Dabie Orogenic Belt After The Collision Tectonic Thermal / Magmatic Evolution Of The Process

The Dabieshan orogen, located between North China plate and Yangtze plate, has become thefocus of geological research due to unique exposure of large-scale HP/UHP rocks and itsrelationship with NNE trending Tanlu fault. Reconstruction of an original distribution of HP/UHPunits and their PTt paths helps in the understanding of orogenic dynamics process. After HP/UHPorogenic exhumation, Dabieshan orogen entered into the period of thermal rebuilding driven byintense extension. This study emphasizes post-collisional tectonic-thermal and magmatic processrather than HP/UHP units themselves through combined geochronology, structure analysis andgeochemistry studies. This study is essential to reconstruct post-date-orogenic evolution history,and also essential to restore the original HP/UHP distribution and exhumation models. Therefore,it can provide detailed constraints on Dabieshan orogenesis mechanism. Our major researchconclusions are as follows.a) Based on regional geochronology, the Dabieshan orogen underwent thermal doming extensionduring 140～85Ma, which resulted in widespread re-melting of metamorphic basement, intensevolcanic activities in North Huaiyang and formation of fault-controlled depressions in Hefeibasin. This thermal doming event can be further divided into two consecutive evolving stages,i.e., the intensifying stage (140～105Ma) and the declining stage (105⁸5Ma). In 140～105Mastage, the thermal doming was mainly concentrated in the Dabie block, and to a less degree, inthe Hong'an block. The Dabie doming structure is configured with Macheng-Yuexi thermalaxis, Yuexi/Luotian thermal cores and their downslide flanks. The orientation of thermal axisruns dominantly parallel to the orogen Strike, and North Huaiyang and HP/UHP units constitutedownslide flanks. Yuexi core differs from Luotian core in both intensity and shaping time. Thedoming process of Dabieshan is characterized by thermal-center's migration along Macheng-Yuexi thermal axis. Consequently, it is speculated to be attributed to convective removal ofthickened orogen-root, which is a process characterized by intermittence, migration, large-scaleand differentiation. During 105～85Ma stage, the dome-shaped figure still structurally existed,but each tectonic units cooled remarkably slow and magmatic activities stagnated gradually.b) Based on structure analysis of present outcrops, the Dabieshan orogen is dominated by thearchitecture of doming extension, which actually formed during Cretaceous period(140～85Ma).This doming architecture is structurally configured with doming cores, detachment faults anddownslide flanks. Moreover, the intensity of extension displays regional differentiation, strongin Dabie block but weak in Hong'an block. In Hong'an block, the doming structure comprisesFutianhe/Kafang cores and Hong'an group/north Huaiyang downslide systems, and Pingtianfan,Xiongdian-huwan, Fengdian-qiliping and Dawu-xishui faults constitute detachment faults. The(low)amphibolite facies chiefly developed within the core area, but the greenschist faciesconcentrated on the doming downslide systems. In Dabie block, its configuration of domingextension includes Luotian/Yuexi cores as well as north Huaiyang/(U)HP downslide units, andYingshan-xinzhou, Mozitan-xiaotian, Taihu-xishui faults obviously exhibit some attributes ofdetachment faults. The amphibolite facies mostly are distributed in the range of doming coreswith larger depth of exhumation, but low-amphibolite facies and greenschist facies come withthe doming downslide systems.c) Apatite fission track (AFT) analysis shows that at 90Ma±the doming structure of Dabieshanwas replaced by another architecture, which is characterized by the thermal fault-depressionintersecting thermal short-axis uplift. This new architecture possibly continued into 90～32Maperiod, although thermal differentiation and small-scale domes occurred in its late stage. Forabove architecture, its driven mechanism is related to a regional differential push-up process,which possibly resulted from some changeable properties of convergence in the western Pacificmargin. This push-up process played an important role in the shaping of modern topography,too. Furthermore, AFt modeling history shows that the orogen of Dabieshan underwent otherthermal events. In detail, the 65～56Ma event found its intense expression near north/southboundary faults of orogen, and much intense in north Huaiyang and Hefei basin. Additionally,the 34～30Ma event almost affected the whole orogen of Dabieshan and its northern basins. The20～13Ma event, differently, thermally disturbed mainly within Dabie and Hong'an blocks, anddecreasing towards north Huaiyang and Hefei basin. These thermal events are presumed to be related to the India-Asia collision tectonics, the Pacific subduction tectonics and the open/closeprocess of South China Sea basin.d) The tectonic/magmatic history of Dabieshan orogen (since Mesozoic era) generally can bedivided into 6 events, i.e., the convergence "island-arc" event(>240Ma), the slab "break-off"event(210Ma±), the compression "thickening" event(early^middle Jurassic), the "delaminationof orogen root"event(early Cretaceous), the far-field "extension" event(85⁵3Ma), and the"mantle plume" event(sine Oligocene epoch). The delamination of orogen root led to intensethermal extension in Dabieshan. Consequently, most of intermediate-basic (sub)volcanics areenriched in Ba,La,K₂O, and components of TiO₂, Nb,P₂O₅ display negative anomaly, andtheir isotopic sources have a strong affinity to EMI mantle and basement rocks. During late-Cretaceous and Paleocene, the K₂O content in volcanics decreased clearly, and their magmaticsources lay between BSE and EMI mantles. This volcanism was possibly under the control ofback-arc extension of western Pacific convergence. Since Oligocene, all volcanic rocks relatedto the mantle plume event represent component of ocean-island basalt, and they have a strongaffinity to PREMA mantle source.e) During late-Jurassic-early Cretaceous period, volcanic rocks with EMI mantle directly resultedfrom combination of delamination material of orogen root and mantle source. In SoutheasternChina, the basic/intermediate volcanism, characterized by EMI-EMII transition source, wastectonically correlated with addition of subducted oceanic sediments, back-arc extension ofwestern Pacific, and uplifting of continental mantle. In the Lower Yangtze, however, volcanismdisplayed transition mantle source between "delaminaton of orogen root" mantle (Dabieshan)"and "back-arc extension" mantle (Southeastern China), and thus, we attempt to link evolutionof Dabieshan orogen with eastern continental margin by tectonic intersection and constraints.The orogen delamination was resulted from superimposed control of orogenic tectonics andextension of eastem continental margin. In late-Cretaceous～Paleocene, volcanic rocks fromDabieshan, Lower Yangtze to Southeast China, approached oceanic-island basalt, and had aclose affinity to BSE mantle. It can be concluded that the volanism(late-Cretaceous～Paleocene)of East China was gradually controlled by an identical mantle source and back-arc extensionalsetting. Since Oligocene, this volcanism is driven by the same mantle source and the sametectonic setting, and their volcanic rocks display the PREMA mantle source and component ofoceanic island basalt. Additionally, some peridotite inclusions imply existence of PREMAmantle in the〉700km depth. We relate these properties of volcanics to the mantle-plume event.f) The Dabieshan orogen is speculated to represent the bivergent-belt architecture rather thanunilateral-wedge-extrusion model. Only detachment faulting, instead of one tectomc boundary,exists between "gay gneiss of North DaNe" and "UHP/HP units of South Dabie", which wastriggered by the thermal doming extension. The Macheng-Yuexi thermal axial represents the deeptrending orientation of thickened orogen root. Mozitan-xiaotian fault possibly serves as one partof remnant suture reconstructed by the thermal doming extension occurred during 140～85Ma.The subduction depth in Dabie block exceeds that in Hong'an block due to oblique convergence,therefore, HP/UHP exhumation scale in Dabie block may be larger than that in Hong'an block.This fact also helps to explain differential thermal extension existed between DaNe block andHong'an block. The Dabieshan orogen might underwent transition from mantle-subduction typeinto indentation type.