Pyroclastic Volcanic Rocks Research Articles

Synthesis of Na–X zeolites from tripolaceous deposits (Crotone, Italy) and volcanic zeolitised rocks (Vico volcano, Italy)

Synthesis of zeolitic minerals (Na–X and HS) by the use of natural materials (naturally zeolitised alkaline volcanic rocks and “Tripoli”) has been achieved. Synthesis was conducted at hydrothermal conditions (80 °C) by the use of alkaline silicates (Na x Si y O z ) and alkaline aluminates (Na x Al y O z ). Chemical treatments were made on “Tripoli” rocks (mainly constituted by opaline silica) coming from Crotone Basin (southern Italy) in order to obtain Na 2 SiO 3 · nH 2O. Volcanic pyroclastic rocks (naturally zeolitised to chabazite and phillipsite) from the Vico Administrative Province (central Italy) were treated to obtain the NaAlO 2 solution. Both products were mixed to gain the gel NaAlSi x O y · nH 2O necessary for the zeolitic synthesis. Mineralogical–crystallographical–chemical and textural–physical characterisation showed that Na–X zeolite synthesis begins after 5 h and reaches its crystallisation climax at 18 h, with a broad field of existence (about 500 h) of the Na–X phase. Then Na–X is substituted by HS (Hydroxysodalite) zeolite. The characterisation shows that microtextural (scanning electron microscope images) and crystallographic values as well as other physical properties (specific surface, density) are comparable to the reputed ones for commercial zeolite. The thermal behaviour (TG-DTA) and IR response also provide consistent evidence of good achievement of Na–X zeolite synthesis. Finally, the wide (in terms of time) range of stability of Na–X zeolite seems to allow transfer to an industrial production scale.

Age and Petrogenesis of Two Late Archean Magmatic Suites, Northwestern Superior Province, Canada: Zircon U-Pb and Lu-Hf Isotopic Relations

The paper presents U–Pb ages for zircon, titanite, and monazite, and Hf isotopic data for zircon, from the rocks of two magmatic suites occurring mostly in the Archean Uchi Subprovince and partly in the neighbouring Berens River and English River subprovinces of the northwestern Superior Province, Ontario. These data, together with observations on the morphologies and, where evident, the inheritance of the zircon crystals, constrain the nature of the sources of the magmas and provide a record of various crustal processes in their evolution. The older of the two magmatic suites formed at 2744–2740 Ma along segments of a common arc system. The suite consists of (1) several trondhjemitic to granodioritic plutons, with εHf values of 6⋅1, intruded into older crust and possibly formed from magma produced by partial melting of subducted, juvenile oceanic crust; (2) an assemblage of dacitic pyroclastic volcanic rocks, with εHf values of 3⋅2–4⋅0, associated with tholeiitic basalts and probably derived from magma melted from arc mantle; and (3) a bimodal assemblage of tholeiitic basalts, rhyolites, and porphyries, also with εHf values of 6⋅1, associated with a volcanogenic massive sulphide deposit and apparently formed by differentiation of mantle-derived basaltic melts at shallow levels in an extensional back-arc setting. The second magmatic suite, formed between 2702 and 2693 Ma, comprises late orogenic plutons and batholiths of dioritic to dominantly granodioritic composition. The characteristics of these intrusions are consistent with a process combining melting of a metasomatized mantle source and subsequent fractional crystallization of the derived magmas at shallow depths. However, most of the studied occurrences show evidence of crustal contamination through various combinations of assimilation of lower-crustal material, assimilation of underthrust sedimentary rocks, and contamination by wall rock materials during the latest stages in the emplacement of the plutons. The involvement of crustal material is indicated by the presence of zircon xenocrysts and by εHf values ranging from 1⋅4 to 4⋅4. Only one intrusion, with an εHf value of 5⋅0 and no xenocrystic zircon, appears to have escaped widespread contamination, perhaps because the ascent of its magma was facilitated by a crustal-scale fracture system.

Miocene alluvial fan-alkaline playa lignite-trona bearing deposits from an inverted basin in Anatolia: sedimentology and tectonic controls on deposition

During the Middle through Late Miocene, over 1200 m of clastics, carbonates and evaporites accumulated in the depositional basin situated in northwestern Ankara province of Central Anatolia. Detailed stratigraphic and sedimentological analyses of the exposed sequence exhibits three major facies: (1) lower alluvial, (2) upper alluvial, and (3) lacustrine. The lower alluvial facies consists primarily of volcaniclastic debris-flow, boulder and channel conglomerate, sandstone, siltstone, mudstone and two lignite horizons. These rock assemblages were deposited in alluvial fans and a braided-river system. The lower and upper lignite seams were laid down in swamps and lake-margin mud-flat environments associated with this river system. The upper alluvial facies is characterized by red conglomerate, fine-grained and cross-bedded sandstone, mudstone and calcareous mudstone units. The dominant depositional environments are interpreted to be alluvial fan, braided-river, and mud-flat. The base of the lacustrine deposits interfingers with the lower alluvial facies and consists of dolomitic claystone, bituminous shale, trona and intraformational conglomerate lithofacies. The sedimentary features of the dolomitic claystone and trona lithofacies suggest deposition in a playa-lake type environment. The bituminous shale is a transgressive deposit laid down in a shallow, but expanded lake environment. Sedimentological and structural characteristics of the intraformational conglomerate lithofacies indicate synsedimentary tectonism, intrabasinal erosion, redeposition and sediment selection toward the deposition centre of the lake environment. The green claystone lithofacies has three units: (a) the claystone-bedded chert-analcitized tuff alternation and siliceous carbonate lithofacies of the lacustrine system, deposited in a perennial lake environment; (b) montmorillonitic olive-green claystone containing mud-cracks, ripple mark and desiccation cracks filled by gypsum, deposited in a saline or brackish ephemeral lake; (c) gypsiferous claystone in a saline mud-flat, and thick gypsum beds including thin glauberite and tenardite lenses, laid down in an ephemeral saline lake. The green claystone and evaporite lithofacies interfinger with fine-grained rocks of the upper alluvial facies and also with Miocene pyroclastic volcanic rocks. During the late Middle Miocene, two northeast-trending depressions isolated by basement rocks were filled rapidly by sediments of the lower alluvial facies. Extensional normal faults were developed at the northern and southern basin margins and controlled deposition of the facies during the Miocene. The basin sediments were folded and faulted as a result of neotectonic compressional stresses associated with movements on the North Anatolian Fault.

Alteración argilica avanzada de rocas piroclasticas en función de su estructura en la zona de rodalquilar (Almeria)

Advanced-argillic alteration process is studied in different types of pyroclastic volcanic rocks: dacitic tuffs, riodacitic ignimbrites and andesitic agglomerates located in the Rodalquilar area (Almeria, SE Spain). Tuffaceous material show massive alteration with vertical zonal patterns. In the lower zones silicification and propylitization processes (quartz, chlorite, illite ± pyrite) predominate and evolve into advanced-argillic paragenesses (illite, kaolinite ± alunite) at intermediate zones and to a quartz-alunite rock in the upper rones. Ferrie aureolaes and limonitic crusts are developed between the intermediate and upper zones. Agglomeratic and ignimbritic rocks show similar a1teration arranged in a more complex fissural patterns probably related to the intense fracturation displayed in that materials. Fissural zones are characterized by the incidence of supergenic processes with an increase in the size, idiomorphism and crystallinity of quartz crystals. Zones of massive argillie a1teration with alunite show important contents in Cl (0.4%), while limonitic crusts and ferrie aureolaes concentrate Sb (30 ppm.) and As (500 ppm.). The Arsenic is also concentrated in quartz levels (80 ppm.). A genetic model related to sea water hydrothermal flow in a «hot spring» environment is discussed: The influence of weathering and supergenic processes in the redistribution of some analyzed minor elements (Cl, As, and Sb) and Fe is discussed toghether with the evolution in morphology and crystallinity of siliceous minerals.

Fission-track dating of glass inclusions in volcanic quartz

We have extended fission-track dating techniques to volcanic pyroclastic rocks, using quartz-bearing uranium-rich glass inclusions. We use both the glass itself and the surrounding quartz as track detectors, the latter having a much higher resetting temperature. We can thus derive the age of the last low-temperature thermal event, as well as that of the most recent deposition of quartz on the walls of the inclusions. This method has been applied to a Sardinian sample and to a Moroccan Precambrian sample.

Marine Geophysical Measurements in Central Puget Sound, Washington: ABSTRACT

Marine seismic refraction (sonobuoy and OBS) and gravity data obtained from the Puget Sound main basin and Lake Washington show a major discontinuity in both seismic velocities and rock densities across a steep (15 mgal/km) gravity gradient striking generally westward through Seattle from the Cascade Range foothills to Hood Canal. North of this gradient is a -129 mgal Bouguer gravity minimum centered over Lake Washington. A least squares inversion analysis of the residual Bouguer gravity field was combined with the refraction velocities to model the subsurface density distribution beneath the central Puget Sound lowland. The data suggest the existence of a 7 to 8-km deep sedimentary basin beneath the gravity minimum north of the steep gradient. The basin is filled with probable Tertiary and Quaternary rocks having densities ranging from 2.0 to 2.6 g/cc. Modeled rock densities beneath the basin (2.7 to 2.8 g/cc) may indicate the presence of volcanic basement rocks. South of the gravity gradient, Tertiary volcanic and intrusive rocks are overlain by Tertiary and Quaternary sedimentary rocks up to 2 km thick. The gravity gradient appears to mark a steep fault or faulted flexure forming the southern boundary of the Tertiary basin lying beneath Lake Washington and Seattle. The gravity model suggests that much of the steepness in the gradient across this feature is due to a near-surface density contrast between a west-trending End_Page 1690------------------------------ belt of high density Tertiary igneous rocks and low density tuffs or pyroclastic volcanic and sedimentary rocks along the southern margin of the basin. End_of_Article - Last_Page 1691------------

Extension of the telluride erosion surface to Washington State, and its regional and tectonic significance

Abstract The Wenatchee Formation of Oligocene age (34 m.y.) rests on a profound unconformity that developed during an extended period of erosion and tectonic quiescence from perhaps 40–34 m.y. B.P. Its occurrence strongly suggests that a stripped erosion surface in the northern Cascade Range, previously considered of Miocene age, is of Late Eocene—early Oligocene age and that it is part of a more widespread surface in the Pacific North-west. The Late Eocene—early Oligocene unconformity/erosion surface is apparently of continental extent, from British Columbia to Mexico. Where it is incised on crystalline basement rocks, it now may be exhumed and exposed as a plateau or level-crested divides of concordant elevation. Where it is developed on sedimentary rocks, it may be found as a profound unconformity. Widespread eruptions of pyroclastic volcanic rocks covered much of the surface in Oligocene time and tended to protect and preserve it. The surface is particularly well documented in the San Juan volcanic field of Colorado. There it was given the name “Telluride peneplain”, which is here broadened to the Telluride erosion surface of continental extent. A magmatic gap indicating a period of magmatic quiescence is generally associated with tectonic quiescence implied by the erosion surface, but the timing varies regionally. Development of the Telluride surface and associated magmatic gaps correlates with general plate reorganization during the Eocene. An erosion surface correlates with a global tectonic transition because erosion rates immediately dominate uplift rates when compressive deformation is slowed or shut off. An analysis of the dynamic balance between erosion rates and plate convergence rates suggests that erosion may maintain a continental surface of low relief if the relative convergence rate falls to about 0.5 cm/yr or less. Gaps in magmatic activity do not correlate as precisely with the 40 m.y. tectonic transition, in part because of the possibility of time lags. A great deal of scattered geologic evidence suggests a major global event at about 40 m.y. BP. The timing does not always precisely correlate with the timing of events in western North America, which suggests that the 40 m.y. tectonic transition was spread over several millions of years: A most impressive global impact of the 40 m.y. transition is the largest drop in eustatic sealevel at about 30 m.y. BP. The 40 m.y. tectonic transition profoundly altered geologic history, including the development of continental landscapes, global sea-level, and global climate. Classical stratigraphic studies remain the best way of documenting its effect on the continental surface. The small remnant of Oligocene rocks in central Washington, overlooked in early reconnaissance studies, is of great geologic significance. Other undiscovered remnants probably exist in western North America.

Criteria for the identification of ancient volcanic arcs

Characteristic features of recent volcanic arcs must be preserved in the rock record to be useful in determining the magmatic affinities of metavolcanic rocks. This paper reviews criteria suggested by others, and proposes new criteria for the recognition of ancient volcanic arc complexes. Major element abundances, which discriminate magma types in recent volcanic rocks, are very susceptible to modification during low-grade metamorphism, and therefore are of limited value for determining magmatic affinities of altered volcanic rocks. Ti and Zr, Cr and the rare-earth elements, are only slightly affected by low-grade metamorphism. These elements show distinctive trends that allow ocean-floor basalts to be discriminated from most volcanic arc basalts. Clinopyroxene phenocrysts are commonly the only unaltered remnant phase present in metavolcanic rocks. Compositions of clinopyroxene phenocrysts from a suite of fractionated volcanic rocks can be employed as a petrogenetic indicator, because each magma series displays a distinctive Ca Fe Mg trend during differentiation. The much greater abundance of pyroclastic volcanic rocks versus flows in modern volcanic are sequences is a preservable criterion for identifying ancient volcanic arcs. Interbedded with the pyroclastic volcanic rocks are thick deposits of graywackes and mudstones. The volcanic arc deposits are overprinted by high-temperature/low-pressure metamorphism. Parallel to and on the seaward side of the volcanic arc metamorphic belt is a belt of low-temperature/high-pressure metamorphic rocks. These two metamorphic belts comprise a paired metamorphic belt that is diagnostic of Pacific-type convergent plate margins. These criteria together distinguish volcanic arc deposits from other volcanic—sedimentary deposits.

The geologic setting of the Ming and other sulfide deposits, Consolidated Rambler Mines, Northeast Newfoundland

Basic (komatiitic) pillowed and massive flows (Gale, 1973) form the base of the succession in the Rambler area, overlain by mafic pyroclastic and pillowed volcanic rocks. A center of silicic pyroclastic volcanism formed during the later stages of mafic volcanicity and the sulfide deposits in the area are related to exhalite activity during the waning stages of this silicic volcanicity. The mafic pyroclastic volcanism was followed by deposition of mafic volcanogenic sediment and/or waterlain tuff and by local extrusion of pillow lava units. The entire sequence is cut by numerous mafic dikes and sills.All the rocks in the area have suffered polyphase deformation and metamorphism in the upper greenschist facies. The first two deformations involved intense strain with 1 2 and F 3 fold axes and to the maximum axis of the D 2 deformation ellipsoid.The Ming massive sulfide deposit is a stratabound Cu, Au, Ag producer and occurs at the top of the silicic volcanic pile. The Rambler deposit is similar to the Ming and both have been compared to the Archean and/or Kuroko types of deposits. The East deposit and the Big Rambler Pond deposit were Cu producers and have been compared to Cyprus-type deposits. It is concluded that the deposits in the Rambler area cannot be classified as a single type and that different types coexist in one environment.

Age of the Precambrian Roraima Formation in Northeastern South America: Evidence from Isotopic Dating of Roraima Pyroclastic Volcanic Rocks in Suriname

Pyroclastic acidic volcanic rocks are intercalated between sedimentary rocks of the Roraima Formation at the Tafelberg in Suriname. From these rocks, a 14-point whole-rock Rb-Sr isochron of 1,599 ± 18 m.y. was obtained with initial 87 Sr/ 86 Sr = 0.7075 ± 0.0032 (47.2 × 10 9 yrs 87 Rb half-life; errors with 95 percent confidence level). K-Ar measurements on five samples give dates ranging from 1,573 to 1,681 m.y. Some 200 m.y. have thus elapsed between the consolidation of the Guiana Shield during the “Trans-Amazonian orogeny” 1,810 ± 40 m.y. ago and the development upon it of the Roraima basin (or basins) of deposition. Isotopic age measurements on dolerite dikes and sills cutting the Roraima sedimentary rocks (the Roraima Intrusive Suite) indicate that the basalt intrusions took place within a relatively short time interval (some 50 m.y. at most) after the deposition of the Roraima Formation.

13. Some new facts of the Kuju volcanic pyroclastic rocks(Abstracts of Papers submitted for the Meetings of the Society)

13. Some new facts of the Kuju volcanic pyroclastic rocks(Abstracts of Papers submitted for the Meetings of the Society)

A study on the zircons from the pyroclastic volcanic rocks

In this paper, the writers studied on the morphological characters of the zircon crystals from the pyroclastic volcanic rocks in Japan and obtained the following results; i) In the pyroclastic volcanic rocks, the zircons are contained in much smaller amount than in the granitic rocks and the grain sizes are mostly very minute ranging from 0.02×0.01mm. to 0.50×0.25mm., and 0.10×0.05mm. in common. ii) The crystal tracht is characterized with Yamamoto's ‘c’-type same as that from granitic rocks and rhyolites. iii) By the I-R-U trigonal diagram of the crystal forms, it was ascertained that the rounded crystals (R) and the broken ones are remarkably contained. The above-mentioned facts lead to us that the original pyroclastic volcanic rock magma might have been contaminated or assimilated by a large amount of sedimentary rocks.

The geology and geochronology of the basement complex of the central Transantarctic Mountains

The central part of the Transantarctic Mountain chain consists of a basement complex of igneous and metamorphic rocks overlain nonconformably by sedimentary rocks of late Paleozoic age, including Permian tillites and coal measures. The basement complex is exceptionally well exposed in the Wisconsin Range of the Horlick Mountains, which are located about 500 km from the South Pole. In this area clastic metasedimentary rocks and overlying metavolcanic rocks of probable pyroclastic origin are intruded by a variety of granitic rocks of the Wisconsin Range batholith, including rapakivi granites and quartz diorites, which are in turn cut by bodies of quartz monzonite, and aplite and pegmatite dikes. The basement complex elsewhere in the central Transantarctic Mountains also includes Cambrian limestones overlain by acid volcanic pyroclastic rocks and lava flows.Age determinations by the Rb–Sr method applied to suites of total rock specimens suggest the presence of two orogenic events accompanied by intrusions of granitic plutons. The first of these occurred about 630 ± 25 m.y. ago and was accompanied by the formation of rapakivi granites. The second took place during the Ordovician Period about 480 ± 10 m.y. ago and involved the intrusion of quartz monzonite and pegmatite dikes. The metasedimentary rocks, which were intruded by both of the granitic rocks, give an apparent age of 460 ± 16 m.y., while the overlying pyroclastic unit was dated at 633 ± 13 m.y. A Precambrian age for the metasedimentary and metavolcanic rocks is consistent with the dating and the field evidence. Rhyolites from localities in the Byrd Mountains and the Long Hills were dated at 489 ± 30 and 498 ± 45 m.y. and are late Cambrian to early Ordovician in age.

On the Geomorphology of the Northern Part of the Tachov Basin and the Bohemian Forest

In 1960 the author carried out a quarternary and engineering-geological investigation of the area that is represented on sheets M-33-73-B-b, d and M-33-74-A-a, b, c, d of the new state map of Czechoslovakia compiled in scale of 1 : 25.000 The present paper treats of the geomorphological results of this investigation carried out in the northern part of the Tachov Basin and the Bohemian Forest. The maped area measures 330 km2. The field work was started with the investigation of the peneplain of the Bohemian Forest which lies at an approximate altitude of 650-750 m. Individually preserved dominants, composed of quartzitic migmatitic gneiss protrude above the peneplain surface (Tišina, Ve skalkách, Štokovský vrch etc.). The basic structural lines of the peneplain body of the Bohemian Forest were formed as early as the Varisian Orogene. After the Varisian faults, movements were resumed at the time of the saxonian tectonic disturbance which made the whole mountain range assume its present character. Towards the end of the Varisian Orogene, old fault lines were filled with hydrothermal quartz or blocked with clastic material. The quartz filling of the faults has the character of morphologically prominant veins or grabens, stretching from the northern environs of Tachov over Tři Sekery (Three Axes) towards Dolní Žandov and over Hazlov to Saxony. Selective stream erosion affected before all healed-up fault lines of the Upper-Rhine (longitudinal) direction as places rendering the least resistance. Valleys of the consequent streams heading for the subsided Tachov Basin are younger. Along the young Marienbad fault line valleys were founded tectonically (sections of the Hamerský and Kosový Brooks). The author consideres the sedimentation depression of the Tachov Basin - filled in its northern part with Tertiary sediments (gravel, sand, clay) - to belong to the accumulation phenomena. Pleistocene terraces have been ascertained along the Hamerský and Kosový Brooks. Two levels could be distinguished (12-18 m and 7-12 m), the thickness of the terrace accumulations reachins 2-4 m. Chronologically they are most probably older than Würm. On their surface they are covered with loeses. The unique occurrence of volcanic pyroclastic rocks of Železná hůrka in the vicinity of Boden is also considered an accumulation phenomenon. The next chapter treats of wheathering processes. Pleistocene weathering asserted itself most strongly in the summit areas of the Bohemian Forest where it formed extensive stone seas. In the Borský Mass it deeply affected the surface granite. Besides the Pleistocene weathering also Tertiary weathering (Chotěnov, Chodová Planá, Hamrníky) took active part especially in chemical decomposition of the alumosilicates in the rocks, Decomposed rocks occur in the form of whitish clayey eluvium along the whole course of the Marienbad fault. They also occur in the vicinity of mineral water springs with exhalations of CO2. The last chapter is reserved for periglacial phenomena. It describes solifluction and cryoturbation which deforms the upper layers of the weathered mantle. Periglacial phenomena are rather scarce in the area under investigation.

OLDER PRECAMBRIAN ROCKS OF THE DIAMOND BUTTE QUADRANGLE, GILA COUNTY, ARIZONA

Rocks intruded by Precambrian granite have been previously described in several districts of central Arizona, but it has not been possible to demonstrate their stratigraphic order. In the Diamond Butte quadrangle five older Precambrian formations can be placed in depositional sequence. The oldest is the Alder Formation, consisting of wacke, slate, quartzite, and conglomerate. It is conformably overlain by the Flying W formation consisting of interbedded basic and acidic volcanic rocks and conglomerate. This is overlain with slight unconformity by the conglomerate, quartzite, and slate of the Houden formation. Above the Houden is the Board Cabin formation composed of porphyritic, pillow, and pyroclastic volcanic rocks, and volcanic sediments. The rhyolites and conglomerates of the Haigler formation complete the sequence. The Haigler is believed to be overlain by still younger rhyolites, and the Alder may be underlain by an older sequence of slate and basic volcanic rocks. This volcanic-sedimentary rock sequence portrays a recurrently unstable marine environment in which a large portion, if not all, of the sediment was derived from contemporary volcanic rocks. Through time the parent magma became enriched in potassium and silicon, and finally engulfed its own extrusive and sedimentary deposit to crystallize at relatively shallow depths as quartz porphyry, granophyre, and granite. Evidence of pre-existing metamorphic or plutonic rocks has not been found, and this depositional-orogenic cycle may represent the initial continental formation in this area.

The Geology of the Country around Bodfean, South-Western Carnarvonshire

The following account deals with a tract of country in Western Lleyn (Carnarvonshire) between Nevin and Pwllheli, chiefly in the parish of Bodfean. It is about 3½ miles long (from west-south-west to east-north-east) and 2 miles wide, and consists chiefly of Ordovician extrusive and pyroclastic volcanic rocks of varied composition associated with marine sediments. The hill of Gam Bodfean that rises steeply to a height of 918 feet is the most prominent feature of the area, although it is overtopped on the north by the larger hills north-east of Nevin and on the south by Carn Fadryn. Moel y Penmaen is a smaller and less conspicuous ridge, about 500 feet high. Most of the remaining country is thickly mantled with glacial drift, which has prevented us from working out the geological sequence and structure as fully as we wished. As this area lies near the margin of the regions which we are separately investigating, we decided to deal with it jointly. Although both authors have taken part in the field work, the first-named is more particularly, concerned with the stratigraphy and palæontology, and the second author with the petrology. With the exception of Garn Bodfean and the recording of fossils from Bodfean Hall, the area has hitherto received little attention from geologists. In 1843 Sedgwick described the Palæzoic region of Lleyn, of which the tract here dealt with forms part, in general terms as ‘composed of greywacke (sometimes passing into a coarse arenaceous rock), and greywacke slate,... and these rocks