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Journal of the Geological Society

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Possible Lizard-derived material in the underlying Meneage Formation

The Lizard Complex rests tectonically on the Meneage Formation within the top of the flysch dominated Gramscatho Group of sediments. The Meneage Formation is a Middle to Upper Devonian sedimentary mélange sequence with interbedded rudites. The possibility that one or both of these lithologies include Lizard-derived debris is examined in detail. The rudaceous debris is characterized by a range of meta-psammitic, pelitic and basic lithologies which cannot be matched with a Lizard source. Parts of this suite, notably granite gneiss and amphibolitic material, are found to be quite different from superficially similar, possible Lizard source rocks. A cratonic source is required. The only widespread type of mélange phacoid with a possible Lizard source is greenstone. Geochemically these have ocean floor tholeiite affinities. Gabbro and amphibolite occur within, and serpentinite near, one particularly large greenstone phacoid. However, no precise geochemical comparison can be made between greenstones and Lizard dykes. Autochthonous oceanic tholeiites in the Gramscatho Group sequence, and the absence of varied and widespread possible Lizard-derived material, suggest that the Lizard Complex is an unlikely source for the allochthonous greenstones. A separate local development of oceanic crust now no longer exposed, and autochthonous volcanics, are postulated as more likely sources.

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