Bipyramidal Quartz Research Articles

First Report of Tuff in Neoproterozoic Bhima Basin: Field Evidence, Petrography and Geochemical Studies

Abstract We report here the presence of tuffs from the Neoproterozoic Bhima basin unreported hitherto. These are exposed near Hotpet, Hulkal, Kanchankavi, and Ukinal area along two different stratigraphic horizons (1) within carbonates of Shahabad Formation and (2) above shale of Hulkal Formation. Tuffs are of two types: 1. Ash fall tuff and 2. Lapilli tuff, occurring as beds, matrix of a conglomeratic unit and patchy discontinuous layers. Physically ash fall tuff is thinly bedded (2-6 cm) and light-weight, lapilli tuff is hard, compact and individual beds are not discernible in the field. Presence of petrological features like bipyramidal quartz, fiamme structures, embayed quartz grains, blue CL colours of quartz grains, volcanic layering and banding, altered glass, and stretched and elongated volcanic glasses, are clear indications of its volcanic affinity. The tuff is anomalously high in silica content ranging from 71.4-91.2wt% and have low alumina value ranging from 0.75-4.11 wt % compared with that of the normal felsic tuffs, rhyolites and high silica rhyolites. REE in the tuffs can be grouped into two types, 1. Ash fall tuff with enriched LREE and fractionated HREE, 2. Lapilli tuff with flat LREE and HREE pattern. Tuff shows high Hf, overall enrichment of large ion lithophile elements (LILE Rb, Th, K and Pb) and depletion in HFSEs (Nb, Ta, Ti, P). The tuff is probably generated from continental crust source and the depositions are episodic and have more than one eruptive pulses of volcanic activity.

Generation of aluminous A-type granite by partial melting of felsic restite: Evidence from Mesozoic granitoids in the southern margin of the North China Craton

Aluminous A-type granites generally show high SiO 2 contents of >70 wt%, and their petrogenesis is still hotly debated with respect to source nature, magmatic processes and tectonic setting . This paper presents a comparison of petrography , geochronology and geochemistry between a Mesozoic syenogranite and its adjacently contemporaneous granitoids in the Xiong'ershan area, the hanging wall of the Paleozoic continental collision zone between the North China Craton and the North Qinling microcontinent. Syenogranites at Taishanmiao are metaluminous to weakly peraluminous, and belong to high-K calc-alkaline to shoshonitic series. They contain embayed bipyramidal quartz crystals , quartz-feldspar intergrowths and high Fe biotite species as well as accessory fluorite , magnetite and ilmenite . They exhibit significantly negative Ba, Sr and Eu anomalies with most δEu values of <0.5, and relatively high contents of Nb, Ta and Zr. They are weakly magnesian to ferroan in lithochemistry and mostly fall into the field of aluminous A-type granite in terms of their high Zr contents and 10,000 × Ga/Al ratios. Furthermore, they have low Y/Nb ratios of <1.20, falling into the A1-type field of the Nb-Y-Ce ternary diagram. The geochronological results indicate that the Taishanmiao syenogranites were emplaced into the late Paleoproterozoic Xiong'er Group volcanics at 119 ± 1 Ma, consistent with emplacement ages of the late stage non-adakitic granitoids at Funiushan in the same area. Whole-rock Nd and zircon Hf O isotope compositions are similar between the two types of granitoids, indicating that they share a common magma source that is composed of the ancient crustal basement in the southern margin of the North China Craton and the juvenile crust of the North Qinling microcontinent. According to their petrographic and geochemical features, the syenogranites are categorized into aluminous A-type whereas the non-adakitic granitoids belong to I-type. Therefore, they were produced by partial melting of felsic restites after extraction of hydrous granitic melts to form the Funiushan I-type granites. As such, the composition of these granitoids is primarily dictated by both source composition and magmatic processes at the ancient convergent plate margin. Although the granitic magmatism would take place in the post-collisional period, the aluminous A-type granite was formed in the terminal stage of post-collisional magmatism. In addition, the post-collisional granites record reactivation of the hanging wall in the continental collision zone. • The Taishanmiao syenogranites belong to aluminous A1-type granite. • The syenogranites were derived from partial melting of the felsic restite. • The felsic restite was produced by extraction of hydrous granitic melts. • The syenogranites are petrogenetically associated with calc-alkaline granitoids. • Granite composition is also dictated by crustal anataxis and melt extraction.

Palaeoclimatic and diagenetic controls based on clay mineralogy and organic matter distribution: The continental rift Cuyo Basin (Triassic), west‐central Argentina

AbstractThe Triassic Cuyo rift Basin is one of the renowned petroleum systems in South America that records distinctive alluvial, lacustrine and fluvial settings with volcanic input. The present study focuses on the clay mineral assemblages that represent the syn‐rift to post‐rift fills. Four clay mineral assemblages are defined based on X‐ray diffraction, optical and electron microscopy analyses. The illite/mica‐dominated assemblage is documented in the syn‐rift I (Cerro de las Cabras Formation). Such mineral phases are inherited and derived from the erosion of pre‐rift units, suggesting that physical weathering associated with a warm and semi‐arid climate played a leading role in their occurrence. Authigenic smectite clays are diagnostic in organic‐lean mudstones of the syn‐rift II (lower Potrerillos Formation) which point to alteration of pyroclastic material under a warm and semi‐arid to humid climate. A warm and humid climate prevailed during the deposition of lacustrine hydrocarbon source rocks (upper Potrerillos and Cacheuta formations). Organic‐rich mudstones show the illite/mica–kaolinite–smectite assemblage in conjunction with amorphous organic matter and restricted terrigenous elements. Diagenetic kaolinite booklets and bipyramidal quartz are attributed to the acidification of pore‐water related to kerogen transformation during burial diagenesis. Vitrinite reflectance values (ca 0.63%) confirm that organic‐rich sediments reached the earliest oil window; however, smectite is devoid of illite layers throughout the Triassic succession, probably related to the absence of potassium in the system and the high crystallinity of the smectite, which delayed the transformation of smectite into mixed layers illite–smectite. Red mudstones of the post‐rift II (the fluvial‐lacustrine Rio Blanco Formation) show the kaolinite‐dominated assemblage where the authigenic kaolinite is interpreted to be derived from the alteration of volcaniclastic detritus and indicative of warm climate with markedly wet periods.

Diagenetic Origin of Bipyramidal Quartz and Hydrothermal Aragonites within the Upper Triassic Saline Succession of the Iberian Basin: Implications for Interpreting the Burial–Thermal Evolution of the Basin

Within the Upper Triassic successions in the Iberian Basin (Spain), the occurrence of both idiomorphic bipyramidal quartz crystals as well as pseudohexagonal aragonite crystals are related to mudstone and evaporite bearing sequences. Bipyramidal-euhedral quartz crystals occur commonly at widespread locations and similar idiomorphic crystals have been described in other formations and ages from Europe, America, Pakistan, and Africa. Similarly, pseudohexagonal aragonite crystals are located at three main sites in the Iberian Range and are common constituents of deposits of this age in France, Italy, and Morocco. This study presents a detailed description of the geochemical and mineralogical characteristics of the bipyramidal quartz crystals to decipher their time of formation in relation to the diagenetic evolution of the sedimentary succession in which they formed. Petrographic and scanning electron microscopy (SEM) analyses permit the separation of an inner part of quartz crystals with abundant anhydrite and organic-rich inclusions. This inner part resulted from near-surface recrystallization (silicification) of an anhydrite nodule, at temperatures that were &lt;40 °C. Raman spectra reveal the existence of moganite and polyhalite, which reinforces the evaporitic character of the original depositional environment. The external zone of the quartz contains no anhydrite or organic inclusions and no signs of evaporites in the Raman spectra, being interpreted as quartz overgrowths formed during burial, at temperatures between 80 to 90 °C. Meanwhile, the aragonite that appears in the same Keuper deposits was precipitated during the Callovian, resulting from the mixing of hydrothermal fluids with infiltrated waters of marine origin, at temperatures ranging between 160 and 260 °C based on fluids inclusion analyses. Although both pseudohexagonal aragonite crystals and bipyramidal quartz appear within the same succession, they formed at different phases of the diagenetic and tectonic evolution of the basin: bipyramidal quartz crystals formed in eo-to mesodiagenetic environments during a rifting period at Upper Triassic times, while aragonite formed 40 Ma later as a result of hydrothermal fluids circulating through normal faults.

Evidence for volcanic ash fall in the Maya Lowlands from a reservoir at Tikal, Guatemala

Powder X-ray diffraction and petrographic analyses of reservoir sediments from Tikal, Guatemala have identified significant quantities of decomposed volcanic ash in the form of smectite and euhedral bipyramidal quartz crystals. X-ray fluorescence trace element content analysis was used to eliminate distant Sahara-Sahel and Antilles sources. The Zr/Y and Ni/Cr ratios of reservoir sediment from Tikal are consistent with a source from Central American volcanism (e.g., Guatemalan and Salvadoran). AMS radiocarbon dating of the smectite and crystalline quartz-rich reservoir sediments show that volcanic ash fell during the Preclassic, Classic, and Postclassic Maya cultural periods. It may now be possible to develop an effective chronology of ash fall at Tikal and the greater Peten.

Cathodoluminescence and trace element zoning in quartz phenocrysts and xenocrysts

Quartz phenocrysts and xenocrysts are a common component of volcanic rocks but rarely contribute to our understanding of magmatic or eruptive processes due to an apparent internal chemical and structural homogeneity. Scanning electron microscopy cathodoluminescence imaging (SEM-CL) and secondary ion mass spectroscopy (ion microprobe) analysis (SIMS) are used here to demonstrate that quartz crystals in volcanic rocks show hitherto unrecognised internal structural and chemical zoning. Two contrasting crystal morphologies have been studied. Porphyritic euhedral bipyramidal quartz crystals in dacite from Bonne Nuit Bay, Jersey, UK show fine-scale growth zoning in SEM-CL. Zones are narrow (1–40 μm wide), planar, parallel to crystal faces, and overgrow more broadly zoned, rounded cores. These crystals are interpreted to have formed at small degrees of undercooling by diffusion-controlled growth. Quiescent magma conditions are indicated by the preservation of an Al-rich diffusive boundary layer which has not been removed by melt shearing during convection. In contrast, rounded and embayed porphyritic quartz grains in a quartz dacite from the Esperanza Mine, E1 Salvador, northern Chile show broad internal SEM-CL zones with irregular boundaries, some of which are truncated by grain margins indicating dissolution. Crystal rounding may occur by dissolution or melting due to thermal or thermal-chemical disequilibrium with surrounding melt, and embayments by gas bubble enhanced convective dissolution are consistent with the development of incipient ocelli textures (strings of amphibole inclusions adjacent to quartz rims). Several episodes of rounding can be recognised in some xenocrysts. Ion microprobe (SIMS) analysis of quartz from both samples shows that A1 contents are higher in areas of bright CL emission. Previous studies of CL in quartz have suggested that several factors control luminescence. Although A1 substitution for Si in SiO4 tetrahedra may be partly responsible for SEM-CL zoning recorded in volcanic quartz, additional mechanisms (including the effect of growth rate on defect density) probably also play a major role.

DISCOVERY OF SALT IN THE VULCAN GRABEN: A GEOPHYSICAL AND GEOLOGICAL EVALUATION

The Paqualin-1 well in Permit AC/P2, Timor Sea, was drilled to test a large structural closure against the flank of an interpreted piercement structure located in the Late Jurassic Paqualin Graben. Prior to drilling the well interpretation of geological, seismic, gravity and magnetic data supported both a salt diapiric and/or an igneous intrusive structural model for the origin of the piercement feature. On drilling the Paqualin-1 well in December 1988, a 627 m thick evaporitic sequence was encountered in the post-rift Tertiary sequence indicating that the well had penetrated a salt overhang close to the main diapiric stock.The age of the evaporitic sequence is unclear but is considered coeval with Palaeozoic salt diapirs in the Bonaparte Basin to the east. Growth of the Paqualin diapir and a similar feature to the south, the Swan structure, which is also interpreted to be a salt diapir, appears to have been triggered initially by the Late Jurassic-Early Cretaceous breakup of the Australian north-west continental margin (doming and pillowing), and then again by a second major tectonic event in the Late Miocene associated with the collision between the Australian and Eurasian plates (diapir- ism and collapse structures).A distinctive cap rock occurs at the top of the evaporitic sequence characterised by an unusual accessory suite of primary and secondary minerals including euhedral magnetite, bipyramidal quartz, biotite, chlorite, sphene, amphibole and feldspar. The high magnetite component is considered responsible for the positive magnetic anomaly observed prior to drilling. The presence of magnetite and the other minerals in the cap rock appears to be related to the presence of exotic igneous material incorporated in the salt stock and the restricted activity of sulphate reducing bacteria in the crest of the diapir.The discovery of the Paqualin salt diapir and the interpretation of a similar structure in the adjacent Swan Graben has lead to the recognition of new play types related to the diapirs.

Depositional environment and mechanism of preservation of microfossils, upper Proterozoic Bitter Springs Formation, Australia

Microfossils preserved in black chert nodules from the upper Proterozoic Bitter Springs Formation of Australia represent the preserved remnants of cyanobacterial communities that inhabited a series of nonmarine saline lakes and ponds. Groundwaters of halite salinity “pickled” the microorganisms and thus inhibited their early bacterial degradation. These same groundwaters were saturated in silica so that upon evaporation the silica and halite precipitated and formed a series of chert concretions, nodules, and bipyramidal quartz crystals. Where the chert nodules formed in areas of buried cyanobacterial mats, further degradation of the buried sheaths and cells was arrested and the microorganisms were preserved.

Petrology and geochemistry of the Paliza Canyon Formation and the Bearhead Rhyolite, Keres Group, Jemez Mountains, New Mexico

Basaltic andesite, andesite, dacite, and rhyodacite of the Paliza Canyon Formation (9.1 to 8.5 m.y.) and the Bearhead Rhyolite (7.1 to 6.5 m.y.) belong to a high-K, calc-alkaline association in the southern Jemez Mountains, New Mexico. The majority of the rocks of the Paliza Canyon Formation contain augite, hypersthene, and plagioclase phenocrysts. Olivine is an important phenocryst in the basaltic andesite, but it is absent in the other rock types. In the rhyodacite, hornblende is a major phenocryst at the expense of the pyroxenes. Characteristically, a very restricted range in the Fe/Mg composition is found for the pyroxenes (augite from 0.33–0.29) and hornblende (0.41–0.47) throughout the whole range of rock types, from basaltic andesite (∼56% SiO 2 ) to rhyodacite (67% SiO 2 ). On the other hand, the Bearhead Rhyolite lacks pyroxene, and it has two feldspars (sanidine and plagioclase) and bipyramidal quartz and biotite. Its SiO 2 content ranges between 76.5% and 77.3%. The Bearhead Rhyolite lies on the two-feldspar surface, or below it, in the sanidine field; the Paliza Canyon rocks plot within the plagioclase volume of the granite system. Both major- and trace-element trends in the Paliza Canyon Formation do not project to the Bearhead Rhyolite composition. Major- and trace-element modeling suggests that the sequence from basaltic andesite to rhyodacite in the Paliza Canyon Formation can be formed by simple fractional crystallization. The Bearhead Rhyolite cannot be formed from the Paliza Canyon rhyodacite by this process, however. Removal of the observed phenocrysts found in the rhyodacite would form a rhyolite enriched in Ba, Hf, Th, U, and light rare earth elements (LREE) and depleted in V and Sc; the Bearhead Rhyolite, however, is depleted in the former group and enriched in the latter. These conclusions are supported by initial 87 Sr/ 86 Sr ratios: three samples of Bearhead Rhyolite have initial ratios of 0.7056, 0.7073, and 0.7076; two samples of Paliza Canyon basaltic andesite, on the other hand, have ratios of 0.7041 and 0.7044. The most likely source for the Bearhead Rhyolite is lower crustal material which may have been fused partially from the heat provided by the andesitic magmas of the Paliza Canyon Formation.

Clay mineralogy of Hijiori and Numazawa Ando soils

Abstract The clay mineralogy of Hijiori and Numazawa Ando soils was investigated with special reference to the occurrence and weathering of hornblende and biotite in the parent materials. The profiles of both soils underlain by pumice beds were developed from tephras of different ages. The ages of the Hijiori and Numazawa pumice beds were approximately 10,000 and 5,000 years old, respectively. The rock types of all the parent materials belonged to dacite and the existence of relatively large amounts of quartz was noted in almost all of the bulk soil samples. The primary mineral compositions were characterized by large amounts of hornblende in the heavy fractions and small amounts of biotite and bipyramidal quartz in the light fractions. Though all the soil samples had similar primary mineral compositions, they were divided into two groups on the basis of their clay mineralogical compositions. The clay fractions were characterized by the presence of relatively large amounts of opallne silica and vermiculit...

Claiborne Group of Central Texas: Record of Middle Eocene Marine and Coastal Plain Deposition: ABSTRACT

Sediments of the Claiborne Group in central Texas were deposited in a variety of nearshore-marine and coastal-plain environments. Fluctuations of the shoreline during middle Eocene time resulted in vertical stacking of 2,600 ft of sediments from contrasting depositional environments. Shoreline fluctuations probably did not result from simultaneous eustatic changes of sea level. They were caused by migrations of large fluvial, fluvio-deltaic, and interdeltaic complexes on the margins of a gently and perhaps uniformly subsiding basin. As a result of migrations of major sediment depocenters, the Claiborne rock record is characterized by high lateral and vertical variability. Each formation of the Claiborne Group commonly is considered to be the product of a single depositional environment which extended laterally for hundreds of miles. For example, the basal formation of the Claiborne, the Carrizo Sandstone, is generally accepted as having been deposited under fluvial conditions close to the shoreline. This assumption is not borne out by detailed field work. An examination of the Carrizo Formation from Bastrop to Freestone Counties reveals that Carrizo sediments were deposited in fluvial, deltaic, and marine environments. In a single outcrop the Carrizo Sandstone may be shown as having been deposited in both barrier bars and delta distributary channels. Such variability also holds true for sediments from each formation comprising the Claiborne Group. Petrographic analysis indicates that the sediment source remained constant during deposition of the Claiborne Group. The presence of phyllite and schist fragments together with the heavy minerals kyanite, staurolite, garnet, and zircon indicates that the source terrane was dominantly metamorphic. Evidence for volcanic activity is particularly strong in the Carrizo Formation and the uppermost Claiborne Yegua Formation. Volcanic minerals in these formations include bipyramidal quartz, euhedral apatite, and bentonite, indicating that the Claiborne coastline periodically was subjected to wind-transported detritus. End_of_Article - Last_Page 1693------------

Petrology, Provenance and Silica Diagenesis of the Penrith Sandstone (Lower Permian) of Northwest England

ABSTRACT The Penrith Sandstone (Lower Permian) of north west England is a continental red bed sandstone, deposited as barchan sand dunes in a hot, arid desert environment. The rock displays a high degree of mineralogical and textural maturity, being a coarse, well sorted orthoquartzite, composed of rounded unstrained, strained and polycrystalline quartz grains, with subordinate feldspar and rock fragments. Distinct petrographic features of the detrital grains suggest derivation from the Millstone Grit (Upper Carboniferous) sandstones of Yorkshire, and perhaps County Durham. In the southern part of the area non-silicified sandstones are interbedded with calcite cemented alluvial fan breccias (brockrams). This contrasts markedly with northern areas where brockrams are absent, in that the sandstones are tightly cemented with secondary quartz, occurring as overgrowths of optically continuous, bipyramidal quartz crystals around detrital grains. Many varieties of overgrowth are present, their structure being pre-determined by the internal structure of the detrital grains. Thus, unicrystaline unstrained and strained, and polycrystalline overgrowths can be recognised. Overgrowths of secondary feldspar occur occasionally on detrital orthoclase and microcline. It is suggested that the source of the secondary quartz is due to the solution of the siliceous dusts produced during aeolian abrasion of the quartz grains. Alkaline desert groundwaters are considered to be responsible for taking the siliceous dust into solution, which, upon evaporation, precipitated the silica as optically continuous quartz overgrowths around the detrital grains. The inverse relationship between silicification of the Penrith Sandstones and brockram distribution is attributed to differences in compositon of the groundwaters in the two facies.

Clay minerals associated with some manganese ore deposits

Three montmorillonites and a serjcite-montmorillonite regular mixed-layer mineral are described. The montmorillonites occur in the altered zone of the country rocks around the manganese-dioxide ore deposits, which were formed by the hydrothermal replacement in the tuffaceous rocks. They bear various colors, and (1) white and (2) brownishred masses from the Shiroiwa mine, Akita Prefecture, and (3) green one from the Miyazaki mine, Miyagi Prefecture, are mineralogically analyzed and shown that (1) and (2) are typical montmorillonite and (3) is beidellitic montmorillonite. The sericite-montmorillonite regular mixed-layer clay-mineral showing 25.2A spacing was found in a narrow fissure traversing the manganese ore deposit in the Paleozoic formation of the Aradani mine, Kyoto Prefecture. It associates with a large quantity of fine-grained bipyramidal quartz. X-ray, D. T. A. and chemical analyses along with heating-effect data show that the clay-mineral is a regular mixed-layer mineral having almost equal percentages of sericite and montmorillonite layers.

On the igneous activities in Kakkonda region, Iwate Pref

In Kakkonda region, the writer observed the dacite eruption as the first products of the igneous activities. This dacite covered the basal Tertiary sediments and developed in large area as the lowermost lava flow. And this dacite is light bluish grey coloured rock, containing huge corroded bipyramidal quartz phenocrysts in the compact fine grained groundmass. The dacite masses with same characteristics are distributed at the neighbourhood of Oshuku and Uguisuzawa districts etc., southward of this region. This fact suggests that the dacite eruption of this type took plate as the first products of so-called new stage igneous activities, covering the late tertiary to quarternary age, at the east side marginal region of the Ou-mountainland, Iwate Prefecture.

Petrography of Salt Dome Cap Rock

Two kinds of cap rock are differentiated: the gypsum anhydrite-cap and the calcite cap. The gypsum is derived from the anhydrite by hydration and is characteristic of the upper part of the cap, where it contains much calcite and sulphur. The deeper, less altered part of the anhydrite cap shows parallel banding, which may be the result of diffusion, and breccia fragments of a parallel-banded, finer-grained anhydrite, which give evidence of sedimentary origin. Therefore, although no detrital minerals have been found in specimens of anhydrite cap, this cap is believed to be of sedimentary origin, brought up by an intrusive salt plug from depth. Possible explanations of the presence or absence of anhydrite caps on salt domes are offered. The calcite cap is the product of repl cement and penetration by calcite of the sedimentary beds adjacent to the gypsum-anhydrite cap and probably of the upper part of the gypsum-anhydrite cap itself. The sulphur in cap rock is characteristically associated with this calcite. The calcite and sulphur are probably the result of reduction of the outer parts of the anhydrite cap by hydrocarbons from adjacent beds. Constituents of minor importance are sulphides, carbonates of an early generation, barite, celestite, bipyramidal quartz crystals, and inclusions of sandstone in anhydrite.