Petrogenesis of massif anorthosites: a perspective from St. Urbain, Quebec
The St. Urbain massif is a post-orogenic anorthosite pluton (approx. 500 km/sup 2/) emplaced within the central high-grade granulite terrain of the Grenville structural province. In contrast to other Grenville anorthosites, primary magmatic features are largely preserved. The massif consists predominantly of andesine anorthosite (AA) of remarkable purity containing abundant plagioclase megacrysts. AA has high K/sub 2/O (approx. 2 wgt.%), very high Sr contents (approx. 1200 ppm) and highly fractionated, low REE contents. Features of AA provide the following insights into anorthosite origins: (1) Crystallization from anorthositic magmas, as evidenced by early crystallization of abundant antiperthitic plag, and igneous emplacement of AA dikes and veins into older, unrelated labradorite anorthosite; (2) in situ crystallization of pyroxene after plag, with no direct evidence of earlier crystallization of mafic minerals from a basaltic parent magma; (3) limited differentiation during crystallization, indicated by small variation in plag and opx and limited variations in plag Sr and REE contents; (4) the involvement of water, suggested by the late igneous crystallization of biotite and the localized grain-boundary replacement of plag by calcic myrmekite (An/sub 80/ + qtz). (5) high temperature, relatively oxidizing conditions, indicated by magmatic hemoilmenite +/- rutile and rare ferropseudobrookite in AA and associated ores. AA crystallized from highly feldspathic, relatively oxidized, somewhat hydrous parent magma with little trapped melt. The development of a hyperfeldspathic parent magma with the requisite geochemical features can be ascribed to hydrous partial melting of mafic (to intermediate) rocks at deep crustal or greater depths, leaving a garnetiferous residue.
- Research Organization:
- Brown Univ., Providence, RI (USA)
- OSTI ID:
- 6576842
- Report Number(s):
- CONF-8510489-
- Journal Information:
- Geol. Soc. Am., Abstr. Programs; (United States), Vol. 17; Conference: 98. annual meeting of the Geological Society of America, Orlando, FL, USA, 28 Oct 1985
- Country of Publication:
- United States
- Language:
- English
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