http://dx.doi.org/10.1016/j.gsf.2016.10.005

The Earth was born as a dry planet without atmosphere and ocean components at 4.56 Ga, with subsequent secondary accretion of bio-elements, such as carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) which peaked at 4.37–4.20 Ga. This two-step formation model of the Earth we refer to as the advent of bio-elements model (ABEL Model) and the event of the advent of bio-elements (water component) as ABEL Bombardment. It is clear that the solid Earth originated from enstatite chondrite-like dry material based on the similarity in oxygen isotopic composition and among other isotopes. On the other hand, Earth's water derives primarily from carbonaceous chondrite material based on the hydrogen isotopic ratio. We present our ABEL model to explain this enigma between solid Earth and water, as well as secondary accretion of oxidizing bio-elements, which became a precursor to initiate metabolism to emerge life on a highly reductive planet. If ABEL Bombardment had not occurred, life never would have emerged on the Earth. Therefore, ABEL Bombardment is one of the most important events for this planet to evolve into a habitable planet. The chronology of ABEL Bombardment is informed through previous researches of the late heavy bombardment and the late veneer model. ABEL Bombardment is considered to have occurred during 4.37–4.20 Ga, which is the concept to redefine the standard late heavy bombardment and the late veneer models. Also, ABEL Bombardment is the trigger of the transition from stagnant lid tectonics to plate tectonics on this planet because of the injection of volatiles into the initial dry Earth.     

http://dx.doi.org/10.1016/j.gsf.2016.07.005

The Hadean history of Earth is shrouded in mystery and it is considered that the planet was born dry with no water or atmosphere. The Earth-Moon system had many features in common during the birth stage. Solidification of the dry magma ocean at 4.53 Ga generated primordial continents with komatiite. We speculate that the upper crust was composed of fractionated gabbros and the middle felsic crust by anorthosite at ca. 21 km depth boundary, underlain by meta-anorthosite (grossular + kyanite + quartz) down to 50–60 km in depth. The thickness of the mafic KREEP basalt in the lower crust, separating it from the underlying upper mantle is not well-constrained and might have been up to ca. 100–200 km depending on the degree of fractionation and gravitational stability versus surrounding mantle density. The primordial continents must have been composed of the final residue of dry magma ocean and enriched in several critical elements including Ca, Mg, Fe, Mn, P, K, and Cl which were exposed on the surface of the dry Earth. Around 190 million years after the solidification of the magma ocean, “ABEL bombardment” delivered volatiles including H₂O, CO₂, N₂ as well as silicate components through the addition of icy asteroids. This event continued for 200 Myr with subordinate bombardments until 3.9 Ga, preparing the Earth for the prebiotic chemical evolution and as the cradle of first life. Due to vigorous convection arising from high mantle potential temperatures, the primordial continents disintegrated and were dragged down to the deep mantle, marking the onset of Hadean plate tectonics.     

Tectonic activity of the early Earth (4.56-3.4(2.7?) Ga)

The paper discusses a possible model of the ancient (Hadean-Archean) Earth’s geodynamic evolution. We believe that the early Earth was characterized by a stagnant lid regime and whole-mantle convection suggesting cells that convect through the whole mantle (from the core-mantle boundary to the lithosphere base). The lid tectonics was perturbed by asteroid-meteorite bombardments that destroyed the primary terrestrial partly granitoid crust. The destroyed crust together with the residual enriched mantle reservoirs sank into the lower mantle. In addition to the crust destruction, the bombardments led to emplacement of a huge proportion of basalt-komatiitic melts, which can be interpreted as mantle overturn events. In the Hadean, the Earth survived frequent large-scale asteroid-meteorite bombardments, which resulted in almost a complete destruction of the primary terrestrial crust. In the Early Archean, the Earth still experienced the same tectonic processes, as in the Hadean; however, meteorite impact was small-scale and the bombardments influenced only a limited area of a common, as it seems to us, subequatorial supercontinent. Those bombardments led to the sagduction of the Archean basalt-komatiiic terrestrial crust, which sank into the mantle, transforming into amphibolite-eclogite rocks giving rise to a tonalite-troondhjemite-granodiorite suite. As preserved in the zircon record, the formation of the Archean mantle-derived magmas occurred as pulses at 4.5, 4.2-4.3, 3.8-3.9, and 3.3-3.4 Ga. These peaks, most likely, correspond to the Hadean-Archean meteorite bombardments. There is evidence of formation of the subcontinental lithospheric mantle (SCLM) beneath the cratons between 3.3 and 3.5 Ga. This SCLM was markedly different from peridotites of modern ophiolites. However, the existence of ophiolitic peridotites indicates that modern style plate tectonic processes were in operation at that time, as we will discuss below. The transition from the early Earth (Hadean-Archean) tectonic style to the recent tectonics occurred between 3.4 (2.7?) and 2.0 Ga.     

华北古陆的形成与构造演化史

以华北古陆为例，论述了地球演化史中经历的三大阶段：（１）古陆的形成阶段（４６００～１８００Ｍａ）：地球形成早期，以地幔对流为主导作用，到早太古宙出现初始古陆核，地幔对流驱动的地体拼贴和板底垫托是陆壳形成的主要方式；中太古宙开始出现一定规模的坳陷盆地，发育了基性火山岩 碎屑岩 镁质碳酸盐岩等表壳岩，同时伴随着大量中基性、花岗质岩浆活动；晚太古宙和早元古宙是陆壳形成的主要时期，并已具现今板块活动特征。地幔热柱与板块构造共同控制着地壳运动。（２）古陆稳定发展阶段（１８００～２５０Ｍａ）：地幔热柱活动较弱，古陆主要表现为缓慢的升降运动（造陆运动）。（３）地球新活动时期（２５０Ｍａ至今）：地幔热柱活动进入一个新的活跃时期。岩石圈发生明显的热减薄，地幔热柱表现为多级演化，并导致盆岭系的形成。     

地质历史中板块构造启动时间

地质历史中板块构造是何时开始启动的长期存在着激烈的争论,最极端的一是认为板块构造在新元古代的800 Ma前开始,二是在冥古宙4.3 Ga就已启动,多数学者认为在太古宙末开始启动。确定板块构造启动时间主要依据以下几方面:(1)地球动力学特点,如地幔的热状态以及粘塑性地幔对流模拟表明,板块构造可能是在地球热和冷停滞状态之间演化的一个相。在太古宙较热的地球中,板片强度低,板片的频繁断离阻止了形成类似现代样式的长期俯冲体系,太古宙的板块构造是短期的、阵发性的;(2)代表俯冲的标志的蛇绿岩、蓝片岩和超高压(UHP)变质地体;(3)具有弧特征的岩石组合,如拉斑玄武岩-安山岩-英安岩-流纹岩及英云闪长岩-奥长花岗岩-花岗闪长岩(TTG)岩套;(4)增生楔中混杂岩和大洋板块地层、前陆盆地、大陆裂谷、双变质带、造山带;(5)与俯冲带关系密切的造山型Au矿、斑岩Cu矿和浅成热液矿床、火山岩型块状硫化物矿床(VHMS),它们最早出现的年龄一致在3.5~3.1 Ga,指示了板块构造的开始;(6)世界不同地区大陆的Ni/Co、Cr/Zn比值随沉积年龄变年轻而降低,陆壳从3.0 Ga前的镁铁质转变为2.5 Ga时的长英质,表明全球板块构造的启动应在3.0 Ga的古中太古代;(7)冥古宙锆石、太古宙金刚石中矿物包裹体及Hf、O、C、N同位素组成研究表明,冥古宙地球表面存在类似板块汇聚边缘,太古宙含有大陆沉积物的海洋岩石圈俯冲进入地幔。     

Extraterrestrial factors and their role in the Earth’s tectonic evolution in the Early Precambrian

The paper is focused on the fundamental problem of influence of extraterrestrial factors on the Earth’s geologic and tectonic evolution. Extraterrestrial factors played a decisive role in the Earth’s genesis, the formation of the first Hadean continental crust, and the beginning of the Archean era. Their significant influence persisted in the later epochs: Even in the Phanerozoic, extraterrestrial factors might have had a considerable influence on the environment. The sialic cores of protocontinental crust (4.4-3.9 Ga) with first-generation greenstone zones (3.8-3.2 Ga) and the global system of granite-greenstone belts (3.1-2.7 Ga) formed in the rotation-plume regime, mainly in the subequatorial hot belt. The formation of these global structures was, to a large extent, influenced by asteroid impacts, which caused the impact-triggered genesis of mantle plumes. Dramatic changes in the subsequent geologic history began at 2.7-2.0 Ga; at 2.0 Ga they terminated with the Moon’s transition to an orbit similar to the present-day one (50 ± 3 Earth’s radii), accompanied by the abrupt slowdown of the Earth’s axial rotation, the termination of formation of the layer D", and the start of recent plate tectonics, which is accompanied by the plume tectonics.     

The geological history and geodynamics of the Earth

The Earth formed through a hot accretion process. Almost simultaneously, the core and the mantle were separated from each other. At the final stages of the accretion process, the outer layer approximately 2000 km thick was molten, thus representing a magma ocean. This magma ocean produced the primary crust of the Earth. Surface waters were precipitated from the atmosphere and released from the crystallizing magma ocean. The plate tectonic processes started at around 4.3 to 4 Ga BP. In the Archean, the overall tectonic mechanism was quite specific, due to substantially higher mantle temperature and thicker oceanic crust. The normal plate tectonics acted during the Proterozoic and Phanerozoic with the periodic assembly of continents, which are known as supercontinent cycles.     

再论“关于板块运动启动时间的争论”

笔者在"关于板块运动启动时间的争论"一文（陆松年等,2016）基础上,参阅公开发表的部分文章,介绍了前板块岩浆洋和停滞盖研究的新进展,并再次讨论有关板块运动启动时间的不同认识和观点。根据太阳系30个类地行星天体图像资料,硅酸盐质行星体在它的生命期由于冷却和岩石圈增厚似乎经历过多种构造样式,包括岩浆洋、多类型停滞盖以及板块构造。硅酸盐类行星由于增生、分异、撞击和放射性活动,使星体变热,发生熔融反应而形成"岩浆洋"。硅酸盐星球体早期很可能是大面积但寿命很短的岩浆洋,之后的星球可能受两种构造模型控制：停滞盖构造和板块构造。已有资料表明,停滞盖构造是类地行星的主要构造类型,而板块构造在类地行星中是十分罕见的。停滞盖构造是一个单一的、围绕天体的板块,基本由硅酸盐质岩石圈组成。停滞盖构造有三种主要变化形态——热管、滴状及柱状体、拆沉及上涌流。板块构造启动时间从 > 4.4Ga到0.85Ga至少有11种不同观点：认为板块构造始于冥古宙的有1种、始于始-古太古代的有3种、始于中-新太古代的有4种、始于古元古代有1种、始于新元古代的有2种。本文重点介绍了地球冥古宙-太古宙板块运动启动的地质学证据和特点。     

超越板块构造——我国构造地质学要做些什么?

对近十年来全球构造学和构造地质学的重要进展进行了简要评述.30年前建立的全球构造理论改变了人们对地球及其演化的认识.作为固体地球统一理论的板块构造主要涉及刚性板块边界之间的变形、地震活动和火山作用.至今还没有完整理论阐明板块运动的驱动力和地幔对流机制.板块边界和板内变形等许多问题仍然无法回答.大陆岩石圈和大洋岩石圈在成分、厚度和力学强度方面有明显的差别, 因此现有板块构造不完全适合于大陆构造.大陆地壳和地幔流变学的综合研究是认识大陆构造和超越板块构造的最佳途径.流变学是大陆造山带几何学和动力学的桥梁.大陆岩石圈对构造作用、重力作用和热作用的响应在很大程度上取决于其流变强度.岩石圈流变性质是岩石圈分层和塑性流动的主导因素.大量透入性变形和巨型大陆造山带内部构造显示非刚性特征.大陆构造和力学行为主要由地壳强度而不是地幔强度所控制.从大陆岩石圈多层性和力学强度不均匀性表征看, 现在是抛弃传统“三明治”构造模式的时候了.面对地球系统科学和地球动力学新思维发展趋势, 多学科综合研究大陆构造(造山带)和加速高水平构造地质学人才的培养是我国构造地质学发展的最紧迫任务      

Magmatism and metallogeny of the Early Earth as a reflection of its geologic evolution

The paper is focused on the evolution of the Earth starting with the planetary accretion and differentiation of the primordial material (similar in composition to CI chondrites) into the core and mantle and the formation of the Moon as a result of the impact of the Earth with a smaller cosmic body. The features of the Hadean eon (ca. 4500–4000 Ma) are described in detail. Frequent meteorite-asteroid bombardments which the Earth experienced in the Hadean could have caused the generation of mafic/ultramafic primary magmas. These magmas also differentiated to produce some granitic magmas, from which zircons crystallized. The repeated meteorite bombardments destroyed the protocrust, which submerged into the mantle to remelt, leaving refractory zircons, indicators of the Early Earth’s geologic conditions, behind.The mantle convection that started in the Archean could possibly be responsible for the Earth’s subsequent endogenous evolution. Long-living deep-seated mantle plumes could have promoted the generation of basalt-komatiitic crust, which, thickening, could have submerged into the mantle as a result of sagduction, where it remelted. Partial melting of the thick crust, leaving eclogite as a residue, could have yielded tonalite-trondhjemite-granodiorite (TTG) melts. TTG rocks are believed to compose the Earth’s protocrust. Banded iron bodies, the only mineral deposits of that time, were produced in the oceans that covered the Earth.This environment, recognized as LID tectonics combined with plume tectonics, probably existed on the Earth prior to the transitional period, which was marked by a series of new geologic processes and led to a modern-style tectonics, involving plate tectonics and plume tectonics mechanisms, by 2 Ga. The transitional period was likely to be initiated at about 3.4 Ga, with the segregation of outer and inner cores, which terminated by 3.1 Ga. Other rocks series (calc-alkaline volcanic and intrusive) rather than TTGs were produced at that time. Beginning from 3.4-3.3 Ga, mineral deposits became more diverse; noble and siderophile metal occurrences were predominant among ore deposits. Carbonatites, hosting rare-metal mineralization, could have formed only by 2.0 Ga. From 3.1 to 2.7 Ga, there was a period of “small-plate” tectonics and first subduction and spreading processes, which resulted in the first supercontinent by 2.7 Ga. Its amalgamation indicates the start of superplume-supercontinent cycles.Between 2.7 and 2.0 Ga, the D″ layer formed at the core-mantle interface. It became a kind of thermal regulator for the ascending already tholeiitic mantle plume magmas. All deep-seated layers of the Earth and large low-velocity shear provinces, called mantle hot fields, partially melted enriched EM-I and EM-II mantles, and the depleted recent asthenosphere mantle, which is parental for midocean-ridge basalts, were finally generated by 2 Ga. Therefore, an interaction of all Earth’s layers began from that time.     

西准噶尔蛇绿岩:古大洋俯冲增生过程的记录

蛇绿岩记录了大洋岩石圈形成、演化、消亡的全过程,是刻画区域板块构造和洋 陆格局演化的关键证据。本文通过系统梳理前人相关研究,总结西准噶尔蛇绿岩最新研究成果,探讨大陆地壳增生方式、恢复古大洋演化历史,从而对西准噶尔构造体制转化提供新制约。西准噶尔地区发育多条震旦纪—石炭纪被构造肢解的蛇绿岩带,具有典型的岩块 基质结构,绝大多数蛇绿岩包括正常洋壳组分和海山/大洋高原残片,其中基性岩具有MORB和OIB的地球化学特征。基于前人研究,本文认为在西准噶尔古大洋发育过程中,发育不同时代与地幔柱有关的海山/大洋高原,同时存在增生型和侵蚀型两类汇聚板块边界。另外,大洋高原增生不仅是大陆地壳增生的有效途径之一,还可能诱发俯冲极性反转和传递。而在大洋高原形成初期,还可能存在地幔柱诱发俯冲起始机制。     

Speculations on evolution of the terrestrial lithosphere–asthenosphere system—Plumes and plates

In the early Earth, accretionary impact heating, including collision with a large, Mars-sized object, decay of short-lived radioisotopes, and (after an initial thermal run-up) continuous segregation of the liquid Fe–Ni core resulted in extensive the melting of the silicate mantle and in the formation of a near-surface magma mush ocean. Progressive, continuous degassing and chemical–gravitational differentiation of the crust–mantle system accompanied this Hadean stage, and has gradually lessened during the subsequent cooling of the planet. Mantle and core overturn was vigorous in the Hadean Earth, reflecting deep-seated chemical heterogeneities and concentrations of primordial heat. Hot, bottom-up mantle convection, including voluminous plume ascent, efficiently rid the planet of much thermal energy, but gradually decreased in importance with the passage of time. Formation of lithospheric scum began when planetary surface temperatures fell below those of basalt and peridotite solidi. Thickening and broadening of lithospheric plates are inferred from the post-Hadean rock record. Developmental stages of mantle circulation included: (a) 4.5–4.4 Ga, early, chaotic magma ocean circulation involving an incipient or pre-plate regime; (b) 4.4–2.7 Ga, growth of small micro-oceanic and microcontinental platelets, all returned to the mantle prior to 4.0 Ga, but increasing in size and progressively suturing sialic crust-capped lithospheric amalgams at and near the surface over time; (c) 2.7–1.0 Ga, assembly of cratons surmounting larger, supercontinental plates; and (d) 1.0 Ga–present, modern, laminar-flowing asthenospheric cells capped by gigantic, Wilson-cycle lithospheric plates. Restriction of komatiitic lavas to the Archean, and of ophiolite complexes ± alkaline igneous rocks, high-pressure and ultrahigh-pressure metamorphic terranes to progressively younger Proterozoic–Phanerozoic orogenic belts supports the idea that planetary thermal relaxation promoted the increasingly negative buoyancy of cooler oceanic lithosphere. The Thickening of oceanic plates enhanced the gravitational instability and the consequent overturn of the outer Earth as cold, top-down oceanic mantle convection. The scales and dynamics of deep-seated asthenospheric circulation, and of lithospheric foundering + shallow asthenospheric return flow evidently have evolved gradually over geologic time in response to the progressive cooling of the Earth.     

克拉通、下地壳与大陆岩石圈——庆贺沈其韩先生百年华诞

把克拉通、下地壳和大陆岩石圈这几个重要的地质名词放在一起做文章的标题,其实只是想强调一个事情,即陆壳形成和稳定化的结果是形成大陆岩石圈。大陆岩石圈是地球圈层的基本单元,是现代板块构造运动的核心构件和核心载体。忽视大陆岩石圈,要讨论地球上大陆与大洋、地壳与地幔、地球的深部圈层与外部圈层、内部圈层间相互作用以及物质与能量的交换等问题都无法深入。大陆岩石的最初形成可能在冥古宙早期已经开始,全球大陆的稳定化即克拉通化一般认为完成于太古宙末,并开启了元古宙的新纪元。对大陆壳的形成机制和演化过程,已经有很多讨论,但是还存在许多争议。而对大陆岩石圈人们知之不多,文献中常常在早期的小的陆壳形成,即陆核或微陆块阶段,就将其与大陆岩石圈的概念混为一谈。岩石圈的形成固然是大陆形成演化的结果,但是岩石圈的物质组成、结构和物理性质等从最初形成到成熟并成为稳定的地球独立圈层并足以承担板块构造的重任,可能经历了不止一个阶段。本文强调应对岩石圈的形成和演化给予更多的关注,并深化研究大陆地壳形成、克拉通化对大陆岩石圈形成的贡献。

     

早期大陆与板块构造启动——前沿热点介绍与展望

板块构造的启动时间和机制,一直是国内外地球科学界关注的焦点,有不少热点文章对此进行了讨论。它涉及的不仅是早期地球的构造机制问题,更关系到整个地球的演化历史、变化过程和演化规律,以及地球的未来。本文对国内外的研究状况、研究重点进行了简单述评,强调地球的"热状态和热演化"是构造机制演化的关键控制因素,提出大陆形成和岩石圈的演化与板块构造起源关联密切,是理解早期板块构造启动的重要研究内容。华北克拉通是代表性的古老大陆,本文对它的研究状况给出了介绍和评述。文章最后展望了"早期大陆与板块构造启动"这一重要科学问题的研究方向,并对相应的研究方法提出了评论和建议性意见。     

俯冲带形成机制的可检验假说和检验方案

五十年前板块构造理论的诞生是地球科学领域的一场革命,它为理解地球如何运作构建了基本框架。过去五十年对该理论的进一步研究告诉我们地质过程最终都是地球热损失的结果。例如,大洋岩石圈板块在洋中脊形成,其运动和增生以及最终通过俯冲带进入地幔导致地幔冷却降温,从而导致大规模的地幔对流。亦即,板块构造的直接驱动力是俯冲大洋岩石圈板块的下沉力。因此,没有俯冲带就没有板块构造,但是俯冲带如何开始仍然有争议。对俯冲起始的研究从未中断,有数值模拟也有地质推断。2014年在西太平洋用三个IODP航次(350、351和352)来检验“自发”和“诱发”俯冲开始的想法。所有这些努力都值得肯定,但这些是无法检验的想法。无法检验意味着没有结果。本文介绍至今唯一可用地质学方法检验的假说,亦即“岩石圈内横向物质组成差异导致的浮力差是俯冲带形成的起因”。这种浮力差位于海底高原的边部和被动大陆边缘,因此这些部位是未来俯冲带起始的必然轨迹。在远离这些部位的正常洋盆内因缺乏浮力差而俯冲带不可能起始。换句话说,“所有岛弧一定有大陆(或海底高原)基底”,这可以通过采集和研究岛弧基底岩石来验证。     

Archean greenstone-tonalite duality: Thermochemical mantle convection models or plate tectonics in the early Earth global dynamics?

Mantle convection and plate tectonics are one system, because oceanic plates are cold upper thermal boundary layers of the convection cells. As a corollary, Phanerozoic-style of plate tectonics or more likely a different version of it (i.e. a larger number of slowly moving plates, or similar number of faster plates) is expected to have operated in the hotter, vigorously convecting early Earth. Despite the recent advances in understanding the origin of Archean greenstone–granitoid terranes, the question regarding the operation of plate tectonics in the early Earth remains still controversial. Numerical model outputs for the Archean Earth range from predominantly shallow to flat subduction between 4.0 and 2.5 Ga and well-established steep subduction since 2.5 Ga [Abbott, D., Drury, R., Smith, W.H.F., 1994. Flat to steep transition in subduction style. Geology 22, 937–940], to no plate tectonics but rather foundering of 1000 km sectors of basaltic crust, then “resurfaced” by upper asthenospheric mantle basaltic melts that generate the observed duality of basalts and tonalities [van Thienen, P., van den Berg, A.P., Vlaar, N.J., 2004a. Production and recycling of oceanic crust in the early earth. Tectonophysics 386, 41–65; van Thienen, P., Van den Berg, A.P., Vlaar, N.J., 2004b. On the formation of continental silicic melts in thermochemical mantle convection models: implications for early Earth. Tectonophysics 394, 111–124]. These model outputs can be tested against the geological record. Greenstone belt volcanics are composites of komatiite–basalt plateau sequences erupted from deep mantle plumes and bimodal basalt–dacite sequences having the geochemical signatures of convergent margins; i.e. horizontally imbricated plateau and island arc crust. Greenstone belts from 3.8 to 2.5 Ga include volcanic types reported from Cenozoic convergent margins including: boninites; arc picrites; and the association of adakites–Mg andesites- and Nb-enriched basalts.Archean cratons were intruded by voluminous norites from the Neoarchean through Proterozoic; norites are accounted for by melting of subduction metasomatized Archean continental lithospheric mantle (CLM). Deep CLM defines Archean cratons; it extends to  350 km, includes the diamond facies, and xenoliths signify a composition of the buoyant, refractory, residue of plume melting, a natural consequence of imbricated plateau-arc crust. Voluminous tonalites of Archean greenstone–granitoid terranes show a secular trend of increasing Mg#, Cr, Ni consistent with slab melts hybridizing with thicker mantle wedge as subduction angle steepens. Strike-slip faults of 1000 km scale; diachronous accretion of distinct tectonostratigraphic terranes; and broad Cordilleran-type orogens featuring multiple sutures, and oceanward migration of arcs, in the Archean Superior and Yilgarn cratons, are in common with the Altaid and Phanerozoic Cordilleran orogens. There is increasing geological evidence of the supercontinent cycle operating back to  2.7 Ga: Kenorland or Ur  2.7–2.4 Ga; Columbia  1.6–1.4 Ga; Rodinia  1100–750 Ma; and Pangea  230 Ma. High-resolution seismic reflection profiling of Archean terranes reveals a prevalence of low angle structures, and evidence for paleo-subduction zones. Collectively, the geological–geochemical–seismic records endorse the operation of plate tectonics since the early Archean.     

南海北部陆缘地壳结构特征及其构造过程

根据“北部湾大陆缘地壳结构ＰＳ转换波测深”等地球物理测量结果,本文研究了南海北部陆缘的地壳结构特征,讨论了其白垩纪以来的构造过程。地球物理测量表明,由陆向海,南海北部陆缘地壳由陆壳、过渡壳变为洋壳,厚度由３４ｋｍ减薄至８ｋｍ左右。垂向上地壳为３层结构模式。陆壳、过渡壳和洋壳的下地壳Ｐ波速度普遍较高。地壳伸展系数的计算表明南海北部陆缘伸展主要发育于陆坡地区。结合区域地质研究,本文认为：南海北部陆缘及     

http://dx.doi.org/10.1016/j.gsf.2016.08.001

Geological observations indicate that there are only a few rocks of Archean Earth and no Hadean rocks on the surface of the present-day Earth. From these facts, many scientists believe that the primordial continents never existed during Hadean Earth, and the continental volume has kept increasing. On the other hand, recent studies reported the importance of the primordial continents on the origin of life, implying their existence. In this paper, we discussed the possible process that could explain the loss of the primordial continents with the assumption that they existed in the Hadean. Although depending on the timing of the initiation of plate tectonics and its convection style, subduction erosion, which is observed on the present-day Earth, might have carried the primordial continents into the deep mantle.     

A new cache of Eoarchaean detrital zircons from the Singhbhum craton,eastern India and constraints on early Earth geodynamics

The dominant geodynamic processes that underpin the formation and evolution of Earth’s early crust remain enigmatic calling for new information from less studied ancient cratonic nuclei.Here,we present U-Pb ages and Hf isotopic compositions of detrital zircon grains from^2.9 Ga old quartzites and magmatic zircon from a 3.505 Ga old dacite from the Iron Ore Group of the Singhbhum craton,eastern India.The detrital zircon grains range in age between 3.95 Ga and 2.91 Ga.Together with the recently reported Hadean,Eoarchean xenocrystic(up to 4.24 Ga)and modem detritus zircon grains from the Singhbhum craton,our results suggest that the Eoarchean detrital zircons represent crust generated by recycling of Hadean felsic crust formed at^4.3-4.2 Ga and^3.95 Ga.We observe a prominent shift in Hf isotope compositions at^3.6-3.5 Ga towards super-chondritic values,which signify an increased role for depleted mantle and the relevance of plate tectonics.The Paleo-,Mesoarchean zircon Hf isotopic record in the craton indicates crust generation involving the role of both depleted and enriched mantle sources.We infer a short-lived suprasubduction setting around^3.6-3.5 Ga followed by mantle plume activity during the Paleo-,Mesoarchean crust formation in the Singhbhum craton.The Singhbhum craton provides an additional repository for Earth’s oldest materials.