Fluid Activity in the Lower Crust and Upper Mantle: Mineralogical Evidence Bearing on the Origin of Amphibole and Scapolite in Ultramafic and Mafic Granulite Xenoliths

A. J. Stolz
Department of Geology, University of Tasmania, GPO Box 252C, Hobart, Tasmania, 7001, Australia

Abstract: Xenoliths in an olivine nephelinite from the McBride Province, North Queensland, include Cr-diopside lherzolites, spinel and garnet websterites, felsic, 2-pyroxene and garnet granulites, and hornblendites. The spinel and garnet websterites are interpreted as crystal segregations from olivine basalt or alkali olivine basalt magma at ∼ 12 kbar followed by isobaric cooling (to approximately 900–1000°C) and subsolidus reequilibration. Garnet and 2-pyroxene granulites are mineralogically and texturally distinct and are considered to represent relatively large degrees of crystallization of basaltic magmas at comparable or slightly lower pressures (8–12 kbar). Mafic and ultramafic xenoliths have been modified to varying degrees following the relatively recent influx of a H2O- and CO2-bearing fluid. Variable amounts of amphibole and mica developed in response to the introduced fluid and it is argued that some hornblendites are the end-products of this process acting on spinel websterites. Felsic and 2-pyroxene granulite xenoliths display only minor evidence of increased PH2O. Mineralogical and textural evidence indicates high-sulphur Ca-rich scapolite in several garnet granulites did not form in response to the increased fluid activities. It is proposed the scapolite was a primary cumulate phase precipitated from alkali basaltic magma under elevated fo2 and fso2 conditions.

Keywords: granulite xenoliths • scapolite • amphibole • lower crust • upper mantle • McBride Province • Queensland • Australia

Mineralogical Magazine; December 1987 v. 51; no. 363; p. 719-732; DOI: 10.1180/minmag.1987.051.363.13
© 1987, The Mineralogical Society
Mineralogical Society (www.minersoc.org)