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  • 1
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    Trachyte-phonolite transition at Dunedin Volcano: Fingerprints of magma plumbing system maturity and mush evolution (2021)
    Elsevier
    Details
    Publication Date: 2021-12-22
    Description: Phonolite-trachyte associations are a common feature of alkaline volcanoes in intraplate settings, and their coexistence challenges closed-system magmatic differentiation scenarios. Here we have investigated the mineralogical and petrochemical features of dikes, lavas, pyroclastic deposits, and comagmatic crystal-rich enclaves outcropping at Dunedin Volcano (Otago region, southern New Zealand). These alkaline magmatic products show both highly and mildly alkaline affinities, trending towards phonolitic and trachytic end-members, respectively. Intermediate rocks are phonotephrites + tephriphonolites (highly alkaline series) and mugearites + benmoreites (mildly alkaline series) with a phenocryst assemblage of clinopyroxene + plagioclase ± amphibole formed at low to mid-crustal levels (i.e., ~29–16 km). Phonolites are porphyritic rocks characterized by alkali feldspar ± amphibole ± clinopyroxene. Their whole-rock compositions are highly enriched in incompatible elements, with variable Ba + Sr contents. A weak negative to slightly positive Eu anomaly is also associated with 87Sr/86Sr ratios of 0.7028–0.7031, which are comparable to those of parental magmas. Geochemical models indicate that phonolites originate as interstitial melts that are generated via abundant alkali feldspar crystallization from a shallow crystalline mush (i.e., ~14–5 km). Strong melt differentiation and extraction is testified by crystal-rich enclaves, as remnants of the mush region. On the other hand, trachytes are phenocryst-poor products strongly depleted in Ba + Sr and with a marked negative Eu anomaly. Trachytes are characterized by 87Sr/86Sr ratios of 0.7040–0.7060, which are different from intermediate rocks and phonolites, and trend towards crustal isotopic compositions. Integrated mass balance, trace element, and energy-constrained modeling confirm that trachytes originate from mildly alkaline magmas interacting with the country rock during feldspar fractionation. We interpret the transition from trachyte to phonolite formation and eruption resulting from the maturation of the plumbing system through accumulation, cooling, and degassing of both highly and mildly alkaline magmas.
    Description: Published
    Description: 106545
    Description: 2V. Struttura e sistema di alimentazione dei vulcani
    Description: 3V. Proprietà chimico-fisiche dei magmi e dei prodotti vulcanici
    Description: 4V. Processi pre-eruttivi
    Description: JCR Journal
    Keywords: Crystalline mush evolution ; Alkaline magmas ; Crystal-rich enclaves ; Trachyte-phonolite transition ; Magma fractionation ; Crustal assimilation ; Dunedin Volcano
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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  • 2
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    Intraplate Basalt Alkalinity Modulated by a Lithospheric Mantle Filter at the Dunedin Volcano (New Zealand) (2021)
    Oxford University Press
    Details
    Publication Date: 2021-12-22
    Description: Systematic variations in the crystal cargo and whole-rock isotopic compositions of mantle-derived basalts in the intraplate Dunedin Volcano (New Zealand) indicate the influence of a complex mantle-to-crust polybaric plumbing system. Basaltic rocks define a compositional spectrum from low-alkali basalts through mid-alkali basalts to high-alkali basalts. High-alkali basalts display clinopyroxene crystals with sector (hourglass) and oscillatory zoning (Mg#61–82) as well as Fe-rich green cores (Mg#43–69), whereas low-alkali basalts are characterized by clinopyroxenes with unzoned overgrowths (Mg#69–83) on resorbed mafic cores (Mg#78–88), coexisting with reversely zoned plagioclase crystals (An43–68 to An60–84 from core to rim). Complex magma dynamics are indicated by distinctive compositional variations in clinopyroxene phenocrysts, with Cr-rich zones (Mg#74–87) indicating continuous recharge by more mafic magmas. Crystallization of olivine, clinopyroxene and titanomagnetite occurred within a polybaric plumbing system extending from upper mantle to mid-crustal depths (485–1059 MPa and 1147–1286°C), whereas crystallization of plagioclase with subordinate clinopyroxene and titanomagnetite proceeded towards shallower crustal levels. The compositions of high-alkali basalts and mid-alkali basalts resemble those of ocean island basalts and are 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), whereas low-alkali basalts have lower incompatible element abundances and isotopic compositions trending towards EMII (87Sr/86Sri = 0.70327–70397, 143Nd/144Ndi = 0.51282–0.51286 and 206Pb/204Pb = 19.278–19.793). High- and mid-alkali basalt magmas mostly crystallized in the lower crust, whereas low-alkali basalt magma recorded deeper upper mantle clinopyroxene crystallization before eruption. The variable alkaline character and isotope composition may result from interaction of low-alkaline melts derived from the asthenosphere with melts derived from lithospheric mantle, possibly initiated by asthenospheric melt percolation. The transition to more alkaline compositions was induced by variable degrees of melting of metasomatic lithologies in the lithospheric mantle, leading to eruption of predominantly small-volume, high-alkali magmas at the periphery of the volcano. Moreover, the lithosphere imposed a filtering effect on the alkalinity of these intraplate magmas. As a consequence, the eruption of low-alkali basalts with greater asthenospheric input was concentrated at the centre of the volcano, where the plumbing system was more developed.
    Description: Published
    Description: egab062
    Description: 2V. Struttura e sistema di alimentazione dei vulcani
    Description: 3V. Proprietà chimico-fisiche dei magmi e dei prodotti vulcanici
    Description: 4V. Processi pre-eruttivi
    Description: JCR Journal
    Keywords: alkali basalts ; Dunedin Volcano ; thermobarometry ; primary magma ; lithospheric mantle filter ; Igneous Petrology ; Thermobarometry ; Mantle melting and metasomatism ; Magmatic plumbing systems
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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  • 3
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    Distinct scheelite REE geochemistry and 87Sr/86Sr isotopes in proximally- and distally-sourced metamorphogenic hydrothermal systems, Otago Schist, New Zealand (2022)
    In:  Ore Geology Reviews
    Details
    Publication Date: 2022-07-21
    Description: The Otago Schist in New Zealand hosts orogenic tungsten (W) mineralization in two types. Proximally-sourced (“proximal”) mineralization in sub- to lower-greenschist facies Mesozoic metasedimentary rocks within the northern flank of the Otago Schist Belt comprise quartz-calcite veins with minor scheelite and few sulfides. Distally-sourced (“distal”) deposits are hosted in major crustal discontinuities within Mesozoic sub-greenschist to upper-greenschist facies metasedimentary rocks and are associated with abundant sulfides ± Au. In-situ trace element (particularly REE, Sr, Mo, Na, As) and in-situ 87Sr/86Sr compositions of scheelite show the scheelite occurring in these different mineralization types to be geochemically distinct. Proximal occurrences are characterised by scheelite with both heterogenous trace element and Sr isotope compositions displaying variations that can be linked to different veins, host rock types and scheelite generations. In contrast, scheelite within distal scheelite deposits tends to have fairly homogenous trace element and Sr isotopic ratios at the deposit to grain scale. The heterogenous compositions of proximal occurrences represent local derivation of diverse components from the sub-millimetre to meter scales and small extents of fluid flow, which resulted in a high sensitivity to sources and local rock compositions and thus limited large-scale equilibration of elemental and isotopic systematics within the scheelite they contain. The larger distal deposits formed by regional leaching by metamorphic fluids, probably at the greenschist-amphibolite facies boundary, followed by homogenisation as they ascended through the crust and leading to deposition of scheelite with rather uniform chemical characteristics. Our data re-enforce the model that tungsten mobility in the Otago Schist involved local-scale mobilisation of W in the shallow crust by breakdown of detrital rutile and scheelite at low metamorphic grade. Burial of these veined scheelite-bearing rocks to temperatures of ∼500 °C enabled remobilisation of W by metamorphic fluids, concurrent with scavenging of Au, As and S, followed by return of the metal-bearing fluids towards mid to upper crustal levels, where they precipitated scheelite.
    Language: English
    Type: info:eu-repo/semantics/article
    Format: application/pdf
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