Composition of Monocrystalline Detrital and Authigenic Minerals, Metamorphic Grade, and Provenance of Torlesse and Waipapa Graywacke, Central North Island, New Zealand

Abstract

Sandstones of the juxtaposed and partially coeval quartzofeldspathic Torlesse terrane and volcanogenic Waipapa terrane of North Island, New Zealand, are generally described as having been derived from silicic continental arc and evolved intermediate volcano-plutonic arc sources, respectively. Modal and chemical compositions of the two terranes differ slightly as a result. From textural considerations, their single-grain (unitary) detrital mineral populations are inferred to have been derived largely from the plutonic components in their sources. Intensive microscopic and electron microprobe study of two representative samples shows that the unitary detrital mineral assemblages in the two terranes are virtually identical, comprising quartz, plagioclase, K-feldspar, white mica, epidote, titanite, pumpellyite, ilmenite, rutile, tourmaline, zircon, and apatite. Detrital chlorite, garnet, and graphite also occur in the Torlesse sample, whereas amphibole, clinopyroxene, and prehnite occur in the Waipapa sample. Authigenic mineral assemblages are also similar, consisting of quartz, albite, chlorite, phengitic mica, epidote, titanite, pumpellyite, pyrite, and calcite. Stilpnomelane and pyrrhotite also occur in the Torlesse sample, and prehnite in the Waipapa specimen. These assemblages define upper prehnite-pumpellyite to lower pumpellyite-actinolite facies conditions (Torlesse) and lower prehnite-pumpellyite facies metamorphism (Waipapa). By comparison with published compositional data for minerals from plutonic, metamorphic, and volcanic rocks, electron microprobe analyses of individual minerals confirm that the unitary detrital grains in both terranes were largely derived from calc-alkaline S-type granitoid plutonic rocks. Contrasts in mineral compositions between the two terranes show that the Torlesse unitary mineral detritus was derived almost entirely from granodiorite, whereas the Waipapa grains originated from a mixed diorite, monzonite, and granodiorite plutonic component in their source. In neither terrane was detritus derived from granite in the strict sense. Although the plutonic components in their sources are lithologically similar, the compositional contrasts seen indicate that they were not coeval or spatial components of the same terrane. Detailed electron microprobe analysis of unitary detrital phases in low-grade metasedimentary rocks thus enables identification of specific source terrane lithotypes, and hence is a valuable complement to existing petrographic, modal, and chemical approaches that define more generalized provenances.