Granitic Rock Samples Research Articles

Rare-Earth-Element and Isotopic Evidence for the Genesis of the Bajiazi Stratabound Sulfide Ore Bodies of Northeast China

This study examines the rare-earth-elemenl and isotopic geochemistry of sulfide ores and associated rocks in the Bajiazi sulfide deposits of northeastern China. The distribution and concentration of rare earth elements (REE) in sulfide ores and associated rocks from the Bajiazi deposits have been determined by inductively coupled plasma spectroscopy (ICP) methods. Birdwing-shaped rare-earth-element profiles are only observed in granitic rocks at Bajiazi. Diverse rare-earth-element profiles of the ores and dolostones are different from those of granite and are interpreted to reflect an early sedimentary-diagenetic mineralization stage during which rare earth elements were added to the ores and host rocks. The characteristic features of the abundances of REE in the Bajiazi samples, including granite and sedimentary host rocks, are demonstrated by a variety of Eu anomalies in the chondrite-normalized REE pattern. Eu is the most mobile element, exhibiting increasingly negative anomalies in granite rock samples relative to the depletion of the sulfide ores and associated sedimentary host rocks, which are considered to be caused by preferential scavenging of Eu from sea water to sediments. The δ34S values of sulfide minerals at Bajiazi generally range from −12.3 to 14.2%, suggesting reduction both of marine sulfate and biogenic sources. The δ34S values from different ore types, sulfide minerals, and mines indicate a sedimentary exhalative origin, although they were, to some extent, homogenized during late overprinting. The late-stage sulfides from Bajiazi are isotopieally remarkably homogeneous and are significantly, although slightly, enriched in δ34S. These results imply multiple sources of sulfur derived from biogenic reduction, and/or sulfate in oceanic and/or connate waters, or from marine evaporites, and/or from magmatic hydrothermal sources. Isotopic temperatures from intersulfide fractionations (pyrite, sphalerite, and galena) range from 75° to 542°C, indicating the Bajiazi sulfide minerals have experienced different mineralizing stages. Galena in the Bajiazi Proterozoic sediment-hosted Pb-Zn sulfide deposits has a very uniform Pb-isotope composition, with 206Pb/204Pb = 16.07 to 16.58, 207Pb/204Pb = 15.00 to 15.66, and 208Pb/204Pb = 36.13 to 36.92, suggesting that it all formed from a common mineralizing fluid. The variation of lead-isotopic values of ores is similar to that of their host sedimentary strata. The lead in the various ore types and host rocks within the Bajiazi district is virtually identical. Lead-isotope signatures, which generally plot very close to model crustal growth curves, characterized by a model age of ∼1350 Ma, indicate a normal, nonradiogenic origin and an Early Proterozoic upper-crustal source for the lead in the ores. That lead was unrelated to the radiogenic lead in the Mesozoic granite. The δ13C values range from +1 to −5% PDB, and the δ18O values from −6 to −15% PDB. Later crystallization generations are enriched in the light isotopes of carbon and oxygen compared to early generations. The characteristics of carbon and oxygen isotopes of the Bajiazi deposits indicate that: (1) primary ores were precipitated in the Proterozoic marine environment; (2) both ores and host carbonates from the Bajiazi district are remarkably homogeneous in their carbon- and oxygen-isotope compositions during later tectonic, conlact-metamorphic stages; (3) although the variations in isotopic composition of carbon and oxygen are relatively small, a significant evolution toward a lighter isotopic composition with advancing diagenetic, tectonic, and contact metamorphic processes is observed; and (4) this evolution is independent of the presence or absence of ore minerals.

Correlation of mineralogical and textural characteristics with engineering properties of selected granitic rocks from Turkey

Granitic rocks show a variety of engineering properties that may affect quarrying operations, tunneling, mining, slope stability and the use of rock as a construction material. The physical and mechanical properties are a function of the mineralogical and textural characteristics of the rock. The purpose of this study is to apply correlation analysis to investigate the relationships between petrographical and engineering properties of granitic rocks. A variety of granitic rock samples from different parts of Turkey were subjected to petrographic studies. The same samples were then tested to determine specific gravity, dry and saturated unit weight, water absorption, effective and total porosity, sonic velocity, Schmidt hardness, point load strength index, uniaxial compressive strength, tensile strength and modulus of elasticity. The relationships between these properties and the petrographical characteristics are described by simple regression analyses. The study revealed that the influence of the textural characteristics on the engineering properties appears to be more important than the mineralogy. It also determined that the types of contacts, grain (mineral) shape and size significantly influence the engineering properties of the granitic rocks.

Reassessment of the north-western border zone of the Palaeoproterozoic Ketilidian orogen, South Greenland

As part of ongoing research into the plate tectonic setting of the Palaeoproterozoic Ketilidian orogen led by the Geological Survey of Greenland and Denmark, four geologists from Denmark and the U.K. re-examined parts of the north-western border zone in July–August 1997. The field work was generously supported by the Danish Natural Science Research Council and the Carlsberg Foundation. One team studied the Proterozoic (Ketilidian) sedimentary and volcanic rocks and the regional structure, working from six inland camps along the variably deformed Archaean–Proterozoic unconformity between Midternæs and Qoornoq and on Arsuk Ø (Fig. 1). A second team investigated the plutonic and kinematic evolution of the Kobberminebugt area at the northwestern margin of the Julianehåb batholith (Fig. 1); the latter forms the central part of the Ketilidian orogen (Chadwick & Garde 1996). In addition, samples of volcanic and granitic rocks were collected for geochemical studies and dating of depositional and tectonic events. The first systematic study of the Ketilidian orogen took place in the 1960s and was largely concentrated in its western and southern parts (Allaart 1976). Essential new data from the central and eastern parts of the orogen were acquired during the Survey’s SUPRASYD project (1992–1996; e.g. Garde & Schønwandt 1994, 1995; Garde et al. 1997; Stendal et al. 1997), which was initiated with the aim of assessing the potential for mineral resources in supracrustal sequences (Nielsen et al. 1993). In the course of the SUPRASYD project a new plate-tectonic model for the entire orogen was also published (Chadwick & Garde 1996), in which the orogen is viewed as the result of oblique convergence between the Archaean craton of southern Greenland and a supposed oceanic plate located south of the present orogen, which was subducted towards the north. Chadwick & Garde (1996) also suggested a new division of the Ketilidian orogen into a ‘Border Zone’ adjacent to the Archaean craton, the ‘Julianehåb batholith’ (formerly the Julianehåb granite) in the central part of the orogen, and the ‘Psammite Zone’ and ‘Pelite Zone’ to the south-east, which largely consist of deformed and metamorphosed erosion products derived from the evolving batholith. The north-western border zone and the Ketilidian supracrustal sequences were mapped in the 1960s by Harry & Oen Ing Soen (1964), Watterson (1965), Bondesen (1970), Higgins (1970), Muller (1974), Berthelsen & Henriksen (1975) and Pulvertaft (1977). It was shown that an Archaean gneiss complex (part of the Archaean craton of southern West Greenland) and Palaeoproterozoic basic igneous rocks (the so-called ‘MD’ (metadolerite) dyke swarms and related intrusions), and the unconformably overlying Ketilidian supracrustal rocks are progressively affected by Ketilidian deformation and metamorphism towards the Julianehåb batholith in the south and south-east. Boundary relationships were reviewed by Henriksen (1969). Where the Ketilidian supracrustal rocks are best preserved at Midternæs and Grænseland, Bondesen (1970) and Higgins (1970) divided them into the Vallen Group, which largely consists of sedimentary rocks (Table 1), and the overlying Sortis Group, in which basic pillow lavas and related doleritic to gabbroic sills predominate. Supracrustal rocks of presumed Palaeoproterozoic age further south have previously been referred to as the Qipisarqo and Ilordleq Groups (Berthelsen & NoeNygaard 1965; Allaart et al. 1969). Based on data collected in the 1960s, the earliest plate-tectonic interpretation of the Ketilidian orogen included a prominent suture in Kobberminebugt (Windley 1991). However, other critical aspects of Windley’s model were not substantiated during the Survey’s studies in 1992–1996 (Chadwick et al. 1994; Chadwick & Garde 1996), and a re-evaluation of the north-west border zone was therefore a natural focus of subsequent investigations.

Geochronology of the northern Idaho batholith and the Bitterroot metamorphic core complex: Magmatism preceding and contemporaneous with extension

Granitic plutonism and extension are broadly contemporaneous in many metamorphic core complexes. However, the relationship between magmatism and extension is rarely unambiguous. The northern Idaho batholith (Idaho-Bitterroot batholith), Montana and Idaho, composes the footwall for most of the Bitterroot metamorphic core complex and thus is an ideal area for assessing the relationships between magmatism and extension. We analyzed zircon from six samples of granitic rock from the Idaho-Bitterroot batholith using the SHRIMP (II) ion microprobe. Three samples of mylonitic granite from the Bear Creek pluton, Lost Horse Canyon, give a weighted mean 206 Pb/ 238 U age of 54.3 ± 0.7 Ma. A protomylonitic granite from the central part of the Bitterroot core complex (also Bear Creek pluton) gives a similar 206 Pb/ 238 U age of 54.6 ± 0.8 Ma. Mylonitic megacrystic granite from Sweathouse Canyon yields an age of 63.6 ± 0.6 Ma. A granite sample from the Lochsa Canyon, in the central Idaho-Bitterroot batholith, gives an age of 56.7 ± 1.0 Ma. Inherited zircon from the granitoids ranges in age from 800 to 1820 Ma, but the majority of grains have formation ages of 1750–1800 Ma. This suggests that Paleoproterozoic crust dominates the source region of the Idaho-Bitterroot batholith. Hornblende 40 Ar- 39 Ar age spectra for mafic dikes intruded during late-stage crystallization of main-phase granite in the central Idaho-Bitterroot batholith suggest crystallization of the main-phase plutons in this area at ca. 57 Ma. New and previously published 40 Ar- 39 Ar and K-Ar apparent ages of biotite and muscovite from the Lochsa River area and the western and central Bitterroot core complex are 50 to 47 Ma. Younger mica ages (46–43 Ma) are restricted to the vicinity of the Bitterroot mylonite zone. These results indicate that the cessation of main-phase magmatism within the Bitterroot metamorphic core complex migrated east with time, and that most of the plutons in the core complex were intruded during the Paleocene and early Eocene. When the regional tectonic setting changed from compression to extension at ca. 50 Ma, the late stages of mid-crustal, peraluminous plutonism appear to have been localized within the Bitterroot core complex. The presence of the youngest mid-crustal plutons in this area may have focused extensional deformation leading to the thick mylonite zone, as a consequence of rheological contrasts with cooler areas to the east and west. A progression of K-Ar and 40 Ar- 39 Ar cooling ages from west to east within the core complex part of the batholith is consistent with top-to-the-east shear indicators in the mylonite zone. Thermochronology indicates that the western part of the Bitterroot metamorphic core complex was below ≈350°C at the same time as the last stage of granite emplacement and metamorphism in the east. Therefore, the transition from mylonitization to brittle deformation to inactivity of the shear zone was progressive from west to east across the core complex from ca. 50 to 44 Ma. These features offer an explanation for the previously enigmatic occurrence of amphibolite facies ductile deformation in the eastern part of the core complex coincident with emplacement of epizonal, alkali-feldspar granite plutons along the western side of the complex.

The importance of apatite composition and single-grain ages when interpreting fission track data from plutonic rocks: a case study from the Coast Ranges, British Columbia

Fission track results determined from granitic rocks exposed within the Coast Ranges of British Columbia indicate that significant variations in apatite chemistry may occur in small plutonic rock samples. To test ages determined earlier by Parrish using the population method of fission track dating, new apparent apatite ages were completed using the external detector method so that single-grain ages could be generated. In most cases, the new apatite ages were similar to the earlier ones (within ±2σ). However, in many cases, it was found that individual samples contained significant variations in single-grain apatite ages. Microprobe analyses showed the spread in ages were directly related to variations in apatite chemistry, such that chlorine-rich apatites retained older ages than fluorine-rich grains. Examination of the single-grain age data suggested that the spread in single-grain ages provided information which constrained the thermal history of the sample and that distributions of single-grain ages reflected the temperatures at which they resided during cooling. These results suggest that it is important to generate more than a few single-grain fission track ages when dating apatite from granitic rocks, since variations in the chlorine concentration in apatite grains may exist in kg-sized granitic rock samples. Therefore, it is no longer a valid assumption that each apatite grain separated from a small granite sample will have a similar chlorine concentration, and in future apatite fission track studies of granitic terrains it is important to recognize that the apatite composition might be heterogeneous when interpreting results.

Analysis of rare earth elements in standard samples of granitic rocks by inductively coupled plasma mass spectrometry after acid digestion and alkali fusion.

Acid digestion and alkali fusion were applied to sample preparation of granitic rocks for the rare earth elements (REE) analysis by inductively coupled plasma mass spectrometry (ICP-MS). Granitic rock samples were decomposed by conventional rock-dissolution techniques, and the residues were fused with the mixture of Na2CO3 and H3BO4. The fusion ensured complete dissolution of acid-resistant minerals. Appropriate amounts of major elements were added to the REE calibration standard solution for matrix matching. The conditions of the instrument were adjusted to minimize the formation of the oxide molecular ions. Fourteen REE in three standard samples of granitic rocks were determined successfully.

Variation of radon-222 concentration in exposure systems air under different conditions of exposure

Simplified, laboratory scale systems, namely ordinary laboratory desiccators and cylindrical containers were tested with regard to their reliability as exposure systems for determining certain parameters of radon emanation from locally obtained crushed granite rock samples. The samples were placed inside the exposure systems. Activity concentration of emanated radon in the exposure systems air increased with increase of mass of granite sample in the desiccator and with length of the exposure period. Activity concentration of radon was high near the granitic source but decreased with vertical distance from it when the exposure system was semiclosed but activity was homogeneous when the system was completely closed. The cylindrical exposure system was used in assessing Ra-226 content in some crushed granitic samples identified as altered alkali granite and found to be: 0.024 Bq g−1 (0.65 pCig−1). Rn-222 emanation rate from the same samples was: 0.013 Bq m−2 s−1 (0.34 pCi m−2 s−1). Saturation density thickness for a mixed sample of pure and alkali granites was found to be 116 g cm−2. The results agree in general with reported observations and support the reliability of the exposure systems used.

Evaluation of Zr and Hf Data for Some Geological Standard Rock Samples

Zirconium and hafnium contents were determined by radiochemical neutron activation analysis for some basaltic and granitic geological standard rock samples. Our values for Zr and Hf are consistent with the literature values for basaltic samples, but are systematically higher than the literature data, especially for Zr, for granitic samples, though their relative ratios were found to be in good agreement with meteoritic values. The present work suggests that the literature values of Zr and Hf are systematically low for some granitic rock standard samples.

Structural relationships and SrNd isotope systematics of polymetamorphic granitic gneisses and granitic rocks from central Rajasthan, India: implications for the evolution of the Aravalli craton

Abstract This paper reviews the state of structural and geochronological knowledge of Precambrian cratonic rocks in part of central Rajasthan, India, and contributes new observations and isotopic data on the structural features and age of granitic rocks in the area. The granitic rocks comprise plutons of two suites (BGC and Sendra suites), which intrude ortho- and paragneisses the Archean Banded Gneiss Complex (BGC) and its inliers, as well as the Proterozoic Delhi Supergroup. Nearly all the plutons show a foliation and a lineation, which are mostly the result of solid state deformation, although a magmatic component of strain is apparent in some of them. The intensity of these structural fabrics varies from weak to extremely strong. Samples of granitic rocks from the BGC suite yield ages from SmNd isotopic data which scatter around a 2.83 Ga reference isochron [ϵNd(t) of ∼+4.1 ], with depleted mantle model ages ranging from 2.6 to 3.0 Ga. Scatter of the data suggests that the BGC is a composite group of gneisses, and confirms the antiquity of the BGC within this part of the Aravalli-Delhi orogenic belt. RbSr isotopic data of the BGC suite lie on a 1.7 Ga reference isochron, indicating a thermal event at that time. Data from plutons of the Sendra suite, on the other hand, show some scatter but define a model II isochron with slope equivalent to an age of 0.966±250 Ga (initial 87 Sr 86 Sr of 0.70390±0.01889, MSWD-311). The scatter of data could reflect open system behavior during a subsequent thermal event (Malani volcanics? ), although independent evidence for such is lacking in the study area. Ages calculated from our RbSr data are within the error of published results from previous workers. Structural observations on granitic rocks within the BGC inlier at Ajmer suggest that the inlier could represent a remobilized diapiric ridge emplaced into the Delhi Supergroup during the 1.7 Ga thermotectonic event contemporaneously with formation of D1 structures in the wall rock, although the radiometric age of the latter structures is not well-constrained. Microtexture, mineralogy, and field relations of granitic rocks of the Sendra suite point to those plutons being emplaced into the Delhi Supergroup during the 0.966 Ga thermotectonic event late syntectonic relative to D2 indicating that D2 is ∼ 0.96–1.0 Ga. D3 represents retrograde deformation synchronous with cooling of the regional thermal framework and local pluton cooling, probably between ∼ 0.96 and 0.90 Ga. Further geochronological, geochemical and structural work on the plutons and wall rock of both suites is necessary to establish a rigorous thermotectonic chronology by which to accurately model the crustal evolution of this part of central Rajasthan.

Fluorine distribution in waters of Nalgonda District, Andhra Pradesh, India

Geochemical and hydrochemical studies were conducted in Nalgonda District (A.P.), to explore the causes of high fluorine in waters, causing a widespread incidence of fluorosis in the local population. Samples of granitic rocks, soils, stream sediments, and waters were analyzed for F and other salient chemical parameters. Samples from the area of Hyderabad city were analyzed for comparison. The F content of waters in areas with endemic fluorosis ranges from 0.4 to 20 mg/l. The low calcium content of rocks and soils, and the presence of high levels of sodium bicarbonate in soils and waters are important factors favoring high levels of F in waters.

Sm-Nd and U-Pb Isotopic Evidence of Juvenile Crust in the Adirondack Lowlands and Implications for the Evolution of the Adirondack Mts.

Sm-Nd investigations of Hyde School Gneiss yield $T_{DM}$ ages only slightly older than U-Pb zircon emplacement ages thus indicating substantial addition of juvenile, calcalkaline crust to the Adirondack lowlands at ca. 1300 Ma. Juvenile crust of similar age has been recognized within the Adirondack highlands, the Elzevir terrane further to the northwest in Canada, and in the Green Mts. of Vermont suggesting the presence of an extensive magmatic arc at that time. While the precise nature of the arc remains uncertain, an Indonesian analogue is favored. U-Pb zircon ages of ca. 1150-1160 Ma are reported for lowlands samples of syenitic and granitic rocks belonging to the anorthosite-mangerite-charnockite-granite (AMCG) suite that also occurs in the highlands, as well as in the Canadian sector of the Frontenac terrane. The regional occurrence of these anorogenic intrusions indicates that the Adirondack highlands and Frontenac terrane, including the Adirondack lowlands, constituted a single, contiguous entity by ca. 1160 Ma. Monazite ages from AMCG rocks and metamorphic zircons from Hyde School Gneiss indicate that the last high grade metamorphism (i.e., upper amphibolite facies) in the Adirondack lowlands occurred at ca. 1150 Ma. The presence of 1050 Ma granulite facies assemblages east of the Carthage-Colton mylonite zone (CCMZ) in the highlands indicates the presence of a discontinuity across the CCMZ, which may manifest ca. 1050 Ma downfaulting of the lowlands block along the northwest dipping CCMZ in response to verthickened crust during the Ottawan orogeny.

高知県南西部柏島‐沖ノ島地域の花こう岩類の岩石学的研究

From the mode of occurrence, texture, and mineral assemblage, granitic rocks in the Kashiwa-jima-Okinoshima district are classified into four groups; namely, the Tanijiri-type granodiorite (abbreviated to TGd), the Tanijiri-type granodiorite porphyry (TGdp), the Moshima-type medium-grained granite (MmGr), and the Moshima-type fine-grained granite (MfGr). The similarity of major and trace element chemistry of the TGd and TGdp, and of the MmGr and MfGr suggests that the petrogenetic processes operating were similar, respectively. The Rb-Sr isotopic analyses of nine whole-rock samples of the Moshima-type granitic rocks (MmGr and MfGr) yield an age Rb-Sr of 16±2 Ma with an initial 87Sr/86Sr ratio of 0.70740± 0.00018. From the data of Nd and Sr isotopic ratios (Fig. 9) and of initial 87Sr/86Sr ratios and reciprocals of the strontium concentration (Fig. 10), it is suggested that the Tanijiri-type granodioritic rocks have been formed by mixing of an original S-type magma and sedimentary rocks of the Shimanto belt, while the Moshima-type granitic rocks have been formed by fractional crystallization of a S-type magma, which has been more felsic than the original S-type magma. From the chemical composition of garnet in the TGdp, it is suggested that the original S-type magma of the Tanijiri-type granodioritic rocks may have been generated by the partial melting of metamorphic rocks of pelitic and psammitic origin at a depth of about 20km. The TGdp shows a distinct porphyritic texture compared with the TGd. Minerals, such as plagioclase, orthopyroxene, cordierite, and biotite in the TGd have slightly wider range of solid solution compared with the minerals in the TGdp. Therefore, it is suggested that the petrogra-phical difference between the TGd and TGdp may have been caused by a difference in the ascent and cooling rate of their magmas.

Dynamic elasticity and strength anisotropy of some surficial rocks

Abstract Rock mass classification systems that comprise both rock material properties and large-scaldiscontinuities are essential in surface mining operations, tunneling and rock foundation engineering. As reported in this paper, point load strength measurements made on limestone, sandstone, granite, basalt and syenite rock samples indicate anisotropy ratios (axial to diametral) ranging approximately from 2.0 to 9.5. In general, direct proportionality exists between measured strength values in the two orthogonal directions. Furthermore, axial and diametral strength values are respectively, directly proportional to the dynamic elastic modulus of the samples. Values for the latter were obtained indirectly through density and sonic wave transit time measurements. Although it is known to be significant in rock excavation processes, rock quality designation (RQD) exhibits no discernible relationship with elastic modulus or point load strength of each of the rock samples tested. Along with the results of previo...

Magnetic susceptibility and relation to initial 87Sr/86Sr for granitoids of the central Sierra Nevada, California

Measurement of the magnetic susceptibility of more than 6000 samples of granitic rock from the Mariposa 1° by 2° quadrangle, which crosses the central part of the Sierra Nevada batholith between 37° and 38°N latitude, shows that magnetic susceptibility values are above 10−2 SI units in the east and central parts of the batholith and drop abruptly to less than 10−3 SI units in the western foothills. In a narrow transitional zone, intermediate values (10−3 to 10−2) prevail. Magnetic susceptibility appears to decrease slightly westward within the zones of both high and low values. Magnetic susceptibility in plutonic rocks is chiefly a function of the abundance of magnetite, which depends, in turn, on the total iron content of the rocks and their oxidation ratio. Lower magnetic susceptibilities of felsic members of Sierran intrusive suites and of some felsic rock units relative to adjacent mafic rock units commonly reflect differences in total iron content, but the differences of magnetic susceptibility that define the regional pattern generally are much larger and are determined chiefly by the oxidation ratios of the rocks. The relatively unaltered condition of the samples and restriction of δ18O to the range of +7 to +10.3‰ indicate that neither hydrothermal fluids nor subsolidus alteration were important in modifying oxidation ratios. Correlations of magnetic susceptibility with initial 87Sr/86Sr suggest that oxidation ratios have been inherited from the source regions for the magmas from which the rocks crystallized. Reduction of Fe3+ to Fe2+ by organic carbon or other reducing substances may also have affected magnetic susceptibility.

Laboratory study of fluid pressure diffusion in rock using acoustic emissions

In many geological situations, there is a need to monitor fluid migration and associated phenomena. Acoustic emissions (AE) that result from fluid pressure diffusion from a borehole can potentially provide useful information about migration paths, local stress states and local strength distributions. However, AE may also result from regional differential stress as well as mechanical stress concentrations associated with the injection borehole. To attempt to differentiate between these possible AE generating situations, we performed several experiments in unjointed granite rock samples containing a borehole. Fluid diffusion into and out of the borehole was achieved, under hydrostatic to high deviatoric far‐field stress states, while concurrently recording digitized waveforms of AE at 20 transducer locations and measuring acoustic velocity changes. Over 2000 AE events were located with a precision better than 2 mm, and focal mechanisms were obtained for the majority of them. Most acoustic emission events could be modeled by a shear displacement source mechanism giving rise to a quadrature polarity pattern usually associated with a double couple. A significant fraction of the events, however, were not so simply modeled and may indicate a combination of extension and shear displacement at the source. Very few pure dilatational or compressional events occurred. Results indicate that diffusing fluids, at pressures less than the local tensile strength, produce useful accompanying AE only in the presence of a large differential stress state surrounding the borehole. It is difficult, however, to distinguish events produced solely as a result of fluid pressure diffusion from those resulting from regionally applied shear stresses. A high‐pressure fluid leak at a metal‐rock interface was easily detected using AE, but mapping of isobaric pore pressure fronts in the presence of background seismicity was less successful. A model is developed to explain the difficulty. The spatial patterns of the acoustic emission locations appeared to be fractal and might be used to infer information about the magnitude and spatial distributions of local strength in the rock.

Prediction of changes in elastic moduli due to water-rock interaction during operation of granitic hdr geothermal system

Young's modulus of granitic rock samples under the simulated conditions of a geothermal reservoir, such as a pressurized high temperature water environment up to 350°C, was measured by means of the compliance calibration technique. The results obtained indicate a significant change in modulus with increasing test temperature and exposure time in a hydrothermal environment, which can be considered as a consequence of an increase in microcrack aperture due to chemical dissolution. The applicability of the laboratory results to hot dry rock geothermal field data is also demonstrated.

Computer Assisted INAA Measurements and Statistical Feature Extraction in Raw Material Explorations

Some series of granitic rock samples have been investigated by the aid of minicomputer spectral analysis after neutron activation. Maintaining an optimal relation between measurement time, precision of analysis, and a high sample through-put as well as a great number of elements, the concentrations of some elements have been determined. The total set of elements should than be reduced by statistical feature extraction in order to obtain an optimal set of elements with the same discrimination power. The presented paper especially concerns with the relation of tin-tungsten ore deposits near the areas of high differentiated granitic rocks (Variscian age).

Epithermal neutron activation analysis using the monostandard method

A method is described for epithermal neutron activation analysis of 17 elements in granite rock samples using a single standard. Gold has been used as a single comparator due to its relatively high resonance integral value (I0=400 barn). In addition, it is preferable to Co in order to obtain a large epithermal activation in a short irradiation. The method of calculation is simple and rapid and can be done using a small calculator. Epithermal activation is able to overcome the difficulty arising from changing irradiation position as well as increasing the number of determinable elements by eliminating the interference from undesired isotopes which have relatively high thermal cross section values (σ0), when reactor neutron flux is used. The coupling of epithermal activation with the monostandard method has the advantage of using a small Cd-cover which overcomes most of the difficulties arising in the relative method with large volume cadmium filters.

阿武隈山地北端部，丸森地域の花こう岩類のＲｂ‐Ｓｒ全岩年代

Rb-Sr ages were determined on 20 whole-rock samples of granitic rocks from the Marumoridistrict, northernmost part of the Abukuma Plateau. The rocks are divided into two groups onthe basis of Rb-Sr isotopic nature; five samples of granitic rocks near the Marumori Bridge, which is hereby named the Marumoribashi granite, give a Rb-Sr whole-rock isochron age of 111.6±2.4 Ma with an initial 87Sr/86Sr ratio of 0.70645±0.00009, whereas all other samples give an age of 106.4±3.5 Ma with an initial ratio of 0.70553±0.00010. Thus the Marumoribashi granite isslightly but significantly older than and genetically different from other granitic rocks of theMarumori district. The mineral isochrons with biotite, K-feldspar, plagioclase and whole-rock give ages of 98.8±4.5 Ma for a sample of the Marumoribashi granite and 99.3±2.0Ma for that of the other group, both of which are identical to each other and slightly younger than the whole-rock ages.

Uranium and thorium contents of Archaean granitoids from the Barberton Mountain Land, South Africa

Abstract One hundred and eighty-five samples of granitic rocks from the Barberton Mountain Land have been analysed for U and Th by instrumental epithermal neutron activation analysis. The samples cover the full range of granitic (sensu lato) compositions and vary in age from 3.5 to 2.5 Ga. The granitoids have been subdivided into three categories, termed magmatic cycles. Each magmatic cycle is considered to have evolved progressively through time by reworking of pre-existing crustal material. The U and Th contents increase progressively from the first magmatic cycle to the third magmatic cycle. The geometric mean U value for the entire sample population is 1.8 ppm (range: 0.13–19.2 ppm) and for Th is 8.8 ppm (range: 0.78–47.6 ppm), yielding a mean Th/U ratio of 4.8. Correction of the raw data for alpha-decay results in significantly different U and Th abundances in the granites at their time of emplacement, a consideration which has a marked effect on the Th/U ratio (corrected Th U ratio = 3.5 ). Comparison of U, Th, Zr, and Ce contents indicates that the mineralogical siting of U and Th may differ from one magmatic cycle to the other. Furthermore, there are indications that some granitic bodies are relatively enriched in U whereas others appear to have suffered U loss.