Metamorphic evolution, mineral chemistry and thermobarometry of schists and orthogneisses hosting ultra-high pressure eclogites in the Dabieshan of central China

As is typical of ultra-high pressure (UHP) terrains, the regional extent of the UHP terrain in the Dabieshan of central China is highly speculative, s ince the volume of eclogites and paragneisses preserving unequivocal eviden ce of coesite and/or diamond stability is very small. By contrast, the comm on garnet (X-Mn = 0.18-0.45)-phengite (Si = 3.2-3.35)-zoned epidote (Ps(38- 97))-biotite-titanite-two feldspars-quartz assemblages in the more extensiv e orthogneisses have been previously thought to have formed under low P-T c onditions of ca. 400 +/- 50 degrees C at 4 kbar, However, certain orthognei ss samples preserve garnets with X-Ca up to 0.50, rutile inclusions within titanite or epidote and relict phengite inclusions within epidote with Si c ontents p.f.u. of up to 3.49 - overlapping with the highest values (3.49-3. 62) recorded for phengites in samples of undoubted UHP schists. These and o ther mineral composition features (such as A-site deficiencies in the highe st Si phengites, Na in garnets linked to Y + Yb substitution and Al F Ti-1 O-1 substitution in titanites) are taken to be pointers towards the orthogn eisses having experienced a similar metamorphic evolution to the associated UHP schists and eclogites. Re-evaluated garnet-phengite and garnet-biotite Fe/Mg exchange thermometry and calculated 5 rutile + 3 grossular + 2SiO(2) + H2O = 5 titanite + 2 zoisite equilibria indicate that the orthogneisses may indeed have followed a common subduction-related clockwise P-T path wit h the UHP paragneisses and eclogites through conditions of P-max at ca. 690 degrees C-715 degrees C and 36 kbar to T-max at ca. 710 degrees C-755 degr ees C and 18 kbar, prior to extensive re-crystallisation and re-equilibrati on of these ductile orthogneisses at ca. 400 degrees C-450 degrees C and 6 kbar. The consequential conclusion, that it is no longer necessary to resor t to models of tectonic juxtapositioning to explain the spatial association of these Dabieshan orthogneisses with undoubted UHP lithologies. has far-r eaching implications for the interpretation of controversial gneiss-eclogit e relationships in other UHP metamorphic terrains. (C) 2000 Elsevier Scienc e B.V. All rights reserved.