Metamorphic grade and gradient from white K-mica bearing sedimentary rocks in the Mosquito Creek Basin, east Pilbara Craton, western Australia

Abstract

Shales and phyllites from the turbidite sequences of the 2.9 Ga Mosquito Creek Formation of the East Pilbara, Western Australia contain varying amounts of paragonite and mixed Na–K micas (MNKMs), the 0 0 l X-ray diffraction reflections of which are unresolved from the 10-Å reflections, and only partly resolved from the 5-Å reflections of white K-mica (WKM). The Kübler index (‘crystallinity’), the full width at half maximum (FWHM) of the WKM, obtained from these composite reflections by applying a three-peak deconvolution procedure, reveals a metamorphic zoning of the Mosquito Creek Formation. The highest, “epimetamorphic”, grade occurs in the — largely Na-mica free — southern part, with lower, medium- to high-anchimetamorphic, grades in the central part, notably in a WSW-ENE anticlinal zone extending from Nullagine to the Blue Spec Mine. The Na-mica free metasediments of the Glen Herring Shale of the Fortescue Group, overlying the Mosquito Creek Formation to theW,show only a slightly lower metamorphic grade. The low b0 lattice parameter of the WKMs indicates a very low metamorphic P/T gradient. The Na-mica bearing metasediments of the Mosquito Creek Formation correspond to a kaolinitebearing protolith, strongly Al-enriched and K-depleted with respect to the presumably granitic-tonalitic source rock.