The tectonic evolution of mantle rocks from the lizardophiolite complex, South-West England

Abstract

In SW England, a highly deformed and metamorphosed assemblage of ultramafic, mafic and granitic rocks is interpreted to represent a fragment of upper mantle and lower oceanic crust: the Lizard Ophiolite Complex. Although the processes involved in the tectonic and chemical evolution of mafic rocks and subsequent emplacement of this complex are well documented, the importance of the tectonic evolution of the peridotites is poorly constrained. Structural field mapping of deformed peridotites, ultramafic and mafic cumulates and amphibolites, combined with geochronological (sensitive high mass-resolution ion micro-probe), microstructural (optical microscope and universal stage) and geochemical (X-ray fluorescence, ICP-MS and electron microprobe) analyses undertaken as part of this thesis have identified evidence of four tectono magmatic events, three of which occurred during the Early to Late Devonian. An earlier episode relates to a fragment of Ordovician basement that became tectonically incorporated within the basal structural unit of the Lizard Ophiolite Complex. A basement, and structurally lowermost unit, comprising granitic (MOWG) and layered meta-sedimentary and meta-basic (OLHS) rocks of earliest Ordovician age (~499-488Ma) is interpreted as fragments of arc-type crust developed in an active magmatic arc during closure of an ocean basin (Rheic ocean?).Tectonic exhumation of mantle along extensional lithosphere-scale mantle shear zones in the Early Devonian or earlier (~397 Ma) may be responsible for the early tectonic evolution of the mantle section of the Lizard Ophiolite Complex. It is proposed that this may have occurred during asymmetric extension associated with continental breakup and oceanic rifting. During exhumation, the high-T and high-P mineral assemblage (~1200 C & 15Kb) of the Lizard peridotites progressively re-equilibrated to conditions of lower T and P (~919-1074 C & 5-6Kb). High temperature (~900-1050 C) deformation of ultramafic and mafic Traboe cumulates is consistent with deformation and metamorphism of early formed oceanic crust in the hangingwall of the inferred shear zone, which may have been located at the base of the crustal sequence close to the Moho. The later evolution of a second generation of oceanic crust sequence shown by gabbro and mafic dyke intrusion in the early to middle Devonian (~ 375 Ma) primarily involved magmatism, but NE-SW directed extension of the oceanic crust may have been predominantly accommodated by low-angle ductile shear zones, suggestive of a magma-starved slow- spreading ridge environment. Emplacement of the Lizard Ophiolite Complex took place during the Middle to Late Devonian (~390-366Ma). Top-to-the-NW thrusting facilitated decoupling of the mantle and emplacement over deformed and metamorphosed oceanic crust. During emplacement, widespread magmatism involved the intrusion of a mixed suite of felsic and mafic magmas that may well have been focused along the detachment surface. The geochemical characteristics of this suite of intrusive rocks suggests that initial emplacement of the Lizard Ophiolite Complex may have taken place in a subduction zone environment. Extensive, apparently extensional reactivation of thrust contacts involved the development serpentine-filled fault zones. It is proposed that infiltration of volatile-rich melts during the early mantle deformation and exhumation along extensional shear zones could have led to significant localisation of strain and weakening of the upper mantle. The main evidence supporting this hypothesis being mylonitic peridotites that demonstrate confirmation of chemical enrichment as a result of melt impregnation. Weakening may have occurred by replacement of strong mineral phases e.g. pyroxene by weaker phases e.g. amphibole i.e. reaction softening, characterised by the development of mylonitic amphibole-bearing peridotites. It is concluded that this and other weakening processes may also be responsible for the development and enhancement of mantle shear zones in other ophiolite complexes and present-day oceanic lithosphere during oceanic rifting.