SHRIMP U–Pb zircon dating of the exhumation of the Lizard Peridotite and its emplacement over crustal rocks: constraints for tectonic models

Abstract

On the Lizard Peninsula of Cornwall, UK, within the European Variscan orogenic belt, the Lizard Peridotite preserves evidence of high-temperature exhumation from c. 15 kbar, 1250–1300°C to <8 kbar, <900°C in marginal peridotite tectonite, and emplacement into mafic and pelitic rocks undergoing regional metamorphism. Following exhumation, the peridotite was intruded by troctolite and gabbro, and after cooling, by NNW–SSE-trending mafic dykes and shallow-dipping mafic sheets. Very shortly after this, the peridotite and associated rocks forming the Lizard Complex, were thrust over crustal rocks and intruded by granite and granitic injection gneiss, assumed to be derived from underlying continental crust. The Lizard Peridotite and associated gabbros have been interpreted as an ophiolite or having developed during extension to form a basin within, or at the margin of, continental crust. Using the ion-microprobe (SHRIMP) we address questions of these multiple events and histories recorded from complex, zoned zircons and their relevance to different tectonic models. Zircons in an metagabbro marginal to peridotite in the NE of the complex (Landewednack or Traboe Hornblende Schists) yield a date of metamorphism of 387±7 Ma. In Landewednack Hornblende Schists structurally below the peridotite in the SW corner of the complex, low-U zircon grew at 379±25 Ma during ‘Barrovian’ amphibolite facies metamorphism. Two samples of Traboe Hornblende Schists, one in the amphibolite-granulite facies thermal aureole of the exhuming peridotite and the other one granulite within the marginal peridotite tectonite, have yielded metamorphic zircons dated at 393±6 Ma, and 386±7 Ma. One inherited old zircon core (two analyses—high U and c. 500 Ma) was detected in the Traboe Hornblende Schist sample from within the peridotite tectonite, and provides a link between them and the continental rocks of the Old Lizard Head Series (which are cut by granodioritic sheets which contain c. 500 Ma, high U, magmatic zircons) and the Landewednack Schists (which have biotite schist layers with c. 500 Ma zircons) in the south of the complex. Kennack Gneiss granite sheets intruded into the peridotite have been dated at 384±4 Ma, 396±10 Ma and 391±8 Ma by U–Pb zircon. Inherited Proterozoic zircons within Kennack Gneiss granites shows that source sialic (zircon-bearing) crust already (tectonically) underlay the peridotite by the time the granites were emplaced. Together with the date of 397±2 Ma on a ‘plagiogranite’ sheet the results show that exhumation of the peridotite, intrusion of gabbros into it and subsequent compression, detachment and stacking of the Lizard Peridotite over crust which melted to give the Kennack Gneiss granites occurred within a short interval between 380 and 400 Ma (Eifelian–Emsian).