Prograde history of the Archean Salma eclogites and piclogites (Belomorian eclogite province)

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Konilov, Alexander
Holding Date 04 October 2008

In the past few years in many places in the northern part of the Belomorian mobile belt, the Baltic shield the bodies of eclogites were found (Uzkaya Salma (67°30’N, 32°20’E), Kuru-Vaara quarry (67°37’N, 31°30’E), and others). Their mineral parageneses are similar to strong retrograde eclogites known in literature (Dirks, Sithole, 1999; Page et al., 2003). Eclogite-facies metamorphism is indicated by widespread Cpx-Pl symplectites interpreted as the products of exsolution of the jadeite component from omphacite during decompression. In a few cases relicts of omphacite contained 32 mole% jadeite have been found amongst the symplectite colonies. Peak metamorphic conditions reconstructed for the Salma eclogite reached about 700oC and 14 -15 kbar. In rare cases the rods of SiO2 in omphacite were also found. It may indicate that peak pressure might be much higher. Eclogites associate with metamorphosed basic-ultrabasic rocks (piclogites according to White, 1997, p.477) and biotite-garnet-kyanite gneisses (metamorphosed adakites) among metatonalites of Belomorian series.
Unusually the eclogites have kept mineral parageneses of a prograde stage of evolution: inside of garnet (sometimes of atoll type) - albite, prehnite, pumpellyite and pumpellyite+actinolite assemblage, phengitic muscovite and zoisite; was found also the chlorite inclusions in clinopyroxenes from matrix as well as in clinopyroxene inclusions inside of garnet, any replacement textures of chlorite after garnet or another minerals was not observed; in a garnet mantle - amphibole with the alumina contents increasing toward periphery (up to 20 wt.% of Al2O3) and clinopyroxene in edge zone.
Chemical zonality in a garnet is a prograde type: Alm48Prp26Grs24 in core and Alm40Prp40Grs20 in an intermediate mantle. Inclusions of omphacite were found only in outer iron-rich zones of the garnet. In the kyanite among Cpx-Pl symplectites (with well known typical corona of plagioclase+spinel around kyanite), an inclusion of hornblende with relatively low alumina content was discovered.
In the piclogites the relics (?) of saponite replaced by talc were found. In same samples in the chromian spinel there are the numerous inclusions of diaspore (containing up to 5 wt.% of Cr2O3). This mineral was studied and confirmed by Raman microprobe. Only submicron grains of Fe-Ni sulphides in diaspore were discovered.
The found relic minerals in the Salma eclogites and piclogites, showing about a prograde history, have good analogies with known in the literature (Lin Wei, Enami, 2006; Tsujimori et al., 2006; Krenn et al., 2004; Giacomini et al., 2005; Feenstra, 1997). By analogy it is supposed that on prograde stage the eclogites were formed in accordance to mechanism of subduction of the oceanic slab that underwent ocean floor metamorphism and perhaps was partially laterized before eclogitization. The work has been supported by Grant RFBR 07-05-00292.