| During the Mesozoic,the South China block（SCB）underwent a crucial geodynamic background transformation,which resulted in intense material exchange and tectonic deformation,inducing the “metallogenic explosion”,and forming the specific mineral system with different spatial and temporal distribution,ore deposit combinations.Therefore,it is an ideal area for studying continental evolution,tectonic-magmatic-metallogenic mechanism.In this paper,the seismic P-wave receiver function（PRF）Common Conversion Point（CCP）and H-k stacking method,and S-wave receiver function（SRF）CCP method are used to study the deep structure and constraint the Mesozoic geodynamic process in SCB: 1)detecting the upper mantle structural and crustal Vp/Vs,revealing the crustal structure and average composition,Lithosphere-Asthenosphere Boundary（LAB）and Mantle Transition Zone（MTZ）structure;finding out whether there is plate subduction,upwelling “hot mantle plume”,asthenosphere upwelling,lithosphere thinning,Moho uplift and its corresponding deep structure characteristics in SCB.2)Explore the geodynamic mechanism of large-scale tectonic-magmatic-metallogenic events in SCB during the Mesozoic,and discuss the “source” characteristics related to the temporal and spatial distribution,and ore deposit combinations of the Mesozoic intracontinental mineral system.The crustal structure and Vp/Vs results illuminate that: Overall,the lateral crustal structure of SCB ranges complex,the crustal thickness increases from east to west,and the eastern part thickens from south to north;the Vp/Vs presents the characteristic of “both flank high with middle low”.1)The Moho of the Cathaysian block is roughly horizontally distributed,with a crustal thickness of 26-34 km,crustal thickness obviously thinning occurs in Nanling,Yunkai and coastal areas;There is no obvious regularity of the bulk crust Vp/Vs distribution,and ranges between 1.66-1.82.Besides,there is an increasing trend from the inland to the offland,it is speculated that which may be related to the Mesozoic multistage compression-extension geodynamic transformation,and dehydration and melting of subducting slabs,leading to upwelling of mantle material,the mantle-derived mafic materials underplating appears in the coastal areas,resulting in the increase of mafic proportion in the lower crust.2)The crustal thickness in the upper Yangtze region in the study area is relatively stable,with an average crustal thickness of about 42 km,the Vp/Vs varies from 1.74 to 1.84,and presenting a positive correlation between Vp/Vs and crustal thickness,inferring that the stable craton has a relatively thick mafic lower crust.3)The lateral crustal structure of the Lower Yangtze region ranges in different geological units,with a thickness of 32-36 km.At the intersection of geological units,the Vp/Vs is relatively high,basically greater than 1.76,and the relatively high mafic proportion crustal may be related to the upwelling of the mantle materials and crustal-mantle interaction caused by the Mesozoic delamination,inducing a large-scale underplating in the lower crust,and formatting multi-level magma chambers in the middle and lower crust.4)The Jiangnan Orogenic Belt presents a transitional features,the thickness of the crust gradually increases from east ca.32 km to west about 40 km,the overall Vp/Vs is relatively low（<1.75）,which may be related to the detachment and thickening of the upper and middle crust and thinning of the lower crust caused by the multi-stage compression-extension geodynamic background transition in the Mesozoic.The research on the lithosphere-asthenosphere boundary（LAB）structure shows that: the thickness of LAB in the southern part of study area is thin in the east and thick in the west,and ranges in different geological units.1)The thickness of the LAB in the southern part of the Cathaysia block is about 90 km,and there is an obvious LAB uplift to approximate 80 km beneath the Nanling areas,indicating that during the Mesozoic,the lithosphere was affected by the subduction and retreat of the Palaeo-Pacific and/or the upwelling of the “hot mantle plume” in the Nanling area,leading to multi-stage compression-extension tectonic transformation,resulting in the differential intracontinental tectonic evolution and the lithosphere thinning.2)The depth of the LAB in the Upper Yangtze block in the study area is about 110 km.It is speculated that the LAB is less modified in associated with the thick and cold lithospheric mantle root of the Sichuan Basin.3)The Jiangnan Orogenic Belt presents transitional character,the LAB depth gradually deepens from east to west.4)The overall thickness of the LAB in the Lower Yangtze region is 80-110 km,the thickness of the lithosphere of the North China block in the west is about 110 km,and the rest of the LAB is basically stable at about 90 km;during the Mesozoic,the Lower Yangtze region was controlled by the low-angle westward subduction of the Paleo-Pacific plate.Due to the blocking of the North China Plate and the Qinling-Dabie Orogenic Belt,the Lower Yangtze region were strongly deformed,forming the Late Mesozoic intracontinental orogeny;with the subduction of the Paleo-Pacific plate,the compression and far-field stress weakened（the angle became steeper/direction changed）,the thickened lithosphere was delaminated,especially at the intersection of the different geological units.The images of the MTZ results show that: Within the South China block,there are large differences in the structure and thickness of the MTZ.1)In the Lower Yangtze region,the d660 is obviously depressed and the thickness of the MTZ is greater than global MTZ average depth（250km）,implying the existence of low temperature disturbance in the MTZ,and reflecting the existence of stagnant slabs.Combined with existing research,it is speculated that the stagnant slabs is the Paleo-Pacific plate/Pacific plate or Mesozoic delaminated continental lithosphere;2)In the 29°N southern part of the Cathaysian block,the d410 and d660 is depressed and uplifted separately in the Nanling and northern Hainan Island,and the thickness of the MTZ is less than250 km,reflecting the high temperature disturbance inside it,and it is speculated that the lateral varied MTZ is related to the upwelling “hot mantle plume”.Combined with previous geological and geophysics studies,our results revealed several insights about the tectonic processes in the South China block: 1)There is a 29°N mantle structural boundary in SCB.2)During the Mesozoic,the Lower Yangtze region,north of the boundary,was affected by the subduction of the Paleo-Pacific plate,which induced a strong intracontinental orogenic process,resulting in lithosphere delamination and asthenosphere upwelling,and finally leading to lithospheric thinning;accompany with the large-scale mantle-derived magma ascending,the crust-mantle material exchange in the lower and middle crust,and the Cu-Au-W polymetallic mineral system was formed in the eastern Qin-hang metallogenic belt,which is characterized by a mixed source of crust and mantle material.With the subducting far-field stress to the Yangtze block,the compression-extension geodynamic transformation leads to severe delamination,which induces the rapid rise of mafic magma along the reactivated deep fault systems.The metallogenic belt in the Middle and Lower Yangtze Cu-Au-Fe polymetallic mineral system was formed,which is dominated by mantle-derived materials,mixed with a small amount of crustal material.3)In the southeastern part of SCB,with the Paleo-Pacific plate subduction in the margin of SCB,during the Late Jurassic,arc volcanic rocks and related crust-mantle materials mixed Au-Cu mineral system were formed in the Wuyi metallogenic belt.At the same time,due to the subduction of the paleo-Pacific plate,in the intracontinental back-arc extension background region,or related to the upwelling of the “hot mantle plume”,the lithosphere in the Nanling area was strongly thinned,the crust-mantle interaction was strengthened,and the overlying crust was partially melted,forming a large number of crustal remelting or anatexis materials Nanling W-Sn rare earth metal mineral system related to highly fractionated granites.During the Cretaceous,due to the back-arc background formed by the retreat of the Paleo-Pacific plate in Wuyishan area,along with the dehydration and upwelling of mantle materials,underplating occurred in the eastern part of the Cathaysian block,and the ore-forming materials ascended along the fault zone,and forming the crust-mantle mixed materials Wuyishan Au-Cu polymetallic mineral system. |
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