Geology of the Cobb Intrusives, Takaka Valley, North‐West Nelson, New Zealand

The igneous complex in the upper Takaka and Cobb valleys is the largest mass (1.5 × 2.5 km) of the Cobb Intrusives, which elsewhere occur as small tenses of mafic and ultramafic rock associated with a belt of Cambrian sediments and volcanics between Mount Arthur and Collingwood in North‐west Nelson, New Zealand. The complex consists of basal peridotite, overlain in turn by orthopyroxenite, websterite, and gabbro, all of which show extensive alteration. Despite this alteration, primary igneous layering and relict igneous textures are preserved in many places. Clinopyroxene is the most abundant relic primary mineral with an average composition of Ca 45 Mg 56 Fe 5 ; most variation in the chemistry of this mineral is because of differences in the amounts of exsolution. Scarce relic olivine is rich in magnesium (Fo 96 ), as is orthopyroxene (En 92 ). Small chromite segregations, with Cr 2 O 3 contents of 45–63%, are found in serpentinised peridotite and orthopyroxenite, and high‐chromium detrital chromite in the Upper Cambrian Lockett Conglomerate indicates a middle Cambrian or earlier age for the Cobb Intrusives. Asbestos has long been known from the complex, while small talc deposits are found at the contacts of serpentinite and more siliceous rocks, and talc‐magnesite rock is quarried as a fertiliser. On the basis of field and petrographic evidence, it is considered that the Cobb Intrusives complex in the Takaka area is a slice of a stratiform intrusion, emplaced at a fairly shallow level during Middle Cambrian time, and hydrothermally altered soon after.