华北板块北缘中段花岗闪长岩-苏长辉长岩的锆石U-Pb年代学、地球化学特征及其形成机制

Geochemical Characteristics and Zircon U-Pb Age of the Granodiorite-Norite Gabbro in the Northern Margin of the North China Block and Their Formation Mechanism

华北板块北缘中段土牧尔台地区发育大量的酸性侵入岩,仅局部出露一些苏长辉长岩岩体.前人对该基性岩岩体的研究较少,且缺少其与周围同时代酸性侵入岩演化关系的讨论.基于年代学和地球化学方法,锆石U-Pb测年结果显示,花岗闪长岩年龄为275.3±2.6Ma,苏长辉长岩为270.1±4.2Ma,两者均为早二叠世产出.苏长辉长岩贫硅(SiO_2=46.2%~49.8%)和高场强元素(Nb、Ti、Zr等),富Mg#(59.16~67.58)和大离子亲石元素(Cs、Ba、Sr等),具有较低的稀土总量和较平缓的配分曲线,及Eu正异常(δEu=1.02~2.41),显示幔源侵入岩的特点;花岗闪长岩SiO_2含量在65.6%~67.0%之间,K_2O含量为3.71%~4.15%,为准铝质系列(A/CNK=0.94~0.98),属于高钾钙碱性I型花岗岩.样品富集轻稀土、大离子亲石元素(Cs、Rb、K等),亏损重稀土元素、高场强元素(Nb、Ti、Th等),存在Eu负异常(δEu=0.61~0.69),具有大陆弧火山岩的特征,同时岩石中存在镁铁质包体,表明其岩浆来源是壳幔混源的.两者的时空关系及地球化学特征显示,基性岩浆来自于受俯冲流体交代的亏损岩石圈地幔,底侵加热地壳产生花岗质岩浆并与之发生混合作用.结合区域研究背景,表明花岗闪长岩-苏长辉长岩岩体形成于俯冲的构造背景下,且在早二叠世,古亚洲洋仍未闭合.

There are many acid intrusive rocks and a few norite gabbros in Tumuertai,located in central-northern margin of the North China Block.The research on the norite gabbro is limited and the evolution between the norite gabbro and cotemporary intrusive rocks has hardly been discussed.Base on the method of chronology and geochemistry,SHRIMP zircon U-Pb data indicate that the formation age of the granodiorite is 275.3±2.6 Ma;LA-ICP-MS zircon U-Pb data indicate that the formation age of the norite gabbro is 270.1±4.2 Ma,and both were formed in the Early-Permian.Geochemical data indicate that the norite gabbro is depleted in SiO2（46.2%-49.8%）and high field strength elements（HFSEs,e.g.,Nb,Ti and Zr）,and is enriched in Mg#（59.16-67.58）and large ion lithophile elements（LILEs,e.g.,Cs,Ba and Sr）,and displays low REE abundances and flat REE pattern with obviously positive Eu anomalies（δEu=1.02-2.41）,which is similar to mantle derived intrusive rocks.These granodiorite samples have SiO2=65.6%-67.0%,K2O=3.71%-4.15%,A/CNK=0.94-0.98,belong to high-K calcalkaline I-type granitoids.They also show strong enrichments in light rare earth elements（LREEs）and LILEs（e.g.,Cs,Rb and K）,and depletions in HFSEs（e.g.,Nb,Ta and Th）and heavy rare earth（HREEs）,with weakly negative Eu anomalies（δEu =0.61-0.69）.Combining with its common mafic inclusions,we consider that its primary magma could be possibly contributed by magma mixing in a continental arc setting.The spatio-temporal relationship of the granodiorite-norite gabbro and their geochemical characteristics suggest that mafic magmas derived from the depleted mantle wedge which was metasomatized by the subducted slab-derived magma.The mafic magmas not only heated to melt the crust,but also mixed with the crustderived melts to form the diverse granitoids.In summary,we conclude that the granodiorite-norite gabbro formed in a subduction tectonic setting and the Paleo-Asian Ocean was not closed in Early Permian.