Meso-cenozoic Lithospheric Thermal-rheological Structure And Deep Magmatic-thermal Process In The WNCC

The North China Craton has experienced large-scale magmatic thermal transformation since the Mesozoic.It has been recognized that the eastern part of the North China Craton(NCC)has been damaged since the Mesozoic.For the central and western part of the NCC,there is a great controversy on its evolution of the Meso-Cenozoic lithosphere,which limits the understanding of the temporal and spatial scope and dynamic mechanism of the destruction of the NCC.Based on the recovery of the current geothermal field and Meso-Cenozoic thermal history in the central and western parts of the NCC,further combined with the deep crustal stratification structure model and thermal-rheological parameters,the Meso-Cenozoic thermal-rheological structures of different tectonic units have been investigated,which reveals the deep thermal structure,rheological properties and evolution of the lithosphere in different periods in the study area.In addition,on the basis of the petrological and geochemical results of Meso-Cenozoic igneous rocks,this paper further explores the magmatic process,tectonic background and source characteristics of those igneous rocks,and discusses the deep magmatic-thermal process and lithospheric dynamic evolution.The results show as follows:1)The thermal history evolution of Meso-Cenozoic in the Ordos Basin and Qinshui Basin was similar.Both the Ordos Basin and Qinshui Basin reached the peak value of heat flow(74?92 mw/m²)at the end of Early Cretaceous,reflecting the affection by the same tectonic thermal event.In the early Mesozoic,the two basins had the lithospheric thermal structure type of"cold mantle and hot crust"with the relatively thick lithosphere.During the peak period of lithospheric thinning in the early Cretaceous,the minimum thickness of the lithosphere was 50?75 km,showing a typical lithospheric thermal structure with the type of"hot mantle and cold crust",and the lithosphere presented as a thermal rheological structure of"banana boat".The heat flow has decreased rapidly and the lithosphere has undergone significant vertical thickening since the late Cretaceous.2)The current thermal rheological structures of different tectonic units in the central and western parts of the NCC are different.The Yishan slope and Weibei uplift of Ordos Basin,the southern and central Qinshui Basin have the rheological characteristics of"strong crust and weak mantle"under the conditions of dry and wet mantle with low level of the total strength of lithosphere(1.89×10¹² N/m?2.78×10¹²)and effective elastic thickness(24 km?30 km).Under the condition of dry mantle,Tianhuan depression,Western thrust belt and Northern Qinshui Basin have the rheological characteristics of"weak crust and strong mantle".The current thickness of thermal lithosphere in the Ordos Basin is thin in the east(80?100 km)and thick in the west(100?160 km).The heat flow ratio of crust and mantle in the west is obviously greater than 1.It has a lithospheric structure of"hot crust and cold mantle",and the temperature of Moho surface is between610?850?.The heat flow ratio of crust and mantle in the middle and south of Qinshui Basin is 0.7?0.93,and the north is 0.95?1.30.At present,the thickness of thermal lithosphere is 90?160 km,which gradually increases from north to south.The present heat flow of deep mantle in the Weihe basin is distributed at 35.4?47.1 m W/m²,and the heat flow ratio between crust and mantle is between 0.65?0.83.It has a lithospheric thermal structure of"cold crust and hot mantle",and the thickness of thermal lithosphere is between 65?95 km.3)Under the analysis of the geology and geochemical characteristics from late Jurassic to early Cretaceous magmatic rocks in central and western of the NCC,the U-Pb ages of Chakou,liulingshan and zhongzhuangpu magmatic rock in the Hunyuan area,Shanxi are 150.9 Ma,137.9 Ma and 115.4 Ma respectively.Monzogranite in the Chakou was formed by the underplating of basic magma,resulting in the partial melting of lower crustal materials.The Syenogranite in Liuleng shan originated from the underplating of mantle-derived magma,and the magmatic mixing occurs with the partial melting of the crust,and the crust derived components account for the main part.Biotite monzogranite in the Zhongzhuangpu was formed under extensional conditions,and felsic magma was formed by partial melting of materials in the crust caused by the upwelling of asthenosphere materials.The formation age of hornblende gabbro and diorite in the Pingshun complex is 127±1 Ma and 128.1±1.6 Ma,respectively.The rock is characterized by low Si O₂ and rich Mg O.Geochemical studies show that gabbro is mainly formed by partial melting of lithospheric mantle,and the original magma of diorite may originate from the mixing of enriched lithospheric mantle melting and lower crustal melting.The Heishitougou basalt in the Ordos is potassic trachyte basalt with low Cr and Ni contents,suggesting that olivine minerals were separated and crystallized in the process of magmatic differentiation.The isotopic characteristics indicate that basalt originated from the partial melting of the enriched lithospheric mantle in the early Cretaceous.4)Under the analysis of the geology and geochemical characteristics of Cenozoic magmatic rocks in central and western of the NCC,the results show that the Zuoyun basalt is alkaline olivine basalt,Helinger is tholeiitic basalt and Datong basalt is with the above two lithos.These Cenozoic basalts have similar characteristics in major and trace elements.Generally,the Si O₂ content of the rock is low,Mg O content is high,and Rb,Ba,K and other large ion lithophile elements,and Nb and Ta are relatively enriched.The isotopic characteristics indicate that these basalts mainly originate from depleted asthenosphere mantle melt and interact with EMI enriched lithospheric mantle.The difference is that the contribution of enriched lithospheric components in different basalt source areas is different.5)Combined with the thermal rheological structure of the lithosphere and the thermal process of deep magma in the NCC,it shows that there was an obvious tectonic thermal event in the middle and west of the NCC in the early Cretaceous,the thickness of the lithosphere was thinned.However,it has a relatively stable thermal lithospheric structure.The main cause for the formation of late Jurassic Early Cretaceous igneous rocks in the NCC is the extension and thinning of the crust and the upwelling of deep thermal materials,resulting in the partial melting of the lithospheric mantle and crustal materials.The Cenozoic basalt was mainly formed in the intraplate tension rift environment,and the partial melting of the asthenosphere mantle.The time of lithospheric thinning and magmatic thermal events in the central and western part of the NCC is basically consistent with the time of lithospheric destruction in the eastern part of the NCC,reflecting that they may be affected by the same dynamic mechanism,that is,they are closely related to the westward subduction and roll back of the ancient Pacific plate.