Behavior of Siderophile and Chalcophile Elements in the Subcontinental Lithospheric Mantle beneath the Changbaishan Volcano,NE China

The mantle xenoliths in the Quaternary ChangbaishanVolcano in southern Jilin Province contain spinel-facies lherzolites. The equilibration temperatures for these samples range from 902℃ to 1064℃ based on the two-pyroxene thermometer of Brey and Kohler （1990）, and using the oxybarometry of Nell and Wood （1991）, the oxidation state was estimated from FMQ-1.32 to -0.38 with an average value of FMQ-0.81 （n = 8）, which is comparable to that of abyssal peridotites and the asthenospheric mantle. ThefO2 values of peridotites, together with their bulk rock compositions （e.g., Mg#, Al2O3, CaO, Ni, Co, Cr） and mineral compositions （e.g., Mg# of olivine and pyroxene, Cr# [=Cr/ [Cr＋Al]] and Mg# [=Mg/[Mg＋Fe2~] of spinel）, suggest that the present-day subcontinental lithospheric mantle （SCLM） beneath the Changbaishan Volcano most likely formed from an upwelling asthenosphere at some time after the late Mesozoic and has undergone a low degree of partial melting. The studied lherzolite xenoliths show low concentrations of S, Cu, and platinum group elements （PGE）, which plot a flat pattern on primitive-mantle normalized diagram. Very low concentrations in our samples suggest that PGEs occur as alloys or hosted by silicate and oxide minerals. The compositions of the studied samples are similar to those of peridotite xenoliths in the Longgang volcanic field （LVF） in their mineralogy and bulk rock compositions including the abundance of chalcophile and siderophile elements. However, they are distinctly different from those of peridotite xenoliths in other areas of the North China Craton （NCC） in terms of Cu, S and PGE. Our data suggest that the SCLM underlying the northeastern part of the NCC may represent a distinct unit of the newly formed lithospberic mantle.

Plate tectonics in the twenty-first century

Plate tectonics was originally established as a kinematic theory of global tectonics,in which the Earth’s rigid outer layer,the lithosphere,consists of different size plates that move relative to each other along divergent,convergent or transform boundaries overlying the ductile asthenosphere.It comprises three elements:rigid lithosphere plates,ductile asthenosphere,and coupled movement systems.It operates through the interlinked processes of continental drift,seafloor spreading and lithospheric subduction,resulting in the generation,modification and demise of lithospheres throughout geological time.The system of lithospheric plates in horizontal and vertical movements forms the spatiotemporal linkages of matter and energy between the surface and interior of Earth,advancing the kinematic theory with a dynamic explanation.While top-down tectonics through lithospheric subduction plays a key role in the operation of plate tectonics,it is balanced for the conservation of both mass and momentum on the spherical Earth by bottom-up tectonics through asthenospheric upwelling to yield seafloor spreading after continental breakup.The gravity-driven subduction of cool lithosphere proceeds through convergence between two plates on one side,and rollback of the subducting slab makes the vacancy for upwelling of the hotter asthenosphere to form active rifting in backarc sites.Plate convergence is coupled with plate divergence between two plates along mid-ocean ridges on the other side,inducing passive rifting for seafloor spreading as a remote effect.Thus,plate tectonics is recognizable in rock records produced by tectonic processes along divergent and convergent plate margins.Although the asthenospheric upwelling along fossil suture zones may result in continental breakup,seafloor spreading is only induced by gravitational pull of the subducting oceanic slab on the remote side.Therefore,the onset and operation of plate tectonics are associated with a series of plate divergent-convergent coupling systems,and they are critically dependent on whether both construction and destruction of plates would have achieved and maintained the conservation of both mass and momentum on the spherical Earth.Plate margins experience different types of deformation,metamorphism and magmatism during their divergence,convergence or strike-slip,leaving various geological records in the interior of continental plates.After plate convergence,the thickened lithosphere along fossil suture zones in intracontinental regions may be thinned by foundering.This causes the asthenospheric upwelling to reactivate the thinned lithosphere,resulting in superimposition and modification of the geological record at previous plate margins.The operation of plate tectonics,likely since the Eoarchean,has led to heat loss at plate margins and secular cooling of the mantle,resulting in the decrease of geothermal gradients and the increase of rheological strength at convergent plate margins.Modern plate tectonics is characterized by the predominance of rigid plate margins for cold subduction,and it has prevailed through the Phanerozoic.In contrast,ancient plate tectonics,that prevailed in the Archean and Proterozoic,is dominated by relatively ductile plate margins for collisional thickening at forearc depths and then warm subduction to subarc depths.In either period,the plate divergence after lithospheric breakup must be coupled with the plate convergence in both time and space,otherwise it is impossible for the operation of plate tectonics.In this context,the creation and maintenance of plate divergent-convergent coupling systems are responsible for the onset and operation of plate tectonics,respectively.Although a global network of mobile belts is common between major plates on modern Earth,it is difficult to find its geological record on early Earth if microplates would prevail at that time.In either case,it is important to identify different types of the geological record on Earth in order to discriminate between the different styles of plate tectonics in different periods of geological history.

Shear wave velocity structure of the crust and upper mantle in Southeastern Tibet and its geodynamic implications

Southeastern Tibet,which has complex topography and strong tectonic activity,is an important area for studying the subsurface deformation of the Tibetan Plateau.Through the two-station method on 10-year teleseismic Rayleigh wave data from 132 permanent stations in the southeastern Tibetan Plateau,which incorporates ambient noise data,we obtain the interstation phase velocity dispersion data in the period range of 5–150s.Then,we invert for the shear wave velocity of the crust and upper mantle through the direct 3-D inversion method.We find two low-velocity belts in the mid-lower crust.One belt is mainly in the SongPan-GangZi block and northwestern part of the Chuan-Dian diamond block,whereas the other belt is mainly in the Xiaojiang fault zone and its eastern part,the Yunnan-Guizhou Plateau.The low-velocity belt in the Xiaojiang fault zone is likely caused by plastic deformation or partial melting of felsic rocks due to crustal thickening.Moreover,the significant positive radial anisotropy(VSH>VSV)around the Xiaojiang fault zone further enhances the amplitude of low velocity anomaly in our VSVmodel.This crustal low-velocity zone also extends southward across the Red River fault and farther to northern Vietnam,which may be closely related to heat sources in the upper mantle.The two low-velocity belts are separated by a high-velocity zone near the Anninghe-Zemuhe fault system,which is exactly in the inner and intermediate zones of the Emeishan large igneous province(ELIP).We find an obvious high-velocity body situated in the crust of the inner zone of the ELIP,which may represent maficultramafic material that remained in the crust when the ELIP formed.In the upper mantle,there is a large-scale low-velocity anomaly in the Indochina and South China blocks south of the Red River fault.The low-velocity anomaly gradually extends northward along the Xiaojiang fault zone into the Yangtze Craton as depth increases.Through our velocity model,we think that southeastern Tibet is undergoing three different tectonic modes at the same time:(1)the upper crust is rigid,and as a result,the tectonic mode is mainly rigid block extrusion controlled by large strike-slip faults;(2)the viscoplastic materials in the middlelower crust,separated by rigid materials related to the ELIP,migrate plastically southward under the control of the regional stress field and fault systems;and(3)the upper mantle south of the Red River fault is mainly controlled by large-scale asthenospheric upwelling and may be closely related to lithospheric delamination and the eastward subduction and retreat of the Indian plate beneath Burma.