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Origin of the Anorthosite-Mangerite Rocks in Southern Quebec

1966; Oxford University Press; Volume: 7; Issue: 1 Linguagem: Inglês

10.1093/petrology/7.1.1

1460-2415

Anthony R. Philpotts,

Paleontology and Stratigraphy of Fossils

Rocks of the anorthosite-mangerite suite were intruded in many places in the southern part of the Grenville province of the Canadian Shield. These rocks, known as the Morin series in southern Quebec, are described from two contrasted areas, one in which there has been intense deformation during the emplacement of these rocks and in the other only mild deformation. Differences in the mineralogy and chemistry of these rocks in the two areas can be related to the degree of deformation. Although many of the rocks of the series still retain their igneous textures, others, especially the mangerites, are largely recrystallized and commonly exhibit a gneissic foliation. The feldspars and pyroxenes which are by far the most abundant minerals of the Morin series are dealt with in detail. From a study of the coexisting pyroxenes and the amount of iron in the plagioclases, evidence is given for a difference in the degree of metamorphism between the two areas. All of the pyroxenes from the more highly deformed area contain considerably more alumina than those from the other area. Coexisting lime-rich and lime-poor pyroxenes are characteristic of almost all of the rocks of the series. The coexistence of subcalcic ferro-augite and inverted pigeonite as iron-rich as Ca25.6Mg22.4Fe52.0 and Ca9Mg23Fe63, respectively, in the unmetamorphosed rocks suggests very high temperatures of crystallization. Analyses of the rocks are compared with those from other anorthosite assemblages. Variation diagrams indicate that the anorthosite-mangerite rocks form a very definite series which is not exactly comparable with any other known rock series. Although the anorthosite-mangerite suite shows strong iron enrichment and is therefore similar to many tholeiitic suites, the basic members of the series are more akin to calc-alkaline rocks. It is concluded that the anorthosite-mangerite rocks are derived from a calc-alkaline parental magma that underwent differentiation in a very dry environment and hence gave rise to the strong iron enrichment trend that is characteristic of tholeiitic suites. Early accumulation of plagioclase from the parental magma to form anorthosites may be due to a probable increase in the size of the field of crystallization of intermediate and sodic plagioclases under high pressures. The fact that intrusion took place during an orogenic period is of great importance, for the rocks in the more deformed area are more differentiated than those in the less deformed area.