太行山南段早元古代基性脉岩的~(40)Ar-~(39)Ar年代学及其构造意义

针对华北克拉通中部地区(即Zhaoetal.(2001)称之的中部带)内元古代未变形变质基性岩墙群重要组成部分的太行山南段基性岩脉开展了较为系统的40Ar-39Ar年代学研究,研究结果表明:区内三条NWW或NNE走向的基性岩脉(99JX-16、99JX-65、99JX-71)在>80%的39Ar累积量基础上分别给出了1765.3±1.1Ma,1774.7±0.7Ma,1780.7±0.5Ma的坪谱年龄。1781～1765Ma的年龄限定了区内基性岩脉的侵位年龄,该年龄也一致于恒山NWW向未变形基性岩脉1769.1±2.5Ma的单颗粒锆石U-Pb年龄。上述资料较好地约束了华北陆块早元古代基性岩脉的形成年龄。这为深入理解华北陆块1800Ma左右的热构造事件和华北早前寒武纪构造演化提供新的年代学资料。     

甘肃礼县新生代火山喷发碳酸岩的发现及意义

甘肃礼县新生代钾霞橄黄长岩中有大量火山喷发碳酸岩出露,本文对该区各种碳酸岩的地质产状,岩石特征,全岩化学及稀土,微量和C、O同位素地球化学进行了研究,结果表明本区碳酸岩属于一种高CaO低碱（Na2O K2O)的火成碳酸岩,其化学成分与东非裂谷乌干达FortPortal地区的碳酸岩相似,其中碳酸岩的火山砾岩和凝灰岩具有高的SiO2和Mg/Ca比值,化学成分显示了硅酸盐和碳酸盐混合的特征,可能代表直接由地幔部分熔融形成的原生碳酸盐岩浆。其它类型碳酸岩的成因与原生碳酸盐岩浆或原生钾霞橄黄长岩浆的液态不混溶作用和／或结晶分异作用有关。碳酸岩及共生的高钾火山岩不是典型的大陆裂谷岩浆作用的产物,其起源和成因与软流圈的上涌有关。碳酸岩的发现为查明本区地幔的组成和性质提供了新的岩石学证据。     

东秦岭钼矿带内碳酸岩脉型钼（铅）矿床地质 地球化学特征、成矿机制及成矿构造背景

东秦岭钼矿带位于华北板块南缘,NW-NWW向的固始―栾川深断裂带控制着钼矿床的空间分布.黄水庵碳酸岩脉型钼(铅)矿床的确定,为本矿带内已有碳酸岩脉型钼(铅)矿床(黄龙铺地区的大石沟、石家湾和桃园等)增添了又一新成员.本矿带不仅钼金属储量居世界已知单个钼矿带之首,而且碳酸岩脉和花岗斑岩两个成矿体系并存,亦是本区钼矿带的一大特色.业已查明,黄水庵和黄龙铺(大石沟)等碳酸岩脉型钼(铅)矿床的δ~(13)C=-5.3‰～-7.0‰,~(87)Sr/~(86)Sr=0.7049～0.7065.同时,方解石富含轻稀土(LREE/HREE=1.8～2.9).辉钼矿以富含Re(平均为110×10~(-6)～244×10~(-6))为特征.基于含矿碳酸岩脉方解石的Sr、Nd、Pb同位素比值(~(87)Sr/~(86)Sr对~(206)Pb/~(204)Pb、~(207)Pb/~(204)Pb对~(206)Pb/~(204)Pb和~(143)Nd/~(144)Nd对~(87)Sr/~(86)Sr)的关系图,我们初步判断本矿带区域陆壳之下可能存在有EMI(富集地幔Ⅰ),这些含矿碳酸岩脉是源于EMI的碱性硅酸盐-碳酸盐熔体-溶液结晶分异的产物,成矿金属Mo、Pb主要来自EMI.根据黄水庵和黄龙铺(大石沟)钼(铅)矿床的成矿年龄(Re-Os年龄分别为209.5 Ma和221 Ma),我们推断,碳酸岩脉型钼(铅)矿床形成于华北和扬子两大板块三叠纪碰撞造山后伸展阶段的晚三叠世时期,而在侏罗纪陆内造山晚期的伸展阶段,形成了晚侏罗-早白垩世的斑岩型和斑岩-矽卡岩型钼矿床(Re-Os年龄介于147～116 Ma).     

Torngat ultramafic lamprophyres and their relation to the North Atlantic Alkaline Province

Geological mapping and diamond exploration in northern Quebec and Labrador has revealed an undeformed ultramafic dyke swarm in the northern Torngat Mountains. The dyke rocks are dominated by an olivine-phlogopite mineralogy and contain varying amounts of primary carbonate. Their mineralogy, mineral compositional trends and the presence of typomorphic minerals (e.g. kimzeyitic garnet), indicate that these dykes comprise an ultramafic lamprophyre suite grading into carbonatite. Recognized rock varieties are aillikite, mela-aillikite and subordinate carbonatite. Carbonatite and aillikite have in common high carbonate content and a lack of clinopyroxene. In contrast, mela-aillikites are richer in mafic silicate minerals, in particular clinopyroxene and amphibole, and contain only small amounts of primary carbonate. The modal mineralogy and textures of the dyke varieties are gradational, indicating that they represent end-members in a compositional continuum.

The Torngat ultramafic lamprophyres are characterized by high but variable MgO (10–25 wt.%), CaO (5–20 wt.%), TiO₂ (3–10 wt.%) and K₂O (1–4 wt.%), but low SiO₂ (22–37 wt.%) and Al₂O₃ (2–6 wt.%). Higher SiO₂, Al₂O₃, Na₂O and lower CO₂ content distinguish the mela-aillikites from the aillikites. Whereas the bulk rock major and trace element concentrations of the aillikites and mela-aillikites overlap, there is no fractional crystallization relation between them. The major and trace element characteristics imply related parental magmas, with minor olivine and Cr-spinel fractionation accounting for intra-group variation.

The Torngat ultramafic lamprophyres have a Neoproterozoic age and are spatially and compositionally closely related with the Neoproterozoic ultramafic lamprophyres from central West Greenland. Ultramafic potassic-to-carbonatitic magmatism occurred in both eastern Laurentia and western Baltica during the Late Neoproterozoic. It can be inferred from the emplacement ages of the alkaline complexes and timing of Late Proterozoic processes in the North Atlantic region that this volatile-rich, deep-seated igneous activity was a distal effect of the breakup of Rodinia. This occurred during and/or after the rift-to-drift transition that led to the opening of the Iapetus Ocean.     

The crater-facies kimberlite system of Tokapal, Bastar District, Chhattisgarh, India

Discovery of diamondiferous kimberlites in the Mainpur Kimberlite Field, Raipur District, Chhattisgarh in central India, encouraged investigation of similar bodies in other parts of the Bastar craton. The earlier known Tokapal ultramafic intrusive body, located beyond the 19-km milestone in Tokapal village along the Jagdalpur–Geedam road, was reinterpreted as crater-facies kimberlite. Its stratigraphic position in the Meso-Neoproterozoic intracratonic sedimentary Indravati basin makes it one of the oldest preserved crater-facies kimberlite systems. Ground and limited subsurface data (dug-, tube-wells and exploratory boreholes) have outlined an extensive surface area (>550 ha) of the kimberlite. The morphological and surface color features of this body on enhanced satellite images suggest that there is a central feeder surrounded by a collar and wide pyroclastic apron. Exploration drilling indicates that the central zone probably corresponds to a vent overlain by resedimented volcaniclastic (epiclastic) rocks that are surrounded by a 2-km-wide spread of pyroclastic rocks (lapilli tuff, tuff/ash beds and volcaniclastic breccia). Drill-holes also reveal that kimberlitic lapilli tuffs and tuffs are sandwiched between the Kanger and Jagdalpur Formations and also form sills within the sedimentary sequence of the Indravati basin. The lapilli tuffs are commonly well stratified and display slumping. Base surges and lava flows occur in the southern part of the Tokapal system. The geochemistry and petrology of the rock correspond to average Group I kimberlite with a moderate degree of contamination. However, the exposed rock is intensely weathered and altered with strong leaching of mobile elements (Ba, Rb, Sr). Layers of vesicular fine-grained glassy material represent kimberlitic lava flows. Tuffs containing juvenile lapilli with pseudomorphed olivine macrocrysts are set in a talc–serpentine–carbonate matrix with locally abundant spinel and sphene. Garnet has not been observed, and phlogopite is very rare. Very limited microdiamond testing (two 18-kg samples) proved negative; however, the composition of chromite grains indicate crystallization in the diamond stability field.     

Decoding Neoarchaean to Palaeoproterozoic tectonothermal events in the Rangnim Massif,North Korea: regional correlation and broader implications

Any knowledge about Archaean to Palaeoproterozoic magmatic and metamorphic events in North Korea has the potential to make a significant difference to our understanding of the early tectonic configuration and evolution of East Asia. This zircon U–Pb dating and Hf isotopic study documents multiple Neoarchaean to Palaeoproterozoic tectonothermal events from the meta-igneous complex in the Machollyong ‘Group’ of the Rangnim Massif. Two tonalitic-trondjemitic gneiss samples record a crystallization age of meta-igneous protoliths at ca. 2.56 Ga and multiple migmatization and metamorphism from 2.52 to 1.85 Ga. A meta-dolerite sample yields a magmatic emplacement age of ca. 1.83 Ga. In situ zircon Hf isotopic data indicate that most zircons from the gneiss samples have ε_Hf(t) values from –16.9 to + 3.1 and crustal model ages from 2.84 to 3.73 Ga, whereas magmatic zircons from the meta-dolerite dike record ε_Hf(t) values from –5.2 to + 5.2 and model ages of 2.05–2.44 Ga. The first-recognized Neoarchaean tonalitic-trondjemitic migmatite complex in the Rangnim Massif, together with previously identified tonalitic-trondhjemitic-granodioritic (TTG) rocks in the Rimjingang Belt and the coeval counterparts in western Gyeonggi massif, represents the oldest crustal nuclei in the Korean Peninsula. The multiple tectonothermal events in this study present reliable evidence not only for attesting to consanguinity of the basement between the Korean Peninsula and the North China Craton but also for defining the influence scope of the late Palaeoproterozoic orogeny in the Korean Peninsula.     

Carbonatites in a subduction system: The Pleistocene alvikites from Mt. Vulture (southern Italy)

We report here, for the first time, on the new finding of extrusive calciocarbonatite (alvikite) rocks from the Pleistocene Mt. Vulture volcano (southern Italy). These volcanic rocks, which represent an outstanding occurrence in the wider scenario of the Italian potassic magmatism, form lavas, pyroclastic deposits, and feeder dikes exposed on the northern slope of the volcano. The petrography, mineralogy and whole-rock chemistry attest the genuine carbonatitic nature of these rocks, that are characterized by high to very high contents of Sr, Ba, U, LREE, Nb, P, F, Th, high Nb/Ta and LREE/HREE ratios, and low contents of Ti, Zr, K, Rb, Na and Cs. The O–C isotope compositions are close to the “primary igneous carbonatite” field and, thus, are compatible with an ultimate mantle origin for these rocks. The Sr–Nd–Pb–B isotope compositions, measured both in the alvikites and in the silicate volcanic rocks, indicate a close genetic relationship between the alvikites and the associated melilitite/nephelinite rocks. Furthermore, these latter products are geochemically distinct from the main foiditic-phonolitic association of Mt. Vulture. We propose a petrogenetic/geodynamic interpretation which has important implications for understanding the relationships between carbonatites and orogenic activity. In particular, we propose that the studied alvikites are generated through liquid unmixing at crustal levels, starting from nephelinitic or melilititic parent liquids. These latter were produced in a hybrid mantle resulting from the interaction through a vertical slab window, between a metasomatized mantle wedge, moving eastward from the Tyrrhenian/Campanian region, and the local Adriatic mantle. The occurrence of carbonatite rocks at Mt. Vulture, that lies on the leading edge of the Southern Apennines accretionary prism, is taken as an evidence for the carbonatation of the mantle sources of this volcano. We speculate that mantle carbonatation is related to the introduction of sedimentary carbon from the Adriatic lithosphere during Tertiary subduction.     

克拉玛依侏罗纪橄榄玄武岩中菱铁矿的成因

克拉玛依市西盆山交界处发育侏罗纪杏仁玄武岩-橄榄玄武岩-含菱铁矿橄榄玄武岩组合,主要由橄榄石、单斜辉石、斜长石(中长石-拉长石为主)、钛铁矿、玄武玻璃及少量磷灰石组成,含菱铁矿橄榄玄武岩中存在原生菱铁矿(≈10%)和少量碱性长石.橄榄石及单斜辉石的Mg#为≈70,单斜辉石属于富Ti普通辉石.含菱铁矿橄榄玄武岩中的菱铁矿常常包裹浑圆状斜长石,且与斜长石呈锯齿状接触.本文提供的岩相学观察表明,菱铁矿与玄武玻璃、斜长石、单斜辉石和钛铁矿呈明显的共生关系.在有些情况下观察到玄武玻璃与菱铁矿充填在斜长石格架的现象.含菱铁矿橄榄玄武岩玻璃中FeO含量最高至29.2%,CaO含量＜1.5%,即熔体具有富Fe贫Ca的特点.早结晶的碳酸盐矿物是以菱铁矿为主的菱铁矿-白云石固溶体(另含少量菱锰矿).斜长石大量结晶促使熔体不断亏损CaO,并导致较晚结晶的碳酸盐矿物向菱铁矿端元演化.菱铁矿发育有明显的成分梯变带(FeO和CaO含量表现出突变).玄武玻璃的Mg#普遍较低且变化范围大,橄榄玄武岩中玻璃的Mg#值最高为64,含菱铁矿橄榄玄武岩中玻璃的Mg#值最低至≈23.依据地质温压计,克拉玛依侏罗纪玄武岩浆的形成温度为1300℃～1340℃,压力为3.0～4.0GPa.基于本文的研究,我们认为克拉玛依侏罗纪含菱铁矿橄榄玄武岩岩浆由含CO2橄榄岩在软流圈中经极低程度部分熔融形成,该岩浆在快速上升过程中通过结晶分异作用形成了菱铁矿.     

Injection and arrest of dykes: implications for volcanic hazards

Dykes are the principal channels through which magma reaches the surface in volcanic eruptions. For this reason dykes observed in the field are commonly assumed to be feeders to lava flows. The actual proportion of dykes reaching the surface is, however, poorly known. In order to develop models for the purpose of estimating volcanic hazard, this proportion must be known. This follows because such models should not only consider the probability of dykes being injected from magma chambers during periods of unrest in the associated volcanoes, but also the probability of the injected dykes being arrested. This paper presents field data on several thousand dykes from Iceland and Tenerife (Canary Islands) indicating that many, and probably most, dykes become arrested at various crustal levels and never reach the surface to feed eruptions. Using the results of analytical and numerical models, it is shown that, for common loading conditions, the stress field in the vicinity of a magma chamber may favour the injection and propagation of dykes while the stress field at a certain distance from the chamber favours dyke arrest. This means that many dykes that are injected from the chamber propagate only for a very limited distance from the chamber to the point where they become arrested. The implication is that during periods of unrest in volcanoes, the probability of volcanic eruption is only a small fraction of the probability of dyke injection from the source magma chamber.     

中天山地块暗色岩墙LA-ICP-MS锆石U-Pb年龄和岩石地球化学特征

中天山地块发育有大量的暗色岩墙,主要走向为北北西。对中天山暗色岩墙进行LA-ICP-MS锆石U-Pb测年,获得的年龄为304.8±1.5Ma,表明其形成于晚石炭世。这些暗色岩墙具有较高的Si O2,较低的Mg、Cr、Ni含量。它们富集大离子亲石元素(Rb、Ba、Th),亏损高场强元素(Nb、Ta、Ti),显示俯冲相关特征。上述特征暗示,中天山暗色岩墙可能是由俯冲板片流体交代的地幔楔部分熔融形成的。这一发现表明,准噶尔洋的闭合应该发生在晚石炭世之后。