内蒙古阿拉善地块北缘查干敖包一带二长花岗岩锆石定年、地球化学特征及其构造意义

查干敖包二长花岗岩位于阿拉善左旗诺尔公苏木查干敖包一带。该岩体是阿拉善地块北缘诺尔公—狼山构造带诺尔公大型复式岩基的主要组成部分。通过LA-ICP-MS锆石U-Pb同位素测年,确定其形成年龄为(278±1)Ma,属早二叠世晚期。二长花岗岩SiO₂含量为72.88%~75.44%,全碱(K₂O+Na₂O)含量平均为8.16%,K₂O/Na₂O主要介于0.87~1.41之间,Al₂O₃含量为12.38%~13.98%,铝饱和指数(A/CNK)介于0.88~1.02之间,在AR-SiO₂图上落入碱性系列区域。上述特征表明查干敖包二长花岗岩为高硅、富钾、准铝质碱性花岗岩。稀土元素球粒陨石标准化曲线明显右倾,轻稀土元素相对富集(LREE/HREE=4.70~14.66),δEu= 0.18~0.87,平均为0.37,呈中等-较强烈负铕异常。明显亏损Nb、Ta、Ti、P等高场强元素,Sr平均值为94.44×10^-6,Yb为3.10×10^-6,呈现非常低Sr、高Yb的特征。岩石地球化学特征表明查干敖包二长花岗岩体形成于后碰撞环境,地幔物质底侵作用下的伸展机制可能启动了这次岩浆侵入事件。结合区域资料推断阿拉善地块所代表的华北板块西北缘在早二叠世晚期已经进入后碰撞演化阶段。

Zircon U-Pb Dating,Geochemical Characteristics,and Tectonic Significance of the Chaganobo Monzogranite in the Northern Margin of Alxa Block,Inner Mongolia

The Chaganobo monzogranite is located in Alxa of Inner Mongolia. The intrusion is the main component of the large Norgong intrusive complex in the Norgong-Langshan tectonic belt, located in the northern margin of Alxa block. The monzogranite is LA-ICP-MS zircon U-Pb dated to be late Early Permian (279±1 Ma). The monzogranite has SiO₂=72.88%-75.44%, total alkali (K₂O+Na₂O)=8.16%, K₂O/Na₂O=0.87-1.41, Al₂O₃=12.38%-13.98%, aluminum saturation index (ASI=A/CNK)=0.88-1.02, falling into the alkaline field in the AR-SiO₂ diagram. These geochemical characteristics show that the Chaganobo monzogranite is high Si-K metaluminous granite. The right-inclining chondrite-normalized REE patterns indicate that they are enriched in LREEs (LREE/HREE=4.70-14.66) and show moderately to strongly negative Eu anomalies (δEu=0.18-0.87, average 0.37). The primitive mantle normalized trace element diagrams are characterized by obvious depletions of HFSEs, such as Nb, Ta, Ti, and P. The rocks contain very low average Sr (94.44×10^-6) and high Yb (3.10×10^-6). Geochemical characteristics show that the Chaganobo monzogranite formed in the post-collision setting, where extensional-related underplating may have started the magma intrusion event. Based on the regional data, we deduce that the northwestern margin of the north China plate (represented by the Alxa block) and the southern margin of the Tarim plate may have entered the post-collisional stage in the Early Permian.