EXSOLUTION TEXTURES OF ORTHOPYROXENE AND CLINOPYROXENE IN HIGH-GRADE BIF OF THE VORONEZH CRYSTALLINE MASSIF: EVIDENCE OF ULTRAHIGH-TEMPERATURE METAMORPHISM

Abstract

Archaean banded iron formation (BIF) of the Voronezh Crystalline Massif (VCM) contains coexisting clino- and orthopyroxenes with exsolution textures. The pyroxene in the VCM BIF is found in two generations, with only the first generation containing such textures. Clinopyroxene contains large (up to 5-10 μm) (0 0 1) orthopyroxene (Opx1L) lamellae in a host clinopyroxene (Cpx1H). This host Cpx, in turn, exsolves into thin (~1 μm) (1 0 0) lamellae of orthopyroxene (Opx2L) and clinopyroxene (Cpx2H). Orthopyroxene exhibits similar exsolution textures with large (up to 50 μm) (0 0 1) clinopyroxene (Cpx1L) lamellae developed in a host orthopyroxene (Opx1H). This clinopyroxene Cpx1L shows further exsolution of thin (1 0 0) Opx2LL lamellae and clinopyroxene (Cpx2LH). Point microprobe analysis, raster-mode microprobe analysis, and microprobe element mapping of mineral grains with a large number of point analysis were used to determine the composition of the exsolution products and the primary chemistry of the coexisting clinopyroxene (CaO = 14.86-17.26 wt%) and pigeonite (CaO = 4.45-6.23 wt%). These pyroxenes crystallized during the peak of metamorphism, and application of the Lindsley geothermometer suggested that they formed at extremely high temperatures of about 1000 °C. Primary very dense CO2-rich fluid inclusions (ρ = 1.152 g cm-3, Th = -49.2 °C) were discovered for the first time in these BIF. With these data, the metamorphic pressure was estimated as 10-11 kbar (depth 36-40 km). Such ultrahigh temperature-high pressure (UHT-HP) conditions for the regional metamorphism of the Precambrian BIF have previously been reported only for Archaean metaironstone from the Napier Complex (Enderby Land, Antarctica). They give an insight into the peak metamorphic conditions of the BIF of the VCM, their burial under thickened continental crust during this period of Earth evolution and suggest a more complicated multistage metamorphic and tectonothermal history for the region than has previously been postulated. © 2006 Blackwell Publishing Ltd.