龙首山直沟门岩体特征及构造环境

Characteristics and tectonic environment of Zhigoumen pluton in Longshoushan area

直沟门岩体(年龄374±38Ma)侵入于华北板块西缘的龙首山岩群,主要由花岗岩、二长花岗岩等岩性组成。根据野外产状和岩相学特征可划分为3个单元,从早到晚依次为:直沟门单元、湾湾圈单元和马路湾单元。岩体的w(SiO2)65.94%～75.18%,w(K2O)2.36%～4.35%,w(Na2O)3.53%～5.47%,总体上w(Na2O)≈w(K2O)。随w(SiO2)增大w(CaO),w(TFeO),w(MgO)减少,所有岩石均具铝弱过饱和,ANKC=1.01～1.09,岩浆类型属钙碱性系列,σ=1.99～2.64,皮可克指数为55.6。Li,Cs,Rb质量分数较低,而Ba,Sr较高,从早到晚大体存在Rb/Sr减少,而Sr,Ba,Co和Ba/K增加。湾湾圈和马路湾单元在∑REE((152.11～171.94)×10-6),∑LREE/∑HREE(24.81～26.92),δEu(0.91～1.10)以及配分模式等方面是相似的,与其相比,直沟门单元的∑REE(46.62×106),∑LREE/∑HREE(3.52)低,而δEu(1.60)高。w(La)与w(La)/w(Sm)大体存在正相关,δEu,∑REE与DI变化也相一致。直沟门岩体形成于祁连造山带碰撞阶段之后,属后碰撞花岗岩。在w(CaO)/w(Na2O)对w(Al2O3)/w(TiO2)图上,与拉克伦褶皱带后碰撞花岗岩的投影区相近。直沟门岩体的岩浆可能源自祁连褶皱带下冲火山岩。

The Zhigoumen pluton(age of 374±38 Ma),intruded into the Longshoushan Group at the western margin of the North China Plate,is composed dominantly of granite,monzogranite et al.. On the basis of the field relations and petrographic data ,the pluton can be divided into three units from early to late:Zhigoumen unit,Wanwanjuan unit and Maluwan unit.The rocks of the pluton have w(SiO_2)=65.94%~75.18%,w(K_2O)=2.36%~4.35% and w(Na_2O)=3.53%~5.47%,w(CaO),w(TFeO) and w(MgO) decrease with increasing w(SiO_2).All rocks are moderately peraluminous with ANKC of 1.01~1.09,and magmatic type belongs to calc-alkalic series with σ=1.99~2.64 and Peacock index=55.6.As compared with typical granites,the trace elements of Li,Cs and Rb are lower,and Sr and Ba higher.There is,from early to late units,a weakly tendency of decrease in Rb/Sr and increase in Sr,Ba,Co and Ba/K.The Wanwanjuan unit is similar to the Maluwan unit in ∑REE(152.11~171.94×10 -6 ),δEu(0.91~1.10)and REE pattern,but the Zhigoumen unit differs from them,it has lower ∑REE(46.62×10 -6 )),lower ∑LREE/∑HREE(3.53)and higher δEu(1.60).Generally,La is positively correlated to La/Sm,and δEu and ∑REE vary consistently with DI.The Zhigoumen pluton,formed after the collision stage of the Qinling Fold Belt ,belongs to post-collision granite type,because the rocks are similar to the granites of the Lachlan Fold Belt in the plots such as w(Cao)/w(Na_2O) vs.w(Al_2O_3)/w(TiO_2).It is possible that the Zhigoumen pluton is derived from the melting of the subducted volcanic rocks of the Qilian Fold Belt.