Miocene intrusive Rocks of Moehau Range, Coromandel

Abstract

An Upper Jurassic, greywacke suite of Moehau Range is intruded by dikes and small plugs of porphyritic andesite and dacite (Kai-iti Porphyrites) and by a composite pluton of granodiorite and quartz-diorite (Paritu Plutonics), the latter having a contact aureole of biotite hornfels with porphyroblastic pyroxene, almandine, quartz, plagioclase and orthoclase; tourmaline occurs on joint faces

The porphyrites have suffered hydrothermal autoalteration and silicification and -have caused chlorite, sericite, rutile, and stilpnomelane to form in greywacke xenoliths. There is a wide age range with respect to the Paritu Pluton, which has produced porphyroblastic hornblende, epidote, and tourmaline in pre-pluton dikes but oligoclase, biotite, and hornblende in extrusive andesite.

The Paritu Pluton is a sub-volcanic, epizonic, composite intrusion of hornblende-pyroxene granodiorite, pyroxene-hornblende quartz diorite, marginal pyroxene tonalite, hornblende-pyroxene quartz diorite, and biotite-pyroxene quartzdiorite porphyry as well as quartz-orthoclase aplites and pegmatitic quartzorthoclase- epidote-calcite druse cavities. The pyroxenes are ferro-hypersthene and titaniferous augite, and the hornblende is an intratelluric replacement of augite and xenocrystic biotite. The pyroxenes have also been selectively deuterically altered to two distinct uralites.

The tonalite has formed from the dissemination of biotite rich xenoliths of greywacke in the granodiorite and quartz-diorite magmas, the xenoliths supplying xenocrystic biotite, apatite, and zircon, and their ingestion causing the growth of an oligoclase zone on plagioclase crystals. An early quartz-diorite porphyry has a marginal facies in which biotite and orthoclase have formed at the expense of hornblende and pyroxene, and an apparent excess of potash feldspar is associated with tourmaline. Late potassic magmatic or metasomatic alteration of plagioclase to sodic perthite along joints has produced the aplitic liquids, and late hydrothermal activity has altered ferromagnesians to aggregates of pennine, calcite, epidote,and sphene, and plagioclase to sericite, calcite, and albite; pyrite is widely disseminated but other metallic sulphides are of minor importance.