Anorthosites and anorthosites: Contrasting plagioclase-rich rocks in the Archaean and Proterozoic
- Washington Univ., St. Louis, MO (United States). Dept. of Earth Planetary Sciences
Anorthosites -- rocks consisting predominantly of plagioclase feldspar -- have figured prominently in at least two distinct intervals of Earth history: the late-Archaean and mid-Proterozoic. Archaean anorthosites (AA) are a key component of high-grade gneiss terranes, where they typically form laterally extensive deformed sheets or sills up to a km thick. In contrast, Proterozoic anorthosites (PA) form plutons or plutonic complexes, and are most abundant in a quasi-continuous belt across NE N. America. In addition to these temporal and structural contrasts, AA and PA display markedly different mineralogical and geochemical properties, including, respectively: (1) equant plagioclase megacrysts vs. tabular megacrysts; (2) highly calcic compositions vs. intermediate to alkalic compositions; (3) amphibole vs. olivine or orthopyroxene as the dominant mafic mineral; (4) the presence of chromite, locally in ore-grade layers vs. massive, cross-cutting Fe-Ti oxide ores; (5) low levels of Sr and Ba vs. high to extreme levels; (6) high levels of ferromagnesian trace elements vs. low levels; (7) Ga/Al values typical of basaltic plagioclase vs. much lower values; and (8) moderately vs. extremely fractionated REE patterns. Given these contrasts, it appears that the only property AA and PA share is their plag-rich nature, suggesting that there must be more than one process (and probably multiple processes) capable of producing anorthosite.
- OSTI ID:
- 7206966
- Report Number(s):
- CONF-9303210-; CODEN: GAAPBC
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Vol. 25:3; Conference: 27. annual Geological Society of America (GSA) North-Central Section meeting, Rolla, MO (United States), 29-30 Mar 1993; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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