The Lizard complex as an ophiolite

Abstract

The Lizard complex of south Cornwall consists of an assemblage of peridotites, gabbros, hornblende schists, metasediments, dolerites and metadolerites and acid-basic banded gneisses (the Kennack gneisses). It is bordered to the north by a zone of chaotic sediments with phacoidal inclusions of pillow lavas, greywackes and quartzites (the so-called Meneage crush zone). This zone can be traced eastwards into the Roseland district of south Cornwall¹. The age of the complex is uncertain. Dates (largely K–Ar) obtained from the Kennack Gneiss and the majority of hornblende schists are mainly between 370 and 390 Myr, although Green^2,3considered the peridotite to be older than the oldest date obtained from the hornblende schists, at 492 Myr. Metadolerite dykes have recently⁴ been given ages of approximately 400 Myr. All the above are thought to represent minimum ages due to the possibility of argon loss during subsequent uplift, cooling and mild metamorphism. Since the work of Green³, the Lizard complex has been regarded by many as the type example of a hot diapiric mantle intrusion into continental crust. Intrusion is thought to have occurred during a period of regional metamorphism and to have resulted in the imposition of a contact dynamothermal aureole on the hornblende schists at the margin of the peridotite body. The gabbro is regarded as a later and unrelated intrusion, and the Kennack gneisses were attributed to intrusion of acid magma along sheared basic dykes in peridotite. Thayer⁵ suggested that the complex was ophiolitic, basing his views on the field relationships of gabbros and peridotites which indicated that the two lithologies might be temporally and genetically related. General comparisons have since been made between the Lizard complex and the well-documented ophiolote sequences^6–10, although little detail has generally been given. I provide here relevant details which suggest that the Lizard complex is ophiolitic in origin.