Lithosphere Structure Of The North China Block

The lithosphere which composed with outer crust and the lithospheric mantle isone of the most fundamental elements in solid Earth science.The continental lithosphere records information on the tectonic and geologic history over 4 Ga.The natureand evolution of the North China Block is one of the most significant topic.To constrain the crustal and lithospheric mantle of the North China Block,detailed petrology and geophysic work has been done.The widespread deep-seated crustal xenoliths at the margins of the North China Block provide an unique opportunity to probe the evolution and structure of continen-tal crust.Petrology crustal structure has been established for several locations such as,Hannuoba at the north margin of the North China Block and Xinyang at the south margin of the North China Block.As the petrology crustal structure and the evolution of the deep crust beneath the southeastern margin are poorly defined.Detailed petrology and U-Pb dating of deep-seated crustal xenoliths have been done in this thesis.The gneissic and amphibolite xenoliths from the Xu-Huai area defined four distinct U–Pb ages populations of >2.4 Ga,2.2-2.0 Ga,ca.1.9 Ga and <1.8 Ga.These new results,combined with previous petrological and geochemical studies and P-T estimates,suggest that the deep crust beneath the southeastern North China Block in the Mesozoic is consists of Neoarchean gneisses which and Paleoproterozoic amphibolies in the middle crust(20-30 km),Mesozoic granulites and eclogites in the lower crust(30-45 km).Zircon ages and Hf isotopes from Mesozoic and Cenozoic xenoliths imply that this lower crust had a complex Precambrian history of accretion and modification beneath the southeastern North China Block,including the Neoarchean initial growth,the repeat Paleoproterozoic reworking event(rifting,high-amphibolies facies metamorphism with the collision and post collision extension).Felsic to mafic granulite xenoliths and disaggregated xenocrysts in the Cenozoic Nushan basalts were analyzed.The xenoliths are 5-15 cm and show fine-grain edequilibrated granoblastic microstructure,consist of plagioclase,quartz,minor hypersthene,diopside and Fe-Ti oxides.Felsic granulites,represent the lowermost middle crust to the upper lower crust.Magmatic zircons in the xenoliths yielded an upper intercept age of 2716 ± 32 Ma and a T DM1 of 3.1-2.9 Ga.The metamorphic zircons were ?2100 Ma and had a similar T DM1 to the magmatic ones.The lowermost crust is represented by mafic granulites.With the exception of three inherited zircons in the granulite,which yielded an upper intercept age of 2455 Ma and T DM1 of ca.2.7 Ga,most zircons are from the late Mesozoic(115 Ma).Combine with the seismic profile and the Cenozoic xenoliths evidence,the delamination of lower crust in the southeastern North China Block resulted in the sharp,flat Moho,with a thin(ca.30 km)felsic to intermediate lower crust in the Cenozoic.Geophysics method is one of the most important way to probe the structure of the lithosphere(crust and lithospheric mantle)for a large region.High-resolution crustal model is required to constrain the lithospheric mantle structure.In this thesis,highresolution crustal model based on all available refraction/reflection seismic profiles,thermal lithosphere structure based on new surface heat flow database and lithospheicmantle density based on gravity data of the North China Block have been established.We present a new digital model(NCcrust)of the seismic crustal structure of the Neoarchean North China Block and it ssurrounding Paleozoic-Mesozoic orogenic belts(30 ?-45 ? N,100 ?-130 ? E).All available seismic profiles,complemented by receiver function in terpretations of crustal thickness,are used to constrain a new comprehensive crustal model NCcrust.The model,presented on a 0.25 ? × 0.25 ? grid,includes the Moho depth,and the internal structure(thickness and velocity)of the crust specified for four layers(the sedimentary cover,upper,middle and lower crust),and the Pn velocity in the uppermost mantle.The crust is thin(30-32 km)in the east while the Moho depth in the western part of the North China Block is 38-44 km.The Moho depth of the Sulu-DabieQinling-Qilian orogenic belt ranges from 31 km to 51 km,with a general westward increase in crustal thickness.The sedimentary cover is 2-5 km thick in most of the region and typical thicknesses of the upper,middle,and lower crust are 16-24 km,6-24 km,and 0-6 km,respectively.We document a general trend of westward increase in the thickness of all crustal layers of the crystalline basement and as an consequence,the depth to the Moho.There is no systematic regional pattern in the average crustal Vp velocity and the Pn velocity.We examine correlation between the Moho depth and topography for seven tectonic provinces in the North China Block and speculate on mechanisms of isostatic compensation.Our new thermal model which constrianed by high resolution crustal model and new surface heat flow data suggestes that the surface heat flow in the North China Block,including the western block,is >60 m W/m 2.Low surface heat flow(30 – 40 m W/m 2)is observed in the northern part and the southern corner of the Trans-North China Orogen together with the western margin of the Western block.Moho temperature ranges from 450 ? C to 600 ? C in the eastern block and in the western block is 550 ? C to 650 ? C.The thermal lithosphere is 100-140 km thick where have the surface heat flow of 60-70 m W/m 2.Based on gravity,seismic and thermal data we constrained the lithospheric mantle density at in-situ and STP condition by remove the gravity effect of topography,sedimentary cover,Moho and Lithosphere-Asthenosphere Boundary variation from the free-air gravity anomaly.The sedimentary covers has average density range from 1.80 g/cm 3 to 2.40 g/cm 3.The average crustal density variaous between of 2.70 g/cm 3 and 2.78 g/cm 3.The gravity effect of surface topography,sedimentary cover,Moho depth are 0 to +150 m Gal,-20 to-120 m Gal and +50 m Gal to-200 m Gal,respectively.The gravity effect of both the thermal and seismic lithosphere-asthenosphere boundary ranges from 20 m Gal to +200 m Gal.The lithospheric mantle residual gravity ranges from-250 m Gal to +100 m Gal.The lithospheric mantle density with values ranges from 3.20 g/cm 3 to 3.26 g/cm 3 at in-situ condition and 3.31 g/cm 3 to 3.41 g/cm 3 at STP condition with a slight difference between values derived from the thermal and seismic LAB.We find that densities in the subcontinental lithosphere of the North China Block are highly heterogeneous.The deep-seated crustal xenoliths beneath the southeastern margin of the North China Block record thermal events from Neoarchean to Cenozoic and indicate a 15 km crustal remove during the Mesozoic.The bulk composition of the crust in the North China Block is similar to other typical Archean blocks over the word.The subduciton/collision of surrounding orogens do not significant effect the bulk composition of the crust.Surface heat flow,thermal lithosphere structure and the lithospheric mantle density of the North China Block suggested that the lithospheric mantle beneath the North China Block experienced vary degree modification.The Eastern block experienced significant modification or destroy.The Western block suffered minor reworking or re-thicken with the mantle cooling after modification.The subduciton/collision of surrounding orogens played an important role in the modification of the lithospheric mantle beneath the North China Block.