青海省阳康地区晚奥陶世花岗岩地球化学特征及其地质意义

Geochemical Characteristics and Geological Significance of Late Ordovician Granite in Yangkang Area of Qinghai Province

青海省阳康地区花岗岩岩石组合为细粒石英闪长岩、黑云母花岗闪长岩。通过对岩石地球化学分析发现,SiO_2含量为54.36%～70.16%,Al_2O_3含量为13.08%～17.01%,K_2O含量为1.57%～3.91%,Na_2O含量为2.37%～3.75%,CaO含量为1.91%～8.09%,里特曼指数σ为1.5～2.61,属偏铝质-过铝质钙碱性花岗岩;稀土总量(∑REE)为121.7×10^(-6)～458×10^(-6),LREE/HREE值为6.81～14.72,表现为轻稀土富集型,δEu值为0.46～1.16,Eu亏损明显,δCe略亏损;富集大离子亲石元素(Rb、Ba、K)和轻稀土元素(La、Ce、Nd),亏损重稀土元素(Yb、Lu)和高场强元素(Nb、Ta),具有明显的Eu和Sr、P、Ti的亏损。在石英闪长岩和花岗闪长岩中分别获得LA-ICP-MS锆石U-Pb年龄分别为(455.5±1.1)Ma和444Ma,形成时代为晚奥陶世。通过构造环境分析,认为阳康地区花岗岩是由挤压向拉伸环境转换的张驰阶段的岩浆侵位,属同造山花岗岩。

The granite rocks in Yangkang area of Qinghai province are fine grained quartz diorite and biotite granodiorite.The geochemical data shows that these rocks have SiO2 content of 54.36%~70.16%,Al2O3 content of 13.08%~17.01%,K2O content of 1.57%~3.91%,Na2O content of 2.37%~3.75%,CaO content of 1.91%~8.09%,and their Ritterman indexes are1.5~2.61,which belong to the peraluminum-peraluminous calcium alkaline granite.Their total amount of rare earth(∑REE)are 121.7×10-6~458×10-6,with LREE/HREE ratios of 6.81~14.72,which are expressed as light rare earth enrichment type.TheirδEu values are 0.46~1.16,with obvious Eu loss and slightlyδCe loss.The large ion lithophile elements(K,Rb,Ba,)and the light rare earth elements(La,Ce,Nd)are enriched,while the heavy rare earth elements(Yb,Lu)and the high field strength elements(Nb,Ta)are loss,with obvious loss of the Eu and Sr,P,Ti.The LA-ICP-MS zircon U-Pb ages of quartz diorite and granodiorite are(455.5±1.1)Ma and 444 Ma respectively,which belong to late Ordovician.Through the analysis of tectonic environment,it is believed that the granite in Yangkang area was formed by the magmatic emplacement in the relaxation stage,which was transformed from the extrusion to the tensile environment,belonging to the syn-orogenic granite.