Provenance of the Murihiku Terrane, New Zealand: evidence from the Jurassic conglomerates and sandstones in Southland

Abstract

Sedimentary petrological, geochemical, and geochronological studies were carried out to reveal the provenance of the Murihiku Terrane, Southland, New Zealand. The Murihiku Terrane comprises Late Permian to Early Cretaceous volcaniclastic sediments that were deposited at the fore-arc or back-arc basin of the eastern Gondwana margin. The strata in the Waikawa district, Southland, are Middle or early Late Jurassic and comprise sandstones interbedded with conglomerates and mudstones. The sediments can be petrologically classified into two groups: one dominated by intermediate-mafic volcanic detritus (type-A), and the other more quartz bearing (15–20% in QFL), dominated by felsic volcanic and plutonic detritus (type-B). The chemistry of detrital clinopyroxene in type-A sediments is compatible with their derivation from island arc volcanics. Whole-rock major- and trace-element analyses suggest that granitoid and gabbroid clasts associated with type-B sediments are also of volcanic arc origin. Geochemical characteristics of the plutonic clasts can be compared closely with the Median Batholith (Median Tectonic Zone). Whole-rock K–Ar ages of the clasts (156±8 and 177±9 Ma for gabbroid clasts and 156±8 and 212±11 Ma for alkali feldspar granite clasts) are consistent with the Median Batholith magma cooling. Small quantities of grandite and pyrope-bearing garnet grains in type-B sediments might be derived from metamorphosed intermediate-mafic volcanic rocks to amphibolite facies. Granitic cataclasite and hornfels are also compatible with derivation from the Median Batholith that was accreted to the Brook Street Terrane.