Cumulate rocks of the Luxmore complex, Fiordland, New Zealand petrology and geochemistry

Abstract

This report presents detailed petrological and geochemical observations of the cumulate rocks of the Luxmore Complex (LC) with an introductory examination of dioritic and granitic rocks to the west of Mt. Luxmore. The cumulate rocks of the LC form an elongate body with a rough concentric zonation from ultramafic rocks to altered gabbroic rocks. This zonation is supported by a mineral and whole rock (Bulk MgO#) chemical traverse through the complex. The cumulate rocks are divided into the Ultramafic-mafic Series and the Gabbro Series. The rocks of the Ultramafic-mafic Series are plagioclase- and amphibole bearing, and are characterised by significant locallised heterogeneity at outcrop and microscopic scale. This heterogeneity is mostly defined by the abundances of cumulus and postcumulus plagioclase and amphibole relative to olivine. Plagioclase-rich layers in the ultramafic-mafic cumulates, with diffuse and poorly formed margins, are interpreted as injection layering. These features, and thenumerous plagioclase-cumulate pods, sills and veins are best explained by the injection of an aluminous basaltic magma into the main ultramafic body before it had consolidated. A plug-like outcrop of dioritic gabbro is considered further evidence for multiple intrusions during the emplacement of the LC. The apparent complexity of the LC is evidence for complicated and multi-event crystallisation histones within sub-volcanic plumbing systems. Fine-grained mafic sills consisting of amphibole, plagioclase, clinopyroxene, olivine and chrome spinel are interpreted as the products of quenched water-saturated melt expelled from the mam cumulate body during cooling. The presence of olivine, chrome spinel, plagioclase and clinopyroxene in the cumulate rocks indicates that they were emplaced from the crystallisation of basaltic magma/s. The proposed crystallisation sequence is olivine + chrome-spinel > plagioclase + Fe-Ti oxides > clinopyroxene > orthopyroxene > amphibole. Magma composition, pressure/temperature and water content mostly drove this crystallisation sequence. The observed crystallisation of plagioclase before clinopyroxene in the Luxmore cumulates is indicative of crystallisation at moderate pressure (0.5-0.8 GPa) and water content, based on a review of experimental results in the literature. The presence of multi-phase coronas, consisting of orthopyroxene, amphibole and spinel, also roughly constrains the depth of emplacement of the Luxmore cumulates to 0.5-0.8 GPa (18-24 km depth). Plagioclase compositions are irratic, particularly in the orthocumulate rocks. This includes fluctuating An% within individual samples, mostly in the range An85-94, and increasingly calcic grain rims adjacent to postcumulus or secondary amphibole. This is likely the result of fluids percolating through the cumulus pile proceeding crystallisation. From the elevated abundances of residual postcumulus amphibole combined with the observed hydrothermal alteration of the later formed rocks, manifested by partial to complete replacement of mafic phases by palegreen amphibole, it is interpreted that the amount of water interacting with the cumulates markedly increased during crystallisation. The range of olivine compositions (F070-81) is low compared to primitive mantle-derived ultramafic rocks and it is suggested that the parent magma was already considerably evolved before emplacement of the exposed cumulate outcrop. The compositions of co-existing olivine and plagioclase from the ultramafic-mafic cumulates is consistent with emplacement of the LC in an arc setting. Clinopyroxene compositions reflect low titanium relative to aluminium, another general characteristic or arc magmas. Electron microprobe analyses of chrome spinel inclusions within olivine indicate that two distinct types of spinel are present within individual grains: an aluminous phase and a Fe +-rich phase, both contain similar amounts of chrome (wt%). Plotted on a spinel tetrahedron, these two compositions representunmixing of an original spinel composition along a compositionally defined solvus. The incorporation of a Fe + -rich phase into the spinel structure is interpreted as evidence for oxidising conditions in the parent magma. The eventual unmixing of the two compositions is likely to have been promoted by prolonged cooling at low (sub solidus) temperatures at moderate depths in the crust. Homogenised spinel separate compositions plot within the spinel solvus, indicating that the end members are formed from compositional unmixing of one original phase. Calculations for oxygen fugacity using olivine and homogenised spinel separates indicate the conditions of crystallisation were oxidising relative to the fayalitemagnetite- quartz (FMQ) buffer. Olivine-spinel temperature calculations indicate sub solidus re-equilibration. Two-pyroxene thermometry applied to pyroxene-bearing rocks also indicates sub-solidus pyroxene re-equilibration temperatures in the range 900-950°C. Combined amphibole-plagioclase thermometry and Al-in-amphibole barometry indicate that quartz-diorite adjacent to the cumulate rocks was emplaced at pressures of 0.35 GPa at 690°C. This point lies on the experimentally tested wet tonalite solidus indicating it is an accurate pressure estimate. Combined with field observations, the pressure estimates are indicative that the cumulate rocks of the LC were tectonically emplaced into the dioritic and granitic rocks to the west of Mt. Luxmore. It is interpreted here that the cumulate rocks of the LC represent a dismembered portion of a much larger system and any definitive size estimates would be speculative. It is also interpreted that within this system, the crystallisation sequence (as observed in the cumulate rocks) may have in part been responsible for the derivation of dioritic and tonalitic magmas that rose to higher levels in the crust. However, from this study, no direct link can be drawn between the cumulate rocks and the dioritic, tonalitic and granitic rocks to the west of Mt. Luxmore. Whole rock Pt-Pd-Au analyses fail to reveal any anomalies, but moderate to high background Cu levels in the Gabbro Series cumulates indicate that these rocks may be the source for Cu mineralisation in minor carbonate breccias throughout the LC. Trace element chemistry of fine-grained mafic sills is indicative of an arc setting and that the cumulate parent magma was generated by low degrees of partial melting of a moderately depleted mantle source. The mafic sill trace element patterns display moderate incompatible element enrichment, a feature of ‘medium-K’ arc magmas. Depletion of HREE in the mafic sill, phenocryst and calculated melt REE patterns, indicates that partial melting occurred in the presence of a phase with Dhree > 1 such as clinopyroxene or garnet. The trace element patterns for the dioritic and granitic rocks also display an arc signature, particularly the enrichment of large-ion lithophile elements and depletion of high field strength elements relative to MORB. The trace element patterns of the Luxmore mafic sills, and many other petrological/geochemical features of the cumulate rocks, display strong similarities with other cumulate complexes from the South Island. Cumulate phenocryst trace-element patterns prove these similarities, indicating a similar source and setting, possibly the same arc system. It is interpreted that these cumulate complexes represent the root zone of an arc system active throughout the Triassic and Jurassic. Show more