Investigation of Archean Ophiolites and Oceanic Crust and Mantle Fragments in Melange, North China Craton: Implications for Archean Tectonics

华北克拉通混杂岩太古代蛇绿岩和大洋壳幔碎片研究：对太古代构造的启示

• 批准号: 0207886
• 负责人: Timothy Kusky
• 金额: $ 9.99万
• 依托单位: Saint Louis University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2002
• 资助国家: 美国
• 起止时间: 2002-06-01 至 2006-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0207886&HistoricalAwards=false
• 关键词: Investigation; Archean; Ophiolites; Oceanic; Crust

Understanding Archean crustal and mantle evolution hinges upon proper identification and characterization of oceanic lithosphere. The PI has discovered and reported a complete, albeit slightly dismembered and metamorphosed, Archean ophiolite sequence in the North China Craton. The top of the ophiolitic succession is marked by sulfide-rich chert and banded iron fon-nation overlying several tens-to one hundred meters of variably deformed pillow lavas. These grade down through a mixed dike/pillow lava section into a 2 km thick I 00% sheeted dike complex mapped continuously for more than 5 km along strike; reconnaissance mapping suggests that the dike complex may extend for more than 20 km. The dikes consist of diabase, basalt and hb-pyx-gabbro. Most have chilled margins developed on their NE sides, but not their SW sides, indicating one-way chilling. The sheeted dike complex is underlain by several km of mixed isotropic and foliated gabbro, which develop compositional layering approximately 2 km below the sheeted dikes, and then over several hundred meters merge into strongly compositionally layered gabbro and olivine-gabbro. The layered gabbro becomes mixed with layered pyroxenite/gabbro marking a transition zone that grades into cumulate ultramafic rocks including dunite, pyroxenite and wehrlite, and finally into strongly defon-fied and serpentinized olivine and orthopyroxene-bearing ultramafic rocks that may be depleted mantle harzburgite tectonites. The PI has obtained a U/Pb zircon age of 2.505 Ga from gabbro of the Dongwanzi ophiolite, making it the world s oldest recognized, laterally- extensive complete ophiolite sequence. Study of this remarkable ophiolite may offer the best constraints yet on the nature of the Archean oceanic crust and mantle, and offer insights to the style of Archean plate tectonics and global heat loss mechanisms. The Dongwanzi ophiolite is one of the largest well-preserved greenstone belts in the Zunhua structural belt (part of the Central orogenic belt),that divides the North China craton into eastern and western blocks. More than I 000 other fragments of gabbro, pillow lava, sheeted dikes, harzburgite, and podifonn-chromite bearing dunite occur as tectonic blocks in a blotite- gneiss matrix in the Zunhua structural belt. The PI interprets these rocks as an Archean ophiolitic melange and recognize that some of the blocks preserve deeper levels of oceanic mantle than the Dongwanzi ophiolite. The PI and co-workers have designed a comprehensive series of studies to fully document the field, structural, geochronological, mineralogical, and chemical characteristics of this ophiolite and related melange. They will assess what these data mean in ten-ns of Archean crustal and mantle evolution, thermal state of the early Earth, and ideas about Precambrian plate tectonics. They regard this as an unsurpassed opportunity to evaluate fundamental properties of Archean crust and mantle, and to understand processes critical for evaluating Archean oceanic crustal and mantle evolution. Understanding Archean oceanic processes in the North China craton will provide a valuable contrast with similar processes recorded in younger ophiolites, indicating how mid-ocean ridge processes may have evolved from a period of high heat production to one of lower heat production. Detailed mapping will accurately delineate the extent, thickness, and relationships between individual units, which is important for understanding mechanisms of Archean sea floor spreading and heat loss. Structural analysis of mantle tectonites will reveal deformation conditions in the Archean oceanic mantle. U/Pb geochronology will be used to establish the age and duration of magmatism represented in the ophiolite and document the structural chronology by dating igneous dikes and plutons that intruded between different events, as deten-nined by mapping. Geochemical analyses will be aimed at determining relationships between the crustal and depleted mantle components of the ophiolite, and comparing these data with younger ophiolites from different tectonic settings, and also from Archean greenstone belts, some of which may include severely dismembered Archean ophiolite fragments. Analyses will include major, minor, trace, REE, and several isotopic systems (in collaboration with geochemists from several cooperating institutions)and will be used to assess crustal and mantle evolution. Archean oceanic lithospheric structure is likely to have been as complex and variable as that in the present-day plate mosaic. Documentation of the characteristics of the Dongwanzi ophiolite and related rocks may serve as the first reference column for Archean oceanic lithosphere, since it is the most complete Archean ophiolite known on the planet.

了解太古宙地壳和地幔演化取决于对海洋岩石圈的正确识别和表征。 PI 在华北克拉通发现并报告了一个完整的太古代蛇绿岩序列，尽管略有分解和变质。 蛇绿岩层序的顶部以富含硫化物的燧石和带状铁矿为标志，上面覆盖着几十到一百米的不同变形的枕状熔岩。 这些坡度穿过混合堤坝/枕状熔岩部分，进入 2 公里厚的 I 00% 片状堤坝复合体，沿走向连续绘制超过 5 公里；勘测显示，堤坝群可能延伸超过20公里。 岩脉由辉绿岩、玄武岩和辉长岩组成。 大多数在其东北侧有冷却边缘，但在西南侧没有，表明单向冷却。 片状岩脉复合体的下方是数公里的混合各向同性和叶状辉长岩，在片状岩脉下方约 2 公里处形成成分层，然后在数百米以上合并成强成分层状辉长岩和橄榄石辉长岩。 层状辉长岩与层状辉石岩/辉长岩混合，标志着一个过渡带，该过渡带分级为堆积超镁铁质岩石，包括纯橄榄岩、辉石岩和辉长岩，最后形成强烈变形和蛇纹石化的橄榄石和含斜方辉石的超镁铁质岩石，这些岩石可能是耗尽的地幔方辉橄榄岩构造岩。 PI从东湾子蛇绿岩辉长岩中获得了2.505 Ga的U/Pb锆石年龄，使其成为世界上公认的最古老的、横向广泛的完整蛇绿岩序列。 对这种非凡的蛇绿岩的研究可能为太古代洋壳和地幔的性质提供迄今为止最好的限制，并为太古代板块构造的风格和全球热量损失机制提供见解。 东湾子蛇绿岩是遵化构造带（中央造山带的一部分）中保存最完好的最大绿岩带之一，将华北克拉通分为东西两块。 超过 000 个其他辉长岩、枕状熔岩、片状岩脉、方辉橄榄岩和含纯铬铁矿的碎片以构造块的形式出现在遵化构造带的黑斑岩-片麻岩基质中。 PI 将这些岩石解释为太古代蛇绿混杂岩，并认识到其中一些区块比东湾子蛇绿岩保存了更深层次的大洋地幔。 项目负责人和同事设计了一系列全面的研究，以全面记录这种蛇绿岩和相关混杂岩的现场、结构、地质年代学、矿物学和化学特征。 他们将评估这些数据对于十纳秒的太古代地壳和地幔演化、早期地球的热状态以及有关前寒武纪板块构造的想法意味着什么。 他们认为这是一个无与伦比的机会来评估太古代地壳和地幔的基本特性，并了解对于评估太古代海洋地壳和地幔演化至关重要的过程。 了解华北克拉通的太古宙海洋过程将与较年轻的蛇绿岩中记录的类似过程进行有价值的对比，表明洋中脊过程可能如何从高产热期演化为低产热期。 详细的绘图将准确地描绘出各个单元之间的范围、厚度和关系，这对于理解太古代海底扩张和热量损失的机制非常重要。 地幔构造岩的结构分析将揭示太古宙大洋地幔的变形条件。 U/Pb地质年代学将用于确定蛇绿岩中代表的岩浆作用的年龄和持续时间，并通过测绘确定不同事件之间侵入的火成岩岩脉和岩体的年代来记录结构年代学。 地球化学分析的目的是确定蛇绿岩的地壳和贫化地幔成分之间的关​​系，并将这些数据与来自不同构造环境以及太古代绿岩带的较年轻的蛇绿岩进行比较，其中一些可能包括严重肢解的太古代蛇绿岩碎片。 分析将包括主要、次要、痕量、稀土元素和几个同位素系统（与多个合作机构的地球化学家合作），并将用于评估地壳和地幔演化。 太古宙海洋岩石圈结构可能与当今板块镶嵌结构一样复杂多变。 洞湾子蛇绿岩及相关岩石特征的记录可以作为太古宙大洋岩石圈的第一个参考柱，因为它是地球上已知的最完整的太古代蛇绿岩。