Anatexis,Deformation and Exhumation Mechanism for UHP Metamorphic Rocks:A Case Study in the North Qaidam and South Altyn UHP Terrane,Western China

Objective In recent years,hydrous silicate melts by dehydrationdriven in situ partial melting constrained from experiments and natural rocks have been increasingly recognized in UHP rocks,indicating partial melting of UHP slab.Partial melting of UHP metamorphic rocks can dramatically affect the rheology of deeply subducted crust and thus play a crucial role in accelerating the exhumation of UHP slabs.

Aragonite from Mulanshan Glaucophane Schist: Implications for Regional Evolution of Southwestern Dabie Mountains, Central China

The aragonite, an index mineral of glaucophane schist facies, has not been confirmed in the Dabie Mountains high pressure and ultrahigh pressure metamorphic belts. The Mulanshan glaucophane schist in Huangpi County, Hubei Province is located in the southwestern Dabie Mountains, Central China. The micron sized intergranular aragonite is confirmed with optical microscope (OM) and electron probe microanalysis (EPMA) in the glaucophane schist. The submicrometer sized ellipse aragonite inclusion is observed by using bright field image (BFI), X ray energy damage spectrograph (EDS) and selected area electron diffraction (SAED) with transmission electron microscope in the quartz lens of glaucophane albite epidote chlorite schist from Mulanshan. The presence of aragonite indicates not only the average geothermal gradient of the Mulanshan glaucophane schist less than 10 ℃/km, which is very close to that of eclogite in the Dabie Mountains metamorphic belts, but also the relatively higher concentration of CO2 during the metamorphic process. In addition, the glaucophane schist free of such index minerals as lawsonite, prehnite and pumpellyite, has been attributed to the effect of CO2 on the stability of calcium aluminum silicate minerals during the low grade metamorphism. EDS and SAED analysis results show that the host of aragonite inclusion is amorphous SiO2. The occurrence of amorphous SiO2 indicates a quick cooling process during the exhumation of the Mulanshan glaucophane schist. These results suggest that the rapid exhumation mechanism of the glaucophane schist, the same as that of eclogite in the Dabie Mountains metamorphic belts, occurred in the geodynamic context of subduction obduction.

The exhumation of high-and ultrahigh-pressure metamorphic terranes in subduction zone:Questions and discussions

In terms of petrology,thermomechanical simulation is an important frontier to study the geodynamic process of the exhumation and uplift of high pressure(HP)to ultrahigh pressure(UHP)metamorphic rocks in subduction zones and collision orogenic belts.Based on the recent petrological studies and numerical modellings for the exhumation of HP to UHP metamorphic terranes,the exhumation mechanisms of HP to UHP metamorphic terranes can be roughly summarized into ten types:channel flow,diapiric exhumation,a coexistence mechanism of channel flow and diapiric exhumation,slab breakoff,multi-stage exhumation,divergent plate motion(including slab rollback and the upper-plate divergent motion away from the subducted plate),overthrust exhumation,overpressure mechanism,wedge-like extrusion and microplate rotation.The exhumation of high-density UHP oceanic eclogites is a relative controversial issue.Some of our recent researches on quantitatively determining the exhumation mechanism of UHP oceanic eclogites using thermomechanical and phase equilibrium modelling was introduced in details in this paper.We obtained the 3-D density evolutions of three-type subducted oceanic materials(MORB,serpentine and oceanic sediments)in P-T space by the methods of phase equilibrium and density calculation.According to the density difference between the metabasic and their surrounding rocks,the exhumed eclogites could be divided into two types.The first category,the self-exhumation eclogites(ρ_(MORB)<ρmantle),which can exhume driven by their own buoyancy,an example is the coesite-bearing oceanic eclogites from Southwest Tianshan.Another is the carried-exhumation eclogites(ρ_(MORB)>ρmantle),which can only be carried back to the surface with the assistance of low-density metasediments and serpentinite due to their negative buoyancy;the coesite-bearing UHP eclogites of Zermatt-Saas in the Western Alps is a typical example.Besides,we further explored the ultimate self-exhumation depth,exhumation mechanisms,the effect of the transition from high pressure to ultra-high pressure on the exhumation process of oceanic eclogites and the spatial distribution of exhumed HP-UHP metamorphic terranes.

The study of subduction channels:Progress,controversies,and challenges

The subduction channel is defined as a relatively thin and weak zone with coherent kinematics between the descending and overriding plates during subduction.The materials in the channel,showing the characteristics of mélange,consist of low-density,low-viscosity,highly sheared metasediments and/or serpentinite-rich matrix with relatively rigid blocks.The channel materials flow downwards and upwards,forming a kind of channel convection.Based on geophysical and geological observations on active oceanic subduction zones,combined with numerical simulations,the subduction channel model was initially set to shallow depths<30 km.With the development of numerical simulation technology and fossil subduction zone investigations,the subduction channel model was extended to depths of 80-100 km to describe deeper geodynamic environments of oceanic subduction zones and explore the exhumation of high to ultrahigh pressure metamorphic rocks.It is also extended to continental subduction zones in order to explain the exhumation of metamorphic rocks in different grades as well as fluid activity and crust-mantle interaction during continental subduction.Although there are a series of differences in material composition and slab/mantle interface property between the upper and lower plates,continental subduction channels exhibit many similarities in structure and dynamics to oceanic subduction channels.Existing studies have provided a fundamental understanding of the structural and dynamic characteristics of subduction channels,but many problems still need to be solved and clarified.In particular,the fundamental characteristics of subduction channels obtained by numerical simulation need to be confirmed and modified through geological investigations of fossil subduction channels.