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- Volume 97 (1974)
- Fascicule 2
- The Harp Lake complex, Labrador, and the Morin complex, Quebec: examples of igneous and meta-igneous anorthositic complexes in the eastern Canadian shield
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The Harp Lake complex, Labrador, and the Morin complex, Quebec: examples of igneous and meta-igneous anorthositic complexes in the eastern Canadian shield
Abstract
The Harp Lake complex underlies about 10,000 km2 in central Labrador. Approximately 75 percent of the complex comprises leucotroctolites, leucogabbros, leuconorites and anorthosite. Layered structures are widespread in these rocks but do not define a simple structural entity. Younger pyroxene- and olivine-bearing adamellites make up most of the remainder of the complex. Olivine gabbros, gabbros and diorites are present at the margins of the anorthositic rocks and occur as dikes and small intrusive masses within the complex. Analyzed pyroxenes and feldspars show continuous compositional ranges from the anorthositic rocks through rocks of intermediate composition to the adamellites. Orthopyroxenes extend from Mg73 to Mg21 and plagioclases from An72 to An6.
The Morin meta-igneous complex in Quebec comprises an anorthositic massif (chiefly anorthosite and leucogabbro) that underlies about 2,500 km2 together with a similar area of closely spatially associated pyroxene quartz monzonites with lesser proportions of gabbro, pyroxene diorite and monzodiorite. Dikes of pyroxene monzodiorite cut only anorthositic rocks and dikes of pyroxene quartz monzonite cut both anorthositic rocks and pyroxene monzodiorites. Analyzed pyroxene and feldspar compositions form continuous ranges from the anorthositic rocks through pyroxene monzodiorites to pyroxene quartz monzonites. Orthopyroxenes extend from Mg70 to Mg36, plagioclases from An53 to An24 and alkali feldspars from Or95 to Or80. The lower degree of iron enrichment in the pyroxenes by comparison with Harp Lake is in accord with a greater abundance of oxide minerals in the rocks and numerous oxide mineral deposits suggesting higher oxygen fugacities in equilibrium with the Morin magmas.
The mineral chemistry of both complexes implies that they resulted from some sort of evolving magmatic process. Periodic tapping of a magma reservoir undergoing fractional crystallization at depth is suggested as a possible mechanism to explain the observed geological relations and the patterns of mineral variation. The Harp Lake complex retains mineral equilibria resulting from subsolidus reactions during cooling from igneous temperatures. The Morin complex reflects mineral equilibria in part characteristic of lower temperature metamorphic conditions, in part a preservation of higher temperature, presumably relict igneous, equilibria.