The Petrology of the Blue Mountain Igneous Complex, Marlborough, New Zealand

Abstract

Blue Mountain is a central type alkali ultrabasic-gabbro ring complex of Middle Cretaceous age intruding Upper Jurassic sediments, Inland Kaikoura Range, Marlborough, New Zealand. The complex (1.5km x 1km) is composed of a layered sequence of olivine pyroxenites, olivine-plagioclase pyroxenites and olivine ferrogabbros which dip steeply inward (around 70°) near the margin and flatten out towards the centre of the intrusion. This simple structure has been slightly modified by faulting and the western part of the complex has been slightly modified by faulting and the western part of the complex has been upthrown about 150 metres. The whole complex has been tilted about 30° to the west. Incomplete ring dykes of titanaugite-ilmenite gabbro and lamprophyre intrude the ultrabasic rocks and the ultrabasic and gabbroic rocks are enclosed within a marginal ring dyke of alkali gabbro. The complex is cut by a swarm of lamprophyre dykes, the majority of which strike E-W and form part of a much larger swarm intruding Jurassic-Cretaceous sediments of the Marlborough area. Field and chemical evidence indicates that the order of emplacement of the various intrusive phase at Blue Mountain was as follows:- ultrabasic and gabbroic rocks; titanaugite-ilmenite gabbro and lamprophyre ring dykes; marginal alkali-gabbro ring dyke; lamprophyre dyke swarm. The igneous rocks have metamorphosed the surrounding greywacke-argillite sediments to a hornfels which extends up to 600 metres from the igneous contact. The metamorphic grade ranges from the albite-epidote-hornfels facies to the upper limits of the hornblende-hornfels facies. The rocks of the complex are mainly composed pf olivine, endiopside, titanaugite, plagioclase, titaniferous hornblende, titanobiotite, ilmenite and titaniferous magnetite. With differentiation in the Blue Mountain rocks the pyroxenes show the trend:- endiopside titanaugite sodic titansalite; the amphiboles show the trend:- titaniferous hornblende kaersutite Fe rich hastingsite. The mineralogy of the Blue Mountain rocks is therefore similar to intrusions of alkali-olivine basalt affinity. Reversals in the composition of cumulate olivine and endiopside in the ultrabasic-gabbroic rocks indicates at least three main injections of magma into the Blue Mountain chamber. At least once in the exposed sequence, during the formation of the gabbroic rocks, the roof of the chamber was fractured, probably resulting in the eruption of magma to higher levels. Within each injection cumulate endiopside shows a progressive enrichment in magnesium which is considered to be the result of the resorption of early-formed olivine. Cumulate olivine and intercumulate titanaugite show a progressive iron enrichment with differentiation. Marked iron and titanium enrichment in the Blue Mountain magma is shown in the ferrogabbros which contain abundant ilmenite with titaniferous magnetite, iron rich olivine, and the extensive development of exsolved ilmenite needles in titanaugite. Calculations of the composition of successive intercumulate liquids in two ultrabasic rocks also show extreme iron enrichment. The major oxides and trace element abundances of all the Blue Mountain rocks show progressive differentiation in the successive injection phases of the complex. This suggests that a lower reservoir that fed Blue Mountain was undergoing a parallel differentiation process and that the magma was periodically injected to higher levels to form the intrusive phases at Blue Mountain. The emplacement of the Blue Mountain Complex was the result of ring fracturing. This was associated with subsidence of a truncated block of country rock within the ring fracture into an underlying magma chamber with injection of magma from the lower chamber into the upper one. Slow subsidence of the central block while crystallization and settling of early formed minerals was in progress resulted in a steepening of the primary igneous dips near the margins of the intrusion. This was associated with the inward and downward movement of cumulates from the steeper slopes to a central area where stable deposition conditions prevailed. The intrusion of the internal titanaugiteilmenite gabbro and lamprophyre ring dykes was probably associated with the subsidence of the ultrabasic and gabbroic rocks. Blue Mountain is considered to be a subvolcanic magma chamber that was formed at a depth of about 4km. It is similar to the central type volcanic-plutonic complexes of the Scottish Hebrides and close comparisons in rock types, structures, mineralogy and geochemistry can be made with the plutonic rocks exposed in the cores of oceanic volcanoes such as the Canary Islands and Tahiti.