Melting experiments on SiO2-rich lamproites to 6.4?GPa and their bearing on the sources of lamproite magmas |
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| Authors: | R H Mitchell A D Edgar |
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| Institution: | (1) Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada, CA;(2) Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada, CA |
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| Abstract: | Summary Supra-solidus phase relations at temperatures and pressures ranging from 800 to 1700 °C and 2 to 6.4 GPa have been determined
experimentally for three silica-rich lamproites: hyalo-leucite phlogopite lamproite (Oscar, West Kimberley); sanidine richterite
lamproite (Cancarix, Murcia-Almeria); and phlogopite transitional madupitic lamproite (Middle Table Mountain, Wyoming). All
samples have extended melting intervals (500–600 °C). Bulk composition has a significant control on the nature of the initial
liquidus phases, with orthopyroxene occurring at low pressures (<4 GPa) in the relatively calcium-poor Oscar and Cancarix
lamproites. At higher pressure (>6 GPa) orthopyroxene is replaced by garnet plus clinopyroxene as near-liquidus phases in
the Oscar lamproite and by orthopyroxene plus clinopyroxene in the Cancarix sample. Clinopyroxene is a near-liquidus phase
at all pressures in the Middle Table Mountain lamproite. Near-solidus phase assemblages at high pressure (>5 GPa) are: clinopyroxene + phlogopite + coesite + rutile + garnet
(Oscar); clinopyroxene + garnet + coesite + K–Ti-silicate (Cancarix); clinopyroxene + phlogopite + apatite + K–Ti-silicate
(Middle Table Mountain). In all compositions olivine is never found as a liquidus phase at any of the temperatures or pressures
studied here. The phase relationships are interpreted to suggest that silica-rich lamproites cannot be derived by the partial
melting of lherzolitic sources. Their genesis is considered to involve high degrees of partial melting of ancient metasomatic
veins within a harzburgitic-lherzolitic lithospheric substrate mantle. The veins are considered in their mineralogy to be
similar to the experimentally-observed, high pressure, near-solidus phase assemblages. The composition of silica-rich primary
lamproite magmas differs between cratons as a consequence of differing mineralogical modes of the source veins and different
relative contributions from the veins and wall-rocks to the partial melts.
Received February 21, 2000; revised version accepted July 3, 2001 |
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