Geochemistry and U-Pb and 40 Ar- 39 Ar geochronology of the Man of War Gneiss, Lizard Complex, SW England: pre-Hercynian arc-type crust with a Sudeten-Iberian connection

Abstract

The Man of War Gneiss is a variably deformed meta-igneous body that ranges in composition from gabbro to tonalite and comprises part of the basal structural slice of the Lizard ophiolite complex, SW England. U-Pb dating of zircon demonstrates that the parental magmas crystallized in the earliest Ordovician (499t.+ s 3 Ma), but 40 Ar- 39 Ar step-heating analysis of amphiboles suggests that at least the northernmost zone of the gneiss was subjected to an amphibolite-grade metamorphic event at c. 374 Ma, probably associated with thrust emplacement of the ophiolite. The gneiss exhibits major, trace and rare earth element variations in conformity with an origin through partial melting of an amphibolitic source containing hornblende and plagioclase, probably with minor garnet and zircon. The elemental abundances and variations are characteristic of granitoids formed in supra-subduction zone environments. A granitoid sill within the adjacent Old Lizard Head Series schists, previously inferred to be related to the Man of War Gneiss, is trondhjemitic and exhibits higher SiO 2 , TiO 2 and MgO, but lower Zr, V, Ba, and Sr contents, and therefore is probably not genetically related to the latter. The sill may record local melting during ophiolite emplacement. The Man of War Gneiss probably represents a structural inlier of pre-Hercynian basement incorporated in the Hercynian nappes of SW England during NNW-directed thrusting. We emphasize the geological and geochronological similarities between variably deformed granitoid units of the Lizard complex, the Sudeten Massif of southern Poland, and the allochthonous complexes of northern Portugal and NW Spain. These granitoid rocks may represent dispersed remnants of Tremadoc arcs that formed oceanward from the northern margin of Gondwana. This is further convincing evidence that at this time Eastern Avalonia was separated from northern Gondwana by an ocean basin that may have been connected with the Tornquist Sea.