Sr-Nd Isotopic Characteristics And Geological Significance Of Late Mesozoic Granites In The Core Of Metamorphic Core Complex In Northern Margin Of North China

The metamorphic core complex(MCC)in the northern margin of the North China Craton is a typical representative of large-scale crustal extension and deformation in Northeast Asia.The formation and evolution of metamorphic core complex are closely related to the tectonic background and crustal extension.Previous studies mainly focus on the fabric characteristics,shear direction,strain,kinematic vorticity and deformation age of the detachment zone.The study on the geochemistry,isotope characteristics and genesis of the core granite,an important part of the nuclear complex,is relatively weak.Therefore,isotope geochemistry of granitic bodies in the core of the typical metamorphic core complex in the northern margin of North China,in order to reveal its genetic relationship with metamorphic core complex,and provide new information for understanding the tectonic evolution history of metamorphic core complex.In this paper,the Late Mesozoic granites in the core of the Yiwulüshan,Yunmengshan,Louzidian and Liaonan MCCs in the northern margin of North China Craton are selected as the research objects.On the basis of systematic collection of major and trace elements and rare earth elements of Late Mesozoic granites in the study area and adjacent areas,combined with field observation,petrographic analysis and Sr-Nd isotopic determination of representative samples,the petrogenesis and geological significance of the core granites of each metamorphic core complex are discussed.The results show that the core body of the metamorphic core complex is mainly composed of biotite granite,monzogranite and granodiorite.Generally,it is rich in silicon and alkali,and belongs to the high-K calc-alkaline series,consistent with the characteristics of type I granite.Most granites are characterized by enrichment of large ion lithophile elements(Rb,Ba,Sr)and light rare earth elements,depletion of high field strength elements(Nb,Ta,Ti)and heavy rare earth elements,high Sr and high Sr/Y,low Y and Yb,and are characterized by adakitic rocks.Isotope data show that the initial(⁸⁷Sr/⁸⁶Sr)_i ratio of the early granites ranges from 0.588 to 0.701,with negative?Nd(t)values(-20.60?-1.75,avg.-13.06).According to the two-stage Nd model age calculation method,the t _DM2 of the two-stage Nd model age ranges from 1146 to 2699Ma,with an average of 2023Ma.The initial ratio of late granite(⁸⁷Sr/⁸⁶Sr)_i is 0.705?0.712,with an average of 0.707,and?Nd(t)value(-20.60?-3.33 with an avg.-16.12).The calculated Nd second-stage model ages range from 1163 to 2408Ma with an average of 2226Ma.These granites may be the product of partial melting of the thickened lower crust under the background of regional extension and decompression,indicating the thinning of the North China Craton during the Late Mesozoic.