Cenozoic Tectono-Magmatic Evolution And Strain Localization Process In The Ailaoshan-Red River Strike-Slip Shear Zone,Western Yunan

The Ailao Shan-Red River（ASRR）strike-slip shear zone is a significant tectonic belt in SE Tibet Plateau.The ASRR shear zone goes through multiple geological historical evolution periods,especially playing an important role in accommodating the Indian–Eurasian plate collision and subsequent post-collisional evolution.A series of high-grade metamorphic rock series and granitic rocks developed in the ASRR shear zone,which are regarded as the best natural laboratory for studying metamorphism,magmatism and tectonic evolution.In this study,we investigate the widely exhumed adakite-like rock,migmatite,leucogranite and mylonite from Ailaoshan（ALS）metamorphic massif.The methods adopted in this paper include optical microscope,cathodoluminescence,zircon U-Pb chronology,whole rock geochemistry,EBSD fabric,seismic properties,kinematic vorticity and rheological parameters.On this basis,the Cenozoic tectono-magmatic evolution and strain localization process of the ASRR shear zone is mainly discussed,and the following main understandings are obtained:1)The adakite-like rock in the ASRR shear zone was formed from the late Eocene to the early Oligocene（45–35 Ma）,which was formed by mixing and partial melting of enriched mantle-derived melt and magmas derived from the thickened ancient crust.Eocene potassic felsic rocks are abundantly developed in the Jinshajiang–Ailaoshan suture zone that stretches from eastern Tibet over western Yunnan to Vietnam.High SiO₂（68.95–72.38%）,K₂O（4.38–5.80%）and low MgO（0.29–0.51%）,Y（5.58–14.19ppm）and Yb（0.4–1.34 ppm）contents as well as high Sr/Y（70–285）and La/Yb（11.5–47.5）values,belonging to high-K calc-alkaline to shoshonitic adakite-like rocks.Zircon U-Pb ages reveal that the adakite‐like rocks with different deformation characteristics emplaced between 39 and 30 Ma.Zircons of these rocks showεHf（t）values ranging from-2–+6,and two-stage model ages T_DM=750–1500 Ma supports the interpretation of its formation by mixing and partial melting of enriched mantle-derived melt and magmas derived from the thickened ancient crust.Temperature and pressure calculated by amphibole-plagioclase thermobarometry reveal average emplacement depths of ca.18 km of the adakite-like rock,indicating exhumation from nearly 18 km depth by apparent vertical movement of ASRR shear zone.2)The migmatites and leucogranites with different deformation record Early Oligocene to middle Oligocene（35–28 Ma）crustal anatexis along the ASRR shear zone,and provides important information for the study of Cenozoic crustal anatexis and tectonic magmatic evolution.The leucogranites contain high SiO₂（72.7–76.3 wt%）and K₂O（4.6–7.4 wt%）and moderate Al₂O₃（13.49–14.49 wt%）and show significant Pb and Sr enrichment anomalies and Nb,Ta,and Ti depletion.The REE content of the mesosomes of the migmatites is higher than that of the leucosomes,and the Eu of the mesosomes and the melanosomes in the migmatites have obvious negative anomaly（Eu/Eu*=0.72–0.85）.The Eu of leucosomes in migmatites has obvious positive anomaly（Eu/Eu*=20.2–21.2）.Combined with the microscopic characteristics of migmatites,which indicate the extensive crustal anatexis along the ASRR shear zone.Most zircons from the migmatite and leucogranites present an inherited core-rim structure,zircon U-Pb ages reveal that Cenozoic crustal anatexis event along the ASRR shear zone at 41–28 Ma.Zircon U-Pb ages of the magmatic zircons from the undeformed leucogranites at 26–23 Ma.The Ti-in-zircon temperature on zircon rims from migmatites and leucogranites ranges mainly from 749 to 781℃,which restricts the temperature of crustal anatexis.3)The Cenozoic crustal anatexis promoted the initiation of the ASRR shear zone in the early stage of left-lateral shearing.Crustal anatexis and magmatism during the Early Oligocene to Late Oligocene in the ASRR shear zone would strongly reduce the strength of the lithosphere,thus controlling the rheological behavior of the lithosphere and playing an essential role in the initial localization of shearing in the lower crust.Ages of the magmatic zircons from the undeformed leucogranites provide the timing of the termination of a high-temperature tectono-thermal event and ductile left-lateral shearing at 29.7–23 Ma.4)Field,meso-and microstructural,deformation temperatures and flow vorticities of the ASRR shear zone provide significant information on the deformation characteristics,strain localization and exhumation mechanism.The kinematic vorticity values calculated by the rotated rigid porphyroblasts,the quartz c-axis fabrics and the oblique grain shape method imply that the general shear-dominated flow（0.5–0.7）and sample shear-dominated flow（～0.95）,respectively.The shortening perpendicular to the flow plane（S）of mylonites is about 0.19–0.34.Crystallographic textures indicate restricted recrystallization of quartz mainly via dislocation creep through activation from rhomb<a>slip system to prism<c>slip system.According to the opening angle thermometer of quartz,the deformation temperature of quartz can be divided into four ranges of～350℃,～400℃～450℃,and～500℃.Rheological parameter estimations suggest differential stress within the range of 15.9–65.1 MPa,the corresponding strain rate ranges of the four temperature ranges are 10^-17–10^-15s-1,10^-14–10^-13s^-1,10^-14–10^-13s^-1and 10^-13–10^-12s^-1,respectively.Combined with the previous zircon U-Pb ages and⁴⁰Ar/³⁹Ar thermochronological data,Middle to Late Oligocene（29.7–23 Ma）the left-lateral shearing along the ASRR shear zone resulted in the rapid extrusion and exhumation of materials in the lower crust.