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3581192 
Journal Article 
Phase relationships and P-T conditions of coexisting eclogite-blueschists and their transformation to greenschist-facies rocks in the Nerkau Complex (Northern Urals) 
Gomezpugnaire, MT; Karsten, L; Sanchezvizcaino, VL 
1997 
1 
Tectonophysics
ISSN: 0040-1951 
ELSEVIER SCIENCE BV 
AMSTERDAM 
276 
1-4 
195-216 
WOS:A1997XZ01400013 
Eclogites and blueschists coexist in the same lithologic sequence in the Nerkau Complex, one of the metamorphic complexes, in the northern Urals, Russia. Two types of eclogites can be distinguished in this complex. (1) Glaucophane-free eclogites, consisting of garnet + omphacite + barroisite + calcic amphibole + albite + epidote + rutile + titanite + quartz. In these rocks, phase compatibilities indicate that barroisite was stable at eclogite conditions, together with garnet and omphacite. This assemblage was overprinted by another consisting of barroisite/Mg-hornblende + albite + epidote, which developed under albite-bearing greenschist conditions. (2) Paragonite-glaucophane-bearing eclogites consist of omphacite + garnet + glaucophane + barroisite + actinolite + paragonite + albite + epidote + titanite +/- rutile + quartz. Textural and chemographic analyses indicate that two main metamorphic episodes took place as well, represented by the assemblages omphacite + garnet + glaucophane and barroisite + albite + chlorite, respectively. The blueschists surrounding the eclogites consist of garnet + glaucophane +/- barroisite + phengite +/- paragonite + chlorite + albite + quartz + titanite +/- rutile +/- ilmenite. Phase relationships indicate that no reaction relates eclogites with blueschists, and that both mineral 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 calculated 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 minimum 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 blueschists, coexisting glaucophane and garnet are stable at similar metamorphic conditions. The final retrogressive transition to greenschist took place at P below 9 kbar and T around 480 degrees C. This results in an almost isothermal decompressional path. 
Urals; eclogites; blueschists; phase relationships; P-T path 
7th Urals Project Workshop 
GRANADA, SPAIN