High-temperature And High-pressure Rheological Experiments Of Dry K-feldspar Polycrystalline Aggregates

The continental subduction zone has become an important window to study the structure,property and behavior of the continental lithosphere since the finding of coesite in metamorphic rocks of supracrustal origin.Felsic gneiss is the main rock type of ultrahigh pressure metamorphic belt in subduction zone,and its rheological properties control the dynamic processes of continental subdction and exhumation.As an important constituent mineral of felsic gneiss,the rheology K-feldspar is much less characterized comparing to those of the other two major constituent minerals（quartz and plagioclase）of felsic gneiss.Systematic high-temperature and high-pressureaxial compression deformation experiments and EBSD/TEM microstructural analyses of dry K-feldspar polycrystalline aggregates have been carried out in this study.In addition,the rheological properties of ultrahigh-pressure gneisses from the Sulu subduction zone have been studied as a comparative study of experiments.The main conclusions are as follows:（1）Systematic axial compression deformation experiments were conducted on a relatively dry（22 ± 2 ppm）K-feldspar aggregates（Or81Ab19）at strain rates of 10^-4 - 10^-5 s^-1,temperatures of 950 - 1100 °Cand a pressure of 1.5 GPa,using a 5 GPa modified Griggs-type deformation apparatus.The flow law of dry K-feldspar aggregates in the dislocation creep domain was obtained for the first time:ε=10^2.4σ^3.0exp（-（368±32）/RT）.The mechanical results show that the creep strength of plagioclase is the highest under the "dry" conditions,followed by K-feldspar,and the strength of both two kinds of feldspar is much higher than that of quartz.Microstructure analyses of the subduction-zone deformed rocks reveal feldspar porphyroclasts surrounded by highly deformed quartz with strong SPOin weakly deformed samples,and plastic deformation of feldsparsin strongly deformed samples,indicating that feldspars are much stronger than quartz.However,when the deformation mechanism of feldspar changes from dislocation creep into subgrain rotation recrystallization accommodated dislocation creep,feldspar becomes weak and bear strain together with quartz.（2）The deformation experimentsat 900 °C showed brittle fractures of K-feldspar,suggesting brittle-ductile transition temperatures of the "dry" K-feldspar polycrystalline aggregate at 1.5 GPa is 900-950 °C.This transition temperature is higher than the brittle-ductile transition temperature of quartz and plagioclase,so the content of K-feldspar in felsic rock has a great influence on its brittle-ductile transition.It has also beenspeculated that K-feldspar-richfelsic rocks may be one of the reasonable explanations for intermediate and deep focus earthquakes.（3）Microstructural analyses on deformed experimental samples and UHP gneisses revealed that,the dominant deformation mechanism of K-feldspar is dislocation creep accommodated by subgrain rotation recrystallization.Recrystallization is the primary cause for grain size reduction.The TEM and EBSD results showed that the dominant slip system is [100]（010）in experimental samples.However,the natural samples show weak K-feldspar CPOs as a result of strong recrystallization.（4）The rheological strength of K-feldspar polycrystalline aggregates is between dry olivine aggregates and garnet amphibolite under the conditions of a natural strain rate（10-14 s-1）.Its strength is 1 order of magnitude lower than those of eclogite and "dry" olivine polycrystalline aggregate,4-5 times of granulite strength,and 1-2 orders of magnitude higher than that of garnet amphibolite.The different rheological strength can make the subduction plate produce obvious vertical stratification in mechanical structure.The peridotite upper mantle and eclogiticlower crusthave great mechanical strength.The rheological strength of middle-lower crustincreases with the increase of content of K-feldspar.The strength difference between crust and underlying upper mantle can be small,resulting in a mechanicallycoupled lithosphere.In contrast,if the content of amphibolite in the middle and lower crust is small,the rheological strength of middle-lower crust can be weak.The great strength difference will make the crust mechanical decoupled with the underlying upper mantle.The achievements of K-feldspar rheological experiments mentioned above provide a quantitative contribution for a complete understanding of the mechanical strength of the continental subduction slab,as well asthe variation of the rheological strength and the transiton from brittle to plastic deformation in the middle-lower continental crustal rocks.