CONSTRAINING PT CONDITIONS OF METAMORPHISM IN ECLOGITE FROM THE SOUTHERN DULAN AREA in THE NORTH QAIDAM ULTRAHIGH-PRESSURE METAMORPHIC BELT

Eclogite occurs with minor mafic and ultramafic rocks as half to several hundred meter blocks within granitic and pelitic gneissic host rock in the Shaliuhe terrane of the Dulan area, located in the eastern end of the North Qaidam ultrahigh-pressure metamorphic belt in northwestern China. Samples of eclogite, gneiss, amphibolite, and ultramafic rocks were collected from across the terrane. Eclogite mineral assemblages vary, but typically consist of Garnet (5-30%) + Clinozoisite (10-40%) + Rutile (1-2 %) + An15-60 Plagioclase (1-5%). Garnet composition is typically 35-45% Pyrope, 30-40% Almandine, and 20-30% Grossular, with almost no Mn content. Differences in eclogite assemblages result from the presence or absence of Phengite, Kyanite, Omphacite, Augite, Hornblende, Margarite, Titanite, Ilmenite, Chlorite, Apatite, Prehnite, Quartz, K-feldspar, and Calcite. Complex textures, including Kyanite and Clinozosite inclusions in Garnet, intergrowths of Titanite + Ilmenite + Rutile, Margarite rimmed by Clinozoisite, exsolution lamellae in Titanite, and anhedral Prehnite laths that rim Clinozoisite are evidence of pro- and retrograde metamorphic evolution. All eclogite samples collected have experienced varying degrees of decompression and/or hydrous alteration, as evidenced by Clinopyroxene + Albite symplectite, commonly further retrograded to Amphibole + Plagioclase, fine-grained Clinozoisite/Epidote replacement, Calcite/Epidote veins, and Hornblende and Chlorite replacement of Garnet. Amphibolite samples may be fully retrograded eclogite and consist of lineated Hornblende + Titanite + Ilmenite + Apatite(?) + Quartz + Plagioclase.

Gneisses can be divided into two primary assemblages, Garnet + Muscovite + Biotite + Quartz + Plagioclase in pelitic gneiss, and K-feldspar + Plagioclase + Quartz+ Muscovite in granitic gneiss. Some samples have a highly mylonitic fabric. Gneiss commonly contains Garnet or Amphibole pseudomorphs after Garnet.

Phase equilibria and geothermobarometry constrain PT histories of rocks in the terrane, yielding estimates of peak and retrograde metamorphic conditions.