Subduction and the geochemical cycle

The present balance of crust creation at ocean ridges and above subduction zones appears to be similar to crust removal processes at subduction zones. Almost 10% of the mass of the upper mantle has been influenced by surface hydrosphere, atmosphere and crustal component contamination. As long as surface components are subducted, then in a slowly cooling planet it is difficult to see how the mass of continents and hydrosphere can increase with time.     

Spatiotemporal relationships of dike magmatism in the Kola region, the Fennoscandian Shield

A brief geological and petrographic characterization of the Early Precambrian dike complexes of the Kola region is given along with data on new estimates of dike age and analysis of their distribution over the entire Fennoscandian Shield. The emplacement of dikes in the Archean core of the shield continued after consolidation of the sialic crust 2.74?C1.76 Ga ago. After the Svecofennian Orogeny, dikes continued to form in the west in the area of newly formed crust, while the amagmatic period began in the Archean domain. The intense formation of dikes in the Svecofennian domain lasted approximately for 1 Ga (1.8?C0.84 Ga). The younger igneous rocks in the crustal domains of different age are less abundant and localized at their margins. A similar distribution of dikes is characteristic of other shields in different continents. This implies that the formation of the sialic crust in the shields is not completed by its consolidation and formation of the craton. For 1 Ga after completion of this process, the crust is underplated by mantle-derived magmas. This process is reflected at the Earth??s surface in the development of mantle-derived mafic and anorogenic granitoid magmatism. The process of crust formation is ended as the subcratonic lithosphere cools and the amagmatic period of the craton history is started. Beginning from this moment, the manifestations of cratonic magmatism were related either to the superposed tectonomagmatic reactivation of the cold craton under the effect of crust formation in the adjacent mobile belts or to the ascent of mantle plumes.     

大陆岩石圈有效弹性厚度的计算及其地质意义

大陆范围内Te值有很大的范围，不同构造单元有不同的Te值。Te值的大小与岩石圈的热结构（热年龄）、壳幔耦合等因素有关。同时Te值和地壳厚度、地表有关矿产的分布、岩石圈地幔的物质组成等也有一定关系。     

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.     

Constraints from Thorium/Lanthanum on Sediment Recycling at Subduction Zones and the Evolution of the Continents

Arc magmas and the continental crust share many chemical features,but a major question remains as to whether these features arecreated by subduction or are recycled from subducting sediment.This question is explored here using Th/La, which is low inoceanic basalts (<0·2), elevated in the continents(>0·25) and varies in arc basalts and marine sediments(0·09–0·34). Volcanic arcs form linear mixingarrays between mantle and sediment in plots of Th/La vs Sm/La.The mantle end-member for different arcs varies between highlydepleted and enriched compositions. The sedimentary end-memberis typically the same as local trench sediment. Thus, arc magmasinherit their Th/La from subducting sediment and high Th/Lais not newly created during subduction (or by intraplate, adakiteor Archaean magmatism). Instead, there is a large fractionationin Th/La within the continental crust, caused by the preferentialpartitioning of La over Th in mafic and accessory minerals.These observations suggest a mechanism of ‘fractionation& foundering’, whereby continents differentiate intoa granitic upper crust and restite-cumulate lower crust, whichperiodically founders into the mantle. The bulk continentalcrust can reach its current elevated Th/La if arc crust differentiatesand loses 25–60% of its mafic residues to foundering. KEY WORDS: arc magmatism; continental crust; delamination; thorium; sediment subduction     

大别造山带构造超压形成的碰撞力学机理

提出了大别山构造超压形成的点碰撞模型，简要分析了大陆碰撞带构造运动引起的粘性介质中粘性应力和平均应力随岩石物性的变化规律。探讨了构造压力对超高压的贡献及对成岩深度的重要意义。研究表明：构造运动引起的岩石圈中的附加压力可能与静岩压力有相同的数量级，大陆造山带两陆块不规则边界的碰撞会引起局部应力集中，产生较大的构造压力，岩石介质的流变学分析表明，在相同外力作用下，岩石圈上部的高粘度性质决定了其在构造活动期间增温效果显著，但增压效果有限；而粘性较低的岩石圈下部则增压效果明显，为此，在下地壳与上地幔之间的低粘度带内有可能发生超高压变质作用。     

The investigation of petrology and geodynamic mafic magmatism in Bijar area, west of Iran (Arabia- Eurasia collision zone)

In northeastern Sanandaj-Sirjan structural zone, the Takab-Ghorveh belt comprises a volcanic province which related to the collision between the Eurasian and Arabian continents. It contains almost Quaternary andesitic basalt to alkali basalt. These alkali basaltes show Strombolian type eruptions. The volcanic rocks in Bijar area represent a range of mafic magmas, re-vealed by mingling and mixing textures. A variety of features suggest that the lava flows before eruption from magma chambers, contaminated by continental crust.     

Continent-ocean differences and a differentiation of the earth

The composition of residual matter after the segregation of the crust from the mantle is calculated. The most probable components of the mantle: garnet-peridotitic, pyrolitic and chondritic, were taken into consideration and the continental, oceanic and olivine-tholeiitic crust segregated from them. The probability of the existence of each of the proposed mantle types may be estimated as based on the obtained residual matter. It is established that the hypothesis of the pyrolitic mantle is the most acceptable.A comparison of the compositions of the continental and oceanic crust makes evident that there exist two types of differentiation processes in the upper mantle. One of them leads to the development of continents, the second of the oceanic areas. In the first case the partial melting in the mantle and the ascent of magma are accompanied by an additional evacuation of silica (?) and especially of potassium. This rise of supplementary light substance embraces the depths as great as 1000 km or more. In the case of oceanic crust such an additional rise of matter is absent since there only partial melting in the mantle takes place and accordingly the depth of the differentiation is much smaller.The differences in the process of the earth's differentiation may be easier explained if the mantle is assumed to be mobile, instead of an immobile mass. It is assumed that a differentiation of the primary matter of the planet takes place near the mantle-core boundary and that the uplift of the much lighter silicate differentiation is in the asthenosphere. On this level the mantle becomes partially melted and the resultant liquid rises into the crust. This liquid is enriched by sialic substances, particularly by potassium and may be by silica.In the regions where the rise of matter from great depth is lacking, continental crust is not formed, and oceanic crust is born.The above comparison leads in particular to the following additional conclusions: the chemical differences of the continental crust and the oceanic crust do not permit the hypotheses of continenta drift, nor of the spreading of the ocean floor and the transformation of the continental crust into oceanic. All these hypotheses become incompatible with the chemical composition of the crust.     

Deep mantle roots and continental emergence: implications for whole-Earth elemental cycling,long-term climate,and the Cambrian explosion

Elevations on Earth are dominantly controlled by crustal buoyancy, primarily through variations in crustal thickness: continents ride higher than ocean basins because they are underlain by thicker crust. Mountain building, where crust is magmatically or tectonically thickened, is thus key to making continents. However, most of the continents have long passed their mountain building origins, having since subsided back to near sea level. The elevations of the old, stable continents are lower than that expected for their crustal thicknesses, requiring a subcrustal component of negative buoyancy that develops after mountain building. While initial subsidence is driven by crustal erosion, thermal relaxation through growth of a cold thermal boundary layer provides the negative buoyancy that causes continents to subside further. The maximum thickness of this thermal boundary layer is controlled by the thickness of a chemically and rheologically distinct continental mantle root, formed during large-scale mantle melting billions of years ago. The final resting elevation of a stabilized continent is controlled by the thickness of this thermal boundary layer and the temperature of the Earth’s mantle, such that continents ride higher in a cooler mantle and lower in a hot mantle. Constrained by the thermal history of the Earth, continents are predicted to have been mostly below sea level for most of Earth’s history, with areas of land being confined to narrow strips of active mountain building. Large-scale emergence of stable continents occurred late in Earth’s history (Neoproterozoic) over a 100–300 million year transition, irreversibly altering the surface of the Earth in terms of weathering, climate, biogeochemical cycling and the evolution of life. Climate during the transition would be expected to be unstable, swinging back and forth between icehouse and greenhouse states as higher order fluctuations in mantle dynamics would cause the Earth to fluctuate rapidly between water and terrestrial worlds.     

冈底斯中段林子宗火山岩岩石地球化学特征

广泛发育在冈底斯岩浆岩带中的林子宗火山岩及其与下伏地层间的区域不整合提供了印度-亚洲大陆碰撞的重要证据.谢通门地区的林子宗火山岩早期以中基性-中性岩为主,夹少量流纹质凝灰岩,晚期以流纹质火山岩为主.岩石学和地球化学研究表明,这套火山岩早期以钙碱性为主,带有较多陆缘火山岩特征,中期开始出现标志陆内活动的钾玄岩,晚期更多地显示了加厚陆壳条件下火山岩的特点,记录了由新特提斯俯冲消减末期过渡到印度-亚洲大陆碰撞的信息.中基性岩浆来源于俯冲带的地幔源区,长英质岩浆形成于加厚地壳的部分熔融.结合区域同位素年龄资料,可以认为林子宗火山岩中高钾流纹质火山岩是印度-亚洲大陆碰撞阶段陆壳缩短加压升温引起部分熔融的产物.     

Lateral density inhomogeneities of the continental and oceanic lithosphere and their relationship with the Earth’s crust formation

This paper presents the results of the study of the free mantle surface (FMS) depth beneath continents and oceans. The reasons for the observed dependence of the FMS depth on the crustal thickness in the continental lithosphere are discussed. The influence of radial variations in the mantle’s density is evaluated. The calculations performed have indicated that the observed dependence of the FMS depth on the crustal thickness is caused mostly by lateral inhomogeneities in the lithospheric mantle, and the size of these inhomogeneities is proportional to the thickness of the crust. The origin of such inhomogeneities can be related to the process of continental crust formation.     

Die großen Gräben: Harmonische Strukturen in einer disharmonisch struierten Erdkruste

A system of intracontinental grabens extends over Western Europe, the Levant and East Africa. Small crustal segments framed by elevated shoulders are sunken along parallel escarpments and disintegrated by antithetic normal faults. The shoulders are risen up as outward tilted blocks and thought to form a closed vault at the base of the crust, corresponding to the wedge block of the graben. Underneath the Rhinegraben exists, as detected by seismic refraction measurements, a pillow-shaped body of material with P-wave velocities of 7.4 to 7.9 km/sec, intercalated between crust and mantle. The taphrogenesis of all larger grabens is assumed to be induced by the formation and growth of subcrustal swells of this type. Also the specific graben volcanism is thought to be connected with the intruded laccolithic body of mantle-derived material. The tensional breakup and the faulting of the warped crustal masses was favoured by the gravity slide of the crust which was uncoupled from the substratum by the intercalated magmatic layer. Along the Red Sea Rift the crust tore completely releasing the basaltic substratum in the inner graben. The pattern of its magnetic anomalies leads to the assumption that the pillow body is recruited by simatic dike injections according to the principle of sea-floor spreading. Therefore, there is a great conformity between the intercontinental and the mid-oceanic rift systems. The supplies of mantle material along the Mid-Atlantic and the Carlsberg Rift are related to the drift of the continental frames. The mid-oceanic rift systems in their medial position to the framing continental margins correspond to the intracontinental graben swarm, equidistant from both fronts of circum-Pacific tectonics. A reciprocating action is presumed between the ascending masses along the rift zones and the suction of masses along the deep-sea trenches and geosynclines. The observed crustal movements imply an equilibrant plastic flow within the upper mantle, probably impelled by mechanical convection currents. The continental unbalance between the Pacific and the anti-Pacific hemisphere is discussed as causing mantle currents. Within this interplay between crust and mantle and between continents and ocean floors, the oceanic crust had obtained harmonical features moulded directly by the deduced mobility. The continental crust, however, is passively stressed, its rocks are affected by heterogeneous deformations from which the continents got its polygenetic multiform fabric.     

8411地区深熔作用的Sr,Nd同位素地质研究

8411地区深熔作用的同位素地质研究表明,J_(1-2)砂岩是以元古代古陆壳供源的湖相沉积岩、岩浆岩为元古代古陆在下地壳局部重熔或重熔-混染的产物。铀的预富集与深熔作用中硅铝质地壳岩浆混染有关。铀矿物和岩浆岩的同位素组成相似。这些研究结果,结合岩石学、矿物学、同位素演化和稀土含量分析,认识到铀来自深源,元古代地层是铀源层。     

大冰期成因探讨

大冰期的形成不仅需要气温低，而且需要大量海水转移到大陆上形成冰川。元古代晚期全球规模白云岩建造形成之后，火山作用在洋壳和陆壳上的表现大不相同：洋壳型火山作用是岩浆静静涌出，大量蒸发海水；陆壳型火山作用是猛烈爆发，烘烤碳酸盐岩层产生二氧化碳喷发形成“干冰制冷机制”，使大气降温。洋壳型火山与陆壳型火山共同作用形成大冰期。震旦纪大冰期后开始了显生宙。大冰期与生物的演化发展密切相关的原因是二氧化碳的大量介入。     

Certain problems of the structure and evolution of transition zones between continents and oceans

A type of continental-oceanic transition zone, referred to as the Columbian transition zone, is distinguished from two other commonly known types of these zones. The subsidence of the Earth's crust, typical of all transition zones, is shown to be connected (by geophysical properties) to the transformation of continental crust into intermediate crust and later into oceanic. The most likely mechanisms of such changes are the basification of continental crust, its foundering, block by block, into the heated upper mantle, and its substitution by new oceanic crust. The evolution of transition zones of the Pacific type is largely influenced by deep faults, which reach down to the level of undepleted mantle. From this level, the volatile products rise to the surface which results in the formation of calc-alkali magmas on island arcs. The Benioff zones are deep faults, whose inclinations are dependent on the density contrasts in the upper mantle on either side of the Benioff zones. The denser mantle flows beneath the mantle of lower density. This phenomenon is depicted by plate tectonics as subduction.On the whole, the evolution of transition zones gives rise to the growth of the oceans at the expense of the continents, though oceanic crust becomes thicker by addition of volcanogenic layers composed of andesite, in the transition zones (type two) of the Pacific type at island arcs.     

Granite subduction: Arc subduction, tectonic erosion and sediment subduction

Continental growth has been episodic, reflecting the episodic nature of mantle dynamics as well as surface dynamics of the Earth, the net result of which is exhibited by the present mantle with two huge reservoirs of TTG rocks, one on the surface continents and the other on the D″ layer on the Core-Mantle Boundary (CMB). During the early half of the Earth history, the felsic continental crust on the surface which formed in an intra-oceanic environment has mostly been subducted into the deep mantle, except in the rare case of parallel arc collision. The growth history of continental crust shows that with its simultaneous formation, a considerable amount must have also been subducted. Such ongoing subduction processes can be seen in the western Pacific region, through tectonic erosion, arc subduction, and sediment-trapped subduction.     

Secular change in lifetime of granitic crust and the continental growth: A new view from detrital zircon ages of sandstones

U-Pb ages of detrital zircons were newly dated for 4 Archean sandstones from the Pilbara craton in Australia, Wyoming craton in North America, and Kaapvaal craton in Africa. By using the present results with previously published data, we compiled the age spectra of detrital zircons for 2.9, 2.6, 2.3,1.0, and0.6 Ga sandstones and modern river sands in order to document the secular change in age structure of continental crusts through time. The results demonstrated the following episodes in the history of continental crust:(1) low growth rate of the continents due to the short cycle in production/destruction of granitic crust during the Neoarchean to Paleoproterozoic(2.9-23 Ga),(2) net increase in volume of the continents during Paleo-to Mesoproterozoic(2.3-1.0 Ga), and(3) net decrease in volume of the continents during the Neoproterozoic and Phanerozoic(after 1.0 Ga). In the Archean and Paleoproterozoic, the embryonic continents were smaller than the modern continents, probably owing to the relatively rapid production and destruction of continental crust. This is indeed reflected in the heterogeneous crustal age structure of modern continents that usually have relatively small amount of Archean crusts with respect to the post-Archean ones. During the Mesoproterozoic, plural continents amalgamated into larger ones comparable to modern continental blocks in size. Relatively older crusts were preserved in continental interiors, whereas younger crusts were accreted along continental peripheries.In addition to continental arc magmatism, the direct accretion of intra-oceanic island arc around continental peripheries also became important for net continental growth. Since 1.0 Ga, total volume of continents has decreased, and this appears consistent with on-going phenomena along modern active arc-trench system with dominant tectonic erosion and/or arc subduction. Subduction of a huge amount of granitic crusts into the mantle through time is suggested, and this requires re-consideration of the mantle composition and heterogeneity.     

Regularities of rare-earth element distribution in the sedimentary shell and in the crust of the earth

After summarizing the vast analytical material it was possible to establish the differences in rare-earth element (REE) distribution in geosynclines and on platforms. It is shown that the heavier REE composition in sediments of geosynclines and the lighter one in sediments of platforms was initially created by processes of endogenic differentiation and then inherited by sedimentary series of these zones. On the basis of the calculation of the REE content in rocks of various shells of the Earth crust the increase of the role of heavy lanthanides into the depths of the crust is shown. The obtained estimate of REE abundance in the lithosphere led to the conclusion that the crust of our planet, despite the repeated reworking of its matter by sedimentary processes, metamorphism and granitization, has inherited features peculiar to products of tholeiitic magmatism—the most widely spread type of mantle fusion.     

风化壳研究的现状与展望

风化壳是岩石圈、大气圈以及水圈、生物圈之间相互作用的界面,能够直接记录地球多圈层演化的信息。利用风化壳的地带性规律重建古环境是地貌学研究的传统内容之一。近年来,单晶矿物激光^40Ar/^39Ar测年技术、“双面”模式以及古地磁法等在风化壳研究中的成功应用,在理论和技术上为恢复大陆剥蚀区高分辨率的环境演变历史创造了条件。利用风化年代学、风化地层学、古地磁学和地球化学等方法对风化壳进行综合研究,不仅可以建立剥蚀区的环境演变序列,为风化期次（事件）与其他全球性构造－气候事件的对比提供了广阔的前景;而且可以用于化学风化（强度和速度）的准确量化,有利于深入理解构造－剥蚀－风化－气候之间相互作用的反馈机制和正确评估人类活动对未来气候的影响能力。     

Experiments simulating surface deformation induced by pluton emplacement

We present centrifuge experiments to study the surface deformation induced by shallow pluton emplacement in a rheologically stratified crust. Sand simulates the topmost brittle crust; plastilina and denser silicone represent more and less competent crustal portions, respectively; lighter silicone simulates a buoyant intrusion. In the models, density differences affect the rate of intrusion but not their evolution or shape, whereas viscosity and strength stratifications control both the shape and rate of the intrusions. With a higher viscosity contrast (10^2–4) between the intrusion and the embedding media, the rise of the lighter silicone induces a laccolith-like intrusion, responsible for doming and thinning of the overburden; an apical depression may form, inducing silicone extrusion. Conversely, with a lower (10¹) viscosity contrast, the overburden and the intrusion exhibit a lens-shaped form, with a broad central depression bordered by an upward flexure towards the periphery. A sag in the floor of the intrusion is commonly observed; no silicone extrusion occurred. The intrusion is a hybrid between a laccolith and a lopolith. The comparison with nature (1) confirms roof uplift as an important means of accommodating space during pluton emplacement and (2) suggests that, where roof uplift plays a major role, pluton emplacement can induce a well-correlated sequence of events at surface: doming, the development of a depression and extrusion.