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

正确识别和表征大洋岩石圈是理解太古宙地壳和地幔演化的关键。 PI在华北地区发现并报道了一个完整的太古宙蛇绿岩序列，尽管有些肢解和变质。 蛇绿岩系的顶部以富含硫化物的硅质岩和条带状铁质岩为标志，其上覆盖着几十到一百米长的变形枕状熔岩。 这些等级通过混合岩墙/枕状熔岩段下降到2 km厚的I 00%席状岩墙复合体，沿着走向连续绘制超过5 km;勘测绘图表明，岩墙复合体可能延伸超过20 km。 脉岩由闪长岩、玄武岩和hb-pyx-辉长岩组成。 大多数有冷缘发展的东北侧，但不是他们的西南侧，表明单向冷却。 席状岩墙复合体的下面是几公里的混合各向同性和叶理辉长岩，其中发展成分分层约2公里以下的席状岩墙，然后超过几百米合并成强烈的成分层状辉长岩和橄榄辉长岩。 层状辉长岩与层状辉石岩/辉长岩混合，形成一个过渡带，过渡带由纯橄榄岩、辉石岩和韦尔岩组成的堆晶超镁铁质岩，过渡到强烈变形和蛇纹化的橄榄石和斜方辉石超镁铁质岩，可能是亏损的地幔方辉橄榄岩构造岩。 PI获得东湾子蛇绿岩辉长岩锆石U/Pb年龄为2.505Ga，是世界上公认的最古老的、侧向广布的完整蛇绿岩序列。 对这一引人注目的蛇绿岩的研究可能提供对太古代洋壳和地幔性质的最佳限制，并提供对太古代板块构造风格和全球热损失机制的见解。 东湾子蛇绿岩是中央造山带遵化构造带中保存最完整的绿岩带之一，它将华北克拉通分为东、西两个地块。 在遵化构造带中，还有1000多个辉长岩、枕状熔岩、席状岩脉、斜长角闪岩和含豆荚状铬铁矿纯橄榄岩的其他碎块作为构造块体出现在黄长岩-片麻岩基质中。 PI将这些岩石解释为太古代蛇绿混杂岩，并认识到一些块体比东湾子蛇绿岩保留了更深层次的海洋地幔。 PI及其同事设计了一系列全面的研究，以充分记录该蛇绿岩和相关混杂岩的野外、结构、年代学、矿物学和化学特征。 他们将评估这些数据在太古代地壳和地幔演化、早期地球的热状态以及关于前寒武纪板块构造的想法方面的意义。 他们认为这是一个无与伦比的机会，以评估太古代地壳和地幔的基本属性，并了解评估太古代海洋地壳和地幔演化的关键过程。 了解华北克拉通的太古代海洋过程将提供一个有价值的对比与类似的过程记录在年轻的蛇绿岩，表明如何洋中脊过程可能已经从一个时期的高热量生产的低热量生产。 详细的测绘将准确地描绘出各个单元的范围、厚度和关系，这对于了解太古代海底扩张和热损失的机制非常重要。 地幔构造岩的结构分析将揭示太古代洋壳地幔的变形条件。 将使用U/Pb地质年代学确定蛇绿岩中所代表的岩浆活动的年龄和持续时间，并通过测定在不同事件之间侵入的火成岩脉和岩体的年龄来记录构造年代学，如制图所确定的。 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的年轻蛇绿岩以及太古代绿岩带的数据进行比较，其中一些可能包括严重肢解的太古代蛇绿岩碎片。 分析将包括主要的，次要的，痕量，稀土元素和几个同位素系统（与几个合作机构的地球化学家合作），并将用于评估地壳和地幔的演变。 太古代海洋岩石圈结构很可能和现今板块镶嵌体一样复杂多变。 东湾子蛇绿岩及其相关岩石的特征可作为太古代海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。

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.