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

Abstract

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 subducting 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 the 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 ρ mantle), which is driven by their own buoyancy, an example is the coesite-bearing oceanic eclogites from Southwest Tianshan. Another is the carried-exhumation eclogites ( ρ MORB> ρ mantle), 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 have 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.