内蒙古科尔沁右翼中旗碱长花岗岩锆石U-Pb年代学、岩石地球化学及其动力学意义

对内蒙古科尔沁右翼中旗碱长花岗岩进行了同位素年代学及岩石地球化学研究。碱长花岗岩LA-ICP-MS锆石U-Pb加权平均年龄为(166±1)Ma,表明该侵入体是中侏罗世侵位形成的。岩石学及地球化学成分显示其属于碱性、具A型花岗岩特征。岩石高硅(w(SiO₂)=74.80%~76.34%)、富碱(w(Na₂O+K₂O)=7.94%~8.71%)、高铁镁比(TFeO/MgO=13.54~24.28)、贫钙(w(CaO)=0.10%~0.21%)、贫镁(w(MgO)=0.08%~0.16%)和低钛(w(TiO₂)=0.07%~0.10%);稀土配分曲线呈现"海鸥式"分布特征,显示强烈的Eu负异常(δEu=0.09~0.17);微量元素特征显示具有较高质量分数的有Zr(128.95×10^-6~156.32×10^-6)、Yb(4.93×10^-6~5.35×10^-6)和Y(40.93×10^-6~56.75×10^-6),较低质量分数的有Sr(23.16×10^-6~37.14×10^-6)、Ba(186.13×10^-6~231.31×10^-6),在微量元素原始地幔标准化蛛网图上显示明显的Sr、Ba和Ti的负异常。以上特征表明,碱长花岗岩为A型花岗岩。岩石具有高的Rb/Sr值(4.26~7.81,平均为6.12)和Rb/Nb值(10.2~14.7,平均为12.7),显示出壳源岩浆的成分特征。综合分析表明,碱长花岗岩为低压下长英质地壳部分熔融的产物。w(Rb)-w(Yb+Ta)图解、w(Rb)-w(Y+Nb)图解、w(Ta)-w(Yb)图解、w(Nb)-w(Y)图解、Ce/Nb-Y/Nb图解、Ce/Nb-Yb/Ta图解及结合区域构造演化研究表明,碱长花岗岩形成于造山后伸展的构造环境,并与松辽盆地及其周围的花岗岩一起暗示松辽盆地是在中侏罗世造山作用之后伸展的构造环境下形成的陆内盆地。

Zircon U Pb Geochronology,Geochemistry and Dynamics of the Alkali Feldspar Granite in Horqin Right Wing Middle Banner of Inner Mongolia,with Implications for the Geodynamic Setting

The study on Geochronology and Geochemistry of the alkali feldspar granite in Horqin Right Wing Middle Banner of Inner Mongolia.Zircon U Pb dating demonstrates that the weighted mean 206 Pb/238 U age for the zircons from the adamellite (syenogranite)is (1 66 ±1)Ma,i.e.the Middle Jurassic.The alkali feldspar granite is slightly peraluminous and belongs to the alkaline series.The alkali feldspar granite is characterized by high SiO2 (74.80% 76.34%),high alkali (7.94% 8.71%), high TFeO/MgO (13.54 24.28),low CaO(0.10% 0.21%),low MgO(0.08% 0.1 6%),and TiO2 (0.07% 0.10%).The alkali feldspar granite is characterized by a “sea-gull”REE pattern with a significant negative Eu anomaly (0.09 0.17).It is rich in Zr(128.95×10 -6 156.32×10 -6 ),Yb(4.93 ×10 -6 5.35×10 -6 ),and Y(40.93×10 -6 56.75 ×10 -6 ),and depleted of Sr(23.1 6 ×10 -6 37.14 × 10 -6 ),Ba(186.13 ×10 -6 231.31 ×10 -6 )and Ti.Thus,the alkali feldspar granite belongs to A-type aluminous granite.The high Rb/Sr (ranging from 4.26 to 7.81,and the average value is 6.12)and Rb/Nb ratios(ranging from 10.2 to 14.7,and the average value is 12.7)of the rock indicate a crustal origin. The comprehensive analysis shows that the alkali feldspar granite was the product of low pressure felsic crust partial melting.In addition,based on Rb (Yb+Ta),Rb (Y+Nb),Ta Yb,Nb Y,Nb Y Ce,Ce/Nb Y/Nb,Ce/Nb Yb/Ta diagrams and combined with regional tectonic evolution,we suggest that the alkali feldspar granite was formed in an extensional setting of the Middle Jurassic orogeny,also the formation of Songliao basin.