ECLOGITES OF BELOMORIAN ECLOGITE PROVINCES: HIGH-PRESSURE OR ULTRA-HIGH-PRESSURE?

There are two eclogite types in Belomorian eclogite province in the NE Baltic shield. They mark the Archean paleosubduction zone at the boundary between Karelia and Kola cratons. Eclogite forming age is before 2,7 Ga. The Salma eclogites have N-MORB composition, and their protolith is ophiolites. Maximal content of jadeite in an examined omphacite is 30 mole %. Estimation using the omphacite composition of peak of the Salma eclogite metamorphism is 15 kb and 750 °C. Other type of eclogite displayed in mafic pods, lenses and undeformed and deformed mafic dykes in Southeastern part of the Belomorian eclogite province in the Gridino area. Maximal content of jadeite in an examined omphacite (only in quartz-bearing eclogites) reaches up to 34 mole %. Petrological investigations the Gridino eclogite displayed the united step-by-step metamorphic P-T trend for all mafic rocks from eclogite (peak pressure 17,5 kb) to amphibolite (7,9-9,6 kb, 530-700 °C) through HP-granulite (13,9-14,8 kb, 800-810 ºC) facies conditions.

We discovered petrological observations that probably can be evidences of a much higher pressure at Belomorian eclogite formation:

(1) Were discovered oriented quartz rods in omphacite, which usually interpreted as exsolution structures of a supersilicic clinopyroxene during decompression of ultra-high-pressure eclogites [Katayama et al., 2000; Tsai, Liou, 2000].

(2) Integral composition of omphacite with quartz rods is identical with dendritic clinopyroxene-plagioclase symplectite with a “single grain” structure and have Ca-Eskola end-member about 5 mole percent.

(3) Jadeite content of omphacite corona around a magmatic clinopyroxene in a silica-poor olivine gabbronorite dykes reaches up to 55-60 mole %. Probably in this case the omphacite had a higher concentration of jadeite because external rims of coronas are composed by symplectite of omphacite, oligoclase with corundum and/or spinel.

(4) Equilibrium mineral paragenesis of Grt+Omph+Opx in orthopyroxene-bearing eclogite over silica-poor gabbronorite gave 22,4 kb at 800 °C by using Grt-Opx geobarometer [Harley, 1984].

(5) Lonsdalite was discovered in the eclogite boudin from Gridino area [Volodichev et al., 2006].