Dyke Rocks Research Articles

Evolution of gold-bearing veins in dykes of the Woods Point dyke swarm, Victoria

The Woods Point dyke swarm comprises hundreds of narrow, subparallel igneous dykes and dozens of pipe-shaped dyke bulges within strongly deformed early Palaeozoic turbidites of the Melbourne trough. Porpylitic alteration accompanied dyke emplacement and was followed by microfracturing induced by high fluid pressures, involving CO2 of magmatic origin, as the dykes solidified. Further stress caused through-going faults having ladder and other patterns. Isotopic studies suggest that metamorphically or geothermally-derived solutions filled the faults and other fractures with quartz and carbonate and altered immediately adjacent dyke rock. However earlier-formed vein and wall rock carbonates retained their magmatic isotopic composition. Fluid inclusions indicate vein deposition began at approximately 400°C with salinities up to 9 weight percent NaCl. Nine sulfide minerals and gold were deposited in the veins after ankerite, sericite and albite, while quartz deposition continued through all stages. Sulfur isotopic determinations indicate the vein sulfur could not have been derived from adjacent sedimentary rocks, nor exclusively from the dykes. Metamorphic waters of marine origin is a viable source for sulfur. Saline and CO2-rich alkaline solutions reacted with the dyke wall rocks and probably evolved chemically prior to deposition of gold. Vug carbonates deposited by meteoric water that leached vein carbonates mark the end of vein formation.

Patapedia: an Appalachian calc-silicate-hosted copper prospect of porphyry affinity

The Patapedia prospect is in several bodies of calc-silicate hornfelses and subordinate marbles in which fracturing and metasomatism are associated with copper-bearing sulphide mineralization. Igneous rocks are restricted to a swarm of thin felsic dykes of calc-alkaline affinity.Prograde metamorphic isograds (anorthite–K-feldspar; tremolite–anorthite; sphene; diopside; and wollastonite) are telescoped into the marginal parts of each metamorphic body, and are estimated from phase equilibrium relationships to have developed at temperatures above 400 °C at XCO2 < 0.5. The prograde metamorphism, which was accompanied by some addition of potassium and a loss of calcium, is interpreted to have been caused by heat transferred from hydrothermal fluids.Garnet- and epidote-bearing assemblages are exclusively metasomatic and formed when conditions of XCO2 < 0.1 prevailed. Epidote-bearing assemblages developed at lower temperatures than those containing garnet, and are associated with the bulk of the copper mineralization. The metasomatic stage, including sulphide deposition, is considered from various lines of evidence (albitization of prograde plagioclase, local marialitic scapolite development, and NaCl-saturated fluid inclusions) to have proceeded from the interaction of the rocks with saline hydrothermal fluids.Dyke rocks show evidence of potassic, sericitic, and propylitic alteration. Those centrally located with respect to the metamorphism are potassically altered. Such areas are also better mineralized.The metasediments display patterns of 13C and 18O depletions that are due in part to the prograde metamorphism and in part to the interaction of the rocks with an isotopically lighter hydrothermal fluid of probable magmatic origin.A model is proposed for the evolution of the deposit and its host, which involves prograde metamorphism of calcareous sediments caused by heat transferred from a largely magmatic hydrothermal system. This fluid altered associated dyke rocks and, after calcite was consumed in the calc-silicate hornfelses, fractured and altered those rocks with concomitant precipitation of copper and iron sulphides. This model is similar to that commonly put forward for the development of porphyry copper deposits.

Youngest Precambrian dyke rocks in North Karelia, east Finland

Youngest Precambrian dyke rocks in North Karelia, east Finland

Nature and significance of beerbachites in the Ballantrae ophiolite, SW Scotland

ABSTRACTMetamorphosed abyssal ocean-floor tholeiitiic rocks, little affected by crustal contamination and probably formed at a spreading ridge, occur as a dyke complex in banded gabbros with oceanic affinities and as small tectonic lenses in serpentinised peridotite. Igneous textures and mineral assemblages have been partly or completely replaced in both dykes and gabbros. Metamorphic development of clinopyroxene and very Ti-rich hornblende at 900-1000°C was followed by a low temperature actinolite-chlorite assemblage and then by pectolite-bearing veins. Geochemical variations resulting from magmatic and metasomatic processes have been distinguished and plots of major and trace elements used to establish tectonic environment. The very high temperature metamorphic mineral growth associated with beerbachite formation indicates that the dyke rocks were in a high heat-flow environment for a considerable time, presumably in the vicinity of a spreading ridge. Obduction of a very hot slab of oceanic lithosphere accounts for the superimposed mineral assemblages.Spatially associated pillow lavas have been affected by crustal contamination and are not comagmatic with the dykes. There are petrochemical characters indicating that some of the lavas are ocean-floor basalts. For other lavas, features suggesting emplacement in a continental or an island-arc environment could also be consistent with development in a marginal basin. However, there is so much variation in composition of pillows from margin to centre, from small to large and from metasomatic activity, that their compositional fields cannot be used for discrimination of basaltic type with the same confidence as for the dykes. Accordingly, it is suggested that the petrochemistry of sheeted dyke complexes, rather than of pillow lavas, be used for this purpose in the study of ophiolites.

Granitic xenoliths in dyke rocks of the Setamai district, southern KitakamiMountains, Northeast Japan

Granitic xenoliths in dyke rocks of the Setamai district, southern KitakamiMountains, Northeast Japan

Distribution of REE and other trace elements between phenocrysts and peralkaline undersaturated magmas, exemplified by rocks from the Gardar igneous province, south Greenland

Partition coefficients for iron-rich olivine and pyroxene, sanidine, nepheline and apatite are reported from peralkaline trachytic to phonolitic dyke rocks and the agpaitic Ilímaussaq intrusion. Partition coefficients for many elements in olivine and pyroxene decrease with increasing peralkalinity and undersaturation of the magma, i.e. with decreasing polymerisation. The REE partition coefficients for olivine and pyroxene also show dependence on the mineral chemistry, i.e. the iron content. Probably due to the larger lattice sites in the iron end-members the heavy REEs enter the small six-coordinated lattice sites with increasing ease as the iron content of the mineral increases. La and Ce partition coefficients for apatite increase with increasing peralkalinity; this condition seems to stabilise a Na-REE-phosphate component in the mineral.

Differentiated dyke rocks of Mer Mundwara, Rajasthan and their metallogenic significance

SummaryThe differentiated dyke rocks of Mer Mundwara comprise a subalkaline series which includes basalts and dolerites, and an alkaline series which includes theralites, porphyritic alkali syenites and nepheline syenites. These intrude into the coarse grained Precambrian granites which, in their turn, are subjacent bodies in the Aravalli system of metamorphic complex of archaean age. Absolute age of basic volcanics is 56±8 Ma, thus relating their emplacement to Deccan Trap volcanism in the Cretaceo-Eocene times in Central and Western India. Petrochemical characteristics of the basic rocks exemplify an alkaline character and so indicate that the basaltic magma of Mer Mundwara is tectonically and genetically different from the Deccan basalts which are in the main tholeiitic. The geological setting, petrography and major and minor element chemistry (Na > K and abundance of Nb, Co, Sn and V) relegate the dyke rock suite to the alkaligabbroic formations of Borodin (1974) with their parent magmatic source in the upper mantle. The alkaline igneous rocks of Mer Mundwara strongly suggest a possible occurrence of rare earths, niobium, zirconium and fluorine in industrial concentrations and also thorium and phosphorus.

Palaeomagnetic data and radiometric ages from the Cretaceous of West Kalimantan (Borneo), and their significance in interpreting regional structure

Potassium-argon ages of biotite and hornblende from sixteen samples of mainly granitic plutonic rocks from West Kalimantan indicate a complex history of magmatism and cooling from the Middle Jurassic to late Cretaceous, with most ages in the range 75 to 112 Ma. Samples from the Schwaner Zone in the northern part of the area give measured ages ranging from 76 to 116 Ma. In the southern region most samples are in the range 79 to 86 Ma, but two are older (127 Ma, early Cretaceous, and 153 Ma, Middle Jurassic). In both areas, concordant ages from biotite and hornblende indicate a well-defined event at about 80 to 85 Ma, which has also been recognized to the north in Sarawak, on islands of the South China Sea, in the Malay Peninsula, south Sumatra, and in the Java Sea. Palaeomagnetic measurements on thirty-nine samples of dyke rocks, lavas, tuffs, granites, hornfels and metatuff, all believed to be late Cretaceous, yield a mean palaeomagnetic pole at 21°E 41°N, and a palaeolatitude for West Kalimantan of 0°. The palaeomagnetic pole does not differ significantly from the Cretaceous pole estimated for the Malay Peninsula. It is concluded that since the Middle Cretaceous West Kalimantan and the Malay Peninsula have behaved as a unit, have remained in much their present latitude, but have rotated anticlockwise about 50 .

Lithium in dyke rocks of a plutonic sequence in the Bohemian Massif (Czechoslovakia and partly in Austria)

The Li content in 129 samples of dyke rocks derived from three different areas of the Bohemian Massis (Bohemian-Moravian Heights and Waldviertel, Eastern Sudeten, central Bohemia) has been determined. The dyke rocks investigated are partly genetically related to the plutonic rocks of the area and their Li content is identical to that of the comparable plutonic rocks, and they belong partly to the lamprophyre series which is in no direct genetic relation with the plutonic rocks. The Li contents of these dykes are elevated. In the calcalkaline series of the dyke rocks the Li content and the Li/Mg ratio increase with progressing differentiation, whereas in the differentiation series of mica lamprophyres, which has an alkalic trend, the Li content and Li/Mg ratio decrease. The Li content of the dykes is controlled by the Mg content of the rock, by its acidity, its content of pneumatolytic components (the grain size), the chemical composition of the constituent micas, and by the F content of the rock.

Early Tertiary flood basalts from Hareøen and western Nûgssuaq, West Greenland

In central West Greenland flood basalts were erupted in the Early Tertiary. On Hareøen and western Nûgssuaq the upper part of the volcanic sequence predominates reaching a thickness of more than 3 km. In these two areas tholeiitic, olivine porphyritic basalts and picritic basalts are followed by tholeiitic basalts with plagioclase as the dominant phenocryst. The content of incompatible elements in the plagioclase porphyritic basalts is low in the early stages, as is the case in most of the older olivine porphyritic lavas in the province; however, after a pause in the volcanic activity, the concentration of these elements is increased by a factor of 3-6. A few thin layers of peralkaline, rhyolitic tuffs are found in the upper part of the plagioclase porphyritic sequence. The tholeiitic lavas are intruded by dykes. Most of these are petrographically the equivalent of the extrusive rocks, but dykes of transitional olivine basalts are presumed to represent a younger magmatic episode from which no lavas are known. The youngest igneous rocks in the area – and perhaps the youngest lavas in all of the West Greenland basalt province – are olivine porphyritic, alumina enriched transitional basalts deposited unconformably on the tholeiitic sequence on Hareøen. Forty-three new chemical analyses of the magmatic rocks are presented as well as micro-probe data on phenocryst and groundmass phases. The diversity of basaltic rocks cannot be explained by low pressure crystal fractionation alone, and it is suggested that the chemical evolution of the lavas and dyke rocks reflect chemical variations in batches of olivine rich magmas injected into the crust from the mantle.

The Ngatutura Volcanics, Southwest Auckland

Abstract Basaltic volcanism occurred in three centres at Ohuka, southwest Auckland, in response to late Pliocene-early Pleistocene fault movements. Eruption took place at, or close to, sea level, and volcanoclastic deposits at each centre include agglomerates, sub-aqueous breccias of terrestrial origin, and laharic breccias. Associated flow and dyke rocks are well-jointed, fine-grained alkali olivine basalts which display little variation in their petrographic, mineralogical, and che:nical characters.

Magnetite-serpentine-calcite dykes at Premier Mine and aspects of their relationship to kimberlite and to carbonatite of alkalic carbonatite complexes

Magnetite-serpentine-calcite rocks at Premier Mine are shown to be igneous dykes which, in some cases, have caused extensive carbonatization of country rock kimberlite. Previous theories of a metasomatic origin for the dykes have probably resulted from studies of carbonatized kimberlite and not of the dykes themselves. The magnetite-serpentine-calcite rock is considered to represent a residual, vapour-rich liquid which separated from crystallizing kimberlite and was subsequently intruded as a separate phase. Though qualifying as carbonatite, the dyke rocks differ radically from typical carbonatite of alkalic carbonatite complexes in that serpentine is an interstitial phase, and in the manner in which they have metasomatized their wall rocks. Thus, no direct relationship toward carbonatite of alkalic carbonatite complexes can be proved.

Phosphorus in lamprophyre and lamproid rocks

The P2O5-content of 300 samples of lamprophyre, lamproid and some acid dyke rocks from three areas of the Bohemian Massif has been determined colorimetrically. The values established range from 0.02% in microgranite to 2.38% in an alkaline minette. A negative correlation exists between P2O5 and SiO2, and Al2O3, respectively, a positive one between P2O5 and TiO2, CaO and F, respectively. In the alkaline dykes most of the calcium is found in apatite. The main factor controlling the distribution of P2O5 in the rocks studied is their saturation with aluminium. Theoretically, this may be explained according to Shcherbina's (1966) acidity concept. If the chemical composition of a dyke remains approximately constant, the same holds good also for phosphorus, in spite of considerable differences in the mode. In the course of the differentiation of the kersantite type magma, P2O5 increases at first in agreement with the Al2O3 saturation of the magma, but falls off in the final stages. Due to local factors, there exist considerable differences between the P2O5- contents of the individual rock types of various areas.

22. On an alkalic dyke rock in Tanegashima Is., south Kyushu, Japan(Abstracts of Papers submitted for the Meetings of the Society)

22. On an alkalic dyke rock in Tanegashima Is., south Kyushu, Japan(Abstracts of Papers submitted for the Meetings of the Society)

Petrology of the dyke rocks of the western portions of Rajpipla hills, Broach district, Gujarat, India

The paper embodies the field, petrographic and petrochemical studies of the dykes occuring within the Deccan basalts, in the Western portions of Rajpipla hills. Major and minor dykes with different trends occur in the area varying in thickness from 2′ to 75′ and traceable lengthwise from few to several miles. The density of the dyke distribution is two per mile.

Chemical variation within three alkaline complexes in South Greenland

Comparison is made between three hypersolvus alkaline complexes of Precambrian age occuring in the same igneous province. Two represent the cores of deeply eroded central volcanoes; the third comprises a suite of trachytic and rhyolitic dykes. In each complex a similar suite of trachytic and comenditic magmas was intruded and the gross mineralogy and chemistry are closely comparable. However, such differences as exist are explicable in terms of variation in cooling rates, controlling the degree to which the rocks represent cumulates and the extent to which excess alkalis in liquid residua were lost to the atmosphere and wall-rocks. In the Kûngnât Fjeld complex where cooling was slowest, alkali loss is inferred to have been greatest and the bulk of the constituent syenites are mildly peraluminous in consequence. In the dyke complex cooling was rapid and alkali loss less severe; the majority of the trachyte/rhyolite dyke rocks are mildly peralkalic. The central ring-complex of Tugtutôq represents an intermediate situation in relation to Kûngnât Fjeld and the dyke complex.

On the rock name beerbachite

SUMMARYIt is suggested that the name beerbachite be reserved for fine- to medium-grained granular dyke rocks of gabbroic composition occurring in coarser-grained gabbro. It should not be used for basic granular hornfelses or xenoliths.

Fluorine in lamprophyre and lamproid rocks

The fluorine content has been determined for 80 igneous dyke rocks (mostly lamprophyre and lamproid rocks; rarely alkaline and strongly acid ones) from two areas at the eastern margin of the Bohemian Massif. The main carriers of fluorine are biotite and amphibole. The highest concentrations of fluorine are found in alkaline and acid dykes. The fluorine content of dykes is higher than the average of other igneous rocks of similar composition.

Petrological studies of some alkalic and peralkalic dyke rocks from the Tugtutôq–Narssaq area

The purpose of current research work on the dyke rocks and, to a lesser extent, the major Gardar intrusions of Tugtutôq and adjacent areas, is to elucidate more fully the development of the peralkine comenditic magrnas. The dyke swarm through the Tugtutoq region is instructive as it displays an apparently continuous series of rock-types from relatively basic microsyenites (or trachytes) to peralkalic microgranites (or comendites).

Plutonic development of the Ilordleq area, South Greenland. Part I: Chronology, and the occurrence and recognition of metamorphosed basic dykes

Evidence is presented which shows the rocks of Ilordleq to be the products of two distinct episodes of plutonic (Read, 1957, p.165) activity. In the first of these episodes the mise-en-place of a large area of synkinematic granitic rocks was affected. A NE striking belt of pre-granite supracrustal rocks is preserved in the north of the area, and isolated remnants of the supracrustal rocks remain as enclaves in the granite. Cutting the metasediments and metavolcanics of the supracrustal group are numerous basic dykes which pre-date the granite. The first plutonic episode was followed by a further period of emplacement of basic dykes, which cut both supracrustal and granitic rocks. Indications of original chilled margins in these dykes, together with their regular form, suggest a marked change in conditions from those prevailing in the preceding plutonic episode. The second period of plutonic activity which followed resulted in a reactivation of much of the granite previously formed, especially in the northern part of the area where basic dykes were deformed and became either disrupted or were migmatised by kinematically controlled metasomatism. The surrounding granite shows evidence of flow in the solid state and was evidently approaching a state in which diapiric movement was possible. In a less reactivated part of the area, porphyroblastic microcline augen granites were formed in those places where the supracrustal enclaves were most strongly deformed. Deformation during each plutonic episode took place by homogeneous strain with resulting change in shape rather than tectonic transport; minor structures in supracrustal, granitic and dyke rocks are homologous. Principal stress axes were similarly oriented during both plutonic episodes. A further period of emplacement of basic dykes took place in the closing stages of the second period of plutonic activity, while the country rocks were still at elevated temperatures and during the operation, at least intermittently, of compressive forces. A conjugate set of shear fractures controlled the emplacement of these dykes; movement along the fractures and emplacement of dykes were essentially contemporaneous and gave rise to 'differentiation by deformation' of the partially consolidated dyke magma, sigmoid foliation of the dykes, and other distinctive features. Further swarms of basic dykes were subsequently emplaced during the Gardar volcanic episode (ca. 1150 x 106 years). Reference is made to published criteria for the recognition of relict dykes, replacement dykes, and dykes emplaced in hot granite, and the possibility of confusion with dykes in reactivated granites is indicated. The suitability of the orogenic cycle as the primary unit of Precambrian chronology is questioned and a less interpretative unit advocated. The Ilordleq chronology is compared with those established by the writer's colleagues elsewhere in South Greenland and possible alternatives suggested to some current correlations. The similarity of many features of the Ilordleq rocks to those of the Lewisian rocks of Scotland is referred to, and the concept of a migrating front of plutonic activity shown to be not applicable to Ilordleq.