塔里木盆地夏河南层状玄武岩的岩石地球化学特征及地质意义

塔里木盆地夏河南玄武岩露头以水平层状分布为最大特征,其玄武岩层和沉积夹层保持了较原始的产状,是研究塔里木盆地二叠纪多期次玄武质岩浆活动的理想场所,是对比盆地内其他玄武质岩浆活动的最佳剖面。本文通过详细的野外和遥感解译工作,报道了夏河南玄武岩的野外产状、岩相学特征和各层玄武岩的地球化学特征,探讨了夏河南玄武岩的岩浆源区和岩浆演化过程。研究认为夏河南玄武岩质岩浆来源于富集的岩石圈地幔,为尖晶石-石榴石二辉橄榄岩经历低程度部分熔融的产物,各层玄武岩的成分差异与部分熔融程度有关,单一层内玄武岩的岩浆演化以分离结晶为主,地壳混染程度较小。夏河南各层玄武岩来源于同一岩浆源区,其特征与柯坪玄武岩相似,可对应于柯坪地区开派兹雷克组的6层玄武岩。研究进一步明确了塔里木盆地早二叠世玄武岩的空间分布特征,证实塔里木早二叠世大火成岩省的玄武质岩浆作用可从柯坪地区延伸到夏河南、塔中、塔西南等地。     

Elemental composition and mineralogical characteristics of coastal marine sediments of Tutuila, American Samoa

Surface sediment samples were collected from 5 pristine coastal areas and 1 potentially contaminated coastal site on Tutuila, the main island of American Samoa, an isolated island group in the South Pacific Ocean. Samples were analysed for total element analysis (15 elements) and mineralogy. The results indicated no evidence of trace element contamination at any site, including Pago Pago Harbour. Inter-site variations could be explained assuming the sediments consisted predominantly of coralline sand and rubble with varying quantities of basaltic materials derived from local catchments.     

昆明西山峨眉山玄武岩风化壳磁化率特征

开展广泛的磁化率特征研究是目前中国南方红色风化壳环境磁学研究的一个重要内容。通过对昆明西山4个峨眉山玄武岩风化壳剖面进行系统的磁化率测定发现,所有剖面磁化率值都呈现自剖面底部向顶部增高的趋势,这一现象与以往流行的玄武岩风化壳磁化率值随风化成土过程的加深而减小的模式有较大出入。磁化率与频率磁化率的对比分析结果进一步表明风化过程中形成的超顺磁颗粒可能是本区玄武岩风化壳(至少顶部60 cm区域)磁化率增强的主要贡献者。     

Multi-stage zeolite facies mineralization in the Hvalfjördur area,Iceland

The Hvalfjördur area, 30 km north of Iceland’s capital Reykjavik, belongs to the sequence of Late Tertiary to early Quaternary flood basalts with minor intercalations of hyaloclastites and rhyolites. The basalts are affected by progressive low-temperature metamorphism, caused by the burial of the lava succession and higher heat flow from nearby central volcanoes. Low-grade zeolite facies metamorphism of basaltic lavas in the Hvalfjördur area results in two distinct mineral parageneses that can be correlated to events in the burial and hydrothermal history of the lava pile. Stage Ia represents syn-eruptive near-surface alteration in which celadonite and silica were precipitated along primary pores. During regional burial metamorphism (stage Ib), hydrolysis of olivine and glass led to the formation of mixed-layer chlorite/smectite clays. The chlorite content of stage Ib phyllosilicate vesicle rims increases with increasing burial depth and temperature. Stage II occurred after the burial and is marked by zeolite mineralization caused by higher heat flow from the Laxárvogur and Hvalfjördur central volcanoes. Altogether 11 different zeolites were found in the Hvalfjördur area: analcime, chabazite, epistilbite, heulandite, laumontite, levyne, mesolite, stilbite, stellerite, thomsonite and yugawaralite. In total, three separate depth and temperature-controlled “zeolite zones” occur in the Hvalfjördur area.     

云南永善茂林玄武岩型铜矿成因

矿床分布于北西向地球化学急变带古火山口。受峨眉山玄武岩控制,赋存于硅质、沥青质、气孔熔岩中。矿化与沥青化、硅化、方解石化密切相关。应为峨眉山玄武岩型同生热液矿床。     

Shallow plumbing systems for small-volume basaltic volcanoes

Eruptive dynamics in basaltic volcanoes are controlled, in part, by the conduit geometry. However, uncertainties in conduit shape and dike-to-conduit transition geometry have limited our predictive capability for hazards assessments. We characterize the subvolcanic geometry of small-volume basaltic volcanoes (magmatic volatile-driven eruptions, 0.1 to 0.5 km³) based on a synthesis of field studies of five basaltic volcanoes exposed to varying degrees by erosion and exhibiting feeder dikes, conduits, and vent areas ≤250 m depth. Study areas include East Grants Ridge (New Mexico, USA), Basalt Ridge, East Basalt Ridge, Paiute Ridge, and Southeast Crater Flat (Nevada, USA). Basaltic feeder dikes 250 to 100 m deep have typical widths of 4–12 m, with smooth host-rock contacts (rhyolite tuff). At depths less than 100 m, heterogeneities in the host rock form preferential pathways for small dike splays and sills, resulting in a 30-m effective width at 50 m depth. The development of a complex conduit at depths less than 70 m is reflected in bifurcating dikes and brecciation and incorporation of the country rock. The overall zone of effect at depths less than 50 m is ≤110 m wide (220 m elongated along the feeder dike). Based on comparisons with theoretical conduit flow models, the width of the feeder dike at depths from 250 to 500 m is expected to range from 1 to 10 m and is expected to decrease to about 1–2 m at depths greater than 500 m. The flaring shape of the observed feeder systems is similar to results of theoretical modeling using lithostatic pressure-balanced flow conditions. Sizes of observed conduits differ from modeled dimensions by up to a factor of 10 in the shallow subsurface (<50 m depth), but at depths greater than 100 m the difference is a factor of 2 to 4. This difference is primarily due to the fact that observed eroded conduits record the superimposed effects of multiple eruptive events, while theoretical model results define dimensions necessary for a single, steady eruption phase. The complex details of magma-host rock interactions observed at the study areas (contact welding, brecciation, bifurcating dikes and sills, and stoping) represent the mechanisms by which the lithostatic pressure-balanced geometry is attained. The similarity in the normalized shapes of theoretical and observed conduits demonstrates the appropriateness of the pressure-balanced modeling approach, consistent with the conclusions of Wilson and Head (J Geophys Res 86:2971–3001, 1981) for this type of volcano.     

Can crystallization of olivine tholeiite give rise to potassic rhyolites?—an experimental investigation

Experiments were conducted to determine whether the rhyolites and basalts of the intraplate silica-saturated potassic suites could be genetically related through crystallization. Extreme crystallization (96–97%) of a high-MgO (10.62 wt%) olivine tholeiite from the Snake River Plain with an initial bulk water content of 0.4 wt% at a mid-crustal pressure of 4.3 kbar generated potassic rhyolitic liquids similar in major element chemistry to those found in the Quaternary rhyolite domes of the Snake River Plain and their plutonic equivalents in the Proterozoic Laramie Anorthosite Complex. Residual liquids comparable in composition to the bulk rock compositions of intermediate rocks found at the Craters of the Moon and Cedar Butte eruptive centers in the Snake River Plain are also generated along this crystallization path. This paper constitutes part of a special issue dedicated to Bill Bonnichsen on the petrogenesis and volcanology of anorogenic rhyolites.     

Slab dehydration and basalt petrogenesis in subduction systems involving very young oceanic lithosphere

Compositions of post-Miocene basalts erupted in the Garibaldi and Central America volcanic arcs exhibit significant correlations with the age of the subducted plate. In general, SiO₂, Al₂O₃, CaO, V, and (Sr/P)_N decrease and FeO, MgO, TiO₂ and Na₂O increase as the age of the subducted plate decreases. Variations in CaO/Al₂O₃, SiO₂, (Sr/P)_N, and Ba are compatible with lesser slab input, and hence less hydrous melting conditions in the mantle wedge in segments of the arcs overlying the youngest oceanic lithosphere. This interpretation is supported by comparison with peridotite melting experiments, which suggest higher melt pressures and temperatures in the mantle wedge above very young oceanic lithosphere. These observations point to a model in which dehydration of the downgoing slab occurs at shallow depths in subduction systems involving oceanic lithosphere younger than about 20 Ma. Because young oceanic lithosphere is relatively warm, little post-subduction heating is required to produce metamorphic reactions that release slab volatiles. Geodynamic models indicate most volatile-liberating reactions will occur within the seismogenic zone in oceanic lithosphere younger than 20 Ma, thus limiting the volatile flux beneath the arc and encouraging drier, higher temperature and higher pressure melting conditions in the mantle wedge in comparison to typical arc systems. Liberation of volatiles in the downgoing plate is strongly dependant on the shear stress on the fault, but is predicted to occur within the seismogenic zone for shear stresses greater than 33 MPa. Similarly, early loss of volatiles is predicted over a wide range of convergence rates, plate dips, and convergence angles. These results are shown to be robust for realistic ranges of slab dip, convergence angle, and shear stress, suggesting that volatile-poor melt generation is a characteristic of modern and ancient arc systems that involve subduction of young oceanic lithosphere.     

Erosion of lithospheric mantle beneath the East African Rift system: geochemical evidence from the Kivu volcanic province

This study presents new major and trace element and Sr–Nd isotopic results for a suite of Miocene–Recent mafic lavas from the Kivu volcanic province in the western branch of the East African Rift. These lavas exhibit a very wide range in chemical and isotopic characteristics, due to a lithospheric mantle source region that is heterogeneous on a small scale, probably <1 km. The chemical and isotopic variations are mostly geographically controlled: lavas from Tshibinda volcano, which lies on a rift border fault on the northwestern margin of the province, have higher values of ⁸⁷Sr/⁸⁶Sr, (La/Sm)_n, Ba/Nb, and Zr/Hf than the majority of Kivu (Bukavu) samples. The range of ⁸⁷Sr/⁸⁶Sr at Tshibinda (0.70511–0.70514) overlaps some compositions found in the neighboring Virunga province, while Bukavu group lavas include the lowest ⁸⁷Sr/⁸⁶Sr (0.70314) and highest _Nd (+7.6) yet measured in western rift lavas. The Tshibinda compositions trend towards a convergence for Sr–Nd–Pb isotopic values among western rift lavas. Among Kivu lavas, variations in ¹⁴³Nd/¹⁴⁴Nd correlate with those for certain incompatible trace element ratios (e.g., Th/Nb, Zr/Hf, La/Nb, Ba/Rb), with Tshibinda samples defining one compositional extreme. There are covariations of isotopic and trace element ratios in mafic lavas of the East African Rift system that vary systematically with geographic location. The lavas represent a magmatic sampling of variations in the underlying continental lithospheric mantle, and it appears that a common lithospheric mantle (CLM) source is present beneath much of the East African Rift system. This source contains minor amphibole and phlogopite, probably due to widespread metasomatic events between 500 and 1000 Ma. Lava suites which do not show a strong component of the CLM source, and for which the chemical constraints also suggest the shallowest magma formation depths, are the Bukavu group lavas from Kivu and basanites from Huri Hills, Kenya. The inferred extent of lithospheric erosion therefore appears to be significant only beneath these two areas, which is generally consistent with lithospheric thickness variations estimated from gravity and seismic studies.     

塔西南玄武岩年代学和地球化学特征及其对二叠纪地幔柱岩浆演化的制约

本文通过对塔西南达木斯剖面中玄武岩进行K-Ar同位素定年,获得年龄为289.6Ma,并结合Ar-Ar坪年龄结果(290.1Ma)和古生物以及沉积特征,认为290Ma的年龄代表了塔西南玄武岩形成于早二叠世,对应于盆地内下二叠统库普库兹满组下段层位的年龄.地球化学特征显示塔西南熔岩为分异的碱性玄武岩并含45%SiO2和4%MgO含量.塔西南玄武岩与盆地内柯坪玄武岩具有相近的主量元素含量和稀土配分与微量元素蜘蛛网图型、无Eu异常、富集轻稀土元素、较高的其它不相容元素(如高场强元素).但塔西南玄武岩比柯坪玄武岩具有较高的A12O2和CaO含量及稀土总量(288×10-6～358×10-6),偏低的Na2O,P2O5和FeO含量.K、Rb和Cs丰度的无系统性变化主要受这些元素丰度本身变化的影响.对其它不活动组分,塔西南玄武岩具有高Ti(Ti/Y=522～624)和Nb含量(30×10-6～40×10-6)及低zr/Nb比值,暗示其来自富集的地幔源区.其Nb含量相对La含量无显著变化以及相对低的Nb/U(近30)和Ce/Pb比值(近15),指示塔西南玄武熔岩来自大陆岩石圈或受一定程度的地壳混染.塔里木盆地大规模的火山喷发以及富集不相容元素的地球化学特征支持这样一种假设,即塔西南玄武岩来自地幔柱火山作用,或由于地幔柱的供热和上升导致富集的岩石圈地幔部分熔融而形成.且岩浆作用过程以部分熔融为主,结晶分异作用较弱.基于塔西南玄武岩和柯坪玄武岩相近的时代、源区成分和/或岩浆作用过程以及处于陆内稳定构造环境,笔者认为塔里木二叠纪玄武岩的分布范围可以从塔里木盆地内的塔中、柯坪一带一直延伸到塔西南地区.