Rock Associations And Its Evolution Of Bixiling And Sidaohe Area In UHP Metamorphic Terrane, Dabie Mountains

The Qinling-Dabie collision zone between the Sino-Korean and Yangtze cratons in central China extends -2000 km from the western Qinling Mountains through the Tongbai to the Dabie Mountains. Its eastern extension has displaced northward by the sinistral Tan-Lu fault to the Sulu region. Dabie collisional orogenic belt is an important portion of the Qinling-Dabie collision zone, and it displays typical feature of a Cordilleran-type metamorphic core complex. It is commonly accepted that the UHP terrane was formed in Triassic by the continental collision between the Sino-Korean and Yangtze cratons.Some conclusions about the rock associations and its evolution of Bixiling and Sidaohe areas in the UHP metamorphic terrane are proposed as follows.(1) The UHP metamorphic terrane are composed mainly of crustal and mantle-derived rocks that have undergone UHP metamorphism, retrograded UHP metamorphic rocks, as well as gneisses and foliated garnet-bearing granite. Firstly is the ecologitic rocks including eclogite, eclogitic amphibolite and amphibolite (symplectite of plagioclase+amphibole). In some localities where large eclogite bodies occur, there is a gradual transition of eclogite→eclogitic amphibolite →amphibolite. Secondly is the gneiss, which is straight country rock of the eclogitic rocks, there is a gradual transition between eclogite and gneiss. Thirdly is the foliated granite, there is a gradual transition between gneiss and foliated granite. In a word, there is a gradual transition of eclogite→gneiss→foliated granite.(2) During the process of evolution from eclogitic rocks to gneiss and foliated granites, the most obvious characteristic showing on garnet mineral is that transition from integrated garnet phenocryst to small particle, the "isolated island" garnet phenocryst, which composed of garnet phenocryst pseudomorphs and retrograde minerals. The typical equilibrium parageneses of the minerals in rocks are distinguished into three metamorphic stages: (1) Grt + Omp + Rut ±Coe (or pseudomorphs ) (Peak metamorphism, only in Bixiling area); (2) Hbl + Pl (which forms symplectite on rim of garnet and omphacite) (amphibolite retrograde metamorphism); and (3) Epi + Hbl ± Bi ± garnet + sodium feldspar (which forms garnet phenocryst pseudomorphs) (epidote-blueschist retrograde metamorphism).(3) During the evolution process from eclogitic rocks to gneiss and foliated granites, with the increase of the SiO2 content, the contents of TiO2, A12O3, MgO, CaO, FeO and P2O5 decrease accordingly, and the contents of K2O and Na2O increase accordingly. Combined to datum of experimental petrology, it exists systematic compositional variation among these three type rocks. Trace elements show the characteristics of crustal source. In REE patterns, from eclogite to gneiss and foliated garnet granites, LREEs show gradual enrichment and the curve of REE patterns change from left-trend to right-trend.(4) During the evolution process from eclogites to gneiss and foliated ganites, Garnet composition have been changed remarkably, especially in contents of spessartine. The contents of...