The Principal-stage Metamorphism Conditions And Chronology Research Of Susong Metamorphic Complex In The Dabie Orogenic Belt

Susong metamorphic complex is located in the southern margin of Dabie orogenic belt,sandwiched between the South Dabie ultrahigh-pressure metamorphic belt and the foreland fold-thrust belt of Yangtze plate.A great deal of research work has been done by predecessors around its metamorphic degree,metamorphic evolution process,metamorphic and formation time,but the following three problems are still unclear: first,what kind of subduction-exhumation process the Susong metamorphic complex has experienced,and whether the subduction depth at the middle-shallow level is universal;Second,whether there are older geological bodies involved in subduction and exhumation;Third,the formation time and structural attribution of its protolith.Therefore,this paper has carried out systematic research on petrography,mineral composition,temperature-pressure conditions,P-T path and zircon geochronology of Susong metamorphic complex,deeply discussed its subduction depth and subduction-exhumation mechanism,protolith formation time and structural attribution,geological significance of ?1.4Ga garnet amphibolite.The main research contents and achievements are as follows:(1)Temperature and pressure conditions in the principal-stage metamorphism: the mineral assemblage of garnet amphibolite,garnet-mica schist and garnet-mica-quartz schist in Susong metamorphic complex is complete,which is suitable for the evaluation of temperature and pressure conditions.On the basis of mineral composition analysis and reasonable selection of metamorphic components in the principal-stage metamorphism,a variety of geological thermobarometer was used for comparison and solution.The given metamorphic temperature and pressure conditions in the main stage are T=470?590? and P=0.8?1.15 GPa,which are in the range of epidote-amphibolite facies to amphibolite facies,indicating that the Susong metamorphic complex generally experienced a subduction depth of middle-shallow level.(2)Metamorphic evolution process: Based on the results of mineral composition and petrographic analysis,the garnet amphibolite in Susong metamorphic complex has four stages of metamorphic mineral assemblages.The temperature and pressure conditions are 582?617? and 0.84?0.91 GPa in M1 stage;respectively.652?669? and 1.04?1.08 GPa in M2 stage;627?642? and 0.58?0.61 GPa in M3 stage;400?500? and0.1?0.3GPa in M4 stage.The constructed clockwise P-T path shows the subductionexhumation pattern of relatively slow subduction in the early stage?rapid exhumation?slow uplift in the late stage.It is suggested that the subduction-exhumation mechanism of Susong metamorphic complex may be: subduction under the drag of oceanic plates in the early stage,decoupling at the depth of middle and lower crust,rapid rise to near superficial surface due to buoyancy or compression,and then slowly uplifted to the surface at last.(3)Chronological study: the garnet amphibolite geologic body with the age of1381±25Ma was first disintegrated in the Biehe area of Susong County,and the recorded magmatic activity of ?1.4Ga was probably in response to the breakup of the Columbia supercontinent.The intrusive age of ultrabasic rocks and diorite dikes in this block is Early Cretaceous.Six groups of ages are given in garnet-mica schist samples,among which the early Mesozoic metamorphic age(weighted average age 211Ma)should represent the retro-metamorphic age in the exhumation stage.detrital zircons have five age peaks of 2469 Ma,2026Ma,1356 Ma,747Ma and 437 Ma,among which two groups of detrital zircons with peak ages of 2469 Ma and 1356 Ma probably come from the geological bodies of ?2.5Ga amphibolite and ?1.4Ga garnet amphibolite in Susong metamorphic complex.The youngest detrital zircon age is 415 Ma,which indicates that the lower limit of protolith sedimentary age of Susong metamorphic complex is Early Devonian.In addition,the age of detrital zircon is consistent with the characteristic ages of Late Archean(2.4?2.7Ga),Paleoproterozoic(?2.0Ga)and Neoproterozoic(700?900Ma)in Yangtze plate,which further indicates that the Susong metamorphic complex has the attribute of Yangtze plate.