Juxtaposition of HP Rocks Exhumed from Different Maximum Pressures After Subduction: P-T Evolution of Lawsonite-bearing Schists from the Chinese SW Tianshan

Whether HP/UHP oceanic metabasites and metasedimentary host rocks are cofacial has been a longstanding question that is fundamental to understanding the evolution of orogenic belts worldwide. Although the Chinese SW Tianshan is a well-known example of an accretionary metamorphic belt in which HP/UHP metabasites occur as boudins, lenses, blocks and thin layers in voluminous host schists, information about the pressure (P)-temperature (T) evolution of rocks in its eastern segment is limited precluding a full understanding of the development of the belt as a whole. In this study, to decipher the evolution of the southwestern Tianshan orogenic belt, we use microstructural evidence and phase equilibrium modeling to determine the peak and retrograde P-T conditions from two lawsonite-bearing micaschists and an enclosed garnet-epidote blueschist at Kekesu in the eastern segment of the belt. We compare the results with published information from eclogites in the same area before considering the wider implications for the orogenic development of the SW Tianshan. In the two micaschist samples, relics of prograde lawsonite are preserved in quartz inclusions in garnet, whereas in the metabasite, evidence of lawsonite is inferred based on polymineralic pseudomorphs included in garnet. For garnet micaschist TK21, which is mainly composed of garnet, phengite/paragonite, albite, chlorite, quartz and relict lawsonite, with accessary rutile, sphene and ilmenite, the maximum P-T conditions for the peak stage are 18.0-19.0 kbar at 480-485 °C. During initial exhumation, the retrograde P-T path passed through metamorphic conditions of 14.5-17.0 kbar and 475-510 °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 sphene, apatite, ilmenite and zircon, the maximum P conditions for the peak stage are > 24.0 kbar at 400-500 °C. During exhumation, the P-T path passed through metamorphic conditions of 17.5-18.5 kbar, 485-495 °C and 14.5-15.0 kbar, 460-500 °C. For garnet-epidote blueschist TK37, which is mainly composed of garnet, glaucophane, epidote, phengite, chlorite, albite and quartz, with accessary sphene, apatite, ilmenite, zircon and calcite, the prograde evolution passed through metamorphic conditions of ~20.0 kbar at ~445 °C to P_max conditions of ~21.5 kbar at 450-460 °C and T_max conditions of 19.5-21.0 kbar, 490-520 °C. During exhumation, the retrograde P-T path passed through metamorphic conditions of 17.5-19.0 kbar at 475-500 °C, before recording P-T conditions of < 17.5 kbar, < 500 °C. The results show that the metabasite and the host micaschists have undergone eclogite faices metamorphism. However, when combined with results from eclogite, the maximum pressure conditions for individual samples are seen to vary by up to 5 kbar. The varieties of the P-T paths from different lithologies show that they were juxtaposed together along the subduction interface during exhumation at ~17.0-15.0 kbar.

This study was financially supported by the National Natural Science Foundation of China (grants 41502043, 41330210, 41520104004) and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (grant CUGCJ1814).