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A granite-gabbro complex from Madagascar; constraints on melting of the lower crust

McMillan, Andy; Harris, Nigel B. W.; Holness, Marian; Ashwal, Lewis; Kelley, Simon and Rambeloson, Roger (2003). A granite-gabbro complex from Madagascar; constraints on melting of the lower crust. Contributions to Mineralogy and Petrology, 145(5) pp. 585–599.

DOI (Digital Object Identifier) Link: http://dx.doi.org/10.1007/s00410-003-0470-1
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Abstract

The Ranomandry Complex is a Neoproterozoic, nested intrusion from central Madagascar composed of a gabbroic core within a coeval peraluminous granite ring intruding pelitic metasediments. Although xenocryst entrainment and magma mixing have both contributed to marginal phases of the granite, the primary melt is characterised by steep LREE/HREE ratios and negligible, or slightly positive, Eu anomalies. Both isotopic and trace element systematics preclude an origin from either partial melting of the metapelitic country rock or from fractional crystallisation of the gabbroic magma. However, trace-element modelling suggests an origin from the dehydration melting of a plagioclase-poor, garnet-bearing metagreywacke at temperatures of 850–900 °C and at lower crustal pressures (>10 kbar). Melting of an enriched subcontinental mantle generated gabbroic magmas that caused advective heating and anatexis at the base of thickened continental crust during their ascent. Transport of the resulting granite magma was facilitated by the pre-existing plumbing system that overcame thermal and mechanical problems associated with both diapirism and self-propagating dykes as mechanisms for long-distance transport of granite magmas. The nested geometry of the intrusions is an indication of a structurally homogeneous lower crust that contains no pre-existing shear zones or fault systems.

Item Type: Journal Article
ISSN: 0010-7999
Academic Unit/Department: Science > Environment, Earth and Ecosystems
Interdisciplinary Research Centre: Centre for Earth, Planetary, Space and Astronomical Research (CEPSAR)
Item ID: 4108
Depositing User: Nigel Harris
Date Deposited: 03 Jul 2006
Last Modified: 02 Dec 2010 19:51
URI: http://oro.open.ac.uk/id/eprint/4108
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