Observations on Lewisian Chronology

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Abstract
Synopsis: Evidence relating to the chronology of the Lewisian is reviewed in five categories in the light of recent differing interpretations. (1) Metamorphic and migmatitic episodes —the restriction of the term Scourian to a time period is urged. ‘ Badcallian’ is proposed for the early Scourian (c. 2600 m.y.) episode, Inverian being well established as the late Scourian (c. 2200 m.y.) episode. The sub-division of the Laxfordian (? 1800–1400 m.y.) is discussed—the younger dates ( c . 1100 m.y.) may refer to a younger orogeny. (2) Scourie dyke suite —although four tectonically separable suites have been proposed, the evidence is consistent with a single suite. (3) Structural episodes and sequences —unjustified correlation has led to over-elaborate structural chronology. (4) Gairloch–Loch Maree sediments —evidence points to a post-Badcallian pre-Inverian age but the existence of large areas of gneisses of this age is doubted. (5) Other basic and ultrabasic intrusions —chronological implications are discussed. Finally it is argued that since the major tectonic re-orientation of the Lewisian seems to have occurred in the Inverian , there is a case for placing an orogenic break within the Scourian.

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