Seismic evidence for thick and underplated late Archaean crust of eastern Dharwar craton

The deep crustal structure of eastern Dharwar craton has been investigated through τ-p extremal inversion of P-wave travel times from a network of seismographs recording quarry blasts. Travel times have been observed in the distance range 30–250 km in a laterally homogeneous lithospheric segment Main features of the inferred velocity-depth relationship include: (a) 29 km thick combined upper and middle crust velocity varying from 6 km/s to 7 km/s, with no observable velocity discontinuity in this depth range; (b) a lower crust (∼ 29–41 km) with velocity increasing from 7.0 to 7.3 km/s; (c) an average upper mantle velocity of 8.1 km/s; and (d) presence of a 12 km thick high velocity crustal layer (7.4 – 7.8 km/s) in the depth range 41–53 km, with a distinct velocity gradient marking a velocity increase of 0.4 km/s. The anomalous 53 km thick crust is viewed as a consequence of magmatic underplating at the base of the crust in the process of cratonization of the eastern Dharwar craton during late Archaean. The underplated material reflects here with the velocity of 7–3 to 7–8 km/s below the depth of 40 km. Our proposition of magmatic underplating is also supported by the presence of large scale I-granitoid, a product of partial melting of the upper mantle material.     

Origin of high-velocity anomalies beneath the Siberian craton: A fingerprint of multistage magma underplating since the Neoarchean

Despite the violent eruption of the Siberian Traps at ~ 250 Ma, the Siberian craton has an extremely low heat flow (18–25 mW/m²) and a very thick lithosphere (300–350 km), which makes it an ideal place to study the influence of mantle plumes on the long-term stability of cratons. Compared with seismic velocities of rocks, the lower crust of the Siberian craton is composed mainly of mafic granulites and could be rather heterogeneous in composition. The very high V_p (> 7.2 km/s) in the lowermost crust can be fit by a mixture of garnet granulites, two-pyroxene granulites, and garnet gabbro due to magma underplating. The high-velocity anomaly in the upper mantle (V_p = 8.3-8.6 km/s) can be interpreted by a mixture of eclogites and garnet peridotites. Combined with the study of lower crustal and mantle xenoliths, we recognized multistage magma underplating at the crust-mantle boundary beneath the Siberian craton, including the Neoarchean growth and Paleoproterozoic assembly of the Siberian craton beneath the Markha terrane, the Proterozoic collision along the Sayan-Taimyr suture zone, and the Triassic Siberian Trap event beneath the central Tunguska basin. The Moho becomes a metamorphism boundary of mafic rocks between granulite facies and eclogite facies rather than a chemical boundary that separates the mafic lower crust from the ultramafic upper mantle. Therefore, multistage magma underplating since the Neoarchean will result in a seismic Moho shallower than the petrologic Moho. Such magmatism-induced compositional change and dehydration will increase viscosity of the lithospheric mantle, and finally trigger lithospheric thickening after mantle plume activity. Hence, mantle plumes are not the key factor for craton destruction.     

Geophysical constraints on mesozoic disruption of North China Craton by underplating‐triggered lower‐crust flow of the Archaean lithosphere

The mechanism of the disruption, both lithospheric thinning and oceanization of the commonly accepted long‐term‐stable Archaean craton, is still an open question. The available models, all imply a bottom to top process. With the construction of a 1660‐km‐long transect across the eastern North China Craton (NCC), we demonstrate that both the P‐wave velocity and density in the lowermost crust beneath the central section are significantly higher than in the corresponding parts of the south and north sections on the transect. These features are interpreted as geophysical signature of lower crustal underplating, which supplies sufficiently high gravitational potential energy to trigger lateral flow of the lower crust. This magma underplating‐triggered bilateral lower crust flow may facilitate the lithospheric thinning by means of asthenosphere upwelling and decompression melting, which infill the gap produced by the lower crust flow. The underplating‐triggered lower crustal flow can provide an alternative mechanism to explain the NCC lithosphere disruption, which highlights the crustal feedback to Archaean lithosphere disruption, from top to bottom.     

Continental crust and lithospheric mantle interaction beneath North China: isotopic evidence from granulite xenoliths in Hannuoba, Sino-Korean craton

Mafic granulite and pyroxenite xenoliths from Cenozoic alkaline basalts at Hannuoba, Hebei Province, North China have been selected for a systematic geochemical and Sr–Nd–Pb isotopic study, which provides a unique opportunity to explore nature of the lower crust and the interaction between the continental crust and lithospheric mantle beneath an Archean craton. The major, compatible and incompatible elements and radiogenic isotopes of these xenoliths suggest great chemical heterogeneity of the lower crust beneath the Hannuoba region. Petrological and geochemical evidences indicate a clear cumulate origin, and most likely, they are related to basaltic underplating in different geological episodes. However, the Sr–Nd–Pb isotopic compositions of the xenoliths reveal a profound enriched source signature (EM I) with some influence of EM II, which implies that some portion of pre-existing, old metasomatized subcontinental lithospheric mantle could have played an important role in their genesis. It is suggested that the interaction between continental crust and subcontinental mantle as manifested by basaltic underplating would be closely related to regional tectonic episodes and geodynamic processes in the deep part of subcontinental lithospheric mantle.     

Episodic widespread magma underplating beneath the North China Craton in the Phanerozoic: Implications for craton destruction

A comprehensive synthesis of U–Pb geochronology and Hf isotopes of zircons from granulite/pyroxenite xenoliths entrained in Phanerozoic magmatic rocks and inherited xenocrysts from the associated lower crust rocks from various domains of the North China Craton (NCC) provides new insights into understanding the Phanerozoic evolution of the lower crust in this craton. Episodic widespread magma underplating into the ancient lower crust during Phanerozoic has been identified throughout the NCC from early Paleozoic to Cenozoic, broadly corresponding to the Caledonian, Hercynian, Indosinian, Yanshanian, and Himalayan orogenies on the circum-craton mobile belts. The early Paleozoic (410–490 Ma) ages come from xenoliths in the northern and southern margins as well as the central domain of the Eastern Block of the craton which mark the first phase of Phanerozoic magma underplating since the final cratonization of the NCC in the Paleoproterozoic. The magmatism coincided with the northward subduction of the Paleotethysian Ocean in the south and the southward subduction of the Paleoasian Ocean in the north. The subduction not only triggered magma underplating but also led to the emplacement of the diamondiferous kimberlites on the craton, marking the initiation of decratonization. The late Paleozoic event as represented by the 315 Ma garnet pyroxenite and/or lherzolite xenoliths in Hannuoba was restricted to the northern and southern margins of the craton, correlating with the arc magmatism continuous associated with the subduction of the Paleotethysian and Paleoasian Oceans and resulting in the interaction between the melts from subducted slabs and the lithospheric mantle/lower crust. The early Mesozoic event also dominantly occurred in the northern and southern margins and was related with the final closure of the Paleotethysian and Paleoasian Oceans as well as the collisional orogeny between the NCC and the Yangtze Craton. The late Mesozoic (ca. 120 Ma) was a major and widespread magmatic event which manifested throughout the NCC, associated with the geothermal overturn due to the giant south Pacific mantle plume. The Cenozoic magmatism, identified only in the dark clinopyroxenite xenoliths in the Hannuoba, was probably induced by the Himalayan movement in eastern Asia and might also have been influenced by the subduction of the Pacific Ocean to some extent. These widespread and episodic magma underplating or rejuvenation of the ancient lower crust beneath the NCC revealed by U–Pb and Hf isotope data resulted from the corresponding addition of juvenile materials from mantle to lower crust, with a mixing of the old crust with melts. The process inevitably resulted in the compositional modification of the ancient lower crust, similar to the compositional transformation from the refractory lithospheric mantle to a fertile one through the refractory peridotite — infiltrated melt reaction as revealed in the lithospheric mantle beneath the craton.     

Stagnant lids and mantle overturns:Implications for Archaean tectonics,magmagenesis,crustal growth,mantle evolution,and the start of plate tectonics

The lower plate is the dominant agent in modern convergent margins characterized by active subduction,as negatively buoyant oceanic lithosphere sinks into the asthenosphere under its own weight.This is a strong plate-driving force because the slab-pull force is transmitted through the stiff sub-oceanic lithospheric mantle.As geological and geochemical data seem inconsistent with the existence of modernstyle ridges and arcs in the Archaean,a periodically-destabilized stagnant-lid crust system is proposed instead.Stagnant-lid intervals may correspond to periods of layered mantle convection where efficient cooling was restricted to the upper mantle,perturbing Earth's heat generation/loss balance,eventually triggering mantle overturns.Archaean basalts were derived from fertile mantle in overturn upwelling zones(OUZOs),which were larger and longer-lived than post-Archaean plumes.Early cratons/continents probably formed above OUZOs as large volumes of basalt and komatiite were delivered for protracted periods,allowing basal crustal cannibalism,garnetiferous crustal restite delamination,and coupled development of continental crust and sub-continental lithospheric mantle.Periodic mixing and rehomogenization during overturns retarded development of isotopically depleted MORB(mid-ocean ridge basalt)mantle.Only after the start of true subduction did sequestration of subducted slabs at the coremantle boundary lead to the development of the depleted MORB mantle source.During Archaean mantle overturns,pre-existing continents located above OUZOs would be strongly reworked;whereas OUZOdistal continents would drift in response to mantle currents.The leading edge of drifting Archaean continents would be convergent margins characterized by terrane accretion,imbrication,subcretion and anatexis of unsubductable oceanic lithosphere.As Earth cooled and the background oceanic lithosphere became denser and stiffer,there would be an increasing probability that oceanic crustal segments could founder in an organized way,producing a gradual evolution of pre-subduction convergent margins into modern-style active subduction systems around 2.5 Ga.Plate tectonics today is constituted of:(1)a continental drift system that started in the Early Archaean,driven by deep mantle currents pressing against the Archaean-age sub-continental lithospheric mantle keels that underlie Archaean cratons;(2)a subduction-driven system that started near the end of the Archaean.     

青藏高原北缘深部地壳结构特征及其形成机制探讨

柴达木盆地－祁连山地区位于青藏高原北缘，同青藏高原主体一样，该区具有多层地壳结构特征，并普遍出现壳内低速层，地壳厚度是华北及华南地区的2倍以上。其形成可能与地壳的横向挤压缩短及幔源物质的底侵作用有关。随着底侵作用增强，地壳厚度加大，岩石圈厚度则越趋于减薄，地壳上部表现为拉张，下部发生壳幔深熔及幔源流体的交代作用，从而导致了地壳低速层，地热和浅源地震的发育。同时，这也是青藏高原出现热壳冷幔的原因之一。     

中国东部岩石圈热状态与流变学强度特征

根据均衡原理制约的地热计算得到中国东部岩石圈的温度分布状态,以40、70、100km和莫霍面深度等温线图以及600°C、1100°C等温面深度的形式表示.同时计算了以1350°C等温面深度表示的中国东部的热岩石圈厚度.结果显示:在扬子克拉通西部四川盆地之下存在160～200km厚的岩石圈根,但在整个华北克拉通之下缺失岩...     

华北东部中生代晚期-新生代软流圈上涌与岩石圈减薄

现今的地幔是由软流圈地幔(热的,主元素饱满、微量元素亏损的,塑性流变性质的)、古老岩石圈地幔(地幔1,冷的,主元素贫瘠、微量元素富集的,刚性的,以方辉橄榄岩为代表)以及现今的岩石圈地幔(地幔2,主元素饱满、微量元素亏损,以二辉橄榄岩为代表,可能包括多时期形成的)组成。古老岩石圈地幔与地幔2样品的共存、100~4·3Ma在地幔内部持续发生的古老岩石圈与软流圈的相互作用以及上述作用的时空不均一性,都表明了岩石圈减薄是软流圈呈“蘑菇云状”大规模上涌的结果。上述事件发生于100Ma以后。软流圈来源的玄武岩大范围喷发并伴随了岩石圈的强烈拉伸是事件发生的主要标志,岩石圈减薄是一个深部地质过程,不像是突发事件。     

Phanerozoic magma underplating and crustal growth beneath the North China Craton

Evidence for post‐Archaean crustal growth via magma underplating is largely based on U–Pb dating of zircons from granulite‐facies xenoliths. However, whether the young zircons from such xenoliths are genetically related to magma underplating or to anatexis remains controversial. The lower‐crustal xenoliths carried by igneous rocks in the Chifeng and Ningcheng (North China Craton) have low SiO₂ and high MgO, indicating that parental melts of their protoliths were of unambiguous mantle origin. The xenoliths contain abundant magmatic zircons with late‐Palaeozoic ages, and have more radiogenic zircon Hf‐isotope compositions and hence younger model ages than ancient crustal magmas and the “reworking array” of the basement rocks. Our data suggest that the granulites represent episodic magmatic underplating to the lower crust of this craton in Phanerozoic time. Considering the observation that regional lowermost crust (~5 km) is mafic and characterized by Phanerozoic zircons, this work reports an example of post‐Archaean crustal growth via magma underplating.     

Ancient deep roots for Mesozoic world-class gold deposits in the north China craton:An integrated genetic perspective

The North China Craton(NCC)hosts some of the world-class gold deposits that formed more than 2 billion years after the major orogenic cycles and cratonization.The diverse models for the genesis of these deposits remain equivocal,and mostly focused on the craton margin examples,although synchronous deposits formed in the interior domains.Here we adopt an integrated geological and geophysical perspective to evaluate the possible factors that contributed to the formation of the major gold deposits in the NCC.In the Archean tectonic framework of the NCC,the locations of the major gold deposits fall within or adjacent to greenstone belts or the margins of micro-continents.In the Paleoproterozoic framework,they are markedly aligned along two major collisional sutures-the Trans North China Orogen and the Jiao-Liao-Ji Belt.Since the Mesozoic intrusions hosting these deposits do not carry adequate signals for the source of gold,we explore the deep roots based on available geophysical data.We show that the gold deposits are preferentially distributed above zones of uplifted MOHO and shallow LAB corresponding to thinned crust and eroded sub-lithospheric mantle,and that the mineralization is located above regions of high heat flow representing mantle upwelling.The NCC was at the center of a multi-convergent regime during the Mesozoic which intensely churned the mantle and significantly en riched it.The geophysical data on Moho and LAB upwarp from the centre towards east of the craton is more consistent with paleo-Pacific slab subduction from the east exerting the dominant control on lithospheric thinning.Based on these results,and together with an evaluation of the geochemical and isotopic features of the Mesozoic magmatic intrusions hosting the gold mineralization,we propose a genetic model that invokes reworking of ancient Au archives preserved in the lower crust and metasomatised upper mantle and which were generated through multiple subduction,underplating and cumulation events associated with cratonization of the NCC as well as the subduction-collision of Yangtze Craton with the NCC.The heat and material input along zones of heterogeneously thinned lithosphere from a rising turbulent mantle triggered by Mesozoic convergent margins surrounding the craton aided in reworking the deep roots of the ancient Au reservoirs,leading to the major gold metallogeny along craton margins as well as in the interior of the NCC.     

地壳与弱化岩石圈地幔的相互作用:以燕山造山带为例

燕山造山带中生代发育4期钙碱性火山活动,它们的源区组成都是受壳幔相互作用的制约,其中髫髻山组和义县组分布广泛,具有代表性．髫髻山组岩性比较单一,地球化学参数变化范围小,岩浆的AFC作用不强烈,源区成分不复杂．依据Kay et al．(1991)的方法,估算了早-中侏罗世燕山地区的地壳厚度为40-45 km.髫髻山组粗安岩是在加厚的地壳 (40-45 km)条件下,源区是含角闪石的石榴石麻粒岩 底侵的基性岩的壳幔过渡带熔融形成．义县组火山岩的源区为下地壳 岩石圈地幔,地幔组分较髫髻山组增加．研究区中生代早期地壳开始加厚,发生下地壳拆沉,进入流变学性质改变了的“弱化的岩石圈地幔”,二者发生作用．岩石圈地幔在中生代晚期受到流体、熔体、地幔矿物中活化的分子水、剪切构造作用,以及温、压条件改变的影响,导致岩石圈地幔发生不均一的局部弱化,为容纳拆沉的下地壳提供了优化场所．推测弱化岩石圈地幔出现于135 Ma以后燕山地区发育的小型拉伸盆地之下,以及对应的小型软流圈底辟体之上．上述模型可以与俯冲带的楔形地幔与俯冲洋壳的相互作用相对比．     

太行山重力梯度带的形成与华北岩石圈减薄的时空差异性有关

通过对比华北太行山重力梯度带两侧新生代玄武岩及其中幔源包体的成分, 发现: (1)华北东、西部新生代玄武岩具有相反的演化趋势, 说明新生代以来西部岩石圈逐渐减薄, 而东部岩石圈逐渐加厚; (2)西部岩石圈地幔组成相对复杂, 年龄多为晚太古代-元古代; 而东部岩石圈地幔组成相对单一, 年龄多为现代值, 少数为元古代; (3)西部壳幔过渡带较厚而东部较薄, 反映两地不同的岩浆底侵作用程度.华北岩石圈组成的空间不均一性可能与岩石圈减薄过程的时空差异有关. 岩相古地理分析说明太行山重力梯度带的雏形形成于早白垩世, 与华北中生代岩浆活动的高峰相吻合.由于岩浆作用与岩石圈减薄作用密切相关, 因此认为华北岩石圈减薄的时空不均一性是形成太行山重力梯度带的重要机制.      

残留板块及地幔柱:怀俄明克拉通的上地幔地震图像(英文)

介绍了近年来天然地震体波和面波层析成像以及接收函数在怀俄明克拉通地区的应用。怀俄明克拉通地区的地壳和上地幔结构可以归结为克拉通成型时期的残存高速度异常以及进行中Yellowstone低速地幔柱。在克拉通南部边界缝合带地区以及在中部和北部下地壳中保存着一些高速度结构。南部边界缝合带地区的高速度倾斜上地幔结构与人工地震剖面LithoProbe在北美各个克拉通边界所记录的上地幔倾斜反射体一致。作为一个可能的上地幔消减板块残留体,这个倾斜上地幔结构显示出板块叠加可能是一个普遍的克拉通成型过程。在克拉通缝合带的莫霍面和上地幔深度,人工地震的研究结果显示板块叠加过程形成一个楔形体,体现出克拉通上地幔的较高粘度系数。接收函数的转换波共转换点叠加技术显示出这种楔形体存在于整个怀俄明克拉通的南部边界。接收函数和基于噪声的瑞利波层析成像图像显示出在克拉通地壳增厚地区存在下地壳高速体。作为早期岩石圈分裂过程残留的火成岩侵入体,这种下地壳高速体存在于较早成型的克拉通北部和中部地区,显示出南部克拉通地区不同的形成机制。克拉通的西部地区受到Yellowstone地幔柱的影响。层析成像显示低速的地幔柱从黄石地区向下延伸到至少500km。在消蚀岩石圈的同时,岩浆侵入体沿着hotspottrack在中地壳大量的形成,并引起下地壳岩石的横向流动。     

Petrology and geochemistry of peridotite xenoliths from the Letlhakane kimberlites, Botswana

The diamondiferous Letlhakane kimberlites are intruded into the Proterozoic Magondi Belt of Botswana. Given the general correlation of diamondiferous kimberlites with Archaean cratons, the apparent tectonic setting of these kimberlites is somewhat anomalous. Xenoliths in kimberlite diatremes provide a window into the underlying crust and upper mantle and, with the aid of detailed petrological and geochemical study, can help unravel problems of tectonic setting. To provide relevant data on the deep mantle under eastern Botswana we have studied peridotite xenoliths from the Letlhakane kimberlites. The mantle-derived xenolith suite at Letlhakane includes peridotites, pyroxenites, eclogites, megacrysts, MARID and glimmerite xenoliths. Peridotite xenoliths are represented by garnet-bearing harzburgites and lherzolites as well as spinel-bearing lherzolite xenoliths. Most peridotites are coarse, but some are intensely deformed. Both garnet harzburgites and garnet lherzolites are in many cases variably metasomatised and show the introduction of metasomatic phlogopite, clinopyroxene and ilmenite. The petrography and mineral chemistry of these xenoliths are comparable to that of peridotite xenoliths from the Kaapvaal craton. Calculated temperature-depth relations show a well-developed correlation between the textures of xenoliths and P-T conditions, with the highest temperatures and pressures calculated for the deformed xenoliths. This is comparable to xenoliths from the Kaapvaal craton. However, the P-T gap evident between low-T coarse peridotites and high-T deformed peridotites from the Kaapvaal craton is not seen in the Letlhakane xenoliths. The P-T data indicate the presence of lithospheric mantle beneath Letlhakane, which is at least 150 km thick and which had a 40mW/m² continental geotherm at the time of pipe emplacement. The peridotite xenoliths were in internal Nd isotopic equilibrium at the time of pipe emplacement but a lherzolite xenolith with a relatively low calculated temperature of equilibration shows evidence for remnant isotopic disequilibrium. Both harzburgite and lherzolite xenoliths bear trace element and isotopic signatures of variously enriched mantle (low Sm/Nd, high Rb/Sr), stabilised in subcontinental lithosphere since the Archaean. It is therefore apparent that the Letlhakane kimberlites are underlain by old, cold and very thick lithosphere, probably related to the Zimbabwe craton. The eastern extremity of the Proterozoic Magondi Belt into which the kimberlites intrude is interpreted as a superficial feature not rooted in the mantle. Received: 19 March 1996 / Accepted: 16 October 1996     

Regional patterns in the paragenesis and age of inclusions in diamond, diamond composition, and the lithospheric seismic structure of Southern Africa

The Archean lithospheric mantle beneath the Kaapvaal–Zimbabwe craton of Southern Africa shows ±1% variations in seismic P-wave velocity at depths within the diamond stability field (150–250 km) that correlate regionally with differences in the composition of diamonds and their syngenetic inclusions. Seismically slower mantle trends from the mantle below Swaziland to that below southeastern Botswana, roughly following the surface outcrop pattern of the Bushveld-Molopo Farms Complex. Seismically slower mantle also is evident under the southwestern side of the Zimbabwe craton below crust metamorphosed around 2 Ga. Individual eclogitic sulfide inclusions in diamonds from the Kimberley area kimberlites, Koffiefontein, Orapa, and Jwaneng have Re–Os isotopic ages that range from circa 2.9 Ga to the Proterozoic and show little correspondence with these lithospheric variations. However, silicate inclusions in diamonds and their host diamond compositions for the above kimberlites, Finsch, Jagersfontein, Roberts Victor, Premier, Venetia, and Letlhakane do show some regional relationship to the seismic velocity of the lithosphere. Mantle lithosphere with slower P-wave velocity correlates with a greater proportion of eclogitic versus peridotitic silicate inclusions in diamond, a greater incidence of younger Sm–Nd ages of silicate inclusions, a greater proportion of diamonds with lighter C isotopic composition, and a lower percentage of low-N diamonds whereas the converse is true for diamonds from higher velocity mantle. The oldest formation ages of diamonds indicate that the mantle keels which became continental nuclei were created by middle Archean (3.2–3.3 Ga) mantle depletion events with high degrees of melting and early harzburgite formation. The predominance of sulfide inclusions that are eclogitic in the 2.9 Ga age population links late Archean (2.9 Ga) subduction-accretion events involving an oceanic lithosphere component to craton stabilization. These events resulted in a widely distributed younger Archean generation of eclogitic diamonds in the lithospheric mantle. Subsequent Proterozoic tectonic and magmatic events altered the composition of the continental lithosphere and added new lherzolitic and eclogitic diamonds to the already extensive Archean diamond suite.     

华北地块南部巨型陆内俯冲带与秦岭造山带岩石圈现今三维结构

豫西横穿秦岭造山带的反射地震为主的综合地球物理探测，发现秦岭现今北界存在华北地块南部自北向南向秦岭的巨型陆内俯冲带，深达Ｍｏｈｏ面以下，与之相伴而生，在中上地壳发育自南向北的逆冲推构造带，千公里东西向延伸，主要发生于晚白垩世１００Ｍａ±，成为秦岭与华北地区块间中新生代重要陆内构造，它是秦岭造山带岩石圈现今三维结构的基本要素和组成部分，秦岭造山带岩石圈现今结构具有流变学分层的“立交桥”三维结构框架模型。显然它们具有统一的动力学背景，是秦岭造山带现今处于印度－青藏、太平洋和欧亚板块的西伯利亚地块等三大构造动力学体系复合部位，导致其从深部地幔动力学的最新调整到上部地壳响应所发生的壳幔等圈层相互作用的综合产物，可能是大陆长期保存、演化的主要途径与形式之一，具有重要的大陆动力学意义，对中国大陆构造、灾害、环境研究也具重要意义。     

Mantle convection and early crustal evolution

The existence of peridotitic komatiites in the Archaean suggests that the Archaean mantle was significantly hotter than the modern mantle. This evidence is contradicted by estimates of Archaean continental geothermal gradients, based on the pressure and temperature recorded in metamorphic rocks, which suggest that there is no marked difference between Archaean and modern continental geothermal gradients.Numerical modelling shows that small changes in the mantle temperature can have an important influence on convection. If the average temperature of the upper mantle is increased by 200°C, convection within the mantle becomes chaotic and an upper mantle partial melt zone encircles the globe. The crust formed during this period will be komatiitic in composition but will be unstable and will be mixed back into the mantle by subduction. Later, when the mantle temperature falls to 100°C above its present level, the upper mantle partial melt zone contracts away from subduction areas.It is suggested that the first primitive felsic magmas were generated at subduction zones. The appearance of these magmas at ～3.8 Ga permitted the formation of buoyant continents and eventually led to crustal thickening. As a consequence of this thickening the proto-continents, consisting of a bimodal suite of basalts and sodic granodiorites, contained two types of latent energy: (1) radioactive energy held in elements such as Th, K and U; and (2) potential energy resulting from the elevation of the continents above sea level. The potential energy of the continents led to sedimentation. The increase in the rate of sedimentation during the Archaean resulted from increased crustal buoyancy. At the same time heat released by radioactive elements in the deep crust built up under the insulating blanket of the upper crust. This caused a major metamorphic, metasomatic and crustal melting event which produced the potassic granites of the late Archaean. Once the radioactive elements had been removed from the lower crust, that region of the continent become tectonically stable. The Proterozoic shelf sediments were deposited at the margins of these stable cratons.Convection models of the Archaean mantle show hot diapirs rising from the boundary layer above the core—mantle interface. We suggest that these diapirs began to melt at a depth of ～ 450 km, giving rise to komatiitic magmas. This model requires the average temperature of the Archaean upper mantle to be ～ 100°C above that of the modern mantle. The similarity between Archaean and modern continental geothermal gradients can be explained if Archaean continents formed above subduction zones.Raising the temperature of the Archaean mantle by 100°C (1) halves the thickness of the oceanic lithosphere, (2) increases the oceanic geothermal gradient at the mid-point of a convection cell, (3) decreases the viscosity of the mantle by at least an order of magnitude. The combination of these effects produces a marked decrease in the strength of the Archaean lithosphere and mantle. Thus the form of Archaean tectonics can be expected to have been very different from modern tectonics.     

Evolution of the Australian lithosphere

The evolution of the Australian plate can be interpreted in a plate‐tectonic paradigm in which lithospheric growth occurred via vertical and horizontal accretion. The lithospheric roots of Archaean lithosphere developed contemporaneously with the overlying crust. Vertical accretion of the Archaean lithosphere is probably related to the arrival of large plumes, although horizontal lithospheric accretion was also important to crustal growth. The Proterozoic was an era of major crustal growth in which the components of the North Australian, West Australian and South Australian cratons were formed and amalgamated during a series of accretionary events and continent‐continent collisions, interspersed with periods of lithospheric extension. During Phanerozoic accretionary tectonism, approximately 30% of the Australian crust was added to the eastern margin of the continent in a predominantly supra‐subduction environment. Widespread plume‐driven rifting during the breakup of Gondwana may have contributed to the destruction of Archaean lithospheric roots (as a result of lithospheric stretching). However, lithospheric growth occurred at the same time due to mafic underplating along the eastern margin of the plate. Northward drift of Australia during the Tertiary led to the development of a complex accretionary margin at the leading edge of the plate (Papua New Guinea).     

华北克拉通中生代破坏前的岩石圈地幔与下地壳

华北克拉通是世界上最古老的克拉通之一,有 38亿年的古老陆壳存在,它经历了复杂的地质变迁,在太古宙末(约2500Ma)基本完成克拉通化,在古元古代(约1900~1850Ma)整体受到了高级变质作用,最终完成了克拉通化。它的东部在中生代发生了重大的构造机制的转变,克拉通基底发生了破坏、置换和再造。在太行山重力梯度带以西的华北克拉通受中生代构造转折的改造程度较低,它们的下地壳和岩石圈地幔结构,大致保持了华北克拉通破坏前的状态。前寒武纪麻粒岩地体代表了掀翻抬升到地表的古元古代下地壳,出露地表的时间大致在1850~1800Ma。中、新生代火山岩中的地幔和麻粒岩捕虏体代表了现代的岩石圈地幔和下地壳的岩石。岩石学、地球化学和地球物理的研究,推测华北克拉通西部的岩石圈厚约200km,地壳厚度约45km~50km,是在古元古代(约1.9Ga)时期终极克拉通化作用形成的,其厚度和结构与全球典型的元古宙克拉通岩石圈相同。而太行山重力梯度带以东的克拉通岩石圈地幔受到程度不等的交代、改造、置换和减薄,下地壳大规模重熔,地壳厚度也发生减薄,指示了强烈的壳幔解耦、物质交换和重新耦合的过程。