地球早期大陆地壳的形成与演化

现代地球岩石圈主要由镁铁质上地幔和长英质地壳两个储集层组成,研究大陆地壳的形成和演化对揭示地球早期地质过程和物质循环、厘定板块构造启动时限具有重要意义。冥古宙—始太古代具有更高的地幔潜能温度和地温梯度,岩浆海冷却形成薄的原始地壳;大洋岩石圈表现为韧性,主要构造机制应为停滞盖层模式,有地幔柱参与。太古宙片麻岩中奥长花岗岩—英云闪长岩—花岗闪长岩（TTG）的出现标志着镁铁质原始地壳向长英质陆壳转变的开始。本文总结了地球早期停滞盖层模式到现代板块构造模式下含水玄武岩部分熔融、结晶分异形成大陆地壳的过程,主要包含幔源岩浆停滞盖层（“自下而上”的热管火山岩和“自上而下”的深成侵入岩构造模式）、增厚镁铁质地壳部分熔融、俯冲洋壳、岛弧及洋底高原部分熔融模式;陆壳的破坏和消减主要受陨石撞击、分层沉降、重力不稳导致拆沉控制;板块构造的出现进一步促进了地球内部的热量扩散,俯冲作用加快了洋壳和陆壳之间的物质循环。最后,结合太古宙变质岩、古老克拉通岩石学特征和锆石Hf、O及全岩Nd、Sr、Ar、Ti同位素组成,讨论了陆壳的形成时间和演化过程： 3.0 Ga之前形成了现有陆壳体积的60%~70%,厚度约为20~40 km;3.0~2.5 Ga,地壳改造速率明显增加,陆壳生长和破坏速率达到动态平衡,表明全球性现代板块构造体制逐渐成为控制大陆形成、裂解和陆壳演化的主要因素。     

地球早期大陆地壳的形成与演化

现代地球岩石圈主要由镁铁质上地幔和长英质地壳两个储集层组成,研究大陆地壳的形成和演化对揭示地球早期地质过程和物质循环、厘定板块构造启动时限具有重要意义。冥古宙—始太古代具有更高的地幔潜能温度和地温梯度,岩浆海冷却形成薄的原始地壳;大洋岩石圈表现为韧性,主要构造机制应为停滞盖层模式,有地幔柱参与。太古宙片麻岩中奥长花岗岩—英云闪长岩—花岗闪长岩(TTG)的出现标志着镁铁质原始地壳向长英质陆壳转变的开始。本文总结了地球早期停滞盖层模式到现代板块构造模式下含水玄武岩部分熔融、结晶分异形成大陆地壳的过程,主要包含幔源岩浆停滞盖层(“自下而上”的热管火山岩和“自上而下”的深成侵入岩构造模式)、增厚镁铁质地壳部分熔融、俯冲洋壳、岛弧及洋底高原部分熔融模式;陆壳的破坏和消减主要受陨石撞击、分层沉降、重力不稳导致拆沉控制;板块构造的出现进一步促进了地球内部的热量扩散,俯冲作用加快了洋壳和陆壳之间的物质循环。最后,结合太古宙变质岩、古老克拉通岩石学特征和锆石Hf、O及全岩Nd、Sr、Ar、Ti同位素组成,讨论了陆壳的形成时间和演化过程：3.0 Ga之前形成了现有陆壳体积的60%～70%,厚度约为20～4...     

钼同位素揭示地球早期大陆地壳的快速生长和破坏北大核心CSCD

地球早期,尤其是冥古代-早太古代时期大陆地壳的性质、范围和产生机制一直是地球科学研究的前沿问题。在已有的大陆地壳生长模式中,有些认为当前大陆地壳的60%~80%在30亿年前就已形成,而有些则倾向于一个渐进的地壳生长模式。不论哪种模式,大都基于锆石的形成年龄或者壳幔系统的放射性同位素演化。然而,由于不能限定再循环物质的量,锆石仅能限定地壳生长的年龄下限。相反,放射性同位素体系演化曲线可有助于了解地幔亏损过程,壳幔分离和地壳再循环作用。基于长寿命同位素(Sr、Nd及Hf)体系在壳幔间的互补性,前人估计要达到当前地壳和亏损地幔间的组成平衡,只需25%~50%的地幔物质经历熔体抽提。     

钼同位素揭示地球早期大陆地壳的快速生长和破坏

正地球早期,尤其是冥古代-早太古代时期大陆地壳的性质、范围和产生机制一直是地球科学研究的前沿问题。在已有的大陆地壳生长模式中,有些认为当前大陆地壳的60%～80%在30亿年前就已形成,而有些则倾向于一个渐进的地壳生长模式。不论哪种模式,大都基于锆石的形成年龄或者壳幔系统的放射性同位素演化。然而,由于不能限定再循环物质的量,锆石仅能限定地壳生长的年龄下限。相反,放射性同位素体系演化曲线可有助于了解地幔亏损过程,壳幔分离和地壳再循环作用。基于     

试论陆壳增生的两种基本模式及其对比

大陆地壳是地球形成演化的必然产物。大陆地壳由不同时代、不同类型、不同规模地体的拼贴而增生；同时已形成的大陆地壳沿着新的断裂分裂、离散而碱小。因此大陆地壳是地体拼贴增生与分裂离散的综合结果。太古代早期，原始陆壳形成后，主要通过环太平洋型与天山型两种基本模式达到陆壳的增生。环太平洋型陆壳增生模式出现于陆块的边缘，由古大陆向大洋方向单向增生，增生年代由老到新，增生地体一般都有较大距离的移置，其增生与板块的俯冲作用密切有关。天山型陆壳增生模式出现在陆块的内部，其形成与陆块的开台作用密切有关，可以但不一定伴随有俯冲作用。当古大陆沿一定方向断裂带分裂、离散。其间形成新的海槽接受碳酸盐岩和正常陆源碎屑沉积物与来自地壳深部或地幔的火山物质。由于壳下应力条件改变，两侧古陆相向运动，海槽中物质受两侧古陆碰撞挤压，形成褶皱造山带，并把两侧的古大陆“焊接”成新的、范围更大的大陆地壳。     

识别洋陆转换的岩石学思路——洋内弧与初始俯冲的识别

阐述了洋陆转化形成的洋内弧与初始弧的岩石组合序列及其地球化学特征,提出岩浆弧是由洋陆转化以及底侵的壳幔转化共同作用形成的认识,前弧环境是洋陆转化形成初生大陆的场所,由特征的类似洋中脊的洋内弧前弧玄武岩类构成。大陆的形成过程如下:从地幔中生长出洋壳,从洋壳中的洋陆转化生长出不成熟的弧陆壳,最后从弧陆壳底侵的壳幔转化中长出成熟的陆壳。这样,地壳的生长和形成主要通过岩浆增生作用来实现。     

中国花岗岩与大陆地壳生长方式初步研究

中国大陆造山带花岗岩可分为东西两个区，西区的中亚造山带、秦祁昆造山带和青藏高原冈底斯造山带为与大洋发育有关的造山带花岗岩，东区主体的东北、华北和华南是形成于中国大陆拼合之后的燕山期造山带花岗岩。根据不同造山带花岗岩的形成背景、地质地球化学特征差异，以阿尔泰、东昆仑、华北燕山、东北和南岭造山带花岗岩为例讨论花岗岩与大陆地壳生长的关系，区分出中国大陆的5种大陆地壳生长方式：阿尔泰式是古亚洲洋背景上形成的古生代对流地幔物质、热输入和上地壳混合为主的方式；东昆仑式是元古代造山带TTG陆壳背景基础上古生代一早中生代对流地幔物质和热输入，改造元古宙造山带基底的方式；东北式是燕山期中亚造山带背景上对流地幔物质和热输入改造显生宙陆壳的生长方式；燕山式是燕山期对流地幔物质和热输入改造太古宙基底的方式；南岭式燕山期对流地幔输入大陆的是以热为主、物质为辅，大陆地壳生长是以陆壳物质再循环为主（零增长）的生长方式。它们构成中国大陆显生宙地壳生长的基本方式。     

华北大陆地壳—上地幔岩石学结构与演化

本文基于出露的前寒武纪变质岩系、中、新生代岩浆活动以及新生代玄武岩中上地幔包体的岩石学与地质压力计研究，结合地球物理测深资料与高温高压下岩石中地震波传播速度的实验成果，提出了华北大陆三个地区(河北平原、太行一五台、鄂尔多斯)的地壳一上地幔岩石学结构，讨论了界面性质及其演化。在强调Vp、 Vs、a结构与岩石学结构共同约束的基础上，有效地识别了不同地区硅铝质陆壳在物质组成上的差异和上地幔低速层或矿物相转变等特征。本文提出壳-幔岩石学结构及其演化，密切地与陆壳主要形成时期的太古一早元古构造岩浆事件相关，又与显生宙构造岩浆事件对它的改造程度有关，壳-幔岩石学结构是我们追索大陆的构造性质及其演化的一个重要记录和科学依据。     

中国东部陆壳洋幔型岩石圈及其形成机制

长期以来中国东部岩石圈具有何种特殊性,其类型是否转变,其形成年代与形成机制等问题一直存在着不同意见。通过系统研究中国东部地质、地球化学与地球物理资料,笔者提出中国东部大陆地壳在侏罗纪晚期,受北美板块WSW向强烈的挤压以及特提斯洋朝东北方向张开的共同作用,使东亚大陆地壳发生逆时针转动,以致中国东部陆壳水平滑移到古老的洋壳或洋幔之上。在中国东部,原来的大陆型岩石圈的边部就出现较薄的陆壳洋幔型岩石圈(陆壳厚30~40 km,洋幔厚40~50 km)。幔源包体的资料表明中国东部岩石圈地幔与软流圈是未经扰动或轻微扰动,它们均发生在太古宙或中新元古代,还没有足够的在中、新生代发生扰动的证据。看来造成上述岩石圈结构类型的转换,不大可能是中、新生代大洋板块俯冲作用的直接结果,也不像是深部热地幔上涌或底侵作用的产物。强构造–岩浆活动源区形成于区域性断层与中地壳或莫霍面的构造滑脱的交切带,只有极少数的断层可深达岩石圈底面。中国东部岩石圈的构造滑脱面主要在中地壳或莫霍面,而不是在软流圈。中国东部中新生代强烈的构造-岩浆作用与内生金属成矿作用,正是受此种特殊岩石圈结构的控制而形成的。总之笔者认为:亚洲与中国大陆东部的岩石圈(包括其中的华北地区)在中、新生代并没有发生"克拉通的裂解",而只是岩石圈的类型发生了变化,出现了一种洋陆过渡类型的陆壳洋幔型岩石圈。     

陆壳的垂向增生

陆壳的垂向增生是幔源物质进入到下地壳和下地壳物质进入到地幔的双向过程 ,前者主要表现为壳下底侵作用 ,后者主要表现为岩石圈规模的拆沉作用 ,其中拆沉作用往往诱发了陆壳下大规模的底侵作用。下地壳部分熔融残余的超镁铁质岩沉入到岩石圈地幔的过程称为陆壳沉没作用 ,它可能是陆壳物质进入地幔的一种重要方式。     

Continental recycling and true continental growth

Continental crust is very important for evolution of life because most bioessential elements are supplied from continent to ocean. In addition, the distribution of continent affects climate because continents have much higher albedo than ocean, equivalent to cloud. Conventional views suggest that continental crust is gradually growing through the geologic time and that most continental crust was formed in the Phanerozoic and late Proterozoic. However, the thermal evolution of the Earth implies that much amounts of continental crust should be formed in the early Earth. This is “Continental crust paradox”.Continental crust comprises granitoid, accretionary complex, and sedimentary and metamorphic rocks. The latter three components originate from erosion of continental crust because the accretionary and metamorphic complexes mainly consist of clastic materials. Granitoid has two components: a juvenile component through slab-melting and a recycling component by remelting of continental materials. Namely, only the juvenile component contributes to net continental growth. The remains originate from recycling of continental crust. Continental recycling has three components: intracrustal recycling, crustal reworking, and crust–mantle recycling, respectively. The estimate of continental growth is highly varied. Thermal history implied the rapid growth in the early Earth, whereas the present distribution of continental crust suggests the slow growth. The former regards continental recycling as important whereas the latter regarded as insignificant, suggesting that the variation of estimate for the continental growth is due to involvement of continental recycling.We estimated erosion rate of continental crust and calculated secular changes of continental formation and destruction to fit four conditions: present distribution of continental crust (no continental recycling), geochronology of zircons (intracontinental recycling), Hf isotope ratios of zircons (crustal reworking) and secular change of mantle temperature. The calculation suggests some important insights. (1) The distribution of continental crust around at 2.7 Ga is equivalent to the modern amounts. (2) Especially, the distribution of continental crust from 2.7 to 1.6 Ga was much larger than at present, and the sizes of the total continental crust around 2.4, 1.7, and 0.8 Ga became maximum. The distribution of continental crust has been decreasing since then. More amounts of continental crust were formed at higher mantle temperatures at 2.7, 1.9, and 0.9 Ga, and more amounts were destructed after then. As a result, the mantle overturns led to both the abrupt continental formation and destruction, and extinguished older continental crust. The timing of large distribution of continental crust apparently corresponds to the timing of icehouse periods in Precambrian.     

Growing primordial continental crust self-consistently in global mantle convection models

The majority of continental crust formed during the hotter Archean was composed of Tonalite-Trondhjemite-Granodiorite (TTG) rocks. In contrast to the present-day loci of crust formation around subduction zones and intra-plate tectonic settings, TTGs are formed when hydrated basalt melts at garnet-amphibolite, granulite or eclogite facies conditions. Generating continental crust requires a two step differentiation process. Basaltic magma is extracted from the pyrolytic mantle, is hydrated, and then partially melts to form continental crust. Here, we parameterise the melt production and melt extraction processes and show self-consistent generation of primordial continental crust using evolutionary thermochemical mantle convection models. To study the growth of TTG and the geodynamic regime of early Earth, we systematically vary the ratio of intrusive (plutonic) and eruptive (volcanic) magmatism, initial core temperature, and internal friction coefficient. As the amount of TTG that can be extracted from the basalt (or basalt-to-TTG production efficiency) is not known, we also test two different values in our simulations, thereby limiting TTG mass to 10% or 50% of basalt mass. For simulations with lower basalt-to-TTG production efficiency, the volume of TTG crust produced is in agreement with net crustal growth models but overall crustal (basaltic and TTG) composition stays more mafic than expected from geochemical data. With higher production efficiency, abundant TTG crust is produced, with a production rate far exceeding typical net crustal growth models but the felsic to mafic crustal ratio follows the expected trend. These modelling results indicate that (i) early Earth exhibited a “plutonic squishy lid” or vertical-tectonics geodynamic regime, (ii) present-day slab-driven subduction was not necessary for the production of early continental crust, and (iii) the Archean Earth was dominated by intrusive magmatism as opposed to “heat-pipe” eruptive magmatism.     

俯冲工厂和大陆物质的俯冲再循环研究

板块的俯冲系统可以比拟为一个工厂。再循环研究强调对俯冲物质各种组分的行为、去向的追踪和定量分析。沉积物俯冲和俯冲侵蚀作用导致陆壳物质返回地幔,初步估算表明,大陆物质返回地幔的速率与岩浆活动导致陆壳生长的速率在数量上大体相当,晚近时期陆壳的净增长速率可能近于零。大洋岛玄武岩地化特征上的多样性提示,沉入下地幔的板片可能从深部卷入地幔柱的源区。俯冲再循环过程对地壳、地幔的动力学和演化产生深刻影响。     

地球历史中矿产资源分布之规律

现有的矿产资源分布资料可以说明地球历史中亲铁和亲铜元素成矿的一般规律。铁矿床在元古宙地核分离之后最为发育。除个别显著成矿高峰外 ,金的储量分布与铁类似。亲铁元素矿物的成矿潜力在古元古代也最大 ,以后逐渐降低 ,只是在显生宙 ,因地壳的再循环和古老矿床中矿石物质的活化而再次增大。元素的行为则完全不同 ,在前寒武纪时期它们的活动性为中等。亲铜矿物矿床的总量 ,在显生宙因再循环而达到最高 ,亲铜矿物矿床成矿能力较之亲铁矿物强得多 ,因为硫化物比铁金属氧化物更为活泼。地球演化的旋回性表现为超大陆形成的周期性 ,它与矿产的分布颇为一致。铁矿、亲铁元素和金成矿作用的突然激增 ,显然是在超大陆形成之后 ,并且与其早期裂解的脉动相对应。亲铜元素堆积的高峰期往往与超大陆的形成期相对应。尽管亲铜元素成矿的第三次脉动与中生代古陆 (Mesogea)的形成并不一致 ,但却与劳亚古陆及冈瓦纳古陆的裂解相一致。此种成矿堆积现象可以借助地幔内非稳态化学密度对流所诱发的地壳发展周期性来解释。根据超大陆形成的周期性推断地幔对流巨旋回的主期应为 80 0Ma ,此种周期性亦反映了具原始地幔印记的亲铁、金和亲铜元素的聚集作用过程。因此 ,地球圈层中地幔对流作用的周期性亦显著地控制了包括地幔     

Evolution of the early Earth: Constraints from Nd---Nd isotopic systematics

Combined ¹⁴⁷Sm---¹⁴³Nd and the now extinct [τ(1/2)₁₄₆=103×10⁶ yr] ¹⁴⁶Sm---¹⁴²Nd isotopic systematics are reported for early Archean gneisses from Greenland (Amîtsoq and Akilia associations), and Canada (Acasta gneiss). Using both field relationships and high resolution U---Pb SHRIMP ion-probe ages, it has been possible to identify the most ancient rocks in these terrains for isotopic analyses. Preliminary ¹⁴²Nd analyses of a still limited number of samples have failed to identify terrestrial ¹⁴²Nd anomalies. Effects, if present, are limited to < 10 ppm and we have thus been unable to confirm the +33±4 ppm ε₁₄₂ value claimed by Harper and Jacobsen (1992a, b) for a single sample. From the lack of ¹⁴⁶Sm---¹⁴²Nd effects we infer that large-scale fractionation events that may have occurred in the first 200 Ma of Earth history did not leave a significant nor widespread imprint on the early Archean mantle or crust. If a terrestrial magma ocean, with associated LREE fractionation, formed as a result of planetary accretion, then it had a lifetime of at most 250 m.y. before being remixed into the Earth's mantle.

The samples analysed in this study have a range of ε₁₄₃ values including highly positive values of up to +4.2. This requires that the earliest known Archean crust was differentiated from a reservoir that was strongly depleted in the LREE as compared with chondritic compositions. In the early Archean it is proposed that the depletions in LREE are a consequence of extraction of a limited fraction of the Earth's continental crust ( < 10%) from the upper 200 km of the mantle. A three reservoir model, consisting of the continental crust, depleted mantle and a more primitive mantle reservoir can be extended to account for both the present-day, as well as the evolving Nd isotopic composition of the Earth's crust and mantle. In contrast to previous models, the rate of growth of the continental crust is used as an input parameter to constrain the concomitant growth and evolution of the depleted mantle reservoir. Recycling of large volumes of bulk continental crust into the mantle is not considered to be an important process, nor is the existence of an additional major enriched component in the early Archean mantle.     

On the scarcity of >3900 Ma detrital zircons in ≥3500 Ma metasediments

Constant volume models for the continental crust require a flux of crustal material back into the mantle (recycling), equal in volume to that of the juvenile igneous suites added to the continental crust throughout time. In growth of crustal volume models, there is not equilibrium between the volume of juvenile crustal additions and any recycling (destruction) of crust. By establishing the proportion of >3900 Ma detrital zircons in early Archaean sediments it might be possible to constrain the relative importance of crustal growth and recycling. Gneiss complexes in western Greenland, northern Labrador and northeastern China contain rare ≥3500 Ma detrital metasediments. In sediments deposited between 3500 and 3600 Ma, ≥3900 Ma zircons have not been detected in a suite of 117 detrital grains. Based on statistical considerations, at the 95% confidence level any ‘missed’ ≥3900 Ma component forms <3% of this suite. Likewise, >3900 Ma detrital grains do not occur amongst 54 detrital grains from (even rarer) 3700–3800 Ma sediments, arguing with 95% certainty that any ‘missed’ ≥3900 Ma component forms <5% of this most ancient suite. If the age spectra of these detrital zircon suites are representative of the complexes in which they reside, then constant volume (recycling=new additions) models require that by 3500 Ma, >97% of >3900 Ma crust was destroyed by recycling. Such an extremely high recycling rate (≈25% of the crust 100 Ma⁻¹) is hard to reconcile with the diversity of initial Nd and Sr isotopic ratios of well preserved early Archaean granitoid suites in the same complexes, because significant average crustal residence times are required to permit the radiogenic isotopic systems to evolve. The most likely interpretation of the detrital zircon record in the Greenland, Labrador and China sediments is that in their provenance areas the volume of continental crust was small at 3900 Ma, and that it grew significantly during the early Archaean. If the measured ≥3500 Ma detrital sediment suites are globally representative, they support growth models for the continental crust in the early Archaean, rather than models involving recycling of a voluminous >3900 Ma sialic crust. Because of its global coverage and the dating of thousands of grains, the age spectra for detrital zircons from 3000–3200 Ma sediments provides a more reliable impression of crustal ages. However, as they were deposited 700–900 Ma after 3900 Ma, the globally small proportion of ≥3900 Ma detrital grains in them (from Jack Hills, Mt. Narryer and Wyoming) can be accommodated in both crustal growth and moderate recycling models.     

Evolution of the 3.65–2.58 Ga Mairi Gneiss Complex,Brazil: Implications for growth of the continental crust in the São Francisco Craton

The composition and formation of the Earth’s primitive continental crust and mantle differentiation are key issues to understand and reconstruct the geodynamic terrestrial evolution, especially during the Archean. However, the scarcity of exposure to these rocks, the complexity of lithological relationships, and the high degree of superimposed deformation, especially with long-lived magmatism, make it difficult to study ancient rocks. Despite this complexity, exposures of the Archean Mairi Gneiss Complex basement unit in the São Francisco Craton offer important information about the evolution of South America’s primitive crust. Therefore, here we present field relationships, LA-ICP-SFMS zircon U-Pb ages, and LA-ICP-MCMS Lu-Hf isotope data for the recently identified Eoarchean to Neoarchean gneisses of the Mairi Complex. The Complex is composed of massive and banded gneisses with mafic members ranging from dioritic to tonalitic, and felsic members ranging from TTG (Tonalite-Trondhjemite-Granodiorite) to granitic composition. Our new data point to several magmatic episodes in the formation of the Mairi Gneiss Complex: Eoarchean (ca. 3.65–3.60 Ga), early Paleoarchean (ca. 3.55–3.52 Ga), middle-late Paleoarchean (ca. 3.49–3.33 Ga) and Neoarchean (ca. 2.74–2.58 Ga), with no records of Mesoarchean rocks. Lu-Hf data unveiled a progressive evolution of mantle differentiation and crustal recycling over time. In the Eoarchean, rocks are probably formed by the interaction between the pre-existing crust and juvenile contribution from chondritic to weakly depleted mantle sources, whereas mantle depletion played a role in the Paleoarchean, followed by greater differentiation of the crust with thickening and recycling in the middle–late Paleoarchean. A different stage of crustal growth and recycling dominated the Neoarchean, probably owing to the thickening of the continental crust by collision, continental arc growth, and mantle differentiation.     

地壳深俯冲与富钾火山岩成因

富钾火山岩是一类兼具壳幔双重地球化学特征的特殊岩石组合 ,它们不可能由亏损或原始地幔所派生 ,成岩过程中必须有地壳物质的参与 ,将地壳物质引入富钾火山岩成岩过程的主要动力机制即是深俯冲作用。洋壳和陆壳均可以通过俯冲进入地幔 ,俯冲地壳物质析出流体对地幔岩石的交代作用是导致富钾火山岩具特殊地球化学特征的主要原因。根据对大别—苏鲁造山带南北两侧晚中生代富钾火山岩的实例研究 ,表明该区火山岩的形成均受到了俯冲洋壳析出流体的交代作用 ,但造山带北侧富钾火山岩的形成还叠加了俯冲的扬子陆壳析出流体的交代作用 ,是多次富集事件综合作用的结果。文中还对富钾火山岩成因研究中值得进一步深入探索的问题进行了讨论。     

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。     

Implications of Nb/U, Th/U and Sm/Nd in plume magmas for the relationship between continental and oceanic crust formation and the development of the depleted mantle

The Nb/U and Th/U of the primitive mantle are 34 and 4.04 respectively, which compare with 9.7 and 3.96 for the continental crust. Extraction of continental crust from the mantle therefore has a profound influence on its Nb/U but little influence on its Th/U. Conversely, extraction of midocean ridge-type basalts lowers the Th/U of the mantle residue but has little influence on its Nb/U. As a consequence, variations in Th/U and Nb/U with Sm/Nd can be used to evaluate the relative importance of continental and basaltic crust extraction in the formation of the depleted (Sm/Nd enriched) mantle reservoir.This study evaluates Nb/U, Th/U, and Sm/Nd variations in suites of komatiites, picrites, and their associated basalts, of various ages, to determine whether basalt and/or continental crust have been extracted from their source region. Emphasis is placed on komatiites and picrites because they formed at high degrees of partial melting and are expected to have Nb/U, Th/U, and Sm/Nd that are essentially the same as the mantle that melted to produce them. The results show that all of the studied suites, with the exception of the Barberton, have had both continental crust and basaltic crust extracted from their mantle source region. The high Sm/Nd of the Gorgona and Munro komatiites require the elevated ratios seen in these suites to be due primarily to extraction of basaltic crust from their source regions, whereas basaltic and continental crust extraction are of subequal importance in the source regions of the Yilgarn and Belingwe komatiites. The Sm/Nd of modern midocean ridge basalts lies above the crustal extraction curve on a plot of Sm/Nd against Nb/U, which requires the upper mantle to have had both basaltic and continental crust extracted from it.It is suggested that the extraction of the basaltic reservoir from the mantle occurs at midocean ridges and that the basaltic crust, together with its complementary depleted mantle residue, is subducted to the core-mantle boundary. When the two components reach thermal equilibrium with their surroundings, the lighter depleted component separates from the denser basaltic component. Both are eventually returned to the upper mantle, but the lighter depleted component has a shorter residence time in the lower mantle than the denser basaltic component. If the difference in the recycling times for the basaltic and depleted components is ∼1.0 to 1.5 Ga, a basaltic reservoir is created in the lower mantle, equivalent to the amount of basalt that is subducted in 1.0 to 1.5 Ga, and that reservoir is isolated from the upper mantle. It is this reservoir that is responsible for the Sm/Nd ratio of the upper mantle lying above the trend predicted by extraction of continental crust on the plot of Sm/Nd against Nb/U.