Petrology And Metamorphic Evolution Of The Diancang Shan-Ailao Shan Metamorphic Complex

The Diancang Shan(DCS)-Ailao Shan(ALS)metamorphic complex belt is located on the southeast margin of the Tibetan plateau,which separated the Yangtze plate to the east and the Indochina plate to the west.The DCS-ALS complex belt with multiple deformation,magmatism and metamorphism events is essential for understanding the evolution of the Paleo-Tethys,subduction-collision-exhumation between the Indian and Eurasian plate,and the Indochina Block extrusion alone the Red River-Ailao Shan(RR-ALS)sinistral strike-slip shearing zone in Tertiary.Primarily for the meta-sedimentary and the meta-basic rocks assisted with granitic rocks and anatectic leucosomes,systematic field geology,petrography,genetic mineralogy,geochemistry,zircon U-Pb dating,and zircon morphology were integrated studied in this thesis.A set of new dates and cognition of the nature and deposited age of protolith,transformation of mineral phases,metamorphic evolution and chronological framework for the meta-sedimentary rocks in the ALS complex belt were obtained and multiple metamorphism-magmatism were ascertained.The mainly new understandings were listed as follows:The Diancang Shan-Ailao Shan metamorphic complex belt mainly comprises meta-sedimentary rocks,granitic gneisses and irregular lenses or dykes meta-basic rocks,extensive mylonitization and anatectic melting overprinted on these rocks.The meta-sedimentary rocks mainly contain AI-rich paragneisses,two-mica schists,quartzites,and dolomitic marbles.The granitic rocks intruded in meta-sedimentary rocks or as lenticular occurs in other rocks,consists mainly of monzogranite,granodiorite,(garnet-bearing)granitic gneisses,granitic mylonite and biotite granite.The meta-mafic rocks are primarily comprised by(garnet-bearing)amphibolite and garnet pyroxenite and can be divided into two groups based on the field occurrences,the first group occurring as dykes intruded into paragneisses or granitic rocks and the second group occurring as irregular lenses within dolomitic marbles.The field occurrences and geochemical studies indicates the protolith of meta-sedimentary rocks were moderately weathered claystone or graywackes,the average CIA index is 70.1 and the A/CNK(molar ratio of Al2O3/(CaO+Na2O+K2O))ranging from 1.53 to 4.06.The REE distribution patterns of the Al-rich paragneisses are similar to those of the NASC and average upper-crustal compositions,indicating the passive continental margin setting.The LA-ICP-MS U-Pb dating of detrital zircons in meta-sedimentary rocks within ALS Group shows two major age peaks at 520?450 Ma and 780?740-Ma,respectively and a inferior peak at 2480?2400 Ma.The youngest U-Pb age population(peak at 520?450 Ma)is considered to constrain the maximum depositional age of the ALS Group to Early-Paleozoic rather than previous believed Paleoproterozoic and the ALS Group does not belong to the basement of Yangtze Block.The metamorphic zircon U-Pb dating reveals that the ALS metamorphic complex underwent Middle-Late Triassic(236 ± 3.7 Ma?211±3.4 Ma)and Eocene-Oligocene(46-27 Ma)metamorphisms.Accompanied with the regional ALRR strike-slip shearing uplift in Oligocene,the rocks in the ALS metamorphic complex belt experienced varying degrees of partial melting including muscovite and biotite dehydration in the meta-sedimentary rocks formed granitic leucosomes.Ti in zircon thermometer of leucosomes shows the rock forming temperature ranging from 670 to 720?.Granitic rocks in Diancang Shan-Ailao Shan metamorphic complex relatively high Na2O+K2O contents(3.25?10.65,average 7.89),compared to typical calc-alkaline rocks.The ratios of K2O/Na2O ranges from 0.48 to 3.57,and are peraluminous to metaluminous with aluminous indices A/CNK ranging from 0.57?1.14,A/CN ranging form 1.10 to 1.70.Samples have variable REE contents but similar chondrite-normalized REE patterns with moderately enriched light REE,morderate negative Eu anomalies(8Eu 0.27?0.81),enrich in large ion lithophile elements(Rb?Ba?Th?U),variable Sr/Y ration 2.27?94.26 and low Rb/Sr ratios 0.15?2.32.The U-Pb dating for the granitic rocks reveals that the ALS metamorphic experienced at least thrree magmatism.The first phase was at Neoproterozoic(779 ± 8.5 Ma?744±6.7 Ma),the second phase was at Middle-Late Triassic(234±2.0 Ma)and the third phase was at Cenozoic(36.7±0.5 Ma?24.5 ± 0.5 Ma).The magmatism in Cenozoic could be further subdivided into two stages of 36.7 ± 0.5 Ma?33.0 ± 0.4 Ma and 29.4 ± 0.4 Ma?24.5 ± 0.5 Maand the former was characterized by enrich alkaline,the Na2O+K2O content ranging from 9.2%to 10.4%,while the latter was characterized by Al enrichment,the ratio of A/CNK in most rocks are above 1.0.The geochemical contrastive studies shows that the granitic rocks with Neoproterozoic ages are similar to continental-arc magmatic rocks in southwest margin of Yangtze Block.These rocks may be drawn into the ALS metamorphic complex in later orogenesis and/or strike-slip shearing.The granite intruded during Middle Triassic was formed by decompressional melting,the tectonic background may be related to the stretch after the collision between Yangtze Block and Indochina Block.The Eocene alkaline-rich granites may be related to the post-collisional extension between Indian plate and Eurasian plate,while the Oligocene granite coeval with the RR-ALS strike-slip shearing,which maybe resulted by upper-middle crust melting during the strike-slip uplift movement.The first group of the meta-basic rocks occurring as dykes is characterized by lower MgO and TiO2 contents,weak negative Eu anomalies.It is enriched in,Rb,Ba Th,U and LREE,and depleted in Nb,Ta,Ti,P high field strength elements and HREE,which similar to those of the island-arc basalts.The second group meta-intermediate rocks occurring as irregular lenses within dolomitic marbles exhibits higher MgO,TiO2 contents,flat REE patterns and no Eu anomalies,suggesting oceanic island basalt affinities.Combined with petrography,genetic mineralogy,metamorphic reactions and phase equilibria modeling studies of the kyanite-bearing garnet-sillimanite-biotite monzonitic gneisses,the mineral assemblage for the peak stage(M1)is characterized by Grt+Ky+Bt+Pl+Kfs+Qtz+Ru with the P-T conditions of 750?800?,10.1?12.6 Kbar by GB-GASP thermobarameter and 800?810? and 13.5?14.0 kbar by phase equilibria modeling.Typical decompressional reaction of Grt+2Ky+Qtz=3An have been identified in the matrix and the garnet porphyroblasts preserved compositional zoning with the CaO contents decreased continuously and MgO contents increased from the core to rim.The mineral assemblage for isothermal decompressional stage(M2)is identified as Grt+ Sil+Bt+Pl+Kfs+Qtz+Ilm,the P-T conditions of 700?750?,6.0?8.0 Kbar by GB-GASP thermobarameter and 830?840? and 8?10.5 kbar by P-T pseudosection.Typical reactions are represented by Ky=Sil and Bt+Sil+Qz ± Grt1±Pl=Grt2+Kfs+Melt.Correspondently,in the meta-basic rocks,the mineral assemblage for the peak stage(M1)is characterized by Grt+Cpx+Pl and Grt+Am+Pl+Mag during the decompressional stage(M2).The reaction may related to Grt+Cpx=Am+Pl+Mag.The calculate results by traditional thermobarameters for the peak stage(M1)of the P-T conditions calculated by traditional thermobarameters are 847?861?,11.9?13.2 kbar.The P-T evolutions suggest the rocks in the ALS metamorphic complex belt experienced high pressure granulite-facies metamorphism and the path is characterized by isothermal decompression clockwise style.The results of previous studies and new dates in this thesis reveal that the detrital material source of meta-sedimentary rocks in ALS metamorphic complex may come from both Yangtze Block and Indochina Block while the Jinsha jiang-Ailao shan-Sone Ma ocean initial open in early Paleozoic.The meta-sedimentary rocks in DCS-ALS metamorphic complex documented Early-Middle Triassic(245?230 Ma)and Eocene(47?36 Ma)two phase high pressure granulite metamorphism.The former phase metamorphism associated with the subduction-collision between Yangtze Block and Indochina Block after the closure of Jinsha Jiang-Ailao Shan-Sone Ma ocean and the later related to Cenozoic intracontinental orogeny,the exhumation of the high pressure granulites may be related to shearing uplift in Oligocene.The features of the P-T-t path for Two-phase metamorphism indicated the DCS-ALS metamorphic complex belt was a composite orogeny.