The Ile Bizard intrusion, Montreal, Quebec—kimberlite or lamprophyre?

Abstract

The Ile Bizard intrusion is a composite diatreme consisting of a pipe-like main body with cross-cutting pipes and dikes. The rock types include an intrusive breccia consisting of a kimberlite-like matrix bearing many xenoliths, a later tuffisite breccia that contains abundant autoliths, and two phases of xenolith-poor tuffisite breccia. All kimberlitic intrusive phases are more aluminous, iron-rich, and less magnesian than typical kimberlites, but more closely resemble kimberlites than alnoites.Xenolith rock types include sedimentary rocks, basement rocks, alloclastic fragments, autoliths, and ultramafic rocks. Phenocrysts and (or) xenocrysts of clinopyroxene, phlogopite, olivine, and magnesian titanomagnetite are common. Ultramafic xenoliths include websterites, orthopyroxenites, dunites, harzburgites, garnet and garne–spinel lherzolites, garnet websterites, and garnet clinopyroxenites. Banded xenoliths represent a cumulate phase in the upper mantle; the remaining ultramafic xenoliths are samples of unmelted mantle material. Temperature and pressure estimates for the garnet lherzolites, ranging from 880 to 980 °C and 2.1 to 2.8 GPa, do not outline a paleogeotherm.Mineralogical and chemical characteristics of the intrusive rocks and the xenolith suite are compatible with those of true kimberlites. The Ile Bizard diatreme is, however, closely related in space and time to the Oka carbonatite complex and Monteregian alkaline activity, and provides an example where a clear distinction between true kimberlites and lamprophyres of so-called central complex kimberlites cannot be made. This intrusion represents the deepest level of generation of Monteregian igneous activity in the form of potassic–utramafic (or kimberlitic) magmatism.