1. The geology and petrography of an Archaean inlier, south of Normanville. 2. The origin of the 'Houghton' granulite.

Abstract

1. Detailed mapping established definite rock units within the Archaean inlier south of Normanville. The distribution of the rock types is suggestive of a major antiformal structure. Only metamorphic layering is recognised in the area. The first Archaean orogeny produced this layering and reached upper amphibolite grade of metamorphism as defined by the minerals sillimanite, garnet, diopside and scapolite. The temperature and pressure conditions were ~750C and 2-3 Kbars. This was established using mineral assemblages and the presence of considerable partial melting, pegmatitic masses and migmatites. This same orogeny was also responsible for the development of a strong schistosity, which was always parallel to the metamorphic layering. A Proterozoic orogeny produced a strong crenulation of the schists. This crenulation is parallel to the fold axis of a major antiform which plunges 35deg to 138deg. The deformation reached biotite grade of metamorphism as shown by the minerals biotite, sericite and epidote, which, commonly replace the higher grade minerals. An antiform overturned to the west was the major structure. Its under limb was sheared away, so that the inlier represents the eastern limb only. The western outcrops of the inlier represent the shear zone. 2. Geochemical analyses of the Houghton granulite suggest a sedimentary origin. This, coupled with its conformability with the surrounding rocks, and its association with a zircon rich meta-quartzite, support this conclusion.