Abstract Mid- to late-Miocene continental arc volcanism on the North Island of New Zealand is found in the Coromandel Peninsula, the Kiwitahi volcanic chain and the Taranaki Basin (Kora Volcano) offshore the western margin of the North Island. Coeval oceanic arc volcanism is also found along the offshore Colville Ridge/Kermadec Ridge north of New Zealand. This Pb–Sr–Nd–Hf isotopic study aims to evaluate mantle sources and potential crustal contaminants along these sections of the Miocene arc system. The Colville/Kermadec Ridge and Kora lavas have the lowest Sr (0.7029–0.7045) and highest Nd (0.51305–0.51292) ratios; the Coromandel and Kiwitahi lavas overlap (Sr = 0.704–0.706; Nd = 0.51268–0.51296). The Colville/Kermadec Ridge, Kora, and Coromandel/Kiwitahi rocks form three distinct arrays on Pb–Pb plots, all above the Northern Hemisphere Reference Line, but none trend towards local Mesozoic basement greywackes. Isotopic and trace element ratio variations suggest that subducted sediments are a component in Coromandel/Kiwitahi mafic lava sources. The younger, southern Kiwitahi lavas have a more depleted mantle source than that for the older, northern Kiwitahi chain. Evolved lavas commonly have interacted with Waipapa basement rocks. Kora rocks have compositions similar to those of back-arc lavas and have been emplaced as sills in a rift environment above a distinct subduction-modified mantle.

Abstract Understanding the formation of volcanic and epiclastic reservoirs is pivotal for exploring geoenergy resources such as geothermal energy, hydrocarbons, and new CO 2 sequestration and hydrogen storage opportunities. This paper examines the processes controlling the quality of pyroclastic and epiclastic reservoirs of the Kora volcano, an extinct stratocone presently buried in the offshore Taranaki Basin, New Zealand. We conduct detailed seismic reflection interpretation, drillcore lithofacies and wireline-log description, petrographic analysis, and analytical tests to generate a unified framework that explains the formation of volcaniclastic reservoirs from basin to pore-scale. Each stage of construction and degradation of the Kora volcano is associated with particular processes that increase or reduce reservoir quality. Primary processes include quench fragmentation, deuteric mineral dissolution, and epiclastic sedimentation. Secondary processes comprise mineral alteration (mainly meteoric; minor hydrothermal and diagenetic), mechanical stress fracturing (mainly tectonic; minor magmatic and burial deformation), and pervasive biogenic cementation. Epiclastic conglomerates present the highest reservoir quality (average 23% porosity and up to 997 mD permeability), followed by lapilli-tuffs and tuff-breccias. In contrast, bioclastic epiclastic sandstones are typically cemented by carbonates and pyrite. Our models and interpretations will increase understanding of the formation of volcaniclastic reservoirs and aid exploration of geoenergy resources in volcanic terrains.

Abstract The lamproites and kimberlites are well known from the Eastern Bastar Craton, Central India. However, a Proterozoic lamprophyre dyke is discussed here, from the Western Bastar Craton (WBC). The field geology, petrographic, mineralogical and whole-rock and in-situ trace element geochemistry of biotite are described to understand the petrogenesis and lithospheric evolution in the WBC. The Thanewasna lamprophyre (TL) is undeformed and unmetamorphosed, intruded into c. 2.5 Ga charnockite and metagabbro but closely associated with c. 1.62 Ga undeformed Mul granite. The TL has a characteristic porphyritic texture, dominated by phenocrysts of biotite, microphenocryst of amphibole, clinopyroxene and a groundmass controlled by feldspar. Mineral chemistry of biotite and amphibole suggest a calc-alkaline (CAL) type, and pyroxene chemistry reveals an orogenic setting. The TL is characterized by high SiO 2 and low TiO 2, MgO, Ni and Cr, consistent with its subcontinental lithospheric origin. The presence of crustal xenolith and ocelli texture followed by observed variations in Th/Yb, Hf/Sm, La/Nb, Ta/La, Nb/Yb, Ba/Nb indicate substantial crustal contamination. Whole-rock and in-situ biotite analysis by laser ablation inductively coupled plasma mass spectrometry show low concentrations of Ni (30–50 ppm) and Cr (70–150 ppm), pointing to the parental magma evolved nature. Enrichment in H 2 O, reflected in magmatic mica dominance, combined with high large ion lithophile element, Th/Yb ratios, and striking negative Nb–Ta anomalies in trace element patterns, is consistent with a source that was metasomatized by hydrous fluids corresponding to those generated by subduction-related processes. Significant Zr–Hf and Ti anomalies in the primitive mantle normalized multi-element plots and the rare earth element pattern of the TL, similar to the global CAL average trend, including Eastern Dharwar Craton lamprophyres. Our findings provide substantial petrological and geochemical constraints on petrogenesis and geodynamics. However, the geodynamic trigger that generated CAL magmatism and its role in Cu–Au metallogeny in the WBC, Central India, is presently indistinct in the absence of isotopic studies. Nevertheless, the lamprophyre dyke is emplaced close to the Cu–(Au) deposit at Thanewasna.