Abstract The c. 450 km-long Brook Street Terrane (pre-Alpine Fault displacement) sheds light on processes of arc magmatism and related sedimentation. A very thick (up to 15 km) succession accumulated south of the Alpine Fault in the Takitimu Mountains during the Early Permian. Predominant arc-flank talus is intercalated with basic extrusive and intrusive igneous rocks. Volcaniclastic sediments mainly accumulated by mass-flow and turbidity current processes. The sediments were mostly derived from differentiated, arc-core, basaltic–andesitic rocks, contrasting with less evolved arc-flank flows and minor intrusions. Some igneous rocks are mildly enriched, supporting an extensional back-arc setting. After volcanism ended, Middle–Late Permian mixed carbonate–volcaniclastic gravity-flow deposits were derived from a non-exposed carbonate platform. Other volcanogenic successions in the south (Bluff, Riverton) represent smaller eruptive centres. In contrast, north of the Alpine Fault (e.g. Nelson), volcanism began with mostly felsic tuffaceous gravity-flow deposits, followed by extrusion/intrusion of clinopyroxene-rich, primitive magmas, related to arc rifting, and ended with an accumulation of a mixed basic–felsic volcaniclastic forearc apron. Taking account of regional comparisons, the Early Permian arc is interpreted as having formed adjacent to Gondwana (on accreted or trapped oceanic lithosphere), whereas the lithologies north of the Alpine Fault represent contrasting Late Permian continental arc magmatism.

Abstract The Dun Mountain ophiolite and related oceanic-arc rocks (Otama Complex) formed above a westward-dipping subduction zone within Panthalassa, with implications for the emplacement of Cordilleran-type ophiolites and arcs elsewhere. The ophiolite is overlain by the Mid–Late Permian Upukerora Formation (up to 850 m), a predominantly very coarse breccia-conglomerate that mainly accumulated by mass flow. Lesser amounts of sediment accumulated from turbidity currents and as background hemipelagic sediments. The succession unconformably overlies ophiolitic basaltic or, rarely, gabbroic rocks after a regional hiatus. Much of the coarse clastic debris was derived from the underlying ophiolite. However, clasts of plagioclase-phyric basalt, felsic volcanics and quartz-bearing intrusive rocks, including plagiogranite, are over-represented compared to the ophiolite. The evolved igneous material was derived from an incipient oceanic arc (the Otama Complex) that bordered or covered the ophiolite, especially in the south. The coarse clastic material accumulated following the activation of north–south-trending, subaqueous, extensional growth faults within the underlying oceanic crust. Large blocks of mainly basalt, diabase and gabbro were also shed down fault scarps from relatively shallow-water to deeper-water settings. Fault-controlled talus accumulated soon after Mid-Permian docking of the ophiolite and oceanic arc with SE Gondwana to initiate the Mid-Permian–Mid-Triassic Maitai continental margin forearc basin.

Abstract Large-volume, high-crystallinity, chemically homogeneous ignimbrites, dubbed ‘monotonous intermediates’, provide a unique opportunity to investigate the evolution of crustal magmatic reservoirs. We present the results of hydrothermal experiments on a dacite from Fish Canyon Tuff (FCT) in Colorado (USA), a classic example of a monotonous intermediate deposit, in order to characterize the variations in chemical and physical properties of hydrous dacite magmas as a function of temperature. The experiments (200 MPa, 720–1100°C) span the inferred pre-eruptive conditions of FCT magmas, and are shown to provide the best match to the chemical and physical properties of the erupted magmas at 790±10°C under conditions at or close to water-saturation. The results show the important effect of water content in controlling the chemical and physical evolution of magma, and the contrasted behaviour of water-saturated v. water-undersaturated magmas. In both cases, however, there is a broad interval of temperature (200°C) over which crystal fraction changes little. By recasting this behaviour in terms of enthalpy, rather than temperature, as the independent variable we show that this interval corresponds to a minimum in the change in crystallinity per unit of energy added or subtracted from the system, such that small perturbations to the heat content of the system (e.g. by cooling or new magma injections) results in very little change in magma properties. The crystal content in this interval is 55–65 wt%, which is close to the phenocryst content (40–55 wt%) of monotonous intermediates. We propose that crystal-rich magmas tend to settle in this ‘petrological trap’, changing little in physical and chemical properties over time as the system grows. Petrological trapping enables very large volumes of intermediate magma to accumulate in the shallow crust until such time as the net buoyancy force of these crystal-rich magma is sufficient to overcome the strength of the roof rocks, leading to a potentially very large eruption.

Sandstone provenance is commonly characterized by point counting thin sections using a petrographic microscope. An analytical tool (QEMScan™: Quantitative Evaluation of Materials by Scanning Electron Microscopy) newly applied to provenance analyses provides complementary data and alternatives for quantifying modal compositions of sandstones. QEMScan combines scanning electron microscope (SEM) imaging and elemental analyses to create mineralogical maps of solid materials. Three different applications of QEMScan (mineralogic maps, bulk mineralogy calculations, and automated disaggregate counts) were compared to traditional (petrographic) point-count results using a test data set of 12 samples from Utah and Mongolia. Results indicate that QEMScan can provide semi-automated and rapid analyses of sandstone provenance. In the case of the manual QEMScan point-count method, the new technique largely removes operator error in grain identification. However, direct comparison to petrographic data currently requires time-consuming image processing, and adjusting QEMScan processors to recognize grain boundaries and complex grain-mineral types. Moreover, comparison of these methods provided a means to assess the operator error associated with point counting. The results of the petrographic and QEMScan methods generated comparable results, indicating that operator error does not significantly affect modal compositions through traditional techniques.