Isotopic constraints on anorthosite genesis and implications for crust-mantle evolution
Crystallization ages of anorthosite massifs, determined from whole-rock and internal Sm-Nd and Rb-Sr isochrons range between about 1.1 and 1.6 Ga, arguing against a discrete anorthosite event. Metamorphic ages of some massifs are as much as 200-300 Ma younger, indicating that the Grenville orogeny was not a causative factor in anorthosite genesis. Variable crustal contamination effects are evident in many massifs, particularly in border zones. In some late-stage ferrogabbros, mafic silicates and/or Fe-Ti oxides are not in isotopic equilibrium with plagioclase, suggesting that crystallization took place both before and after contamination. The most isotopically primitive materials are Al-rich opx megacrysts. Isotopic data to date are compatible with a two-stage model involving (1) emplacement of basaltic magma into lower crustal chambers where fractionation and accumulation of olivine and Al-rich opx, and eventually plagioclase took place, and (2) detachment and ascent of buoyant anorthositic mushes to upper crustal emplacement sites. Besides being useful as indicators of Proterozoic mantle evolution, anorthosites can be used as tracers to map our basement types through which they were emplaced.
- Research Organization:
- Lunar and Planetary Institute, Houston, TX (USA)
- OSTI ID:
- 6576341
- 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
CRYSTALLIZATION
GEOCHEMISTRY
ISOTOPE RATIO
MINERALIZATION
EARTH CRUST
PETROGENESIS
EARTH MANTLE
AGE ESTIMATION
BASALT
CHEMICAL COMPOSITION
GEOLOGIC HISTORY
GEOLOGIC MODELS
NEODYMIUM ISOTOPES
OLIVINE
ORIGIN
RUBIDIUM ISOTOPES
SAMARIUM ISOTOPES
STRONTIUM ISOTOPES
ALKALI METAL ISOTOPES
ALKALINE EARTH ISOTOPES
ALKALINE EARTH METAL COMPOUNDS
CHEMISTRY
GABBROS
IGNEOUS ROCKS
IRON COMPOUNDS
IRON SILICATES
ISOTOPES
MAGNESIUM COMPOUNDS
MAGNESIUM SILICATES
MINERALS
OXYGEN COMPOUNDS
PHASE TRANSFORMATIONS
PLUTONIC ROCKS
RARE EARTH ISOTOPES
ROCKS
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
TRANSITION ELEMENT COMPOUNDS
VOLCANIC ROCKS
580400* - Geochemistry- (-1989)
580300 - Mineralogy
Petrology
& Rock Mechanics- (-1989)