Abstract

Bouguer gravity anomalies over the Lizard have been interpreted in terms of a lower tectonic sheet of hornblende schists and metasediments, structurally overlain by a sheet of peridotite and gabbro. The upper tectonic unit shows a lateral variation from harzburgite peridotite, gabbro and a dyke complex in the east to lherzolite peridotite and cumulate complex in the west. This might indicate structural separation into eastern and western units, or primary igneous variation. At Lizard point, the minimum thickness of the lower Landewednack hornblende schists, on the basis of residual gravity anomalies, is about 700 m, while in the eastern ophiolite unit the minimum thickness of the gabbro and hornblende schists is about 1.5 km. The western lherzolite peridotite is generally less than 400 m thick, whereas the eastern harzburgite peridotite is possibly 600 m thick. A strong aeromagnetic anomaly, with a maximum over the peridotite SW of Coverack, extends both offshore and to the NW, across the mapped contact of the Crousa gabbro and western peridotite, and over the Traboe cumulate complex. At ground level, this anomaly appears to be discontinuous. Although the maximum surface magnetic anomalies occur over thin horizons in the Traboe cumulate complex, a zone of discontinuous, high frequency, high amplitude anomalies extends across the mapped Crousa gabbro and indicates a zone of varied gabbroic and associated rocks which mark the real southern boundary of the Crousa gabbro.

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