微量元素和Pb同位素对西藏雅鲁藏布构造带日喀则蛇绿岩形成环境的制约

西藏南部雅鲁藏布构造带分布有一系列蛇绿岩体。人们对这些蛇绿岩体的形成环境仍然存在较大的争议。雅鲁藏布构造带中段日喀则蛇绿岩路曲和大竹曲岩体镁铁质岩石的微量元素和Pb同位素特征指示其母岩浆起源于亏损地幔源区。这些镁铁质岩石的La/Sm和Sm/Yb比值显示其岩浆产生于尖晶石二辉橄榄岩地幔经过大约10%部分熔融作用。综合岩相学和全岩主量元素特征暗示这些镁铁质岩石形成于无水玄武质岩浆。而且这些镁铁质岩石的微量元素和REE元素配分模式均非常相似于N-MORB,除了弱Nb-Ta负异常。这些特征表明路曲和大竹曲岩体形成于大洋中脊环境。此外,路曲和大竹曲镁铁质岩石的Pb同位素结果指示其地幔源区与印度洋MORB地幔域具有相似的地球化学特征。这些镁铁质岩石N-MORB标准化微量元素模式显示弱Nb-Ta负异常可能是由于其地幔源区交代了古老的俯冲带物质。     

海南岛新生代玄武岩成因的PGE与Os同位素地球化学制约

本文对海南岛新生代玄武岩开展了PGE和Re-Os同位素体系的系统研究,结合全岩主量、微量元素和Sr-Nd-Pb同位素数据,取得了如下认识:（1）海南岛新生代玄武岩根据PGE含量和配分模式可以划分为两类,第一类PGE总量为1.88×10^-9～4.54×10^-9,配分模式为相对平坦型（Pd/Ir<3.5）,高Os含量和低¹⁸⁷Os/¹⁸⁸Os值,代表了来自亏损软流圈地幔的熔体;第二类PGE总量为0.35×10^-9～2.67×10^-9,配分模式为向右上倾型（Pd/Ir>5.0）,低Os含量和高¹⁸⁷Os/¹⁸⁸Os值,反映其熔融地幔源区包含了大量的再循环洋壳;（2）海南岛新生代玄武岩PGE地球化学特征受控于岩浆演化过程中硫化物的熔离作用;（3）Os-Pb同位素模拟结果暗示海南岛新生代玄武岩再循环洋壳的年龄在100～300 Ma之间,为近期/同期俯冲的大洋板片。本次研究为进一步理解海南岛新生代玄武岩源区的再循...     

西藏日喀则白马让蛇绿岩:亚洲大陆边缘的小洋盆

藏南雅鲁藏布蛇绿岩被认为是新特提斯大洋岩石圈的残留。该带中段的日喀则白马让蛇绿岩是保存较完整的蛇绿岩岩块之一。该蛇绿岩主要由橄榄岩、蛇纹岩、镁铁质侵入岩和玄武岩组成,缺堆晶岩系。镁铁质侵入岩主要呈辉绿岩脉、岩床和少量的岩墙产出。辉绿岩脉在整个蛇绿岩层序中均有分布,侵入橄榄岩的部分岩脉已经变为变辉绿岩和异剥钙榴岩。辉绿岩床(墙)向上逐渐过渡为玄武岩。局部可见日喀则群整合覆盖在玄武岩之上。地球化学分析显示不同产状的镁铁质岩均属于低钾或中钾的拉斑玄武岩,亏损Nb、Ta、Ti和LREE,具有弧前玄武岩(FAB)或弧后盆地玄武岩(BABB)的特征,它们的Ti/V和Yb/V的比值与BABB或正常大洋中脊玄武岩(N-MORB)相似,Sr-Nd-Pb同位素数据指示了亏损地幔(DM)与富集地幔(EM)过渡的源区。镁铁质岩野外产出关系和地球化学特征表明,白马让蛇绿岩的镁铁质岩组合可能形成于SSZ环境。考虑到超镁铁质岩、镁铁质岩和日喀则群在空间上的连续性,认为白马让蛇绿岩可能是起源于亚洲大陆边缘俯冲带上的洋盆,属于原地系统,而非外来的构造岩片。     

辽东-胶东半岛三叠纪镁铁质岩浆岩:记录从洋壳俯冲到大陆碰撞的构造转换

大陆碰撞造山带中出露的镁铁质岩浆岩,特别是形成于洋陆转换和大陆碰撞关键时期的镁铁质岩浆岩,对理解从大洋俯冲到大陆碰撞化学地球动力学过程的转变,以及发展板块构造理论具有重要意义.本文通过对苏鲁造山带和华北东南缘(胶东和辽东地区)三叠纪镁铁质岩浆岩同位素年代学和地球化学的系统总结,概括出从大洋俯冲到大陆碰撞过程中古洋壳和大陆地壳物质再循环的岩石地球化学记录.早-中三叠世洋岛型镁铁质岩浆岩属于同碰撞岩浆岩,具有洋岛型微量元素特征和弱富集的放射成因同位素组成,记录了先前俯冲古特提斯洋壳来源的熔体与上覆地幔楔橄榄岩的相互作用;晚三叠世岛弧型镁铁质岩浆岩属于同折返岩浆岩,具有弧型微量元素特征和相对富集的放射成因同位素组成,记录了随后俯冲的华南陆壳来源的熔体与上覆华北岩石圈地幔之间的相互作用.因此,辽东-胶东半岛三叠纪镁铁质岩浆岩记录了大陆俯冲带不同类型的壳幔相互作用及其形成的化学动力学过程.     

滇东南八布杨万铜矿床硫化物Re-Os同位素年龄及其地质意义

滇东南八布地区发育一套晚古生代超镁铁质-镁铁质杂岩,是认识古特提斯分支洋构造演化的重要窗口,而其中以火山岩为赋矿围岩的杨万铜矿床,被视作区域找矿的突破点。目前,对这套超镁铁质-镁铁质杂岩的构造属性及赋存铜矿床的成矿时代及成因联系还存在争议。通过对杨万铜矿床中黄铜矿和黄铁矿进行Re-Os同位素定年,获得Re-Os等时线年龄为269±3 Ma,表明该矿床形成于早中二叠世,与火山岩(～270 Ma)大致同时;而初始187Os/188Os值为0.31±0.17,暗示成矿时存在热液流体与古海水的相互作用。区内玄武岩围岩和其它超镁铁质-镁铁质岩石的地球化学特征对比分析显示,八布超镁铁质-镁铁质杂岩为一套N-MORB型蛇绿岩组合,代表了古特提斯分支洋盆的洋壳残片。杨万铜矿床应划归为与古特提斯分支洋裂解有关的火山成因块状硫化物(VMS)矿床,其周边地区具有较好的找矿潜力。     

班公湖MOR型蛇绿岩Re-Os同位素特征对班公湖-怒江特提斯洋裂解时间的制约

西藏北部班公湖MOR型蛇绿岩主要由角砾状的地幔橄榄岩和玄武岩组成,其中地幔橄榄岩主要是低Cr#尖晶石相含单斜辉石(Cpx)方辉橄榄岩和少量不含Cpx的方辉橄榄岩,玄武岩具有MORB地球化学特点。岩石地球化学特征和二元混合模拟计算表明,含Cpx方辉橄榄岩是由较为亏损的方辉橄榄岩与玄武质熔体发生反应再富集形成的,玄武质熔体和方辉橄榄岩的混合比例为1∶9至1∶4。9个含Cpx方辉橄榄岩样品(含5个重复测试样)的Re和Os含量分别为0.19×10-9~1.49×10-9和2.91×10-9~5.40×10-9,187Re/188Os变化范围为0.169±0.009(2σ)~1.833±0.183(2σ),187Os/188Os变化范围相对较小,介于0.121 13±0.000 44~0.128 53±0.000 36(2σ)之间。含Cpx方辉橄榄岩的Re-Os参考等时线年龄为254±28 Ma。由于不同比例熔体的加入造成橄榄岩具有不同的Re/Os比值,因而不同含Cpx方辉橄榄岩样品具有不同187Os/188Os比值。样品的Re含量与烧失量中的H2O没有相关性,说明蛇纹石化对样品Re-Os体系的影响可以忽略,Re-Os同位素体系在低温地质作用下能够保持相对封闭。参考等时线年龄可能代表亏损方辉橄榄岩与玄武质熔体发生反应的时间,即含Cpx方辉橄榄岩的形成年龄,它表明在该时期特提斯洋经历了一次构造热事件。这一构造岩浆热事件的时间与早期定义的班公湖-怒江特提斯洋的裂解时间晚二叠世至早三叠世较为一致,推测本文MOR型蛇绿岩地幔橄榄岩的Re-Os同位素年龄可能代表班公湖-怒江特提斯洋开始裂解的时间。     

强亲铁元素高温地球化学与天体化学及研究进展

强亲铁元素(HSE)包括铂族元素(PGE):Os、Ir、Ru、Rh、Pt、Pd)及Re、Au,它们对金属与硫化物相具有强烈的亲和性,这种独特性质使其成为高温地球化学与天体化学研究中的优秀示踪剂。此外,强亲铁元素内含两种长半衰期(¹⁸⁷Re-¹⁸⁷Os,t_1/2=41.6 Ga;¹⁹⁰Pt-¹⁸⁶Os,t_1/2=469 Ga)和一种短半衰期(¹⁰⁷Pd-¹⁰⁷Ag,t_1/2=6.5 Ma)放射性同位素衰变体系,可以用作地球与天体演化的时钟。在地幔部分熔融或分离结晶期间,PPGE(Rh、Pd、Pt)与Re、Au倾向于进入熔体相,IPGE(Os、Ir、Ru)则更多保留在残余固相。因此,通过地幔包体、地幔构造岩、镁铁质与超镁铁质火山岩的HSE丰度与Os同位素可约束地幔组成和相应的岩浆过程,以及制约地球早期演化过程。随着分析技术的改进,HSE与Re-Pt-Os同位素在行星增生与分异、核幔相互作用、壳幔分异、洋陆转换、克拉通形成与破坏、岩石圈俯冲再循环以及Ni-Cu-PGE矿床成因等方面均显示了巨大的应用潜力。     

东秦岭松树沟超镁铁岩侵位机制及其构造演化

东秦岭松树沟蛇绿岩主要由镁铁质-超镁铁质岩石组成。镁铁质岩类的Sm-Nd全岩等时年龄为1030±46(2δ)Ma,ε_Nd(t)=+5.7±0.2,代表了蛇绿岩的形成时代。超镁铁质岩石由不同成因的橄榄质糜棱岩和中粗粒橄榄岩组成,橄榄质糜棱岩是地幔橄榄岩经历复杂变形并多次部分熔融的残余体,具LREE亏损特征,其中发育橄榄石高温位错构造和高温组构以及低温位错构造和低温组构。中粗粒橄榄岩具LREE略富集的分布特征,是地幔橄榄岩残余体再次部分熔融熔体分离结晶的产物。野外地质、地球化学、构造变形特征均表明超镁铁岩块是因洋壳俯冲而底辟侵位于上覆玄武岩中的地幔橄榄岩残余体。综合分析认为,松树沟蛇绿岩经历了古陆块裂解或洋脊扩张(1271-1440Ma)-洋壳形成(1030-1271Ma)-洋壳俯冲消减-橄榄岩块底辟侵位(983Ma)-蛇绿岩构造侵位及其后构造变形叠加改造的复杂演化过程。     

长江中下游中生代安山质火山岩记录的新元古代大洋板片-地幔相互作用

大陆弧安山岩的形成是大洋板片向大陆边缘之下俯冲的结果,但是在具体形成机制上存在很大争议.针对这个问题,对长江中下游地区中生代安山质火山岩及其伴生的玄武质和英安质火山岩进行了系统的岩石地球化学研究,结果对大陆弧安山质火成岩的成因提出了新的机制.分析表明,这些岩石形成于早白垩世,它们不仅表现出典型的岛弧型微量元素分布特征,而且具有高度富集的Sr-Nd-Hf同位素和高的放射成因Pb以及高的氧同位素组成.通过全岩和矿物地球化学成分变化检查发现,地壳混染和岩浆混合作用对其成分的富集特征贡献有限,而其岩浆源区含有丰富的俯冲地壳衍生物质才是其成分富集的根本原因.虽然这些火山岩的喷发年龄为中生代,但是其岩浆源区形成于新元古代早期的华夏洋壳俯冲对扬子克拉通边缘之下地幔楔的交代作用.大陆弧安山岩地幔源区中含有大量俯冲洋壳沉积物部分熔融产生的含水熔体,显著区别于大洋弧玄武岩的地幔源区,其中只含有少量俯冲洋壳来源的富水溶液和含水熔体.正是这些含水熔体交代上覆地幔楔橄榄岩,形成了不同程度富集的超镁铁质-镁铁质地幔源区.在早白垩纪时期,古太平洋俯冲过程的远弧后拉张导致中国东部岩石圈发生部分熔融,其中超镁铁质地幔源区熔融形成玄武质火山岩,镁铁质地幔源区则熔融形成安山质火山岩.因此,大陆弧安山岩成因与大洋弧玄武岩一样,可分为源区形成和源区熔融两个阶段,其中第一阶段对应于俯冲带壳幔相互作用.      

西藏日喀则地区雅鲁藏布蛇绿岩地球化学特征及其源区性质

日喀则地区的蛇绿岩是西藏南部雅鲁藏布蛇绿岩带出露较好的蛇绿岩之一。对日喀则地区白朗蛇绿岩的主量、微量元素及Sr-Nd同位素研究表明,其基性岩石为钙碱性系列,主要氧化物具有低钛(0.6%~1.1%)和富镁(6.6%~8.7%)、高铝(15.3%~16.0%),以及烧失量普遍较高(2.8%~4.6%)的特征。岩石微量元素配分型式与N-MORB类似,又具有岛弧玄武岩的地球化学特征,表明蛇绿岩受到了俯冲作用的影响。Sr-Nd同位素特征表明源区为略富集的MORB型地幔。白朗蛇绿岩所代表的特提斯地幔域与印度洋地幔域具有相似的地球化学性质,进一步证实了现今的印度洋继承了特提斯地幔域的地球化学特征。     

特提斯洋对流上地幔Os同位素组成估算的新方法及初步运用

对流上地幔Os同位素组成的准确估算是运用Re-Os同位素体系探讨地幔演化的基础。前人研究主要是以地幔橄榄岩为研究对象,由于地幔橄榄岩Os同位素存在明显的不均一性,因而直接影响估算值的准确性。对流上地幔中包含的不同亏损程度的难熔组分在部分熔融过程中难以熔融,对形成的熔体相的Os同位素组成贡献很少或者没有。因此,与对流上地幔具有相同的Os同位素组成初始值的早期分离结晶岩石(如堆晶岩),结合堆晶岩中锆石的准确定年,可以用来估算对流上地幔Os同位素组成。本文根据这一方法测试了那曲地区弧后盆地堆晶岩的Os同位素组成和锆石U-Pb年龄,推测那曲地区新特提斯洋对流上地幔Os同位素组成为碳质球粒陨石型的。根据这一模型,对比了罗布莎和东巧铬铁矿岩、含矿围岩以及不含矿围岩的Os同位素特征,揭示出矿石及围岩均具有古老大陆岩石圈地幔信息,而不含矿围岩(泽当岩体)的Os同位素组成为碳质球粒陨石型的,无古老大陆岩石圈地幔信息。     

Advances in Re-Os Isotopic Dating in Geochronology of Hydrocarbon Accumulation

Rhenium-Osmium (Re-Os) isotopic dating applied to the geochronology of hydrocarbon accumulation has been the forefront of international research in the last 10 years and has achieved significant achievements. However, our current researches are lacking in Re-Os isotopic chronology on the geochronology of hydrocarbon accumulation. The successful application of the Re-Os geochronology has enabled the determination of accurate and precise depositional ages for Organic-Rich Sedimentary rocks (ORS), the establishment of timing constraints of oil generation or migration, and the fingerprinting of the source of oil. On the basis of literatures study both at home and abroad, this article mainly dealt with the principle of Re-Os isotopic dating and analyzed the accumulation processes of Re and Os in a seawater-sediment system. Meanwhile, the test samples, sample preparation and analytical methodology of Re-Os isotopic dating were summarized. The results revealed that the initial ¹⁸⁷Os/¹⁸⁸Os ratio could constrain the age of oil generation or migration, and effectively evaluated the relationship between source rock and oil. The radiogenic ¹⁸⁷Os/¹⁸⁸Os ratio showed a positively correlated relationship with source rock age. This article proposed that hydrocarbon mature, biodegradation and water washing had minimal or no effect on data scatter about Re-Os isochron. However, the effects of postdepositional mobility of Re and Os, variations in the initial ¹⁸⁷Os/¹⁸⁸Os ratio, and thermochemical sulfate reduction could result in data scatter. The reproducibility and accuracy of Re-Os age was also closely related to the sample particle size and sample amount. Compared with other isotopic dating, the biggest advantage of Re-Os isotopic dating is to study geological body related to accumulation process, and to obtain direct evidence of hydrocarbon accumulation period. From what was discussed above, the article discussed the existing problems and put forward the future hot research and development direction of Re-Os isotope system, so as to promote the deep development of relative theory and technology of Re-Os isotopic dating.     

Trace and Minor Elements in Smithsonite from Huoshaoyun Nonsulfide Zinc Deposits in Southwest China: A LA–ICP–MS Study

The geodynamic setting of the Xigaze ophiolite has long been debated. Structural and geochemical evidence suggest the Xigaze ophiolite was formed at a slow‐spreading ridge (Nicolas et al., 1981; Liu et al., 2016). Based on incompatible element concentrations, the Xigaze ophiolite volcanics are consistent with the ubiquitous subduction signature in suprasubduction zone (Bedard et al., 2009; Hebert et al., 2012; Dai et al., 2013). It is noteworthy that the Xigaze ophiolite is different from the Geotimes and Lasail and Velly units from Oman ophiolite, respectively. The mafic rocks of the Xigaze ophiolite generally resemble typical N‐MORB and Geotimes volcanics in composition except for slight depletions of Th and Nb (Fig.1a). Although the Xigaze rocks have similar Th and Nb concentrations to Lasail and Velly rocks, most incompatible elements in the Xigaze rocks are comparable to N‐MORB. Petrography in gabbro of Xigaze ophiolite shows that euhedral plagioclases are enclosed by clinopyroxenes suggesting that these minerals have crystallized from an anhydrous magma (Sisson and Grove, 1993). Although the Xigaze volcanic rocks are slightly depleted in Th and Nb, they have MORB‐like trace element characteristics implying that they are derived from an anhydrous MORB magma at spreading centre. Godard et al. (2006) suggested that the mantle source of the Oman ophiolite have element and isotopic characteristics similar to Indian Ocean MORB, where the mantle preserved some older slab materials. A negative Nb anomaly of Oman Geotimes volcanic rocks may be resulted from contamination of the slab materials via decompression melting of the convecting mantle. Moreover, the Xigaze rocks have 1.27–3.18 of (Th/Nb)_N ratios similar with those of Geotimes volcanics ((Th/Nb)_N =0.51–2.77) and lower than those of Lasail and Velly units ((Th/Nb)_N =2.12–6.35). These features suggest that the Xigaze ophiolite may have formed at the spreading centre.     

石油系统Re-Os同位素体系封闭性研究进展

Re-Os同位素定年在石油生成及富有机质沉积岩的绝对定年方面取得了良好的应用效果,并且石油的Os同位素初始值还可以作为油源示踪的指标。分析了Re-Os同位素定年方法在富有机质体系中应用的原理,探讨了影响同位素体系封闭性的各种因素,指出熟化作用、生物降解及水洗作用、脱沥青作用不会影响Re-Os同位素体系的封闭性,而硫酸盐热化学还原作用、成矿流体作用及幔源岩浆混染会扰动Re-Os同位素体系。目前石油系统中Re-Os同位素研究存在的主要问题可归结为2个方面:1石油系统有机质中Re-Os同位素的地球化学行为;2Re-Os同位素年龄的精度及其指示意义。对于这2个方面,开展有机相及现代海藻与Re-Os同位素相关性的研究将有助于了解Re和Os在有机质中的具体富集形式;对控制Re-Os同位素分馏的因素(如沉积环境及有机质类型)的研究将会有助于鉴别哪些地层以及什么样的样品能用来进行Re-Os定年。     

Geochemical and zircon U–Pb age constraints on the origin of the Mesozoic Xigaze ophiolite,Yarlung Zangbo suture zone,SW China

The Mesozoic Xigaze ophiolite is a key to understanding the tectonic evolution of the Yarlung Zangbo suture zone. Although many studies have been reported, the formation age and petrogenesis of the Xigaze ophiolite remain controversial. In this paper, new geochronological and geochemical data for mafic dikes (diabase, dolerite), lavas, and gabbros of the Xigaze ophiolite are provided to constrain the origin of the Xigaze ophiolite. Combined with previous studies, three new zircon U–Pb ages of samples from two gabbro and one dolerite samples show that the Xigaze ophiolite was produced at two distinct stages of 174–149 Ma and 137–123 Ma. Whole-rock geochemical data indicate that these rocks exhibit N-MORB-like features, but the gabbros are more depleted in trace elements and belong to cumulates. Geochemical characters, combined with their positive ε_Nd(t) values (+3.2 to +9.6), suggest that these samples originated from depleted mantle sources with minor influence of slab-derived fluids. Considering the previous studies on the Yarlung Zangbo suture zone, the Xigaze ophiolite was likely generated in an active continental margin fore-arc basin with a multistage model associated with the northward subduction of the Yarlung Zangbo Neo-Tethys Ocean beneath the Lhasa terrane. The Middle–Late Jurassic ophiolitic massifs (174–149 Ma) were produced as the result of slab rollback and were followed by subsequent slab break-off at ~ 150 Ma. The fore-arc lithosphere may be frozen at ~150–137 Ma, consistent with the termination of the Gangdese arc magmatism during this period. The Early Cretaceous ophiolitic massifs (137–123 Ma) were developed in relation to the reinitiation of the Neo-Tethyan oceanic lithosphere subduction, the retreat of the subduction zone, and the creation of a fore-arc basin with strong hyperextension in a new cycle.     

Origin and geodynamic significance of Tertiary postcollisional basaltic magmatism in Serbia (central Balkan Peninsula)

Tertiary basaltic magmatism in Serbia occurred through three episodes: (i) Paleocene/Eocene, when mostly east Serbian mafic alkaline rocks (ESPEMAR) formed, (ii) Oligocene/Miocene, dominated by high-K calc–alkaline basalts, shoshonites (HKCA–SHO) and ultrapotassic (UP) rocks, and (iii) Pliocene episode when rocks similar to (ii) originated. In this study, the geodynamics inferred from petrogenesis of the (i) and (ii) episodes are discussed.

The ESPEMAR (62–39 Ma) occur mainly as mantle xenolith-bearing basanites. Their geochemical features, such as the REE patterns, elevated HFSE contents and depleted Sr–Nd isotope signatures, indicate a relatively small degree of melting of an isotopically depleted mantle source. Their mantle-normalized trace element patterns are flat to concave and “bell-shaped”, characteristic of an OIB source free of subduction component. ⁸⁷Sr/⁸⁶Sr_i and ¹⁴³Nd/¹⁴⁴Nd_i isotope ratios (0.7030–0.7047 and 0.5127–0.5129, respectively) indicate a depleted source for the ESPEMAR similar to the European Asthenospheric Reservoir (EAR).

The HKCA–SHO rocks (30–21 Ma) occur as basalts, basaltic andesites and trachyandesites. They show enrichment in LILE and depletion in HFSE with all the distinctive features of calc–alkaline arc-type magmatism. This is coupled with somewhat enriched Sr–Nd isotope signature (⁸⁷Sr/⁸⁶Sr_i=0.7047–0.7064, ¹⁴³Nd/¹⁴⁴Nd_i=0.5124–0.5126). All these features are characteristic of subduction-related metasomatism and fluxing of the HKCA–SHO mantle source with fluids/melts released from subducted sedimentary material.

UP rocks (35–21 Ma) appear as (i) Si-rich lamproites and related rocks and (ii) olivine leucitites and related rocks. UP rocks have high-LILE/HFSE ratios with enrichment for some LILE around 1000× primitive mantle, troughs at Nb and Ti, and peaks of Pb in their mantle-normalized patterns. They also show highly fractionated REE patterns (La/Yb up to 27, La_N up to 400). The isotopic ratios approach crustal values (⁸⁷Sr/⁸⁶Sr_i=0.7059–0.7115 and ¹⁴³Nd/¹⁴⁴Nd_i=0.5122–0.5126), and that signature is typical for ultrapotassic rocks worldwide.

The Paleocene/Eocene episode and formation of the ESPEMAR is referred to as asthenospheric-derived magmatism. This magmatism originated through passive riftlike structures related to possible short relaxational phases during predominantly collisional and compressional conditions. The Oligocene/Miocene episode and formation of HKCA–SHO and UP rocks were dominated by lithospheric-controlled magmatism. Its origin is connected with the activity of a wide dextral wrench corridor generated along the axis of the Dinaride orogen which collapsed in response to thickened crust caused by earlier compressional processes.

To explain conditions of these two magmatic events, a three-stage geodynamic model has been proposed: (1) subduction–termination/collision stage (Paleocene/Eocene), (2) collision stage (Eocene) and (3) postcollision/collapse stage (Oligocene/early Miocene).     

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.