Lizard complex

Abstract: A stratigraphy for south Cornwall is combined with an analysis of the chronology of thrust nappe emplacement. The resulting model suggests that northward overthrusting of nappes of flysch and olistostrome, ophiolite, and continental basement is related to the closure of an oceanic basin in south Cornwall involving southward subduction throughout the Devonian. The subsequent continental collision between the Normannian High and part of Laurentia occurred in the Upper Devonian with deformatian continuing until late Carboniferous times.

Abstract: An Sm-Nd mineral isochron from an olivine gabbro yields an age of 375 ± 34 Ma (MSWD = 0006) which represents the formation age of the Lizard Igneous Complex Associated dolerite dyke suites have ɛ Nd375 values between +89 and +117 and record derivation from a recently trace element-enriched, but heterogeneous, source region The petrology and Sm-Nd isotope systematics of the Lizard peridotites record the local infiltration of a magma Significantly, the Sm-Nd isotope systematics of the ‘metasomatized’ peridotites are the same as calculated sources of the dolerite dyke suites: their sources are probably equivalent at depth Finally, the magmatic evolution of the Lizard Complex is likened to the initial development of the Red Sea, with early trace element-enriched volcanism followed by MORB-Iike activity

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 Green2,3considered 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 sequences6–10, although little detail has generally been given. I provide here relevant details which suggest that the Lizard complex is ophiolitic in origin.

Abstract: The Gramscatho flysch sequence of south Cornwall, including the melanges of the Meneage Formation, and the Lizard ophiolite are described in relation to the development of the Gramscatho basin The timing and geotectonic evolution of this feature are considered in its regional context within the Rhenohercynian zone of Northwest Europe This zone is interpreted as an intracontinental dextral transform system that developed during the Devonian closure of the so-called Massif Central Ocean A basin and rise topography is attributed to extension and compression at offsets on major faults E–W extension of the Gramscatho pull-apart basin in Middle and Upper Devonian times is believed to have caused rifting and the development of ocean crust from which the Lizard ophiolite was assembled in an oceanic setting at a ridge-transform junction The Meneage Formation was generated as a result of intrabasinal uplift on transcurrent faults A major change of plate motions took place when the Massif Central Ocean finally closed at the end of the Devonian and the weak Rhenohercynian zone became a site of crustal convergence from which thrust nappes were transported northwards Foreland basins were sequentially developed and deformed by thin-skinned tectonics as the system advanced northwards during the Carboniferous