Structure of the Constant Gneiss near Cape Foulwind, south-west Nelson, and its bearing on the regional tectonics of the West Coast

Abstract

The Constant Gneiss at Cape Foulwind and Tautanga Bay is a coarse-grained rock of predominantly granitic to granodioritic composition with abundant feldspar phenocrysts and occasional banding of light and darker (more biotite rich) gneiss. Two foliations have been mapped. The first is a plane of flattening and contains associated lineations trending in the NW-SE quadrants. The orientations of this foliation, the lithological banding, and the trains of aligned and elongate meta-sedimentary fragments are coincident; their particular characteristics lead to the conclusion that the gneiss is the result of the partial melting of sediments of greywacke type, an origin similar to that of the gneiss further south near Charleston. In Siberia Bay, a iater plutonic event of probable Cretaceous age has caused melting and movement of the older gneiss along a NE-SW trending zone. The movement of this newer granitic rock imposed a second foliation on the older gneiss together with strong NE-SW lineations; gneiss in the movement zone has been strongly flattened. Implications for the structure of the West Coast in general are discussed in the light of the structure of the gneiss.