A. H. CLARK1, D. J. SCOTT2, H. A. SANDEMAN1,2, A. V BROMLEY3 & E. FARRAR1
The long-controversial age of the Lizard ophiolite is resolved by U-Pb dating of three single magmatic zircons from a plagiogranite dyke near Porthkerris Point. This demonstrates that the axial oceanic crust was undergoing extensional ductile shearing, mylonitization, amphibolite-facies metamorphism and, probably, anatexis at 397 +/- 2 (2sigma) Ma. In conjunction with published K-Ar, ^sup 40^Ar- ^sup 39^Ar and Rb-Sr data for the complex, this Early Devonian age predicates a protracted interval between intra-oceanic magmatism and deformation, on the one hand, and obduction during Late Devonian, northwestward displacement of the Normannian High, on the other.
Keywords: Cornwall England, Hercynian Orogeny, ophiolite, U Pb, plagiogranite
Recognition of the ophiolitic nature of the Palaeozoic Lizard Complex (Fig. 1) by Thayer (1969) Strong et al. (1975), Bromley (1976, 1979) and Kirby (1979) has had a major impact on models for the stratigraphic and geotectonic history of the Rhenohercynian Zone of the external Variscides. Although now accepted as constituting a strongly deformed section of oceanic crust and upper mantle, incorporating metabasalts with MORB-like compositions (Floyd et al. 1976; Davies 1984), the environment in which this lithosphere was generated remains uncertain. Thus, it has been variously proposed that the ophiolite formed: in a major ocean separating the Gondwanide and Avalonian plates (McKerrow & Zeigler 1972); in an ensialic back-arc basin (Floyd 1982); or in an extensional basin delimited by an intracontinental dextraltransform fault system (Barnes & Andrews 1986). In the last case, the Lizard Complex would be sensibly coeval with the thick, largely flyschoid sequence of the Middle Upper Devonian Gramscatho Group with which it is now juxtaposed across the Lizard Boundary Fault, and which is envisaged to have accumulated in a `leaky-transform situation' (Barnes & Andrews 1986, p. 117) prior to Late Devonian NW-directed thrusting. In contrast, Holder & Leveridge (1986) and Leveridge et al. (1990) interpret the ophiolite as a fragment of earliest Devonian oceanic lithosphere which was detached through the convergence of the Normannian High and Laurentia in the Gedinnian Siegenian, the Gramscatho flysch subsequently accumulating in the foredeep during continued NW overthrusting.
Within the ophiolite itself, the development of a wide range of predominantly ductile shear-zones (Bromley 1979; Vearncombe 1980; Gibbons & Thompson 1991) has been ascribed to crustal-scale extensional tectonism close to a slow-spreading oceanic ridge (Roberts et al. 1993; Hopkinson & Roberts 1995), while Jones (1997) has defined an ensuing history of intra-oceanic nappe development in the structurallylowest part of the ophiolite, followed by emplacement along a major low-angle detachment. High-grade dynamothermal metamorphism (Jones 1994) is inferred to have occurred late in the emplacement of the ophiolite, but final obduction of the complex is considered to have taken place at temperatures too low to affect pumpellyite-actinolite grade assemblages in the contiguous Gramscatho Group (Barnes & Andrews 1984).
The uncertainties regarding the geotectonic setting of the Lizard Complex reflect not only the complexities inherent in the generation and emplacement of oceanic lithosphere, but also the problematic geochronological database for both the ophiolite itself (see summary in Sandeman et al. 1995) and the oldest strata exposed in the Gramscatho succession (Barnes & Andrews 1986). A Rb Sr isochron date of 369 +/- 12 (Isigma) Ma for the Kennack Gneiss (Styles & Rundle 1984), inferred to record metamorphism, and a Sm-Nd isochron date of 375 +/- 34 (1sigma?) Ma for the Crousa Downs gabbro (Davies 1984), assigned to magmatic crystallization, have been widely accepted as evidence …