Quantitative Constraint Of Deformed Graphite On Tectono-thermal Process And Significance Of Rheological Weakening

The precise determination of the tectono-thermal evolution from deep to shallow processes in the orogenic belt is a hot and difficulty topic in geological research.Graphite has typical mineral properties of layered structure and very low friction coefficient.And the graphization process of amphous caebonaceous material with increasing temperature is irreversible,which means that graphite can record metamorphic-deformation events.However,there is still a lack of sufficient understanding of the formation of natural deformed graphite-bearing rock and its macroscopic to microstructural characteristics for the deformation of rocks and minerals,and their rheological weakening behavior and mechanism.In this contribution,we select different types of metamorphic-deformation rocks in the Cenozoic Ailaoshan-Red River shear zone,especially graphite-bearing rocks and minerals.On the basis of detailed field observations,using optical microscope（OM）,scanning electron microscope（SEM）with electron back-scattering diffraction（EBSD）,cathodoluminescence（CL）,Raman spectra,carbon isotope mass spectrometer,to carry out analysis in detailed deformation macro-microscopic and submicroscopic structural characteristics,mineral lattice preferred orientation（LPO）,graphite Raman spectrum temperature,origin of carbon material.This research has mainly obtained the following results and understandings:1.Four types of graphite-bearing rocks are determined.According to the graphite metamorphic-deformation characteristics,graphite-bearing rocks in the study area is divided into four types,e.g.,type 1 middle to high-grade metamorphic,and weakly deformed to undeformed graphite-bearing pelitic gneiss,granitic gneiss and marble;type 2 medium to high-grade metamorphic and strongly deformed mylonitic graphite-bearing schist,marble;type 3 low-grade metamorphic and strongly deformed graphite-bearing schist and marble;and type 4 lower-grade metamorphic and weakly deformed to undeformed graphite-bearing phyllite and limestone.It is found that from the core of the shear zone to the boundary,graphite shows obvious ordered sheets to form gneissosity and mylonitic foliation,or forms micro shear zones in strongly deformed rocks,which represents obvious rheological weakening characteristics.2、The slip systems of graphite represented high tempearature to low temperature stage are obtained,and accurate metamorphic-deformation conditions in the study area is limited.Through detailed microstructure and EBSD analysis of different types of graphite-bearing rocks,the slip system of deformed graphite switching from prism<a>,rhomb<a>to basal<a>are proposed,which is resemble to that of deformed quartz.The metamorphic temperature calculated by graphite Raman spectra thermometer,and the deformation temperature indicated by quartz slip system and c-axis open angle（OA）are consisitent with each other.The left-lateral strike-slip deformation and peak metamorphic tempertures in deep metamorphic zone are concentrated between450-630℃,which is same as temperatures of the middle part of the shear zone indicated by garnet feldspar thermometer（500-650℃）;the deformation and peak metamorphic temperatures in shallow metamorphic zone are concentrated between 300-400℃.In addition,the peak metamorphic temperature of the deep metamorphic rock zone tends to decrease gradually from south to north with 432-633℃,401-607℃and 434-543℃,respectively.3.A quantitative method to qualify tectono-thermal evolution is proposed.Establishment of a new graphite Raman thermometer is applied well in the study area using the Raman parameter（D1/G）_Areavalue as the independent variable:T*=-25.836x³+142.73x²-323.57x+640,R2=0.9787,where x is（D1/G）_Areavalue,and R²is confidence interval.This thermometer is suitable for greenschist facies to amphibolite facies of graphite-bearing rocks,and only mylonitization could lead to the temperature underestimation of the penetrated slip surface heat which is sub-parallel to mylonitic surface composed of enriched graphite layer.4.A three-stage tectono-thermal evolution from high temperature to low temperature is established in the study area.The origin properties of graphite are obtained,as the organic carbon isotope analysis of graphite rocks shows that both gneiss and schist are organic origins,while graphite carbon materials in marble come from inorganic decarbonation.Combining graphite metamorphic-deformation characteristics,Raman peak temperature,EBSD slip system,origin of carbon in graphite,differential paleostress and strain rate,we propose three tectono-thermal stages in the Ailaoshan-Red River strike-slip fault zone,e.g.,D1-weakly deformed period above the high-grade metamorphic zone(e.g.,type 1 rocks with peak Raman temperature of432-633℃,deformed quartz grains represent prism<a>and rhomb<a>slip system,differential paleostress of 17-22MPa,and strain rate of 10^-13～10^-14s^-1),D2-high strain period superimposed above the weak deformed domain(e.g.,type 2 rocks with peak Raman temperature of 388-557℃,deformed quartz grains represent prism<a>,rhomb<a>to basal<a>slip system and superpostions of low-temperature rhomb<a>to basal<a>on high-temperature prism<a>slip system,differential paleostress of 37-42MPa,and strain rate of 10^-12～10^-13s^-1)and D3-strong/weakly deformed period distributed in the low-grade metamorphic zones(e.g.,type 3 and type 4 rocks with peak Raman temperature of 330-440℃,deformed quartz grains represent basal<a>slip system and superpostion of low-temperature basal<a>on high-temperature prism<a>slip system,differential paleostress of～67MPa,and strain rate of～10^-13s^-1).