Geology Group Diary

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    Meeting on 9 September 2015 Notes on Geology of Devon and Cornwall

    The geology of the south west peninsula of Devon and Cornwall is dominated by the granite intrusions (plutons) that extend from Dartmoor through Bodmin Moor, St Austell, Carnmenellis and Land’s End to the Scilly Isles. These granite outcrops are linked underground since they are part of the same large batholith that was intruded some 290 Ma during the Variscan earth movements. When the magma cooled it slowly crystallised at around 600ºC producing crystals of quartz, mica and orthoclase feldspar. The feldspar crystallised first and formed large crystals called phenocrysts. The heat from the intrusion baked the surrounding rocks (known as killas in Cornwall) forming a metamorphic aureole or zone of baking. During the later phase of magma cooling, hot mineralising fluids emanated from the magma chamber and produced hydrothermal veins as both gangue and metallic  minerals were precipitated. These minerals crystallize out at different temperatures…tin at 5500C; copper at 5000C; lead-zinc at 4000C; Gangue minerals including quartz or calcite form at circa 2000C and are often formed in layers within the metallic vein. High temperature minerals occur nearest to the granite whilst copper, lead and zinc were precipitated at successively lower temperatures farther away from the granite. Another form of mineralisation known as pneumatolysis led to the alteration of feldspars in some granites by rising hot water vapours during the later stages of  magma crystallisation. The hydrated feldspars in the granite produced kaolin or china clay which is mined extensively around St Austell.
    The granite landscape is dominated by tors, outcrops of weathered jointed rock that form the summits of the granite moorlands. The physical appearance of the tors can be described as accumulations of large irregular blocks resting on the granite outcrops, as for example at Haytor on Dartmoor. Large blocks litter the slopes having been detached from the main outcrop. Many researchers consider that the tors evolved under periglacial conditions during the late Devensian when the area would have been just beyond the maximum ice limit. On the other hand, Linton (1955) proposed that tors were formed by a two-stage process, involving deep tropical weathering during the Palaeogene followed by solifluction and mass movement during the Pleistocene.
    During early Devonian times, around 400 Ma, a slice of the Earth’s mantle-crust boundary was slowly thrust upwards and northwards and welded on to the ORS continental margin bordering the Rheic ocean.. This fragment of ocean crust (known as ophiolite) now forms the Lizard peninsula, composed of ultra mafic rocks including gneiss, gabbro and serpentine. The latter is a coarse grained igneous rock derived from peridotite, the mantle material. When polished, serpentine shows an interlocking mosaic of green and black olivine crystals.
    The sedimentary rocks of north and mid Devon and NE Cornwall are Upper Carboniferous in age and are known as the Culm Measures. The term culm derives from a local name for thin sooty coals that occur within thick sequences of mudstones and sandstones. The sediments were deposited in the Rheic ocean and later deformed by the Variscan orogeny as illustrated by the chevron folds at Millook on the north coast of Cornwall.

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