扬子克拉通北缘新元古代大陆岩石圈脱层作用信息——毕机沟镁铁质-超镁铁质侵入体同位素地球化学指示

毕机沟侵入体作为扬子克拉通北缘已知分布面积最大的镁铁质－超镁铁质侵入岩体，在扬子克拉通岩石圈演化研究中具有重要地位。除在七十年代开展过部分矿产资源调查工作外，长期缺乏深入研究。本文在Ｓｍ－Ｎｄ和Ｒｂ－Ｓｒ年代学工作基础上厘定了毕机沟侵入体的侵位和后期构造热事件时代；根据侵入体源区地幔组成特征和地幔端元组成，显示出毕机沟岩体源区具有与大陆岩石圈脱层作用有关的成因信息，结合晚元古代扬子克拉通北缘在构造体制、幔源岩浆性质和地壳增生方式等发生突变的地质背景以及现代大陆岩石圈脱层作用研究成果，提出了区内新元古代可能存在大陆岩石圈脱层作用的初步认识     

扬子克拉通北缘中、新元古代洋壳俯冲及壳幔再循环作用的同位素地球化学证据

报道和总结了近期陕西汉中碑坝和西乡地区重要地质体系统同位素年代研究成果，在重新厘定的年代学基础上，就扬子克拉通北缘同位素特征及其地质意义进行剖析，依据该区岩浆作用的上地幔源区性质及演化规律，提出了中、新元古代扬子克拉通北缘存在与现代板块运动类似的洋壳俯冲和壳幔再循环作用的同位素地球化学证据     

扬子克拉通北缘中,新元古代洋壳俯冲及壳幔再循…

报道和总结了近期陕西汉中碑坝和西乡地区重要地质体系统同位素年代学研究成果，在重新厘定的年代学基础上，就扬子克拉通北缘同位素特征及其地质意义进行剖析，依据该区岩浆作用的上地幔源区性质及演化规律，提出了中、新元古代扬子克拉通北缘存在与现代板块运动类似的洋壳俯冲和壳幔再循环作用的同位素地球化学证据。     

扬子克拉通北缘元古宙基底同位素地质年代学和地壳增生历史：Ⅰ．后河群和西乡群

多种同位素年代学方法综合研究表明，扬子克拉通北缘中段基底岩系后河群和西乡群变质地层的原岩形成时代并非早期推测或报道的新元古代．后河群斜长角闪岩原岩的形成时代应为古元古代，并有太古宙岩石存在的可能．后河群混合岩化变质作用发生于２１４１Ｍａ．西乡群底部白勉峡组属古元古代，而中上部孙家河组为中元古代末期．普遍存在的７００～８００Ｍａ的Ｋ－Ａｒ和Ｒｂ－Ｓｒ年龄反映了区内于晋宁期发生了一次大规模的构造热事件     

扬子克拉通北缘新元古代岛弧花岗岩类成分极性…

对扬子克拉通北缘新古代岛弧花岗岩类成分极性的地球化学研究表明，在ＳｉＯ２含量相近的条件下，由北向南岩石中Ｋ２Ｏ、Ｋ２Ｏ＋Ｎａ２Ｏ，Ｋ２Ｏ／Ｎａ２Ｏ逐渐增加，而ＣａＯ逐渐降低；从北向南，微量元素Ｒｂ，Ｂａ，Ｔｈ，Ｚｒ，Ｎｂ，Ｔａ，ΣＲＥＥ及Ｒｂ／Ｔｈ，Ｒｂ／Ｎｂ，ΣＬＲＥＥ／ΣＨＲＥＥ（Ｌａ／Ｙｂ）Ｎ等逐渐增加或具增加趋势，而Ｓｒ，Ｃｏ，Ｎｉ和Ｓｒ／Ｂａ具逐渐降低或具降低趋势；同位素组成成由北向南δ     

扬子克拉通北缘元古宙基底同位素地质年代学和地壳增生历史：Ⅱ．火地垭群

扬子克拉通地壳的生成始于太古宙，但依据目前的研究成果，其主体形成于元古宙，对位于扬子克拉通北缘，分布在碑坝地区重要的基底岩系之一的火地垭群铁船山组火山岩进行了Ｓｍ－Ｎｄ和Ｒｂ－Ｓｒ同位素地球化学研究．同位素年代学数据表明，铁船山组岩系形成于约（１６６８５）Ｍａ的中元古代，并源于高亏损的地幔岩浆库（ｄ（ｔ）７．５３．１０），且其亏损地幔模式年龄ＴＤＭ（（１．６８±０．１０）Ｇａ）与Ｓｍ－Ｎｄ等时年龄是一致的．Ｒｂ－Ｓｒ同位素年代学揭示出在新元古代早期（约（８６０±１２）Ｍａ的晋宁期）铁船山组岩系受到了构造热事件的改造．该事件与区内大规模的基性－超基性和碱性、中－酸性岩浆活动的同时出现在时间上是一致的．这些晋宁期岩浆作用有着以下共同特征：（１）岩浆源于亏损程度大为降低的岩浆库；（２）模式年龄ＴＤＭ与铁船山组火山岩的形成时代相近．在本区的西乡地区，同为中元古代形成的西乡群底部的白勉峡组火山岩与其周围的晋宁期基性－超基性和碱性、中－酸性岩浆岩在年代学特征、岩浆库性质和亏损地幔模式年龄等多方面与碑坝地区铁船山组火山岩及其周围晋宁期岩浆岩的特征是类似的．这些同位素地球化学特征所揭示的区内地壳增生和晋宁期构造历史进一     

内蒙古喀喇沁堆晶岩捕虏体和寄主闪长岩的同位素年龄

喀喇沁地区们于大兴安岭中生代构造岩浆岩带与华北克拉通交汇部位。研究区的闪长岩侵入到太古宙基底中。闪长岩中含有二辉岩、辉长岩、苏长岩、角闪岩、斜长岩等不同组分的镁铁质－超镁铁堆晶岩捕虏体和基性麻粒岩捕虏体,它们是幔源玄武质岩浆底侵作用的产物。分别用Ｒｂ－Ｓｒ法、Ｋ－Ａｒ法和Ａｒ－Ａｒ法,对堆晶岩捕虏体及其寄主岩全岩或单矿物进行了详细的同位素地质年代学研究,共获得各种年龄数据８件,橄榄辉石岩的全岩Ｒｂ     

侵入火山碎屑岩中的铷—锶系统

对泛高加索山脉前脊的华力西晚期流纹质侵入凝灰岩，利用Ｒｂ－Ｓｒ同位素素统来重建侵入火山碎屑岩的历史，在侵入火山碎屑岩建造中，石英和未蚀变黑云母中熔融包裹体的Ｒｂ－Ｓｒ系统可作为岩浆期的指示剂；钾长和钠长石的同位素系统可作为热液期的指示剂；绿泥石的Ｒｂ－Ｓｒ系统可作为风化物的成岩作用期的指示剂。     

安徽黄梅尖岩体热历史及其与成矿关系：同位素证据

本文根据全岩Ｒｂ－Ｓｔ，锆石Ｕ－Ｐｂ和角闪石、黑云毒、钾长石Ｋ－Ａｒ同位素年龄综合测定结果，再造了安庆－庐江石英正长岩带中黄梅尖岩体的冷却史。矿物对氧同位素地质测温结果证实，扩散作用是控制同位素体系封闭的主导因素。     

黑龙江碾子山晶洞碱性花岗岩岩体年龄及其意义

用Ｋ－Ａｒ法和和Ｒｂ－Ｓｒ等时线法测定了碾子山晶洞碱性花岗岩的年龄，花岗岩及其围岩安山岩、正长斑岩、碱流岩、玄武岩和闪长岩脉的年龄分别为１２３、１４１、１３５、１２０、１０８和８０Ｍａ。该期碱性花岗岩的侵位反映了大兴安岭南部早白垩世以形成拗陷盆地为物质特征的构造岩浆活动。碱性花岗岩与其主要围岩构成了同一Ｒｂ－Ｓｒ等时线，表明它们是同源的；Ｓｒ同位素初始值和其他同位素证据表明它们是由上地幔来源的母岩     

扬子崆岭新太古代壳-幔地球化学特征及其与华北克拉通-大别造山带的对比

认识华北与扬子克拉通太古宙时期是否属同一岩石圈块体以及大别造山带的归属对研究中国大陆早期演化历史具有重要地质意义．对新太古代斜长角闪岩和ＴＴＧ片麻岩组合样品系统的元素和同位素地球化学研究结果表明,新太古代扬子崆岭与华北陆块存在岩石圈地幔性质上的明显差异,其ＴＴＧ片麻岩的岩浆形成环境与条件也区别显著．湖北大别杂岩在一系列地球化学特征上表现出与扬子崆岭不同的特殊性,而与扬子北缘后河杂岩地球化学特征相似．崆岭斜长角闪岩、ＴＴＧ片麻岩和华北ＴＴＧ片麻岩的Ｕ,Ｔｈ元素组成特征符合其Ｐｂ同位素填图成果,但华北斜长角闪岩Ｕ,Ｔｈ元素组成特点与其Ｐｂ同位素填图成果不一致     

Petrology and geochemistry of spinel peridotite xenoliths from Hannuoba and Qixia, North China craton

We report mineralogical and chemical compositions of spinel peridotite xenoliths from two Tertiary alkali basalt localities on the Archean North China craton (Hannuoba, located in the central orogenic block, and Qixia, in the eastern block). The two peridotite suites have major element compositions that are indistinguishable from each other and reflect variable degrees (0–25%) of melt extraction from a primitive mantle source. Their compositions are markedly different from typical cratonic lithosphere, consistent with previous suggestions for removal of the Archean mantle lithosphere beneath this craton. Our previously published Os isotopic results for these samples [Earth Planet. Sci. Lett. 198 (2002) 307] show that lithosphere replacement occurred in the Paleoproterozoic beneath Hannuoba, but in the Phanerozoic beneath Qixia. Thus, we see no evidence for a compositional distinction between Proterozoic and Phanerozoic continental lithospheric mantle. The Hannuoba xenoliths equilibrated over a more extensive temperature (hence depth) interval than the Qixia xenoliths. Neither suite shows a correlation between equilibration temperature and major element composition, indicating that the lithosphere is not chemically stratified in either area. Trace element and Sr and Nd isotopic compositions of the Hannuoba xenoliths reflect recent metasomatic overprinting that is not related to the Tertiary magmatism in this area.     

华北克拉通晚中生代壳-幔拆离作用: 岩石流变学约束

大陆岩石圈的流变学结构对于岩石圈深部过程(壳/幔过程)有着深刻的影响,直接表现在岩石圈壳-幔结构与浅部构造上.本文注意到华北克拉通晚中生代岩石圈减薄期间地壳的伸展、拆离与减薄在不同地区的宏观、微观构造及地壳岩石流变学等方面的差异表现与区域变化,以及现今和晚中生代时期岩石圈厚度的不均匀性.讨论了以水为主体的地质流体的存在对于岩石圈流变性的影响.综合克拉通东部与西部地壳/地幔厚度变化特点以及下地壳和上地幔含水性特点,阐述了晚中生代时期华北克拉通岩石圈内部壳幔耦合与解耦的规律,提出了华北岩石圈壳-幔拆离作用模型以解释华北克拉通晚中生代岩石圈减薄的基本现象与深部过程.提出区域性伸展作用是岩石圈减薄的主要动力学因素,东部地区在晚中生代伸展作用过程中壳-幔具有典型的解耦性,上部地壳、下部地壳和岩石圈地幔的变形具有显著差异性.而西部区壳幔总体具有耦合性,下地壳与岩石圈地幔共同构成流变学强度很高且难以变形的岩石圈根.     

Integrated models of diamond formation and craton evolution

Two decades of diamond research in southern Africa allow the age, average N content and carbon composition of diamonds, and the dominant paragenesis of their syngenetic silicate and sulfide inclusions to be integrated on a cratonwide scale with a model of craton formation. 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 mid-Proterozoic and display little correspondence with the prominent variations in the P-wave velocity (±1%) that the mantle lithosphere shows at depths within the diamond stability field (150–225 km). Silicate inclusions in diamonds and their host diamond compositions for the above kimberlites, Finsch, Jagersfontein, Roberts Victor, Premier, Venetia, and Letlhakane show a regional relationship to the seismic velocity of the lithosphere. Mantle lithosphere with slower P-wave velocity relative to the craton average correlates with a greater proportion of eclogitic vs. 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. The oldest formation ages of diamonds support a model whereby mantle that became part of the continental keel of cratonic nuclei first was created by middle Archean (3.2–3.3 Ga or older) mantle depletion events with high degrees of melting and early harzburgite formation. The predominance of eclogitic sulfide inclusions in the 2.9 Ga age population links late Archean (2.9 Ga) subduction–accretion events to craton stabilization. These events resulted in a widely distributed, late Archean generation of eclogitic diamonds in an amalgamated craton. 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. Similar age/paragenesis systematics are seen for the more limited data sets from the Slave and Siberian cratons.     

早前寒武纪地质及深成构造作用研究进展

早前寒武纪地质的研究进展主要表现在准大陆克拉通早期构造演化,克拉通及古老造山带深层结构,元古代超大陆恢复对比、早期地壳性质及生长等主要问题上开展多学科研究计划的实施。其中,同位素年代学,特别是锆石Ｕ－Ｐｂ方法,地震反射、Ｐ－Ｔ计算及古地磁研究对前寒武纪地质学的进展具有重要的推动作用。和个古陆克拉通区域地质学的持质研究积累,不断产生新的认识,这种新的科学思想涉及到早期陆壳组成及区划,太古代克拉通化历史,太古代－元古代界限及性质,元古代造山带网络与克拉进陆块拼合,大陆下地壳剖面及其组成等同题。华北早前寒武纪地质演化研究中的重要问题包括：华北麻粒岩相带与克拉通基底构造的关系,克拉通基底构造区域,早期陆壳性质及其记录的重大构造一热事件幕,华北克拉通与世界典型陆块构造演化对比等。     

The age of continental roots

Determination of the age of the mantle part of continental roots is essential to our understanding of the evolution and stability of continents. Dating the rocks that comprise the mantle root beneath the continents has proven difficult because of their high equilibration temperatures and open-system geochemical behaviour. Much progress has been made in the last 20 years that allows us to see how continental roots have evolved in different areas. The first indication of the antiquity of continental roots beneath cratons came from the enriched Nd and Sr isotopic signatures shown by both peridotite xenoliths and inclusions in diamonds, requiring isolation of cratonic roots from the convecting mantle for billions of years. The enriched Nd and Sr isotopic signatures result from mantle metasomatic events post-dating the depletion events that led to the formation and isolation of the peridotite from convecting mantle. These signatures document a history of melt– and fluid–rock interaction within the lithospheric mantle. In some suites of cratonic rocks, such as eclogites, Nd and Pb isotopes have been able to trace probable formation ages. The Re–Os isotope system is well suited to dating lithospheric peridotites because of the compatible nature of Os and its relative immunity to post-crystallisation disturbance compared with highly incompatible element isotope systems. Os isotopic compositions of lithospheric peridotites are overwhelmingly unradiogenic and indicate long-term evolution in low Re/Os environments, probably as melt residues. Peridotite xenoliths from kimberlites can show some disturbed Re/Os systematics but analyses of representative suites show that beneath cratons the oldest Re depletion model ages are Archean and broadly similar to major crust-forming events. Some locations, such as Premier in southern Africa, and Lashaine in Tanzania, indicate more recent addition of lithospheric material to the craton, in the Proterozoic, or later. Of the cratons studies so far (Kaapvaal, Siberia, Wyoming and Tanzania), all indicate Archean formation of their lithospheric mantle roots. Few localities studied show any clear variation of age with depth of derivation, indicating that >150 km of lithosphere may have formed relatively rapidly. In circum-cratonic areas where the crustal basement is Proterozoic in age kimberlite-derived xenoliths give Proterozoic model ages, matching the age of the overlying crust. This behaviour shows how the crust and mantle parts of continental lithospheric roots have remained coupled since formation in these areas, for billions of years, despite continental drift. Orogenic massifs show more systematic behaviour of Re–Os isotopes, where correlations between Os isotopic composition and S or Re content yield initial Os isotopic ratios that define Re depletion model ages for the massifs. Ongoing Sr–Nd–Pb–Hf–Os isotopic studies of massif peridotites and new kimberlite- and basalt-borne xenolith suites from new areas, will soon enable a global understanding of the age of continental roots and their subsequent evolution.     

Linking continental deep subduction with destruction of a cratonic margin: strongly reworked North China SCLM intruded in the Triassic Sulu UHP belt

The collision between the North and South China cratons in Middle Triassic time (240–225 Ma) created the world’s largest belt of ultrahigh-pressure (UHP) metamorphism. U–Pb ages, Hf isotope systematics and trace element compositions of zircons from the Xugou, Yangkou and Hujialing peridotites in the Sulu UHP terrane mainly record a ~470 Ma tectonothermal event, coeval with the Early Paleozoic kimberlite eruptions within the North China craton. This event is interpreted as the result of metasomatism by fluids/melts derived from multiple sources including a subducting continental slab. The peridotites also contain zircons with ages of ~3.1 Ga, and Hf isotope data imply a component ≥3.2 Ga old. Most zircon Hf depleted mantle model ages are ~1.3 Ga, suggesting that the deep subcontinental lithospheric mantle beneath the southeastern margin of the North China craton experienced a intense mid-Mesoproterozoic metasomatism by asthenospheric components, similar to the case for the eastern part of this craton. Integrating data from peridotites along the southern margin of the craton, we argue that the deep lithosphere of the cratonic margin (≥3.2 Ga old), from which the Xugou, Yangkou and Hujialing peridotites were derived, experienced Proterozoic metasomatic modification, followed by a strong Early Paleozoic (~470 Ma) tectonothermal event and the Early Mesozoic (~230 Ma) collision and northward subduction of the Yangtze craton. The Phanerozoic decratonization of the eastern North China craton, especially along its southern margin, was not earlier than the Triassic continental collision. This work also demonstrates that although zircons are rare in peridotitic rocks, they can be used to unravel the history of specific lithospheric domains and thus contribute to our understanding of the evolution of continental cratons and their margins.     

A Re–Os isotope and PGE study of kimberlite-derived peridotite xenoliths from Somerset Island and a comparison to the Slave and Kaapvaal cratons

The concentrations of platinum-group elements (PGE; Os, Ir, Ru, Pd and Pt) and Re, and the Os isotopic compositions were determined for 33 lithospheric mantle peridotite xenoliths from the Somerset Island kimberlite field. The Os isotopic compositions are exclusively less radiogenic than estimates of bulk-earth (¹⁸⁷Os/¹⁸⁸Os as low as 0.1084) and require a long-term evolution in a low Re–Os environment. Re depletion model ages (T_RD) indicate that the cratonic lithosphere of Somerset Island stabilised by at least 2.8 Ga, i.e. in the Neoarchean and survived into the Mesozoic to be sampled by Cretaceous kimberlite magmatism. An Archean origin also is supported by thermobarometry (Archean lithospheric keels are characterised by >150 km thick lithosphere), modal mineralogy and mineral chemistry observations. The oldest ages recorded in the lithospheric mantle beneath Somerset Island are younger than the Mesoarchean (>3 Ga) ages recorded in the Slave craton lithospheric mantle to the southwest [Irvine, G.J., et al., 1999. Age of the lithospheric mantle beneath and around the Slave craton: a Rhenium–Osmium isotopic study of peridotite xenoliths from the Jericho and Somerset Island kimberlites. Ninth Annual V.M. Goldschmidt Conf., LPI Cont., 971: 134–135; Irvine, G.J., et al., 2001. The age of two cratons: a PGE and Os-Isotopic study of peridotite xenoliths from the Jericho kimberlite (Slave craton) and the Somerset Island kimberlite field (Churchill Province). The Slave–Kaapvaal Workshop, Merrickville, Ontario, Canada]. Younger, Paleoproterozoic, T_RD model ages for Somerset Island samples are generally interpreted as the result of open system behaviour during metasomatic and/or magmatic processes, with possibly the addition of new lithospheric material during tectono-thermal events related to the Taltson–Thelon orogen. PGE patterns highly depleted in Pt and Pd generally correspond to older Archean T_RD model ages indicating closed system behaviour since the time of initial melt extraction. Younger Proterozoic T_RD model ages generally correspond to more complex PGE patterns, indicating open system behaviour with possible sulfide or melt addition. There is no correlation between the age of the lithosphere and depth, at Somerset Island.     

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     

Origin of layered continental mantle (Karelian craton, Finland): Geochemical and Re–Os isotope constraints

Studies of mantle xenolith and xenocryst studies have indicated that the subcontinental lithospheric mantle (SCLM) at the Karelian Craton margin (Fennoscandian Shield) is stratified into at least three distinct layers cited A, B, and C. The origin and age of this layering has, however, remained unconstrained. In order to address this question, we have determined Re–Os isotope composition and a comprehensive set of major and trace elements, from xenoliths representing all these three layers. These are the first Re–Os data from the SCLM of the vast East European Craton.

Xenoliths derived from the middle layer B (at  110–180 km depth), which is the main source of harzburgitic garnets and peridotitic diamonds in these kimberlites, are characterised by unradiogenic Os isotopic composition. ¹⁸⁷Os/¹⁸⁸Os shows a good correlation with indices of partial melting implying an age of  3.3. Ga for melt extraction. This age corresponds with the oldest formation ages of the overlying crust, suggesting that layer B represents the unmodified SCLM stabilised during the Paleoarchean. Underlying layer C (at 180–250 km depths) is the main source of Ti-rich pyropes of megacrystic composition but is lacking harzburgitic pyropes. The osmium isotopic composition of layer C xenoliths is more radiogenic compared to layer B, yielding only Proterozoic T_RD ages. Layer C is interpreted to represent a melt metasomatised equivalent to layer B. This metasomatism most likely occurred at ca. 2.0 Ga when the present craton margin formed following continental break-up. Shallow layer A (at  60–110 km depth) has knife-sharp lower contact against layer B indicative of shear zone and episodic construction of SCLM. Layer A peridotites have “ultradepleted” arc mantle-type compositions, and have been metasomatised by radiogenic ¹⁸⁷Os/¹⁸⁸Os, presumably from slab-derived fluids. Since layer A is absent in the core of the craton, its origin can be related to Proterozoic processes at the craton margin. We interpret it to represent the lithosphere of a Proterozoic arc complex (subduction wedge mantle) that became underthrusted beneath the craton margin crust during continental collision  1.9 Ga ago.