229-10 Garnet-rich Fingers in the Chipman dike Swarm, Athabasca Granulite Terrain: Implications for the evolution of hydrous mafic rocks in the lower crust.

Session: Crustal Petrology (Posters)

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Abstract:

The early Proterozoic Chipman dikes form part of a major NE striking mafic dike swarm near the eastern edge of the Rae tectonic province, in the Canadian Shield. The 1- to 10-m wide dikes were emplaced into the ca. 3.0 Ga Chipman tonalite. The dikes are dominated by hornblende + plagioclase, but a variety of metamorphic textures from garnet-forming reactions have been observed including: solid-state reactions producing garnet + clinopyroxene, and partial melting reactions yielding garnet and a tonalitic melt. Calculated metamorphic pressures of 1-1.2 GPa suggest that these reactions preserve the lower crustal evolution of the hydrous mantle-derived magmas. One distinctive reaction texture involves 2-4 cm wide, finger-shaped protrusions into the dikes. These ‘fingers’ consist of fine-grained garnet, hornblende and quartz inside a sheath of coarse-grained hornblende with isolated euhedral garnet. Most fingers originate at dike-tonalite contacts and penetrate several meters into the dike. Each finger has a very similar bulk composition to the surrounding hb-pl matrix, suggesting that they formed from a solid state reaction rather than the intrusion of new material. The fingers and matrix differ significantly in peak P-T conditions: 820 ℃, 1.25 GPa (finger) compared to 700 ℃, 1.0 GPa (matrix). Phase equilibria modelling suggests that assemblage changes from matrix to finger can be induced by 60-80% dehydration of the bulk composition. Modelling other garnet-forming reactions in the Chipman dikes indicates that subtle differences in dike composition and the degree of dehydration may control the particular reactions which occur. The remarkably straight, fracture-like character of the fingers suggests that fluids may have entered or exited the dike through propagating fractures. Our findings suggest that the dikes may have cooled to the point of embrittlement before reheating to metamorphic temperatures, perhaps multiple times. We suggest that a low-H2O fluid from adjacent tonalite may have entered the dike through fractures and induced the dehydration reaction: hb + pl → hb + Grt + Qtz. Temperature fluctuations may explain the apparent barometric discrepancies between the fingers and matrix. Overprinting textures and cycling temperatures in the Chipman dikes are interpreted to reflect a history of reheating, dehydration and melting during emplacement and underplating of the lower crust.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9837

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