P-T evolution and tectonic significance of lawsonite-bearing schists from the eastern segment of the southwestern Tianshan, China

Robust quantification of pressure (P)-temperature (T) paths for subduction-related HP/UHP metamorphic rocks is fundamental in recognizing spatial changes in both the depth of detachment from the down-going plate and the thermal evolution of convergent margin sutures in orogenic belts. Although the Chinese southwestern (SW) Tianshan is a well-known example of an accretionary metamorphic belt in which HP/UHP metabasites occur in voluminous host metasedimentary schists, information about the P-T evolution of these rocks in the eastern segment is limited, precluding a full understanding of the development of the belt as a whole. In this study at Kekesu in the eastern segment of the SW Tianshan, we use microstructural evidence and phase equilibrium modelling to quantify the peak and retrograde P-T conditions from two lawsonite-bearing micaschists and an enclosed garnet-epidote blueschist; for two of the samples we also constrain the late prograde P-T path. In the two micaschist samples, relics of prograde lawsonite are preserved in quartz inclusions in garnet, whereas in the metabasite, polymineralic aggregates included in garnet are interpreted as pseudomorphs after lawsonite. For garnet micaschist TK21, which is mainly composed of garnet, phengite/paragonite, albite, chlorite, quartz and relict lawsonite, with accessary rutile, titanite and ilmenite, the maximum P-T conditions for the peak stage are 18.0-19.0 kbar at 480-485 degrees C. During initial exhumation, the retrograde P-T path passed through metamorphic conditions of 15.0-17.0 kbar at 460-500 degrees C. For garnet-glaucophane micaschist TK33, which is mainly composed of garnet, glaucophane, phengite/paragonite, albite, chlorite, quartz, relict lawsonite and minor epidote, with accessary titanite, apatite, ilmenite and zircon, the maximum P conditions for the peak stage are >24.0 kbar at 400-500 degrees C. During exhumation, the P-T path passed through metamorphic conditions of 17.5-18.5 kbar at 485-495 degrees C and 14.0-17.5 kbar at 460-500 degrees C. For garnet-epidote blueschist TK37, which is mainly composed of garnet, glaucophane, epidote, phengite, chlorite, albite and quartz, with accessary titanite, apatite, ilmenite, zircon and calcite, the prograde evolution passed through metamorphic conditions of similar to 20.0 kbar at similar to 445 degrees C to P-max conditions of similar to 21.5 kbar at 450- 460 degrees C and T-max conditions of 19.5-21.0 kbar at 490-520 degrees C. During exhumation, the rock passed through metamorphic conditions of 17.5-19.0 kbar at 475-500 degrees C, before recording P-T conditions of <17.5 kbar at <500 degrees C. These results demonstrate that maximum recorded pressures for individual samples vary by as much as 6 kbar in the eastern segment of the SW Tianshan, which may suggest exhumation from different depths in the subduction channel. Furthermore, the three samples record similar P-T paths from similar to 17.0 to 15.0 kbar, which suggests they were juxtaposed at a similar depth along the subduction interface. We compare our new results with published information from eclogites in the same area before considering the wider implications of these data for the orogenic development of the belt as a whole.