赣东北婺源地区中生代花岗岩体的构造背景研究

Research on tectonic setting of Mesozoic granitoids in the Wuyuan county area,northeastern Jiangxi province

邦彦坑岩体、段莘岩体和青山岭岩体位于江南造山带东段。邦彦坑花岗岩特征为高硅(SiO_(2)=65.56%~71.36%);富铝(Al_(2)O_(3)=14.12%~16.58%);低镁(MgO=1.08~1.33);富钾(K_(2)O=3.42%~4.56%);弱的Eu负异常(δEu为0.65~0.92);(La/Yb)_(N)较高(13.92~19.09);特征元素Y含量较低(13.8×10^(-6)~17.9×10^(-6));富集元素Sr(335.9×10^(-6)~484×10^(-6));高的K_(2)O/Na_(2)O(1.07~1.62);低的Mg^(#)(0.23~0.27)、Cr(6.83~13.02)、Ni(3.81~6.13)、Ce/Pb比值(1.60~3.46),明显的Nb、Ta负异常,指示其为壳源,C型埃达克岩,为构造加厚下地壳基性岩的部分熔融形成。段莘和青山岭花岗岩有富硅、富钾、贫钙、贫镁和过铝质的特征,LREE与HREE均表现出平缓的右倾趋势,强烈的Eu负异常。呈现出经典右倾“V”字形,Rb、Th、Ta等不相容元素富集,Zr轻微亏损,Ba、Sr、Eu强烈负异常,10000×Ga/Al=3.12~3.44,属于是A_(2)型花岗岩,为后造山伸展作用的产物。赣东北印支期造山后有地壳加厚现象,由晚侏罗世到白垩世早期区域构造应力发生转变,由构造加厚的挤压环境开始转变为造山后伸展环境。

The Bangyankeng,Duanxin and Qingshanling granitic plutons intrude in the eastern part of Jiangnan orogenic belt.The Bangyankeng pluton shows high SiO_(2) content(65.56%~71.36%),rich in aluminum(Al_(2)O_(3)=14.12%~16.58%),and poor in MgO(1.08 to 1.33).It is also rich in potassium(3.42%~4.56%),and weak negative Eu anomaly(δEu=0.65~0.92),(La/Yb)_(N) has a higher value of 13.92~19.09.As the content characteristic element,Y is with a low value of 13.8×10^(-6)~17.9×10^(-6).Sr is of 335.9×10^(-6)~484×10^(-6),with the significant negative anomalies of Nb and Ti.Various characteristics indicate that Bangyankeng granite belongs to C-type adakites with the partial melting of mafic crust formation.The DuanXin and Qingshanling granites are rich in Si,K,poor in Ca,Mg and peraluminous characteristics.The LREE and HREE exhibit moderate right orientation,strong negative anomalies of Eu.It shows classic right-slope"V"shape.The incompatible elements Rb,Th,Ta are high,and Zris slight loss.Ba,Sr,Eu show strong negative anomaly,together with Ga/Al=3.12~3.44.The geochemistry indicates that DuanXin and Qingshanling rock belong to A_(2)-type granite,as the product of postorogenic extensional.The crust in northeast of Jiangxi was thickened after the Indosinian orogeny and then suffer the regional tectonic stress shifts during the Late Jurassic to Early Cretaceous.The compression under the tectonic thickening transfers to the post-orogenic extension.