Beyond crystal mushes: evidence for uptake of high-T pyroxene antecrysts from mid- to upper crustal andesites into tephras from the Central Plateau, New Zealand. Issue 2 (3rd July 2021)

• Record Type: Journal Article
• Authors: Zellmer, Georg F.; Iizuka, Yoshiyuki; Lormand, Charline; Moebis, Anja
• Is Part Of: New Zealand journal of geology and geophysics. Volume 64:Issue 2/3(2021)
• Page Start: 443
• Page End: 455
• Publication Date: 2021-07-03
• Subjects: Tongariro -- Ruapehu -- differentiation -- delamination -- relamination -- primary andesites -- thermobarometry -- MELTS modelling; Geology -- New Zealand -- Periodicals; Geology -- Periodicals; Geophysics -- Periodicals; Geofysica; Geologie; Geology; Geophysics; New Zealand; Periodicals; 559.3
• DOI: 10.1080/00288306.2020.1848883
• Languages: English
• ISSNs: 1175-8791

Abstract

ABSTRACT: Pyroxene crystals from recent Central Plateau tephras are used to deduce their formation conditions through two-pyroxene thermobarometry. Crystals return pseudo-pressures and pseudo-temperatures that are artefacts of uptake of antecrysts formed at a range of crustal levels by isobaric cooling of previously intruded magmas. MELTS modelling of tephra glass compositions shows that pseudo-PT conditions are reproduced at oxygen fugacities above the nickel-nickel oxide buffer (NNO+1, NNO+2), mid- to upper crustal pressures (100–400 MPa), and temperatures between c . 900°C and >1100°C. Modelled crystals from the deep crust (800 MPa) are restricted to clinopyroxenes. However, these display chemical equilibrium with shallow orthopyroxenes at higher pseudo-PT conditions than observed in Central Plateau pyroxenes. The data indicate uptake of high-temperature pyroxenes at mid- to shallow crustal levels into ascending andesitic melts and thus preclude the presence of long-lived crustal mush zones (<1000°C) as a source for the crystal cargo of the Central Plateau tephras studied here. Further, the apparent absence of deep crustal pyroxene antecrysts does not preclude models of arc andesite genesis without a 'deep crustal hot zone' beneath the Central Plateau. Generation and ascent of primary andesites from a heterogeneous mantle wedge is therefore a possible scenario at the southern Hikurangi margin.