A petrological study of the Mandamus igneous complex, North Canterbury

Abstract

The Mandamus Igneous Complex is the basal remnant of an alkaline volcano of probable Middle Cretaceous age. The complex is a product of three distinguishable phases of igneous activity. An early eruptive phase built a basalt-trachybasalt-trachyandesite-trachyte volcano (Korari Volcanics). Thin vertical sheets similar in composition to the volcanics were intruded into the surrounding country rock (Torlesse Supergroup). Towards the end of the eruptive phase magma began to intrude the base of the volcano and subsequently cooled slowly to form plutonic rocks (Mandamus Intrusives). This later intrusive phase is made up of a number of separate injections of progressively more differentiated magma from an underlying source. An alkali gabbro (Mg) was probably the first to be intruded; followed by a syenodiorite (Md). Both of these small stocks were engulfed and partly assimilated by a later alkali syenite (Ms) which was sufficiently large to differentiate further in place producing peralkaline syenites. Dykes of peralkaline microsyenite and lamprophyric rocks (kersantites, minettes, camptonitic basalts) were intruded after consolidation of the alkali syenite. The final liquid derived from the underlying magma source was a peralkaline undersaturated (agpaitic) phonolite crystallising sodalite, aegerine and sanidine and which was intruded either as a dyke or plug into the earlier igneous rocks. Incorporation of xenoliths of these rocks produced a contaminated sodalite phonolite. Extreme differentiation in the underlying magma chamber was accompanied by a build up of volatiles which produced a vapour pressure sufficient to pierce the roof. The pent up gases drilled a vent (diatreme) up through the Mandamus Complex and probably reached the surface as a violent explosive eruption. Rocks dislodged by the uprushing gases formed the breccias and agglomerate that now fill the vent (Dove Volcaniclastics). A separate olivine gabbro intrusion (Hut Gabbro) crops out to the NE of the main complex but probably represents the lower part of another intra-volcanic pluton which has since been isolated by erosion. Chemically the Mandamus alkaline rocks belong to a moderately potassic lineage characterised by high K2O and K2O:Na2O ratios, generally between 1:2 and 1:1. Iron enrichment is slight in the passage from gabbro, through the intermediate syenodiorite, to alkali syenite and is probably caused by early precipitation of iron oxides. The alkali syenite has a composition appropriate to the low temperature area in the system Ne-Ks-Qz-H2O at pH2O = 1000 kg/cm2; the production ofa peralkaline syenite by fractional crystallisation of the alkali syenite magma is demonstrated by plotting the successive rock compositions in this system. The occurrence of tourmaline in the contact aureole surrounding the Mandamus complex is thought to be due to the migration of boron from the residual alkali syenite magma into the greywackes and argillites of the Torlesse Supergroup. Tourmaline is prevented from crystallising within the Mandamus Complex by the undersaturated nature of all the igneous rocks.