Distribution by Scientific Domains
Distribution within Earth and Environmental Science

Kinds of Granite

  • s-type granite

  • Terms modified by Granite

  • granite outcrop

  • Selected Abstracts

    Environmental Isotopes as Indicators for Ground Water Recharge to Fractured Granite

    GROUND WATER, Issue 6 2004
    Article first published online: 9 OCT 200
    First page of article [source]

    Genesis and Mixing/Mingling of Mafic and Felsic Magmas of Back-Arc Granite: Miocene Tsushima Pluton, Southwest Japan

    RESOURCE GEOLOGY, Issue 1 2009
    Ki-Cheol Shin
    Abstract The Middle Miocene Tsushima granite pluton is composed of leucocratic granites, gray granites and numerous mafic microgranular enclaves (MME). The granites have a metaluminous to slightly peraluminous composition and belong to the calc-alkaline series, as do many other coeval granites of southwestern Japan, all of which formed in relation to the opening of the Sea of Japan. The Tsushima granites are unique in that they occur in the back-arc area of the innermost Inner Zone of Southwest Japan, contain numerous miarolitic cavities, and show shallow crystallization (2,6 km deep), based on hornblende geobarometry. The leucocratic granite has higher initial 87Sr/86Sr ratios (0.7065,0.7085) and lower ,Nd(t) (,7.70 to ,4.35) than the MME of basaltic,dacitic composition (0.7044,0.7061 and ,0.53 to ,5.24), whereas most gray granites have intermediate chemical and Sr,Nd isotopic compositions (0.7061,0.7072 and ,3.75 to ,6.17). Field, petrological, and geochemical data demonstrate that the Tsushima granites formed by the mingling and mixing of mafic and felsic magmas. The Sr,Nd,Pb isotope data strongly suggest that the mafic magma was derived from two mantle components with depleted mantle material and enriched mantle I (EMI) compositions, whereas the felsic magma formed by mixing of upper mantle magma of EMI composition with metabasic rocks in the overlying lower crust. Element data points deviating from the simple mixing line of the two magmas may indicate fractional crystallization of the felsic magma or chemical modification by hydrothermal fluid. The miarolitic cavities and enrichment of alkali elements in the MME suggest rapid cooling of the mingled magma accompanied by elemental transport by hydrothermal fluid. The inferred genesis of this magma,fluid system is as follows: (i) the mafic and felsic magmas were generated in the mantle and lower crust, respectively, by a large heat supply and pressure decrease under back-arc conditions induced by mantle upwelling and crustal thinning; (ii) they mingled and crystallized rapidly at shallow depths in the upper crust without interaction during the ascent of the magmas from the middle to the upper crust, which (iii) led to fluid generation in the shallow crust. The upper mantle in southwest Japan thus has an EMI-like composition, which plays an important role in the genesis of igneous rocks there. [source]

    40Ar/39Ar Dating of Xuebaoding Granite in the Songpan-Garzê Orogenic Belt, Southwest China, and its Geological Significance

    Yan LIU
    Abstract: Thus far, our understanding of the emplacement of Xuebaoding granite and the occurrence and evolution of the Songpan-Garzê Orogenic Belt has been complicated by differing age spectra results. Therefore, in this study, the 40Ar/39Ar and sensitive high resolution ion micro-probe (SHRIMP) U-Pb dating methods were both used and the results compared, particularly with respect to dating data for Pankou and Pukouling granites from Xuebaoding, to establish ages that are close to the real emplacements. The results of SHRIMP U-Pb dating for zircon showed a high amount of U, but a very low value for Th/U. The high U amount, coupled with characteristics of inclusions in zircons, indicates that Xuebaoding granites are not suitable for U-Pb dating. Therefore, muscovite in the same granite samples was selected for 40Ar/39Ar dating. The 40Ar/39Ar age spectrum obtained on bulk muscovite from Pukouling granite in the Xuebaoding, gave a plateau age of 200.1±1.2 Ma and an inverse isochron age of 200.6±1.2 Ma. The 40Ar/39Ar age spectrum obtained on bulk muscovite from Pankou granite in the Xuebaoding gave another plateau age of 193.4± 1.1 Ma and an inverse isochron age of 193.7±1.1 Ma. The 40Ar/39Ar intercept of 277.0±23.4 (2,) was very close to the air ratio, indicating that no apparent excess argon contamination was present These age dating spectra indicate that both granites were emplaced at 200.6±1.3 Ma and 193.7±1.1 Ma, respectively. Through comparison of both dating methods and their results, we can conclude that it is feasible that the muscovite in the granite bearing high U could be used for 40Ar/39Ar dating without extra Ar. Based on this evidence, as well as the geological characteristics of the Xuebaoding W-Sn-Be deposit and petrology of granites, it can be concluded that the material origin of the Xuebaoding W-Sn-Be deposit might partially originate from the Xuebaoding granite group emplacement at about 200 Ma. Moreover, compared with other granites and deposits distributed in various positions in the Songpan-Garzê Orogenic Belt, the Xuebaoding emplacement ages further show that the main rare metal deposits and granites in peripheral regions occurred earlier than those in the inner Songpan-Garzê. Therefore, 40Ar/39Ar dating of Xuebaoding granite will lay a solid foundation for studying the occurrence and evolution of granite and rare earth element deposits in the Songpan-Garzê Orogenic Belt. [source]

    40Ar- 39Ar Age of K-feldspar from K-feldspar Granite in the Qiaohuote Copper Deposit, Bayanbulak, Xinjiang, and Its Geological Significance

    WANG Zhiliang
    Abstract, By using the 40Ar- 39Ar chronological method to date K-feldspar from K-feldspar granite in the Qiaohuote copper district, the authors obtained a plateau age of 274.78±0.44 Ma and an isochron age of 272.7±3.0 Ma. Because there is no tectonic deformation overprinted or hydrothermal alteration in the K-feldspar granite intrusion after its emplacement, the 40Ar- 39Ar age represents the crystallization age of K-feldspar in K-feldspar granite, i.e. the late crystallization age of the K-feldspar granite intrusion, which indicates that the K-feldspar granite formed in the intraplate extensional stage during the Early Permian. Moreover, based on the spatial relationship between the K-feldspar granite intrusion and copper orebodies, variations of copper ore grade, REE characteristics of K-feldspar granite and copper ores, and H and O isotopic compositions of fluid inclusions in copper ores, the metallogenesis of the Qiaohuote copper deposit is directly related to intrusive activities of the K-feldspar granite, and thus the crystallization age of K-feldspar in the granite approximately approaches the metallogenic epoch of the Qiaohuote copper deposit. [source]

    Sm-Nd, Pb-Pb and Rb-Sr Isotopic Dating and Its Dynamic Implications for the Proterozoic Augen Granite in the Yunkai Area, Western Guangdong Province

    PENG Songbai
    Abstract The samples of ductile-rheologic deformational augen granite from the Yunkai uplift area, western Guangdong province, were determined by the whole-rock Sm-Nd, Pb-Pb and Rb-Sr isotopic dating to have an Sm-Nd isochron age of 1414±68 Ma, a Pb-Pb isochron age of 1388±90 Ma and a Rb-Sr isochron age of 490±36 Ma. The first two ages are interpreted as the formation age of this suite of granite and the last age represents the timing of the tectono-thermal event of Caledonian ductile-rheologic shear partial melting. It is indicated that in the study area not only an orogeny took place in the Caledonian, but also a more important tectono-magmatic activity occurred in the Meso-proterozoic there, which may be related to the subduction-collision between the Yangtze block and Cathaysia block. [source]

    Geochemistry of K-feldspar and Muscovite in Rare-element Pegmatites and Granites from the Totoral Pegmatite Field, San Luis, Argentina

    RESOURCE GEOLOGY, Issue 4 2009
    Julio Oyarzábal
    Abstract The geochemistry of K-feldspar for K, P, Sr, Ba, Rb, Cs, Ga, and of muscovite for the same elements plus Nb and Ta, was used for proving the parental relationships of S-type granites and LCT (Li, Cs, Ta) rare-element pegmatites in the southernmost pegmatitic field of the Pampean pegmatite province in Argentina. The variation of K/Rb-Cs, K/Cs-Rb, K/Rb-Rb/Sr, K/Rb-Ba in K-feldspar from the granites and pegmatites show that they form an association with the evolutional sequence: granites , barren- to transitional pegmatites , beryl type, beryl-columbite-phosphate pegmatites , complex type of spodumene subtype pegmatites , albite-spodumene type , albite type pegmatites. This sequence reflects the regional distribution of the different magmatic units. The Ta-Cs diagram for muscovite reveals that none of the studied pegmatites exceed the threshold established in previous studies for being considered with important tantalum oxide mineralization. The granites and pegmatites constitute a rare-element pegmatitic field in which different magmatic units form a continuous fractionation trend, extended from the less evolved granitic facies to the most geochemically specialized pegmatites [source]

    Late Jurassic,Early Cretaceous Plutonism in the Northern Part of the Precambrian North China Craton: SHRIMP Zircon U,Pb Dating of Diorites and Granites from the Yunmengshan Geopark, Beijing

    Yuruo SHI
    Abstract: The Yunmengshan Geopark in northern Beijing is located within the Yanshan range. It contains the Yunmengshan batholith, which is dominated by two plutons: the Yunmengshan gneissic granite and the Shicheng gneissic diorite. Four samples of the Yunmengshan gneissic granite give SHRIMP zircon U,Pb ages from 145 to 141 Ma, whereas four samples of the Shicheng gneissic diorite have ages from 159 Ma to 151 Ma. Dikes that cut the Yunmengshan diorite record SHRIMP zircon U,Pb age of 162±2 and 156±4 Ma. The cumulative plots of zircons from the diorites show a peak age of 155 Ma, without inherited zircon cores, and the peak age of 142 Ma for granite is interpreted as the emplacement age of the Yunmengshan granitic pluton, whose igneous zircons contain inherited zircon cores. The data presented here show that there were two pulses of magmatism: early diorites, followed c13 Ma later by true granites, which incorporated material from an older continental crust. [source]

    Multi-phase evolution of gnammas (weathering pits) in a Holocene deglacial granite landscape, Minnesota (USA)

    David Domínguez-Villar
    Abstract The morphometry of 85 gnammas (weathering pits) from Big Stone County in western Minnesota allows the assessment of the relative ages of the gnamma population. The ratio between maximum and minimum depths is independent of the initial size of the cavity and only depends on the weathering evolution. Therefore, the distribution of depth ratios can be used to assess the gnamma population age and the history of weathering. The asymmetrical distribution of depth ratios measured in Big Stone County forms three distinct populations. When these sets are analyzed independently, the correlation (r2) between maximum and minimum depths is greater than 0·95. Each single population has a normal distribution of depth ratios and the average depth ratios (, -value) for each population are ,1 = 1·60 ± 0·05, ,2 = 2·09 ± 0·04 and ,3 = 2·42 ± 0·08. The initiation of gnamma formation followed the exhumation of the granite in the region. This granite was till and saprolite covered upon retreat of the ice from the Last Glacial Maximum. Nearby outcrops are striated, but the study site remained buried until it was exhumed by paleofloods issuing from a proglacial lake. These Holocene-aged gnammas in western Minnesota were compared with gnammas of other ages from around the world. Our new results are in accordance with the hypothesis that , -values represent the evolution of gnammas with time under temperate- to cold-climate dynamics. Phases of the formation of new gnammas may result from changes in weathering processes related to climate changes. Copyright © 2007 John Wiley & Sons, Ltd. [source]

    Patterns of damage in igneous and sedimentary rocks under conditions simulating sea-salt weathering

    C. Cardell
    Abstract A saline-spray artificial ageing test was used to simulate the effects produced in granites and sedimentary rocks (calcarenites, micrites and breccia) under conditions in coastal environments. Three main points were addressed in this study: the durability of the different kinds of rock to salt decay, the resulting weathering forms and the rock properties involved in the weathering processes. For this, mineralogical and textural characterization of each of the different rocks was carried out before and after the test. The soluble salt content at different depths from the exposed surfaces was also determined. Two different weathering mechanisms were observed in the granite and calcareous rocks. Physical processes were involved in the weathering of granite samples, whereas dissolution of calcite was also involved in the deterioration of the calcareous rocks. We also showed that microstructural characteristics (e.g. pore size distribution), play a key role in salt damage, because of their influence on saline solution transport and on the pressures developed within rocks during crystallization. Copyright © 2003 John Wiley & Sons, Ltd. [source]

    Weathering rates of buried machine-polished rock disks, Kärkevagge, Swedish Lapland

    Colin E. Thorn
    Abstract Some 48 dolomite, 37 granite, and 19 limestone machine-polished disks were buried at soil horizon boundaries in 16 soil pits to maximum depths of 0·5,0·6 m within Kärkevagge, a presently periglacial, glaciated trough in Arctic Sweden. The objective of the ongoing study is to determine near-surface ,potential' weathering rates beneath birch forest, Dryas heath, heath, meadow, solifluction meadow, and willow vegetation cover types. Selected disks were excavated after 1 year, and all disks were excavated after 4 or 5 years. The limestone disks weathered fastest, followed closely by the dolomite disks, but the granite disks weathered at much slower rates. Mean annual percentage mass loss over 4 or 5 years was 0·473 ± 0·145% (two standard errors) for dolomite disks, 0·032 ± 0·005% (two standard errors) for granite disks, and 1·104 ± 0·446% (two standard errors) for limestone disks. For dolomite disks the Dryas heath and heath environments weathered the most slowly, a trend that was generally followed by the granite disks. Poor drainage (i.e. increasing wetness) and decreasing pH were found to promote dolomite weathering rates in a statistically significant manner. Surficial and subsurface weathering rates at individual sites were generally found to be correlated for dolomite, but not granite. Copyright © 2002 John Wiley & Sons, Ltd. [source]

    Strontium isotopic characterization of the Palmottu hydrosystem (Finland): water,rock interaction and geochemistry of groundwaters

    GEOFLUIDS (ELECTRONIC), Issue 3 2003
    Philippe Negrel
    Abstract The Palmottu hydrosystem is located in a granitic host rock in southern Finland. Along well-defined pathways in the fractured crystalline rock, strontium isotopes are used to trace the degree of water,rock interaction (WRI) and mixing processes in groundwaters. The 87Sr/86Sr ratios range between 0.716910 and 0.735606 in the surface waters and between 0.719991 and 0.750787 in the groundwaters, but are between 0.720 and 0.735 in most of the samples. Moreover, the results show a lack of correlation between the water chemistries determining the classification into different water-types (Na,Cl, Na,SO4, etc.) and the results of the strontium (Sr) contents and Sr isotopic ratios. From a WRI standpoint, this implies that the Sr behaviour is independent of the water chemistry; the occurrence of large 87Sr/86Sr variations is site specific and mainly dependent on the lithology. A model to determine the 87Sr/86Sr ratio of water after interaction with granite was developed. This model is based on the assumption that Sr was derived from three minerals: plagioclase, K-feldspar and biotite. The results of the calculation indicate that around half of the water analysed within the Palmottu hydrosystem can be explained by the weathering of the granites. However, clearly lower 87Sr/86Sr are observed in waters when compared to the calculated 87Sr/86Sr and other sources of Sr, with low 87Sr/86Sr, rather than the calculated granite,water interaction, which may be suspected. When comparing the 87Sr/86Sr and ion ratios (Ca/Na, Mg/Na, Sr/Na, Cl/Na), the scattering of the data can be explained by the presence of four end-members: a brine component (low 87Sr/86Sr and Ca/Na ratios,), a deep granitic component (high 87Sr/86Sr ratios and low Ca/Na ratios,), a subsurface component (intermediate 87Sr/86Sr ratios associated with high Ca/Na ratios,) and a surface end-member:snow and river drainage (low 87Sr/86Sr and low Ca/Na ratios,). These extreme end-members define a series of WRI-mixing line within a rather complex hydrosystem. [source]

    Age and Origin of Upland block Fields on Melville Peninsula, Eastern Canadian Arctic

    Lynda A. Dredge
    Block fields have developed on gently graded uplands of granite and gneiss on central and southern Melville Peninsula. The location of block fields is not controlled by elevation, but rather by areas covered by cold,based ice during the last glaciation. Block fields consist either of angular boulders, sorted circles 3,4 m across having blocky rims and central areas of weathered grus, concentrations of openwork boulders, or in the southeast, of immature bouldery till. The block fields are primarily relict features predating the last glaciation, produced by weakening of bedrock by weathering along joints, followed by frost heave, although some block fields have been modified by meltwater from regional glacier down,wasting. Frost processes are active in the modern environment but the extent of riven bedrock and the size of recent patterned ground forms on postglacial surfaces are insufficient to account for the forms in the block fields. [source]

    The effect of inertial coupling on seismic reflection amplitudes

    Ashish Arora
    ABSTRACT A problem of reflection and transmission of elastic waves at a plane interface between a uniform elastic solid half-space and a porous elastic half-space containing two immiscible fluids is investigated. The theory developed by Lo, Sposito and Majer for porous media containing two immiscible fluids is employed to find out the reflection and transmission coefficients. The incident wave is assumed to propagate through the uniform elastic half-space and two cases are considered. In the first case, a beam of plane longitudinal wave is assumed to be incident and in the second case, a beam of transverse wave is assumed to be incident at the interface. By taking granite as impervious elastic medium and columbia fine sandy loam containing air-water mixture as porous medium, reflection and transmission coefficients are obtained. By neglecting the inertial coupling coefficients, these coefficients are reduced to those obtained by Tomar and Arora using the theory of Tuncay and Corapcioglu. It is found that the inertial coupling parameters significantly affect the phase speeds and the amplitude ratios of the transmitted waves. [source]

    Geophysical exploration for interlayer slip breccia gold deposits: example from Pengjiakuang gold deposit, Shandong Province, China

    Z. Qingdong
    ABSTRACT Interlayer slipping breccia-type gold deposit , a new type of gold deposit, defined recently in the northern margin of the Jiaolai Basin, Shandong Province, China , occurs in interlayer slip faults distributed along the basin margin. It has the features of large orebody thickness (ranging from 14 m to 46 m, with an average thickness of 30 m), shallow embedding (0,50 m thickness of cover), low tenor of gold ore (ranging from 3 g/t to 5 g/t), easy mining and ore dressing. This type of gold deposit has promising metallogenic forecasting and potential for economic exploitation. A ground gamma-ray survey in the Pengjiakuang gold-ore district indicates that the potassium/thorium ratio is closely related to the mineralization intensity, i.e. the larger the potassium/thorium ratio, the higher the mineralization. The gold mineralized alteration zone was defined by a potassium/thorium ratio of 0.35. A seismic survey confirms the location of the top and bottom boundaries and images various features within the Pengjiakuang gold mineralization belt. The gold-bearing shovel slipped belt dips to the south at an angle of 50,55° at the surface and 15,20° at depth. The seismic profile is interpreted in terms of a structural band on the seismic section characterized by a three-layered model. The upper layer is represented by weakly discontinuous reflections that represent the overlying conglomerates. A zone of stronger reflections representing the interlayer slip fault (gold-bearing mineralized zone) is imaged within the middle of the section, while the strongest reflections are in the lower part of the section and represent metamorphic rocks at depth. At the same time, the seismic reflection survey confirms the existence of a granite body at depth, indicating that ore-forming fluids may be related to the granite. A CSAMT survey showed that the gold-bearing mineralized zone is a conductive layer and contains a low-resistivity anomaly ranging from 2 ,m to 200 ,m. [source]

    Seismic anisotropy in granite at the Underground Research Laboratory, Manitoba

    Gordon M. Holmes
    The Shear-Wave Experiment at Atomic Energy of Canada Limited's Underground Research Laboratory was probably the first controlled-source shear-wave survey in a mine environment. Taking place in conjunction with the excavation of the Mine-by test tunnel at 420 m depth, the shear-wave experiment was designed to measure the in situ anisotropy of the rockmass and to use shear waves to observe excavation effects using the greatest variety of raypath directions of any in situ shear-wave survey to date. Inversion of the shear-wave polarizations shows that the anisotropy of the in situ rockmass is consistent with hexagonal symmetry with an approximate fabric orientation of strike 023° and dip 35°. The in situ anisotropy is probably due to microcracks with orientations governed by the in situ stress field and to mineral alignment within the weak gneissic layering. However, there is no unique interpretation as to the cause of the in situ anisotropy as the fabric orientation agrees approximately with both the orientation expected from extensive-dilatancy anisotropy and that of the gneissic layering. Eight raypaths with shear waves propagating wholly or almost wholly through granodiorite, rather than granite, do not show the expected shear-wave splitting and indicate a lower in situ anisotropy, which may be due to the finer grain size and/or the absence of gneissic layering within the granodiorite. These results suggest that shear waves may be used to determine crack and mineral orientations and for remote monitoring of a rockmass. This has potential applications in mining and waste monitoring. [source]

    Calibration of a discrete element model for intact rock up to its peak strength

    Yuannian Wang
    Abstract When three dimensional, bonded discrete element models (DEMs) are deployed to model intact rock, a basic question is how to determine the micro parameters that control macro properties of the modeled rock. After briefly describing the authors' DEM code, this paper describes algorithms to calibrate the model's micro parameters against standard laboratory tests, such as uniaxial and triaxial tests. Sensitivity analysis is used to identify the deformability micro parameters by obtaining relationships between microscopic and macroscopic deformability properties. The strength model parameters are identified by a global optimization process aimed at minimizing the difference between computed and experimental failure envelopes. When applied to the experimental results of Lac du Bonnet granite, this calibration process produced a good agreement between simulated and experimental results for both deformability and strength properties. Copyright © 2009 John Wiley & Sons, Ltd. [source]

    Analytical simulation of the dynamic compressive strength of a granite using the sliding crack model

    H. B. Li
    Abstract A sliding crack model is employed to simulate rock strength under dynamic compression. It is assumed that the growth and nucleation of a sliding crack array presented results in the shear fault failure and dominate the mechanical properties of rock material. The pseudo-tractions method is used to calculate the stress intensity factor of the sliding crack array under compression. With the utilization of a dynamic crack growth criterion, the growth of the sliding crack array is studied and the simulated strengths of a granite under dynamic compression are correspondingly obtained. It is concluded that the simulated rock strengths increase with increasing strain rates at different confining pressures, and the rising rates have a trend to decrease with increasing confining pressures. It is also indicated that the simulated rock strengths increase with increment of confining pressure at different strain rates, and the rising rates are almost identical at different strain rates. The simulation results are validated by the experimental data for the granite. Copyright © 2001 John Wiley & Sons, Ltd. [source]

    Adhesion of Listeria monocytogenes to materials commonly found in domestic kitchens

    Pilar Teixeira
    Summary The aim of this work was to investigate the adhesion of Listeria monocytogenes ATCC 15313 to glass, granite, marble, polypropylene from a bowl (PPb), polypropylene from a cutting board (PPcb) and stainless steel (SS), which are materials commonly used in kitchens. Marble and granite were chosen because they are applied as kitchen bench covers and pavements in many countries and there are no literature reports on their behaviour in terms of microbial adhesion. The effect of surface hydrophobicity and roughness on the adhesion process was also analysed. The results showed that the highest extent of adhesion of L. monocytogenes occurred to stainless steel, followed by glass and in less extent to the other materials studied. However, it was not possible to establish a correlation between surface hydrophobicity or roughness and the extent of adhesion of L. monocytogenes. The adherence of L. monocytogenes should be dependent on other factors, like the presence of exopolymers and surface charge. [source]

    Trends in woody vegetation cover in the Kruger National Park, South Africa, between 1940 and 1998

    H. C. Eckhardt
    Changes in the cover and density of shrubs and trees were assessed from aerial photographs (1940, 1974 and 1998) as well as from fixed-point photographs taken in 1984 and 1996 in the Kruger National Park, South Africa. Woody cover (trees and shrubs combined) increased by 12% on granite substrates but decreased by 64% on basalt substrates over the past 58 years. Both these figures are expressed in terms of the initial values, respectively. The density of the large tree component of woody vegetation decreased on both substrates. Woody vegetation cover declined as fire return periods became shorter, but the relationship was weak. The increases in woody plant density and cover on granite are thought to be the result of decreased competition from grasses, which in turn is a result of overgrazing by wild herbivores whose numbers have been kept high through the provision of surface water. These effects were not seen on the relatively nutrient-rich basalts, where grasses can recover rapidly even after heavy grazing. The decline in overall woody cover on basalts is interpreted as a result of regular, short-interval prescribed burning over the past 40 years, while the universal decline in large trees seems to result from an interaction between regular, frequent fires and utilization by elephants. The implications for management are discussed. Résuné On a évalué les changements du couvert et de la densité des arbustes et des arbres à partir de photos aériennes (1940, 1974 et 1998) et de photos prises d'un endroit fixe en 1984 et en 1996 dans le Parc National Kruger, en Afrique du Sud. Le couvert boisé (arbustes et arbres pris ensemble) a augmenté de 12% sur les substrats granitiques mais diminué de 64% sur les substrats basaltiques, au cours des 58 dernières années. Ces deux chiffres sont exprimés en termes de valeurs initiales, respectivement. La densité de la composante de grands arbres de la végétation boisée a diminué sur les deux substrats. Le couvert végétal boisé a diminué alors que les périodes de récupération entre les feux raccourcissaient, mais la relation est faible. On pense que l'augmentation de la densité et du couvert boisés sur le granite est le résultat d'une diminution de la compétition exercée par les herbes qui elle, résulte d'un surpâturage des herbivores sauvages dont le nombre a été maintenu élevé par l'apport d'eau de surface. On n'a pas observé ces effets sur les basaltes relativement riches en nutriments, car les herbes peuvent y repousser rapidement même après un pâturage intense. On interprète le déclin du couvert boisé global sur le basalte comme le résultat des feux provoqués régulièrement et avec de brefs intervalles depuis 40 ans, alors que le déclin général des grands arbres semble être le résultat d'une interaction entre les feux, réguliers et fréquents, et la présence des éléphants. On discute de ce que cela implique pour la gestion. [source]

    Zeolites in fissures of granites and gneisses of the Central Alps

    Abstract Six different Ca-zeolite minerals are widespread in various assemblages in late fissures and fractures in granites and gneisses of the Swiss Alps. The zeolites formed as a result of water,rock interaction at relatively low temperatures (<250 °C) in the continental upper crust. The zeolites typically overgrow earlier minerals of the fissure assemblages, but zeolites also occur as monomineralic fissure fillings. They represent the youngest fissure minerals formed during uplift and exhumation of the Alpine orogen. A systematic study of zeolite samples showed that the majority of finds originate from three regions particularity rich in zeolite-bearing fissures: (i) in the central and eastern part of the Aar- and Gotthard Massifs; (2) Gibelsbach/Fiesch, in a fissure breccia located at the boundary of Aar Massif and Permian sedimentary rocks; and (3) in Penninic gneisses of the Simano nappe at Arvigo (Val Calanca). Rail and road tunnel construction across the Aar- and Gotthard Massif provided excellent data on zeolite frequency in Alpine fissures. It was found that 32% (Gotthard NEAT rail base tunnel, Amsteg section) and 18% (Gotthard road tunnel) of all studied fissures are filled with zeolites. The number of different zeolites is limited to six species: laumontite, stilbite and scolecite are abundant and common, whereas heulandite, chabazite and epistilbite occur occasionally. Calcium is the dominant extra-framework cation, with minor K and Na. Heulandite and chabazite contain Sr up to 29 and 10 mol.% extra-framework cations respectively. Na and K contents in zeolites tend to increase during growth as a result of changes in fluid composition and/or temperature. The K enrichment of stilbite found in surface outcrops compared to subsurface samples may indicate late stage cation exchange with surface water. Texture data, relative age sequences derived from fissure assemblages and equilibrium calculations show that the Ca-dominated zeolites precipitated from fluid with decreasing temperature in the order (old to young = hot to cold): scolecite, laumontite, heulandite, chabazite and stilbite. The necessary components for zeolite formation are derived from dissolving primary granite and gneiss minerals. The nature of these minerals depends, among other factors, on the metamorphic history of the host rock. Zeolites in the Aar Massif derived from the dissolution of epidote, secondary calcite and albite that were originally formed during Alpine greenschist metamorphism from primary granite and gneiss assemblages. Zeolite fissures occur in areas of H2O-dominated fluids. This is consistent with equilibrium calculations that predict a low CO2 tolerance of zeolite assemblages, particularly at low temperature. [source]

    The formation of foliated (garnet-bearing) granites in the Tongbai-Dabie orogenic belt: partial melting of subducted continental crust during exhumation

    L. ZHANG
    Abstract Foliated (garnet-bearing) (FGB) granites are associated closely with and are usually the major wall rocks of the high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks in the Tongbai-Dabie region, the mid segment of the Qinling-Dabie-Sulu orogenic belt in central China. These granites appear either as small plutons or as veins, which commonly intrude into or surround the HP and UHP metamorphic eclogites or gneisses. The veins of FGB granites usually penetrate into the retrograded eclogites or gneisses along the foliations. Condensation rims can occasionally be found along the margins of granite veins. These granites are rich in Si and alkali with high Ga/Al ratios, and depleted in Ca, Mg, Al, Ti, Sc, V, Ni, Co, Cr and Sr, which are similar to A-type granites. In a chondrite normalized diagram, the samples are light rare earth elements enriched with different extent of negative Eu anomaly. Moreover, Rb, Nb, Ta, Sr, P and Ti show different degrees of negative anomalies, whereas Ba, K, La, Zr and Hf show positive anomalies in the primitive mantle normalized diagram. Negative anomalies of Eu and Sr indicate strong influence of plagioclase. In conventional discrimination diagrams, these FGB granites belong to the A-type granite, with geochemical characteristics affinitive to post-collisional granites. The ,Nd (230 Ma) values (,15.80 to ,2.52) and TDM values (1.02,2.07 Ga) suggest that magma for the FGB granites were derived from a heterogeneous crustal source. Therefore, the FGB granites may provide clues for deciphering the formation of post-collisional granites. It is proposed that the magma of the FGB granites both in the HP and UHP units was formed in an extensional tectonic setting slightly post-dating the HP and UHP metamorphism, most likely as a result of decompressional partial melting of UHP retrograded eclogites during exhumation. [source]

    Experimental deformation of partially melted granite revisited: implications for the continental crust

    Abstract A review and reinterpretation of previous experimental data on the deformation of partially melted crustal rocks reveals that the relationship of aggregate strength to melt fraction is non-linear, even if plotted on a linear ordinate and abscissa. At melt fractions, , < 0.07, the dependence of aggregate strength on , is significantly greater than at , > 0.07. This melt fraction (, = 0.07) marks the transition from a significant increase in the proportion of melt-bearing grain boundaries up to this point to a minor increase thereafter. Therefore, we suggest that it is the increase of melt-interconnectivity that causes the dramatic strength drop between the solidus and a melt fraction of 0.07. We term this drop the ,melt connectivity transition' (MCT). A second, less-pronounced strength drop occurs at higher melt fractions and corresponds to the breakdown of the solid (crystal) framework. This is the ,solid-to-liquid transition' (SLT), corresponding to the well known ,rheologically critical melt percentage'. Although the strength drop at the SLT is about four orders of magnitude, the absolute value of this drop is small compared with the absolute strength of the unmelted aggregate, rendering the SLT invisible in a linear aggregate strength v. melt-fraction diagram. On the other hand, the more important MCT has been overlooked in previous work because experimental data usually are plotted in logarithmic strength v. melt-fraction diagrams, obscuring large strength drops at high absolute strength values. We propose that crustal-scale localization of deformation effectively coincides with the onset of melting, pre-empting attainment of the SLT in most geological settings. The SLT may be restricted to controlling flow localization within magmatic bodies, especially where melt accumulates. [source]

    Ordovician high-grade metamorphism of a newly recognised late Neoproterozoic terrane in the northern Harts Range,central Australia

    I. S. Buick
    Abstract Granulite facies rocks from the northernmost Harts Range Complex (Arunta Inlier, central Australia) have previously been interpreted as recording a single clockwise cycle of presumed Palaeoproterozoic metamorphism (800,875 °C and >9,10 kbar) and subsequent decompression in a kilometre-scale, E-W striking zone of noncoaxial, high-grade (c. 700,735 °C and 5.8,6.4 kbar) deformation. However, new SHRIMP U-Pb age determinations of zircon, monazite and titanite from partially melted metabasites and metapelites indicate that granulite facies metamorphism occurred not in the Proterozoic, but in the Ordovician (c. 470 Ma). The youngest metamorphic zircon overgrowths from two metabasites (probably meta-volcaniclastics) yield 206Pb/238U ages of 478±4 Ma and 471±7 Ma, whereas those from two metapelites yield ages of 463±5 Ma and 461±4 Ma. Monazite from the two metapelites gave ages equal within error to those from metamorphic zircon rims in the same rock (457±5 Ma and 462±5 Ma, respectively). Zircon, and possibly monazite ages are interpreted as dating precipitation of these minerals from crystallizing melt within leucosomes. In contrast, titanite from the two metabasites yield 206Pb/238U ages that are much younger (411±5 Ma & 417±7 Ma, respectively) than those of coexisting zircon, which might indicate that the terrane cooled slowly following final melt crystallization. One metabasite has a second titanite population with an age of 384±7 Ma, which reflects titanite growth and/or recrystallization during the 400,300 Ma Alice Springs Orogeny. The c. 380 Ma titanite age is indistinguishable from the age of magmatic zircon from a small, late and weakly deformed plug of biotite granite that intruded the granulites at 387±4 Ma. These data suggest that the northern Harts Range has been subject to at least two periods of reworking (475,460 Ma & 400,300 Ma) during the Palaeozoic. Detrital zircon from the metapelites and metabasites, and inherited zircon from the granite, yield similar ranges of Proterozoic ages, with distinct age clusters at c. 1300,1000 and c. 650 Ma. These data imply that the deposition ages of the protoliths to the Harts Range Complex are late Neoproterozoic or early Palaeozoic, not Palaeoproterozoic as previously assumed. [source]

    X-ray Diffraction Investigations of Microstructure of Calcium Hydroxide Crystallites in the Interfacial Transition Zone of Concrete

    Valeri S. Harutyunyan
    This work presents theoretical and experimental X-ray diffractometry investigations of the morphology and deformation state of calcium hydroxide (Ca(OH)2, CH) crystallites in the interfacial transition zone of concrete. Based on the developed theoretical approach, the distributions of CH crystallites and their coherent domains over strains and sizes are reconstructed. The average amounts of planar defects, cracks, and possible stacking faults within a CH crystallite are estimated. A comparative analysis is conducted for the morphology of CH texture depending on the type of aggregate particles (granite and smoky quartz) used. [source]

    Geology and geochemistry of shallow drill cores from the Bosumtwi impact structure, Ghana

    Daniel Boamah
    The interior of the structure is largely filled by the 8 km diameter Lake Bosumtwi, and the crater rim and region in the environs of the crater is covered by tropical rainforest, making geological studies rather difficult and restricted to road cuts and streams. In early 1999, we undertook a shallow drilling program to the north of the crater rim to determine the extent of the ejecta blanket around the crater and to obtain subsurface core samples for mineralogical, petrological, and geochemical studies of ejecta of the Bosumtwi impact structure. A variety of impactite lithologies are present, consisting of impact glassrich suevite and several types of breccia: lithic breccia of single rock type, often grading into unbrecciated rock, with the rocks being shattered more or less in situ without much relative displacement (autochthonous?), and lithic polymict breccia that apparently do not contain any glassy material (allochtonous?). The suevite cores show that melt inclusions are present throughout the whole length of the cores in the form of vesicular glasses with no significant change of abundance with depth. Twenty samples from the 7 drill cores and 4 samples from recent road cuts in the structure were studied for their geochemical characteristics to accumulate a database for impact lithologies and their erosion products present at the Bosumtwi crater. Major and trace element analyses yielded compositions similar to those of the target rocks in the area (graywacke-phyllite, shale, and granite). Graywacke-phyllite and granite dikes seem to be important contributors to the compositions of the suevite and the road cut samples (fragmentary matrix), with a minor contribution of Pepiakese granite. The results also provide information about the thickness of the fallout suevite in the northern part of the Bosumtwi structure, which was determined to be ,15 m and to occupy an area of ,1.5 km2. Present suevite distribution is likely to be caused by differential erosion and does not reflect the initial areal extent of the continuous Bosumtwi ejecta deposits. Our studies allow a comparison with the extent of the suevite at the Ries, another well-preserved impact structure. [source]

    Sustainability of the dimension stone industry in Zimbabwe,challenges and opportunities

    Oliver Maponga
    Abstract The rapid expansion of the dimension stone and black granite industry in Zimbabwe in the 1990s, a result of increasing domestic and international demand for black granite, has ushered in new challenges and opportunities for producers and beneficiaries alike. This article investigates the social, environmental and economic challenges emanating from the export boom, the economic opportunities presented by the expanding market and the potential role the robust industry could play in regional economic development and poverty alleviation. The article evaluates the long-term sustainability of the dimension stone industry and recommends the reinvestment of revenues into diversification of the regional economic base, upgrading of local infrastructure, further processing of rough stone before export and encouragement of micro-businesses to enhance sustainability. The author argues for modification of economic incentives to incorporate environmental and social objectives in order to enhance sustainability. The need for a holistic approach to policies affecting the exploitation, processing and marketing of granite resources and their products is recognized. More research on the operations of the industry is called for. [source]

    Respiratory effects of exposure to low levels of concrete dust containing crystalline silica

    E. Meijer MD
    Abstract Background Dusts containing crystalline silica are generated in mining, construction, glass, granite and concrete production industries. The association between exposure to low levels of concrete dust containing crystalline silica and reduction in lung function, was evaluated in a cross-sectional study. Methods The study was carried out among 144 concrete workers, from two factories, with exposure assessment of respirable dust and silica by personal samplers. Results of respiratory questionnaires and standardized measurements of lung function were compared with the results in a control population. Multiple linear regression analysis was used in selecting factors that predict (age and standing height standardized residual) lung function. Results The average concentration of respirable dust in both factories was 0.8 mg/m3 and 0.06 mg/m3 for respirable silica. The average silica content of the dust was 9%. The average cumulative dust exposure was 7.0 mg/m3 year and cumulative silica exposure was 0.6 mg/m3 year. Significant associations between exposure to concrete dust and a small lung function (FEV1/FVC ratio, MMEF) loss were found, independent of smoking habits and of a history of allergy. Conclusions Our results indicate that, concrete workers with chronic obstructive pulmonary symptoms and/or work-related lower respiratory symptoms are at risk of having a reduction in lung function (FEV&1/FVC ratio) outside the 5th percentile of the external reference population, and therefore, of mild chronic obstructive pulmonary disease, at respirable concrete dust levels below 1 mg/m3 with a respirable crystalline silica content of 10% (TWA, 8 hr). Am. J. Ind. Med. 40:133,140, 2001. © 2001 Wiley-Liss, Inc. [source]

    REE, Mn, Fe, Mg and C, O Isotopic Geochemistry of Calcites from Furong Tin Deposit, South China: Evidence for the Genesis of the Hydrothermal Ore-forming Fluids

    RESOURCE GEOLOGY, Issue 1 2010
    Yan Shuang
    Abstract The Furong tin deposit in the central Nanling region, South China, consists of three main types of mineralization ores, i.e. skarn-, altered granite- and greisen-type ores, hosted in Carboniferous and Permian strata and Mesozoic granitic intrusions. Calcite is the dominant gangue mineral intergrown with ore bodies in the orefield. We have carried out REE, Mn, Fe, and Mg geochemical and C, and O isotopic studies on calcites to constrain the source and evolution of the ore-forming fluids. The calcites from the Furong deposit exhibit middle negative Eu anomaly (Eu/Eu*= 0.311,0.921), except for one which has an Eu/Eu* of 1.10, with the total REE content of 5.49,133 ppm. The results show that the calcites are characterized by two types of REE distribution patterns: a LREE-enriched pattern and a flat REE pattern. The LREE-enriched pattern of calcites accompanying greisen-type ore and skarn-type ore are similar to those of Qitianling granite. The REE, Mn, Fe, and Mg abundances of calcites exhibit a decreasing tendency from granite rock mass to wall rock, i.e. these abundances of calcites associated with altered granite-type and greisen-type ores are higher than those associated with skarn-type ores. The calcites from primary ores in the Furong deposit show large variation in carbon and oxygen isotopic compositions. The ,13C and ,18O of calcites are ,0.4 to ,12.7, and 2.8 to 16.4,, respectively, and mainly fall within the range between mantle or magmatic carbon and marine carbonate. The calcites from greisen and altered granite ores in the Furong deposit display a negative correlation in the diagram of ,13C versus ,18O, probably owing to the CO2 -degassing of the ore-forming fluids. From the intrusion to wall-rock, the calcites display an increasing tendency with respect to ,13C values. This implies that the carbon isotopic compositions of the ore-bearing fluids have progressively changed from domination by magmatic carbon to sedimentary carbonate carbon. In combination with other geological and geochemical data, we suggest that the ore-forming fluids represent magmatic origin. We believe that the fluids exsolved from fractionation of the granitic magma, accompanying magmatism of the Qitianling granite complex, were involved in the mineralization of the Furong tin polymetallic deposit. [source]

    Mineralogical and Geochemical Characterization of Beryl-Bearing Granitoids, Eastern Desert, Egypt: Metallogenic and Exploration Constraints

    RESOURCE GEOLOGY, Issue 2 2009
    Hamdy M. Abdalla
    Abstract Mineral chemistry and geochemical characteristics of beryl-bearing granitoids in Eastern Desert of Egypt, were examined in order to identify the metallogenetic processes of the host granitoids. The investigated Be-bearing granitoids and type occurrences are classified into two groups: (i) peraluminous, Ta , Nb + Sn + Be ± W-enriched, Li-albite granite (e.g. Nuweibi and Abu Dabbab); and (ii) metasomatized, Nb >> Ta + Sn + Be ± W ± Mo-enriched alkali feldspar granite (i.e. apogranite; e.g. Homr Akarem, Homr Mikpid and Qash Amir). In these two groups, beryl occurs as stockwork greisen veins, greisen bodies, beryl-bearing cassiterite ± wolframite quartz veins, dissemination, and miarolitic pegmatites. Beryl of the Be-granitoids, particularly those of miarolitic pegmatites, contains appreciable contents of Fe, Na, and H2O. An important feature of the Be-apogranites is the occurrence of white mica as the sole mafic mineral in the unaltered alkali feldspar granite in lower zones. Presence of white mica as volatile-rich pockets suggests that the melt underwent disequilibrium crystallization, rapid nucleation rates, and exsolving and expulsion of volatiles. [source]

    The Cornubian Batholith: an Example of Magmatic Fractionation on a Crustal Scale

    RESOURCE GEOLOGY, Issue 3 2006
    Bruce W. Chappell
    Abstract. The Cornubian Batholith comprises six major and several smaller bodies of S-type granite in southwestern England. These late-Variscan granites comprise two-mica granites, and much less abundant Li-mica granites that are restricted to one of the major bodies (St Austell) and smaller bodies. Some of these intrusive rocks are associated with major Sn mineralization. This paper is concerned with the geochemistry of the two-mica granites, which are felsic, strongly peraluminous, and have a high total alkali content and low Na:K. Rocks with very similar compositions to these granites occur elsewhere, including the Variscan granites of continental Europe, and in southeastern Australia. In detail all of the major plutons of this batholith have distinctive compositions, except for Bodmin Moor and Carnmenellis which cannot be discriminated from each other compositionally. A comparison with experimental data shows that the granites attained their major element composition under conditions of crystal-liquid equilibrium, with the final melt being saturated in H2O, at temperatures close to 770d,C and pressures about 50 MPa. That temperature estimate is in good agreement with values obtained from zircon saturation thermometry. The specific minimum-temperature composition excludes the possibility of widespread transfer of elements during hydrothermal alteration. Minor elements that are relatively very abundant are Li, B, Cs and U, while F, Ga, Ge, Rb, Sn, Ta, W and Tl are quite abundant and P is high for felsic rocks. Sr, Ba, and the trace transition metals Sc to Zn, are low, but not as low as they commonly are in very felsic granites. These trace element abundances, and the EL2O-saturation, resulted from the fractional crystallization of a melt derived by the partial melting of feldspathic greywackes in the crust. The Cornubian granites have compositions very similar to the more felsic rocks of the Koetong Suite of southeastern Australia, where a full range of granites formed at the various stages of magmatic fractionation postulated for the Cornubian granites, can be observed. The operation of fractional crystallization in the Cornubian granites is confirmed by the high P abundances in the feldspars, with P contents of the plagioclase crystals correlating with Ab-con-tent Most of the granites represent solidified melt compositions but within the Dartmoor pluton there is a significant component of granites that are cumulative, shown by their higher Ca contents. The Cornubian plutons define areas of high heat flow, of a magnitude which requires that fractionated magmas were transported laterally from their sources and concentrated in the exposed plutons. The generation of these granite plutons therefore involved magmatic fractionation during the stages of partial melting, removal of unmelted material from that melt, and fractional crystallization. During the later stages of those processes, movement of those magmas occurred on a crustal scale. [source]