Journal of Petrology, Volume 40, Issue 2: February 1999.

Some Phase Equilibrium Constraints on the Origin of Proterozoic (Massif) Anorthosites and Related Rocks

JOHN LONGHI¹, JACQUELINE VANDER AUWERA², MIRANDA S. FRAM³ AND JEAN-CLAIR DUCHESNE²

¹LAMONT-DOHERTY EARTH OBSERVATORY, RT. 9W, PALISADES, NY 10964, USA
²L.A. GÉOLOGIE, PÉTROLOGIE, GÉOCHIMIE, UNIVERSITÉ DE LIÈGE, 4000 LIÈGE, BELGIUM
³DEPARTMENT OF GEOLOGY, UNIVERSITY OF CALIFORNIA, DAVIS, CA 95616, USA

Experimental data in the range of 1 bar to 13 kbar enable us to map 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 along the trace of the plagioclase + two-pyroxene cotectic from 10 to 13 kbar, but also straddles the thermal divide on the plagioclase + pyroxene liquidus surface. This condition requires mafic source regions, such as lower continental crust or foundered mafic plutons, for liquids parental to massif anorthosites and associated mafic intrusions.

Keywords: anorthosite;lower crust;megacryst; thermal divide; troctolite

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