite-hosted melt inclusions from the Stillwater Complex

Spandler, Carl, Mavrogenes, John, and Arculus, Richard (2005) Origin of chromitites in layered intrusions: evidence from chromite-hosted melt inclusions from the Stillwater Complex. Geology, 33 (11). pp. 893-896.

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Chromitites from layered mafic intrusions are of great economic importance, yet the origin of these deposits remains enigmatic. We describe multiphase silicate inclusions trapped within chromite grains from the G chromitite seam of the Stillwater Complex, Montana, United States. These inclusions are interpreted to represent melt trapped during chromite growth and hence provide information on chromitite formation. Most reheated inclusions have variable quench textures and chemical compositions that are consistent with variable degrees of mixing between a high-Mg basaltic parental magma and a Na-rich trondhjemitic melt. The trondhjemite is suggested to derive from partial melting of mafic or metasedimentary country rocks. Based on these inclusions, we outline a model for chromitite formation involving ponding of a new pulse of primitive magma at the roof of the Stillwater magma chamber, followed by localized partial melting and assimilation of the country rock. The newly formed hybrid melts become oversaturated in chromite, leading to extensive chromite crystallization. Chromitite horizons are proposed to form from dense chromiterich plumes that periodically sink down from the roof zone to settle out as layers at the basal cumulate mush zone. Numerous radiogenic isotope studies, together with the widespread occurrence of similar multiphase inclusions in chromite from other cumulate complexes, indicate that assimilation of country rock by primitive magma may be a critical mechanism for forming chromitites in many layered intrusions.

Item ID: 8628
Item Type: Article (Research - C1)
ISSN: 1943-2682
Keywords: chromitite, melt inclusions, Stillwater Complex, parent magmas, layered intrusions
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Funders: Australian Research Council
Date Deposited: 25 Feb 2010 00:44
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040307 Ore Deposit Petrology @ 100%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 100%
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