Isotopic constraints on anorthosite genesis and implications for crust-mantle evolution

Conference · Tue Jan 01 04:00:00 EST 1985 · Geol. Soc. Am., Abstr. Programs; (United States)

OSTI ID:6576341

Ashwal, L D

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-
• Conference Information: Journal Name: Geol. Soc. Am., Abstr. Programs; (United States) Journal Volume: 17
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580300 -- Mineralogy
Petrology
& Rock Mechanics-- (-1989)
580400* -- Geochemistry-- (-1989)
AGE ESTIMATION
ALKALI METAL ISOTOPES
ALKALINE EARTH ISOTOPES
ALKALINE EARTH METAL COMPOUNDS
ANORTHOSITES
BASALT
CHEMICAL COMPOSITION
CHEMISTRY
CRYSTALLIZATION
EARTH CRUST
EARTH MANTLE
GABBROS
GEOCHEMISTRY
GEOLOGIC HISTORY
GEOLOGIC MODELS
IGNEOUS ROCKS
IRON COMPOUNDS
IRON SILICATES
ISOTOPE RATIO
ISOTOPES
MAGNESIUM COMPOUNDS
MAGNESIUM SILICATES
MINERALIZATION
MINERALS
NEODYMIUM ISOTOPES
OLIVINE
ORIGIN
OXYGEN COMPOUNDS
PETROGENESIS
PHASE TRANSFORMATIONS
PLUTONIC ROCKS
RARE EARTH ISOTOPES
ROCKS
RUBIDIUM ISOTOPES
SAMARIUM ISOTOPES
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
STRONTIUM ISOTOPES
TRANSITION ELEMENT COMPOUNDS
VOLCANIC ROCKS