Geochronology And Geochemistry Characteristics Of TTG Gneiss In The Bengbu Area Along The Southeast Margin Of North China Craton And Their Implications

The TTG is an important component of the Archean craton crust which also has extremely far-reaching significance on the study of the early plate formation,revolution and the growth pattern.Bengbu area is located at the southeastern margin of the North China Craton?NCC?that belong to one of China's three largest craton.A suit of TTG gneiss distributed in this study area is researched by the petrography,zircon LA-ICP-MS U-Pb dating,petrogeochemistry and the isotope geochemistry.While all kinds of geochemical characteristics,diagenetic age,petrogenesis,genesis and the tectonic environment are discussed in this article.The main lithologies of the TTG gneiss from the Bengbu are mainly granitic gneiss.And the clear gneissosity are constitute by the directional alignment of the dark-colored mineral.The rock samples belong to the representative TTG which correspond with the tonalite,trondhjemite and granodiorite after the original rocks recovering.According to the data of the LA-ICP-MS zircon U-Pb dating of the Shizishan and Xiangmiaocun TTG gneiss,the weighted average age of²⁰⁷Pb/²⁰⁶Pb are respectively2510±18Ma and 2625±12Ma.These results show that the TTG gneiss are belong to the products of the Neoarchean magmatism that corresponding to the ages of the TTG gneiss from other area in the North China Craton.The Al₂O₃ of the TTG gneiss from Bengbu is less than 15%which belong to the low-Al type TTG.These samples of calc-alkaline series are rich in SiO₂?70.05%⁷6.86%,average value is 71.42%?and Na₂O?2.88%⁸.08%,average value is 4.79%?,while the low K₂O is0.11%³.18%and Na₂O/K₂O>1.They could also show the characteristics of the metaluminous-peraluminous.The low contents of MgO,TiO₂ and P₂O₅ are 0.12%².79%,0.08%¹.25%and 0.001%⁰.14%.And the Mg^#is relative higher,range from 14.59 to52.85.The SiO₂ displays a positive correlation relationship with K₂O and Na₂O,while anegative correlation relationship with Fe₂O₃,P₂O₅,TiO₂,MgO and MnO in the Harker diagram.The REE characteristics of the TTG gneisses from Bengbu area show a high total content which have a enrichment of LREE,the?La/Yb?_N range from 1.76 to 58.69?average value is 11.71?.The fractionation of the LREE is greater than the HREE,and LREE/HREE is 2.37²2.86?average value is 8.28?,?La/Sm?_N is 1.37⁴.35.The samples also display a strong Eu negative anomalies.The trace element of the Bengbu TTG gneiss enriched in Ba,U and Pb,but poor in HFSE,like Ti,Nb and Ta.The low value of Y,?Yb?_N,Rb/Sr,Sr/Ba and the high Zr/Hf,Sr/Y,?La/Yb?_N,Nb/Ta all show that the TTG gneiss may form in the partial melting of the basaltic crust,meanwhile accompany the garnet,hornblende and rutile residue.The value of(⁸⁷Sr/⁸⁶Sr)_i of Bengbu TTG gneiss range from 0.675940 to 0.703821?average value is 0.694494?,and the(¹⁴³Nd/¹⁴⁴Nd)_i range from 0.509919 to 0.510072?average value is 0.509987?.The calculated?_Nd?t?range from-5.1 to-2.1?average value is-3.78?,Nd model age(T_DM2)range from 2.54 to 2.78Ga?average value is 2.68Ga?.The characteristics of Hf isotopes of the zircons from the Shizishan and the Xiangmiaocun TTG gneiss show that the?_Hf?t?is+4.4⁺9.2 and-1.3⁺6.4.And the Hf model age of the two plutons are 2.46².65Ga and 2.75².97Ga respective.According to the two kinds of isotopic datas,this article considers the basalt crust that experice the partial melting should be the juvenile crust.Therefore,this article suggests that the TTG gneiss from Bengbu area may have formed by partial melting of a juvenile basalt crust in the environment of island arc or the continental marginal arc.And this 2.5-2.6Ga magmatic events could represent a significant crust-growth record,meanwhile finishing the collisions of various micro-continents of the North China Craton.