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

Abstract

In this recent contribution, Nutman et al. (2001) provide the first SHRIMP U–Pb zircon age data for the Lizard Peridotite and its immediate envelope, and demonstrate the usefulness of ion-microprobe geochronology in the unravelling of the complex thermo-tectonic history inherent in the emplacement of such assemblages of plausibly oceanic rocks onto continental strata. Building on the research of Cook et al. (2000), the authors distinguish clearly between the Lizard Peridotite proper , a mass of c. 15 kbar mantle tectonite, and the contiguous Crousa Downs complex of gabbro, troctolite and sheeted mafic dykes, emplaced into the peridotite following its exhumation and cooling. Thus, although exhibiting many lithologies and structures characteristic of ophiolites, the “oceanic” part of the Lizard Complex is clearly at least bipartite. The studies of Cook et al. (2000) and Nutman et al. (2001) rationalize and clarify several problematic relationships in this extensively studied, but still ambiguous suite. We understand that the editorial handling of Nutman et al. ’s paper overlapped with the publication in this journal of our detailed account of the petrogenesis of the Kennack Gneiss, which incorporates an evaluation of the tectonic context of this controversial unit on the basis of an IDTIMS single-zircon date (Sandeman et al. 2000). We consider, none the less, that several aspects of the new contribution require comment. These include Nutman et al. ’s interpretations of our earlier documentation of the petrology and age relationships of the Kennack Gneiss (Sandeman et al. 1995), the Man of War Gneiss (Sandeman et al. 1997) and the Porthkerris Plagiogranite (Clark et al. 1998 b ), as well as our overview of the emplacement history of the Lizard Complex (Clark et al. 1998 a ), an extended abstract cited but not referenced by Nutman et al. We employ herein the revised Palaeozoic time-scale of McKerrow …