26-4 Trans-crustal Magma Diversification using Amphibole Chemometry from Fiordland, New Zealand

Session: Subduction Zones and Their Volcanic Arcs: Initiation and Evolution, Structure, Metamorphism, Magmatism

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Abstract:

Understanding how felsic melts are generated and transported within magmatic arc systems is key to resolving how continental crust accumulates and evolves. A key problem is that deep-crustal processes that drive magma differentiation and mobilization remain poorly understood. The Misty pluton in Fiordland, New Zealand provides an opportunity to investigate an exhumed and tilted vertical cross section of lower-arc crust. Ongoing research divides the Misty Pluton into 5 zones based on structural and geochronological characteristics, from the deepest Zone 1 in the west progressing to the shallowest Zone 5 to the northeast. Using samples spanning Zones 2, 3, and 4, this study examines amphibole chemistry in 13 felsic dikes that preserve snapshots of melt migration and evolution across ~20-35 km of crustal paleodepths. The dike thicknesses range from ~0.5 cm to 10 cm, with some reaching ~1 m. Dike textures range from coarse-grained to pegmatitic, with a mineral assemblage dominated by plagioclase and amphibole, with abundant epidote, biotite, and accessory apatite and Fe-Ti oxides, accompanied by quartz and titanite. Results from the deeper Zone 2 samples show amphiboles are pargasite, magnesio-hastingsite, and rarely tschermakitic pargasite, with crystallization temperatures ranging from 880 to 940° C. Amphibole equilibrium melts determined from the Zone 2 amphiboles are dacitic in composition. Zone 3 amphiboles are pargasite, with crystallization temperatures ranging from 930 to 970° C, and amphibole equilibrium melts are andesitic in composition. Taken together, the differences in melt composition between magmatic zones may reflect distinct pulses from the same source, having undergone different depth-dependent fractional crystallization and assimilation. We suggest that melts that traveled through dikes from deeper paleodepths have undergone more extensive differentiation than those from shallow levels, suggesting that magmas at depth cool more slowly, with long residence times facilitating geochemical diversification.

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