CAJ | 학술논문

嵩山地区登封群是华北克拉通南部古老结晶基底的重要组成部分,由一套火山-沉积成因的表壳岩系组成,形成于新太古代.斜长角闪质岩石广泛发育于登封群表壳岩中,同时,也以包体形式普遍存在于TTG片麻岩体内部.二者主量元素差别不大,SiO_2含量为45%～63%,富Fe_2O_3、Al_2O_3、CaO,TiO_2(0.5%～1.11%)含量较低,原岩为亚碱性玄武岩、安山岩.二者的微量元素特征稍有差别,登封群斜长角闪岩REE配分形式平坦,轻重稀土基本无分异((La/Yb)_N=0.99～2.07),基本无Eu异常(δEu≈1);Ti负异常,Nb、Ta、Y负异常不明显,Ba、Sr呈现正异常,显示洋中脊和岛弧拉斑玄武岩特征;在Cr-Y、Ta/Yb-Th/Yb、Zr/Y-Nb/Y图解中位于洋中脊向岛弧玄武岩的过渡区域;ε_(Nd)(t)=4.43,显示源岩来自亏损地幔.而TTG片麻岩中斜长角闪岩包体的LREE富集,Eu负异常明显(δEu)=0.46～0.87);大离子亲石元素Rb、Cs、Ba明显高于登封群中的斜长角闪岩,除了Ti含量稍低外,Zr、Nb和Y含量范围和登封群斜长角闪岩相似,Nb、Ta和Y呈负异常,具有岛弧玄武岩特征;ε_(Nd)(t)=2.56和4.08,显示源岩来自亏损地幔,反映有地壳物质的混染.登封群斜长角闪岩及斜长角闪岩包体原岩的源区物质有所不同,在汇聚板块边缘洋壳俯冲条件下,前者是地幔楔部分熔融的产物,形成于弧后盆地环境;后者可能是随着俯冲作用的进行,小部分板片熔融开始发生,形成的熔浆混染亏损地幔部分熔融形成的熔浆.地球化学特征显示登封群形成的地球动力学背景是汇聚板块边缘洋壳的俯冲,反映当时陆壳以水平方式增生.
숭산지구등봉군시화북극랍통남부고로결정기저적중요조성부분,유일투화산-침적성인적표각암계조성,형성우신태고대.사장각섬질암석엄범발육우등봉군표각암중,동시,야이포체형식보편존재우TTG편마암체내부.이자주량원소차별불대,SiO_2함량위45%～63%,부Fe_2O_3、Al_2O_3、CaO,TiO_2(0.5%～1.11%)함량교저,원암위아감성현무암、안산암.이자적미량원소특정초유차별,등봉군사장각섬암REE배분형식평탄,경중희토기본무분이((La/Yb)_N=0.99～2.07),기본무Eu이상(δEu≈1);Ti부이상,Nb、Ta、Y부이상불명현,Ba、Sr정현정이상,현시양중척화도호랍반현무암특정;재Cr-Y、Ta/Yb-Th/Yb、Zr/Y-Nb/Y도해중위우양중척향도호현무암적과도구역;ε_(Nd)(t)=4.43,현시원암래자우손지만.이TTG편마암중사장각섬암포체적LREE부집,Eu부이상명현(δEu)=0.46～0.87);대리자친석원소Rb、Cs、Ba명현고우등봉군중적사장각섬암,제료Ti함량초저외,Zr、Nb화Y함량범위화등봉군사장각섬암상사,Nb、Ta화Y정부이상,구유도호현무암특정;ε_(Nd)(t)=2.56화4.08,현시원암래자우손지만,반영유지각물질적혼염.등봉군사장각섬암급사장각섬암포체원암적원구물질유소불동,재회취판괴변연양각부충조건하,전자시지만설부분용융적산물,형성우호후분지배경;후자가능시수착부충작용적진행,소부분판편용융개시발생,형성적용장혼염우손지만부분용융형성적용장.지구화학특정현시등봉군형성적지구동역학배경시회취판괴변연양각적부충,반영당시륙각이수평방식증생.
The Dengfeng Group in Songshan area is a part of the ancient crystalline basement in the southern part of the North China Craton.The Group mainly consists of metamorphosed volcanogenie and sedimentogenie supracrustal rocks,in which the amphibolite is a very common type.The amphibolite includes two types which occur as rock body in Dengfeng Group and as enclaves in TTG gneiss,respectively.Both types have similar major components,they are all rich in Fe_2 O_3,Al_2O_3 and CaO but poor in TiO_2(ranging from 0.50% to 1.11%),and have SiO_2 content ranging from 45% to 63%.These features indicate that they are orthometamorphite and are similar to sub-alkaline basalts and andesite.Nevertheless,the trace and rare earth element characteristics of amphibolites body in Dengfeng Group and amphibolites enclaves in TrG gneiss are different,the former has relatively fiat REE pattern((La/Yb)_N is from 0.99 to 2.07)with no Eu anomalies(δEu) is about 1)on the chondrite-normalized REE patterns,and exhibit negative Ti but positive Ba and Sr anomalies with slightly negative Nb,Ta and Y anomalies on the primitive-mantle normalized spidergrams,similar to MORB and island are tholeiite.The whole rock Nd isotopic compositions(ε_(Nd)(t)is 4.43)of the amphibolite body indicate that they may originate from a depleted mantle.In contrast,the enclaves in TTG gneiss have relatively higher HREE/HREE ratios with negative Eu anomalies(δEu) = 0.46 to 0.87).It has higher Rb,Cs and Ba contents but lower Ti content than the amphibolites body,and exhibits negative Nb,Ta and Y anomalies,similar to island arc basalts.The whole rock Nd isotopic compositions(ε_(Nd)(t)are 2.56 and 4.08)indicate that the enclaves may originate from a depleted mantle with the assimilation of older crust.We propose that the amphibolite body in Dengfeng Group may have formed by partial melting of mantle wedge in a subduction zone and erupted through the back-arc basin,whereas the enclaves in TTG geniss may have formed by partial melting of mantle wedge with assimilation of melts formed by slight partial melting of subducted oceanic slab.The geochemical characteristics of the both types of amphibolites indicate that the geodynamie background of Dengfeng Group is oceanic slab subducting in a convergent plate margin,reflecting that the continent crust in the Neo-Archean could grow in a parallel way.