Comparative Study On Nature Of Subcontinental Lithosphere Mantle In Northeastern Margin Of The North China Craton And Northeast Orogenic Belt

It is one of the important scientific problems of deep Earth dynamicsto conduct inversion for the evolutionary history of the continentallithosphere. The changes have occurred in the nature, thickness andthermal state of the eastern lithosphere of North China Craton sincePaleozoic. Compared with the cratons in other areas of the world, NorthChina Craton reflects its own differences, and many geologists have fewcontroversies for the cognizance of this geological event. However,different points of view still exist in the time, mechanism and deepprocess of the occurrence of the great change. The previous studiesusually took the marginal areas of North China Craton as the studyhotspots, while the northeastern margin of North China Craton is close toPacific subduction plate and northeast orogenic belt, and obviously thestudy work on the nature of the mantle of the subcontinental lithosphereof the northeast orogenic belt similarly has important significance. Inview of this, in this paper, in addition that the peridotite xenoliths of theCenozoic basalts in Kuandian, Longgang and Changbai Mountain areas of the northeastern margin of North China Craton were selected as thestudy objects, we also selected the spinel phase peridotite xenoliths inAer-Chaihe area of northeast orogenic belt Daxinganling as the studyobject, conducted the studies on the nature and evolutionary process ofthe subcontinental lithosphere mantle under different tectonicenvironments through the detailed petrology and geochemistry as well asthe redox state (O2) of the mantle in the study area, and carried out thedetailed comparative study on the formation mechanism.The chemical characteristics of the major elements of the wholerock and minerals in the study area show that, the nature of the mantle ofthe northeastern margin of North China Craton overall is dominated bythe nascent oceanic mantle, while the Aer-Chaihe lithosphere mantleretains more remnants of ancient mantle. The equilibrium temperature ofthe study area also shows the following heterogeneity: The equilibriumtemperature of the peridotite in Longgang-Changbai Mountain-Kuandian areas of the northeastern margin of North China Craton is low,being close to the equilibrium temperature of the peridotite in Changleand Shanwang areas of eastern North China Craton; the equilibriumtemperature range of the mantle peridotite in Aer-Chaihe area is similarto the equilibrium temperature range of the peridotite in Hebi area ofHenan of North China Craton; both lherzolite and harzburgite have thedistinctions of high temperature and low temperature, the minority lowtemperature samples present the characteristics of the lithosphericmantle which accepts transformation; while the high temperaturelherzolite samples are speculated to be the results after the interactionbetween asthenosphere and lithosphere.The characteristics of the trace elements of the clinopyroxene in thestudy area show that, the overall partial melting degree of thenortheastern margin of North China Craton is similar to the low partial melting degree experienced by the nascent and full-type peridotite inShanwang area, besides, the northeastern margin of North China Cratonexperienced the metasomatism of low degree, and the metasomaticmedium was the silicate fluid/melt. The data of the Aer-Chaihe areashow that, the partial melting degree of most samples in the study area issimilar to that of the refractory and defective Hebi peridotite, showinghigh partial melting degree. The discussions about the metasomatism ofthe Aer-Chaihe study area show that, the metasomatic process of thestudy area is relatively complex, the main bodies of its metasomaticmedium are silicate fluids/melts, very few samples have the metasomaticresidues of carbonate melt and the metasomatic characteristics ofwater-rich fluids, and it is speculated that the water-rich fluids comefrom the deep asthenosphere, while not from the influence of the ancientsubduction slabs.The results of the discussions about the redox state of the mantle inthe study area show that, the range value of the oxygen fugacity (O2) ofthe subcontinental lithosphere mantle of the northeastern margin of NorthChina Craton is equivalent to the range value of the oxygen fugacity ofthe subcontinental lithosphere mantle affected by the asthenosphere, andit is speculated that the formation of the nascent mantle in the study areais related to the activities of the asthenosphere. However, the Aer-Chaihearea has a considerable number of the transition samples of thegarnet-spinel phase. After discussions, it is thought that, the oxygenfugacity calculated by using the peridotite samples which do not reach themineral chemical equilibrium (spinel-garnet transition samples) is invalid,and it cannot truly reflect the redox state of the mantle in the study area.The content characteristics of the pro-copper and pro-iron elementsof the subcontinental lithosphere mantle in Longgang-ChangbaiMountain-Kuandian areas of the northeastern margin of North China Craton show that, the sulfides are not the main occurrence matters ofPGE, while the alloys, silicate minerals and oxides are more inclined tobe the occurrence matters of PGE. The low content of Cu, S and PGEmay indicate that compared with other areas inside North China Craton,the nature of the mantle of the northeastern margin of North ChinaCraton is characterized by heterogeneity.After the discussions were conducted for the formation mechanismof the mantle in the study area, it is thought that, the respectivelyeastward and southward composite subduction of western Pacific plateand Mongolia-Okhotsk oceanic plate result in the imbalance anddelamination of the ancient lithosphere of the northeastern margin ofNorth China Craton, while cause the diapir and upwelling of the hot andfull asthenosphere substances below, thereby leading to the formation ofthe nascent lithosphere mantle of the northeastern margin of North ChinaCraton. Besides, the eastward and westward subduction of westernPacific plate is the fundamental factor of the delamination of thelithosphere mantle. However, the processes which are similar to thedestruction and transformation of the northeastern margin of North ChinaCraton for the protogenetic lithosphere mantle do not exist in thelithosphere mantle of the Aer-Chaihe area. The phenomenon that thestudy area retains a considerable number of ancient lithosphere mantleshow that the evolutionary history of the lithosphere mantle of theAer-Chaihe area significantly differs from the destruction and activationprocesses experienced by the northeastern margin of North China Craton.