Abstract

Examination of a suite of Lewisian gneisses from NW Scotland has revealed the sequential development of microcrack networks sealed by minerals deposited from fluids passing along them. The sealants were deposited during periods of both localized and regional fluid permeation through the gneisses. The study has shown that both microcracks and shear zones may remain open as permeable pathways over several fluid permeation phases; and that an open porosity within crystalline rock may be maintained, between permeation events, by partially sealed microcracks. The separate microcrack sets, identified by their mineral sealing phases have widespread regional distributions.

Several magnetite sealing events, specific to lithologies and structures, occurred during pre-Torridonian times associated with the Scourian and Laxfordian metamorphic cycles.

Four main post-Laxfordian regional sealing events have been identified, characterized by the mineral suites calcite + K-feldspar; prehnite + albite + calcite; pumpellyite + quartz + calcite; and stilpnomelane, respectively. Of these, calcite + K-feldspar is pre-, or earliest, Torridonian. Prehnite + albite + calcite, and pumpellyite + quartz + calcite are associated with Torridonian events: a model of a downward permeating hydrothermal system involving non-marine ground water is envisaged to account for their formation at about 1000–800 Ma, before the deposition of the Applecross Formation Later stilpnomelane, along with less well developed K-feldspar; barite; and pyrite, may either be a result of this system or perhaps a later one associated with the Cambrian marine transgression of the craton at about 600 Ma.

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