Basement controls upon basin development in the Caledonian foreland, NW Scotland

Abstract The Hebridean basins are part of a compartmentalized half-graben developed in the hanging wall of the partially reactivated Outer Isles Fault. The importance of geological inheritance in the development of these basins can be demonstrated clearly using the widespread exposure of metamorphic basement around the basin margins. The basement structures have been analysed using thematically mapped Landsat images in conjunction with selective field studies. Results of such studies have been integrated with maps generated from the interpretation of offshore multichannel seismic reflection profiles to produce an architectural framework for basin development.
It can be demonstrated that the principal basement faults originated in the early Proterozoic as mid-crustal shear zones and that they have subsequently been partially reactivated during post-Caledonian basin development beginning in the Carboniferous and probably also during an earlier period of basin development in the late Proterozoic (the Torridonian). It is the geometry of the pre-existing basement structures that has controlled the three-dimensional shape of the sedimentary basins and the spatial and temporal distribution of the basin fill.     

Calc--Silicate Diffusion Zones between Marble and Pelitic Schist

Four distinct calc-silicate zones are observed between interlayeredmarble and pelite in roof-pendant (Gile Mountain Fm.) of a graniticintrusive at Lake Willoughby in northern Vermont. The zonesare characterized by garnet, coarse diopside-clinozoisite, fineclinozoisitediopside and amphibole-plagioclase. The bulk compositionsof the zones, calculated from modal data and microprobe analyses,deviate significantly from a gradational mixture marble andpelite. The observed zonal sequence is close to that predictedby a simple model of cation diffusion metasomatism. Possiblelocations of the initial marble-pelite boundary are suggestedon the basis of bulk compositional data and petrographic evidence.Calculated mass balance relations for the formation of the zonesby diffusion at constant volume are unrealistic for reasonablelocations of the initial boundary. Calculated mass balance relationsat constant AL₂O₃ suggest a volume decrease of about one thirdduring formation of the diffusion zones wholly from the pelite.In addition the data suggest that nearly equal volumes of initialmarble and pelite have disappeared, leading to a total volumedecrease of near 50 percent. The observations suggest that diffusionof calcium was dominant and that diffusion of aluminium, ironand magnesium was relatively unimportant. Evaluation relativediffusional mobilities is shown to be possible but intractablewith the quality available thermochemical data and activity-concentrationrelations of crystalline solutions.     

Studies of Igneous and Tectonic Textures and Layering in the Rocks of the Gosse Pile Intrusion, Central Australia

The Gosse Pile mafic-ultramafic intrusion is a laycred igneousbody, the upper part of which was involved in localized ductiledeformation (low angle faulting) soon after crystallization.A complete gradation between rocks showing typically igneoustextures and layering and those showing typically tectonic,or metamorphic, textures and layering can be observed. Textures and preferred orientations in the undeformed part ofthe intrusion are very similar to those found in other layeredigneous bodies. Orthopyroxenes show very strong preferred orientationsin some parts of the body. The metamorphic textures and layering of the deformed rocksare identical to ‘flow-layering’ which is used asan essential criterion for distinguishing ‘alpine-type’from ‘stratiform’ bodies. The Gosse Pile Body thusdemonstrates that at least some of the ‘alpine-type’bodies may be produced byin situ deformation of ‘stratiform’bodies.     

Alluvial fan and lacustrine sediments from the Stephanian A and B (La Magdalena, Ciñera—Matallana and Sabero) coalfields, northern Spain

Vertical sequence analysis within 1500-2500 m thick coarse-grained coalfield successions allows six sedimentary associations to be distinguished. These are interpreted in terms of depositional environments on, or related to alluvial fans which fringed a fault bounded source region. (i) Topographic valley and fanhead canyon fills: occurring at the bases of the coalfield successions and comprising sporadically reddened, scree, conglomeratic thinning and fining upward sequences, and fine-grained coal-bearing sediments. (ii) Alluvial fan channels: conglomerate and sandstone filled. (iii) Mid-fan conglomeratic and sandstone lobes: laterally extensive, thickly bedded (1-25 m) and varying from structureless coarse conglomerates and pebbly sandstones, to stratified fine conglomerates and cross-bedded sandstones. (iv) Interlobe and interchannel: siltstones, fine-grained sheet sandstones, abundant floras, thin coals and upright trees. (v) Distal fan: 10 cm-1.5 m thick sheet sandstones which preserve numerous upright trees, separated by silt-stones and mudstones with abundant floras, and coal seams. The sheet sandstones and normally arranged in sequences of beds which become thicker and coarser or thinner and finer upwards. These trends also occur in combination. (vi) Lacustrine: coals, limestones, and fine-grained, low-energy, regressive, coarsening upward sequences. Proximal fan sediments are only preserved in certain basal deposits of these coalfields. The majority of the successions comprise mid and distal alluvial fan and lacustrine sediments. Mid-fan depositional processes consisted of debris flows and turbulent streamflows, whilst sheetfloods dominated active distal areas. A tropical and seasonal climate allowed vegetation to colonize abandoned fan surfaces and perhaps resulted in localized diagenetic reddening. Worked coals, from 10s cm-20 m thick, occur in the distal fan and lacustrine environments. These alluvial fan deposits infill‘California-like’basins developed and preserved along major structural zones. In many of their characteristics, in particular the occurrence of thinning and fining, and thickening and coarsening upward sequences and megasequences, these sediments have similarities to documented ancient submarine fan deposits.     

植被冠层热点的季相和日相变化

随着热点卫星Triana的即将发射，地表热点季相和日相变化的研究变得日益重要。以几种植被冠层为例，采用计算机模拟的方法考查冠层热点的季相和日相变化。     

Progressive Metamorphism of Metabasic Rocks from the Haast Schist Group of Southern New Zealand

Progressive mineralogical changes are described for metabasicrocks through a Barrovian-type metamorphic series ranging fromgreenschist to amphibolite facies in the Southern Alps of NewZealand. Wet chemical and electronmicroprobe analyses of coexisting phasesillustrate (a) that chlorite composition becomes progressivelymore Mg-rich towards higher grades. Chlorite and biotite zonechlorites have Mg/Fe <1.00, while in the oligoclase zonethe chlorite Mg/Fe >2.00. (b) Biotite and epidote compositionsshow no systematic variation with metamorphic grade and arecontrolled by bulk rock composition. For epidote, compositionis directly related to oxidation states during metamorphism.(c) Zoning profiles from atoll and normal porphyroblastic almandine-richgarnets are illustrated, and their relationship to compositionalchanges with metamorphic grade discussed, (d) Coexisting compositionsof albite and oligoclase through the garnet zone outline theform of the peristerite solvus. Myrmekitic textures in oligoclaseare ascribed to release of silica during progressive eliminationof albite. Element distribution between coexisting minerals is graphicallyinvestigated. Isogradic samples show very similar element distributions,suggesting general equilibration. Distribution of any elementbetween two phases for the facies series as a whole, however,is clearly influenced by changing concentrations of other ionsin one or both minerals. In particular for pairs containingcalciferous amphibole, the distribution coefficient is dependentupon tetrahedrally co-ordinated Al⁺³ of the amphibole.     

Petrology of Ocellar Lamprophyres from Western Otago, New Zealand

The southern part of a lamprophyre dyke swarm from West Otago,New Zealand comprises monchiquites and camptonites. Dykes arecompositionally similar to volatile-rich nephelinite or nephelinebasanite, and contain leucocratic ocelli and irregular veinsegregations of nepheline benmoreite composition. Ocelli arecommonly zoned with an upper globule or centrally disposed coreof analcime-carbonate and an alkali feldspar-kaersutite-richlining. Virtually identical zoning is developed in minerals common toboth lamprophyre host and ocellus. Kaersutite is progressivelyenriched in Fe, Mn, Al_vi, and Na + K, and depleted in Mg, andTi towards the rim. Titanaugite is concentrically zoned to aegirine-augite,and is sector zoned with prism sectors enriched in Ti and Al(and to a lesser extent in Fe and Na) compared to basal sectors.Brown micas are zoned from titanphlogopite cores to titanbiotiteand titaniferous lepidomelane rims. In monchiquites glass occurs as an intersertal phase and asan ocellus infilling. The glass is phonolitic and contrastssharply with the composition of the ultrabasic host, indicatingthat production of extreme differentiates without intermediatecompositions is possible by fractional crystallization. Identicalglass compositions in both lamprophyre groundmass and ocelluspreclude liquid immiscibility. Instead, it is suggested thatocelli are segregations of residual liquids drawn into gas vesiclesas a consequence of reduction of the vapour phase, volume duringcooling. The analcime-carbonate globule within the ocellus isinterpreted as the condensed vapour phase. At Haast River, in the northern part of the swarm the dyke associationof peridotite, camptonite, trachyte, tinguaite, and carbonatiteis readily explained in terms of this model. Fractional crystallizationof camptonite results in phonolitic late stage liquids which,if separated from the lamprophyre host crystallize as sodalitetinguaites, leaving a more ultrabasic residuum whose compositionis equivalent to damkjernitic peridotite. Vesiculation of thecamptonite results in separation of a volatile phase (fluidimmiscibility) which is carbonate and alkali-rich, the sourceof both carbonatites and the agent responsible for fenitization.     

Reactions in Amphibolite, Greenschist and Blueschist

Mineral assemblages in which chlorite [CHL], epidote [EPI],clinoamphibole [AMP], plagioclase [PLG] and quartz [QTZ] aremajor phases are characteristic of many low-grade mafic schists.The possible heterogeneous reactions in such an assemblage maybe separated into two types, exchange reactions and net-transferreactions. Only the latter alter significantly the modal proportionsof the minerals. A set of linearly independent reactions defines a reaction spaceof as many dimensions as there are independent reactions. Thespace defined by the net-transfer reactions alone is a sub-spacethat can be portrayed in three dimensions for the above assemblage.A procedure is presented herein that gives a set of independentreactions that may be taken as basis reactions for definingsuch a reaction space. All other reactions that can be writtenfor this assemblage, as well as observed whole-rock reactions,can be portrayed as vectors in these reaction spaces. Thesevectors connect the region (mineral facies) accessible to theabove assemblage. The whole-rock reactions of Laird (1980) relatinggreenschist, blueschist and various low-grade amphibolites fromVermont, provide informative examples, as do the whole-rockexperiments of Liou et al. (1974). Although reaction spaces apply to both equilibrium and disequilibriumassemblages the reactions selected as basis vectors correspondone-for-one to the chemical conditions for equilibrium thatmust obtain in any fully equilibrated assemblage. The set selectedis one that provides maximum sensitivity for geothermometric,geobarometric and geohygrometric purposes.     

Correlated petrologic and geochemical characteristics of CO3 chondrites

Abstract— Many properties of CO3 chondrites have been shown previously to have resulted from thermal metamorphism; petrologic subtypes 3.0–3.7 have been assigned to members of the group. Additional properties that correlate with the metamorphic sequence but seem to have resulted from hydrothermal alteration include the modal abundance of amoeboid olivine inclusions (AOI), chondrule size, the types of refractory inclusions and whole rock O isotopic composition. The percentage of rimmed AOI increases with petrologic subtype. The rims most likely formed during hydrothermal alteration. The previously reported correlation between AOI abundance and chondrite subtype is probably an artifact due to the difficulty in recognizing small unrimmed AOI in the least metamorphosed CO3 chondrites. Because large (≥ 200 μm size) porphyritic chondrules have nearly the same mean size in all CO3 chondrites, it seems likely that the correlation between chondrule size and subtype is due to alteration of the smallest chondrules to the point of unrecogizability as complete objects in the more metamorphosed CO3 chondrites. The previously reported decrease in the proportion of melilite-rich refractory inclusions with increasing petrologic subtype may have resulted from more extensive hydrothermal alteration in CO3.4–3.7 chondrites that converted primary melilite into Ca-pyroxene, andradite and nepheline. Alteration probably caused the preferential occurrence of ¹⁶O-poor oxygen isotopes in the more metamorphosed whole rock samples.     

Rates and causes of proglacial river terrace formation in southeast Iceland: an application of lichenometric dating techniques

The immediate proglacial areas of most of the Oraefajökull outlet glaciers in southeast Iceland are characterized by well-developed river terraces, formed by the recent downcutting of the major meltwater streams. This paper examines the rates and causes of dissection in two contrasting cases, using lichenometric dating to establish the ages of individual terraces. An age–size curve for the aggregated Rhizocarpon sub-genus is developed from lichen measurements on dated recessional moraines, and is compared with similar curves obtained by previous workers. Levelling profiles of the terraces are then used in conjunction with the lichenometric dates to determine mean rates of net erosion between each dated surface, and to study the associated variations in channel slope. The results obtained for the Svinafellsá show that the timing and rates of downcutting have been closely related to frontal movements of the Svinafellsjökull glacier. The Kotá terraces, however, may have been formed independently of glacier fluctuations, and are thought to represent stages in the gradual recovery of the stream from the aggradational effects of the 1727 jökulhlaup.