张家口水泉沟正长岩杂岩体成因的REE和Sr、Nd、Pb同位素证据  

REE, Sr, Nd, and Pb Isotopic Evidence for the Petrogenesis of the Shuiquangou Syenite Complex in NW Hebei Province, China

作　　者：包志伟[1] 赵振华[1] 张佩华[1] 王一先[1] 

机构地区：[1]中国科学院广州地球化学研究所,510640

出　　处：《地质论评》2003年第6期596-604,共9页Geological Review

基　　金：国家自然科学基金NSFC(编号49633110);中国科学院留学回国人员择优基金(编号2001092419324)

摘　　要：水泉沟杂岩体位于尚义 赤城断裂的南侧,侵位于太古宙桑干群变质岩中。岩体形成于晚加里东至早海两期。其稀土元素含量为8.543×10^(-6)～211.6×10^(-6),随着岩石的CaO、MgO、FeO、Fe_2O_3含量降低,SiO_2、K_2O、Na_2O含量增大,稀土含量减小,稀土分布模式由右倾直线型变为近平直的“～”型。在铅同位素构造模式图上杂岩体落在地幔铅同位素演化线附近。在ε_(Nd)-ε-(Sr)图解上处于地幔演化线的下方。杂岩体的钕模式年龄低于围岩桑干群的形成年龄。岩体可能来源干上地幔与下地壳太古宙变质杂岩混熔作用所形成的正长岩岩浆。The Shuiquangou syenite complex, consisting mainly of alkali syenite, quartz alkali syenite, augite hornblende syenite and syenite, is located on the southern side of the Sangyi-Chicheng fault on the northern margin of the North China Craton. The syenite complex was emplaced in the Archaean metamorphic rocks and formed during the late Caledonian-early Hercynian. The total REE contents of the rocks vary from 8. 54×10-6 to 212×10 6, while the highest values are found in the augite hornblende syenite occurring in the west part of the complex, which are comparable to those of the surrounding metamorphic rocks of the Archaean Sanggan Group. The REE contents decrease with the decrease of CaO, MgO, FeO and Fe2O3 contents,and the increase of SiO2, K2O, and Na2O contents from the augite hornblende syenite to quartz alkali syenite. The chondrite normalized REE patterns shift from right-dipping lines to flat '~'-type patterns without obvious Eu anomalies. On the plumbotectonics model, the rocks are plotted around the mantle evolutionary line with lead isotopic compositions of 206Pb/204Pb= 16. 457 - 17. 718; 207Pb/204Pb= 15. 270 -15. 665; 208Pb/204Pb = 36. 539 - 37. 764. The Nd model ages of the syenitic rocks range from 1. 38 to 2. 29 Ga, lower than the age of theArchaean metamorphic rocks. The values of eNd(t) and esr(t) vary in the ranges of -3. 41^-13. 7 and -0.94^+27. 4 respectively, corresponding to the positions under the mantle array in the eNd(t)-eSr(t) diagram. It can be deduced that the syenite complex was derived from the melting of both mantle material and Archaean metamorphic rocks in the lower crust, which was triggered by the upwelling of the mantle, alkali fluids and the rise of regional geothermal gradient in accordance with the large-scale thrusting from the late Caledonian to the early Hercynian.

关 键 词：正长岩 稀土元素 Sr、Nd、Pb同位素 岩石成因 冀西北 

分 类 号：P597[天文地球—地球化学]