Mingling of multiple crystals mushes in the Fogo batholith

Graham, Benjamin (2017) Mingling of multiple crystals mushes in the Fogo batholith. Masters thesis, Memorial University of Newfoundland.

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Abstract

This field, petrographic, and geochemical study examines mingling of compositionally similar rocks at multiple scales. Evidence of complex magma mingling in a multi-component crystal mush reservoir is preserved at Wild Cove East, located along the northeast shoreline of Fogo Island, Newfoundland. The irregular contacts and lack of chilled margins between units, the back-intrusion of younger units by older units, the similar composition of units, and an overlap in U/Pb zircon ages suggest all units interacted as viscous crystal mushes at similar temperatures in the shallow crust. Abundant rounded to ellipsoidal magmatic enclaves, of which there are at least three different populations based on composition and crystallinity, appear to represent separate magmas that were entrained either as earlier mush material or crystal-poor intrusions that experienced break-up at depth. Evidence of exchange of liquid at deeper levels is preserved both in the field and at the mineral-scale, where it is highlighted by abrupt compositional spikes in traverses across early forming minerals such as plagioclase and pyroxene. Heterogeneity in textures and composition of both major minerals (plagioclase and pyroxene) and an accessory mineral (zircon) suggest processes such as crystal exchange and capture affected mush host rocks, enclaves, and other intrusions in the study area earlier in their histories at deeper levels.

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