Origin And Tectonic Implications Of An Early Paleozoic Tectonic Mélange In The Northwestern Yunkai Domain,south China

The geological evolution of the Early Paleozoic Wuyi-Yunkai orogen in South China is a scientific question of a long-standing debate.The Yunkai Domain is situated in the southwestern part of the Cathaysia Block,which is one of the crucial areas to document the Early Paleozoic geodynamic processes in South China.The presence of the Early Paleozoic extensive anatectic granitoids and locally charnockite and granulite in the Yunkai Domain,and the presence of tectonically juxtaposed fragments of Early Paleozoic flysch,associated with the mafic to intermediate slices or blocks in the northwestern Yunkai Domain,which provide an important window to throw light on the Early Paleozoic tectonic nature of the Wuyi-Yunkai orogen and the Early Paleozoic tectonic evolution,South China.In this contribution,we mainly focus on the Early Paleozoic NE-NEE-striking metamorphosed and deformed flysch unit,associated with the coeval mafic to intermediate volcanic rock slices and granites in the northwestern Yunkai Domain.We carried out an integrated study of field relationships,structural observations,petrography,whole-rock geochemistry,zircon U-Pb geochronology,and Lu-Hf isotope systematics to provide their affinity,petrogenesis,formation ages and tectonic setting.The main results are as follows:（1）We documented an Early Paleozoic accretionary tectonic mélange belt in the northwestern Yunkai Domain.Based on the differences in composition,ages,metamorphism and deformation,the mélange belt can be further subdivided into two tectonostratigraphic units by the Luchuan-Cenxi-Luoding-Guangning Fault Zone.From northwest to southeast these are:the Ordovician to Devonian lower-grade metamorphic siliciclastic unit and the Cambrian to Ordovician greenschist to amphibolite facies flysch unit.The most notable features of the tectonostratigraphic slices in the northwestern Yunkai mélange belt are the northwestward younging,northwestward thrusting,northwestward migration of ductile（deep）to brittle（shallow）deformation,and northwestward transition of granulite-amphibolite facies（high-medium-grade）to greenschist facies（lower-grade）metamorphism.（2）The northwestern Yunkai mélange belt consists predominantly of tectonically juxtaposed fragments of Early Paleozoic flysch,arc volcanic rocks,and a forearc ophiolite.The arc volcanic rocks consist of basaltic-andesite,andesite,and dacite with a mainly sanukitic composition and also include The Guizi Nb-enriched basalts.They are characterized by enrichment of LREEs,LILEs,Pb and depletion of HFSEs,suggesting a continental forearc or a continental arc tectonic setting.The ophiolitic fragments consist of MORB-like metabasalt and metadolerite/metagabbro.They have slightly depleted to flat LREE patterns and are characterized by enrichment of LILEs and Pb and depletion of HFSEs,indicating a continental forearc setting.Zircon U-Pb analyses yield 460–443Ma and 455–437 Ma ages for the arc volcanic rocks and ophiolitic fragments,respectively,suggesting that they formed in the Late Ordovician to Early Silurian.Both the sanukitic volcanic rocks and ophiolitic fragments possess negative to positive zirconε_Hf（t）values（–11.0 to+2.3）,indicating that they may have been generated by partial melting of an old subarc mantle wedge source metasomatized by slab-derived fluids and/or melts.They are likely to have originated from the subduction of a young and hot oceanic slab beneath the subcontinental mantle wedge.（3）We report for the first time the presence of Early Paleozoic Shiwo plagiogranite,they crop out as massive granite or veins associated with altered amphibolite.The plagiogranite possess contents of SiO₂（75.08–80.10 wt.%）,Al₂O₃（11.70–13.25 wt.%）,low ratios of K₂O/Na₂O（0.03–0.04）and low values of A/CNK（1.01–1.15）,suggesting that they belong to low-K tholeiite series weakly aluminous plagiogranite.They have enriched LREE,flat HREE patterns and are characterized by enrichment of LILEs（e.g.,Th,U）and depletion of HFSEs（e.g.,Nb,Ti）,indicating a continental arc setting.The plagiogranite have relatively low TiO₂ and La contents,and the diagrams of La-SiO₂ and TiO2-SiO2 show that the plagiogranite is formed by partial melting.In addition,they are characterized by low Sr,high Yb contents and low Sr/Y ratios,suggesting that the plagiogranite is origined from partial melting of the gabbroic oceanic crust during migration.Zircon U-Pb analyses yield 459 Ma and 442 Ma ages for the plagiogranite and amphibolite,respectively.Both the plagiogranite and amphibolite possess negative to positive zirconε_Hf（t）values（–6.0 to–0.7 and–3.6 to+2.9,respectively）,indicating that they may have been generated by partial melting of an old subarc mantle wedge source metasomatized by slab-derived fluids and/or melts.Recognition of the Early Paleozoic Shiwo plagiogranite and combined with the Shiwo and Nuodong SSZ type ophiolite fragements,suggest the existence of the Late Ordovician ocean crust（459 Ma）in the northwestern Yunkai Domain.（4）Zircon U-Pb analyses yield 454 Ma,440 Ma and 442 Ma ages for the Guizi mylonitic granite,Pingzheng gneissic granite and Huilong granite respectively,suggesting that they formed in the Late Ordovician to Early Silurian.They are characterized by I-S transition type,S-type and High differentiated I-type granites,respectively.The tectonic distinguish diagrams（e.g.,（La/Yb）_N-Yb_N and TiO₂-SiO₂）show that they formed in a continental arc setting.They possessε_Hf（t）values of–2.9 to+5.2、–12.8 to–7.2 and–6.4 to–2.4,indicating that they may have been generated by variable source and added by various degrees of mantle source.Combined with previous studies,we conclude that the Early Paleozoic granites are mainly composed of S-type monzonite granit,generated from the partial melting of Meso-Neoproterozoic sandstone;some granites are characterized by typical TTG（trondhjemite,tonalite,granodiorite）series rocks.The TTGs belong to I-type and I-S transitin type granites,predominantly generated from the partial melting of crustal source with some input of the mantle source.We propose that the origin of the Early Paleozoic granites in the Yunkai Domain are closely related to the subuduction-collision orogeny and the post-orogenic extension,collapse and lithospheric delamination.（5）Based on the temporal and spatial characteristics of the Early Paleozoic tectono-magmatic activities and metamorphic events in Yunkai area,South China,we propose the following Early Paleozoic tectonic-magmatic-metamorphic evolution scenario for the Yunkai Domain.（1）Subduction initiation（≥460 Ma）,the Huanan Ocean began to subduct beneath the Yunkai terrane（arc）from NW to SE;（2）subduction stage（460–440Ma）,the young and hot subducting oceanic slab and overlying sediments dehydrated and melted at different depths,producing fluids and/or melts with different geochemical characteristics.They reacted with the subarc mantle wedge,leading to the partial melting of the mantle wedge and the formation of the sanukitic volcanic rocks（460–443 Ma）and NEBs（453 Ma）.Subsequently,the forearc oceanic crust（455–437 Ma）formed in an extensional environment,probably in response to the upwelling asthenosphere driven by slab rollback.As subduction progressed,more crustal material was transported to the mantle wedge,only fragmented pieces of volcanic arc,oceanic crust and flysch were tectonically incorporated into the accretionary wedge.（3）Collision and post-collision stages（≤440 Ma）,the collision between the Yangtze Block（or Yunnan and Guangxi-North Vietnam terrane）and the Yunkai arc gave rise to the crustal thickening and post-orogenic extension and lithospheric delamination.Voluminous granitoids（TTG）and minor high-temperature pelitic granulite,charnockite and gabbro plutons（e.g.,423 Ma Longhugang gabbros in extension setting）were generated at this stage.The documents of Early Paleozoic ophiolitic fragments,subduction-related volcanic rocks,TTGs,high grade metamorphic rocks,and accretionary complex structures（including oceanic plate stratigraphy）,which are collectively consistent with subduction-collision processes between the Yangtze and Cathaysia blocks in the Early Paleozoic.The Residual Qin-Fang Sea Basin in the southwestern continued to survive until the Early Mesozoic（Indosinian）.（6）The Yangtze and Cathaysia blocks were located along the northern margin of Gondwana in Early Paleozoic.A series of continental/micro-continental blocks in SE Asia experienced gradual southward subduction and accretion to the northern Gondwana margin in the Early Paleozoic（480–400 Ma）times,resulting in the closure of Proto-Tethys Ocean.The Early Paleozoic（460–400 Ma）southward（southeastward）subduction-collision orogeny between Yangtze and Cathaysia would be coherent with the SE Asian tectonic framework in the Early Paleozoic.