华北板块北缘中段早石炭世构造属性：察哈尔右翼后旗高分异Ⅰ型花岗岩地球化学的制约

摘要

This study focuses on the petrology and geochemistry of the monzonitic granites from the Cahayouhouqi area in Inner Mongolia.Tectonically it is located in the central-northern margin of the North China craton.These rocks have high content of silica,kalium and alkalis,low abundance of calcium,phosphorus,iron,magnesium;w(P2 O5 )decreases with the increasing of w(SiO2 );A/CNK equals 0.96 1.15;and differentiation index DI is between 90.36 92.96.The granites consist mainly of perthite,plagioclase,quartz,minor biotite,no mafic alkaline minerals and Al-rich minerals;their accessory minerals are zircon,magnetite,apatite,sphene and ilmenite.Combined with the petrological and geochemical characteristics,the granites can be classified as a highly fractionated I-type granite.The low content of ∑REE ((46.8 94.4)× 10-6 ),Th ((1.74 2.39)× 10 -6 ),and U ((0.27 0.39)× 10-6 ),as well as their trace element discrimination diagrams indicate that the magma source derived from the lower crust,and their source-rock was lithosphere mantle.The chondrite normalized patterns are right declined with a serious positive Eu anomaly (δEu=0.83 3.51).They are enriched in the large ion lithophile elements (LILEs,e.g.,Cs,Rb,Ba,and K),and depleted of the high field-strength elements (HFSEs,e.g.,Nb,Ta,P,and Ti).Their geochemical characteristics indicates that the monzonitic granites emplaced in an active continental margin as a result of the Early Carboniferous subduction of Paleo-Asian Ocean towards the northern margin of the North China craton.%察哈尔右翼后旗二长花岗岩岩体位于华北板块北缘中段。岩体富 SiO2、富 K、富碱、低 Ca 和 P、贫 Fe 和 Mg，w（P2 O5）与 w（SiO2）呈负相关，铝指数（A／CNK）为0．96～1．15，分异指数为90．36～92．96；主要造岩矿物为条纹长石、斜长石和石英，其铁镁矿物主要为黑云母，未出现碱性铁镁矿物和富铝矿物，副矿物为锆石、磁铁矿、磷灰石、榍石和钛铁矿；可见察哈尔右翼后旗二长花岗岩属高分异钙碱性 I 型花岗岩类。结合较低的 w（∑ REE）（（46．8～94．4）×10－6）、w （Th）（（1．74～2．39）×10－6）、w （U）（（0．27～0．39）×10－6）和微量元素判别图解，二长花岗岩岩浆源区可能为下地壳，源岩可能为岩石圈地幔。岩石轻重稀土分馏较强（（La／Yb）N ＝6．52～28．39），δEu 以正异常为主（0．83～3．51），富集大离子亲石元素（LILEs，Cs、Rb、Ba 和 K），亏损高场强元素（HFSEs，Nb、Ta、P 和 Ti）。地球化学特征反映了二长花岗岩具有火山弧岩石特征，且岩体侵入新元古界埃迪卡拉系什那干群，表明岩体形成于活动大陆边缘弧环境，其侵位反映了古亚洲洋的俯冲岩浆事件。