阴山陆块南缘新太古代末岩浆弧的向西延伸:来自乌拉特中旗和乌拉特后旗地质年代学和地球化学的证据

Late Neoarchean magmatic arc extends westward in the southern of Yinshan Block:Evidence from geochronology and geochemistry of the Wulatezhongqi and Wulatehouqi area

阴山陆块出露了较为完整的新太古代末TTG(奥长花岗岩-花岗闪长岩-英云闪长岩)岩系、绿岩带和麻粒岩-紫苏花岗岩等高级地体,是研究华北克拉通这一时期构造背景的热点地区,尤以固阳-武川地区(基底出露最好)最为典型,但其它地区的研究则相对薄弱。本文对固阳-武川、乌拉特中旗和乌拉特后旗变质中酸性岩和斜长角闪岩进行了岩石学、地球化学、年代学及锆石Hf同位素的相关研究,对这三个地区新太古代末不同类型变质岩浆岩的成因及构造背景做出了初步的限定。固阳-武川和乌拉特中旗地区的变质中酸性岩形成时代为2491~2545Ma,变质时代为2485Ma。原始地幔标准化蛛网图中均表现出明显的Nb、Ta、Ti和HREE亏损以及Pb、Th和U的富集。在球粒陨石标准化稀土元素配分图中轻重稀土中等-强分异((La/Yb)_(N)=3.99~119.9),重稀土分异较小((Gd/Yb)_(N)=1.32~5.20)。锆石ε_(Hf)(t)主体在+0.9到+7.1范围内,t_(DM)~C主体在2.67Ga至3.03Ga范围内,表明变质中酸性岩石原岩的母岩浆主体来自中-新太古代古老地壳的再循环,也有少量新生地壳物质的加入。乌拉特后旗地区的斜长角闪岩形成时代为2495~2527Ma,原始地幔标准化蛛网图中均表现出明显Nb、Ta和Ti的亏损,Rb、Pb、Th和U的富集。在球粒陨石标准化稀土元素配分图中轻重稀土中等分异((La/Yb)_N=9.49~14.2),重稀土相对平坦((Gd/Yb)_(N)=1.81~2.19)。锆石ε_(Hf)(t)值为+1.7~+6.3,单阶段模式年龄为2.62~2.73Ga,指示其主要来自新太古代末亏损地幔。在总结前人阴山陆块南缘新太古代末岩浆岩的地球化学与同位素数据的基础上,我们认为这期岩浆事件是与俯冲相关的岩浆作用,其范围不仅局限于固阳-武川地区,可以向西南延伸350km至乌拉特后旗地区。

The Yinshan Block comprises relatively complete Late Neoarchean tonalite-trondhjemite-granodiorite(TTGs),greenstone belt rocks and granulite-charnockite associations,and is the key study area of the North China Craton during this period for the tectonic evolution.The Guyang and Wuchuan regions have been extensively studied due to their good basement exposures,but the researches in other area are relatively weak.In this paper,the petrologic,geochemical,geochronologic,and zircon Hf isotopic studies of the metagranitic rocks and amphibolites from the Guyan-Wuchuan,Wulatezhongqi and Wulatehouqi area have been carried out,and the petrogenetic and tectonic backgrounds of different types of metamorphic magmatic rocks of the Late Neoarchean in the three regions are preliminarily defined.The meta-granitic rocks in the Guyang-Wuchuan and Wulatezhongqi regions formed at 2491~2545 Ma and metamorphosed at 2485 Ma.Primitive mantle-normalized trace-element diagrams show that the meta-granitic samples are depleted in Nb,Ta,Ti and HREE,and are enriched in Pb,Th,and U.The chondrite-normalized REE patterns show that the differentiations of light and heavy rare earth elements are moderate to strong((La/Yb)N=3.99~119.9),and the differentiations of heavy REEs are small((Gd/Yb]N=1.32~5.20).The zircon Hf isotopic compositions(εHf(t)=0.9~7.1,tDMC=2.67~3.03 Ga)indicate that the parent magma of the meta-granitic rocks originates from the recycling of the Meso-to Neo-archean crust,with the addition of a small amount of juvenile addition.The formation ages of the amphibolites in the Wulatehouqi area are 2495~2527 Ma.Primitive mantlenormalized trace-element diagrams show obvious depletions of Nb,Ta and Ti,and enrichments of Rb,Pb,Th and U.The chondritenormalized REE patterns show that moderate differentiation of light and heavy REEs((La/Yb)_(N)=9.49~14.2)and relatively flat of heavy REEs((Gd/Yb)_(N)=1.81~2.19).The zircon Hf isotopic compositions(εHf(t)=1.7~6.3,tDM1=2.62~2.73 Ga)indicate that they originated mainly from the depleted mantle in the Neoarchean.On the basis of summarizing the geochemical and isotopic data of the Late Neoarchean magmatic rocks on the southern margin of the Yinshan Block,we suggest that the magmatic event is subductionrelated,whose scope is not limited to Guyang-Wuchuan region,but extends to the southwest about 350 km to the Wulatehouqi area.