Integrated petrologic, geochemical and experimental investigation of intraplate alkaline magmas at Dunedin Volcano: constraints on mantle sources, differentiation trends and magmatic processes

Intraplate volcanic settings have long been recognized for the compelling diversity of magma compositions if compared to plate boundary magmatism. Alkaline volcanoes are typically characterized by an impressive variety of crystalline cargoes carried by their magmas, witnessing a complex source-to-surface path. Understanding intraplate alkaline volcanoes is a task of fundamental scientific value, as it relates to the very processes of magma genesis and differentiation. Moreover, it is crucial for urban development, as they are often the sites of human activity, where they provide natural resources as well as constituting a natural hazard. The Dunedin Volcano (South Island of New Zealand) represents a classic case study in which some of the early concepts on the differentiation of intraplate alkaline suites were originally proposed. To investigate processes that lead to magma generation, polybaric storage and magma emplacement, in this thesis whole-rock major elements, trace elements and isotopic compositional data have been collected by means of X-Ray Fluorescence (XRF), Inductively Coupled Plasma Mass Spectrometer (ICPMS) and Multicollector ICPMS, respectively. These have been integrated with mineral-scale studies of intrusive and eruptive products, performed with Electron Probe Micro Analyzer (EPMA) and Laser Ablation ICPMS. The composite volcano erupted magmas with silica-saturated to undersaturated affinity, the more abundant eruptive products being represented by primitive (alkali basalts) and highly differentiated (trachyte-phonolite) magmas. Basalts are characterized by a range of alkali contents, incompatible element abundances, isotopic compositions, as well as mineral populations. High-alkali basalts and mid-alkali basalts have complexly zoned clinopyroxene crystals (Mg#65-82), rich in incompatible elements and characterized by FOZO-HIMU isotopic signatures (87Sr/86Sri = 0.70277-0.70315, 143Nd/144Ndi = 0.51286-0.51294, and 206Pb/204Pb = 19.348-20.265). Low-alkali basalts are characterized by clinopyroxenes (Mg#69-84) with resorbed mafic cores (Mg#78-88), and plagioclase crystals (An43-84), and have lower incompatible element abundances and isotopic compositions that trends toward EMII (87Sr/86Sr = 0.70327-70397, 143Nd/144Ndi = 0.51282-0.51286, and 206Pb/204Pb = 19.278-19.793). Partial melting models indicate that the variable alkalinity and isotopic composition of basaltic rocks result from interaction of low-alkali melts derived from fertile asthenospheric mantle domains with melts derived from the metasomatized lithospheric mantle. Basaltic magmas and intermediate magma compositions (phonotephrite, mugearite, tephriphonolite and benmoreite) crystallized at depths mostly comprised between the lower and the mid-crust (13-27 km). Basaltic rocks crystallized olivine + clinopyroxene + oxides, while intermediate magmas are produced after fractionation of clinopyroxene + amphibole + plagioclase + titanomagnetite. Accumulation of crystals segregated from the intermediate magmas is witnessed by a suite of crystal-rich mafic enclaves (gabbros, amphibolites and clinopyroxenites), entrained by the ascending magmas. At shallow crustal levels (5-11 km) phonolites and trachytes are produced by different processes. Trachytes differentiated from benmoreite magmas, following significant feldspar fractionation (~50 wt.%) and small amount of country rock assimilation (≤10 wt.%). Phonolites instead differentiated from tephriphonolites, and represent the interstitial liquids of crystal mush systems, constituted by crystalline networks of alkali feldspars and minor amphibole, represented by syenitic and feldspar-rich enclaves. Trachyte and phonolite may represent two stages in the development of Dunedin Volcano plumbing system, where crustal assimilation becomes more difficult as a crystal mush forms and shields later injections in the upper crust. Differentiation trends have been constrained by high pressure–high temperature piston cylinder experiment, geochemical models and thermodynamic modelling, and results indicate that two distinct differentiation trends coexist, as supported by previous studies. The silica–saturated magmatic lineage is related to mid- and high-alkali basalts parental compositions crystallizing at 15-25 km, 1050-1200°C with H2O>3 wt.% and evolves as mugearitic to benmoreitic magmas crystallize clinopyroxene + amphibole + plagioclase + titanomagnetite at mid- to upper crustal levels. The highest silica-enrichment observed in trachytes (SiO2>60 wt.%) relates to the lowest crystallization temperatures (850-950°C) and to high water contents (H2O>6 wt.%). Silica-undersaturated magmas differentiate after high-alkali basalts at high pressure (20-30 km) and high temperatures (1100-1250°C) in relatively water-poor magmas (H2O ≤ 2.5 wt.%) and fractionate clinopyroxene + olivine + titanomagnetite. Further fractionation of clinopyroxene + amphibole + titanomagnetite from phonotephrite produced tephriphonolite magmas, whose higher volatile budget (H2O ≥ 4.5 wt.%) and lower temperatures (900-1000°C) caused abundant crystallization of alkali feldspars ± biotite ± amphibole in the shallow crust, resulting in the formation of highly crystalline regions (≥60 wt.% crystals) with alkali enriched interstitial melts of phonolitic composition. The variety of alkaline rock compositions and coexisting crystalline populations are not only related to fractionation along differentiation trends, but are strongly influenced by a range of complex processes as magma transfers across diverse storage regions. Different magmas mixed and entrained crystalline material segregated from previous magma batches. The multidisciplinary approach used in this thesis allowed the reconstruction of an integrated source-to-surface history of Dunedin Volcano magmas, with the aim of contributing to our understanding of the processes that control the origin and differentiation of intraplate alkaline systems on a global scale