The metamorphic evolution of MgCr type Norwegian garnet peridotites

Abstract

Mineralogical and microstructural data are presented which demonstrate that the garnet peridotites variably preserved in Alpine-type bodies within the Western Gneiss Region of Norway have had a prolonged, 7-stage, evolutionary history. High-temperature Al-pyroxene ± spinel protolith assemblages (ass. I) are only rarely preserved but predate the coarse-grained P max garnet lherzolite assemblages (ass. II). Porphyroclastic textured samples demonstrate subsequent recrystallisation to a lower-pressure 5-phase garnet + spinel lherzolite assemblage (ass. III). Most samples show development of kelyphites of intimately intergrown replacement pyroxenes + spinel (ass. IV) around garnets. These are in turn frequently overgrown by late coarser-grained coronas of orthopyroxene + pargasittic amphibole + spinel (ass. V). The chrome-depleted secondary garnets are interpreted to have predated kelyphite formation contrary to most previous interpretations. Assemblages VI and VII reflect the late-stage stability at reduced temperatures of firstly aluminous chlorite and finally serpentine + talc assemblages. As the peridotites may contain up to six generations of certain mineral phases (notably orthopyroxene) and individual grains are frequently compositionally zoned, considerable care has been exercised over the selection of mineral compositions appropriate to the various mineral equilibria used to deduce the overall P- T path followed by these rocks. Finally, general aspects of the origin and tectonic setting of these rocks and of the age of the various observed assemblages are discussed. Whilst the early assemblages (I and II) appear to be mid-Proterozoic in age, the later assemblages (III å VII) are probably all Caledonian).