The Geological Significance of the Deformation and Geochronology of the Xiaotian–Mozitan Shear Zone in the Dabie Orogenic Belt(East-Central China)

The Xiaotian–Mozitan Shear Zone(XMSZ)is the boundary of the Dabie High-grade Metamorphic Complex(DHMC)and the North Huaiyang Tectonic Belt.It was deformed in ductile conditions with a top-to-NW/WNW movement.Geothermometers applied to mineral parageneses in mylonites of the shear zone give a temperature range of 623–691°C for the predeformation and 515–568°C for the syndeformation,respectively,which indicates a retrograde process of evolution.A few groups of zircon U-Pb ages were obtained from undeformed granitic veins and different types of deformed rocks in the zone.Zircons from the felsic ultramylonites are all magmatic,producing a weighted mean 206Pb/238U age of 754±8.1 Ma,which indicates the time of magmatic activities caused by rifting in the Neoproterozoic.Zircons from the granitic veins,cutting into the mylonites,are also of magmatic origin,producing a weighted mean 206 Pb/238 U age of 130±2.5 Ma,which represents the time of regional magmatic activity in the Cretaceous.Zircons from the mylonitic gneisses are of anatectic-metamorphic origins and are characterized by a core-mantle interior texture,which yielded several populations of ages including the Neoproterozoic ages with a weighted mean 206 Pb/238 U age of 762±18 Ma,similar to that of the felsic ultramylonites and the Early Cretaceous ages with a weighted mean 206Pb/238U age of 143±1.8 Ma,indicating the anatectic metamorphism in the Dabie Orogenic Belt(DOB).Based on integrated analysis of the structure,thermal conditions of ductile deformation and the contact relations of the dated rocks,the activation time of the Xiaotian–Mozitan Shear Zone is constrained between~143 Ma and 130 Ma,during which the DOB was undergoing a transition in tectonic regime from compression to extension.Therefore,the deformation and evolution of this shear zone plays an instrumental role in fully understanding this process.This research also inclines us to the interpretation of it as an extensional detachment,with regard to the tectonic properties of the shear zone.It may also be part of a continental scale extension in the background of the North China Block’s cratonic destruction,dominated by the subduction and roll-back of the Paleo-Pacific plate,but more detailed work is needed in order to unravel its complicated development.

Crustal structure of the eastern Dabie orogenic belt from the seismic tomography and wide-angle reflection studies

The studies of seismic tomography and wide-angle reflection have been carried out to reveal the velocity structUrebeneath the eastern Dabie orogenic belt. The result from the seismic tomography shows the high velocity bodiesmight be positioned to a depth of only about 1 .5 km below sea level within the Dabie ultra-high pressure metamorphic (UHPM) belt; the fan-profile shows the Shuihou-Wuhe fault, the demarcation between the South Dabieand the North Dabie, slopes to the south-west at a dip angle of about 45° in the bottom of upper crust. The wideangle reflection shows the middle crustal boundaries and the complex features from the lower crust.

Transpressive Deformation across Tongbo-Dabie Orogenic Belt

Deformation characteristics of the dextral Dashankou ductile shear zone , parallel to the Tongbo-Dabie Orogenic Belt and regional stretching lineation patterns within the blueschist belt in Northern Hubei , China , are examined at various scales , respectively . The new data , combined with those obtained in the Wudangshan area , indicate a transpressive deformation involving a WNW directed nearly horizontal shearing accompanied by shortening across the orogenic belt . A kinematic model is proposed for development of the deformation within the Tongboshan segment . Based on these findings it can be seen that the convergence and collision between the North China Craton and the Yangtze Craton may locally be oblique during the Proterozoic Jinningian Orogeny . This oblique collision is probably related to a certain extent to the shapes of the older craton margins .

Retrogressive Microstructures: A Key to Post-Collisional Uplift and Extension Tectonics of Ultrahigh-Pressure Metamorphic Rocks from Dabie Orogenic Belt, China

A wealth of retrogressive microstructures have been discovered from the UHP metamorphic rocks in Dabie orogenic belt, namely, the ultrahigh-pressure (URP) eclogites, jadeite quartzites and kyanite-zoisite-quartz vein. The most important are pseudomorphic replacements of UHP minerals like coesite, the corona reaction textures iuduced by solid-solid reactions as well as the corona and symplectites induced by reactions involving fluid. According to the textural relationships the sequence of mineral Paragenesis and the metamorphic stages in the UHP eclogites can be delineated; the mineral geobarothermometry of the various stages of retrograde metamorphism is studied and a clockwise, nearly isothermal decompressive metamorphic PT-trajectory for the UHP eclogites can be reconstructed. In terms of the PT-trajectory the two stage post collision uplirt and exhumation processes are reflected. When the UHP metumorphic rocks extruded to the lower-middle crust partial melting happened which bad in turn caused tke crustal extension and the further exhumation or the UHP metomorphic rocks. Based on the field strain analysis combined with geochronological data a scenario or post collision uplift aud exhumation model is presented.

Discovery of the mid-Cretaceous sedimentary rocks from the ultrahigh-pressure terrane,Dabie Orogenic Belt,and its tectonopaleogeographic implications

The Dabie Orogenic Belt, located in east-central China, is known for the greatest outcrop area of the ultrahigh-pressure metamorphic rocks in the world and is typical for studying deep continental subduction and continent-continent collision. However, since lower-grade metamorphic rock occurrence in the core of the ultrahigh-pressure terrane was reported, it has become a new point of concern. This study reported new results. The so-called lower-grade metamorphic rocks are divided into sedimentary rocks and meta-volcanics.The sedimentary rocks consist mainly of fine-grained turbidites formed under an anoxic deep-water lacustrine basin. Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry U-Pb detrital zircon dating indicates that their maximum deposit age is 119.0 ± 1.1 Ma belonging to the mid-Cretaceous Aptian. U-Pb detrital zircon ages from the sedimentary rocks cluster in three age groups: 1) 2773.8-1550.0 Ma(45.0%);2)868.8-622.3 Ma(23.0%);3) 147.0-100.0 Ma(31.5%). And the provenance and tectonic settings of these sedimentary rocks are discussed by synthesizing U-Pb detrital zircon ages, initial Hf isotope ratios, and trace elements of the zircons. Source rocks of the first group were mainly composed of the Paleoproterozoic basement derived from the reworking of the Paleoarchean-Neoarchean Yangtze continental crust. For the second group, its source rocks consisted of Neoproterozoic volcanics mainly from an island arc system, which derived from the reworking of older continental crust in the northern Yangtze Block. The third group’s source rocks were chiefly composed of Cretaceous rift igneous rocks derived from the derivation through melting of middle-aged continental crust in the Dabie Orogenic Belt. The meta-volcanics have weighted mean U-Pb ages from 751.0 ± 16 Ma to 786.3 ± 5.9 Ma, and their protoliths formed under a Middle Neoproterozoic island arc system in the northern Yangtze Block. The meta-volcanics underwent a Triassic subduction of different depths and then an exhumation, but the sedimentary rocks did not. The findings suggest that the ultrahigh-pressure terrane had suffered violent subsidence during the mid-Cretaceous, and a four-stage evolution model of the Dabie Orogenic Belt was established accordingly.

Thermal Evolution of the Tanlu Fault Zone on the Eastern Margin of the Dabie Mountains and Its Tectonic Implications

Five samples of muscovite from mylonites of the earlier Tanlu ductile shear zone on the eastern margin of the Dabie Mountains yield 40Ar/39Ar ages ranging from 178 Ma to 196 Ma. Three of them have reliable plateau ages of 188.7±0.7 Ma, 189.7±0.6 Ma and 192.5±0.7 Ma respectively, which indicates a syn-orogenic, sinistral strike-slip thermal event. This displacement movement derived from the continent-continent collision of the North and South China blocks took place in the Early Jurassic and after uplifting of high-pressure to ultrahigh-pressure slabs to the mid-crust. It is suggested that during the collision the Tanlu fault zone was an intracontinental transform fault caused by differential subduction speeds. The 40Ar/39Ar ages of mylonite whole-rock and muscovite from the later Tanlu ductile shear zone suggest another sinistral strike-slip cooling event at 128 Ma. During this strike-slip faulting, large-scale intrusion and doming uplift occurred in the eastern part of the Dabie orogenic belt. Data of K-feldspar 40Ar/39Ar MDD and apatite fission-track analysis from metamorphic rocks indicate two high-speed cooling events experienced by the Tanlu fault zone, which took place at 90 Ma and 45-58 Ma respectively. They correspond respectively to two phases of extensional activities in the Late Cretaceous and Eogene as well as development of the Qianshan fault-controlled basin to the east of the Tanlu fault zone. The cooling times recorded by K-feldspar and apatite show that the uplifting in the Dabie orogenic belt occured earlier than that on the eastern margin occupied by the Tanlu fault zone. The above phenomena suggest that the uplifting of the Dabie orogenic belt during the Late Cretaceous to Eogene was not controlled by the Tanlu normal faulting, but as a result of the lithospheric delamination.

Detrital Composition of Lower-Middle Jurassic Sandstones from Huangshi,Hubei:Implication for Provenance Types and Geotectonics

The detrital modes of the sandstones from the south basin of Dabie（大别） orogenic belt during the Early-Middle Jurassic are complex due to multiple supplying.Detrital compositions of medium-coarse sandstones from the Lower-Middle Jurassic,Huangshi（黄石）,show three main provenances：recycled ororgen,volcanic arc（arc orogen）,and continental block on the bases of the Dickinson discriminating diagrams of modal composition and analysis of sandstone petrofacies.Combined with sparse published paleocurrent data,geotectonic setting of the study area,and the unpublished detrital zircon age data,the positions of the main provenances are inferred as follows：coastal orogenic belt（COB） in the East China provided detritus as the recycled orogen provenance;Dabie orogenic belt（DOB） to the north was the source of volcanic arc（arc orogen） materials;and the continental block clasts were likely from the uplift of the Yangtze basement（UYB）（Jiangnan（江南） uplift）.Six sedimentary cycles are recognized in the studied section,one in Wuchang（武昌） Formation,and five in Huajiahu（花家湖） Formation.During the deposition,the three main provenances play different roles.The quantitative petrofacies analysis revealed distinct changes of the effect from each source,that is,61.9%（Cycle Ⅰ）→54.5%（Cycle Ⅲ）→50%（Cycle Ⅳ）→57.9%（Cycle Ⅴ）→14.3%（Cycle Ⅵ）,14.3%（Cycle Ⅰ）→21.2%（ Cycle Ⅲ）→40%（Cycle Ⅳ）→36.8%（Cycle Ⅴ）→14.3%（Cycle Ⅵ）,and 23.8%（Cycle Ⅰ）→24.2%（Cycle Ⅲ）→10%（Cycle Ⅳ）→5.3%（Cycle Ⅴ）→71.4%（Cycle Ⅵ） from COB,DOB,and UYB,respectively.The results show that（1） COB behaved in considerable activation during the EarlyJurassic but weakened in the Middle Jurassic;（2） the vigorous tectonic uplift of DOB initiated in Middle Jurassic;and（3） UYB was in stabilization basically from the Early to Middle Jurassic.