Abstract

Basalts and sedimentary rocks in the eastern half of the Labrador trough are profusely intruded by gabbro sills. In the Ahr Lake area, Quebec, where the igneous rocks were studied in detail, the gabbros are divided into normal gabbros, leopard rocks, and metagabbros. Metagabbros, which are confined to the eastern part of the area, are equivalent to normal gabbros except for two unusual varieties, leuco- and melanometagabbros.

Normal gabbro sills have differentiated into olivine-bearing lower, and pegmatitic upper, portions. The pegmatite of one sill (albite gabbro pegmatite) has sodic plagioclase in contrast with andesine in other pegmatites and a mesostasis interpreted as devitrified glass. Glass formation is attributed to volatile deficiency in the magma, but development of sodic plagioclase may have been due to later metamorphism facilitated by the presence of glass.

Leopard rock is a coarse-grained feldspathic gabbro spotted with 6- to 15-cm aggregates of altered plagioclase. Sills of leopard rock have narrow zones of medium-grained, sparsely prophyritic gabbro along their margins, and some are composite with intrusions of normal gabbro along their centers. Leopard rock is probably equivalent to normal gabbro with a high concentration of plagioclase, and it may have formed by partial melting of the mantle at high pressures or under conditions of moderately high water pressure. Magma with a load of suspended plagioclase may assume a flow structure upon intrusion such that plagioclase is concentrated in a central sheet and the margins are free of plagioclase clusters. Thus the marginal zones with few feldspathic clots are explained.

Melano- and leuco metagabbros occupy the lower and upper parts respectively of two sills and are separated by a narrow gradational zone. Each also occurs separately. Melanometagabbro is predominantly amphibole but shows relic olivine structures; leucometagabbro is predominantly altered plagioclase. Sills of melano- and leucometagabbros cannot be due to normal differentiation, and their origin is uncertain.

Petrographic, mineralogical, spectrographic, and chemical data for gabbros and basalts are given. Normal gabbros are tholeiites which show an iron-rich trend similar to the Skaergaard trend but with negligible alkali enrichment. Basalts of the area are equivalent to the parent magma of the normal gabbros, and differentiation was in situ. Low potassium and strontium are characteristic of the province. Trace-element behavior is similar to that in other tholeiitic provinces.

Metamorphism decreases westerly across the Ahr Lake area from quartz-albite-epidote-biotite subfacies of the greenschist facies through quartz-albite-muscovite-chlorite subfacies to subgreenschist facies rocks. Local development of pumpellyite and prehnite in the latter may indicate the zeolite facies.

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