Genetic Relationship And Structural Significance Of Cenozoic Metamorphism-Anatexis And Leucogranite In The Ailaoshan-Red River Shear Zone,Western Yunnan

The Sanjiang region on the southeastern margin of the Qinghai-Tibet Plateau,with the Eastern Himalayan tectonic junction as the turning point,experienced significant crustal thickening,tectonic compression,block relocation and strike-slip shearing in the Cenozoic.A series of large-scale ductile shear zones associated high grade metamorphic complexes have been exposed in the Sanjiang region.The Ailao Shan–Red River shear zone(ASRR-SZ)as the boundary of the Indochina Block(or Simao–Lanping Block)and the Yangtze Block,shows great effect during the IndiaAsia collision.A large value high–grade metamorphic rocks,migmatites,and leucogranites were exposed in the shear zone.These rocks not only preserve key information of the metamorphism,crustal melting,and tectonic information n before or during large–scale strike–slip shearing,but also provide insights into the timing and processes of post–collision.However,there are very few studies on the metatexite migmatite compared to structural characteristics,P–T conditions,and geochronology until now.Especially the genetic relationships among the metamorphic evolution,tectonic deformation,and anatexis remain poorly understood.In this contribution,we present investigations on the high-grade metamorphic rocks and leucogranites in the ASRR-SZ.This study has mainly achieved the main conclusions as follows:1.The three stages of regional metamorphic evolution history have been constrained.The metamorphic petrology shows that the metapelitic migmatites in the Ailaoshan metamorphic massif recorded the granulite facies metamorphism and anatexis.The Cenozoic metamorphism of the Ailaoshan metamorphic massif has been divided into three stages by the petrographic analysis: the prograde metamorphic stage M1,with the P-T condition of 650-670 ℃ and 6.6-7.8 kbar;the peak metamorphic stage M2,with the granulite facies metamorphism of 830 ℃,8.5 kbar;the late stage M3 of melt-residue reaction,with the P-T condition of 680 °C,4.8 kbar.The metamorphic evolution of the metabasic rocks is similar to that of metapelitic migmatites in the Ailaoshan metamorphic massif.The metapelitic migmatites in the Diancangshan metamorphic massif recorded the high-pressure(HP)granulite facies metamorphism and anatexis.The phase diagrams of the migmatite indicate the peak metamorphic stage equilibrated at approximately 810 ℃ and 11 kbar,with the maximum melt production of 18 mol%.The retrograde metamorphic stage is constrained by the two segments of 765–780 ℃,7.5–8.2 kbar and 660–670 ℃,4–5kbar.Based on the research results of the Diancangshan and Ailaoshan metamorphic massif,the peak metamorphic stage of the ASRR-SZ is granulite facies at least,and HP granulite facies in the local area.The clockwise near isothermal decompression P-T path indicates the ASRR-SZ was created by the intracontinental orogeny,and the dehydration melting of muscovite and biotite is triggered by the heating process of lower crustal thickening.2.The genetic relationship of the regional migmatites and leucogranites has been determined.Field observations suggest widespread partial melting in the DCS complex through the existence of metatexite migmatites,diatexite migmatites,and large proportions of leucogranites.The structural relationships of these rocks indicate how the melt was generated from the lower crust,segregated out of the protolith,migrated to the shallow crust,and emplaced there.Among these rocks,the migmatites show abundant in-situ partial melting microscopic evidence.Phase diagrams show that the melt production of the Ailaoshan migmatites is about 22 mol%,and the melt production of the Diancangshan migmatites is about 18 mol%.These melts are mainly generated by the dehydration melting of muscovite and biotite during heating,most of which have migrated out of the protolith and become the source for the regional leucogranites.The following features of the leucogranites in the ASRR-SZ reveal these leucogranites are generated by the lower crust anatexis: the aluminum saturation index of 1.00-1.17,the peritectic garnets and anatexis zircon,the Th/U ratio in zircons is usually less than 0.1,the Eu content varying greatly with both positive and negative anomaly(δEu = 0.14-5.9),the Nb/Ta ratio lower than that of chondrites varying greatly(3.16-29.47).The leucogranites have low Sr,Ba and high Rb.The Rb/Sr ratio shows linear negative correlation with Ba and Sr concentrations,which indicate the genesis of muscovite dehydration-melting.The result from the leucogranites is consistent with that from the migmatites: The leucogranites in the ASRR-SZ are mainly generated by the dehydration melting of muscovite in the migmatites.3.The P-T-t-d-melt path for the ASRR-SZ has been determined.U-Pb chronological analysis shows that the granulite facies metamorphism and associated anatexis in the ASRR-SZ have a prolonged history: The beginning time of the peak stage is earlier than 36.3 Ma,with maximum melt production of 22 mol%.The ASRRSZ experienced a clockwise P-T path.The Ti-in-zircon thermometer of leucogranites show that the temperature of leucogranites decreases with age,which indicates the evolution of anatexis is from the high temperature stage(900 ℃)to the low temperature stage(500 ℃),and eventually cooled to the subsolidus.The minimum age of crystallization of leucogranite is about 20.5 Ma,indicating the end of regional anatexis.Both the peak stage and the retrograde stage of high grade metamorphic rocks are affected by the left-lateral shearing.4.This thesis gives an insight to re-examine the tectonic significance of the ASRR-SZ in the Cenozoic India-Eurasian collision.Metamorphic and geochemistry studies reveal the similar P-T-t-melt evolution between the ASRR shear zone and the Great Himalayan Sequence.This study proposed that the ASRR shear zone may not serves just as the accommodation belt for the eastward extrusion of Eurasian continental blocks as the traditional concept,instead it had undergone crust thickening together with the Himalayan orogenic belt in Cenozoic.5.This thesis discussed the relationship between anatexis and deep crustal shear zones.The relationship between the activity of shearing and the intrusion of leucogranite in the ASRR-SZ suggests that the shear zones nucleated in the melt–bearing crust and served as conduits for the ascent of channeled melts to higher crustal levels.The reginal shearing and anatexis form positive feedback in a long-time scale,which shows great significance for many problems such as the localized initiation,the expansion,and the rheological properties of the deep crust.