Acid Volcanics Research Articles

Refined Proterozoic evolution of the Gawler Craton, South Australia, through U-Pb zircon geochronology

Through the application of both conventional U-Pb zircon analyses and small-sample U-Pb isotopic analyses, the nature and timing of tectonic events leading to the formation of the Gawler Craton have been defined more precisely. Constraints on deposition of Early Proterozoic iron formation-bearing sediments have been narrowed down to the period 1960-1847 Ma. Deformed acid volcanics, including the economically important Moonta Porphyry, have zircon ages of ∼ 1790 and 1740 Ma. The voluminous acid Gawler Range Volcanics and correlatives to the east were erupted over a short interval at 1592 ± 2 Ma, and were intruded by anorogenic granites at ∼ 1575 Ma. Small-sample zircon analyses proved to be an extremely valuable adjunct to conventional analyses, generally yielding more-concordant data which forced a curved discordia through an upper intercept slightly younger than from a conventional straight-line discordia.

Sedimentation patterns of the Proterozoic Delhi Supergroup, northeastern Rajasthan, India, and their tectonic implications

The Proterozoic Delhi Supergroup occupies three NE-SW trending linear belts in the northeastern part of the Aravalli mountain range, where its 10 km thick volcano-sedimentary sequence is classified into unconformity-based three groups: the Raialo, the Alwar, and the Ajabgarh. The palaeocurrent patterns and palaeoenvironmental setting of these rocks indicate multiple palaeoslopes and locate the provenance to the east and south. The interrelationship of sedimentary facies and palaeoenvironments of the Delhi Supergroup can best be explained by an evolving intracratonic rift-basin model, comparable to several well-studied continental rift basins. The initial down-sag produced a N-S trending basin which received thick carbonate facies of the lower Raialo Group. Associated with early rifting, conglomerate-sandstone facies association of the middle Raialo developed in isolated grabens. The oligomict conglomerate was deposited along the border faults in the wave and tidal reworking zones and graded to beach sandstone towards the basin centre. Continued extension across the basin axis resulted in outpouring of basic to acid volcanics of the upper Raialo. Renewed rifting and associated major vertical tectonism produced some new grabens and caused wider submergence at the onset of the Alwar sedimentation. Coarse clastics of fluvial origin developed along fault scarps in the proximal southern parts as 100–2800 m thick conglomerate bodies grading to marginal marine sandstones northwards and towards the graben centres. Subsequently, the intervening highlands between grabens were gradually exposed. Another sea-level rise under the influence of a still wider downwarping resulted in submergence of all the interbasin highlands and unification of all the smaller basins into a wider basin, wherein the carbonate and psammo-pelitic assemblage of the Ajabgarh Group was deposited on multi-lagoonal tidal flats. Sedimentation was intermittently interrupted by basin-wide subaqueous volcanism.

U–Pb ages of early and middle Proterozoic volcanism and metamorphism in the Makkovik Orogen, Labrador

Zircon U–Pb dating on rhyolites from three different localities in the upper Aillik group, a volcano-sedimentary sequence of the Makkovik Orogen in Labrador, indicates two distinct pulses of volcanic activity at about 1860 and 1807 Ma. The individual rhyolite ages of [Formula: see text], 1856 ± 2, and 1807 ± 3 Ma are 40–150 Ma older than Rb–Sr ages determined previously on the same rocks. A slight scatter of zircon data observed for one of the rhyolites and the presence of old zircon cores suggest that the felsic magmas were derived in part from older continental material. Analyses of monazite and titanite from two anatectic rocks, a migmatite and leucogranite from a gneiss terrain adjacent to the supracrustals, document a period of high-grade metamorphism in the interval 1794 ± 2 to 1761 ± 2 Ma. This period is considered the terminal phase of the Makkovikian Orogeny. Zircons in the migmatite yield minimum ages ranging from about 1800 to 2340 Ma, indicating a derivation from latest Archean to early Proterozoic basement rocks.Zircon dating on a rhyolite from the Bruce River Group within the ~1650 Ma old Trans-Labrador batholith yields an age of 1649 ± 1 Ma. This age suggests that Bruce River acid volcanism was related to Trans-Labrador batholith emplacement.

Sedimentology of the Westmoreland Conglomerate, southern McArthur Basin, Northern Territory, Australia

The Westmoreland Conglomerate is the oldest formation of the Middle Proterozoic Tawallah Group, the basal group of the McArthur Basin sequence. It unconformably overlies Early Proterozoic granite, acid volcanics and metamorphics, and comprises up to 1800 m of coarse, immature sandstone and conglomerate. The Westmoreland Conglomerate is divided into five fining-upward sequences deposited in a braided river/alluvial fan environment. The main source of sediments was located in the northeast; the sediment-bearing rivers originated in the rising highlands of the northward extension of the Mt Isa Orogen. Their course was controlled by two east-trending palaeohighs, separated by about 20 km of lowlands. The southern high is represented by the Murphy Inlier and the northern high by a belt of east-trending gravity lows interpreted as granitic basement.

Sedimentology of the Westmoreland Conglomerate, southern McArthur Basin, Northern Territory, Australia

The Westmoreland Conglomerate is the oldest formation of the Middle Proterozoic Tawallah Group, the basal group of the McArthur Basin sequence. It unconformably overlies Early Proterozoic granite, acid volcanics and metamorphics, and comprises up to 1800 m of coarse, immature sandstone and conglomerate. The Westmoreland Conglomerate is divided into five fining-upward sequences deposited in a braided river/alluvial fan environment. The main source of sediments was located in the northeast; the sediment-bearing rivers originated in the rising highlands of the northward extension of the Mt Isa Orogen. Their course was controlled by two east-trending palaeohighs, separated by about 20 km of lowlands. The southern high is represented by the Murphy Inlier and the northern high by a belt of east-trending gravity lows interpreted as granitic basement.

Geochemical stratigraphy of the Paraná Continental Flood Basalts: borehole evidence

Stratigraphic studies can provide an insight into the poorly understood geodynamic processes for the generation and emplacement of large continental flood basalt provinces by revealing the structure and sequential evolution of the lava field. Such studies are still in their infancy in the Paraná, where lack of sufficient topographic relief has confined work mainly to the well-exposed coastal escarpment in south-east Brazil. In this paper, a new approach is taken by analysing drillcore chippings from four oil-company boreholes in the central Paraná basin. Major and trace element analyses distinguish three geochemical units by their low, intermediate, and high Ti0 2 contents respectively, and demonstrate the validity of geochemical data from such material. Each borehole retains a coherent stratigraphy with the transition from low-Ti to high-Ti then intermediate-Ti lavas with time, and together they show an overlapping sequence of geochemical units dipping northwards, which could suggest a northward migrating source for the Paraná magmatism in this central-north region. The available data on geochemical stratigraphy of the Paraná, is summarised in a north-south cross-section through the lava field that shows the salient features of the Paraná stratigraphy; the stacking of units in the northern boreholes; the complex transition between low- Ti and high-Ti magma types in the coastal profiles of Santa Catarina state; and the concentration of acidic volcanics along the present day continental margin.

Seismic anisotropy in mylonites: an example from the Mannin Thrust Zone, southwest Connemara, Ireland

Mylonites associated with the Mannin Thrust zone of southwesternmost Connemara formed when the high-grade metamorphic rocks typical of most of the Connemara massif were thrust to the southeast over low metamorphic grade (low greenschist facies?) acid volcanics and volcaniclastic sediments, while being metamorphosed in the epidote-amphibolite facies. Triaxial and biaxial ultrasonic velocity measurements of mylonite specimens from a 240 m borehole have established that there is significant seismic anisotropy up to about 11% when comparing velocities perpendicular and parallel to the foliation. This would ultimately lead to a reflection coefficient of about 0.02 when comparing the mean “isotropic” seismic velocity with that perpendicular to the foliation. The finely striped, discontinunous mineral lithons that define mylonitic foliation, but which form no real and continuous surfaces, could interact with seismic energy to produce “reflections” that do not relate to lithological contacts within the rocks but to a tectonically induced, orientated acoustic impedance. However, the results support the work of others in suggesting that on its own the fabric would not produce the high amplitude reflections observed on deep seismic lines and other mechanisms need to be investigated.

Tourmaline-rich Rocks Associated with the Submarine Hydrothermal Rosebery Zn-Pb-Cu-Ag-Au Deposit and Granites in Western Tasmania, Australia

The stratiform massive Zn-Pb sulphide Rosebery deposit of western Tasmania is hosted by metamorphosed deformed acid volcanics and sediments of the Cambrian Mt. Read Volcanics. Tourmalinite, a boron-rich siliceous sulphide facies iron formation, overlies and occurs as an exhalite facies equivalent of the massive sulphides. The orebody is partially replaced by post deformation tourmaline-bearing pyrrhotite-pyrite rocks associated with an alteration facies comprising magnetite-pyrite-tourmaline-phlogopite and the host metavolcanics are transgressed by quartz-tourmaline veins and tourmaline-filled joints. Tourmalinite and tourmaline in alteration zones are associated with other base metal deposits in the area. Tourmaline also occurs as fault-fill and in granitic rocks and associated Sn-W mineralization nearby. Tourmaline associated with the Cambrian massive sulphides is schorl > dravite in contrast to schorl in the Devonian granites.

Geothermal activity in Turkey

Turkey is located on the Alpine tectonic belt with many grabens, acidic volcanism, hydrothermal alteration zones, numerous hot springs and fumaroles. The data gathered indicate that Turkey has a high geothermal energy potential. Geothermal research began in the 1960s implemented by the Mineral Research and Exploration General Directorate (MTA). Exploration began at Denizli-Kizildere in 1968. Further studies revealed the field to be commercially exploitable and a geothermal power-plant with a capacity of 20 MW started electricity generation in 1984. Currently there are numerous fields being explored and developed for electrical and non-electrical uses.

Stream sediment geochemistry of Guberunde Horst, “Gongola basin”, Upper Benue Trough, Nigeria

The crystalline basement complex of Guberunde Horst is mainly composed of granitic rocks, into which acid volcanics of the Burashika group of rocks were intruded during the Jurassic. The Horst itself was produced in the Lower Albian by uplift of the basement between two series of NE-SW striking border faults. Most of the fractures which pre-dated the uplift are filled with rhyolite and aplite of the Burashika group. A total of 374 stream sediment samples were collected over an area of 700 km 2. Each sample was analysed for Cu, Pb, Zn, Co, Ni, Fe, Mg, Mn and Ca; in addition, the pH was determined in each. Condescriptive and various multivariate statistical methods were employed in the interpretation of the resultant geochemical data. The various association of elements exhibits a four-factor model as follows:

The nature and field relations of the Ordovician Maumtrasna Formation, County Mayo, Ireland

AbstractThe nature and field relationships of the Maumtrasna Formation of the Partry Mountains, Co. Mayo, show that the formation underlies the regionally mappable Glenummera Formation, which is of Llanvirn age, and it is laterally equivalent to the Rosroe and Derrylea Formations, also of Llanvirn age. The formation rests unconformably on two different volcanic domains, the basic volcanics of the Bohaun Volcanic Formation to the northwest (of unknown age) and the largely acidic volcanics and sediments of the Tourmakeady Volcanic succession to the southeast (Arenig). It is not the youngest formation in the Ordovician succession of the South Mayo Trough, as was previously thought.This largely conglomeratic unit forms a coarsening upward sequence deposited on alluvial fans which were inundated by a marine transgression at the base of the Glenummera Formation. Clasts in the conglomerates are mainly of granite and porphyry with a minor metamorphic component. The source is interpreted as an arc rooted on a metamorphic basement which lay to the south and east. The suggestion that this area was present day Connemara finds little support. This area may expose some of the ‘hidden’ geology of the Midland Valley of Scotland.

Geochemistry of the felsic metavolcanic rocks of the Wakeham Group: a metamorphosed peralkaline suite from the eastern Grenville Province, Quebec, Canada

The Wakeham Group is a suite of low-grade (greenschist facies) rocks located in the eastern Grenville Province. In order of volumetric importance, it consists of sandstones, acid volcanics, and gabbro dykes and sills. This report deals with the acid volcanic member of the group. Primary textural features have in large part been preserved. The minerals present include stilpnomelane, white mica, albite, titanite, and zircon. Calculations using 33 chemical analyses show that 29 of the compositions have normative corundum. The rocks would therefore appear to be predominantly peraluminous; however, incompatible trace-element data show affinities with other anorogenic, peralkaline suites. It is proposed that alkali loss, probably during the subsequent Grenvillian orogeny, imposed a peraluminous composition on originally peralkaline rocks. The Wakeham Group cannot be definitively correlated with other metavolcanic rocks found in the region.

UPb zircon dating of volcanic rocks, close to the Silurian-Devonian boundary, from Vendée (western France)

The detrital volcanic horizons in Vendée, called “porphyroïdes” correspond to an important period or rhyolitic volcanism in the Armorican part of the Western European Hercynian Belt. Their stratigraphical assignment is still debated. Using the UPb zircon dating method, acid volcanic and volcanic clastic rocks from the Mareuil-sur-Lay Formation in Vendée are found to be 405 ± 5 Ma old. Interbedded black cherts yield tubular elements of unknown organic origin, up to now only previously recorded in the Upper Silurian and the Early Devonian of Bohemia (Czechoslovakia) and Artois (northern France). On the other hand, closely related carbonaceous remains have been discovered in other south Armorican “porphyroödes” formations. Therefore, the age of 405 ± 5 Ma is an additional datum for the understanding of the calibration of the Silurian-Devonian boundary. Thereby, there exist at least two major episodes of acidic volcanism with porphyritic characteristics. One, well represented in the Iberian Peninsula (Ollo de Sapo), is situated at the base of the Paleozoic; it results from a period of crustal melting at the end of the Ca-domian Orogeny. The other, of Siluro-Devonian age, is studied in this paper. It has a calc-alkaline trend and is marked by ancient crustal contamination. It is linked to an acidic volcanism related to an active continental margin, contemporaneous with eo-Hercynian metamorphic events. Les “porphyroïdes” de Vendée appartiennent à un important épisode de volcanisme rhyolitique, connu dans le Massif Armoricain et dans d'autres parties de la Chaîne Hercynienne d'Europe occidentale. L'attribution stratigraphique de ces volcanites et volcanoclastites a été l'objet de nombreuses controverses. Deux méthodes différentes ont été utilisées ici pour tenter de résoudre ce problème de datation. Ainsi, en Vendée, le matériel rhyolitique de la Formation de Mareuil-sur-Lay a fourni un âge UPb sur zircons de 405 ± 5 Ma. Des niveaux phtanitiques, interstratifiés dans cette formation, ont d'autre part livré des micro-restes charbonneux, d'affinité incertaine, mais, pour l'instant, uniquement cités dans le Silurien supérieur et le Dévonien inférieur de Tchécoslovaquie et du Nord de la France. Des éléments organiques similaires existen également dans des niveaux phtanitiques associés aux “porphyroïdes” de Bretagne méridionale. L'âge de 405 ± 5 Ma obtenu sur les zircons de Mareuil-sur-Lay vient compléter d'autres datations concernant le calibrage de la limite Silurien-Dévonien. En Europe de l'Ouest, deux épisodes majeurs de volcanisme acide à caractère porphyrique peuvent au moins être dénombrés. L'un (“Ollo de Sapo”), représenté en Péninsule Ibérique, se place à la base du Paléozoique et résulte d'une fusion crustale fini-cadomienne. L'autre, étudié dans ce travail, se situe près de la limite Silurien-Dévonien. Il montre une tendance calco-alcaline et porte la trace d'une contamination crustale ancienne. Ce volcanisme acide parait liéà l'existence d'une marge continentale active, contemporaine d'événements métamorphiques éohercyniens.

The preservation and palaeoecology of the Lower Carboniferoussilicified plant deposits at Esnost, near Autun, France

The mineralogy and preservation history of the anatomically preserved plants (permineralizations) of Lower Carboniferous age (Upper Visean) from Esnost, near Autun, France is described. The plants are all preserved in silica and are associated with acid volcanism. The permineralized plants occur within a range of different sediments each of which is characterized by a specific floral association and has a distinct mineralization. The structure of the mineralization is dependent on whether fossilization occurred within a site of growth of a plant or at a site of deposition of plant fragments. The timing of mineralization, whether early or late, also influenced the form of the crystals. The most common plant-bearing lithology at Esnost is the Archaeocalamites «peat. This lithology contains fine roots which are interpreted as the terminal roots of Archaeocalamites. These were important in providing channels through the semi-compacted peat for the permineralizing siliceous fluids. The Botryopteris cherts, dominated by one species of coenopterid fern, were fossilized at a much earlier stage giving exceptionally good preservation of the anatomical detail. The third most important plant-bearing sediment is the Diplolabis chert. This represents mineralization at the site of deposition of fragments of this fern. Within this site, mineralization was slow and episodic involving the considerable loss of organic material from the plant organs, resulting in some cases in a mineralization approaching petrifaction. Fusain was rare in the deposits but occurred within the Diplolabis cherts and the fusain-bearing chert where it is associated with a series of microorganisms of unknown affinities. The relationship of the sediments, plants and mineralization offers a basis for reconstructing the original ecology of the plants within the volcanic terrain. These plant deposits are compared to those of the same age, associated with basaltic volcanism, at Pettycur, Fife, Scotland.

Contents, volume 26, 1985

The late Proterozoic Malani bimodal volcanics constitute the largest suite of anorogenic acid volcanics in India. The volcanism took place during 745±10 Ma ago, succeeding the granitic activity of Abu pluton and ceased before the onset of Marwar sedimentation.On the basis of field evidences, three stages of igneous activity have been recognised. Volcanics of the first stage are mostly basalt with occasional andesite or trachybasalts. These are subsequently covered by the voluminous outpouring of peralkaline and peraluminous rhyolite, basalt, dacite and trachyte flows. The third stage ceased with the outburst of ash flow deposits.The dominant felsic volcanics are rhyolites and rhyodacites spread over an area of about 31, 000 km2. The other rock types associated with rhyolite are trachytes, dacites, pitchstone, welded tuff, vitric, lithic and crystal ash, ignimbrite, obsidian, pyroclastic slates, agglomerate, volcanic breccia and volcanic conglomerates. Majority of the acid volcanics are high potassic and a few are calcalkaline or low potassic in composition.Feldspar geothermometry suggests the temperature of equilibrium to be above 650°C. Similar results were obtained by magnetite-ulvospinel geothermometry. Oxygen fugacity is estimated to be about 10−18 under FMQ-Ni-NiO buffer conditions.Malani volcanism was essentially under terrestrial conditions, although deposition by aqueous conditions are also indicated. The volcanic eruptions have been through fissures, shield volcanoes and central cones. The volcanism was triggered in an extensional tectonic regime of continental crust, where geotherm was raised by the repeated influx of basic magma. The initial basaltic magma was possibly generated at deeper depth by ‘hot spot’ activity. This magma while migrating upwards supplied additional heat for the partial melting of lower sialic crust resulting in the generation of felsic magma. The crustal extension has helped in the upward advancement of the felsic magma.

Gawler Range magmatism—Further isotopic age data

Abstract Zircon U‐Pb age determinations have been made on Gawler Range type acid volcanics from Saint Francis Island, Great Australian Bight (1631±3 Ma), and the crosscutting Hiltaba Granite of the Gawler Ranges (1514± 32 24 Ma). The time of Gawler Range Volcanism is extended back by at least 100 Ma (140 Ma at St Francis Island) from earlier measurements, and now approaches that of volcanism in the Broken Hill, Willyama Sequence. Rb‐Sr total rock isochrons on such material give post‐crystallization ages, and initial 87Sr/86Sr ratios obtained from non‐perfect isochrons are misleading petrogenetic indicators. The St Francis Island acid volcanics appear to be I‐type magmas whereas associated, mildly alkaline granitoids show some A‐type character.

GEOQUÍMICA ISOTÓPICA EM VULCÂNICAS ÁCIDAS DA BACIA DO PARANÁ, E IMPLICAÇÕES GENÉTICAS ASSOCIADAS

Three samples representatives of the acidic volcanism ot the Parana Basin, collected in lhe States of PR, SC and RS, were dated by Rb-Sr and K-Ar methods. Cbemical analysis classify the three occurrences as belonging to the group enriched in phosphorus, titanium and LIL elements (HPT), oompared to the other (LPT) defined for the Parana Basin. Mineral isochrons show the cogeneticity of the components, and the resultant Rb-Sr age is systematically higher than the value obtained by K~Ar method in the plagioclase phenocrysts. Sr initial ratio indicates a common source for the three occurrences.

Sm-Nd and U-Pb dating of Early Proterozoic mafic-felsic volcanism in Northernmost Sweden

A Sm-Nd mineral isochron of 1932 ± 45 Ma on a Kiruna greenstone is defined by greenschist facies metamorphic minerals, apparently older than the regional Svecokarelian metamorphism (< 1880 Ma). However, this age agrees within error with U-Pb zircon ages of 1909 ± 16 17 Ma on quartz-bearing prophyries which overlie but are, in part, intercalated with basic volcanics. If these basic volcanics, following previous studies, are considered to be contemporary with the Kiruna greenstones, the present data suggest that eruption of both basic and acid volcanics and associated hydrothermal metamorphism took place over a short time interval close to 1.9 Ga. The greenstone sample is depleted in light rare-earths and has high initial 145Nd/ 144Nd indicating a source in which such depletion has existed for a long time. In contrast, an albite diabase sample with a somewhat younger mineral age (1.80 ± 0.08 Ga) is light rare earth enriched and has a correspondingly low initial Nd isotopic composition.

The origin of the early Proterozoic supracrustal rocks in the Garpenberg district, south central Sweden

Abstract The early Proterozoic supracrustal sequence in the Garpenberg district, south central Sweden, consists mainly of quartz-feldspar gneissses (‘leptites’) and subordinate amphibolites, quartz gneisses, mica schists, skarns and marbles. Stratabound iron and sulfide deposits are intercalated within this sequence. Most of the quartz-feldspar gneisses and amphibolites are interpreted as acid and basic metavolcanic rocks. The quartz gneisses and mica schists represent the metamorphic equivalents of originally altered felsic volcanic and sedimentary rocks. The geologic development of the Garpenberg district was dominated by extensive acid volcanism of calc-alkaline affinity, which was interrupted by periods of intense hydrothermal activity, ore deposition and sedimentation. Most of the volcanic activity occurred was submarine in a shallow-water environment. Subsequently, the supra crustal sequence was affected by the Svecokarelian orogeny (1800–1750 Ma) and the rocks were metamorphosed in the amphibolite ...

Archaean calc-alkaline volcanism in the Pilbara Block, Western Australia

Archaean calc-alkaline volcanics occur in two distinct settings in the Pilbara Block. Within the ca. 3500 Ma old Warrawoona Group, sequences of intermediate to acid volcanics are interlayered with an extensive sequence of komatiites and tholeiitic basalts. In the western Pilbara, the Whim Creek Group, a younger sequence of intermediate to acid volcanics is associated with terrigenous sediments and minor mafic lavas. Both suites have typical calc-alkaline mineralogy and chemical compositions. The major and trace element chemistry of the most mafic lavas (basalts and basaltic andesites) is consistent with derivation by partial melting of the mantle and a spectrum of fractional crystallization trends (involving the observed phenocryst phases) appears to relate more evolved members to parent magmas. The fractionation processes involved in the evolution of the calc-alkaline suites operated at higher fluid pressures than those involved in the development of associated komatiitic and tholeiitic lava sequences. The setting of volcanism in the Pilbara has a number of broad similarities (and some differences) to modern volcanically active continental margins in which growth of new continental crust appears to be dominated by calc-alkaline plutonism with calc-alkaline volcanism playing a subordinate role. However, more detailed information is required before direct comparisons with modern environments can be made with confidence or rejected in favour of non-uniformitarian models for Archaean crustal evolution.