约3800 Ma外弧群的地球动力学环境,伊苏阿(格陵兰)

IF 1.9 3区 地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY
A. Nutman, C. Friend, V. Bennett, M. V. Kranendonk, A. Chivas
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引用次数: 2

摘要

35公里长的弧形Isua表壳带(ISB,格陵兰岛西南部)包含了世界上最大的始新世火山和沉积序列遗迹。ISB大致分为:(i)3720至3690 Ma岩石的北部内弧群,和(ii)约3800 Ma岩石的南部外弧群,其北侧以高度构造化的约3750 Ma划分沉积单元为界。两组之间的边界是形成于3685至3660 Ma之间的糜棱岩。尽管存在普遍的高应变、角闪岩相变质作用和层平行位错,这些位错可以使一些单元变薄或完全切除,但外弧组1%(定性评估)的较低变形域包含残余沉积和火成岩结构。结合锆石U-Pb地质年代学和全岩地球化学,可以重建外弧群的岩性序列和地球动力学背景。外弧群的下部主要由镁铁质火山岩组的变玄武岩角闪岩组成,其中很少保存的枕状结构表明它们主要是水下喷发,也表明它们面向地层。它们喷发了3800 Ma以上,因为它们首先被3803±3 Ma的细粒浅成英云闪长岩次心线片侵入(Crowley,2003),然后被3795至3791 Ma的粗粒英云闪长岩-花岗闪长岩侵入,后者沿ISB南侧形成一个大的变形深成岩体。该地层之后是沉积地层,其基底由不连续的稀有薄紫红色石英岩组成,具有3890至3805 Ma的碎屑锆石。覆盖层由白云岩、泥灰岩和硅质岩组成。尽管它们受到变质作用和交代作用的广泛改造,产生了滑石或透闪石的广泛生长,但残留的分级沉积分层、化学和同位素特征表明原始沉积原岩。这些岩石中的碎屑锆石的年龄范围为约3820至3805 Ma。该单元显示出从具有明显的海水状微量元素特征的“纯”化学沉积岩向上过渡到越来越多地被长英质物质污染的岩性,长英质材料被局部保存为分级层,这被解释为火山成因输入的增加。沉积岩之后是长英质火山岩组,这是一个广泛的单元,主要由3807至3802 Ma的片状长英质钾质蚀变岩组成,具有富含碳酸盐的夹层和矿脉。局部保存的安山岩单元,具有分级分层、块状泡状熔岩、复相角砾岩、再吸收石英斑晶和fiammé,证明了火山和火山沉积原岩。对这些岩石及其锆石的全岩地球化学和氧同位素分析表明,长英质火山原岩主要在地表环境中经历了大规模蚀变,可能是在陆上喷发之后。块状火山岩在ISB西部最为常见,这表明该带的这一部分靠近火山中心。利用这些地层数据,我们得出结论,ISB火山岩和沉积岩是在类似板块构造的移动地球动力学机制中形成的,而不是在停滞的盖层机制中。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Geodynamic Environment of the ca. 3800 Ma Outer Arc Group, Isua (Greenland)
The arcuate, 35 km long Isua supracrustal belt (ISB, southern West Greenland) contains the world's largest remnants of Eoarchean volcanic and sedimentary sequences. The ISB is broadly divided into: (i) the northern Inner Arc Group of 3720 to 3690 Ma rocks, and (ii) the southern Outer Arc Group of ca. 3800 Ma rocks which is bounded on its northern side by the highly tectonized ca. 3750 Ma Dividing Sedimentary Unit. The boundary between the two groups is a mylonite formed between 3685 and 3660 Ma. Despite the generally high strain, amphibolite facies metamorphism and layer-parallel dislocations that can thin or altogether excise some units, domains of lower deformation comprising ≪1% (qualitative assessment) of the Outer Arc Group contain relict sedimentary and igneous structures. Combined with zircon U-Pb geochronology and whole rock geochemistry, this enables the Outer Arc Group lithological sequence and geodynamic setting to be reconstructed. The lower part of the Outer Arc Group is dominated by metabasaltic amphibolites of the Mafic Volcanic formation in which rarely-preserved pillow structures indicate both their predominantly subaqueous eruption and also their stratigraphic facing. They erupted >3800 Ma, because they were first intruded by subconcordant sheets of fine-grained hypabyssal tonalite dated at 3803±3 Ma (Crowley, 2003) and then by coarser-grained 3795 to 3791 Ma tonalite-granodiorite, which forms a large deformed pluton along the south side of the ISB. This formation is succeeded by the Sedimentary formation whose base consists of discontinuous rare, thin fuchsitic quartzites with 3890 to 3805 Ma detrital zircons. Overlying is a diverse package of dolostones, marls and siliceous rocks. Although they are extensively modified by metamorphism and metasomatism, producing widespread growth of talc or tremolite, relict graded sedimentary layering, chemical and isotopic signatures indicate originally sedimentary protoliths. Detrital zircons in these rocks range in age from ca. 3820 to 3805 Ma. This unit shows an upwards transition from ‘pure' chemical sedimentary rocks with distinct seawater-like trace element signatures into lithologies increasingly contaminated by felsic material that is locally preserved as graded layers, which are interpreted as an increasing volcanogenic input. Succeeding the sedimentary rocks is the Felsic Volcanic formation, an extensive unit of mostly schistose 3807 to 3802 Ma felsic potassic-altered rocks with carbonate-rich interludes and veins. Locally-preserved andesitic units with graded layering, massive vesicular lavas, polymict breccias, resorbed quartz phenocrysts and fiammé, attest to volcanic and volcano-sedimentary protoliths. Whole rock geochemistry and oxygen isotope analyses on these rocks and their zircons indicate predominantly felsic volcanic protoliths that experienced massive alteration in a surficial environment, probably following subaerial eruption. Massive volcanic rocks are commonest in the west of the ISB, suggesting this part of the belt was proximal to a volcanic center. Using these stratigraphic data, we conclude that the ISB volcanic and sedimentary rocks formed in a mobile geodynamic regime resembling plate tectonics, and not within a stagnant lid regime.
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来源期刊
American Journal of Science
American Journal of Science 地学-地球科学综合
CiteScore
5.80
自引率
3.40%
发文量
17
审稿时长
>12 weeks
期刊介绍: The American Journal of Science (AJS), founded in 1818 by Benjamin Silliman, is the oldest scientific journal in the United States that has been published continuously. The Journal is devoted to geology and related sciences and publishes articles from around the world presenting results of major research from all earth sciences. Readers are primarily earth scientists in academia and government institutions.
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