Modelling Diverse Processes in the Petrogenesis of a Composite Batholith: the Central Bohemian Pluton, Central European Hercynides

The multiple intrusions making up the Central Bohemian Plutonin the Central European Hercynides have petrographic and geochemicalfeatures consistent with the presence of four main granitoidsuites. Major-element, trace-element and Sr–Nd isotopiccompositions are used to model their petrogenesis. Partial meltingof metabasic rocks or of a CHUR-like mantle source are interpretedto have produced melts parental to the most primitive calc-alkalineSázava suite. Interaction of basic with more acidic magmasfollowed by extensive amphibole–plagioclase-dominatedfractionation accounts for the production of trondhjemites.Alternatively, the trondhjemites correspond to small-degreemelts of a metabasic source. AFC (assimilation–fractionalcrystallization) modelling with a paragneiss as a contaminantand increasing D_Nd values simulates the characteristics of theBlatná suite. Closed-system fractionation of stronglyenriched mantle-derived magmas or their interaction with leucograniticmelts is deduced for the petrogenesis of the shoshonitic     

气候环境变化研究中影响粘土矿物形成及其丰度因素的讨论

粘土矿物在形成过程中受构造运动、气候、盆地规模、地表母岩、土壤、植被、地貌、介质环境、风以及成岩作用等多种因素的影响,这些因素对地层中粘土矿物的类型和含量的影响程度不一。构造运动和气候是影响粘土矿物形成的两个主要因素,对沉积物中的粘土矿物形成和含量以及影响粘土矿物形成的其他次要影响因素有着控制作用。在构造活动稳定的状态下,气候则成为影响粘土矿物形成和含量的决定性因素,其他因素尽管也有不同程度的影响,但可以看成是相对稳定不变的,粘土矿物携带的主要是气候环境变化的信息。地表水系的复杂程度引起的沉积物的混合程度影响粘土矿物信号的清晰度,沉积埋藏成岩作用对粘土矿物的影响也不容忽视。     

Involutions in the Middle Pleistocene (Anglian) Barham Soil, eastern England: a comparison with thermokarst involutions from arctic Canada

Involutions in the early Anglian Barham Soil at Newney Green, Essex, and Badwell Ash, Suffolk, in eastern England, are attributed to soft-sediment deformation during an episode of regional thermokarst development. The involutions show a striking resemblance in morphology and size to thermokarst involutions within a palaeo-thaw layer at Crumbling Point, western arctic Canada. By analogy with the thermokarst involutions, the involutions in the Barham Soil are reinterpreted to have formed by loading during the melting of an ice-rich layer at the top of Anglian permafrost. This period of thermokarst development may have coincided with an episode of intra-Anglian climatic amelioration. Reinterpretation of the Barham Soil involutions implies that many other Pleistocene involutions in Britain may have formed during periods of thermokarst development rather than by active-layer cryoturbation.     

Famines and cataclysmic volcanism

The gases released by some large volcanic eruptions in history (e.g. Santorini in the seventeenth century BC) have led to famine. Similar events are likely in the future but could be made worse by the huge quantities of material already in the atmosphere as a result of industrial and domestic Processes.     

The Petrology and Geochemistry of Vesteris Seamount, Greenland Basin--an Intraplate Alkaline Volcano of Non-Plume Origin

Vesteris Seamount is a solitary alkaline volcano in the GreenlandBasin some 280 km NW of Jan Mayen. Topographic and geophysicalstudies have shown no sign of an associated plume trace. Evidencefrom ash layers in sediment cores around the volcano and datingof dredged samples show that it has been active in Quaternarytimes. The lavas from Vesteris studied here consist of basanites,tephrites, mugearite, and alkali basalts. Crystal fractionationmodels are consistent with the generation of the tephrites andmugearite from a basanitic parent. Extensive kaersutite fractionationis required late in the fractionation sequence to produce theextreme mugearite composition. Na-Al-Fe-rich green cores tomany clinopyroxene phenocrysts at Vesteris suggest a fractionationhistory beginning at high pressure in the mantle. Differencesbetween Vesteris and Jan Mayen in the ratios of highly incompatibletrace elements such as Ce/Pb and Rb/Cs, which will not normallybe fractionated from one another during mantle melting, suggestthat the two are not derived from the same source. Relativelyunradiogenic Sr isotope ratios (compared with Bulk Earth), andhighly incompatible trace element patterns similar to thosefor St. Helena, suggest that Vesteris magmas are derived froma depleted, asthenospheric source. We propose that the Vesterisbasanites are very low degree partial melts ({small tilde}1%)of this source, most probably those which give rise to the seismiclow-velocity zone (LVZ). Such small-degree melts may preferentiallytap small-scale heterogeneities in the asthenosphere. Vesterislies at the intersection of two major structural trends in theGreenland Basin—(1) a zone of major reorientation of spreadingdirection on the Mohns Ridge north of Jan Mayen and (2) theextension of the Kolbeinsey Ridge axis. We propose that a combinationof the extensional stress fields related to these two lineamentsproduces sufficient dilation of the lithosphere at Vesteristo allow magmas from the LVZ to reach the surface.     

The morphological dynamics of intertidal megaripples in the Mawddach Estuary, North Wales, and the implications for palaeoflow reconstructions

The morphology and migration rate of tidal bedforms are important because of their use in interpretation of modern and ancient sediment transport regimes. Tidal flow, megaripple morphology and migration were studied in the mesotidal Mawddach Estuary, North Wales, to examine the veracity of published flow-bedform relationships, quantify spatial variations in migration and assess consequences for palaeoflow reconstruction. Two transects were surveyed along a megarippled intertidal shoal (mean grain size 280 μm) for a period of 22 semi-diurnal tidal cycles. A vertical array of current meters recorded tidal current profiles at the centre of one of the transects. Flood tidal currents dominate at Fegla Fach shoal, with peak velocities over 1 m s^?1 at spring tides, and 0.5 m s ^?1 at neaps, and bed sediment transport was also flood-dominated. Over the lunar cycle, the morphology of the megaripples on the survey lines was divisible into three phases: 1 the neap mode-consisting of near-moribund two-dimensional (2-D) flood-orientated megaripples of wavelength c. 6 m and height c. 0.2 m; 2 a transitional mode-where, on rising tidal ranges, scour pits formed and developed into 3-D megaripples which underwent net migration with the flood tide; 3 the spring mode-consisting of 3-D megaripples of wavelength c. 4 m and height c. 0.2 m. Despite complete re-orientation by the ebb tide, these were recognizable from one low water survey to the next, and net migration was c. 1 m per tide with the flood tide. We infer the presence of the equilibrium ‘spring tidal form’ occurring as flood-orientated megaripples during the flood tide. The data support previously reported separation of 2-D and 3-D megaripples at a depth to grain size ratio of 8000, and at a depth-mean velocity of the dominant tide (U_maxdom) of 0.75-0.8 m s^?1. A migration threshold exists at U_maxdom of c. 0.53-0.57 m s^?1. Measures of migration which might be used on preserved sections have been applied to the data. These measures systematically overestimated bedform migration at most stages of the lunar cycle (by <25% at spring tides and <140% in the post-spring transition period), but were accurate when the megaripples had developed into their 'spring tidal form’. There is significant variation of migration rates within the survey populations. We conclude that whilst the occurrence of megaripple cross-sets may be used as a palaeoflow indicator, and sedimentary structures associated with 2-D to 3-D transitions may also be indicative of palaeoflows, there are likely to be significant uncertainties involved in using tidal bundles as an indication of sediment transport rates.     

COMPARISON OF SIGNAL PROCESSING TECHNIQUES FOR ESTIMATING THE EFFECTS OF ANISOTROPY1

Three-component recordings of shear-waves in exploration surveys provide an opportunity to measure crustal anisotropy, which may be important in estimating the geometrical and physical parameters of reservoir rocks. VSPs are particularly important for this purpose as they are less subject to the complex interactions of the shear wavefield with the free surface. The first stage in characterizing the subsurface anisotropy requires that the distinctive phenomenon of shear-wave splitting must be examined for every arrival at each geo-phone. This effect may be defined by two parameters: the polarization of the leading shear-wave and the time-delay between corresponding split shear-waves. A variety of techniques have been designed to estimate these parameters of shear-wave splitting. Here, we classify the published techniques into four main categories and review their properties. Representative procedures from each group are applied to a common synthetic data set contaminated with signal-generated noise. The results allow some general statements to be made about the utility of these methods for processing shear-waves in VSP data.     

OPTIMAL ESTIMATION OF CRACK-STRIKE1

The accuracy of estimating crack-strike from the algebraic equivalent of a popular technique, the dual source cumulative technique (DCT), for analysing shear-wave splitting in seismic experiments is evaluated for earth models permeated by different alignments of micro-cracks. A complementary analysis is performed using another analysis procedure, the dual-independent source-geophone technique (DIT), to investigate any benefits of the alternative formulation. The investigation considers synthetic vertical seismic profile (VSP) and reflection data for an earth model with two layers over a half-space, and three different classes of crack-strike variation with depth: uniform crack-strike, an abrupt change of crack-strike between the upper and lower layer, and a continuous increase over both layers. The synthetic data for zero-offset and near-offset VSPs and a reflection profile are computed using a full-wave modelling package in which equivalent anisotropic media simulate distributions of aligned vertical, parallel, water-filled microcracks. Estimates from the two techniques agree for the constant crack-strike model, but differ for the VSP data with crack-strike changes. The asymptotic behaviour of the two angular parameters θ_G and θ_S from DIT suggest that it may be used to determine crack-strike under appropriate circumstances in these VSPs, when the time-delay between the split shear-waves for the layer of interest exceeds the peak period of the wavelet. In this limit, θ_G tends to follow the crack-strike change with θ_S tending to a constant value, whereas DCT will give a misleading value between the upper and lower crack-strike. Although the behaviour of DIT is not understood in all cases, θ_G and θ_S values from the VSP data always appear to diverge near the point where an abrupt crack-strike change takes place. This could be used as a qualitative indicator for layer stripping. Both techniques agree for the reflection data as the recorded data matrix is necessarily symmetric, but still give misleading results for deeper layers in the presence of crack-strike changes. This study suggests that more care should be taken when designing and analysing experimental configurations for detecting crack properties in reservoir rocks, to consider the response and resolution limits of the analysis techniques. A note of caution is offered to those who directly interpret polarization estimates as crack-strike.     

Biolithogenesis of Microcodium: elucidation

Petrographic studies of Tertiary and Pleistocene caliche from the western Mediterranean show some unusual calcite structures. These structures were designated Microcodium elegans Glück 1912. New data are presented which question earlier interpretations with regard to the origin of this structure. The new discovery of Microcodium in Recent soils extends its stratigraphic range into the Holocene. Retention of fine detail in Recent samples, revealed by light microscopy and SEM, has suggested an origin hitherto unconsidered, calcification of mycorrhizal associations. Ancient and Recent Microcodium fabrics are compared; sufficient preservation of ultrastructure in the Ancient indicates a homologous origin. Environmental, stratigraphic and palaeoecological significance of Microcodium is discussed: correct recognition indicates existence of a palaeosol, and hence is a valuable criterion for recognition of continental conditions, cessation of sedimentation, subaerial exposure, and time-equivalent horizons. In particular. Microcodium is a characteristic component of caliche in the western Mediterranean. A review of the literature suggests that its presence may have been overlooked or misinterpreted in other parts of the world and, thus, may be more widespread than hitherto suspected. This study, in its embryonic stage of development. illumines the potential importance of biolithogenesis within terrestial carbonates.