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The banded Lewisian gneisses of Mingulay, with minor amphibolites, show the effects of at least five phases of folding and related deformation. The formation of the banding in the gneisses is associated with the earliest recognized phase of deformation during which the main metamorphic reconstitution took place, with the strong linear (L1) and planar (S1) fabric elements related to tight isoclinal folds (F1). The largest folds (F2), which exercise strong control on the gross attitude of the banding, were formed during a second phase of deformation. They are asymmetrical in form with secondary axial planar foliation (S2) and their axial planes controlled the uprise of locally derived quartzo-feldspathic pegmatitic material. Pseudotachylite formation preceded the three subsequent phases of deformation in which the folds (F3, F4, F5) were formed. These folds are generally open with little or no related mineral reconstitution. Interference structures resulting from the superimposition of these sets of open folds on the asymmetrical folds are common and account for the variation in attitudes of the lithological layering. The most common and largest pegmatitic veins, derived from deeper crustal levels, are situated in the hinge zones of the fourth fold set.

Major- and trace-element proportions are consistent with a derivation of the amphibolites from basic igneous rocks. Metamorphic segregation resulted in the formation of hornblendite pods and balls and associated quartzo-feldspathic material.

The gneisses are generally comparable to granodiorite in chemical composition and in parts show gradations to more potassium-rich quartzo-feldspathic veins of pegmatitic aspect which are largely concordant. Many of the discordant pegmatites have abundant potassium feldspar. The genesis of the different types of quartzo-feldspathic rock is related to the varying roles of extraction and redeposition of the most soluble substance under conditions of heterogeneous pressure, partial melting, and potassium metasomatism.

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