| The Dabie orogen is a continental collision-type belt that exposes the largest area of ultrahigh-pressure(UHP)rocks on the world.It contains various types of high-pressure(HP)to UHP rocks,as well as related low-grade metamorphic rocks.As long as UHP index minerals like coesite and diamond were recognized in the Dabie metamorphic rocks,numerous tetctonic geological,petrological,geochronological and geochemical investigations have been performed on these UHP rocks to discuss and constrain their protolith genesis,ages and nature,metamorphic ages and evolutional processes.Furthermore,integrated studies related to subduction,collision and exhumation processes and the crust-mantle interaction within subduction zone in the Dabie orogen have been carried out,and some important achievements have been made in this contribution.Nevertheless,investigations on the relatively low-grade rocks within the Dabie orogen are rare so far.As to their tectonic property,protolith formation ages and genesis,metamorphic evolution processes and the relationships with continental deep subduction and the UHP rocks,systematic in-depth investigations and uniform acknowledgements are still lack.Therefore,this study has carried out integrated investiagations on petrology,geochronology,elemental and isotopic geochemistry of the relatively low-grade meta-granitoids and meta-basic rocks within the Central Dabie UHP zone(CDZ)and the Susong complex zone,confirming that their protoliths belong to the Yangtze Block and most of them were formed from the large scale magmatic event along the northern margin of the Yangtze block at Neoproterozoic,similar to those of the UHP rocks in the region.These low-grade rocks also underwent the Triassic subduction,therein some rocks(slices)detached from the subducted plate and exhumed at shallow levels during the initial stage of the subduction,while most rocks(slices)in the Susong complex zone located at the last portion of the subducting plate,and exhumed at the latest stage of subduction.In addition,based on field and petrographic observations,as well as preliminary petrological studies,rodingite has been recognized in the Susong complex zone.This lithology is for the first time identified in the Dabie orogen,indicating that the ancient oceanic crust should be existed in the area before the Triassic continental subduction.The main contents and achievements are summarized as follows:1.Zircon U-Pb age,Hf isotope and bulk-rock Sr-Nd-Pb isotope analyses indicate,that the meta-granitoids in the CDZ and Susong complex zone are mainly derived from the late Archean to Palaeoproterozoic lower crust,while the meta-basic rocks are mainly from enriched mantle,with minor contamination of lower crustal materials.Inherited zircon ages and zircon Hf model ages mainly concentrated at?2.5 Ga and?2.0 Ga,concordant with the formation and reworking times of the basement of the Yangtze plate.Most of the meta-granitoids and meta-basic rocks have Neoproterozoic protolith ages that are highly consistent with the UHP meta-igneous rocks in the area,indicating that these rocks are also products of the Neoproterozoic large scale magmatic event.Zircon U-Pb discordias yield Triassic lower intercept ages,consistent with the bulk-rock Rb-Sr“isochron" ages and the Dabie UHP metamorphic ages,confirming that these rocks experienced the Triassic subduction.In this regard,the protoliths of the meta-granitoids and meta-basic rocks may be counterparts of the protoliths of the UHP rocks,but the former suffered evidently weaker influence and modifications than the latter by later metamorphism,thus they are more suitable to trace or constrain the protolith nature and genesis of the UHP rocks.In addition,there are two types of protolith ages for the granitic gneisses from the Susong complex zone,i.e.,late Archean(2.5-2.7 Ga)and Neoproterozoic(770-830 Ma).2.Major and trace elemental,as well as Sr-Nd-Pb isotopic features indicate that the meta-basic rocks in the CDZ are typical of continental within-plate basalt,corresponding to mantle plume or continental rift tectonic settings,while the meta-granitoids in this area are products of post-collisional partial melting of thickened lower crust.These features reflect the tectonic setting of this area transforming from plate convergence to continental rifting at Neoproterozoic,probably related to the assembly and breakup of the supercontinent Rodinia.Zircon U-Pb dating shows that the meta-granitoids formed at?819 Ma and the meta-basalts formed at?772 Ma,and these two groups of age may represent the terminal of the Rodinia assembly and the beginning of the disassociation.The Neoproterozoic granulite-facies metamorphic ages is?768 Ma,probably related to the mantle-sourced magmatic underplating derived from the continental rifting.3.Based on petrological observations and geothermometers studies,the peak metamorphic conditions of the meta-granitoids and meta-basic rocks from the CDZ and the Susong complex zone are epidote amphibolite-facies to amphibolite-facies,indicating that during the Triassic subduction processes,these rocks did not enter the deep crust along with the UHP rocks;combined with zircon U-Pb ages,part of the rocks were detached from the subducted plate at the initial subduction stage,while most rocks(slices)in the Susong complex zone located at the trailing edge of the subducting plate,and exhumed at latest.The conclusions therefore provide new constraints for the intra-plate multi-slice detachment and multi-plate successive exhumation model of the Dabie orogen.4.Two lithologies typical of oceanic crust,rodingite and(serpentinized)peridotite have been for the first time recognized in the Susong complex zone.Zircon U-Pb dating of the rocks have obtained age clusters of 1410± 11 Ma,1078±22 Ma and 798±62 Ma,corresponding to oceanic formation,rodingitization and the closure of the oceanic basin.In addition,rodingite is the specific lithology in the Susong complex zone that has not been recognized in other rock units within the Dabie orogen,thus its Triassic metamorphic age 221 ±3 Ma represents the peak metamorphic age of the Susong complex zone,later than those of the UHP rocks in the Dabie orogen. |
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