ISSN:
1432-0967
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
Abstract Several gneissic xenoliths occur in granite which has been intruded along the contact between an anorthosite body and gneisses in the Nain complex, Labrador. One of these xenoliths is a ferro-aluminous gneiss with ovoids, 1–3 cm in diameter, consisting of an intimate mixture of fine, granular cordierite-olivine-spinel-orthopyroxene-plagioclase-ilmenite-pyrrhotite enclosed by poikilitic grains of garnet and orthoamphibole. Biotite occurs as both granular and poikilitic grains. The textures indicate a two-stage contact metamorphic history characterized by the simplified reaction: cordierite+olivine+spinel+plagioclase +orthopyroxene→garnet+orthoamphibole Although, on the basis of texture, the reactants represent the early stage assemblage and the products represent the final stage, the distribution coefficient data for Fe-Mg indicate that, except for orthopyroxene, all of the minerals were stable and in equilibrium during the final stage. The coexistence of cordierite and olivine indicates that pressure was less than 3 kbar. The phase relationships agree well with the experimental work of Hsu and Burnham (1969) on the almandine-pyrope join at 2 kbar and indicate a temperature of 800° C for the first stage and 615° C for the second stage of contact metamorphism. This close agreement also suggests that the ovoids at one time may have been garnet porphyroblasts that were decomposed during the first stage. These data correlate well with the field relations: the first-stage, high-temperature effects were likely caused by the intrusion of the anhydrous anorthositic body and the second-stage, low-temperature effects by the invasion of the volatile-rich granite.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00378263
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