Stratigraphic correlation of the Awahab and Tafelberg Formations, Etendeka Group, Namibia, and location of an eruptive site for flood basalt volcanism

Detailed field and geochemical investigations in the vicinity of the type section of the Tafelberg Formation of the early Cretaceous Etendeka Group, NW Namibia, have revealed the existence of a large eruptive vent in the lower part of the regional volcanic sequence produced by Strombolian and Vulcanian eruptions. The vent is filled with the thick, differentiated, ponded Kudu-Run olivine-rich basaltic flow, which has a distinctive low Zr/Y geochemical signature as well as a Tafelberg-type tabular basalt and the Nil Desperandum latite. Field evidence indicates that the Kudu-Run basalt and the latite were erupted from fissures located within the vent. Associated with the vent is an extensive pyroclastic apron extending from the vent edge and which is interbedded with the regional stratigraphy. Blocks of Precambrain basement lithologies occur within this deposit and indicate that the vent was excavated to a depth of at least 350 m below the palaeosurface at that time. The original Tafelberg Formation type section described by Erlank et al. [Erlank, A.J., Marsh, J.S., Duncan, A.R., Miller, R.McG., Hawkesworth, C.H., Betton, P.J., Rex, D.C. 1984. Geochemistry and petrogenesis of the Etendeka volcanic rocks from SWA/Namibia, 195–247. In: Erlank, A.J. (Ed.), Petrogenesis of Volcanic Rocks of the Karoo province. Special Publication of the Geological Society of South Africa, vol. 13, 395 p.] the Tafelberg Gully section, crosses from the lower part of the regional sequence into the intra-vent sequence and returns to the regional sequence higher up. In doing so it includes some of the localized intra-vent flows and excludes a number of flows which are part of the regional sequence in its lower part, thus rendering it inappropriate as a type section. A revised type section for the Tafelberg Formation is described by combining the upper part of the Tafelberg Gully section with a new section of 14 flows at the base of the regional sequence in the Tafelberg North (TBN) section some 2 km N of the Tafeleberg Gully. Distinctive flows in the TBN section can be mapped southwards where their precise stratigraphic relationship to the northward-thinning Springbok and Goboboseb Quarts Latite members of the Awahab Formation can be demonstrated. These stratigraphic relationships are entirely consistent with palaeomagnetic reversal stratigraphy and demonstrate that the same N–R–N polarity sequence occurs in the type sections of the Awahab and Tafelberg formations. Thus, the Awahab and Tafelberg magma systems were contemporaneous but the Tafelberg system outlived that of the Awahab. The Awahab system was built from eruptive centres located S of the Huab River whereas the Tafelberg vents were located further north.     

Un exemple de volcanisme calco-alcalin de type orogénique mis en place en contexte de rifting (Cambrien de l'oued Rhebar,Meseta occidentale,Maroc)

The Middle Cambrian calc-alkaline Oued Rhebar volcanic complex (western Meseta, Morocco) compares with rocks originated in orogenic contexts. The La/Nb ratios are relatively high (5.2), suggesting a lithospheric mantle origin. The La/Ta ratios, higher than 26, and the negative Nb anomaly indicate a lithospheric source contaminated by the continental crust. These rocks were generated in the Mesetian Mid-Cambrian rift and would have inherited their orogenic signature from the partial melting of a previously metasomatized mantle. To cite this article: H. El Hadi et al., C. R. Geoscience 338 (2006).     

安徽霍山戴家河—六安毛坦厂地区中生代的火山作用

根据详细填图和研究资料,将区内中生代火山岩系划分为两个喷发旋回,分别属钙碱性系列和碱性系列,并对其岩石化学和地球化学特征等作了较系统的总结。查明了区内火山活动特点,恢复和圈定了数处火山机构,并对其喷发机制、演化规律进行了探讨。     

Physical controls and resulting morphological forms of Quaternary ice-contact volcanoes in western Canada

Volcanism in association with large quantities of ice manifests itself in a variety of morphological forms developed under differing physical conditions. These physical conditions include the location, amount, thickness, and type of confining ice, the location and quantity of trapped water, and the surrounding topography. Ice in the form of thick continental ice sheets, thinner alpine glaciers or even perennial accumulations of snow can influence the resulting morphological form of the volcano. The chemical composition of the erupting magma, effusion rate and total erupted volume will also affect the shape of the resulting edifice. Quaternary volcanoes influenced by many of these factors are found in western Canada. The morphological forms range from tuyas to subglacial mounds to stratovolcanoes. This paper provides a review of ice-contact volcanism and volcanic products formed in these environments using Canadian examples for illustration.     

Stratigraphy and volcanology of the Türkbükü volcanics: products of a stratovolcano in the Bodrum Peninsula,SW Anatolia

The Middle‐Upper Miocene Bodrum magmatic complex of the Aegean region, southwestern Turkey, is mainly represented by intermediate stocks, lavas, pyroclastic and volcaniclastic deposits. Monzonitic stocks and connected porphyry intrusions and extrusions are the first products of the magmatism. These are followed by a volcanic succession consisting of andesitic‐latitic lavas, autobrecciated lavas, pyroclastic and volcaniclastic deposits. The final stage is represented by basaltic and basaltic andesitic flows and dykes intruded into previous units. The volcanic succession crops out in the northern part of the Bodrum peninsula. In the lower part of this succession are widespread pyroclastic deposits, composed of pyroclastic fall and flow units, alternating with epiclastic deposits. Grain size, volume and thickness of the pyroclastic deposits were mainly controlled by the type, magnitude and intensity of the eruption. Further up the section, there are two horizons of debris avalanche deposits forming the coarsest and thickest deposits of the volcaniclastic succession. The debris avalanche deposits indicate at least two different flank collapses coeval with the volcanism. The stratigraphy and map pattern of these volcanic units imply that the northern part of the Bodrum peninsula was the north‐facing flank of a stratovolcano during the mid‐Late Miocene. Copyright © 2006 John Wiley & Sons, Ltd.     

http://www.sciencedirect.com/science/article/pii/S1674987114000681

We present the disaster-forced biological evolution model as a general framework that includes Darwinian "phylogenic gradualism",Eldredge-Gould's "punctuated equilibrium",mass extinctions,and allopatric.parapatric,and sympatric speciation.It describes how reproductive isolation of organisms is established through global disasters due to supernova encounters and local disasters due to radioactive volcanic ash fall-outs by continental alkaline volcanism.Our new evolution model uniquely highlights three major factors of disaster-forced speciation:enhanced mutation rate by higher natural radiation level,smaller population size,and shrunken habitat size(i.e.,isolation among the individual populations).We developed a mathematical model describing speciation of a half-isolated group from a parental group,taking into account the population size(N_e),immigration rate(m),and mutation rate(μ).The model gives a quantitative estimate of the speciation,which is consistent with the observations of speciation speed.For example,the speciation takes at least 10~5 generations,if mutation rate is less than10~(-3) per generation per individual.This result is consistent with the previous studies,in which μ is assumed to be 10~(-3)-10~(-5).On the other hand,the speciation is much faster(less than 10~5 generations)for the case that μ is as large as 0.1 in parapatric conditions(m μ).Even a sympatric(m ~ 1) speciation can occur within 10~3 generations,if mutation rate is very high(μ~1 mutation per individual per generation),and if N_e 20-30.Such a high mutation rate is possible during global disasters due to supernova encounters and local disasters due to radioactive ash fall-outs.They raise natural radiation level by a factor of 100-1000.Such rapid speciation events can also contribute to macro-evolution during mass extinction events,such as observed during the Cambrian explosion of biodiversity.A similar rapid speciation(though in a much smaller scale) also has been undergoing in cichlid fishes and great African apes in the last several tens of thousand years in the current African rift valley,including the origin of humankind due to the radioactive ash fall-outs by continental alkaline volcanism.     

松辽东缘石碑岭中生代盆地的含煤沉积旋回

松辽东缘石碑岭中生代含煤盆地含煤沉积形成于盆地演化的中期阶段。沼泽体系中粗碎屑(河流)沉积与含煤细碎屑(沼泽)沉积构成了含煤沉积旋回。火山沉积事件对7、6含煤旋回层的形成及煤的聚积具有控制作用。含煤旋回层的研究对层序地层学的亚层序的划分是重要的。     

吕梁地区古元古代的裂陷型火山作用及其地质意义

吕梁山中部的古元古代吕梁群(2 300 Ma)和野鸡山群(2 100 Ma)均由下部的碎屑建造和上部的火山建造组成.在火山建造中以基性火山岩为主, 酸性火山岩较少.该区的玄武岩中MgO和P2O5含量低,而Feo*,K2O+Na2O和LREE含量高, 类似于大陆溢流玄武岩.酸性火山岩则以LIL和ΣREE富集和高(La/Yb)N比值为特点.沉积环境和火山岩的地球化学特征表明, 吕梁群和野鸡山群火山岩形成于陆内或大陆边缘裂谷环境.正的εNd(tDM)值(+3.0左右)说明玄武岩的母岩浆来源于亏损地幔, 而近于零的εNd(tFM) 值则表明原始岩浆曾受到大陆地壳物质的混染.古元古代裂谷型火山作用的出现意味着在此之前(太古宙末)华北地区已形成了具有相当规模的大陆地壳.     

新疆库鲁克塔格地区早寒武世磷质岩的形成条件

根据地层层序与岩石学特征,对新疆库鲁克塔格地区下寒武统西山布拉克组磷质岩的形成环境,物质来源和形成机理进行了分析。认为本区磷质岩形成于从浪基面附近至氧化界面附近一个较宽泛的环境带中,具有热液和生物两个方面的物质来源。来自火山和生物的含磷物质由上升洋流带至沉积地点经化学沉淀形成磷质岩。而后又经历了水流与波浪的作用,以及重力产生的滑塌事件的改造。西山布拉克组沉积时期经历了三次海平面的升降变化,磷质岩是早期海平面变化的产物。沉积过程发生在高级别的海侵背景之下。