华北东部中、新生代岩石圈地幔的不均一性：来自橄榄石的组成填图结果

华北中、新生代玄武质火山岩和基性脉岩携带的地慢橄榄岩捕虏体中橄榄石和／或橄榄石捕虏晶系统的组成填图显示华北东部中、新生代岩石圈地幔存在明显的时空分布规律和不均一性。这与通过岩石圈地幔源基性岩石的地球化学反演获得的华北中生代岩石圈地幔的时空不均一性及其块体特征完全一致。太行山和鲁皖地区新生代岩石圈地慢的差异演化主要反映古老地幔橄榄岩与熔体相互作用时熔体性质和来源的不同。同时,橄榄石Fo填图还揭示了郯庐断裂对华北东部中、新生代基性岩浆活动及其岩石圈地幔演化的重要制约作用。而且,华北东部中生代岩石圈减薄后尚存古老岩石圈地幔残留。因此,华北东部岩石圈减薄的整体拆沉模式很难成立。     

用玄武岩组成反演中-新生代华北岩石圈的演化

玄武岩的化学组成与地幔源区特征、部分熔融程度、地幔温度和岩石圈厚度等多个因素有关,因此可以用来反演深部地幔的演化。文中简要地阐述了用玄武岩组成获得岩石圈厚度及其变化的方法,并总结了有关华北中—新生代岩浆演化的两个最主要特征:(a)晚中生代岩浆活动经历了由早期的源自富集地幔的岩浆向后期亏损地幔起源岩浆的转变,而两个阶段为一岩浆间隙期(~10Ma)所分隔;(b)华北东、西部新生代玄武岩具有相反的碱性强度随时间的变化趋势。这些岩浆演化特征可以用岩石圈减薄过程中地幔地温梯度的逐渐升高、岩石圈地幔中富集组分在短时间内的不可再生以及岩石圈盖效应来解释。该认识为华北岩石圈减薄的时间尺度和机制以及减薄作用的时空不均一性提供了新的制约。     

华北地区东部岩石圈导电性结构研究——减薄的华北岩石圈特点

华北古大陆克拉通解体、岩石圈减薄的深部过程,对于建立中国大陆中新生代演化动力学模型是亟待深入研究的重要科学问题,因而"华北克拉通破坏"也就成了当前学术界的热门话题。为了研究"华北克拉通破坏"首先需要给出较准确的华北岩石圈结构,这必须依靠包括超宽频带高精度大地电磁深探测在内的现代深部地球物理探测技术。2001和2005年在华北地区东部布置了应县—商河(HB-MT01)、文水—日照(HB-MT02)大地电磁测深剖面进行研究。研究结果表明,在华北地区东部沿地壳-上地幔探测剖面可划分为4个电性区,区内发现有下地壳高导带和上地幔高导层存在。文中依据研究区壳-幔电性结构特征,推断华北地区东部地壳和上地幔之间发生过大规模构造运动,导致壳-幔之间存在解耦现象。研究结果还发现,华北东部确实存在岩石圈减薄区,其岩石圈厚度约50~80km厚。岩石圈明显减薄的区域包括北太行山隆起、华北裂谷带北部和鲁西断隆,其范围比原先认识到的要复杂,并非以太行山重力梯度带为界划分东、西两区,简单地认为东区即是减薄区。此外,在华北地区东部的现代高精度大地电磁探测结果也进一步证明了地球物理观测对于大陆动力学研究的重要性,这使人们更加认识到在今后的研究中必须强调地球物理-地质-地球化学之间的有机结合。     

辉南新生代玄武岩中捕虏体橄榄岩矿物化学与华北岩石圈地幔演化

内容提要：在对吉林辉南新生代玄武岩中捕虏体橄榄岩详细的岩相学和矿物主量元素研究基础上,重点分析了单斜辉石激光原位微量元素,并讨论了辉南陆下岩石圈地幔的性质及其在华北克拉通破坏过程中的意义。橄榄石、单斜辉石和尖晶石的Mg#和Cr#均表明该区陆下岩石圈地幔主体是饱满的,同时也存在少量过渡和难熔型地幔。单斜辉石REE配分形式包括LREE亏损、倒U字型REE和LREE富集等不同类型。这些橄榄岩是地幔经历不同程度的部分熔融作用(低于10%)和复杂地幔交代作用的产物。交代介质主体是硅酸盐熔体,但个别样品有碳酸岩熔体交代的残留。橄榄石Mg#和平衡温度无相关性,说明辉南陆下岩石圈地幔不存在明显的分层而是交叉的。这些地幔特征和华北东部其他地区新生代的主体地幔性质相似,是熔-岩反应、侵蚀作用和上涌软流圈物质冷却转变置换的综合结果。     

内蒙古集宁新生代玄武岩中橄榄岩包体矿物化学特征及其地幔演化意义

在无水"干"体系的四矿物相橄榄岩中,单斜辉石由于熔点低而通常记录着丰富的部分熔融作用和地幔交代作用信息。在详细岩相学特征和组成矿物主元素分析基础上,重点对内蒙古集宁玄武岩中橄榄岩包体的单斜辉石进行了激光原位微量元素研究。通过与新生代时华北具古老克拉通地幔特征(如鹤壁)和具新生岩石圈地幔特征(如山旺)的橄榄岩对比,讨论了新生代时集宁地区陆下岩石圈地幔性质及其形成和演化机制。集宁地区岩石圈地幔是相当于原始地幔经过较低程度的部分熔融抽取形成的,除个别样品的部分熔融程度5%外,多数样品为5%～10%。橄榄岩的平衡温度与橄榄石Mg#关系表明新生代时集宁地区的陆下岩石圈地幔是不均一的,无明显分层现象,表现为饱满与过渡型地幔的共存。这种主体饱满并兼有过渡型地幔的不均一现象,可能是软流圈物质对古老地幔进行不均匀侵蚀、改造和置换作用的结果。     

华北克拉通东北缘岩石圈深部物质组成的不均一性: 来自吉林南部中生代火山岩元素及Sr-Nd同位素地球化学的证据

本文报导了吉林南部果松组和三棵榆树组火山岩的全岩K-Ar和角闪石40Ar/39Ar定年结果和岩石地球化学资料,并讨论了吉林南部早白垩世火山岩岩浆源区性质以及空间变异.定年结果显示,果松组和三棵榆树组火山岩的形成时代分别为130.2±0.3Ma和118.3±1.9Ma.果松组火山岩主要由玄武岩-玄武质粗面安山岩-粗面安山岩-英安岩组成;它们的SiO2含量介于46%～64%,Mg#介于31～50之间,Al2O3含量介于14.9%～18.9%之间,全碱含量(Na2O+K2O)介于4.61%～9.23%之间,属于亚碱性系列,具钙碱性演化趋势;并以富集大离子亲石元素(LILEs)和轻稀土元素(LREEs),亏损重稀土元素(HREEs)和Nb、Ta、Ti等高场强元素(HFSEs)为特征;(87Sr/88Sr)i值和εNd(t)值分别介于0.7065～0.7077和-2.67～-19.71之间.果松组火山岩的成分具有较好的空间变异趋势,由东向西,火山岩的基性程度增高,东部果松组火山岩具有较高的(87Sr/86Sr)i值,而西部具有较低的εNd(t)值.三棵榆树组火山岩由粗面安山岩和粗面英安岩组成;三棵榆树组火山岩的SiO2含量介于55.5%～65.8%之间,Mg#介于42～50,Al2O3含量介于15.0%～15.7%,全碱含量偏高(Na2O+K2O=6.93%～9.24%),主体属于亚碱性系列,具钙碱性系列的演化趋势;并以较高的Th/U(5.36～5.82)、Ba/Nb(50.2～120.0)、(La/Yb)N(32.9～47.9)和Sr/Y(50.0～72.4)比值为特征;它们的(87Sr/86Sr)i值和εnd(t)值分别变化于0.7056～0.7057和-8.99～-19.71之间.上述特征揭示,果松组火山岩(130Ma)的形成主要与古太平洋板块的俯冲作用有关,岩浆来源于受流体/熔体交代的地幔楔,东西部受到不同程度陆壳物质的混染.与果松组火山岩相比,三棵榆树组火山岩(118Ma)形成于俯冲背景下的相对引张环境,岩浆来源于受交代的岩石圈地幔,但受到深部陆壳物质的混染.     

Oxidized Late Mesozoic subcontinental lithospheric mantle beneath the eastern North China Craton: A clue to understanding cratonic destruction

Despite the critical influence of oxidation state on the geochemical and geodynamic evolution of Earth, its impact on the longevity of cratons is poorly understood. To address this issue, we investigated the redox state of the Late Mesozoic subcontinental lithospheric mantle (SCLM) beneath the eastern North China Craton (NCC), which was destroyed during the Phanerozoic. We report the occurrence of high-Fo olivine (Fo > 87, where Fo = atomic Mg/(Mg + Fe²⁺)) within Early Cretaceous basalts; these olivines show extremely low Ti (<60 ppm) contents, high δ¹⁸O (5.8‰–7.2‰) values, and relatively cool crystallization temperatures (1125 to 1218 °C). These features support derivation of the lavas from highly refractory and cold SCLM. The relatively low partition coefficients of vanadium between olivine and whole rocks (0.019–0.025) indicate a high fO₂ for the Early Cretaceous basalts and their mantle sources (ΔQFM = +1.0 and ΔQFM = +1.5, respectively), and, consequently, an oxidized Late Mesozoic SCLM beneath the eastern NCC. The high degree of oxidation of the mantle was caused by the ingress of hydrous melts and/or fluids released from a subducting slab during the Phanerozoic. We propose that oxidization of the SCLM softened the mantle, which triggered the removal of the cratonic root beneath the eastern NCC. The results highlight the crucial role of oxidation state in craton stability.     

Secular evolution of the lithospheric mantle beneath the eastern North China craton: evidence from peridotitic xenoliths from Late Cretaceous mafic rocks in the Jiaodong region,east-central China

The Mesozoic lithospheric mantle beneath the North China craton remains poorly constrained relative to its Palaeozoic and Cenozoic counterparts due to a lack of mantle xenoliths in volcanic rocks. Available data show that the Mesozoic lithospheric mantle was distinctive in terms of its major, trace element, and isotopic compositions. The recent discovery of mantle peridotitic xenoliths in Late Cretaceous mafic rocks in the Jiaodong region provides an opportunity to further quantify the nature and secular evolution of the Mesozoic lithospheric mantle beneath the region. These peridotitic xenoliths are all spinel-facies nodules and two groups, high-Mg# and low-Mg# types, can be distinguished based on textural and mineralogical features. High-Mg# peridotites have inequigranular textures, high Mg# (up to 92.2) in olivines, and high Cr# (up to 55) in spinels. Clinopyroxenes in the high-Mg# peridotites are generally LREE-enriched ((La/Yb)_N>1) with variable REE concentrations, and have enriched Sr–Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr = 0.7046–0.7087; ¹⁴³Nd/¹⁴⁴Nd = 0.5121–0.5126). We suggest that the high-Mg# peridotites are fragments of the Archaean and/or Proterozoic lithospheric mantle that underwent extensive interaction with both carbonatitic and silicate melts prior to or during Mesozoic time. The low-Mg# peridotites are equigranular, are typified by low Mg# ( < 90) in olivines, and by low Cr# ( < 12) in spinels. Clinopyroxenes from low-Mg# peridotites have low REE abundances (ΣREE = 12 ppm), LREE-depleted REE patterns ((La/Yb)_N < 1), and depleted Sr–Nd isotopic features, in contrast to the high-Mg# peridotites. These geochemical characteristics suggest that the low-Mg# peridotites represent samples from the newly accreted lithospheric mantle. Combined with the data of mantle xenoliths from the Junan and Daxizhuang areas, a highly heterogeneous, secular evolution of the lithosphere is inferred for the region in Late Cretaceous time.     

Thermal and petrological structure of the lithosphere beneath Hannuoba, Sino-Korean Craton, China: evidence from xenoliths

Deep-seated xenoliths entrained in the Hannuoba basalts of the northern Sino-Korean Craton include mafic and felsic granulites, mantle wall-rock from spinel– and garnet–spinel peridotite facies, and basaltic crystallisation products from the spinel-pyroxenite and garnet-pyroxenite stability fields. The mineral compositions of the xenoliths have been used to estimate temperatures and, where possible, pressures of equilibration, and to construct a geothermal framework to interpret the upper mantle and lower crustal rock-type sequences for the region. The xenolith-derived paleogeotherm is constrained in the depth interval of 45–65 km and like others from areas of young basalt magmatism, is elevated and strongly convex toward the temperature axis. Two-pyroxene granulites give the lowest temperatures and garnet pyroxenites the highest, while the spinel lherzolites fall between these two groups. The present-day Moho beneath the Hannuoba area is defined at 42 km by seismic data, and coincides with the deepest occurrence of granulite. Above this boundary, there is a lower crust–upper mantle transition zone about 10-km thick, in which spinel lherzolites and mafic granulites (with variable plagioclase contents) are intermixed. It is inferred that this underplating has resulted in a lowering of the original pre-Cenozoic Moho (then coinciding with the crust–mantle boundary, CMB) from about 30 km to its present-day position and was due to intrusions of basaltic magmas that displaced peridotite mantle wall-rock and equilibrated to mafic granulites. Trace element patterns of the diopsides (analysed by laser ablation-ICPMS) from the Cr-diopside series spinel lherzolites and associated layered xenoliths (spinel lherzolites and pyroxenites) indicate a fertile uppermost mantle with moderate depletion by low degrees of partial melting and little evidence of metasomatic activity. The similarity in major and trace element compositions of the minerals in both rock types suggests that the layered ultramafic xenoliths formed by mantle deformation processes (metamorphic segregation), rather than by melt veining or metasomatism.     

Geochronology and geochemistry of Cenozoic basalts from eastern Guangdong, SE China: constraints on the lithosphere evolution beneath the northern margin of the South China Sea

The ⁴⁰Ar–³⁹Ar dating reveals three episodes of basaltic volcanism in eastern Guangdong of SE China since the late Eocene (i.e., 35.5, ~20 and 6.6 Ma). The Miocene alkali olivine basalts (~20 and 6.6 Ma) have OIB-like trace element characteristics, which is coupled with low (⁸⁷Sr/⁸⁶Sr)_i, high ε_Nd(t), and high ε_Hf(t). In contrast, the late Eocene basalts (35.5 Ma) have overall characteristics of “Island Arc” basalts with strong negative Ta–Nb–Ti anomalies in the primitive mantle-normalized multi-element diagram with high (⁸⁷Sr/⁸⁶Sr)_i, negative ε_Nd(t), and relatively low ε_Hf(t). All basalts have unexpectedly high ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb, delineating a DUPAL signature in the sources. The late Eocene Arc-like basalts may reflect contributions of relict ancient metasomatized mantle lithosphere that melted as the result of extension-induced asthenospheric upwelling and heating, whereas the Miocene OIB-like basalts may represent partial melting of the asthenospheric mantle beneath the thickened lithosphere. We propose that the Cenozoic basaltic volcanism in eastern Guangdong records an overall lithospheric thickening process beneath SE China, that is, a continental rift system from its maximum extension in the late Eocene to its waning in the Miocene. This interpretation is consistent with the evolution of the South China Sea, whose origin is most consistent with the development of a passive continental margin. The seafloor spreading of the South China Sea during ~ 32–16 Ma may not result from the effect of the “Hainan” mantle plume, but rather played a positive role in allowing the mantle plume to express on the surface.     

华北中、新生代岩石圈地幔的交代作用：含金云母地幔岩提供的证据

本文对华北克拉通三个不同地区（河北汉诺坝、内蒙古集宁三义堂、河南鹤壁）新发现的含金云母尖晶石二辉橄榄岩和尖晶石橄榄单斜辉石岩捕虏体进行了详细的矿物组成、单斜辉石的微量元素和Sr-Nd同位素研究.通过与相同地区不含金云母尖晶石二辉橄榄岩捕虏体的系统对比发现通常含金云母的地幔橄榄岩比不含金云母的地幔橄榄岩岩富Al2O3、CaO、NaO、K2O、TiO2,但相对贫镁;其单斜辉石的LREE更为富集,但Sr、Nd同位素组成则相对亏损.这说明地幔交代作用不仅能够造成地幔橄榄岩的玄武质组分和稀土元素的富集,而且亦能够造成全岩和橄榄石Mg#的降低和同位素组成的相对亏损.捕虏体的Rb-Sr等时线年龄暗示地幔交代作用发生在中、新生代;其交代熔体来源于软流圈.同时说明华北新生代岩石圈地幔普遍存在的主、微量元素和同位素组成类似于“大洋型”岩石圈地幔的特征很可能是岩石圈地幔橄榄岩与软流圈来源的熔体的大规模反应的结果,而非真正意义上的新增生的岩石圈地幔.     

华北克拉通东部中生代高Mg闪长岩——对岩石圈减薄机制的制约

在华北克拉通东部鲁西—徐淮地区,存在一套辉石闪长岩-二长闪长岩-花岗闪长岩组成的adakitic岩石。锆石SHRI MP和LA-ICP-MS U-Pb定年结果表明它们形成于早白垩世(130~132Ma)。该类岩石具有较高的MgO含量(质量分数为1·46%~9·76%)、高的Mg#值(0·46~0·68)和高的Sr/Y值(主体介于30~52之间,个别高达410)。这些特征类似于由俯冲大洋板片部分熔融形成的adakitic岩石。然而,它们所表现出来的相对较高的87Sr/86Sr初始比值(0·7051~0·7077)和较低的εNd(t)值(-4·43~-15·92)则反映岩浆形成或演化过程中应有陆壳物质的参与。徐淮地区该类岩石中榴辉岩类捕虏体和石榴石捕虏晶的存在和鲁西辉石闪长岩中众多地幔橄榄岩捕虏体的发现,以及这些捕虏体中普遍发育富硅质交代作用,由此可以判定该类岩浆应起源于拆沉下部大陆地壳的部分熔融及其在上升过程中与地幔橄榄岩的反应,石榴石作为残留相。华北克拉通东部早白垩世adakitic岩石的存在以及榴辉岩类捕虏体的年代学表明,中生代早期曾存在一次重要的陆壳加厚过程,之后相继出现的加厚岩石圈的拆沉应是中生代岩石圈减薄的主导机制。     

木吉村斑岩铜矿成矿作用:华北克拉通中生代岩石圈减薄的响应

木吉村铜(钼、金)矿位于华北克拉通太行山北段中生代岩浆岩带,为大型隐伏斑岩型矿床.矿体赋存于侏罗纪髫髻山组与火山岩相关的闪长玢岩次火山岩相中.木吉村髫髻山组安山岩中岩浆锆石的两组U-Pb年龄分别为(150.5±3.0) Ma和(140.8±3.0) Ma,与北京西山髫髻山组火山岩锆石U-Pb年龄范围(156.6～137...     

Geochemistry of Mesozoic intracontinental basalts from Yunnan, southern China: Implications for geochemical evolution of the subcontinental lithosphere

Summary Southwestern Yunnan, comprising the Yangtze and Shan-Thai microcontinents and the Simao block, has successively undergone subduction of an oceanic plate, followed by a collision of the microcontinents and intracontinental rifting associated with basaltic volcanism during Late Paleozoic to Mesozoic.The Triassic Nanjian basalts, erupted on the Yangtze microcontinent, have more enriched isotopic ratios and higher LREE/HFSE and LREE/HREE ratios. This suggests the existence of an enriched subcontinental lithosphere under the Yangtze microcontinent which stabilized over long periods of the earth's history (> 2Ga).The Middle Jurassic Simao basalts have more depleted geochemical features and also have element enrichments characteristic of a subduction zone environment, although the basalts were erupted in an intracontinental graben. It may be inferred that the lithospheric mantle of the Simao block was modified by subduction processes during Latest Carboniferous to Late Triassic prior to the onset of the Middle Jurassic continental rifting. The lack of correlation between depletion of HFSE, Y and HREE, and relative enriched Nd isotopic ratios suggests that the source depletion of the Simao basalts is not an old feature and has been contemporaneous with the subduction-related enrichment through mantle metasomatism shortly before the basalts were produced.The Middle Jurassic Baoshan basalts which erupted during the continental rifting on the Shan-Thai microcontinent have an Sr-Nd isotopic composition similar to the bulk earth and higher concentrations of incompatible trace elements. These features suggest that the subcontinental lithosphere under the Shan-Thai microcontinent underwent mantle metasomatism just prior to eruption of the Baoshan basalt.

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Geochemie mesozoischer interkontinentaler Basalte aus Jünnan, Südchina: Hinweise auf die geochemische Entwicklung der subkontinentalen Lithosphäre

Zusammenfassung Südwestjünnan umfaßt den Jangtse und den Shan-Thai Mikrokontinent und den Simao Block. Das Gebiet wurde von aufeinander folgenden Subduktionsphasen einer ozeanischen Platte betroffen, auf die Kollision der Mikrokontinente und interkontinentales Rifting folgte. Dieses war mit basaltischem Vulkanismus während des späten Paläozoikums bis ins Mesozoikum assoziiert.Die triassischen Nanjian-Basalte, die auf dem Jangtse Mikrokontinent eruptierten, haben mehr angereicherte Isotopenverhältnisse und höhere LREE/HFSE und LREE/HREE Verhältnisse. Dieses weist auf eine angereicherte subkontinentale Lithosphäre unter dem Jangtse Mikrokontinent hin, die sich während langer Perioden der Erdgeschichte stabilisierte (>2Ga).Die mittel jurassischen Simao-Basalte haben eine mehr verarmte geochemische Signatur aber auch Elementanreicherungen, die für ein Subduktionszonen-Milieu charakteristisch sind, obwohl die Basalte in einem interkontinentalen Graben ausgetreten sind. Man kann daraus schließen, daß der lithosphärische Mantel des Simao-Blockes durch Subduktionsprozesse während des jüngsten Karbons bis in die späte Trias vor dem Beginn des mittel-jurassischen kontinentalen Riftings modifiziert worden war. Das Fehlen einer Korrelation zwischen der Anreicherung von HFSE, Y und HREE und relativ angereicherter Nd-Isotopenverhältnisse weist darauf hin, daß die Verarmung der Quelle der Simaobasalte nicht weit zurückreicht. Sie dürfte viel eher gleichaltrig mit der subduktions-bedingten Anreicherung durch Mantel-Metasomatose kurz vor der Entstehung der Basalte sein.Die mittel-jurassischen Baoshan-Basalte, die während des kontinentalen Riftings auf den Shan-Thai Mikrokontintent eruptierten, haben eine Sr-Nd-Isotopensignatur, die ähnlich der Gesamterde ist, jedoch höhere Konzentrationen inkompatibler Spurenelemente zeigt. All dies legt nahe, daß die subkontinentale Lithosphäre unter dem Shan-Thai-Mikrokontinent kurz vor der Eruption der Baoshan-Basalte von Mantel-Metasomatose betroffen worden ist.

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With 8 Figures

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Visiting Fellow, Geology Department, Australian National University, Australia     

鹤壁新生代玄武岩源区及成因: 地球化学和Sr-Nd-Hf同位素证据

大陆碱性玄武岩是研究大陆地幔、软流圈和岩石圈、壳幔相互作用的窗口。鹤壁碱性玄武岩为华北中部新生代玄武岩重要组成部分,其源区和成因对了解新生代期间华北克拉通中部岩石圈地幔改造提供了重要依据。鹤壁新生代玄武岩主要为碱性玄武岩,贫硅(SiO₂=45.0%~47.8%)富镁(MgO=7.3%~8.5%)和全碱(Na₂O+K₂O=4.6%~6.3%),富集轻稀土元素,轻、重稀土元素强烈分馏,无明显Eu异常。富集大离子亲石元素(LILEs)和高场强元素(HFSEs)。在原始地幔标准化蛛网图上,具有明显的Nb、Ta正异常,其中Nb/U=35~48、La/Nb=0.4~0.7、Ba/Nb=3.6~11。 全岩的Sr、Nd、Hf同位素比值分别为⁸⁷Sr/⁸⁶Sr_i=0.7036~0.7044,ε_Nd(t)=+0.4~+5.4, ε_Hf(t)=+5.0~+9.7,显示鹤壁新生代玄武岩具有洋岛玄武岩(OIB)的地球化学特征和同位素组成。岩相学特征和地球化学特征表明鹤壁新生代玄武岩是软流圈地幔(>80km)小程度部分熔融形成的熔体与富集的岩石圈地幔相互作用后的结果,地壳物质混染和结晶分离作用不显著。     

Mechanism and timing of lithospheric modification and replacement beneath the eastern North China Craton: Peridotitic xenoliths from the 100 Ma Fuxin basalts and a regional synthesis

Lithospheric thinning beneath the eastern North China Craton is widely recognized, but the mechanism and timing of the thinning are contentious. New data on peridotitic xenoliths from the Cretaceous (∼100 Ma) Fuxin basalts at the northern edge of the craton have been integrated with data from other localities across the craton, to provide an overview of the processes involved. The Fuxin peridotite xenoliths can be subdivided into three types, which can also be recognized in other xenolith suites across the craton. The dominant Type 1, lherzolites with olivine Mg^# ∼90, represents fertile mantle (5-12% partial-melt extraction) that makes up much of the Late Mesozoic-Cenozoic lithosphere beneath the craton. Type 2 consists of magnesian (olivine Mg^# >92) harzburgites, interpreted as shallow relics of the Archean cratonic mantle. Type 3, minor lherzolite xenoliths with olivine Mg^# ∼86 reflect the interaction of the lithosphere with magmas similar to the host basalts. In-situ Re-Os data on sulfides in xenoliths from Hebi (4 Ma, interior of the craton) and Hannuoba (22 Ma, northern edge of the Trans-North China Orogen within the craton) basalts give model ages of 3.1-3.0, 2.5, 2.2-2.1, 1.4 and 0.8 Ga, These correspond to the U-Pb ages of zircons from early Mesozoic (178 Ma) peridotitic xenoliths at the southern margin of the craton, and record events during which the Archean lithospheric mantle was modified. The dominance of fertile peridotite xenoliths in the 100 Ma Fuxin basalts indicates that the mantle replacement beneath the eastern North China Craton at least partly took place before that time. The regional synthesis suggests that Mesozoic-Cenozoic lithospheric thinning and mantle replacement was heterogeneously distributed across the North China Craton in space and time. Lateral spreading of the lithosphere, accompanied by asthenospheric upwelling and melt-peridotite interaction, is the most probable mechanism for the lithospheric thinning beneath the eastern part of the craton. Subsequent cooling of the upwelled asthenosphere caused some re-thickening of the lithosphere; this overall more fertile and hence denser lithosphere resulted in widespread basin formation.     

华北克拉通东部中生代岩石圈减薄的过程与机制:中生代火成岩和深源捕虏体证据

基于最新的同位素年代学资料 ,华北克拉通东部中生代的岩浆作用可划分成四个阶段 ,即晚三叠世 ( 2 0 5～ 2 2 5Ma)碱性岩浆作用 ;中晚侏罗世 ( 1 5 5～ 1 6 0Ma)花岗质岩浆作用 ;早白垩世 ( 1 1 2～ 1 32Ma)双峰式岩浆作用和晚白垩世 ( 92～ 73Ma)碱性玄武质岩浆作用。徐淮地区中生代侵入岩中榴辉岩捕虏体的发现及其地质年代学资料 ( 2 1 9Ma)表明 ,华北克拉通东部中生代早期曾发生过一次重要的陆壳加厚过程。俯冲板片的断离以及高压—超高压变质岩的快速折返和晚三叠世 ( 2 0 5～ 2 2 5Ma)的碱性岩浆作用的存在均暗示 ,华北克拉通中生代岩石圈减薄已经开始。拆沉作用则是引起中生代早期岩石圈减薄的主要机制。中、晚侏罗世 ( 1 5 5～ 1 6 0Ma)花岗质岩浆作用形成于造山期后的伸展环境 ,代表了中生代岩石圈减薄的继续和发展。早白垩世 ( 1 1 2～ 1 32Ma)双峰式岩浆作用表明中生代岩石圈减薄达到了峰期。而幔源纯橄岩捕虏体中富硅质熔体的交代作用和玄武岩的高87Sr/ 86Sr值、低ε(Nd ,t)值特征表明 ,软流圈对岩石圈底部的化学侵蚀可能是导致该阶段岩石圈减薄的主导机制。晚白垩世 ( 92～ 73Ma)碱性玄武质岩浆作用和“海洋型”地幔捕虏体的存在代表了等温面的下降和岩石圈地幔的增生     

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。     

华北克拉通东部显生宙地幔演化

华北克拉通东部显生宙以来的地幔可以划分为3种类型:克拉通型地幔,大陆活动带型地幔和大陆裂谷型地幔。1 700 Ma—古生代末,地幔属于克拉通型:ε(Nd,t)值高于-5,为弱富集型;层圈相互作用以幔源的熔体和/或流体与古老的岩石圈地幔的作用为主,但规模较小,范围局部。100 Ma以前的中生代地幔属于“大陆活动带型”:ε(Nd,t)值低,在-5以下,为富集型;地幔中含有地壳的组分,层圈相互作用以下地壳与弱化的岩石圈地幔之间的作用为主;发生的时间为190~100 Ma,高峰期在130 Ma左右;发生的部位邻近莫霍面,导源的岩浆多为钙碱性系列,部位浅,活动范围广泛。100 Ma至新生代,地幔属于“大陆裂谷型”:为亏损型的软流圈地幔,ε(Nd,t)值高,几乎均为正值。层圈相互作用转变为软流圈岩石圈地幔之间的作用,转变的时间具有约40 Ma的过渡时期,前锋开始于100~109 Ma,导源的岩浆大致沿NWW和NEE向的大型断裂带分布。进一步证实了软流圈地幔上隆的不均匀性和主动性。