Mapping and Petrochemistry of the Chatham Island Volcanics

Abstract

The Chatham Islands lie approximately 900km east of Christchurch, New Zealand at the end of a submarine ridge, the Chatham Rise. Bathymetry, sea floor samples, geophysical studies, and Paleozoic-Mesozoic rocks outcropping on the Chatham Islands suggest that the island group is situated on 20 - 25km thick continental crust, extending east from New Zealand. Igneous rocks outcropping on the Chatham Islands comprise intra-plate alkali basaltic lavas and pyroclastic rocks of Cretaceous, Eocene - Oligocene, and Pliocene age. This study is the first to undertake detailed mapping and petrology of the igneous rocks, and has involved three and a half months mapping and sampling, followed by whole-rock chemical and mineralogical analysis of representative samples from each of the three suites. The resulting data have been used to interpret the petrogenesis, by comparison of results with published data from other alkaline provinces, and computer modelling of magmatic processes. The Cretaceous volcanic rocks (Southern Volcanics) outcrop on the southern third of the two main islands (Chatham and Pitt Islands) and as isolated flows on northern Chatham Island. On southern Chatham/Pitt Islands, volcanic rocks comprise rare alkali olivine basalt, abundant hawaiite, mugearite, benmoreite and trachyte. Field relations interpreted in the light of geophysical evidence suggest that these rocks have been erupted from a vent system situated in present day Pitt Strait, between the two main islands. Early erupted melanocratic flows extend more than 10km from the vent system, whereas later more leucocratic flows, pyroclastics and intrusives are more localised. Petrography, mineralogy, whole-rock chemistry and modelling suggest the suite is a result of first mafic then felsic dominated fractional crystallisation of a hydrous magma in a low-pressure regime. Partial melting models, based on analyses of near-primary liquids indicate a non-chondritic amphibole and apatite-bearing garnet lherzolite source with small (15%) degrees of partial melting for the majority of rocks. Gabbro nodules in one flow are accidentally incorporated fragments, which crystallised from a hy normative liquid at P from 2 to 6kb. Eocene - Oligocene volcanic rocks (Northern Volcanics) outcrop as a series of vents aligned east - west across northern Chatham Island. These rocks are poorly exposed and comprise deeply weathered limburgite and rare trachyte breccia and blocks. Modelling suggests the derivation of the most chemically primitive limburgites by 10% partial melting of an amphibole and apatite bearing lherzolite enriched 1.5x chondrite. Element variations and Rayleigh fractionation modelling suggests that the less primitive limburgites are due to low pressure clinopyroxene fractionation, the end product of which could be trachyte. The Pliocene volcanic rocks comprise basanitoid lavas and breccias on northern Chatham Island (Rangitihi Volcanics), breccias with minor lavas on and near Pitt Island, and a phonolite intrusion at Pyramid Rock (Rangiauria Volcanics). Basanitoids show a restricted range in chemistry and mineralogy, but contain a diverse suite of inclusions. These comprise gabbro, hornblende pyroxenite, and clinopyroxenite nodules, and diopside, enstatite, sodian salite, and amphibole megacrysts. Mineral chemistry and textural relations suggest that diopside and enstatite megacrysts are disaggregated fragments of gabbro, which has been incorporated at lower crustal levels during magma ascent. Gabbro xenoliths frequently show signs of cataclasis and recrystallisation. Hornblende pyroxenite nodules and amphibole megacrysts most likely result from the reaction of a LIL-enriched fluid with anhydrous peridotite, this mantle material represented by clinopyroxenite nodules. Varying proportions of anhydrous/hydrous minerals in inclusions attest to varying degrees of reaction, and compare favourably with suites recorded from California and France. Modelling of partial melting processes is consistent with a non-chondritic source, similar in composition to that of Cretaceous and Eocene-Oligicene rocks. Depletion of lavas in compatible trace elments has been modelled by fractional crystallisation of clinopyroxene and olivine. All three phases of volcanism can be temporally related to changes in the velocity of the Pacific Plate relative to the Antarctic plate. It is suggested that the east-west alignment of the Cretaceous magma chamber, and Eocene – Oligocene vents was controlled by the regional fold axes of basement schist, and that a change in plate velocity resulted in crustal rupturing and the release of magma, although there does not seem to be any structural control on Pliocene volcanism. The intimate association of crustal flexuring, mantle metasomatism, and volcanism has been popularised by several recent studies. There is adequate evidence from the Chatham Island volcanics to support these ideas, which satisfy the timing and spatial aspects of volcanism, as well as accounting for the diversity of the rocks.