The Geology of the Takaka Igneous Complex, North-West Nelson, New Zealand

Abstract

The Takaka igneous complex intrudes sedimentary and volcanic rocks of Early to Middle Cambrian age and consists of basal peridotite, overlain by orthopyroxenites, websterites, and gabbros, all of which show considerable hydrothermal alteration. Despite this alteration, primary igneous layering and relict cumulus features are preserved in many places, and it is considered that serpentinization occurred at essentially constant volume. Serpentinization reduces chemical contrast between certain rock types (e.g. peridotite and orthopyroxenite), but at the same time it increases the differences among rocks of a single type by causing a re-distribution of elements (especially Fe and Mg). In a few places, talc deposits are found at the contacts of serpentinite with intrusive syenite and grandodiorite that have been dated by the K-Ar method at 280 and 370 m.y., respectively. Of the relict primary minerals, clinopyroxene is the most abundant, with an average composition of Ca45Mg50Fe5; most variation in the chemistry of this mineral is due to differences in the amounts of exsolution it contains, Clinopyroxene chemistry also suggests a genetic relationship between the ultramafic rocks of the Takaka complex and the volcanics nearby. Scarce relict olivine is rich in magnesium (Mg96) as is orthopyroxene (Mg92). Small chromite segregations, with variable but sometimes very high Cr, are found in serpentinized peridotites and orthopyroxenites, and high Cr detrital chromite in the Late Cambrian Lockett Conglomerate indicates a probable middle Cambrian age for the complex. On the basis of field, petrographic, and chemical evidence, it is concluded that the Takaka complex is a slice of a stratiform intrusion, emplaced at a fairly shallow level during middle Cambrian time, and thoroughly altered hydrothermally soon after.