Number Of Dykes Research Articles

Rifting Kinematics Produced by Magmatic and Tectonic Stresses in the North Volcanic Zone of Iceland

In the North Volcanic Zone of Iceland, we studied with the greatest possible detail the complete structural architecture and kinematics of the whole Theistareykir Fissure Swarm (ThFS), an N-S-trending, 70-km-long active rift. We made about 7500 measurements along 6124 post-Late Glacial Maximum (LGM) extension fractures and faults, and 685 pre-LGM structures. We have collected the data over the last six years, through extensive field surveys and with the aid of drone mapping with centimetric resolution. In the southern sector of the study area, extension fractures and faults strike mainly N10°-20°, the opening direction is about N110°, and the dilation amount is in the range 0.1-10 m. In the central sector, faults and extension fractures strike mainly N00-10°, the opening direction is N90-100°, and the dilation amount is 0.1-9 m. In the northern sector, extension fractures and faults strike N30-40°, the opening direction is about N125°, and the dilation amount is 0.1-8 m. The variations in strike are attributable to two processes: the interaction with the WNW-ESE-striking Husavik-Flatey transform fault and Grímsey Oblique Rift (Grímsey lineament), and the structural inheritance of older NNE- to NE-striking normal faults. Most extension fractures show a minor strike-slip component: a systematic right-lateral component can be accounted for by the interaction with the WNW-ESE-striking fault zones and the regional, oblique opening of the rift. We regard dyke propagation as a possible cause for the more complex strike-slip components measured at several other fractures. Cumulated dilation and fracture frequency decrease along the rift with distance away from the Theistareykir volcano, situated in the central sector of the ThFS. This is interpreted as a decrease in the number of dykes that are capable of reaching great distances after being injected from the magma chamber.

A review of anisotropy of magnetic susceptibility analysis of Indian dykes: Implications for magma emplacement

The analysis of Anisotropy of Magnetic Susceptibility (AMS) is a powerful and rapid technique to examine the preferred orientations of mineral (magnetic) fabrics and can indicate the nature of a magma transport (vertical or lateral). The relationship between magnetic fabric and geometry of a dyke swarm enables us to understand magma emplacement processes. Depending on the mutual relationship of magnetic fabric and individual dyke geometry, mode of magma transport is interpreted. The knowledge on the nature of magma transport combined with information on geometry, magmatic overpressure and geochemistry enable us to comment on dyke emplacement processes, the location of possible feeders, syn-emplacement and post-emplacement deformations and prevailing stress regime during emplacement. A number of dykes and dyke swarms have been emplaced into the Indian shield at different points in time. Their ages vary from the Mesoarchean to Tertiary. We present here a review of three case studies where AMS technique was applied to the samples collected from Indian dykes. Two case studies are on the Proterozoic dykes that intruded into the Dharwar craton and the third case study is on Mesozoic dykes that punctured the South Indian Granulite Terrain (SIGT). The dykes generally show “normal” anisotropy fabric to indicate vertical magma emplacement with few exceptions where lateral/inclined magma flow was suggested or the results were inconclusive. We present here a critical review on the interpretation of such “anomalous” fabrics and comment on further studies that can be carried out to extract more information from such results.

Paleomagnetism, U–Pb geochronology, and geochemistry of Marathon dykes, Superior Province, and comparison with the Fort Frances swarm

We report the first detailed study of the paleomagnetism, U–Pb geochronology and major element geochemistry of Paleoproterozoic north-trending Marathon dykes north of Lake Superior. The paleomagnetic and geochemical results demonstrate that Marathon dykes can be divided into two subsets, one of normal magnetic polarity, the other of reverse polarity. Normal and reverse Marathon paleomagnetic poles, at 43°N, 196°E (dm = 9°, dp = 7°, number of dykes N = 16) and 51°N, 175°E (dm = 9°, dp = 6°, N = 12), respectively, are statistically distinct and may indicate different ages of normal and reverse dyke emplacement, A U–Pb baddeleyite age of [Formula: see text] Ma has been obtained at a normally magnetized Marathon paleomagnetic site. The reversely magnetized Marathon dykes are undated, but have a paleopole rather close to that of the reversely magnetized [Formula: see text] Ma Fort Frances dykes and major element geochemical signatures as portrayed on Jensen plots that are identical to those of the Fort Frances swarm. Therefore, reverse Marathon and Fort Frances dykes could define a giant radiating dyke swarm focused south of Lake Superior, supporting models that associate these dykes with Paleoproterozoic rifting along the southern margin of the Superior Province. The Marathon and Fort Frances paleopoles continue a northwesterly trend in southern Superior Province paleopoles, which has recently been defined by results for [Formula: see text] Ma Senneterre dykes and 2167 ± 2 Ma Biscotasing dykes. This trend contrasts with previous widely used polar wander paths for the same period that young in the opposite direction and illustrates the importance of collaborative studies of paleomagnetism and U–Pb geochronology.

The Tchivira-Bonga alkaline-carbonatite complex (Angola): Petrological study and comparison with some brazilian analogues

The Tchivira alkaline-carbonatite complex, of Lower Cretaceous age (130-138 Ma), crops out in the Precambrian crystalline basement of southwestern Angola as a circular intrusion covering an area of about 100 km 2 . The plutonic complex forms a roughly circular concentric pattern and comprises, from the outside inward, nepheline syenites, ijolites/urtites and carbonatites. The western side of the complex is cut by a second intrusion of alkali gabbros and syenites. Bonga, an isolated plug a few kilometres northeast of Tchivira, consists exclusively of carbonatites with an area of about 12 km 2 . A number of dykes, ranging in composition from tephrite to alkali basalt, hawaiite, mugearite, phonolite and trachy-phonolite, are widespread in the complex and in the basement country rocks. Field relationships and petrological data can be used to define two magmatic suites: 1) nepheline syenite-ijolite-carbonatite, and 2) alkali gabbro-syenite, both of which evolved essentially by fractional crystallization under nearly closed system conditions. Mass balance calculations account for the generation of the various rock-types in both suites, starting from basanitic and alkali basaltic parental magmas, respectively. Ca-carbonatite melts may be genetically linked to nepheline syenite by liquid immiscibility processes, and may themselves undergo fractional crystallization -mostly of calcite- leading to differentiated Mg-carbonatites. Initial 87 Sr/ 86 Sr isotopic ratios mostly range from 0.7038 to 0.7050 in the nepheline syenite-ijolite suite, and from 0.7046 to 0.7059 in the alkali gabbro-syenite suite. Significantly higher isotopic ratios (0.7073-0.7255), which are observed in some dykes and in the border facies of the intrusion, may be related to contamination by the granitic basement. The age relations, petrology, Sr isotopes, rock associations and spatial arrangements of the Tchivira-Bonga complex are comparable with those of the south Brazilian alkaline-carbonatite complexes, particularly Juquia and Jacupiranga. Such a comparison suggests a common geodynamic and petrogenetic setting for the South Brazilian and Angolan alkaline magmatic suites just before the opening of the South Atlantic Ocean

Geochemistry and geochronology of three plutons from central Uruguay: tectonic implications for The Transamazonian orogeny

The Arroyo Grande Belt is the smallest of the three metamorphic belts recognized in south-central Uruguay and is located along the northern margin of the Piedra Aha Terrane. The belt is host to a number of granitic intrusions, three of which (the Marincho Complex, the Arroyo Grande Pluton and the South Granite) are discussed in this report. The three intrusions vary in composition from homblendite to leucogranite and have themselves been intruded by a number of dykes, also of variable composition. The Arroyo Grande Pluton formed in an extensional environment whereas the main portion of the Marincho Complex formed in a compressional tectonic regime. The main unit of the Marincho Complex has been dated at 2,291 ± 65 Ma whereas a leucogranite member of the mass has been dated at 2,067 ± 24 Ma, thereby demonstrating that the Marincho Complex is a composite mass. A leucogranitic dyke, intrusive into the Marincho Complex, yields an age of 1,969 ± 25 Ma. The leucogranitic South Granite has been dated at 2,180 ± 50 Ma. The Arroyo Grande Pluton has not been dated. The available data indicate three periods of emplacement of granitic plutons in southern and central Uruguay: 1. Archean, involving greenschist facies metamorphism; 2.2,290 Ma - 2,180 Ma.- the time of emplacement of massive granitoids; 3. 2,040 Ma -1,894 Ma. - emplacement of predominantly small granitic dykes.

Geochemistry and geochronology of three plutons from central Uruguay: tectonic implications for the transamazonian orogeny

The Arroyo Grande Belt is the smallest of the three metamorphic belts recognized in south-central Uruguay and is located along the northern margin of the Piedra Aha Terrane. The belt is host to a number of granitic intrusions, three of which (the Marincho Complex, the Arroyo Grande Pluton and the South Granite) are discussed in this report. The three intrusions vary in composition from homblendite to leucogranite and have themselves been intruded by a number of dykes, also of variable composition. The Arroyo Grande Pluton formed in an extensional environment whereas the main portion of the Marincho Complex formed in a compressional tectonic regime. The main unit of the Marincho Complex has been dated at 2,291 ± 65 Ma whereas a leucogranite member of the mass has been dated at 2,067 ± 24 Ma, thereby demonstrating that the Marincho Complex is a composite mass. A leucogranitic dyke, intrusive into the Marincho Complex, yields an age of 1,969 ± 25 Ma. The leucogranitic South Granite has been dated at 2,180 ± 50 Ma. The Arroyo Grande Pluton has not been dated. The available data indicate three periods of emplacement of granitic plutons in southern and central Uruguay: 1. Archean, involving greenschist facies metamorphism; 2.2,290 Ma - 2,180 Ma.- the time of emplacement of massive granitoids; 3. 2,040 Ma -1,894 Ma. - emplacement of predominantly small granitic dykes.

Palaeomagnetism of the Ragunda intrusion and dolerite dykes, central Sweden

Abstract The large Ragunda intrusion (1320 Ma) and dolerite dykes cutting the intrusion near its western margin have been extensively sampled for palaeomagnetic study. After comprehensive alternating field and thermal demagnetisation, the intrusion yields a mean direction D = 20°, I= 46° (α95 = 7°); both polarities are present. Two stages of dyke intrusion are identified from their magnetisations: a small number of dykes have directions similar to the Ragunda intrusion and the remainder yield a mean D = 182°, 7=24° (α95= 14°); both polarities are present in each case and neither dyke suite correlates with the magnetisations of post-Jotnian (ca. 1200 Ma) dolerites from elsewhere in the Fennoscandian Shield. Magnetisations reside in different phases of titanomagnetite, pyrrhotite is a locally important remanence carrier, but maghemite, although sometimes present, has not been identified as a significant carrier. The palaeomagnetic poles help to clarify the form of the apparent polar wander path for the Fenn...

The Permo-Carboniferous dyke-swarm of northern Argyll and its bearing on dextral displacement on the Great Glen Fault

Contour-maps of (i) percentage crustal-extension (dilation) and (ii) number of dykes per kilometre, for a regional linear WNW-trending Permo-Carboniferous dyke-swarm of northern Argyll, indicate a dextral displacement of the axis of the swarm at the Great Glen Fault by 7–8 km. The dykes are olivine-dolerites, camptonitic dolerites, camptonites and monchiquites. They are distinguished from similarly trending Tertiary dolerite dykes chiefly by their field-characters. Isotopic dates of eight dykes exhibit a range of ages, although five of these agree within the limits of experimental error, giving a mean of 285 ± 5 Ma. K-Ar ages of two Caledonian dykes are also reported. Since the Tertiary dyke-swarm of Skye is not laterally displaced at the Great Glen Fault, the dextral slip dates from between the dyke-swarms. Comparatively lower intensities of both Permo-Carboniferous and Tertiary dykes south of the fault suggest considerable downthrow to the southeast. At least part of the normal faulting is of post- or syn-Tertiary dyke-swarm date, though the Great Glen Fault may, in addition, have acted as a barrier to the southerly continuation of the Skye dyke-swarm.

Metamorphosed and deformed pegmatites and basic dykes in the Lewisian complex of the Outer Hebrides and their geological significance

The central zone of the Outer Hebrides is composed of a gneissic (‘grey-gneiss’) complex containing evidence of the earliest as well as many of the later events in the history of the Lewisian. In northern South Uist, an old metamorphic complex is intruded by pegmatites which were strongly deformed by a tectogenesis equated here with the Scourian of more than 2600 m.y. ago. It is concluded that much of the original material of the greygneiss may represent the products of a pre-Scourian orogenic cycle of possible Katarchaean (>3000 m.y.) age. Widespread post-Scourian basic (tholeiitic) dykes which cut the grey gneiss are thought to be equivalent to the Scourie dykes of the Scottish Mainland. These intrusions were subjected to granulite-facies metamorphism, regarded as Early Laxfordian, followed by retrogressive amphibolite-facies metamorphism with local folding and migmatization, all of Late Laxfordian date (1500 to 1600 m.y.). A number of dykes described from Benbecula and South Uist illustrate stages of Laxfordian deformation in which perfectly preserved cross-cutting relationships are progressively obliterated, the final products in the southern zone of South Uist being highly deformed and disrupted amphibolite masses with concordant margins in mobilized Laxfordian migmatitic gneiss.

VIII.— The Islay — Jura Dyke Swarm

1. Introduction Portions of the Islay—Jura dyke swarm have been dealt with petrographically in the Geological Survey Memoirs covering sheets 19, 27 and 28. In the Knapdale Jura and Kintyre memoir, Flett (1911, p. 117) coined the term “crinanite” for the ophitic analcite-olivine-dolerites so abundantly represented in the area, but it is perhaps unfortunate that the name indicates their occurrence round Crinan — that is in the Mull swarm — whereas the analysed type rock was taken from Jura and is a member of the Islay—Jura swarm. A more general treatment of the swarm was given by Tyrrell (1928, pp. 238-239) who considered that the Islay—Jura swarm was connected with a plutonic focus in Arran in spite of the gradual diminution in the number of dykes in the direction of that island. McCallien (1932, pp. 52-55) suggested that on the other hand the Arran swarm should be divided into (1) the Islay—Jura swarm consisting almost entirely of crinanites and olivine dolerites and (2) the Arran swarm, which is compound and associated with more than one centre. He suggested further that the focus of the Islay—Jura swarm may be N.W. of Islay since the dykes become more numerous in that direction, but that the complex Arran swarms comprising crinanitic and tholeiitic injections of different ages, has two centres, one being North Arran and the other (dominantly tholeiitic) to the south of the island. Richey (1939, p. 425) in his comprehensive survey of the dykes of Scotland, accepts McCallien’s suggestion as to This 250-word extract was created in the absence of an abstract

Flow Structure and Laminar Flow in Bostonite Dykes at Armidale, New South Wales

AbstractWest of Armidale occur a number of dykes which intrude Woolomin Group greywackes and which are genetically associated with Permian granite. The rocks are similar to bostonites and quartz-bostonites and show remarkably developed flow structures which indicate the condition of the magma at the time of intrusion. It is shown that intrusion probably took place by laminar flow despite the occurrence of a pseudo-turbulent structure in one dyke and it is suggested that evidence of turbulent flow is unlikely to be preserved in intrusions even if such flow did occur.

The Killough-Ardglass Dyke Swarm

I. Introduction The dykes described in this paper are those occurring on the coast of County Down, Northern Ireland, between Killard Point in the north and Inner Dundrum Bay in the south-west (Pl. XX); the length of coast-line is approximately 20 miles. The country rock of the district is entirely of hard, compact Lower Palaeozoic grits and slates, highly folded and faulted and considerably affected by glacial action, which tend to give a very bold and rugged shoreline. Exceptions occur in only one or two places; Ballyhornan Bay is occupied by a wide stretch of sand with few rock exposures; Dundrum Bay includes a very long stretch of sand with few outcrops of solid rock. The fact that a number of dykes are found in the few outcrops that are exposed in these covered stretches suggests that the solid rock concealed by the sand may contain many more. South of Dundrum Bay, the Palaeozoic grits and slates again rise above tide-level and the rocky coast-line is traversed by the numerous dykes of the Mourne swarm, which have already been described by the present authors (1935). In the same paper the geographical relations of the dyke swarms of the Tertiary Province of Northern Ireland have been discussed, and consequently need not be referred to here. The first survey of the dykes of this swarm was completed by Traill and Egan (1871), who gave a brief account of some of the dykes. No further attention appears to have been given to the

On some dykes in east Aberdeenshire

Within the bounds of the ‶Peterhead″ Granite, a mass some 53 square miles in extent, a number of dykes are exposed. With three exceptions, these occur on the coastal section, but, as the geological formations inland are greatly obscured by drift and alluvium, attempts to establish proof of the persistence or otherwise of the dykes have failed, unless in the case of one rock type which appears to stretch as a dyke from Buchan Ness, beyond the limits of the granite, to the village of Rhynie. Previous work on these intrusions is confined to notes on exposures of dolerite and quartz porphyry in the Memoir of the Geological Survey accompanying sheet 87 (1),1 records of dolerite in the Memoir of 76 (2), and descriptions, together with an analysis, of quartz-dolerite given in Memoirs 86 and 96 (3), the presence of most of the dykes not being mentioned. Twenty-five occurrences of dykes within the granite were examined. Two of these were quartz dolerites, nine felsites and quartz porphyries, and fourteen porphyrites. They all show chilled margins against the granite, the latter being apparently unaffected by the intrusions. Eight quartz-dolerite dykes outside the limits of the granite were also examined. The Quartz Dolerites. After examination of the two exposures of this rock within the granite other eight dolerite dykes, mapped by the Geological Survey between Rhynie and Buchan Ness, were investigated in the field and further dealt with as a series in the laboratory. Dr H. H. Read suggests, in the Huntly

XIV. Notes on the Intrusive Rocks of West Renfrewshire

Introduction. Throughout western Renfrewshire the rocks exposed are practically all of Old Red Sandstone or Carboniferous age. Sediments of Calciferous Sandstone age outcrop along the shore from Port-Glasgow to Wemyss Bay, and also to the south-west of Loch Thom, where they are underlain by Old Red Sandstone. A syncline with an approximately east and west axis occurs south of Greenock, but further to the south-west, Old Red Sandstone sediments are exposed in the middle of an anticline, the axis of which trends north-north-east and south-south-west. The remainder of the county is composed of the lavas and tuffs of Calciferous Sandstone age, together with a number of isolated intrusions and the volcanic vents of the Misty Law district. As the last named should more properly be considered in connection with the vents of north Ayrshire (Wilson, 1916) we do not propose to treat them in detail here. The intrusions may be divided into three groups. I. Basaltic Rocks of Calciferous Sandstone Age.—These comprise the sills of Lunderston Bay and Gryfe Castle, and a number of dykes in the Gourock and Kilmacolm district. The field evidence indicates that these are later than the sediments, while their petrological character assigns them to the Calciferous Sandstone volcanic episode. II. Felsitic Rocks of Calciferous Sandstone Age.—These include the sill at Craigmushet, a number of intrusions to the south-west of Loch Thom and a sill (?) near Kilmacolm. With the exception of the last, these are all intrusive into the sediments, but there is little doubt This 250-word extract was created in the absence of an abstract