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Geological Society, London, Special Publications; 1986; v. 24; p. 331-349;
DOI: 10.1144/GSL.SP.1986.024.01.29
© 1986 Geological Society of London

Mineral reactions in xenoliths from the Colorado Plateau; implications for lower crustal conditions and fluid composition

J. R. Broadhurst

Department of Geology, University of Sheffield, Mappin Street, Sheffield, S1 3JD UK

A study of lower crust and upper mantle xenoliths from Moses Rock Diatreme, Utah, has led to the discovery of evidence for low temperature/high pressure, in situ hydration of pyroxene and garnet granulites in isolated parts of the lower crust of the Colorado Plateau; one of the characteristic reactions being: Plagioclase->Zoisite+Paragonite+Quartz+Albite This was followed by a second hydration at lower temperature and pressure, under more oxidizing conditions, probably associated with the incorporation of the xenoliths into the diatreme. Under these conditions, pyroxenes were replaced by fibrous amphiboles and the plagioclases were rimmed by chlorite and epidote. Upper mantle xenoliths show similar histories of cooling and hydration. The origin of the cold hydrating fluid is linked to the presence of low T/high P group ‘C’ eclogites in the suite, xenoliths which may represent relics of a subducted/underplated fragment of the Farollan plate.