Diorite and associated intrusive and metamorphic rocks between Port William and Paterson Inlet, Stewart Island, and on Ruapuke Island

Abstract

Abstract The area mapped consists mainly of heterogeneous dioritic rocks formed by partial assimilation of hornblende hornfels by leucodiorite magma. Field relations and petrography indicate the following sequence: eruption of calc-alkalic and alkalic lavas; emplacement of gabbro, diorite, trondhjemite, and potassic granite, and associated aplite and pegmatite; and injection of basic to intermediate dikes. Small disrupted and hornfelsed fragments of basic lavas and gabbroic intrusive rocks are correlated with the Brook Street and Eglinton Volcanics and probably formed part of an extensive lower Permian volcanic arc. A considerable variety of migmatitic rocks of dioritic aspect is present. Minor trondhjemite probably resulted from crystal fractionation of leucodiorite magma. Potassic granite may be connected with the extensive granite forming much of the southern part of Stewart Island. The full age range of the rocks is unknown, but intrusive activity probably extended from the Permian into the Triassic. Dikes were injected during much of this period. The dioritic rocks have chemical and mineralogical features in common with the hornfelsed lower Permian rocks, and an origin for them by remelting of volcanic rocks depressed to the base of the crust during the Mesozoic is suggested. The structure of the area and the mechanics of intrusion of the dioritic and related rocks are briefly discussed.