Formation Of Dykes Research Articles

Seismological studies on the mechanism of the large Tolbachik fissure eruption, 1975–1976

Seismological observations provided consistent information on the course and mechanism of the complicated large fissure eruption at Tolbachik volcano in Kamchatka from July 6, 1975 to December 10, 1976. Seismicity indicates that the initial magnesian basalts were rising ten days before the eruption from depths of more than 20 km. The formation of new feeding dykes was accompanied by earthquake swarms which decreased sharply one to two days before the opening of new eruptive fissures. The seismological data indicate that the main source of the different erupted basalts (2 km3) was a vast system (diameter ca. 80 km) of hydraulically connected magma chambers located in the lower crustal layers or in the crust-mantle transition layer.

An interpretation of irregular dyke form in the Itivdleq shear zone, West Greenland

Around the settlement of Itivdleq the general form of the Kangamiut dykes varies according to whether the intrusions occur within or outside the pre-dyke Itivdleq shear zone (fig. 1; Nash, this volume). Outlined below is an interpretation of the origin and development of the different dyke forms in the shear zone and some brief remarks on the significance of the results.

Seismic activity associated with the September 1977 deflation of the Krafla central volcano in northeastern Iceland

The September 1977 deflation of the Krafla volcano was one of a series of such events that has been in progress since December 1975. The operation of portable seismographs in the active region and favorable location of the main seismic activity with respect to the permanent seismograph network in northeastern Iceland allow a more detailed study of this event than of most others. Continuous volcanic tremor appeared on the local seismographs shortly before 16:00 (UTC) on September 8, 1977. Deflation of the volcano began at the same time. A small basaltic eruption broke out on a 0.9-km-long fissure near the northern rim of the caldera at about 18:00. Earthquake activity increased soon after the beginning of the tremor and the first earthquakes were located in the caldera region. The earthquake activity then migrated southwards along the Krafla fault swarm with a rate of about 0.5 m/s, and culminated shortly before midnight with 8 earthquakes larger than magnitude 3 located near the Námafjall geothermal area 8 km south of the center of the caldera. Shortly after, small amounts of basaltic pumice were erupted through a 1138m-deep drill hole near Námafjall. Depths of earthquakes were 0–6 km in the northern part of the hypocentral zone and 0–4 in the southern part. The first-motion pattern of P-waves suggests dip-slip faulting on steeply dipping fault planes, consistent with the extensive normal faulting observed at the surface. The magnitude-frequency relationship was nonlinear and changed during the earthquake sequence. The seismological data strongly support the interpretation that deflation of the Krafla volcano is associated with lateral migration of magma from the caldera region and formation of dykes in the Krafla fault swarm.

Geological and physical characteristics of some magnetic sources in lower proterozoic metasediments pine creek geosyncline N.T.

Regional aeromagnetic surveys in the Pine Creek Geosyncline, show that much of the region has a fairly flat magnetic field, and few deep seated magnetic anomalies are evident. The most prominent features are zones of narrow curvilinear anomalies which occur over Lower Proterozoic metasediments. The geological and physical characteristics of the sources of some of the magnetic anomalies over the Lower Proterozoic metasediments have been investigated by: detailed interpretation of magnetic anomalies, geological mapping, ground magnetic surveys, laboratory physical property measurements, and mineralogical studies of rock samples and drill core. In the central part of the Pine Creek Geosyncline most of the prominent curvilinear anomalies result from strongly remanently magnetised iron sulphides in the Golden Dyke Formation sediments. In the east, in the Alligator Rivers Uranium Field, the prominent anomalies result from inductively magnetised magnetite in Cahill Formation schists. In the west, in the Litchfield Block, prominent anomalies result from inductively magnetised iron oxides in unnamed schists. The regional distribution of magnetic iron facies in the Lower Proterozoic metasediments might be a useful tool to assist in formulating hypotheses regarding the geological history of the Pine Creek Geosyncline.

DISCUSSION OF PREVIOUS TWO PAPERS

DR M. J. L e B as offered his congratulations to Dr Walker on his lucid presentation of the role of density during magmatic ascent. However, density effects alone cannot explain the majority of continental basalts which have been extruded through less dense materials. That density control can be important is shown at Carlingford and Skye, where layered gabbros intrude as sheets above sialic crust and at the base of the Tertiary lava pile: Bailey’s mechanism for ring dyke formation is almost impossible here. Dr Walker likened the inclined sheets of Iceland to cone sheets, but the former are planar and dip to the riftzone which gives a different stress field from that of the conical sheets in the Hebrides. It seems that cone sheets raise their central mass of country rock as a result of vertical forces, and that this mechanism is well exemplified by highlevel carbonatite complexes. How did he reconcile the fact that his model appears to depend on acid preceding basic magma, contrary to the relations at most Hebridean centres? Had he considered the mechanical strength of the crust through which magma penetrates: the nature and extent of joints and fractures in strata along which magma can find weaknesses? Finally, did he distinguish between the different effects, apart from viscosity, of ‘wet’ and ‘dry’ magmas, which can either soften the envelope of an intrusive boss followed by deformation and folding, or give brittle-type fracture of the country rocks permitting sheet intrusion? DR R. R. S kelhorn : It was very

A NEW OCCURRENCE OF FERRIFEROUS SEPIOLITE FROM THE AKAN-CHO, EASTERN HOKKAIDO, JAPAN

A new occurence of sepiolite found as a probable hydrothermal alteration product of plagioclase porphyrite, which intrudes into tuffaceous sandstone of Palaeogene age, is described. Although X-ray powder data and thermographs of the mineral completely coincide with previously reported sepiolite, chemical analysis indicates that the mineral is rich, in Fe3+ and Fe2+. The structural formula based on Brauner-Preisinger's model is calculated to be (Si11.48Ti0.01Al0.51) (Al0.26Fe3+1.54Fe2+0.30Mg4.45Mn0.15) O32Ca0.19K0.69Na0.77 Accordingly this mineral may be called as ferriferous sepiolite. The ferriferous sepiolite mass is found in a dyke form of average 30cm, up to 150cm wide, cutting Nuibetsu formation (Oligocene) in Akan-cho, eastern Hokkaido, Japan.

The dynamic emplacement of cone sheets

Explanations of the formation of radial dykes, ring dykes and cone sheets are reviewed and the explanations of the formation of cone sheets are found to be unsatisfactory. Ring dykes develop on shear fractures formed in the overlying rocks when the magma subsides as explained by E.M. Anderson. Radial dykes and cone sheets form as a result of the upward pressure of the magma, and the problem is to explain the development of the two very different stress distributions which gave rise to the two kinds of intrusion. The radial dykes are simple hydraulic tension fractures formed periodically during the upwelling of the magma. It is suggested that the cone sheets occupy shear fractures formed as the result of dynamic stresses arising from the rapid expansion of a magma undergoing retrograde boiling.

Dolomitization and the genesis of the Woodcutters lead-zinc prospect, Northern Territory, Australia

The Woodcutters L. 5 lead-zinc prospect in the Northern Territory, Australia, occurs in the Golden Dyke Formation, a sequence of carbonaceous siltstone, dolomite, and greywacke forming part of the Lower Proterozoic Goodparla Group, which was deposited on an Archaean granitic basement. An attempt has been made to show how those factors which are considered to have been significant in the formation of dolomite were also important in the genesis of the Woodcutters deposit. These factors are: 1. An evaporitic environment which favoured dolomite formation concentrated lead and zinc in the overlying solutions. 2. The base metals were further concentrated, and fixed in the sediments, by co-precipitation with the precursors of dolomite, Mg-calcite and/or aragonite. 3. The formation of dolomite during diagenesis resulted in either a structural change if the precursor was aragonite, or an ordering if the precursor was Mg-calcite. The dolomite could not accommodate the relatively large amount of base metal associated with its precursors, and as a consequence, during dolomitization these were released to the pore solutions. The metals in the pore solutions possibly complexed with organic materials such as those from the degradation of algal protein, and so remained in solution during lithification. During folding, the metal-enriched solutions were transported to fractures, and metal sulphides precipitated when the organic complexes became unstable. After lithification the carbonate-quartz-sulphide veins were zones of weakness along which shearing took place, probably over a considerable period of time. This shearing, as well as slight rise in temperature, resulted in fracturing, recrystallization, and reaction between the first-formed simple sulphides to produce the ore in its present form.

Granulite facies metamorphosed basic dykes of the Torngat Mountains, Labrador

Summary In the Four Peaks region of Labrador Precambrian hornblende granulite facies metamorphosed basic dykes discordantly cross the foliation of garnet-hornblende-pyroxene granulites with sillimanite-garnet-biotite-quartz-feldspar gneiss. Most of the dykes are schistose or banded, but some are massive and undeformed with blastophitic textures. Mineral analyses are given for the equilibrium assemblage in one analysed clinopyroxene-orthopyroxene-amphibole-garnet-plagioclase- quartz blastophitic dyke. Dyke intrusion occurred after formation of the host rock foliation, while the metamorphism was about 1400 Myr ago.

Carbonatite-alkalic complex of Panwad-Kawant, Gujarat, and its bearing on the structural characteristics of the area

Carbonatite-alkalic rocks occur in the form of dykes and small volcanic plugs in the area, with major central type volcanic activity restricted to Amba Dongar. The trappean flows of Blanford and Bose are identified as plagioclasecalcite rocks. An attempt is made to explain the origin of these rocks which are extensively cut by dykes of alkaline rocks, carbonatites and dolerites. By far the dominant lavas are «fissure phonolites» (Wright, 1963) and tinguaites. The chemical analyses of these rocks show that the magma is mainly of continental sodic alkaline suite, probably turning sodi-potassic, a suggestion drawn from the occurrence of lamprophyres and pseudoleucite tinguaites, and the higher potassium contents of some rocks. Bagh sediments are mainly represented by sandstones which show mild contact effects with carbonatite, especially in the south.

Contrasted types of metamorphism of basic intrusions in the Precambrian basement of the Tasîussaq area, South Greenland

Metamorphosed rocks of three distinct episodes of basic intrusion can be recognised in the Precambrian basement of the Tasiussaq area, South Greenland. The oldest intrusions, represented by sills and dykes, are pyriclasites and biotitepyriclasites; the second episode intrusions, in the form of dykes, are pyroxenemetadolerites and the third episode intrusions, represented by dykes and small bodies, are metagabbros, metadolerites, metanorites and amphibolites. The metamorphic nature of the rocks of the three episodes is a reflection of age. Chemical and modal analyses of rocks from the three episodes are presented. Fresh diorite sills and dolerite dykes represent later episodes of Precambrian basic intrusion. The basic rocks depict the varying types of metamorphic conditions which affected the area in Precambrian time, and these are seen to differ from the established metamorphic history in areas to the north-west in South Greenland. The pyriclasites and biotite-pyriclasites have been derived through granulite facies metamorphism; the pyroxene-metadolerites by dipsenic metamorphism under conditions corresponding to the amphibolite facies and the metagabbros, metadolerites, metanorites and amphibolites through amphibolitisation during amphibolite facies metamorphism. It is suggested that the metamorphism producing the pyroxene-metadolerites (Sanerutian in age) was controlled by dipsenic conditions inherited from earlier granulite facies metamorphism (Ketilidian in age). This implies that the Ketilidian and Sanerutian metamorphisms in the Tasiussaq area are not separated by a long span of time and that the break in plutonism marked by the pyroxene-metadolerites cannot be regarded as a significant cratogenic hiatus between two separate plutonisms. The importance of water in controlling trends in the metamorphism of dolerites is stressed. The 1st episode intrusions have undergone severe changes since intrusion and no palimpsest features indicative of primary texture or mineralogy remain. The majority of the 2nd episode intrusions display a granular texture, but some display sub-ophitic and relic sub-ophitic textures. The 3rd episode intrusions display a range from ophitic, sub-ophitic and microporphyritic textures to relic stages of these textures. The 1st episode intrusions were emplaced into a geosynclinal pile of sediments and were probably connected with the volcanicity which occurred at the end of sedimentation. The 2nd and 3rd episode intrusions were emplaced into granitic and metamorphic rocks at a later stage in the same 'geological cycle'. Both the 2nd and 3rd episode intrusions are considered to indicate trends in the crust towards brittle conditions marking temporary partial withdrawals of the thermal front. Their preserved ophitic and sub-ophitic textures are not indicative of emplacement and crystallisation in cratogenic conditions.

The deformation and granitisation of Ketilidian rocks in the Nanortalik area, S. Greenland

The Nanortalik peninsula, situated between the fjords of Tasermiut and Sarqâ, is largely composed of Ketilidian schists, quartzites and volcanic rocks. All these rocks are more or less strongly folded. The folding took place probably in three successive phases during the Ketilidian period : A first deformation resulting in folds with NNE trending axes, was followed by a second major phase of folding with NW axes. This second folding was essentially plastic. A third deformation, acting probably on a more rigid mass, was characterised by the formation of fracturec1eavage. Third-period folds possess very long wavelengths; their axes are oriented NNE to NE. Migmatisation started probably during the second deformation period resulting in the formation of many dykes and veins of pegmatite and aplite. Four generations of Ketilidian pegmatites can be recognised. Most of them appear to have been formed by metasomatic replacement. It seems that during the Ketilidian orogeny, the evolution of the schists and gneissic schists tended to a granodioritic composition. Potassium metasomatism only became active at the end of the Ketilidian period. In the NE part of the Nanortalik peninsula, three Sanerutian granites can be observed. These granites are similar in composition (quartz-microline-biotite), but possess different ages and textures. The time interval between the last Ketilidian deformation and the emplacement of the first Sanerutian granite was marked by the intrusion of several metadoleritic dykes. The first and principal Sanerutian granite usually shows an indistinct foliation due to numerous oriented inc1usions. Field evidence indicates that this granite was formed mainly by replacement of volcanic rocks. Chemical analyses show that large amounts of K, Si and Na have been supplied to produce the granitisation of the volcanic rocks. The second Sanerutian granite is characterised by a coarse porphyroblastic texture and appears to have been emplaced partially by the intrusion of a melt and partially by a subsequent replacement of the host-rock. Finally, the last Sanerutian granite displays all the characteristics of a pure intrusive body. It is generally very fine-grained and forms many cross-cutting dykes.

XIII.—Granites and Associated Rocks of the Eastern Part of the Western Redhills Complex, Isle of Skye

SynopsisThe country lying between the head of Loch Ainort and the northern end of the Blaven Range in central Skye forms part of a Tertiary igneous complex. Country rocks in the area are Jurassic sediments. The earliest igneous rocks are basalt lavas which show the effects of thermal metamorphism by a later intrusion of eucrite and gabbro. This latter, which is part of the Blaven mass, has features of a layered intrusion such as igneous lamination, repetition of layers rich in certain minerals and comparable textures. It may be correlated with zones of the Cuillin layered intrusion. The eucrites are cut by a cone sheet swarm. Adjacent to the eucrite is a vent breccia which contains eucrite blocks, is not cut by cone sheets and is, therefore, of later age than the eucrite. The major part of the area under discussion is formed from four granitic ring dykes and a hybrid suite with ring dyke form, all of which belong to the Western Redhills intrusive complex; in addition, part of a fifth granitic ring dyke, which probably belongs to the Eastern Redhills intrusive complex is included in the area. The granites are mainly fayalite-hedenbergite types with a variety of phenocrysts of felspar and features characteristic of high level intrusions such as granophyric texture and varying optical and chemical properties of the felspar indicating higher temperature modifications. Three new chemical analyses of the granites and one of ferrohedenbergitic pyroxene are presented. The granites may be regarded as partial remelts of the underlying basement gneiss and a simple investigation of thermal requirements indicates that the heat necessary for the partial melting could be provided by a mass of basic rock presumed from geophysical evidence to underlie the complex. Various acid and basic minor intrusions in the area are also described.

On the dolerite and allied rocks of Semi, Yamagata Prefecture

So called “Semi dolerite” is represented by sills and dykes of seven masses. The writer classified those dolerites geologically and petrologically in two groups, namely Semi dolerite and Nagasawa dolerite. Semi dolerite intrudes in the lower part of Nagao formation and the upper part of Semi formation as sill form, but rarely in Nozoki formation in dyke form. Then, Nagasawa dolerite intrudes in the lower part and extrudes in the middle part of Nagasawa formation. Sills and dykes of Semi dolerite show chilled margins at the lower and upper contact with the intruded sediments. The thickness of the largest sill is 24.4 meters, while its chilled base is 0.8 meters and its upper chilled margin is 0.5 meters thick. When the thickness of sill is over about 20 meters, the petrographyic and chemical compositions vary remarkably from the margins to the central and upper part in one sill, by differentiation “in situ”. The writer found a differentiation sequences in Semi dolerite sill as follows, namely, chilled margins, normal dolerite, felsic dolerite (dioritic laver in the upper part), dolerite pegmatite and granophyric segregation vein. Nagasawa dolerite sills 18.6 meters in thickness, and the grain size is relatively small and uniform throughout the whole body. As a characteristic nature, it includes numerous xenocryst of quartz, skeleton like plagioclase and hornblende like matters. The chemical composition of Nagasawa dolerite, comparing with the felsic differentiates of Semi dolerite, has the next characteristics: SiO2 component of the latter is equall to the one of the former, while Fe2O3+FeO component is poor, and A12O3 and MgO components are richer in the latter than in the former. Judging from the chemical composition as well as the mineralogical compositions, Nagasawa dolerite is believed to have been derived from the original magma of Semi type dolerite not by the differentiation process, but by the assimilation of the felsic materials as probably the granitic materials.

The Dynamics of Faulting and Dyke Formation with Application to Britain. E. M. Anderson

<i>The Dynamics of Faulting and Dyke Formation with Application to Britain</i>. E. M. Anderson

The Dynamics of Faulting and Dyke Formation with Applications to Britain. By E. M. Anderson. Second edition revised. Pp. x + 206, with 39 Text-figs. Oliver and Boyd, Edinburgh, 1951. Price 22s. 6d. - Structural Geology. By M. P. Billings. Eighth printing. Pp. xi + 473, with 336 figs. (including several exercises to be torn out), and 19 halftone plates.

The Dynamics of Faulting and Dyke Formation with Applications to Britain. By E. M. Anderson. Second edition revised. Pp. x + 206, with 39 Text-figs. Oliver and Boyd, Edinburgh, 1951. Price 22s. 6d. - Structural Geology. By M. P. Billings. Eighth printing. Pp. xi + 473, with 336 figs. (including several exercises to be torn out), and 19 halftone plates. Sir Isaac Pitman and Sons, Ltd., London (Prentice Hall, Inc., New York), 1952. Price 50s. - Outlines of Structural Geology. By E. Sherbon Hills. Third edition revised. Pp. xi + 182, with 107 text-figs. and 4 half-tone plates. Methuen and Co., Ltd. Price 12s. 6d. - Structural and Field Geology. By James Geikie. Sixth edition revised (as was the fifth) by R. Campbell and R. Craig. Pp. xxiv + 397, with 148 text-figs. and 69 half-tone or coloured plates. Oliver and Boyd, Edinburgh. Price 37s. 6d. - Volume 90 Issue 4

The Dynamics of Faulting and Dyke Formation with Application to Britain. E. M. Anderson

<i>The Dynamics of Faulting and Dyke Formation with Application to Britain</i>. E. M. Anderson

The Dynamics of Faulting and Dyke Formation: with Applications to Britain

THIS book can be warmly recommended to geologists, who will read it with great interest and profit. The theory of principal stresses had long been applied by engineers to explain fractures in materials, but it was the author who first applied it in explanation of the main types of rock fractures or faults well known to geologists. From field experience these had been classified into three main types called strike faults, dip faults and oblique faults which conformed to neither of these directions. With increasing knowledge it was found that the majority of those in the first class were overthrusts while those of the second class were mainly normal faults. It was only later that the oblique faults were better understood ; many of these have now been proved to be associated with some horizontal movement parallel to the earth's surface, and they have been called tear faults, transcurrent faults (the name favoured by the author), and a variety of other names. The Dynamics of Faulting and Dyke Formation: with Applications to Britain By Dr. E. M. Anderson. Pp. xii + 191. (Edinburgh and London: Oliver and Boyd, 1942.) 15s. net.

On a felsite sill near Aberdeen

Larger igneous masses are generally accompanied by minor intrusions in the form of dykes, sills, and sometimes laccoliths. Attention has recently been drawn, in some detail, to numbers of these in recent geological literature dealing with Scotch areas, as in the Survey Memoirs on the Blair Atholl and the Mid-Argyll districts. A characteristic intrusion of this nature which has never been described occurs near Aberdeen, and seems worthy of a brief notice. It crops out on the coast about two miles south of Aberdeen, and is beautifully exposed by the denudation of the coast-line. Although it does not appear on the Survey map (1 in. 77), it forms a continuous wall-like section which can be traced round the cliffs for a distance of about 700 yards. The rock forms a typical sill. It is intruded amongst banded Gneisses, which have a general dark grey aspect, and, as the sill itself is a bright red rock, the contrast is sharp and striking, and produces, indeed, a picturesque bit of coast scenery. As one follows the sill round, it is found, at the southern end of the exposure, to be quite thin, only a foot or two in breadth, and obscured by surface soil and grass. Thence it widens out, and passes, northwards, round a promontory, up the south side of a short gully, enters a cave, and emerges on the north side of the same gully as a fine long vertical wall. Then it rounds another promontory, and runs along both

Tinguaites from EIfdalen and Ruphachthal: Basalts from Madagascar and the Soudan

In connection with the investigation of ægirine-anorthoclase rocks from Abyssinia, belonging to the grorudite-tingusite series, an examination was made of specimens of the so-called green phonolite from Elfdalen, which in the British Museum collection accompanied the more ordinary brown and red porphyry from that locality. In the descriptions of the well-known porphyry quarries of Elfdalen, mention is made of this green variety as occurring only in boulders. Törnebohm was the first to record the occurrence of these rocks in situ, and to describe their characters, They were found in the form of dykes in the neighbourhood of Heden, near Särna, Dalarne, Sweden, and were interesting as containing cancrinite in clear, well-defined porphyritic crystals as undoubtedly a primary constituent.