Chemical trends in massif anorthosites: comparison with Stillwater trends
Formation of massif anorthosites may begin with equilibrium crystallization of suspended plagioclase and sinking mafic minerals. Thick Stillwater anorthosites formed that way and show two chemical trends. In one, all elements are strongly correlated or anticorrelated with MgO and can be modeled precisely as mixtures of cumulus plagioclase, adcumulus plagioclase and pyroxene, magnetite, and trapped liquid. Layers developed from meter-sized packets of crystal mush, each slightly unique in composition, without chemical gradients across thick units ANI or ANII. In the other trend, SiO/sub 2/ and Al/sub 2/O/sub 3/ are deficient, a result of dilution by FeO, TiO/sub 2/, volatiles, and trace elements, all apparently introduced from brines. Marcy anorthosites retain no evidence of a cumulus or adcumulus mafic component. Most samples fall within an envelope of two chemical trends, perhaps indicating two trapped liquids. Neither liquid was necessarily parental to the plagioclase; both may have been carrier liquids that mobilized the feldspar during development of the massif. Metamorphism has altered the composition of some plagioclase and may have extended the range for elemental migration beyond the scale of hand specimens, so trends are evident only when data for many samples are examined.
- Research Organization:
- Washington Univ., St. Louis, MO (USA)
- OSTI ID:
- 6596423
- 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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Related Subjects
ANORTHOSITES
CHEMICAL COMPOSITION
COMPARATIVE EVALUATIONS
MINERALIZATION
MINERALOGY
BRINES
CHEMICAL ANALYSIS
CRYSTALLIZATION
GEOCHEMISTRY
MAGNESIUM OXIDES
MAGNETITE
METAMORPHISM
PETROGENESIS
PYROXENES
ALKALINE EARTH METAL COMPOUNDS
CHALCOGENIDES
CHEMISTRY
GABBROS
IGNEOUS ROCKS
IRON COMPOUNDS
IRON ORES
IRON OXIDES
MAGNESIUM COMPOUNDS
MINERALS
ORES
OXIDE MINERALS
OXIDES
OXYGEN COMPOUNDS
PHASE TRANSFORMATIONS
PLUTONIC ROCKS
ROCKS
SILICATES
SILICON COMPOUNDS
TRANSITION ELEMENT COMPOUNDS
580300* - Mineralogy
Petrology
& Rock Mechanics- (-1989)
580400 - Geochemistry- (-1989)