PHASE-RELATIONSHIPS AND P-T CONDITIONS OF COEXISTING ECLOGITE-BLUESCHISTS AND THEIR TRANSFORMATION TO GREENSCHIST-FACIES ROCKS IN THE NERKAU COMPLEX (NORTHERN URALS)

Eclogites and blueschists coexist in the same lithologic sequence in t he Nerkau Complex, one of the metamorphic complexes, in the northern U rals, Russia. Two types of eclogites can be distinguished in this comp lex. (1) Glaucophane-free eclogites, consisting of garnet + omphacite + barroisite + calcic amphibole + albite + epidote + rutile + titanite + quartz. In these rocks, phase compatibilities indicate that barrois ite was stable at eclogite conditions, together with garnet and omphac ite. This assemblage was overprinted by another consisting of barroisi te/Mg-hornblende + albite + epidote, which developed under albite-bear ing greenschist conditions. (2) Paragonite-glaucophane-bearing eclogit es consist of omphacite + garnet + glaucophane + barroisite + actinoli te + paragonite + albite + epidote + titanite +/- rutile + quartz. Tex tural and chemographic analyses indicate that two main metamorphic epi sodes took place as well, represented by the assemblages omphacite + g arnet + glaucophane and barroisite + albite + chlorite, respectively. The blueschists surrounding the eclogites consist of garnet + glaucoph ane +/- barroisite + phengite +/- paragonite + chlorite + albite + qua rtz + titanite +/- rutile +/- ilmenite. Phase relationships indicate t hat no reaction relates eclogites with blueschists, and that both mine ral assemblages can be stable at the same metamorphic conditions. The occurrence of these assemblages side by side could thus be controlled by the bulk-rock composition. Metamorphic conditions have been calcula ted for each lithology with different, but complementary methods. The two types of eclogites were stable between 450 degrees and 490 degrees C or 500 and 537 degrees C, depending on the thermometer used, and mi nimum pressures of 10 to II kbar. These values compare favourably with 530 +/- 20 degrees C and 14.5 +/- 1 kbar obtained with a P-T diagram calculation and a forward-modelling approach, In the case of blueschis ts, coexisting glaucophane and garnet are stable at similar metamorphi c conditions. The final retrogressive transition to greenschist took p lace at P below 9 kbar and T around 480 degrees C. This results in an almost isothermal decompressional path.