Microanalysis of large volume silicic magma in continental and oceanic arcs: Melt inclusions in Taupo Volcanic Zone and Kermadec Arc rocks, South West Pacific

摘要

The petrogenesis of large volume silicic arc magmas are investigated through microanalysis of four major eruptives of variable size from the continental Taupo Volcanic Zone and one from the oceanic Kermadec Arc. Orthopyroxene, plagioclase and quartz-hosted melt inclusions and groundmass glass display a range in major element (e.g. SiO_2 = 74-79 wt.%, CaO = 0.2-2.5 wt.% and FeO = 0-3 wt.%) and trace element compositions (e.g. Sr= 17-180 ppm and Ba= 140-1500 ppm). Healy melt inclusions are lower in K_2O and Ce/Yb relative to Taupo Volcanic Zone melt inclusions reflecting the lack of continental lithosphere in the oceanic setting. Quantitative trace element modelling of Healy melt inclusions indicates fractional crystallisation is the dominant process responsible for the generation of silicic magma at Healy seamount, although remobilisation of a crystal mush body and/or crustal anatexis cannot be discounted. The chemical heterogeneity of melt inclusions in the continental Taupo Volcanic Zone indicates that the observed crystal populations present in silicic magmas are composed of both phenocrysts and antecrysts and thus derived from polygenetic sources, including mature crystal mush zones that are amalgamated together prior to eruption. This is further enforced through cathodoluminescence imaging of a selection of quartz hosts from the Whakamaru Ignimbrite that allows the location of melt inclusions to be established. This reveals that the most evolved Whakamaru melt inclusions are located in the core of quartz crystals and the least evolved Whakamaru melt inclusions are co-genetic with the groundmass glass in the rim of the crystals. In contrast, Taupo melt inclusions and groundmass glasses are all co-genetic indicating that all crystals grew from this final magma batch prior to eruption. A general trend of increasing magma evolution (to higher SiO_2, Rb/Sr) with erupted volume suggests that the larger magma bodies present in continental crust incorporate more crustal material through assimilation and/or as partial melting of crust.