Origin of the Anorthosite-Mangerite Rocks in Southern Quebec

Rocks of the anorthosite-mangerite suite were intruded in manyplaces in the southern part of the Grenville province of theCanadian Shield. These rocks, known as the Morin series in southernQuebec, are described from two contrasted areas, one in whichthere has been intense deformation during the emplacement ofthese rocks and in the other only mild deformation. Differencesin the mineralogy and chemistry of these rocks in the two areascan be related to the degree of deformation. Although many of the rocks of the series still retain theirigneous textures, others, especially the mangerites, are largelyrecrystallized and commonly exhibit a gneissic foliation. Thefeldspars and pyroxenes which are by far the most abundant mineralsof the Morin series are dealt with in detail. From a study ofthe coexisting pyroxenes and the amount of iron in the plagioclases,evidence is given for a difference in the degree of metamorphismbetween the two areas. All of the pyroxenes from the more highlydeformed area contain considerably more alumina than those fromthe other area. Coexisting lime-rich and lime-poor pyroxenesare characteristic of almost all of the rocks of the series.The coexistence of subcalcic ferro-augite and inverted pigeoniteas iron-rich as Ca_25.6Mg_22.4Fe_52.0 and Ca₉Mg₂₃Fe₆₃, respectively,in the unmetamorphosed rocks suggests very high temperaturesof crystallization. Analyses of the rocks are compared with those from other anorthositeassemblages. Variation diagrams indicate that the anorthosite-mangeriterocks form a very definite series which is not exactly comparablewith any other known rock series. Although the anorthosite-mangeritesuite shows strong iron enrichment and is therefore similarto many tholeiitic suites, the basic members of the series aremore akin to calc-alkaline rocks. It is concluded that the anorthosite-mangerite rocks are derivedfrom a calc-alkaline parental magma that underwent differentiationin a very dry environment and hence gave rise to the strongiron enrichment trend that is characteristic of tholeiitic suites.Early accumulation of plagioclase from the parental magma toform anorthosites may be due to a probable increase in the sizeof the field of crystallization of intermediate and sodic plagioclasesunder high pressures. The fact that intrusion took place duringan orogenic period is of great importance, for the rocks inthe more deformed area are more differentiated than those inthe less deformed area.