Igneous rocks of the Brook Street Terrane, New Zealand: implications for Permian tectonics of eastern Gondwana and magma genesis in modern intra-oceanic volcanic arcs

Abstract

The Brook Street Terrane of South Island, New Zealand, is a remnant of a primitive intra-oceanic arc system of Permian age. The terrane consists largely of volcanogenic sequences that contain plagioclase- and clinopyroxene-phyric basalts, high-MgO ankaramite dikes, and basaltic to andesitic volcaniclastic and sedimentary rocks. Dacites and rhyolites are relatively rare. Intruding the sequences are thick dolerite dikes, trondhjemite plutons, and numerous small cumulate complexes. The cumulate complexes contain early-formed olivine- and clinopyroxene-rich ultramafic cumulates overlain by anorthite and hornblende-bearing gabbros. There is convincing geological evidence to support earlier interpretations of a direct correlation between the Brook Street Terrane and the Gympie Terrane of Queensland, and we present geochemical data to support correlation with the Teremba Terrane of New Caledonia. It is likely that these terranes are exposed sections of an extensive island-arc system that was active in the Pacific in Permian times. Dislocation of the arc probably occurred during accretion to the Gondwana margin and subsequent Gondwana breakup. The major and trace el ement geochemistry of a range of mafic dikes and flows from along the terrane precludes significant geochemical variation along the terrane and shows that the majority of magmas were primitive island-arc tholeiites. The Bluff Complex is an exception and may have formed in a back-arc or arc-rift environment. The geochemistry, petrology, and field relations indicate that most of the intrusive and volcanic rocks are directly related products of upper crustal magmatic differentiation. Primary magma types include high-MgO ankaramites and trondhjemites that are suggested to have formed by partial melting of lower crustal clinopyroxene-rich cumulates and gabbros, respectively. The parental ankaramites fractionated to form the bulk of the Brook Street Terrane, including the mafic-ultramafic cumulates and evolved melts of high-Al basalt to andesite composition. The Brook Street Terra ne is an excellent analogue for modern island-arc systems and allows for the evaluation of magmatic processes that operate at the subvolcanic level of arcs. The wide distribution of ankaramites in the Brook Street Terrane indicates that parental magmas in island arcs may be more primitive than is currently recognised. Furthermore, partial melting of arc lower crustal cumulates before delamination may be crucial to the development of arcs and the evolution of the continental crust.