Dikes from Ortler,Sarntal Alps and Brixen granite: Mineralogy,chemical composition and petrogenesis

Summary The magmatic dikes of Ortler, Sarntal Alps and in the Brixen granite are of calc-alkaline nature with low values of FeO and TiO₂ (< 1 wt. %), corresponding to typical orogenic volcanic series generated at converging plate margins. The Order dikes display a chemical variation basalt/basaltic andesite (Ortler-Cevedale)-acidic andesites (Töll/Meran)-rhyolite (Hoher Dieb). The presence of magmatic garnets (in the andesites of Töll and in the rhyolites of Holier Dieb), and the calc-alkaline nature of the dikes imply a deep origin involving subduction processes.The⁸⁷Sr/⁸⁶Sr ratio are high ranging from 0.71015 ± 10 (2 ) in the dikes of the Ortler-Campo Crystalline Complex to 0.72120 ± 10 in the dikes of the Sarntal Alps. The high⁸⁷Sr/⁸⁶Sr ratio may imply crustal contamination.The Ortler dikes have strongly partitioned REE patterns Ce_N/Yb_N=21–35 whereas the Sarntal dikes have Ce_N/Yb_N values ranging from 5–10. In comparison, the Oetztal dikes are characterized by high FeO and TiO₂ (> 1 wt.%) values and a less partitioned REE pattern with Ce_N/Yb_N = 2–5 (where N indicates chondrite normalized values).

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Die gänge des ortler-massivs, der sarntaler alpen und des brixner granits: mineralogie, chemismus und petrogenese

Zusammenfassung Die magmatischen Gänge in den Ortler- und Sarntaler Alpen sowie im Brixner Granit, haben kalkalkalischen Chemismus mit niederen Werten von FeO und TiO₂ (< l Gwt.%), sie entsprechen vulkanischen Serien die typisch sind für konvergierende Plattengrenzen.Die Gänge im Ortler-Gebiet zeigen eine chemische Variation von Basalten bis zu basaltischen Andesiten im W (Ortler, Cevedale), über saure Andesite (Töll, Meran) bis zu Rhyoliten (Hoher Dieb) im O. Das Auftreten von magmatischen Granaten an Andesiten bei Töll und in Rhyoliten am Hohen Dieb, sowie der kalkalkalische Chemismus der Gänge, sprechen für ihre Herkunft aus großer Tiefe im Zusammenhang mit einer Subduktion.Die⁸⁷Sr/⁸⁶Sr-Verhältnisse sind hoch und variieren von 0.71015 ± 10 (2 ) im Ortler-Campo-Kristallin bis 0.72120± 10 in den Sarntaler Gängen. Das hohe⁸⁷Sr/⁸⁶Sr-Ver-hältnis spricht für eine Beimengung von Krustenmaterial.Die Seltenen Erden der Ortler-Gänge zeigen eine starke Fraktionierung CeN/YbN = 21-35 im Gegensatz zu den Gängen im Sarntal, die ein CeN/YbN-Verhältnis von 5-10 aufweisen. Im Vergleich dazu weisen die Gänge im Ötztal-Stubaier Altkristallin hohe FeO- und TiO₂(> 1 Gwt.%)-Werte und eine geringere REE-Fraktionierung von CeN/YbN = 2-5 auf.

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With 8 Figures     

Geochemistry of Mesozoic dolerite dikes from eastern North America

Major and trace element analyses of over one hundred Mesozoic dolerite dikes from eastern North America have established three main chemical types: 1) olivine-normative; 2) high-TiO₂ quartz-normative; and 3) low-TiO₂ quartz-normative; and a less common high-Fe₂ O₃ ^* ( Fe as Fe₂O₃) quartz-normative type. Quartz-normative dikes predominate from Nova Scotia to Maryland whereas olivine-normative dikes predominate in North and South Carolina. In Virginia and Georgia these types occur in approximately equal abundance.The high-Fe₂O₃ ^* quartz-normative type may be a result of local differentiation. The other quartz-normative types are chemically distinct from each other and probably evolved from different parental magmas. The olivine-normative type may be representative of these parental magmas, and either the parental magmas overlap in composition or only one magma is represented by analyzed olivine-normative dikes.Simple crystal fractionation models coupled with constraints on liquidus phases imposed by recent experimental studies reveal that 1) all three quartz-normative types can be derived from the olivine-normative type by the removal of slightly different cumulate assemblages, but not by contamination with any common crustal composition, and 2) the two-main quartz-normative types are related to each other by neither crystal fractionation nor contamination processes. According to the models, any of the quartz-normative types can be derived from the olivine-normative type by 60–70% accumulation, with the cumulate consisting primarily of 50% plagioclase, 25–30% olivine, and 15% clinopyroxene.The concept of vertical inhomogeniety with respect to incompatible elements in the upper mantle source areas is invoked as a possible explanation for the chemically distinct parental magmas. The spatial distribution of the chemical types and the gross outcrop pattern of the dike swarm clearly indicate that the tectonic environment of the northern Appalachian region differed from that of the southern part during the early Mesozoic.     

New carbonatite complex in the western Baikal area,southern Siberian craton: Mineralogy,age, geochemistry,and petrogenesis

A dike–vein complex of potassic type of alkalinity recently discovered in the Baikal ledge, western Baikal area, southern Siberian craton, includes calcite and dolomite–ankerite carbonatites, silicate-bearing carbonatite, phlogopite metapicrite, and phoscorite. The most reliable ⁴⁰Ar–³⁹Ar dating of the rocks on magnesioriebeckite from alkaline metasomatite at contact with carbonatite yields a statistically significant plateau age of 1017.4 ± 3.2 Ma. The carbonatite is characterized by elevated SiO₂ concentrations and is rich in K₂O (K₂O/Na₂O ratio is 21 on average for the calcite carbonatite and 2.5 for the dolomite–ankerite carbonatite), TiO₂, P₂O₅ (up to 9 wt %), REE (up to 3300 ppm), Nb (up to 400 ppm), Zr (up to 800 ppm), Fe, Cr, V, Ni, and Co at relatively low Sr concentrations. Both the metapicrite and the carbonatite are hundreds of times or even more enriched in Ta, Nb, K, and LREE relative to the mantle and are tens of times richer in Rb, Ba, Zr, Hf, and Ti. The high (Gd/Yb)_CN ratios of the metapicrite (4.5–11) and carbonatite (4.5–17) testify that their source contained residual garnet, and the high K₂O/Na₂O ratios of the metapicrite (9–15) and carbonatite suggest that the source also contained phlogopite. The Nd isotopic ratios of the carbonatite suggest that the mantle source of the carbonatite was mildly depleted and similar to an average OIB source. The carbonatites of various mineral composition are believed to be formed via the crystallization differentiation of ferrocarbonatite melt, which segregated from ultramafic alkaline melt.     

Rb-Sr ages of Precambrian dolerite and alkaline dikes,southeast Mysore state,India

Rb-Sr isochron ages have been determined for two suites of Precambrian dikes in the Bidadi-Harohalli area of southeast Mysore State. Whole-rock samples of unmetamorphosed dolerites yield an age of 2420±246 (2σ) m.y., which is a minimum value for the intruded Peninsular Gneiss and Closepet Granite. The dolerite magma originated in the mantle, as indicated by the initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7012±0.0010 (2σ). A suite of alkaline dikes, also referred to as felsite and feldspar porphyry dikes, has an age of 832±40 (2σ) m.y., which correlates with the intrusion of the Chamundi Hill Granite and the feldspar porphyry dikes near Srirangapatnam. One of the alkaline dikes has a K-Ar age of 810±25 m.y., indicating an absence of subsequent thermal events in the area.     

山东中生代基性脉岩的元素地球化学及其成因

研究区脉岩 SiO2含量变化范围为 48.63%～ 56.02% ,岩性上以煌斑岩、辉绿岩和辉长岩为主 ,以富集轻稀土元素 (LREE)和大离子亲石元素 (LILE)、亏损重稀土元素 (HREE)和高场强元素 (HFSE)为特征.主元素和微量元素研究表明 ,基性脉岩源区存在明显的地壳混染作用和少量富碳酸岩交代作用,该交代作用可能与扬子下地壳物质的参与有关.     

Late Paleozoic gabbroids of western Transbaikalia: U-Pb and Ar-Ar isotopic ages,composition, and petrogenesis

We provide new isotope-geochronological evidence for the synchronous occurrence of Late Paleozoic basic and granitoid magmatism in western Transbaikalia; this is a strong argument for the contribution of mantle magmas to granitoid petrogenesis. The Late Paleozoic basic rocks originated from the phlogopite-garnet-bearing lherzolitic mantle, which melted under “hydration conditions.” The specific features of Late Paleozoic magmatism in western Transbaikalia were determined by the combination of the activity of a low-energy mantle plume with the final stage of the Hercynian orogeny in space and time. At the early stage of magmatism, during the formation of the Barguzin granites,the plume had only a thermal influence on the crustal rocks heated as a result of Hercynian fold-thrust deformations. The mixing of mantle basic and crustal salic magmas at different levels marked the transition from crustal to mixed (mantle-crustal) granites, which include all post-Barguzin complexes (probably, except for alkali granites). In the geologic evolution of Transbaikalia, the Late Paleozoic magmatism was postorogenic, but it was initiated and influenced by the mantle plume.     

Mineralogy,geochemistry and petrogenesis of Kurile island-arc basalts

Whole-rock (major- and trace-element) and mineral chemical data are presented for basaltic rocks from the main evolutionary stages of the Kurile island arc, NW Pacific. An outer, inactive arc contains a Cretaceous-Lower Tertiary sequence of tholeiitic, calcalkaline and shoshonitic basalts. The main arc (Miocene-Quaternary) is dominated by weakly tholeiitic, with lesser, alkalic basalts. The mineralogy of Kuriles basalts is characterised by An-rich plagioclases, a continuous transition from chromites to titanomagnetites, pyroxenes with low Fe³⁺ contents and without strong Fe-enrichment, abundance of groundmass pigeonites and the absence of amphiboles. There is an increase in K₂O contents both along-arc (northwards) and towards the reararc side. The basalts show an exceptionally wide but continuous range of K₂O contents (0.1–4.7%) which correlate with other LIL element contents. Tholeiitic basalts with low LIL element contents, La/Yb and Th/U, but high K/ Rb, P₂O₅/La and Zr/Nb were derived from depleted, lherzolitic mantle which had suffered fluid metasomatism by K, Rb, Cs, Sr, Ba, Pb and H₂O only. Alkali basalts are also thought to be derived from depleted mantle but melt metasomatism involved addition of all LIL elements to a garnet lherzolite mantle. The Kuriles basalts and their mantle sources range continuously between these two end-member compositions. The metasomatic fluids/melts were probably released by early dehydration and later melting within subducted oceanic lithosphere though the process is not adequately constrained.     

Origin and differentiation of Triassic dolerite magmas,North Carolina,USA

Ten dolerite dikes intruded into Triassic fault troughs in the Piedmont area of North Carolina have been analyzed for the contents of major elements plus selected trace elements. The average composition of the initial magma, as indicated by four chill margins for major elements and three for trace elements, is: SiO₂, 48.6%; Al₂O₃, 16.9%; TiO₂, 0.57%; Fe₂O₃, 3.30%; FeO, 6.72%; MgO, 10.59; CaO, 10.42%; Na₂O, 2.03%; K₂O, 0.20%; MnO, 0.20%; Rb, 2.6 ppm; Sr, 133 ppm; Zr, 46 ppm; Th, 0.4 ppm; and U, below detection limit of approximately 0.1 ppm. One large dike (BP) exhibits a Palisades-type of differentiation by crystal settling of olivine, and the comparatively thick JY dike shows development of micropegmatite toward the center; the smaller dikes, however, are relatively homogeneous across their width. Study of the relationship between SiO₂ content and the ratio FeO+Fe₂O₃/MgO+ FeO+Fe₂O₃ indicates that most dikes crystallize under conditions of decreasing oxygen pressure, but the differentiation trend of the JY, RD, and RS dikes indicates either constant or increasing oxygen pressure during their evolution.Statistical comparison of the composition of the initial dolerite magmas with a variety of basalt types around the world suggests that the North Carolina dolerites are far more similar to oceanic or oceanic margin tholeiites than to continental tholeiites. The North Carolina rocks are distinctly different from plateau basalts but are similar to the chill zones of the Precambrian Bushveld and Stillwater lopoliths. The comparatively low contents of Th, U, and Sr, plus the relatively high K/Rb ratio all support the possibility that the magmas for the North Carolina dolerites evolved in a dominantly oceanic environment. It seems distinctly possible that continental-type crust and mantle did not exist in the Appalachian Piedmont area in Triassic time, even after major orogeny and the concurrent formation of granitic intrusions.     

湖北铜绿山铜铁矿床中煌斑岩的矿物学特征及岩石成因初探

湖北铜绿山矽卡岩型铜铁矿床处于长江中下游铁铜成矿带西部的鄂东南矿集区。最近我们在该矿区工作中采集到两条侵入于含矿石英二长闪长玢岩中的煌斑岩脉。本文对这些煌斑岩脉进行了详细的镜下岩相学观察,并对其中造岩矿物黑云母、辉石和长石进行了电子探针成分分析。根据这些数据本文论述了煌斑岩的物源、形成环境以及岩浆体系的物理化学条件。研究表明,煌斑岩中云母为富镁黑云母,其形成的岩浆体系处于中低氧逸度(近Ni-Ni O);黑云母与全岩都具有高钛特征,表明煌斑岩的物源深度可能来自较深部地幔(如软流圈)。煌斑岩中辉石为单斜辉石,环带较为发育,有韵律环带和正环带,其韵律环带的形成可能是由于岩浆温压环境存在振荡性变化而形成的。铜绿山矿床中煌斑岩属于云煌岩;其岩浆来源具幔源性,受到地壳的混染作用不强。     

Microbial activity and chemical weathering in the Middendorf aquifer,South Carolina

We use reactive transport modeling to better understand the kinetics of chemical weathering in the Cretaceous Middendorf aquifer of South Carolina, USA, and the relationship of this process to subsurface microbial activity. We constructed a model accounting for the kinetics of mineral dissolution and precipitation, ion exchange, and the CO₂ and bicarbonate produced by iron reducing and sulfate reducing bacteria in the aquifer. We then fit the model to observed trends in the chemical composition of groundwater along the aquifer by adjusting the rate constants for the kinetic reactions considered. The modeling portrays weathering in the Middendorf as a slow process by which groundwater gradually reacts toward equilibrium with minerals in the aquifer. The rate constants predicted are 6 to 7 orders of magnitude smaller than measured in laboratory experiments and 3 to 4 orders of magnitude less than those inferred from weathering rates in soils. The rate constants are smaller even than expected by projecting observed trends with the duration of weathering to the geologic age of the Middendorf. Weathering is driven largely by biological activity: about half the acid consumed is CO₂ derived from the recharge area, and about half is supplied by iron reducing bacteria in the aquifer; only about 1% of the acid is of atmospheric origin, from CO₂ dissolved in rainwater.     

RbSr study of dolerite dikes and psammite from the Western Gneiss region of Norway

The Saetra Nappe, consisting of feldspathic psammite cut by pre-tectonic dolerite dikes, is a distinctive tectonostratigraphic unit of the Western Gneiss Region of Norway. Although the rocks were metamosphosed at amphibolite facies, RbSr whole-rock sf three well preserved dikes a date of $\text{745 ± 37}\text{Ma}$ with initial ${}^{87}\text{Sr}\text{/}{}^{86}\text{Sr}\text{= .7046}$, interpreted to record the dike intrusion. Analysis of small psammite samples collected in rows parallel to the foliation and in columns perpendicular to it, indicate that the RbSr isotopic systems were not equilibrated on the whole-rock scale. The Strata Nappe provides evidence of a strong Caledonian deformation/metamorphic event in the Western Gneiss Region of Norway.     

磷灰石化学组成研究进展: 成岩成矿过程示踪及对矿产勘查的指示

磷灰石广泛分布于各类岩石和矿床中, 根据其F、OH以及Cl阴离子种类可分为氟磷灰石、氯磷灰石和羟基磷灰石。磷灰石对其自身结构的变形以及化学取代有良好的耐受性, 一系列元素可以替代磷灰石中的Ca、P、F、Cl和OH, 因此其化学组成复杂。磷灰石中Ca位存在两种不同的阳离子位置, 影响不同元素在这两种位置取代的因素包括离子半径、价态以及磷灰石的种类等。岩浆过程中, 稀土(REE)、Y、Sr是磷灰石中的相容元素, 而F、Cl、OH的相容性随物理化学条件不同而变化。相较于元素在磷灰石与熔体相间的分配研究, 目前对于元素在磷灰石与流体相间的分配研究较少。磷灰石化学组成可以判别母岩类型: 如F、Cl、As、V含量可以区分Ⅰ型与S型花岗岩; REE、Sr、Y、Mn、As、Th的含量、轻稀土(LREE)的富集程度和球粒陨石标准化稀土元素配分模式可以用于判别碳酸岩、二辉橄榄岩、镁铁质岩石等岩石类型。利用磷灰石的F、Cl和CO₃等挥发分的含量, 并结合Sr-Nd同位素组成, 可以识别地幔源区及其流体交代作用。磷灰石Sr/Th与La/Sm的变化可用于指示岩浆来自脱水板块或熔融沉积物。磷灰石的主微量元素(Mn、Eu)含量或元素比值(F/Cl)能够反映岩浆的分异程度, 而且Ce/Pb和Th/U比值可以反映岩浆形成过程中流体参与程度。磷灰石中的变价元素如Eu、Ce、Ga和S含量及δEu和δCe可用于推断岩浆的氧化还原状态。由于磷灰石对热液交代作用非常敏感, 所以磷灰石的结构和化学组成已广泛用于成矿过程研究。在与花岗岩有关的关键金属矿床中, 磷灰石的主、微量元素含量和Sr-Nd同位素比值常常用于区分不同花岗质岩浆的源区特征, δCe、δEu也可以用于指示花岗质侵入体的氧化还原状态; 斑岩有关的多金属矿床F、Cl、S、Mn含量以及F/Cl比值能够指示成矿岩浆-热液的来源及演化, δCe、δEu可以用于指示岩浆热液的氧逸度; 铁氧化物-铜-金和铁氧化物-磷灰石矿床中磷灰石结构、共生稀土矿物结合Na、F、Cl以及REE含量能够反映与成矿有关的岩浆-流体作用以及流体性质。磷灰石中Mn、Fe、REE、Y、F、Cl、SO₃的含量或比值的差异可用于判别矿化与未矿化的岩石或者蚀变类型, 并用于区分不同矿床类型等, 对矿产勘查具有一定指示意义。磷灰石化学组成结合机器学习方法, 可以更好地识别磷灰石的来源。综上所述, 磷灰石作为一种新兴的指示矿物, 除了在成岩和成矿过程示踪方面体现出独特优势, 在矿产勘查应用方面也具有重要的应用前景。

     

Origin,age and petrogenesis of Neoproterozoic composite dikes from the Arabian‐Nubian Shield,SW Jordan

The evolution of a Pan‐African (c. 900–550 Ma) suite of composite dikes, with latite margins and rhyolite interiors, from southwest Jordan is discussed. The dikes cut the Neoproterozoic calc‐alkaline granitoids and high‐grade metamorphic rocks (c. 800–600 Ma) of the northern Arabian‐Nubian Shield in Jordan and have been dated by the Rb‐Sr isochron method at 566±7 Ma. The symmetrically distributed latite margins constitute less than one‐quarter of the whole dike thickness. The rhyolite intruded a median fracture within the latite, while the latter was still hot but completely solidified. The dikes are alkaline and bimodal in composition with a gap in SiO₂ between 61 and 74 wt%. Both end members display similar chondrite‐normalized rare earth element patterns. The rhyolites display the compositional signature of A‐type granites. The (La/Lu)_N values are 6.02 and 4.91 for latites and rhyolites, respectively, and the rhyolites show a pronounced negative Eu anomaly, in contrast to the slight negative Eu anomaly of the latites. The chemical variability (e.g. Zr/Y, Zr/Nb, K/Rb) within and between latites and rhyolites does not support a fractional crystallization relationship between the felsic and mafic members of the dikes. We interpret the magma genesis of the composite dikes as the result of intrusion of mantle‐derived mafic magma into the lower crust in an extensional tectonic regime. The mafic magma underwent extensive fractional crystallization, which supplied the necessary heat for melting of the lower crust. The products of the initial stages of partial melting (5–10%) mixed with the fractionating mafic magma and gave rise to the latite melts. Further partial melting of the lower crust (up to 30%) produced a felsic melt, which upon 50% fractional crystallization (hornblende 15%, biotite 5%, feldspars 60%, and quartz 20%) gave rise to the rhyolitic magma. Copyright © 2004 John Wiley & Sons, Ltd.     

Boninitic and tholeiitic basaltic lavas and dikes from dispersed Jurassic East Othris ophiolitic units,Greece: petrogenesis and geodynamic implications

ABSTRACT

Pillow lavas, massive lava flows, and sub-volcanic dikes of tholeiitic basaltic composition are found to be members of the Vrinena, Aerino, Eretria, and Velestino dispersed Middle–Upper Jurassic ophiolitic units in East Othris. The Vrinena and Eretria ophiolitic units appear to have been emplaced onto the Pelagonian continental margin during the Upper Jurassic–Lower Cretaceous, whereas the Aerino and Velestino units seem to have been finally emplaced during post-Palaeocene times. Geochemically these are divided into two groups: Group I includes subduction-related boninites and low-Ti basalts from the Vrinena and Aerino units, and Group II high-Ti basalts show spreading-type characteristics occurring in the Eretria and Velestino units. Primary magma of the Group I volcanics appears to have been formed after high partial melting degrees (~18%) of a highly depleted harzburgitic mantle source, under relatively high temperatures (mantle potential temperature ~1372°C). Petrogenetic modelling also suggests that the primary magma of the Group II volcanics were formed after lower partial melting degrees (~7%) of a moderately depleted mantle source. The petrological and geochemical data from the East Othris dispersed and diversely emplaced ophiolitic units provide evidence of a common intra-oceanic supra-subduction zone (SSZ) origin within the Pindos oceanic strand of the Western Tethys. Specifically, Group I lavas and dikes from Vrinena seem to represent the extrusive part of an almost complete fore- to island-arc ophiolitic sequence. Dikes of Aerino most likely correspond to fore-arc magmatic material that intruded within exhumed serpentinized ultramafic rocks through a subduction channel that developed close to the slab and towards the fore-arc and the accretionary prism. The Group II volcanics either corresponded to a fore-arc magmatic expression, which extruded earlier than Group I volcanics and prior to the establishment of a mature subduction zone, or represent back-arc to island-arc magmatism that was contemporaneous to the fore-arc magmatic activity during rollback subduction.     

Mineralogy,geochemistry and petrogenesis of the Khopoli mafic intrusion,Deccan Traps,India

The Khopoli intrusion, exposed at the base of the Thakurvadi Formation of the Deccan Traps in the Western Ghats, India, is composed of olivine gabbro with 50–55 % modal olivine, 20–25 % plagioclase, 10–15 % clinopyroxene, 5–10 % low-Ca pyroxene, and <5 % Fe-Ti oxides. It represents a cumulate rock from which trapped interstitial liquid was almost completely expelled. The Khopoli olivine gabbros have high MgO (23.5–26.9 wt.%), Ni (733–883 ppm) and Cr (1,432–1,048 ppm), and low concentrations of incompatible elements including the rare earth elements (REE). The compositions of the most primitive cumulus olivine and clinopyroxene indicate that the parental magma of the Khopoli intrusion was an evolved basaltic melt (Mg# 49–58). Calculated parental melt compositions in equilibrium with clinopyroxene are moderately enriched in the light REE and show many similarities with Deccan tholeiitic basalts of the Bushe, Khandala and Thakurvadi Formations. Nd-Sr isotopic compositions of Khopoli olivine gabbros (εNd_t?=??9.0 to ?12.7; ⁸⁷Sr/⁸⁶Sr?=?0.7088–0.7285) indicate crustal contamination. AFC modelling suggests that the Khopoli olivine gabbros were derived from a Thakurvadi or Khandala-like basaltic melt with variable degrees of crustal contamination. Unlike the commonly alkalic, pre- and post-volcanic intrusions known in the Deccan Traps, the Khopoli intrusion provides a window to the shallow subvolcanic architecture and magmatic processes associated with the main tholeiitic flood basalt sequence. Measured true density values of the Khopoli olivine gabbros are as high as 3.06 g/cm³, and such high-level olivine-rich intrusions in flood basalt provinces can also explain geophysical observations such as high gravity anomalies and high seismic velocity crustal horizons.     

Neogene basanites in western Kamchatka: Mineralogy,geochemistry, and geodynamic setting

Neogene (N ₁ ² -N ₂ ¹ ?) K-Na alkaline rocks were found in western Kamchatka as a subvolcanic basanite body at Mount Khukhch. The basanites have a microphyric texture with olivine phenocrysts in a fine-grained doleritic groundmass. The olivine contains inclusions of Al-Cr spinel. The microlites consist of clinopyroxene, plagioclase, magnetite, and apatite, and the interstitial phases are leucite, nepheline, and analcime. The Mount Khukhch basanites are characterized by elevated concentrations of MgO, TiO₂, Na₂O, and K₂O, high concentrations of Co, Ni, Cr, Nb, Ta, Th, U, LREE (La_N/Yb_N = 10.8?12.6, Dy_N/Yb_N = 1.4?1.6) at moderate concentrations of Zr, Hf, Rb, Ba, Sr, Pb, and Cu. The values of indicator trace-element ratios suggest that basanites in western Kamchatka affiliate with the group of basaltoids of the within-plate geochemical type: Ba/Nb = 10?12, Sr/Nb = 17?18, Ta/Yb = 1.3?1.6. The basanites of western Kamchatka show many compositional similarities with the Miocene basanites of eastern Kamchatka, basanites of some continental rifts, and basalts of oceanic islands (OIB). The geochemistry of these rocks suggests that the basanite magma was derived via the ～6% partial melting of garnet-bearing peridotite source material. The crystallization temperatures of the first liquidus phases (olivine and spinel) in the parental basanite melt (1372–1369°C) and pressures determined for the conditions of the “mantle” equilibrium of the melt (25–26 kbar) are consistent with the model for the derivation of basanite magma at the garnet depth facies in the mantle. The geodynamic environment in which Neogene alkaline basaltic magmas occur in western Kamchatka was controlled by the termination of the Oligocene—Early Miocene subduction of the Kula oceanic plate beneath the continental margin of Kamchatka and the development of rifting processes in its rear zone. The deep faulting of the lithosphere and decompression-induced magma generation simultaneous with mantle heating at that time could be favorable for the derivation of mantle basite magmas.     

辽南斑状金伯利岩岩相学特征及其成因研究

辽南金伯利岩岩区是我国最大的原生金刚石矿产区,该区金刚石主要寄主岩石类型为斑状金伯利岩。橄榄石是金伯利岩中最重要的造岩矿物,根据其结构特征可以分为橄榄石粗晶、橄榄石斑晶以及基质中微细粒三个世代。本文将岩相学特征和前人研究成果相结合,构建辽南斑状金伯利岩岩浆起源、上升、喷发和成岩模型,探讨各世代矿物的形成过程。具体包括:深部交代地幔部分熔融,形成初始碳酸盐岩浆;初始岩浆上升过程中捕获的岩石圈地幔橄榄岩不断溶解(形成橄榄石粗晶),岩浆成分发生改变,成为金伯利岩岩浆;金伯利岩岩浆迅速上升侵位,至地表处爆破喷发,最后冷却固结形成包含粗晶及其他两个世代橄榄石的斑状金伯利岩。     

赣杭构造带金衢盆地燕山期基性脉岩地球化学特征及成因探讨

赣杭构造带金衢盆地内发育有燕山期基性脉岩,岩石类型主要为辉绿(玢)岩.K-Ar年代学研究显示基性脉岩产出时代为69.5～131.7Ma,为白垩纪岩浆活动的产物.SiO2含量范围为46.70%～50.23%,K2O+Na2O为4.01%～7.82%,可分为碱性和亚碱性岩两类.微量元素相对富集大离子亲石元素(LILE)(K...     

Models of basalt petrogenesis: Lower Keweenawan diabase dikes and middle Keweenawan Portage Lake Lavas,upper Michigan

Lower Keweenawan diabase dikes from Marquette-Baraga Counties, and middle Keweenawan Portage Lake Lavas from upper Michigan, can each be subdivided into two chemically distinct groups: a low TiO₂-P₂O₅ group characterized by higher A1₂O₃ content, higher Mg ratio, and lower total Fe and REE abundances than a high TiO₂-P₂O₅ group. Both groups, which are indistinguishable in the field, are enriched in the LREE relative to the HREE, and have similar normalized REE abundance patterns.The systematic variation of the REE's within the subgroups is consistent with a model of fractional crystallization of the observed phenocryst phases, olivine and plagioclase. The unique TiO₂/P₂O₅ content of the subgroups cannot be explained by magma mixing or fractionation from a single primary liquid. Thus, two mantle sources of differing depth and REE abundance are required to produce the TiO₂-P₂O₅ subgroups which are intercalated in the lava pile. Similar chemical trends within the dikes have been interpreted to mean that the same magmatic processes must have been operative during their emplacement in the early opening stages of the Keweenawan rift in Michigan.     

Carbonatite-hosted niobium deposit at Aley,northern British Columbia (Canada): Mineralogy,geochemistry and petrogenesis

The Aley Nb deposit in northern British Columbia, Canada, is hosted by metamorphosed calcite and dolomite carbonatites of anorogenic affinity emplaced in Lower Paleozoic sedimentary carbonate rocks in the Devonian. Primary Nb mineralization consists of pyrochlore (commonly comprising a U–Ta-rich and F-poor core) and ferrocolumbite developed as discrete crystals and replacement products after the pyrochlore. These phases and associated heavy minerals (apatite ± magnetite ± zircon ± baddeleyite) precipitated early in the magmatic history and probably formed laterally extensive cumulate layers up to at least 1.5 m in thickness. Fractionation of copious amounts of pyrochlore is reflected in the chemical composition of the carbonatites and their constituent minerals, which show large variations in Nb/Ta value, but a near-chondritic Zr/Hf ratio. Alkali-rich metasomatic rocks (in particular, fenites and glimmerites) associated with the carbonatites are barren; the bulk of Nb in these rocks is contained in rutile, phlogopite and, to a much lesser extent, amphibole. When the passive margin of North America became the zone of plate convergence in the Cretaceous, the host carbonatites were strongly deformed, which is manifested in structures and textures indicative of grain comminution, ductile flow, folding and, locally, brecciation. The structure and continuity of the cumulate units enriched in Nb minerals were profoundly affected by these processes. Interaction of the carbonatites with crustal fluids of complex chemistry resulted in extensive dolomitization, replacement of the pyrochlore and ferrocolumbite by fersmite, and development of hydrothermal parageneses consistent with the lower greenschist-facies conditions. At these late evolutionary stages, Nb was mobilized only to a very limited extent and sequestered in a variety of minerals (fersmite, euxenite, Mg-rich ferrocolumbite and Nb-bearing rutile) typically occurring as scarce minute crystals associated with hydrothermal dolomite, quartz and chlorite. Progressive enrichment of the deformed dolomite carbonatites in heavy C and O isotopes relative to primary calcite, coupled with changes in the trace-element composition of Nb phases, indicate that the fluids were equilibrated with the wall-rock sedimentary rocks hosting the Aley deposit and were capable of transporting F⁻, (PO₄)^3 −, U, Th and rare-earth elements, but not Nb.