Geochemistry of ultramafic and calc-llkaline lamprophyres from the Lac Shortt area,Quebec

Summary Petrographic and geochemical data are presented for the Lac Shortt lamprophyre dyke swarm. The primary mineralogy in all samples has been largely destroyed either by alteration at the time of emplacement or by subsequent metamorphism. Most of the dykes are ultramafic lamprophyres. Extended trace element plots (ETP) for these dykes feature a characteristic double dome pattern, with one dome extending between Sr and Zr and the other between Zr and Yb. One ultramafic dyke was selected for detailed study. The ETP and rare earth element (REE) curves for the analyses of this dyke are essentially parallel, indicating the relative immobility of both groups of elements during the subsequent events. Significant correlation coefficients between the high field strength elements using phosphorus as representative of these elements, and the major element concentrations suggests that these were also relatively immobile. Pearce plots and principal component analyses reveal that olivine and apatite fractionation controlled the chemical variation across the control dyke. A plot of Nb anomalies against silica for Precambrian ultramafic and cafe-alkaline lamprophyres reveals two clear groupings—the former has no such anomalies in contrast to the latter which possesses this feature. A calc-alkaline lamprophyre dyke from the Lac Shortt Mine features significant Nb depletion on ETP suggesting that magmas with low Si0₂ content Si0₂ = 35.85 weight and chondrite normalized La = 1458) may be related to this catagory of lamprophyre.

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Geochemie von ultramaftschen und kalkalkalinen Lamprophyren aus dem Lac Shorn Gebiet, Quebec

Zusammenfassung Petrographische and geochemische Daten für den Lac Shortt Lamprophyr-Schwarm werden vorgelegt. Die primare mineralogische Zusammensetzung in allen Proben ist großteils entweder durch Umwandlung zur Zeit der Platznahme oder durch anschließende Metamorphose zerstört worden. Bei den meisten Gängen handelt es sich um ultramafische Lamprophyre. Extended trace element plots (ETP) für diese Gänge zeigen ein charakteristisches Double Dome Pattern, wobei ein Dom sich zwischen Sr und Zr und der andere zwischen Zr und Yb erstreckt. Ein ultramafischer Gang wurde für detaillierte Untersuchungen ausgewählt. Die ETP und seltenen Erdenkurven (REE) für die Analysen dieses Ganges sind im wesentlichen parallel, und weisen auf die relative Immobilität beider Elementgruppen während der anschließenden geologischen Entwicklung hin. Signifikante Korrelationskoeffizienten zwischen den Elementen großer Feldstärke, wobei Phosphor als repräsentativer Vertreter benützt wurde, und den Hauptelementgehalten zeigt, daß auch diese relativ immobil waren. Pearces Plots und Analysen der Hauptkomponenten zeigen, daß Olivin und Apatit-Fraktionierung die chemische Variation über einen Gang bestimmt. Ein Plot von Nb Anomalien gegen Kieselsäure für präkambrische ultramafische und kalkaline Lamprophyre zeigt daß diese in einer Gruppe vorhanden sind, in der anderen fehlen. Ein kalk-alkaliner Lamprophyrgang aus der Lac Shortt Mine zeigt signifikante Nb-Verarmung bei ETP; dies weist darauf hin, daß Magmen mit niedrigem Si0₂-Gehalt Si0₂ = 35,85 Gew.% und chondrit-normalisiertes La = 1458) mit dieser Art von Lamprophyren in Beziehung gesetzt werden können.

     

Alkaline and ultramafic carbonate lamprophyres in Central Bohemian carboniferous basins,Czech Republic

Summary Dykes of alkaline and ultramafic carbonate lamprophyres (monchiquite-ouachitite and aillikite) intrude the Carboniferous sediments of Central Bohemia. Their characteristic feature is the presence of substantial amounts of a sodalite group mineral (presumably haüyne), carbonate and barite. Isotopic compositions of ¹³C (–3.4 to –6.2) and ¹⁸O ( + 13.5 to + 15.9) indicate that the carbonate is of primary magmatic origin and that fluids were formed at temperatures of 500 to 350°C. High contents of CO₂ (3.6 to 17.6 wt.%) and incompatible elements, high LREE/HREE ratios (30.0 to 57.7), and low Rb/Sr (0.025 to 0.078) and⁸⁷Sr/⁸⁶Sr (0.7038 to 0.7042) ratios suggest the ultramafic lamprophyres are related to deep-seated carbonated magmas of mantle origin. Low degree of partial melting ( < 1%) of the mantle peridotite is envisaged, with additional transport of fluids rich in incompatible elements into the crustal chamber. Alkaline lamprophyres are fractionated derivatives which originated from the same source. Magma intrusion from different levels of a magma chamber into fracture zones under horizontal tension without a central intrusion could result in variability in lamprophyre composition (cumulates or evolved derivatives).

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Alkalische und ultramafische Karbonat-Lamprophyre der zentralen böhmischen Karbonbecken, Tschische Republik

Zusammenfassung Alkalische und ultramafische Karbonat-Lamprophyrgänge (Monchiquit-Ouachitit und Aillikit) intrudieren in karbonische Sedimente Zentralböhmens. Sie sind charakterisiert durch das Auftreten von beträchtlichen Mengen an Mineralen der Sodalith-Gruppe (v.a. Haüyn), Karbonaten und Baryt. Ihre Isotopenzusammensetzung von ¹³C (–3.4 bis –6.2) und ¹⁸O (13.5 bis 15.9) zeigt, daß die Karbonate primär magmatischen Ursprungs sind und daß assoziierte Fluide in einem Temperaturbereich von 350 bis 500°C gebildet wurden. Hohe Gehalte an CO₂ (3.6 bis 17.6 Gew. %), inkompatiblen Elementen, hohe LREE/HREE-Verhältnisse (30.0 bis 57.7), sowie niedrige Rb/Sr—(0.025 bis 0.078) und⁸⁷Sr/⁸⁶Sr-(0.7038 bis 0.7042) Verhältnisse lassen vermuten, daß die ultramafischen Lamprophyre mit karbonatischen Magmen des Mantels in Beziehung stehen. Eine niedrige Aufschmelzungsrate ( < 1%) von Mantelperidotit mit zusätzlichem Transport von, an inkompatiblen Elementen angereicherten Fluiden, in die krustale Magmenkammer wird diskutiert. Alkalische Lamprophyre sind als stärker fraktionierte Magmen, die derselben Quelle entstammen, zu verstehen. Die Intrusion der aus verschiedenen Bereichen der Magmenkammer stammenden Magmen in durch horizontale Dehnung verursachte Störungszonen könnte das Fehlen eines zentralen Intrusionskörpers und die unterschiedliche Zusammensetzung der Lamprophyre (Kumulate oder entwickelte Derivate) erklären.

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With 8 Figures     

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.     

Hydrogeochemistry of sulfur isotopes in the Kalix River catchment,northern Sweden

The³⁴S-to-³²S ratio in dissolved SO₄ has been studied in the Kalix River, Northern Sweden, and its catchment. Weekly sampling over 17 months revealed temporal variations from +5.3‰ up to +7.4‰ in the δ³⁴S values in the river. Snow and rain samples showed lower δ³⁴S values (average +5.6‰ and +5.0‰, respectively). The atmosphere is the major source for S in surface waters in the catchment, and the heavier δ³⁴S values in the river are a result of SO₄ reduction within the catchment.Most of the temporal variations in the δ³⁴S value in the river are caused by a mixing of water from the mountain areas (relatively light δ³⁴S) and the woodland. The δ³⁴S value is relatively heavy in the woodland tributaries because of bacterial SO₄ reduction in peatland areas influenced by groundwater.The highest δ³⁴S values were measured during the spring flood, in June and in November. These heavy δ³⁴S values are related to different types of water with diverse origins.The heavy δ³⁴S values coinciding with the early spring flood originate from peatland areas in the woodland. Relatively heavy δ³⁴S values (up to +14.4‰) were registered in mire water. Smaller variations of the δ³⁴S value during summer and early autumn most likely were caused by the input of ground-mire water during heavy rains. A correlation between increased TOC concentrations and increased δ³⁴S values was observed.The heavy δ³⁴S values in June and November probably originate from SO₄ reduction in bottom water and sediments in lakes within the catchment. Bottom water, enriched in³⁴SSO₄, was transported in the river during the spring and autumn overturn.     

Petrogenesis of early cretaceous carbonatite and ultramafic lamprophyres in a diatreme in the Batain Nappes,Eastern Oman continental margin

Allochthonous carbonatite and ultramafic lamprophyre occur in a diatreme at the beach of the Asseelah village, northeastern Oman. The diatreme consists of heterogeneous deposits dominated by ‘diatreme facies’ pyroclastic rocks. These include aillikite and carbonatite, which intrude late Jurassic to early Cretaceous cherts and shales of the Wahra Formation within the Batain nappes. Both rock types are dominated by carbonate, altered olivine, Ti–Al–phlogopite and Cr–Al–spinel and contain varying amounts of apatite and rutile. The carbonatite occur as fine-grained heterolithic breccias with abundant rounded carbonatite xenoliths, glimmerite and crustal xenoliths. The aillikite consists of pelletal lapilli tuff with abundant fine-grained carbonatite autoliths and crustal xenoliths, which resemble those in the carbonatite breccia. The aillikite and carbonatite are characterized by low SiO₂ (11–24 wt%), MgO (9.5–12.4 wt%) and K₂O (<0.3 wt%), but high CaO (18–22 wt%), Al₂O₃ (4.75–7.04 wt%), Fe₂O_3tot (8.7–13.8 wt%) and loss-on-ignition (24–30 wt%). Higher CaO, Fe₂O_3total, Al₂O₃, MnO, TiO₂, P₂O₅ and lower SiO₂ and MgO content distinguish carbonatite from the aillikite. The associated carbonatite xenoliths and autoliths have intermediate composition between the aillikite and carbonatite. Mg number is variable and ranges between 58 and 66 in the carbonatite, 66 and 72 in the aillikite and between 48 to 64 in the carbonatite autoliths and xenoliths. The Asseelah aillikite, carbonatite, carbonatite xenoliths and autoliths overlap in most of their mineral parageneses, mineral composition and major and trace element chemistry and have variable but overlapping Sr, Nd and Pb isotopic composition, implying that these rocks are related to a common type of parental magma with variable isotopic characteristics. The Asseelah aillikite, carbonatite and carbonatites xenoliths are LREE-enriched and significantly depleted in HREE. They exhibit similar smooth, subparallel REE pattern and steep slopes with (La/Sm) _n of 6–10 and relative depletion in heavy rare earth elements (Lu = 3–10 chondrite). Initial ⁸⁷Sr/⁸⁶Sr ratios vary from 0.70409 to 0.70787, whereas initial ¹⁴³Nd/¹⁴⁴Nd ratios vary between 0.512603 and 0.512716 (εNd _i between 2.8 and 3.6). ²⁰⁶Pb/²⁰⁴Pb _i ratios vary between 18.4 and 18.76, ²⁰⁷Pb/²⁰⁴Pb _i ratios vary between 15.34 and 15.63, whereas ²⁰⁸Pb/²⁰⁴Pb _i varies between 38.42 and 39.05. Zircons grains extracted from the carbonatite have a mean age of 137 ± 1 Ma (95% confidence, MSWD = 0.49). This age correlates with large-scale tectonic events recorded in the early Indian Ocean at 140–160 Ma. Geochemical and isotopic signatures displayed by the Asseelah rocks can be accounted for by vein-plus-wall-rock model of Foley (1992) wherein veins are represented by phlogopite, carbonate and apatite and depleted peridotite constitutes the wall-rock. The carbonatite and aillikite magmatism is probably a distal effect of the breaking up of Gondwana, during and/or after the rift-to-drift transition that led to the opening of the Indian Ocean.     

Between carbonatite and lamproite—Diamondiferous Torngat ultramafic lamprophyres formed by carbonate-fluxed melting of cratonic MARID-type metasomes

New U-Pb perovskite ages reveal that diamondiferous ultramafic lamprophyre magmas erupted through the Archean crust of northern Labrador and Quebec (eastern Canada) between ca. 610 and 565 Ma, a period of strong rifting activity throughout contiguous Laurentia and Baltica. The observed Torngat carbonate-rich aillikite/carbonatite and carbonate-poor mela-aillikite dyke varieties show a large spread in Sr-Nd-Hf-Pb isotope ratios with pronounced correlations between isotope systems. An isotopically depleted component is identified solely within aillikites (⁸⁷Sr/⁸⁶Sr_i = 0.70323-0.70377; εNd_i = +1.2-+1.8; εHf_i = +1.4-+3.5; ²⁰⁶Pb/²⁰⁴Pb_i = 18.2-18.5), whereas some aillikites and all mela-aillikites range to more enriched isotope signatures (⁸⁷Sr/⁸⁶Sr_i = 0.70388-0.70523; εNd_i = −0.5 to −3.9; εHf_i = −0.6 to −6.0; ²⁰⁶Pb/²⁰⁴Pb_i = 17.8-18.2). These contrasting isotopic characteristics of aillikites/carbonatites and mela-aillikites, along with subtle differences in their modal carbonate, SiO₂, Al₂O₃, Na₂O, Cs-Rb, and Zr-Hf contents, are consistent with two distinctive metasomatic assemblages of different age in the mantle magma source region.Integration of petrologic, geochemical, and isotopic information leads us to propose that the isotopically enriched component originated from a reduced phlogopite-richterite-Ti-oxide dominated source assemblage that is reminiscent of MARID suite xenoliths. In contrast, the isotopically depleted component was derived from a more oxidized phlogopite-carbonate dominated source assemblage. We argue that low-degree CO₂-rich potassic silicate melts from the convective upper mantle were preferentially channelled into an older, pre-existing MARID-type vein network at the base of the North Atlantic craton lithosphere, where they froze to form new phlogopite-carbonate dominated veins. Continued stretching and thinning of the cratonic lithosphere during the Late Neoproterozoic remobilized the carbonate-rich vein material and induced volatile-fluxed fusion of the MARID-type veins and the cold peridotite substrate. Isotopic modelling suggests that only 5-12% trace element contribution from such geochemically extreme MARID-type material is required to produce the observed compositional shift from the isotopically most depleted aillikites/carbonatites towards enriched mela-aillikites.We conclude that cold cratonic mantle lithosphere can host several generations of contrasting vein assemblages, and that each may have formed during past tectonic and magmatic events under distinctively different physicochemical conditions. Although cratonic MARID-type and carbonate-bearing veins in peridotite can be the respective sources for lamproite and carbonatite magmas when present as the sole metasome, their concomitant fusion in a complex source region may give rise to a whole new variety of deep volatile-rich magmas and we suggest that orangeites (formerly Group 2 kimberlites), kamafugites, and certain types of ultramafic lamprophyre are formed in this manner.     

Geochemistry and petrology of ultramafic lamprophyres from Schirmacher Oasis, East Antarctica

Summary Petrographic and geochemical data are presented for some ouachitites from the Schirmacher Oasis. The studied dike intruded into the metamorphic basement of the East Antarctic craton. The ouachitite samples contain substantial amounts of zoned phlogopite and diopsidic to salitic pyroxene as phenocrysts. K-rich nepheline is the predominant phase of the matrix. The rocks are characterized by low SiO₂ concentrations (up to 36.5 wt.%), low mg# and high abundances of Al²O³, alkalis and volatiles. Trace element compositions of the Schirmacher ouachitites differ from those of ultramafic lamprophyres from other regions in that they contain significantly lower Cr and Ni concentrations and relatively low enrichments in LILE and LREE. The studied rocks have low¹⁴³Nd/¹⁴⁴Nd and enhanced⁸⁷Sr/⁸⁶Sr ratios. Pb isotope compositions of the ouachitites closely resemble those of MORB. The ¹⁸O values range between 4.1 and 4.7⁰/₀₀ which may be due to interactions with meteoric water. Because the Schirmacher ouachitites are only represented by a few samples it is impossible to discuss the petrogenesis of this magma. However, according to trace element and isotopic characteristics, it can be ruled out that Schirmacher ouachitites and associated minettes (documented inHoch, 1997;Hoch andTobschall, 1998) were derived from the same mantle reservoir.

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Geochemie und Petrologie von Ouachititen aus der Schirmacher Oase, Ostantarktika

Zusammenfassung In dieser Arbeit werden petrologische und geochemische Daten von Ouachititen aus der Schirmacher Oase vorgestellt. Der beprobte Gang intrudierte in das metamorphe Basement des ostantarktischen Kratons. Zonierte Phlogopite und diopsidische bis salitische Pyroxene sind die häufigsten mafischen Einsprenglinge. K-reiche Nepheline bilden die Hauptmineralphase der Grundmasse. Charakteristisch für die Ouachititproben sind niedrige SiO₂-Konzentrationen (bis 36.5 Gew.%) und mg#, aber hohe Gehalte an Al₂O₃ Alkalien und volatilen Bestandteilen. Die Spurenelementzusammensetzung der Schirmacher Ouachitite unterscheidet sich von UML-Proben aus anderen Regionen durch ihre extrem geringen Cr- und Ni-Konzentrationen und die vergleichsweise niedrigen Anreicherungen an LILE und LREE. Die bearbeiteten Gesteine zeigen niedrige¹⁴³Nd/¹⁴⁴Nd- und erhöhte⁸⁷Sr/⁸⁶Sr-Verhältnisse. Die Pb-Isotopie der Ouachitite entspricht der von MORB. 6180-Werte variieren zwischen 4.1 und 4.7⁰/₀₀, was auf eine eventuelle Beeinflussung durch meteorische Wässer hindeutet. Da von den Schirmacher Ouachititen nur wenige Proben für eine Bearbeitung zur Verfügung standen, ist es nicht möglich, die Petrogenese des Magmas zu diskutieren. Aber aufgrund der Spurenelement- und Isotopensignaturen kann ein gemeinsames Mantelreservoir der Schirmacher Ouachitite und Minette (beschrieben inHoch, 1997;Hoch undTobschall, 1998) ausgeschlossen werden.

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With 6 Figures     

Petrology of ultramafic lamprophyres from the Beaver Lake area of Eastern Antarctica and their relation to the breakup of Gondwanaland

Summary Mesozoic melilite-bearing ultramafic lamprophyres are developed as sill, dyke and plug-like intrusive bodies in the East Antarctic Beaver Lake area. They consist of varying amounts of olivine, melilite, phlogopite, nepheline, titanomagnetite and perovskite as major phases, accompanied by minor amounts of apatite, carbonate, spinel, glass and, rarely, monticellite. The rocks are mineralogically and geochemically broadly similar to olivine melilitites, differing in higher CO₂ and modal phlogopite and carbonate contents. The ultramafic lamprophyres are MgO-rich (13.4–20.5 wt%) and SiO₂-poor (32.8–37.2 wt%), indicative of a near-primary nature. Major and trace element features are consistent with minor fractionation of olivine and Cr-spinel from melts originating at depths of 130–140 km. Primary melts originated by melting of upper mantle peridotite which had been veined by phlogopite + carbonate + clinopyroxene-bearing assemblages less than 200 Ma before eruption. The presence of the veins and their time of formation is required to explain high incompatible trace element contents and growth of ⁸⁷Sr/⁸⁶Sr, leaving ¹⁴³Nd/¹⁴⁴Nd unaffected. The major element, compatible trace element, and most radiogenic isotope characteristics are derived from melting of the wall-rock peridotite. The depth of about 130 km is indicated by the presence of phlogopite rather than amphibole in the veins, by control of the REE pattern by residual garnet, by the high MgO content of the rocks, and by the expected intersection of the rift-flank geotherm with the solidus at this depth. The higher CO₂ contents than are characteristic for olivine melilitites favoured the crystallization of melilite at crustal pressures, and suppressed the crystallization of clinopyroxene. The Beaver Lake ultramafic lamprophyres are a distal effect of the breakup of Gondwanaland, too distal to show a geochemical signature of the Kerguelen plume. Upward and outward movement of the asthenosphere-lithosphere boundary beneath the Lambert-Amery rift led first to the production of phlogopite- and carbonate-rich veins, and later to the generation of the ultramafic lamprophyres themselves. Received March 31, 2000; revised version accepted September 3, 2001     

白云鄂博碳酸岩墙碳氧同位素地球化学

对内蒙古白云鄂博 REE- Fe- Nb矿床周围碳酸岩墙中共存的方解石和白云石进行了 C和 O同位素分析。结果表明，方解石和白云石的δ 13C值变化范围一致，均为－ 3.5‰～－ 7.3‰，落在正常地幔δ 13C值范围 (－ 5‰± 2‰ )内；而它们的δ 18O值可分为两组，第Ⅰ 组为 9.5‰～ 18.0‰，第Ⅱ 组为 20.6‰～ 22.6‰，均远大于正常地幔δ 18O值范围 (5.7‰± 1.0‰ )。第Ⅰ 组低δ 18O值样品中共存白云石与方解石之间的 C和 O同位素分馏均为负值，因此处于热力学不平衡状态，指示它们自形成后受到过后期热液蚀变，与先前的岩石学观察一致。相反，第Ⅱ 组高δ 18O值样品中白云石与方解石之间的 C和 O同位素分馏均为正值，处于热力学平衡状态，指示它们自形成后未受到后期热液蚀变，因此可能沉淀于晚期低温高δ 18O值流体。第Ⅰ 组碳酸岩墙中白云石的 C和 O同位素组成不呈线性分布，指示碳酸岩浆并非由幔源碳酸盐与沉积碳酸盐混合形成。应用水-岩交换模型计算得到，第Ⅰ 组碳酸岩在侵位后经历了碳酸岩浆期后热液的不均一蚀变，蚀变温度约在 220～ 800℃之间，蚀变流体的 CO2/H2O比值较小 (1/500)，但水 /岩比值变化较大 (10～ 400)。由于低温下方解石与热液之间的碳氧同位素交换速率大于白云石，导致这部分碳酸     

山西省早白垩纪碳酸岩元素同位素和锆石U-Pb定年研究

碳酸岩广泛出露在华北克拉通的山西省。此文中,针对研究区的碳酸岩墙,我们给出新的地球化学、Sr-Nd同位素和锆石U-Pb年龄。LA-ICP-MS锆石定年结果显示,该岩墙的侵位年龄为132.9Ma,全岩K-Ar年龄为131.3~132.6 Ma。碳酸岩墙具有非常一致的主量元素组成,富集轻稀土元素和大离子亲石元素(Ba、U、Pb、Sr),以及亏损K和高场强元素(Ta、P和Ti)。另外,该岩墙具有相对一致的(87Sr/86Sr)i(0.7079~0.7083)和负的εNd(t)(-16.7~-15.2)。以上地球化学特征表明,该岩墙为大陆地壳和此大陆岩石圈地幔混合时期,受明显地壳混染的下岩石圈地幔的部分熔融作用。     

Petrology of Late Jurassic allochthonous ultramafic lamprophyres within the Batain Nappes,Northeastern Oman

ABSTRACT

Late Jurassic ultramafic lamprophyre (UML) sills and dikes occur as 3 km-long intrusions within the allochthonous Whara Formation of the Batain nappes, eastern Oman. The sills and dikes comprise macrocrystic phlogopite and spinel-bearing aillikite and damtjernite. Aillikite is a light grey, massive fine-grained tuffaceous rock with euhedral laths of mica, while damtjernite is a dark grey, medium- to coarse-grained rock with abundant pelletal lapilli and globular segregationary textures. Both lithologies are composed of calcite, phlogopite, apatite, magnetite, spinel, diopside, and richterite. Orthoclase occurs only within damtjernite. The rocks are strongly silica undersaturated (17.6–33.7 wt.% SiO₂), with low MgO (4.7–10.2 wt. %) and high Al₂O₃ (3.5–8.6 wt.%). The aillikites are distinguished from the damtjernites by their lower SiO₂, Al₂O₃, and Na₂O abundances, and their higher MgO, CaO, and P₂O₅ contents. The rare earth element (REE) patterns of both rock types are similar and show strong light REE (LREE) enrichment. Both are enriched in Ba, Th, U, Nb, and Ta, with normalized concentrations of up to 1000 times those of primitive mantle. Relative depletions are apparent for high REE (HREE), K, Rb, Pb, Sr, P, Zr, and Hf. The rocks have initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70435–0.70646, whereas initial ¹⁴³Nd/¹⁴⁴Nd ratios vary between 0 · 512603 and 0 · 512716 (εNd_i 2.6–3.2). Pb isotopic ratios are more varied among the aillikites and damtjernites: ²⁰⁸Pb/²⁰⁴Pb_i = 38.97–39.39 and ²⁰⁷Pb/²⁰⁴Pb_i = 15.35–15.58, ²⁰⁶Pb/²⁰⁴Pb_i = 18.08–18.96. The abundance of phlogopite, apatite, and rutile and enrichment in LREEs, Ba, Th, U, Nb, and Ta in the Sal UMLs suggest metasomatic enrichment of these rocks following a low degree of partial melting of a depleted source region. Ar–Ar age dating of phlogopite macrocrysts from the aillikites and damtjernites (154 and162 Ma, respectively) correlates with large-scale tectonic events recorded in the proto-Indian Ocean at 140–160 Ma.     

Geochronological and geochemical constraints on the petrogenesis of alkaline ultramafic dykes from southwest Guizhou Province,SW China

Geochronological, geochemical and whole-rock Sr–Nd isotopic analyses have been completed on a suite of alkaline ultramafic dykes from southwest (SW) Guizhou Province, China with the aim of characterising their petrogenesis. The Baiceng ultramafic dykes have a LA-ICP-MS zircon ²⁰⁶Pb/²³⁸U age of 88.1 ± 1.1 Ma (n = 8), whereas two phlogopites studied by ⁴⁰Ar/³⁹Ar dating methods give emplacement ages of 85.25 ± 0.57 Ma and 87.51 ± 0.45 Ma for ultramafic dykes from Yinhe and Lurong, respectively. In terms of composition, these Late Mesozoic ultramafic dykes belong to the alkaline magma series due to their high K₂O (3.31–5.04 wt.%) contents. The dykes are characterised by enrichment of light rare earth element (LREE) and large-ion lithosphile elements (LILEs) (Rb and Ba), negative anomalies in high field strength elements (HFSEs), such as, Nb, Ta and Ti relative to primitive mantle, low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7060–0.7063) and positive ε_Nd(t) values (0.3–0.4). Such features suggest derivation from low degree (< 1%) partial melting of depleted asthenospheric mantle (garnet-lherzolite), and contamination to various degrees (～ 10%) by interaction with upper crustal materials.     

Alteration associated with the gold-bearing quartz veins at Middagsberget,northern Sweden

The mesothermal Au deposit at Middagsberget in northern Sweden is associated with irregular stock-works of quartz veins occurring in shear zones across a dioritic intrusion. Alteration in the shear zones is characterized by sericitization, chloritization, the presence of sulphides and several generations of veins and small-scale fractures. Small fractures which are filled with variable amounts of quartz, carbonate, sericite, chlorite, albite and sulphides, are particularly abundant in zones having a high Au content. In general, these fractures are younger than the major quartz veins and were apparently important for strong Au-enrichment. Au is associated with arsenopyrite and it occurs as droplets or interfillings in the arsenopyrite or at the crystal surface; together with pyrrhotite as inclusions in arsenopyrite; as free grains in silicates but close to arsenopyrite; or in very small fractures in carbonates. The Au-mineralization is associated with elevated contents of As, S, Sb and W and depletion of Cu, Zn, Ge and Bi. The entire altered areas are enriched in Au compared with the < 1 to 15 ppb found in the host intrusion. During alteration an initial increase in Na or K was accompanied by a weak enrichment of Au and associated elements together with a depletion of elements such as Ca and Mg. In Au-rich samples this alkali enrichment was overprinted by the carbonate-and sulphide-bearing fractures, which often led to an increased Ca-content. An clear zonation of alteration types has not been found. The host rock has been altered by several generations of fluids: the shear zones were repeatedly ruptured and new small-scale fractures were opened. The different fluids did not, therefore, flow through identical paths although they penetrated the same major zones. This resulted in a complex pattern of variously superimposed alterations.     

Redox cycling of iron and manganese in sediments of the Kalix River estuary, Northern Sweden

Iron and manganese redox cycling in the sediment — water interface region in the Kalix River estuary was investigated by using sediment trap data, pore-water and solid-phase sediment data. Nondetrital phases (presumably reactive Fe and Mn oxides) form substantial fractions of the total settling flux of Fe and Mn (51% of Fe_total and 84% of Mn_total). A steady-state box model reveals that nondetrital Fe and Mn differ considerably in reactivity during post-depositional redox cycling in the sediment. The production rate of dissolved Mn (1.6 mmol m^–2 d^–1) exceeded the depositional flux of nondetrital Mn (0.27 mmol m^–2 d^–1) by a factor of about 6. In contrast, the production rate of upwardly diffusing pore-water Fe (0.77 mmol m^–2 d^–1) amounted to only 22% of the depositional flux of nondetrital Fe (3.5 mmol m^–2 d^–1). Upwardly diffusing pore-water Fe and Mn are effectively oxidized and trapped in the oxic surface layer of the sediment, resulting in negligible benthic effluxes of Fe and Mn. Consequently, the concentrations of nondetrital Fe and Mn in permanently deposited, anoxic sediment are similar to those in the settling material. Reactive Fe oxides appear to form a substantial fraction of this buried, non-detrital Fe. The in-situ oxidation rates of Fe and Mn are tentatively estimated to be 0.51 and 0.16–1.7 mol cm^–3 d^–1, respectively.     

Shoshonitic and ultrapotassic post-collisional dykes from northern Karakorum (Sinkiang, China)

High-K calc-alkaline, shoshonitic and ultrapotassic post-collisional dykes of Neogene age have been found in the remote and little known region of northern Karakorum located around the Shaksgam valley, north of the K2-Gasherbrum range (China). The dykes derive from more or less comparable basic magma(s) and display rather unusual petrographic and geochemical characters with respect to the other K-rich rocks. The geochemical data are consistent with derivation of the basic magma(s) from small degrees of partial melting of garnet-lherzolites previously enriched in incompatible elements of crustal origin possibly during the subduction of the Indian plate beneath Karakorum. The spectrum of compositions reflects fractional crystallization governed by an early removal of clinopyroxene, phlogopite, plagioclase, garnet±amphibole followed by the precipitation of abundant alkali feldspar, amphibole±apatite±quartz. Additionally, assimilation of crustal rocks during magma ascent contributed to the unusual compositional characteristics and is suggested by the abundance of corroded quartz ( ± plagioclase) xenocrysts and by the occasional presence of granitic xenoliths.

An apparent connection exists between magmatism and tectonism in the complex Karakorum Fault Zone (KFZ). It is suggested that, during the Neogene, the strike-slip KFZ and some adjacent post-metamorphic faults transiently behaved as extensional fault systems down to deep levels, triggering ascent and emplacement of the K-rich magma. The subsequent (re)activation of a compressive and transcurrent regime determined the rapid and recent uplift of the more primitive lamprophyres occurring in the plutonites and metamorphites of the upper Baltoro Glacier and K2-Gasherbrum range, relative to the more fractionated and contaminated lamprophyres injecting the shallower rocks of the Shaksgam valley area.     

阿拉善地块北缘雅布赖地区基性岩墙地球化学、年代学和Sr-Nd同位素特征

雅布赖地区位于阿拉善地块北缘,雅布赖-诺日公-红谷尔玉林构造带西端,该地区晚古生代岩浆岩内广泛出露基性岩墙。锆石LA-ICP-MS U-Pb定年结果显示,雅布赖地区基性岩墙成岩年龄为268±1.6 Ma,时代归属为中二叠世早期,岩石地球化学分析表明,该地区基性岩墙∑REE=143.01×10~(-6)～278.8×10~(-6),LREE/HREE=7.43～10.16,(La/Yb)_N=7.27～12.31,轻、重稀土分馏明显,具不明显的负铕异常(δEu=0.84～1.05),在稀土元素球粒陨石标准化配分模式图上,配分曲线右倾,重稀土较平坦;在微量元素原始地幔标准化蛛网图中,表现出相对富集LREE、Rb、Ba、K、Sr等大离子亲石元素,亏损Nb、Ta、P、Ti等高场强元素。基性岩墙初始~(87)Sr/~(86)Sr比值变化于0.707403～0.707986之间,ε_(Nd)(t)值介于-2.9～-4.7之间。分析认为,雅布赖地区基性岩墙岩浆来源为板片俯冲作用改造的富集岩石圈地幔的部分熔融,形成于碰撞后伸展的构造背景。     

Olivine in ultramafic lamprophyres: chemistry,crystallisation, and melt sources of Siberian Pre- and post-trap aillikites

We studied olivines from the Devonian pre-trap (the Ilbokich occurrence) and the Triassic post-trap (the Chadobets occurrence) carbonate-rich ultramafic lamprophyres (UMLs) in the southwestern portion of the Siberian craton. On the basis of detailed investigations of major, minor, and trace-element distributions, we have reconstructed the main processes that control the origins of these olivines. These include fractional crystallisation from melt, assimilation, and fractional crystallisation processes with orthopyroxene assimilation, melt-reaction diffusive re-equilibration, alkali enrichment, and CO₂ degassing of the melt. Furthermore, we inferred the composition of the sources of the primary UML melt and their possible correlations with proto-kimberlitic melts, as well as the influence of the Triassic Siberian plume on the composition of the lithospheric mantle. The main differences between olivines from the Ilbokich and the Chadobets aillikites were that the olivines from the former had more magnesium-rich cores (Mg# = 89.2?±?0.2), had Mg- and Cr-rich transition zones (Mg# = 89.7?±?0.2 and 300–500 ppm Cr), had lower Ni (up to 3100 ppm) and Li (1.4–1.5 ppm), and had higher B (0.8–2.6 ppm) contents, all at higher Fo values (90–86), relative to the olivines from the latter (Mg# = 88–75; 200–300 ppm Cr; up to 3400 ppm Ni; 1.4–2.4 ppm Li; 0.4–2.2 ppm B). The Siberian aillikite sources contained a significant amount of metasomatic material. Phlogopite-rich MARID-type veins provided the likely metasomatic component in the pre-trap Devonian Ilbokich aillikite source, whereas the Triassic Chadobets aillikitic post-trap melts were derived from a source with a significant carbonate component. A comparison of UML olivines with olivines from the pre-trap and post-trap Siberian kimberlites shows a striking similarity. This suggests that the carbonate component in the aillikitic source could have been produced by evolved kimberlite melts. The differences in the lithospheric metasomatic component that contributed to pre-trap and post-trap aillikitic melts can be interpreted as reflections of the thermal impact of the Siberian Traps, which reduced phlogopite-bearing metasomes within the southwestern Siberian sub-continental lithospheric mantle.