Some phase equilibrium constraints on the origin of Proterozoic (massif) anorthosites and related rocks

Publication Type  Journal Article
Year of Publication  1999
Authors  Longhi, J.; Vander Auwera, J.; Fram, M. S.; Duchesne, J. C.
Journal Title  Journal of Petrology
Volume  40
Issue  2
Pages  339-362
Journal Date  Feb
ISBN Number  0022-3530
Accession Number  ISI:000078832000006
Key Words  anorthosite; lower crust; megacryst; thermal divide; troctolite; sokndal layered intrusion; low-ca pyroxene; liquidus equilibria; continental-crust; magma chamber; sw norway; crystallization conditions; isotopic composition; grenville province; marginal r
Abstract  

Experimental data in the range of I bal to 13 kbar enable us to may the liquidus equilibria relevant to Proterozoic (massif) anorthosites and related mafic rocks. Massif anorthosites are widely believed to have formed by accumulation of plagioclase into high Al basaltic liquids, Mantle-derived basaltic liquids, fractionating at pressures sufficiently high (10-13 kbar) to crystallize the highly aluminous orthopyroxene megacrysts typically observed in anorthosite massifs, reach plagioclase saturation at low normative silica contents. Peritectic-like equilibria (e.g, liq + opx --> plag + cpx + sp) and a thermal divide on the plagioclase + pyroxene liquidus surface ensure that mantle-derived liquids become nepheline normative with further crystallization and crustal assimilation at depth. Such liquids cannot Produce the full range of troctolitic-noritic to troctolitic-gabbroic mineral assemblages observed in anorthosite massifs without extensive low-pressure granite assimilation. Conversely the array of plausible anorthosite Parental liquids not only lies alone the trace of the plagioclase + two-pyroxene cotectic from 10 to 13 X-bar, but also straddles the thermal divide on the plagioclase + pyroxene liquidus surface. This condition requires mafic sou,ce regions, such as lower continental crust or foundered mafic plutons, Sor liquids Parental to massif anorthosites and associated mafic intrusions.

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