丹东地区古元古代晚期花岗岩对胶-辽-吉带造山作用的制约

Constraints of late Paleoproterozoic granites in Dandong area on Jiao-Liao-Ji Belt orogenesis

华北克拉通古元古代胶-辽-吉带造山过程保存了丰富的变质-变形、岩浆-构造热事件与成矿作用记录,带内广泛存在的高压麻粒岩变质作用演化p-T-t轨迹呈顺时针,指示胶-辽-吉带经历了俯冲-碰撞-后碰撞折返伸展造山过程,但这时期的俯冲碰撞或陆-陆碰撞与之后伸展造山作用的时代目前仍有争议。本文对丹东地区大楼房古元古代花岗岩进行了系统的岩相学、锆石U-Pb年代学、全岩主微量元素和Lu-Hf同位素测试分析。研究结果显示,岩浆锆石加权平均年龄1873~1869 Ma,获得岩体的侵位时限为古元古代。大楼房岩体以片麻状黑云母二长花岗岩为主,具有高SiO_(2)(72.39%~74.20%)、中等Al_(2)O_(3)(14.66%~15.40%)、中等CaO(1.35%~1.84%)和低MgO(0.07%~0.35%)含量特征,稀土配分曲线右倾型,具强烈的Eu正异常;微量元素富集Rb、Th、U、K等大离子亲石元素,亏损P、Ti等高场强元素,为准铝-弱过铝质高钾钙碱性系列,具有与Ⅰ型花岗岩类似的岩石地球化学特征。εHf(t)值为-1.76~+1.45,锆石Hf同位素二阶段模式年龄为2607~2425 Ma,代表岩浆源区不仅来自地壳物质再循环,还有新太古代-古元古代物质加入到地壳中或有新生地壳的生成。不同于碰撞后伸展造山作用的花岗质岩石,丹东地区古元古代晚期片麻状二长花岗岩岩石地球化学特征与俯冲作用有关的活动大陆边缘弧花岗岩特征类似,且与北邻的集安地区泥质麻粒岩变质年龄相近,暗示胶-辽-吉带在古元古代晚期可能仍然存在俯冲碰撞作用,这对探讨该造山带的造山作用及其后的碰撞后伸展作用的过程和时限提供了重要约束。

As a Paleoproterozoic orogenic in the North China Craton,the Jiao-Liao-Ji Belt preserves rich records of metamorphism magmatism,sedimentation and metallogeny.The p-T-t paths of the widespread high pressure granulites within this belt reveal the subduction-collision-exhumation geodynamic process.However,the timing of continental collision and subsequent extension is still controversial.In this paper,the petrography,zircon U-Pb geochronology,major trace elements and Lu-Hf isotopes of the whole rocks of the Paleoproterozoic granites in Daloufang Dandong area are systematically analyzed.The results show that the weighted average age of the magmatic zircons is 1873~1869 Ma,and the emplacement time of the acquired granites is Paleoproterozoic.The Daloufang Pluton mainly consists of gneissic biotite monzogranite,with characteristics of high SiO_(2)(72.39%~74.20%),medium Al_(2)O_(3)(14.66%~15.40%),medium CaO(1.35%~1.84%)and low MgO(0.07%~0.35%).The REE distribution curve is right-leaning with strong Eu positive anomaly.Trace elements are enriched in large ion lithophile elements such as Rb,Th,U,K,and depleted in high field strength elements such as P,Ti,etc.It is characterized by high potassium calc-alkaline series of quasi-aluminum to weak peraluminous,and has similar petrogeochemical characteristics to I-type granites.TheεHf(t)values range from-1.76 to+1.45,and the two-stage model ages range from 2607 to 2425 Ma,indicating that the magmatic source area is not only from crustal material recycling,but also from neoarchean to Paleoproterozoic material added to the crust or the formation ofnew crust.The geochemical characteristics of late Paleoproterozoic gneisses monzogranite in Dandong area are similar to those of active continental margin arc granites related to subduction,and similar to the metamorphic age of argillaceous granulites in Ji’an area in the north.It is suggested that the Jiao-Liao-Ji Belt may have been experienced collision in the late Paleoproterozoic,which provides important constraints for the exploration of the orogeny of the Jiao-Liao-Ji Belt and the process and time limit of the post-collision extension.