The geology of the western margin of the Tapuaenuku plutonic complex, Marlborough, New Zealand

Abstract

The Tapuaenuku Plutonic Complex is a 5x7 km central-type alkaline intrusion emplaced into Torlesse greywacke in the Inland Kaikoura Ranges, Marlborough, New Zealand. A radial dyke swarm is centred on the western margin of the complex but was not fed directly from the exposed complex. Rb-Sr and titanite/zircon fission track dating yield a concordant crystallization age of 95±10 my for the pluton that dates this episode of mid-Cretaceous igneous activity related to extensional tectonism. Fission track dating constrains the pluton to have been intruded at a depth less than 4 km and then up to 23 Ma to have resided at a depth of 3 to 7 km. Apatite fission track dates record an Early Miocene period of uplift related to the onset of compressional tectonism as the present day plate boundary system developed in the Marlborough region. A 3 km basin-shaped sequence of layered rocks (LS) comprises 70 % of the exposed plutonic rocks. The Layered Series has been intruded by a number of minor intrusive phases, particularly along the western margin of the complex. In order of intrusion these phases are: 1. Staircase Intrusives (SI); 2. Lower Hodder Gabbro and other minor gabbroic intrusives (LHgb & mgb); 3. Red Hills Breccia Pipe (Bp); 4. Hodder Intrusives (HI); 5. a suite of lamprophyre and phonotephrite-phonolite dykes. The LS is divided into the Lower (LLS) and Upper Layered Series (ULS). The LLS (1650 m thick) is a sequence of alternating zones of uniform olivine-clinopyroxene melanogabbro/pyroxenite and macrorhythmically layered zones comprised of 2 to 20 m thick 'mafic' (olivine-clinopyroxene gabbro) and 'felsic' (Fe-Ti oxide-clinopyroxene-plagioclase gabbro) layers. Hydrous minerals (biotite and kaersutite) are common intercumulus phases in the LLS. The ULS (1350 m thick) is a macrorhythmically layered unit dominated by 2 to 50 m thick alternations of 'mafic' and 'felsic' rocks. Plagioclase is more abundant in the ULS and the 'mafic' layers may contain cumulus plagioclase and Fe-Ti oxides near the top of the section. Hydrous minerals are uncommon in the ULS. Apparently the magmas from which the ULS crystallized were deficient in volatiles relative to those of the LLS. Two large dykes of monzogabbro and monzonite intrude the ULS parallel to the layering in the cumulates. Enigmatic orthopyroxene-bearing gabbros from the top of the ULS appear to imply that the host magma had crossed the critical plane of silica-saturation. Three influxes of magma occurred during the crystallization of the LS. Two influxes of mafic magma during crystallization of the LLS resulted in abrupt changes in cumulus mineral chemistry to more primitive compositions (higher Mg contents in olivine and clinopyroxene and higher Cr in clinopyroxene). Bulk rock compositions also record the influxes as discernable increases in Cr, Ni and MgO contents. Between influxes crystallization drove the magma to more evolved compositions. The ULS represents the final major influx, but of a more felsic and volatile-poor magma. Increases in incompatible element ratios such as Ce/Rb, Zr/Rb, Zr/K2O and Zr/Ba that, unlike absolute abundances of elements reflect the magma composition, through the LLS cumulates require intercumulus biotite and kaersutite to have influenced the chemistry of the evolving magma chamber by in situ crystallization. Compositional convection, involving the return of light residual intercumulus melt at advanced stages of solidification, is envisaged to have been the physical process responsible for return of residual melt from the solidification zone. The above ratios decrease through the ULS, in accordance with the incompatibility of these elements in cumulus phases. Extensive plagioclase crystallization in the ULS (which would probably increase residual melt density in the solidification zone inhibiting compositional convection) and the lack of biotite and kaersutite crystallization are responsible for this difference. The geochemistry of the LS highlights the important, and often neglected, effect that intercumulus (sub-liquidus) phases in a magma chamber can have on erupted magmas. The SI comprise a complicated stock comprising a suite of intermingled non-cumulate gabbroic/trachybasalt lithologies that intrude a cumulate gabbroic sheath. The LHgb is a non-cumulate arcuate body, dipping steeply outwards from the centre of the intrusion, that represents the upper part of a marginal ring dyke. As magma and/or volatile pressure increased during crystallization of the underlying magma chamber, a large intrusive pipe of heterogeneous breccia (Bp) was violently emplaced as a diatreme along the western margin of the complex. The HI cut the Bp and comprise strongly silica-undersaturated monzonite and sodalite syenite. A suite of residual volatile- and alkali-enriched lamprophyre dykes generated during crystallization of the trachybasalt magma in the magma chamber cut the LS rocks and the other intrusives and appear to have been parental to a highly evolved lineage of phonotephrite-phonolite dykes. The radial dyke swarm surrounding the complex is a mildly alkaline lineage (trachybasalt-trachyandesite) that has a composition presumed to be similar to that from which the cumulate rocks crystallized. Major element chemical variations in the dykes are principally controlled by olivine-clinopyroxene-plagioclase-Fe-Ti oxide-apatite crystallization. Initial 87Sr/86Sr isotope ratios for the complex are low (0.70281-0.70342) and suggest derivation of these magmas from a mantle that has recently been enriched in incompatible trace elements. Initial ratios for the complex are uniform, except for rocks of the SI, which are characterised by slightly higher initial ratios (0.70303-0.70342). Limited correlation of 87Sr/86Sr initial ratios with element ratios sensitive to greywacke assimilation (Nb/Rb & Nb/K2O) suggest the involvement of upper crust in the petrogenesis of these magmas. Chemical variation in the complex, from highly silica-undersaturated rocks to silica-oversaturated rocks, is largely the result of in situ and/or fractional crystallization. The appearance and timing of kaersutite crystallization (and hence dependence on intensive variables such as the volatile content of the magma) during the evolutionary sequence (trachybasalt-trachyandesite) is critical in the generation of these chemically diverse magmas. Again, this highlights the vital role intercumulus or sub-liquidus minerals can have on a crystallizing magma.