9 - Microcoesites and microdiamonds in Norway: An overview

Summary

Abstract

The known occurrences in Norway of micron-sized coesite or diamond crystals and their overall geologic context, including possible petrologic and geodynamic models, are briefly reviewed in essay style. Some implications of problems concerning conventional and unconventional models for the creation or exhumation of UHPM rocks are briefly discussed.

Definite or deduced coesite occurs as microinclusions within garnet or clinopyroxene in several eclogites of the Norwegian Coesite-Eclogite Province (CEP) which has been described as “a fragmented infolded nappe” within the larger Caledonized terrane known as the Western Gneiss Region (WGR) in the western part of South Norway. These microcoesites confirmed the preexistence of UHPM conditions that had previously been deduced from other petrologic data for certain eclogite localities in the WGR.

Microdiamonds have recently been reported from two nearby localities of gneiss within the WGR situated further North than the definite microcoesite occurrences of the “Selje District,” but well within the probable limits of the CEP. Unfortunately the petrogenetical significance of these microdiamonds is still uncertain, as several very different hypotheses can be proposed, such that it cannot yet be established whether or not they again confirm the preexistence of UHPM conditions. In any case they provide sufficient data for the definition of an embryonic Diamond Gneiss Province (DGP), and they also underline the importance of parts of the WGR, as well as of the discipline of micromineralogy (cf. micropalaeontology), for a better understanding of deep metamorphic processes during orogenic events.