Peak P-T conditions and metamorphic evolution of the UHP eclogite from the western Dabieshan

Category Tectonic & Seismotectonic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Wei, Chunjing; Zhang, Jingsen
Holding Date 09 September 2008

The UHP eclogites from Xinxian County, Western Dabieshan, are composed of garnet, omphacite, glaucophane, kyanite, epidote, phengite and quartz (coesite or its pseudomorph), and rutile (+/- talc and paragonite). These eclogites contain both glaucophane and kyanite, the index minerals for low-T and medium-T types of eclogite, respectively, which is not common in other UHP terranes. It is still ambiguous if epidote in the eclogite represents the peak metamorphic stage or the retrograde stage.
Pseudosections calculated in NCKFMASH system for three representative samples with Thermocalc 3.25 indicate that there is a transitional range at T = 600 - 640°C and P = 2.5 - 3.3GPa for glaucophane and kyanite to be coexisted. Using pseudosection approach, the peak conditions for the UHP eclogites were estimated as P = 2.85 - 2.95 GPa and T = 625 -630°C, by the isopleth for rim compositions of garnets, and the peak mineral assemblage involves garnet + omphacite + glaucophane + kyanite + lawsonite + coesite and phengite. At this peak condition, the predicted compositions and modal proportions for the main minerals garnet, omphacite and phengite are in a good agreement with the measured ones, indicating metamorphism at this peak stage has reached an equilibrium state.
After the peak stage, the UHP eclogites experienced an isothermal decompression. Metamorphic evolution during this stage involves (i) coesite → quartz, (ii) lawsonite→kyanite + zoisite/epidote + water, (iii) talc occurring in relatively Mg-rich rocks, (iv) appearance of paragonite in rocks with higher water contents, and (v) further growth of glaucophane. Correspondingly, omphacite and phengite partially changed their compositions but garnet trivially changed. These led to forming the mineral assemblages garnet + omphacite + glaucophane + zoisite/epidote + kyanite + quartz +/- phengite +/- paragonite +/- talc observed in the rock, which represent a PT range of 2.0 - 2.2 GPa and 600 - 630 °C. Moreover, calculations in the NCKFMASH system indicate that the UHP eclogites contains a large amount of water at peak stage by the presence of lawsonite and the early decompression is characteristic of lawsonite dehydration. The dehydrated water favors the further growth of glaucophane. After the water was consumed out, the retrograde evolution of the eclogite entered into a symplectite stage.