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
NEW ZEALAND JOURNAL OF GEOLOGY AND GEOPHYSICS(2021)
摘要
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 degrees C and >1100 degrees 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 degrees 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.
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关键词
Tongariro, Ruapehu, differentiation, delamination, relamination, primary andesites, thermobarometry, MELTS modelling
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