辽宁海城地区老虎沟基性岩墙锆石U-Pb-Hf同位素、地球化学特征及其地质意义

Petrogenesis and Tectonic Significance of Laohugou Mafic Dykes in Haicheng,Liaoning Province:Evidence from Zircon U-Pb-Hf Isotopes and Whole-rock Geochemistry

胶-辽-吉活动带是华北板块重要的古元古代造山带,与南北两侧的狼林地块和龙岗地块通过断层接触,共同构成了东部陆块。造山带内分布有大量的变沉积岩系,并在多期岩浆-变质事件的改造下,经历了十分复杂的构造演化过程。其中,在辽宁海城—草河口地区分布着大量古元古代变质基性岩墙,分别为北东—南西走向的什司县基性岩墙群和北西—南东走向的老虎沟基性岩墙群。什司县基性岩墙与北辽河群中的浪子山岩组和里尔峪岩组呈互层状产出于胶-辽-吉带内,老虎沟基性岩墙高角度侵入到龙岗地块东南缘新太古宙片麻岩中。LA-ICPMS锆石U-Pb-Hf同位素测试结果显示老虎沟基性岩经历(1 864±15)Ma的变质作用,锆石Hf同位素一阶段模式年龄(T_(DM1))为2 040~2 293Ma,代表了老虎沟基性岩从地幔中抽取出来的时间。以上测试结果与辽阳地区什司县基性岩年代学记录和Hf同位素数据一致。主、微量元素地球化学分析结果亦表明,老虎沟基性岩墙与什司县基性岩墙的地化特征基本一致,均具有右倾型的稀土配分模式,选择性富集大离子亲石元素和亏损高场强元素,暗示两者形成于相似的构造环境。在Nb/Yb-Th/Yb图解中,老虎沟基性岩和什司县基性岩样品均具有Th的强烈富集,指示了俯冲作用的存在。但是老虎沟基性岩具有相对较高的Nb/Zr值,说明其源区经历过硅酸盐熔体的交代富集作用,而什司县基性岩低的Nb/Zr和变化较大的Th/Zr值指示其源区受到了流体的交代作用。综上分析,老虎沟基性岩和什司县基性岩形成于同一构造背景,二者源区分别受到了硅酸质熔浆和流体的交代作用,随后受1 930~1 850Ma的俯冲碰撞造山作用的影响发生了变质。

As one of important Paleo-proterozoic orogenic belts formed in North China craton,theJiao-Liao-Ji belt was deposited massive meta-sedimentary rocks,underwent a complex tectonic evolution process with the modification by multiple stages of magmatic and metamorphic events,and it merged with Langrim Block in south and with Longgang Block in north to form the Eastern Block.A swarm of Paleoproterozoic metamorphic mafic dykes was exposed in the Haicheng-Caohekou area,Liaoning Province,including NE-SW trend Shensixian mafic dykes swarm and NWSE trend Laohugou mafic dykes.Shensixian mafic dyke swarm was outcroped in the Jiao-Liao-Ji Belt,intercalated with the Langzishan and Lieryu Formations of the North Liaohe Group,while the Laohugou mafic dykes was intruded into the Neoarchean gneisses along the Southeastern margin of the Longgang block with high angle.LA-ICP MS zircon U-Pb dating and LA-MC-ICP MS Lu-Hf isotopic analyses on the sample from the Laohugou mafic dykes yielded a metamorphic age of(1 864±15)Ma,and TDM1(Hf)ages(Hf model ages)of 2 040~2 293 Ma,which are comparable with the record of the Shensixian mafic dyke swarm.Whole-rock major and trace element analyses display the comparability between the Shensixian and Laohugou dykes,both of them are characterized with the enrichment of the LREE and LILE and depletion of the HSFE,implying that the Shensixian and Laohugou mafic dykes were formed in the similar tectonic environment.In the Nb/Yb-Th/Yb diagram,the geochemical data of the Laohugou and Shensixian mafic rocks display Th enrichment,indicating a subduction-related metasomatism.But Laohugou mafic rocks have higher Nb/Zr ratios than the Shensixian mafic rocks,implying the metasomatismby silicate melt in the source region,while the low Nb/Zr ratios with variable Th/Zr ratios of the Shensixian mafic rocks indicate a fluid metasomatism in the source.In conclusion,Laohugou and Shensixian mafic dykes were formed in the same tectonic background,both of them were matasomatised by the silicate melts and fluids,respectively,which were metamorphosed during the subduction and collision event in 1 930~1 850 Ma.