GEOLOGY AND GEOPHYSICS OF THE TYPE ANORTHOSITE, CHATEAU-RICHER, QUEBEC

Anorthosite was named in 1862 by T. Sterry Hunt following his detailed study at Chateau-Richer. Thus, in a broad sense, the anorthosite ther e merits recognition as a sort of type locality of this distinctive pl utonic rock. The anorthosite at Chateau-Richer constitutes an oval bod y 125 km2 in area, located 30 km NE of Quebec City. It is composed nea rly wholly of andesine antiperthite, with accessory pyroxene, biotite, ilmenite, and apatite. The Chateau-Richer anorthosite is more alkali- rich than average massif anorthosite or the classical Marcy type. Its composition closely resembles that of the nearby St-Urbain anorthosite . Foliation dips outward at the edges of the body, and is concordant w ith foliation and layering in enclosing high-grade country rocks. Towa rd the center of the body, foliation is horizontal or dips only gently . At first glance, the anorthosite seems to constitute a dome; however , the structural asymmetry of the body, the irregular distribution of rock types around its periphery, and the prevailing shallow plunge of lineations suggest that the Chateau-Richer anorthosite makes up a rela tively thin sheet now exposed in a doubly plunging antiform. According to gravity and magnetic models, the body is a sheet no more than 2 km thick, and somewhat thinner at its center than near its edges. The ab sence of associated mafic or ultramafic rocks is in keeping with the v iew that the Chateau-Richer anorthosite has been uprooted from its sou rce, possibly a layered mafic intrusion, to be emplaced tectonically a t its present setting.