Recent progresses in metamorphic P-T-t path of orogenic eclogites

Slab subduction and exhumation are essential components of plate tectonic theory and they are important branches of solid earth science study. High pressure (HP) and ultrahigh pressure (UHP) metamorphic rocks in subduction/collision orogens have experienced complex geological processes involving subduction to deep earth followed by exhumation to shallow crust. Deciphering P-T-t path of HP-UHP metamorphic rocks is not only an important research topic in HP-UHP metamorphism, but also a key approach for revealing the tectonic evolution of subduction/collision orogens. Eclogite, one of the representative of HP-UHP metamorphic rocks, typically preserves several stages of mineral assemblage, which can integrally record crucial information of orogenic evolution. Therefore, orogenic eclogite can be served as a kind of ideal rock type for retrieving P-T-t path of HP-UHP metamorphism. Metamorphic mineral assemblage of different stages can be recognized through detailed petrographic observation and mineral composition analysis, thus the metamorphic P-T evolution of a rock can be decoded by using applicable thermobarometers and/or phase equilibria modelling. For an eclogite, two key problems are involved in depicting its P-T-t path, i.e., how to obtain multistage metamorphic ages and how to link these metamorphic ages (t) to corresponding metamorphic conditions (P-T). In this paper, four potential accessory minerals (zircon, rutile, titanite and allanite) suitable for U-Pb dating in eclogites are reviewed in terms of their crystallization conditions and closure temperature, to evaluate the metamorphic ages they possibly record. The result shows that dating a single kind of accessory mineral can hardly obtain multistage metamorphic ages under some circumstances. Therefore, combined dating of several accessory minerals or applying multichronometric methods are potential solutions for the first problem above. For in situ dating of separated accessory minerals, trace elements and Raman mineral inclusion analyses are not adequate to constrain the crystallization conditions of the dating minerals in some cases. Detailed analysis of petrographic texture of the accessory minerals in the eclogites can clarify the genetic relationship between them and other major minerals, which further constrains their crystallization conditions and thus geological meanings of their ages. Therefore, texturally controlled in situ microanalysis (i.e., petrochronology method) of the dating mineral is a key approach for coupling the results of the metamorphic petrology (P-T) and geochronology (t). The petrochronology method should be applied to future study more frequently. Meanwhile, further study should be carried out, in order to investigate the effects of some geological processes (e.g., metamorphic decomposition/recrystallization, fluid/melt metasomatism, brittle/ductile deformation) on the isotopic systems of the accessory minerals. The timing of metamorphism includes not only the absolute age determined by radiometric geochronology, but also the timescale of the metamorphic process. Due to the limitations of analytical precision, some metamorphic processes with the short duration (= 1Myr) can hardly be resolved by radiometric dating methods. Modelling of compositional diffusion, textural evolution and visco-elastic rheology of some related minerals are potential methods to reveal these metamorphic events, which contributes to a refined metamorphic P-T-t path and comprehensive understanding on the petrogenesis of orogenic eclogites.

Automated summary

Slab subduction and exhumation are crucial in plate tectonic theory and solid earth science. Decoding the P-T-t path of high pressure-ultrahigh pressure metamorphic rocks is important for understanding the tectonic evolution of subduction/collision orogens. Petrographic observation, mineral composition analysis, and geochronology methods are applied to decipher the metamorphic ages and conditions.