麻豆传媒

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B. Sc. Honours Thesis

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Frenchman's Barn, near Arisaig, Nova Scotia, is an enigma. It contains some of the textural characteristics of ash flows and lava flows and some which are not common to either. Part of the problem is that many of the original textures in the Barn have been destroyed by the later alteration and recrystallization textures now found in the Barn.

The Barn is one cooling unit consisting of three major rock types, a lower smooth-banded rock type containing elongated amygdules and sheets of devitrified glass, a massive rock type above, containing parallel, discontinuous, quartz-stringers and an upper contorted-banded rock type containing sporadic banding and occasional lithic inclusions.

It is thought that Frenchman's Barn was deposited as a lava flow because of such textural features as smooth, continuous banding at the base of the section, sporadic contorted banding at the top, and a lack of easily identifiable pumice fragments, glass shards and lapilli.

Frenchman's Barn was probably deposited as a very hot lava which was ponded soon after extrusion. The cooling effect of the ground and the force of the overlying magma resulted in smooth, continuous banding at the base. The cooling effect of the air and a limited amount of movement resulted in sporadic contorted flow banding near the top of the flow. The central, massive portion was the last to freeze and as a result relatively coarse-grained quenched crystals with sutured boundaries developed.

The quartz stringers of the massive rock type are thought to have formed as a result of shear stresses causing weaknesses or fractures in the rock which were later filled by a quartz-rich vapour phase.

Alkali enrichment (metasomatism ?) has occurred in parts of the barn, probably during regional metamorphism.

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Pages: 82
Supervisor: D. B. Clarke

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