Formation And Evolution Of Xinlitun Neoarchean Magmatic Complex In Xingcheng,Liaoning Province

Xinlitun area of Xingcheng, Liaoning Province located in the eastern margin of North China Craton,which is characterized by widely exposed Archean rock. Xinlitun magmatic complex is composed chiefly of lithology and field occurrence. Compared with other strongly metamorphosed and deformed rocks, this one showed weaker tectonic deformation and metamorphism deformation.So it is the perfect place to keep original geological characteristics.Through the research of Petrography, Zircon U-Pb geochronology, Zircon Lu-Hf isotope, geochemistry,mineral chemistry,we summarized several important characteristics as follows:(1) According to rock characteristics and field occurrence, porphyritic biotite monzogranite, medium-fine quartz diorite, fine syenogranite, granite pegmatite, metagabbroid and diabase are the main rock types in the research area. The later two presented as fine veins intruded into the former one,which are the main rock units and widely distributed.(2) LA-ICP-MS zircons U-Pb dating yielded three concordant ages of 2496±18Ma for porphyritic biotite monzogranite, 2490±19Ma for medium-fine quartz diorite and 2479±29Ma for fine syenogranite, indicating the instrusive time is Late Neoarchean.(3)Based on their contact relationships in the field,we can infer that porphyritic biotite monzogranite intruded firstly,then medium-fine quartz diorite, fine syenogranite and granite pegmatite in turn,and diabase at last.(4) The geochemical characteristics show that, porphyritic biotite monzogranite shares some features with TTG suite,which is belongs to albite-rich rock. Besides, the average ratios of Nb/Ta is 10.95.It is very close to the character of the Late Neoarchean crust. Electron microprobe analyses show that there are two kinds Plagioclase and K-feldspar,which formed in different periods.The generation of the different minerals suggest that their formation mainly related to anataxis. In conclusion, porphyritic biotite monzogranite maybe a product of partially melting of crustal material.Medium-fine quartz diorite shares some features with archean granite diorite, belongs to the albite-rich rock.Besides,the REE patterns are characterized by the enrichment of LREEs,depletion of HREEs. The average ratios of Nb/Ta is 20.13,whicht is very close to the primitive mantle. It indicates that medium-fine quartz diorite is formed by crust-mantle migmatization.Fine syenogranite and granite pegmatite came from the same magmatic crystallization differentiation. According to REE patterns, these two kinds of rock are rich in Rb,Th,U and depleted in Ba relatively. All the evidence suggests that fine syenogranite and granite pegmatite might be derived from crust source.Metagabbroid is the most obvious type of rock derived from enriched mantle. And all the pyroxenes in metagabbroid has been turned into hornblende. Combining with the characteristics of blastogabbroic texture, We speculate that the metagabbroid is the product of metamorphism after the mantle-derived magma emplacement.The last, diabase dyke cut through other rock types and still remains ophitic texture. In conclusion, diabase is the lastest rock type in Xinlitun magmatic complex and representing the end of thermo-tectonic event.(5)Combined with the results of zircon Lu-Hf isotope(εHf(t)=-1.12~+6.13,),we can draw the conclusion that magmatic complex is the product of mantle material contaminated by juvenile crust. According to the two-stage model age(TDM2(Hf)=2594~2937Ma, peak value=2800Ma),we can draw a conclusion that the crust-mantle differentiation events happed in 2800 Ma.(6)With a comprehensive consideration on many many factors, Xinlitun magmatic complex is divided into three types.One is homogeneous rock(medium-fine quartz diorite),another is syn-plutonic mafic dykes,the last one is quartz monzodiorite enelave.But the number of mafic enclaves is really small.This may be related to mantle material modification.(7) From the above we can come to the conclusion that the Xinlitun magmatic complex is the result of multi geological process,such as anataxis, magma mixing and fractional crystallization.