Petrology And Metamorphic Evolution Of The Diancangshan-Ailaoshan Metamorphic Complex Belt, Southeastern Tibetan Plateau

Diancangshan-Ailaoshan metamorphic complex belt, located in the margin between the Yangtze Plate and Indochina block, southeastern Tibetan Plateau, is a significant tectonic belt. It is a unique window to study complicated magmatism, metamorphism and deformation during geodynamic processes of subduction-exhumation-collision between Indian and Eurasian plate. Comprehensive studies on field geology, petrography, genetic mineralogy and isotopic geochronology have yielded several new constraints on the nature of protolith, transformation of mineral phase, metamorphic evolution and chronological framework of meta-sedimentary rocks and meta-mafic rocks.Diancangshan-Ailaoshan metamorphic complex belt is mainly composed of granitic gneisses and meta-sedimentary rocks with irregular lenses or blocks of meta-mafic rocks. The meta-sedimentary rocks could be subdivided into Al-rich schists, gneisses and marble, while the meta-mafic rocks are comprised of amphibolite and garnet pyroxenite. The geochemical data show that meta-sedimentary rocks are slightly higher in SiO2, but lower in Al2O3 compared to the typical Al-rich gneissic rocks of khondalite worldwide, and enriched in LREE, depleted in HREE with pronounced negative Eu anomalies (Eu/Eu*= 0.55-0.75). Protoliths of meta-sedimentary rocks, including claystone, siltstone and minor greywacke, are supposed to be formed in the passive continental margin. Amphibolites with high Al, and low Ti and Zr contents, are characterized by flat REE patterns with slightly negative or no Eu anomalies, which are depleted in high field strength elements, and show very similar trace element compositions to those of island-arc basalt. These data indicate that the protolith of amphibolites was possibly formed in an island arc tectonic setting related to subduction-collision geodynamic process. Garnet porphyroblasts in Al-rich schist well preserve compositional zoning with abundant fine-grained inclusions of Qtz, Pl, Ms, St and Ilm in the core and few coarse-grained inclusions of Qtz, PI, St, Ky, Ms and Ilm in the mantle and rim, respectivley. CaO and MnO contents decrease continuously from garnet core until rim, ranging from 6.24 wt% to 1.86 wt% and 4.79 wt% to 0.73 wt% respectively, and FeOT and MgO contents show the opposite trends, ranging from 28.35 wt% to 34.43 wt%, and 1.39 wt% to 4.22 wt% respectively. A typical decompression reaction of Grt+ 2Ky +Qtz = 3An has been identified in the matrix of Al-rich gneissic rocks. Based on the petrography, genetic mineralogy and metamorphic reactions, together with traditional geothermobarometric calculation, and THERMOCALC and winTWQ analyses, a clockwise P-T path with four metamorphic stages and distinct mineral assemblage have been identified within Diancangshan-Ailaoshan metamorphic rocks. Early low-T mineral assemblage, characterized by garnet+plagioclase+muscovite+quartz+staurolite ± kyanite ± biotite ± K-feldspar (meta-sedimentary rocks), was preserved in garnet porphyroblasts and formed at T = ～ 510℃ and P=~5.5 kb during prograde process (M1). With increase of temperature and pressure, early low-T mineral assemblage was continuously replaced by High-T mineral assemblages (M2), characterized by garnet+biotite+kyanite/sillimanite+plagioclase+quartz, garnet ± sillimanite+plagioclase ± K-feldspar+quartz (meta-sedimentary rocks), and garnet+clinopyroxene+plagioclase (meta-mafic rocks), which record peak amphibolite-granulite facies conditions of T = 700-760℃ and P= 7.5 ～9.6 kb.Then, typical decompression reaction, and partial melting of hydrous minerals (e.g. biotite) and felsic minerals occurred at the isothermal decompression stage (M3), and formed mineral assemblage of garnet+ biotite+sillimanite+plagioclase (rich in Ca)+quartz (meta-sedimentary rocks), and amphibolite+ plagioclase ± clinopyroxene ± quartz (meta-mafic rocks) at P-T conditions of 650-750℃ and 5.0~7.1 kb. Finally, newly fined-grained biotite and muscovite with the mineral assemblage of biotite+muscovite+ plagioclase ± K-feldspar ± garnet+quartz (meta-sedimentary rocks) was formed at the late retrograde cooling stage (M4) and yielded P-T conditions of T= 512-648℃ and P= 4.0~5.0 kb. Zircon U-Pb, and biotite and muscovite Ar-Ar analyses reveal four discrete and meaningful ages for the Diancangshan-Ailaoshan metamorphic complex:(1) Neoproterozoic protolith age (780-750 Ma) for meta-mafic rocks (amphibolite) and Paleozoic protolith age (< 500 Ma) for meta-sedimentary rocks; (2) 44-37 Ma for peak amphibolite-facies metamorphism (M2); (3) 32-26 Ma for isothermal decompression retrogressive overprint (M3); and (4) 24-14 Ma for late amphibolite facies retrogression. These data together with previous studies reveal that protoliths of volumineous granitic gneisses, together with Neoproterozoic meta-mafic rocks and Paleozoic sedimentary rocks, were subducted to depths of-20 km at 65-55 Ma and recrystallized at early an amphibolite facies condition (Mi). Then these metamorphic rocks were further subducted to depths of 30-35 km at 44-37 Ma and experienced a peak amphibolite-granulite facies metamorphism (M2). About 10-12 Ma later, these amphibolite-granulite facies metamorphic rocks were exhumed to mid-crustal levels (20-25 km) at 32-26 Ma and overprinted by isothermal decompression retrogression (M3), and finally the reotrogressive rocks were continuously uplifted to shallow crustal-levels (-15 km) at 24-14 Ma and underwent a late amphibolite facies retrogression (M4) and strong ductile shear deformation simultaneously. The establishment of P-T-t path for the Diancangshan-Ailaoshan metamorphic complex belt has an important significance for revealing the formation and evolution, metamorphic histories and magmatic events of southeastern Tibetan plateau.