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Higgins, Michael D., mdhiggin@uqac.uquebec.ca, Sciences de la Terre, Université du Québec à Chicoutimi, 555 boulevard de l'Université, Chicoutimi, QC, G7H 2B1
Anorthosite is a rare rock globally, but is common in Grenville province. Why is this so? Massif-type anorthosites are defined by compositions rich in plagioclase, megacrysts of Al-rich pyroxene, association with rapakivi granite, charnockite and mangerite (AMCG suite) and by their coarse and variable textures. The parental magmas were formerly considered to be mantle-derived basalts or plagioclase-component enriched basalts. However, the extreme variability of their initial strontium and neodymium isotopic ratios have instead indicated a strong crustal component. There seems to be a consensus on a lower crustal protolith formed essentially of underplated basaltic magmas. However, some of the more mafic components of the suite may have more of a mantle component. The heat source for magma generation may be mantle plumes, or it may be related regional convergence. Partial melting produces magma pools at the base of the crust. Plagioclase crystallises and floats to the upper part of the chamber whereas mafic minerals sink back into the mantle. The plagioclase charged magmas must rise rapidly to entrain the Al-rich pyroxene megacrysts and carry them upwards. At higher levels in the crust plagioclase separates from the liquid to form the plutons. Textural coarsening will refine the anorthosite, removing other components and producing the extreme heterogeneity characteristic of anorthosite massifs. AMCG suite rocks are commonly associated with sutures on two scales: Most anorthosite massifs seem to occur close to the Archean-Proterozoic boundary in Mesoproterozoic rocks. Within these regions there is also an association with shear-zones. The necessary heat flow for partial melting, plagioclase floatation, emplacement and textural coarsening may have only been possible during the Proterozoic. Earlier the heat flow was too high and there was insufficient time for these processes, later there was not enough time. The association with shear zones may be important also. Thrusting may have heated and melted tongues of lower crust forced into the mantle. The plagioclase-charged magmas then rose along the shear-zones until they reached the brittle-ductile transition zone where they spread out laterally to form their present pancake-shaped intrusions. Rapakivi granites may also reflect the prolonged cooling that optimized conditions for textural coarsening. There are a number of different economic deposits associated with anorthosite massifs. Sulphide deposits occur around the margins, some associated with late, higher temperature troctolitic intrusions. These areas may have been favoured because they were less competent. It is also possible that late interstitial liquids were forced from the magmas during deformation associated with the last stages of solidification. Elsewhere in the world anorthosites are associated with deposits of gem-stones - particularly sapphire, ruby and beryl. Finally, the most highly-valued dimension stone is an anorthosite with a blue labradoresence.