Granitic Region Research Articles

Validation of treatments for conservation of native rhododendrons, the symbol of Satoyama, in the dry granite region of Japan

To develop an appropriate method of conservation for native Rhododendron sections Brachycalyx and Tsutsusi, the symbol of Satoyama, field experiments were performed in the dry granite region of Japan. When carpet-type landscape of native rhododendrons was desired, all plants were cut at 20 cm above the ground level of all trees and shrubs, and herbs and ferns were weeded (once and three times, respectively). When shrubby-type landscape of native rhododendrons was desired, all plants, excluding native rhododendrons, were cut at 20 cm above the ground level of all trees and shrubs, herbs and ferns were weeded, and litter was swept. After 3 years of monitoring of the percentage and depth of crowns with flower buds, the following major results were obtained: Cutting all plants excluding native rhododendrons was effective to maintain the depth of crowns with flower buds. However, weeding and sweeping of litter on the ground caused desiccation of surface soil, which induced a transitory decrease in the percentage of crowns with flower buds. One weeding was effective in maintaining the depth of crowns with flower buds; however, the second and third weedings had no distinct effect. In dry granite regions, considerable attention to desiccation of surface soil is critical, as opposed to limiting attention to maintenance of sunlight as is common practice.

Seismic signatures of a proterozoic thermal plume below southwestern part of the Cuddapah Basin, Dharwar craton

Origin and evolutionary history of the Cuddapah Basin in SE India has remained a subject of considerable speculation whether it was evolved through vertical tectonic movements, extentional stretching or even cometary impact. Based on detailed seismic and other geophysical studies (Gravity, magnetotelluric and heat flow), we have delineated signatures of a possible deep seated mantle plume below southwestern part of the Cuddapah Basin, which may have been responsible for the 1.1 Ga kimberlitic magmatism in the eastern part of the Dharwar craton (EDC). The thermal anomaly associated with this mantle plume appears to have resulted into 15–20 km thick magmatic underplating (Vp: 7.10–7.30 km/s; density 3.07–3.16 g/cm3) below the Parnapalle region of the southwestern Cuddapah Basin, which also coincides with the high gravity and high conductivity anomaly. The massive underplating led to thickening of the crust to about 40–44 km below southwestern part of the Cuddapah Basin, compared to about 34± 2 km in the surrounding regions of EDC, indicating thermal restructuring of the crust / mantle boundary. This plume, which was apparently active in an area of about 500 km radius, may have also affected the Closepet granitic region, which is ∼100 km west of Cuddapah Basin.

옥천대 우라늄 광화대 부근 퇴적암 지하수의 수리화학적 특성

A hydrochemical comparative study of groundwater in uraniferous sedimentary rock of the Ogcheon belt was carried out to investigate the genetic relationship between uraniferous groundwater of Daejeon area and uraniferous sedimentary rocks of the Ogcheon zone. The groundwater shows weak alkaline pH values rangingfrom 6.4 to 8.1 and low Eh values ranging from -50 to 225 mV. The groundwaters to Ca- type that shows high concentration of and due to the dissolution of carbonate mineral in limestone. The concentration of uranium in the groundwater was measured very low below , while it was detected as much as in the mine waste water. The low Eh value of groundwater is one of the main causes of low uranium concentration of groundwater in uraniferous sedimentary rocks in the Ogcheon belt. It is suggested that the uranium of groundwater in granitic region of Daejeon area was not mainly provided from uraniferous sedimentary rocks in the Ogcheon belt.

Thinning of granitic-gneissic crust below uplifting hyderabad granitic region of the eastern Dharwar craton (south Indian shield): Evidence from AMT/CSAMT experiment

Hyderabad granitic region (HGR) forms one of the most unusual geotectonic segment of the south Indian shield. Analysis of multiparametric geological, geophysical and IGS-GPS studies have earlier suggested that this region is neotectonically uplifting at a rapid rate. We propose that consequent to such uplift, only a thin veneer of surface granitic layer now remains. In order to quantitatively examine the thickness of highly resistive granitic-gneissic crust below HGR, a audio-magnetotelluric (AMT) / controlled source AMT (CSAMT) experiment was conducted at three separate locations, situated about 50 km east of Hyderabad. The study reveals a maximum thickness of 5.5 to 6.0 km for the granitic — gneissic crust beneath HGR, which is underlain by thick low resistive exhumed intermediate (granulitic ?) crust. This finding is in sharp contrast to that of a thick granitic-gneissic crust (15–20 km) usually found in comparable late Archaean terrains elsewhere.

금산 지역 토양 차이에 의한 인삼 중 무기 원소의 함량 변화

Geochemical relationships between ginsengs and soils from three representative soil types, shale, phyllite and granite regions, from Keumsan were examined. High elements were shown at the granite and shale areas of the weathered soils, the phyllite areas of the cultivated soils and the shale areas of the host rocks. Tl was enriched in ginsengs grown in the shale areas, Cs and B in the phyllite areas, and Be and Cd in the granite areas. Positive correlations were dominated by the shale areas. These relationships can be explained for mineral characteristics within the soils, and their behaviors related to the physio-chemical conditions. High elements were shown in the 2 year ginsengs of the shale areas, and 4 year ginsengs of the phyllite and granite areas in comparisons with ginsengs of the different ages from the same areas. These differences can be explained with ages of the ginsengs, solubilities of the minerals and physio-chemical differences within soils. The content differences of high elements such as Cs, Tl and Be were found between soils and ginsengs. Overall, these results suggest that components of ginsengs grown in the granite areas are chemically similar to the soils.

Sediment budget for subalpine watersheds, Lake Tahoe, California, USA

Abstract Pinpointing erosional ‘hot spots’ or sediment source areas within a watershed can be difficult because of the highly non-linear and episodic nature of effective runoff and sediment transport. Continuous monitoring of stream networks can provide insight into sediment source areas not possible from routine sampling by capturing episodic events and integrating entire storm events. This paper presents the results of a three-year study of spring snowmelt runoff in watersheds of Lake Tahoe, California, USA, using a network of nine nephelometric turbidometers. We estimate that montane badlands, comprising 1.2% of the Ward Creek watershed (25 km 2 ), contributed 10–39% of snowmelt-derived suspended sediment load. The badlands are comprised of poorly-vegetated and deeply-incised gullies formed in parent material of Miocene volcanic mudflow breccia. The larger (29.5 km 2 ) Blackwood Creek watershed also showed high relative erosion rates from volcanic terrain. Tributaries draining forested granitic and metasedimentary headwater regions had minimal sediment loads. Monitoring networks indicated that temporary main channel storage regulated release of fine sediment eroded from steep headwater slopes. This lag between erosion and delivery may lead to underestimation of gully erosion contribution to sediment budgets.

Paleomagnetism of fluorite veins in the Devonian St. Lawrence granite, Newfoundland, Canada

Paleomagnetic results are reported from the Devonian St. Lawrence granite and the fluorite veins within the granite or its related porphyry dikes. Paleomagnetic analysis of 359 specimens from 29 sites shows that there is no statistically significant difference at 95% confidence between the site mean characteristic remanent magnetization (ChRM) directions carried by pyrrhotite and (or) magnetite of the granite and fluorite veins. No post-granite metamorphic event is known in the area. Thus, these concordant directions indicate that the granite and fluorite veins are coeval and retain a primary remanence. However, the mean ChRM direction of 25 vein and granite sites yields an Early Pennsylvanian paleopole of ∼316 Ma on the North American apparent polar wander path at 35.5°N, 129.2°E (A95 = 3.4°) that is younger than the 374 ± 2 Ma Devonian U–Pb zircon age for the granite. Two intersecting vertical vein sets indicate that the St. Lawrence granite has not been tilted since emplacement of the fluorite veins. Therefore, the difference between the measured and expected paleopoles is interpreted to result from a postemplacement counterclockwise rotation of ∼17° of the St. Lawrence granite region about a vertical axis. Analysis of Carboniferous paleopoles in the northern Appalachian orogen indicates the rotation at the St. Lawrence area is Late Devonian during the Acadian orogeny.

Concentrations of radon and its daughter products in and around Bangalore city

Indoor radon and its progeny levels were measured during 2005-06 in Bangalore rural district and in Bangalore City by using Solid State Nuclear Track Detector (SSNTD)-based twin cup dosemeters, and the activity of radium present in soils and rocks was measured by using HPGe detector. Fifty dwellings of different types were chosen for the measurement. The dosimeters containing the detector (LR-115 Type II Film) used in each house were fixed 2 m above the floor. After an exposure time of 90 days, films were etched to reveal tracks. From the track density, the concentrations of radon were evaluated. The value of radon concentration in the indoor air near granite quarries varies from 55 to 300 Bq.m(-3) with a median of 155 Bq.m(-3) and its progeny varies from 0.24 to 19.6 mWL with a median of 8.4 mWL. In Bangalore City, the concentration of radon varies from 18.4 to 110 Bq.m(-3) with a median of 45 Bq.m(-3) and its progeny varies from 1.62 to 11.24 mWL with a median of 4.15 mWL. Higher concentrations of radon and its progeny were observed in granite quarries compared with Bangalore City. The main reason for the higher indoor radon and its progeny concentration is due to the mining activity and the types of the bedrock. The concentration of radon mainly depends on the activity of radium present in soils and rocks and the types of building materials used. The activity of radium varies in granitic regions of Bangalore rural district from 42.0 to 163.6 Bq.kg(-1) with a median of 112.8 Bq.kg(-1). The concentrations of indoor radon and its daughter products and equivalent effective dose are discussed.

Earth mortars and earth-lime renders

As superficies arquitectonicas em terra desempenham um papel decorativo, para alem da funcao meramente protectora dos paramentos. Na arquitectura vernacula portuguesa, as argamassas em terra eram tradicionalmente usadas na elevacao de paredes em alvenaria de pedra, enquanto os rebocos em terra eram aplicados nas paredes interiores. A pedra calcaria existe apenas em algumas zonas do pais e consequentemente a cal nao estava disponivel, especialmente nas regioes de xisto e granito onde a alvenaria em pedra era uma tecnica corrente. Na regiao centro litoral de Portugal, o procedimento para extinguir a cal consistia na mistura de terra arenosa com cal aerea viva, posteriormente regada e usada como argamassa e reboco. Este era tambem o procedimento para estabilizar as terras usadas na producao de adobes. Os edificios construidos em adobe eram posteriormente rebocados com argamassas em cal e terra e decorados com argamassas de cal em pasta e areia muitas vezes com acabamento colorido em barramento de cal. A ornamentacao do tipo classico na arquitectura em terra dos seculos XVIII e XIX foi em muitos paises influenciada pelo manual de Francois Cointeraux (1740-1830) - Les Cahiers d'Architecture Rurale (1793), um guiao frances para a construcao e arquitectura em terra. Este manual chegou a Portugal no inicio do seculo XIX, mas nunca foi traduzido para a lingua portuguesa. Referencias a decoracao para as casas construidas em terra sao devidamente explicadas neste manual, assim como os procedimentos para a aplicacao de argamassas em terra e cal na ornamentacao das paredes em terra. De facto essas indicacoes sao coincidentes com as que encontramos na arquitectura de adobe em Portugal na regiao do centro. O objectivo deste artigo e mostrar alguns casos de argamassas e rebocos em terra existentes na arquitectura vernacula elevada em alvenaria de pedra e explicar os metodos usados na producao de argamassas em terra e cal, assim como exemplos de rebocos existentes na arquitectura vernacula de adobe em Portugal.

Topographical Effects on Soil Respiration in a Deciduous Forest-The Case of Weathered Granite Region in Southern Kyoto Prefecture-

We monitored soil respiration at a valley bottom and a ridge above that valley in an experimental forest and quantitatively estimated the effect of topography on soil respiration. Compared to the valley bottom, the ridge showed a lower rate of soil respiration in summer and a higher rate in winter; the seasonal difference was caused by soil drying in summer and soil warmth in winter. Both the valley bottom and ridge had annual soil respiration rates of approximately 6.2 tC ha-1 year-1. However, the annual value for the ridge included the estimated respiration acceleration due to warm soil temperature (0.68 tC ha-1 year-1) and deceleration caused by soil drying (0.62 tC ha-1 year-1).

Spatial Distribution of Coda Q in South Korea

Analyzing spectral attenuation of coda waves, we first mapped coda Q- values in the crust over the whole of South Korea. During the period from 1995 to 2004, we selected 574 north-south-component seismograms with epicentral distances less than 100 km from 328 earthquakes with magnitudes between 1.4 and 5.2. We estimated coda Q-values using the single isotropic scattering model at center fre- quencies of 1.5, 3, 6, 9, 12, 15, and 18 Hz, and found significant spatial variation over all frequency ranges and strong frequency dependence of coda Q in the region. The estimated coda Q-value at 1 Hz (Q0) and the g-value ranges are 80-300 and 0.4-1.1, respectively. The values have strong correlation with the regional geology in the Korean peninsula. Low Q0-values are mainly obtained in the regions com- prising sedimentary strata in southeastern South Korea, whereas granite regions in the northern part of South Korea show high Q 0 . The Q 0 -values in the study area agree well with those of the eastern China and Kyushu, western Japan. Furthermore, our g-values are also in good agreement with those of Japan. Online material: Locations of seismometers, list of event-station pairs, and linear regression of QC measurements.

Heterogeneity in crustal structure across the Southern Granulite Terrain (SGT): Inferences from an analysis of gravity and magnetic fields in the Periyar plateau and adjoining areas

The study region forms the western part of the Madurai block (southern block) and shares several lithological characteristics of the Proterozoic exhumed South Indian Granulite Terrain (SGT). The crustal structure of the area has been derived from gravity data, constrained partly by aeromagnetic data. The Bouguer anomaly map of the region prepared based on detailed gravity observations shows a number of features (i) the Periyar lineament separates two distinctly different gravity fields, one, a high gravity gradient tending to be positive towards the coast in south west and significant gravity lows ranging from − 85 to as low as − 150 mGal in the NE covering a large part of the Periyar plateau (ii) within the broad gravity low, three localised circular anomalies of considerable amplitude occur in the region of Munnar granite. A magnetic low region in the central part coincides with the area of retrogressed charnockites and the major lineaments suggestive of a genetic link and considerable downward extent. The crustal models indicate that the upper layer containing exhumed lower crustal rocks (2.76 gm/cc) is almost homogeneous, most part of the gravity field resulting from variations in intracrustal layers of decharnockitised hornblendic gneisses and granite bodies. Below it, a denser layer (2.85 gm/cc) of unknown composition exists with Moho depth ranging from 36 to 41 km. The structure below the region is compared with that of two other segments of the SGT from which it differs markedly. The Wynad plateau forming the western part of the Northern Block of the SGT is characterised by a heterogeneity due to the presence of contrasting crustal blocks on either side of the Bavali shear zone, possibly a westward extension of the Moyar shear zone and presence of high density material in the mid-to-lower crustal portions. The crust below the Kuppam–Palani transect has a distinctive four-layer structure with a mid-crustal low density layer. The differences in crustal structure are consistent with the different tectonic settings of the three regions discussed in the paper. It is suggested that the crustal structure below the Kuppam–Palani transect corridor is not representative of the SGT as a whole, an aspect of great relevance to intra-continental comparisons and trans-continental reconstructions of continent configurations of the Gondwanaland.

Re-examination of the boron isotopic composition of tourmaline from the Lavicky granite, Czech Republic, by secondary ion mass spectrometry: back to normal. Critical comment on “Chemical and boron isotopic compositions of tourmaline from the Lavicky leucogranite, Czech Republic” by S.-Y. Jiang et al., Geochemical Journal, 37, 545-556, 2003

A re-examination of tourmaline from the Lavicky leucogranite by secondary ion mass spectrometry revealed a very homogeneous boron isotopic composition both on grain scale and on the outcrop scale. Tourmaline from tourmaline-quartz-feldspar orbicules and from tourmaline-quartz-feldspar veins show identical boron isotopic compositions of δ11B=-10.77 ± 1.24‰. This value is similar to the value for average continental crust. A comparison of the Lavicky tourmaline with δ11B values of magmatic tourmaline available in the literature additionally demonstrates its rather ordinary character. The extremely negative δ11B values and the boron isotopic fractionation during the magmatic-hydrothermal transition proposed in an earlier study (Jiang et al., 2003) are not supported by our data. The source region of the Lavicky granite and its geochemical evolution did probably not involve any evaporitic material.

Characterization of Atmospheric Aerosol in the Northern Foot of Mt. Fuji, Central Japan

The unique characteristics of atmospheric aerosol in the northern foot of Mt. Fuji, central Japan were first clarified. The Mt. Fuji (an altitute of 3776 m) is the largest basaltic stratovolcano in the quaternary period in Japan. The aerosol measurements were carried out at an altitute of 1100 m from June 2000 to April 2001. Ambient aerosol in the predominant area of a typical volcanic rock like basalt was referred to as a basaltic aerosol in this study. Fifteen elements (Na, Mg, Al, Si, K, Ca, Ti, V, Cr, Mn, Fe, Cu, Zn, Ba, Pb) of major to trace in the aerosol samples were determined by X-ray fluorescence spectrometry. Total mass concentration (< 10 μm) of the basaltic aerosol showed the higher values in summer and spring rather than autumn to winter, and the seasonal variation pattern differed widely from that of general urban aerosol. The behavior of the basaltic aerosol was mainly controlled by mineral particles throughout the year, so that a typical anthropogenic-derived element like Pb was very rarely detected. Even V, Cr and Zn which have been generally considered to be typical anthropogenic-derived elements, showed crustal-like behaviors. A concentration ratio of Si/Al showed markedly a uniqueness of the basaltic aerosol. From a comparison with atmospheric aerosol Si/Al ratio in granitic region being an exact opposite geology, a correlation plot of Si/Al ratio against Si concentration was made. It showed a big regional difference available for source identification of atmospheric soil particles. The chemical and geological characteristics of the basaltic aerosol are very useful for the novel characterization of atmospheric soil particles.

Magnetic resonance sounding (MRS) and resistivity characterisation of a mountain hard rock aquifer: the Ringelbach Catchment, Vosges Massif, France

ABSTRACTThe Ringelbach catchment, which has been studied since 1975, is highly representative of the crystalline Vosges massif, where the water supply is mainly derived from small aquifers in heterogeneous superficial formations and in the weathered and fissured bedrock. Vertical electrical soundings (VES), resistivity imaging profiles and magnetic resonance soundings (MRS) were applied in order to investigate the 3D structure and estimate the hydrodynamic characteristics of the subsurface, which is composed of granites partly covered by Triassic sandstone.Despite poor MRS signal‐to‐noise conditions, the general structure of the catchment is defined using geophysics (electrical methods and MRS combined) and geology. MRS also makes it possible to map the water volume per unit surface within the different geological formations and structural blocks. The corresponding maps are proposed as a basis for evaluating the water storage within the catchment and improving the understanding of its hydrological functioning. The initially proposed geological model is refined and a four‐block structure intersected by faults is defined. The schema of a gradual stratified weathering of the granite from surface to depth is not clearly reflected in the resistivity images, partly because of insufficient investigation depth. It is completed with highly weathered, deeply rooted fractured zones associated with faults in order to explain deeply sited low‐resistivity zones.The MRS water content in the weathered granite of Ringelbach appears abnormally low in comparison with the higher values observed in similar resistivity settings in other granite regions. Because of the rough catchment topography, these low values may be attributed to unsaturated weathered formations on hillsides or saturated, fine material (colluvial deposits, highly weathered granite) in the valley bottom. The control‐wells and laboratory measurements on core samples proposed in the next step of this research will attempt to calibrate the MRS results and validate the proposed structural and weathering schema. Re‐interpreted and validated geophysical results will then be used as effective input for improved hydrological modelling.

Granite fracturing and incipient pollution beneath a recent landfill facility as detected by geoelectrical surveys

A resistivity survey using Wenner array was carried out in June 2000 in a granite region of Northern Portugal, where an active landfill is operating since 1998, to detect the possible spread of contamination. This survey was complemented with a self-potential (SP) survey, a dipole–dipole (DD) array profile and azimuthal Vertical Electrical Sounding arrays (VES). The location of these profiles was highly constrained by the available space in the landfill facility and by the available geological data, mainly fracturing. Significant groundwater circulation was detected, which is characterized by a low resistivity zone (<400 Ω m), with a fairly well defined configuration. Chemical analysis of water samples collected in boreholes inside the landfill facility and on springs around it confirmed the presence of water contamination. The presence of a very well delimited anomaly with low resistivity (<200 Ω m) just beneath the leachate collector system strongly suggests that the groundwater contamination is due to a landfill leak. Results of azimuthal VES are consistent with the structural data obtained outside the landfill, revealing that the strikes of the prevailing fracture systems inside the landfill are generally NW–SE to NNE–SSW, which seems to facilitate the downward propagation of contaminants.

Natural radiation doses to the population in a granitic region in Spain.

The global average effective dose from natural radiation sources is estimated to be 2.4 mSv y(-1). Nevertheless, local variations in the radiation exposure may differ by orders of magnitude. In this paper, we study a rural area in western Salamanca (Spain) where doses are potentially well above the stated average value. Its geology consists mostly of granite and schist, presenting various uranium mineralisations, some of which were exploited in former years. Water samples, both surface and groundwater, were collected in selected villages and measured by liquid-scintillation spectrometry. 222Rn in dwellings was determined in dwellings by means of short-term measurements, using Picorad vials and long-term measurements with CR-39 detectors. The committed effective doses to the inhabitants due to indoor 222Rn and 222Rn and 226Ra in drinking water were assessed using the dose conversion factors proposed in the UNSCEAR 2000 report.

Unusual lithospheric structure and evolutionary pattern of the cratonic segments of the South Indian shield

The southern Indian shield, characterised by several prominent geological and geophysical features, can be divided into three distinct tectonic segments: Western Dharwar craton (WDC), Eastern Dharwar craton (EDC) and Southern Granulite terrain (SGT). With the exception of WDC, the entire crust beneath EDC and SGT has been remobilized several times since their formation during the mid- to late Archeans (3.0–2.5 Ga). In order to understand the evolutionary history of these segments, a multiparametric geological and geophysical study has been made which indicates that the south Indian shield, characterized by a reduced heat flow of 23–38 mW/m2 has a much thinner (88–163 km) lithosphere compared to ∼200–450 km found in other global shields. In the EDC-SGT terrain, high velocity upper crust is underlain by considerably low mantle velocity with a thick high conductive/low velocity zone sandwiched at mid crustal level. Our study reveals that the entire EDC region is underlain by granulite facies rocks with a density of about 2.85 to 3.16 g/cm3 at a shallow depth of about 8 km in the southern part and at even shallower depth of about 1 to 2 km below the Hyderabad granitic region in the north. Cratonic mantle lithosphere beneath EDC may contain a highly conductive, anisotropic and hydrous metasomatic zone between the depth of 90 and 105 km where estimated temperatures are in the range of 850–975°C. It is likely that before the early Proterozoic, the entire south Indian shield was a coherent crustal block which subsequently got segmented due to persistent plume-induced episodic thermal reactivations during the last 2.7 Ga. These reactivations led to self destruction of cratonic roots giving rise to negative buoyancy at deeper levels which may have been responsible for crustal remobilisations, followed by regional uplifting and erosion of once substantially thick greenstone belts. Consequently, the crustal column beneath the EDC has become highly evolved and now corresponds closely to SGT at depth.

Petrology of basement-dominated terranes: II. Contrasting isotopic (Sr, Nd, Pb and O) signatures of basement-derived granites and constraints on the source region of granite (Damara orogen, Namibia)

Major and trace element and Nd, Sr, O and Pb isotope data from granites of the high-grade central part of the Damara orogen (Khan and Outlet gorge areas, Namibia) indicate a dominantly lower crustal origin. Based on their appearance in field, the granites can be divided into red granites, white granites and grey granodiorites. Red granites and some of the white granites from the Khan area are isotopically evolved (initial ε Nd: −12.5 to −18.9) and were likely derived from metaigneous sources with late Archaean to early Proterozoic crustal residence ages. Other white granites are less evolved (initial ε Nd: −6.5 to −8.8) and were likely derived from metasedimentary sources that are similar to the country rock metapelites. Grey granodiorites from the Khan and Outlet gorge area are also isotopically evolved (initial ε Nd: −9.9 to −13.1) but are derived from metaigneous sources with younger, late Proterozoic crustal residence ages. Major and trace element data do not support closed-system fractional crystallization processes for all samples; however, some chemical features (i.e., decreasing Rare Earth Element (REE) abundances with increasing SiO 2) underline the importance of crystal fractionation processes for each distinct magmatic pulse. Isotope data do not support mixing of different crust-derived melts or assimilation of crustal rocks by a mafic magma. Instead, highly evolved Pb isotope compositions, strongly negative ε Nd values and radiogenic Sr isotope compositions argue for an undepleted basement as a potential source. The most likely model involves mainly partial melting of different basement rocks of Archaean to Proterozoic age at different levels within the crust. Only some granites could be derived by melting of metasedimentary rocks of the Pan-African cover sequence. The consistency of the chemical data with a crustal anatectic origin and the range in radiometric ages suggests that they intruded simultaneously with crustal thickening; however, some of them may have been emplaced during extensional tectonics c. 40–50 Ma later than the main period of crustal thickening. The heating events that promoted melting of fertile deep-crustal rocks might have been caused either by the inferred high heat productivity together with crustal thickening during the main periods of orogeny, or by delamination of mantle lithosphere during the final extensional stages of the Pan-African orogeny.

GRANITIC PEGMATITES OF THE O'GRADY BATHOLITH, N.W.T., CANADA: A CASE STUDY OF THE EVOLUTION OF THE ELBAITE SUBTYPE OF RARE-ELEMENT GRANITIC PEGMATITE

The first confirmed Canadian occurrence of the elbaite subtype of rare-element granitic pegmatite has been encountered within the O’Grady batholith, approximately 100 km NNW of Tungsten, N.W.T. The batholith, part of the Selwyn plutonic suite, is a mesozonal, hornblende-bearing, metaluminous composite intrusion with lesser amounts of pegmatitic granite and felsitic satellite dykes. On the whole, pegmatite bodies show a mildly NYF-type geochemical and mineralogical signature (allanite- and magnetite-bearing), but in the region of the pegmatitic granite, they grade toward a distinct LCT-type signature (elbaite-bearing). The melts parental to the pegmatite bodies were more voluminous, more alkaline, less aluminous and less reduced than is the norm for melts parental to LCT-type pegmatite. Differentiation of these melts sequentially produced hornblende granite, pegmatitic leucogranite and pegmatite, as shown by major-, minor- and trace-element geochemistry of K-feldspar, plagioclase, micas, tourmaline, amphibole and a variety of accessory phases. Lithium-mineralized pegmatite is recognized by the presence of multicolored elbaite, by lepidolite, and in the diversity of characteristic accessory phases such as danburite, hambergite, stibiocolumbite, pollucite, a nanpingite-like mineral, and bismutite. The various stages of the evolution of the batholith can be well documented in similar variations in mineral chemistry, and result in one of the few well-characterized case studies of the elbaite subtype.