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1.
Intercalations of muscovite-quartz rocks have been investigated in leucocratic orthogneisses of the Svratka crystalline complexes (western Moravia). General characteristics for distinguishing apogranite greisens affected by regional metamorphism from metapelitic mica schists are given. The regionally metamorphosed greisens at Jimramovské Paseky are described in greater detail. — The greisens originated through greisenization of paraschists and afterwards affected by regional metamorphism can not be identified unequivocally due to lack of precise knowledge of their pregreisenization starting material.  相似文献   

2.
Recent field campaign in the southern Menderes Massif in southwestern Turkey revealed that the so-called ‘core of the massif’ comprises two distinct types of granitoid rocks: an orthogneiss (traditionally known as augen gneisses) and leucocratic metagranite, where the latter is intrusive into the former and the structurally overlying ‘cover’ schists. These differ from one another in intensity of deformation, degree of metamorphism and kinematics. The orthogneiss display penetrative top-to-the-N–NNE fabrics formed under upper-amphibolite facies conditions during the Eocene main Menderes metamorphism (MMM), whereas foliation and stretching lineation exists in the leucocratic metagranites but are not strongly developed. The leucocratic metagranites show evidence of syn- to post-emplacement deformation in a series of weakly developed top-to-the-S–SSW fabrics formed under lower greenschist-facies (?) conditions. Leucocratic metagranite bodies occur all along the augen gneiss–schist contact in the southern Menderes Massif; they are emplaced as sheet-like bodies into country rocks (previously deformed and metamorphosed during a top-to-the-N–NNE Alpine orogeny) along a ductile extensional shear zone, located between orthogneisses and metasediments, which was possibly active during emplacement. The data presently available indicate that emplacement and associated ductile extensional deformation occurred during Late Oligocene–Early Miocene time. These results confirm previous contentions that there are Tertiary granites in this part of the Menderes Massif.  相似文献   

3.
Concordant igneous-looking bands of ferruginous bulk composition occur in a highly aluminous Precambrian metasedimentary series composed predominantly of kyanite quartzite. The bands consist of quartz, staurolite, and magnetite (partially martitized) with accessory amounts of muscovite, chlorite (pseudomorphous after biotite), chloritoid, apatite, and monazite. Quartz is found in three types (I–III) differing in appearance as well as in origin. Staurolite, in combination with quartz-II, shows peculiar radial sieve textures caused by mimetic crystallization after preexisting chloritoid rosettes. The chloritoid has been largely consumed, either by a reaction with hypothetical former kyanite to produce staurolite+ quartz with rock composition unchanged, or, possibly, by metasomatic introduction of oxygen (oxidation) to yield staurolite+quartz+magnetite; the remaining chloritoid, however, persisted in stable equilibrium with the other minerals of the rock. The staurolite quartzites are thus considered to represent original sedimentary bands which have undergone several stages of recrystallisation and (possibly) metasomatic modification during their metamorphic history. Their igneous aspect results from annealing crystallisation during a late static, i.e. postdeformational, thermal event of regional metamorphism.Chemical analysis of the staurolite shows no unusual features. For all staurolites plotted there is a positive relationship of the excess H+ over 2.0 and the Si+4-deficiency in the unit cell. This suggests partial substitution of 4 H+ for Si+4.The formation of staurolite in regional metamorphic rocks with excess silica, low alkali contents, and (FeO+MgO)/Al2O3 ratios < 1 showing chloritoid at lower grades appears to be governed, in many cases, by the reaction chloritoid+Al-silicate=staurolite+quartz+H2O.The assemblage chloritoid-staurolite may be stable in regional metamorphism over a limited pressure-temperature range.  相似文献   

4.
The Voltýov gold deposit in Central Bohemia is situated in the contact zone of granitic rocks of Variscan age and the Upper Proterozoic and Lower Paleozoic metasediments of the Krásná Hora Metamorphic Islet — the relict of an original sedimentary cover. Goldbearing quartz mineralization of vein and stockwork types is developed in rocks of both geological units, its form depending upon lithology. While ore shoots in metasediments and orthogneiss form generally regular bodies several tens of meters thick, those developed in granodiorite are rather irregular, without any indication of the tectonic control of the mineralization. The mineralized zone peters out extremely rapidly downwards, with a marked decrease both in extent and in gold contents. The gold-bearing mineralization is formed by two generations of gold, both of high fineness (over 900) and very fine-grained (5–20 m). The gold is accompanied by accessory arsenopyrite, maldonite and other Bi-Te minerals, scheelite, and exceptionally chalcopyrite, pyrite, and sulphosalts. Gold contents are 2–3 g/t on average. In addition, late quartz-carbonate mineralization, with U and Se minerals and remobilized gold, is also present. Detailed soil prospecting (25 × 50 m grid) of the deposit detected a large and constrasting gold halo with gold contents exceeding 2 ppm (–200 mesh fraction), accompanied by higher concentrations of characteristic pathfinders — above all As and Bi. The halo extent exceeds the area of the ore outcrop by several times. The origin of gold mineralization in the deposit is explained by remobilization of gold from orthogneiss by the intrusion of the Central Bohemian Pluton.  相似文献   

5.
During low-grade regional metamorphism, pyrrhotite can form from gypsum by the reaction: CaSO4·2H2O+Fe sol. 2+ +2Corg.FeS+Ca sol. 2+ +2H2O+2CO2. This reaction takes place in the anchizone, below 350°C and might be initiated by the thermal dehydration of gypsum (200°C) and aided by the generation of gaseous hydrocarbons. Evidence for the reaction is the occurrence in dolomitic layers in the Ballachulish Slate, East Larroch quarry, Argyll of lath and swallow-tail shaped quartz+dolomite+pyrrhotite pseudomorphs after gypsum. Quartz+pyrrhotite bodies in slate represent replaced and deformed (mainly flattened) crystals, concretions and possible veinlets of gypsum. Pyrite porphyroblast growth, after the peak of metamorphism and under relatively high fS2 conditions, failed to destroy some early pyrrhotite because it is encapsulated in quartz. Pyrite-silicate reactions and hydrothermal exhalations have been suggested previously to account for pyrrhotite-enriched horizons in regionally metamorphosed rocks. Replacement of gypsum by pyrrhotite is an additional explanation for pyrrhotite-enriched horizons, especially in dolomitic and graphitic lithologies.  相似文献   

6.
Investigation of the greisen occurrence at Cetoraz, Bohemian-Moravian Heights, which underwent a regional metamorphism, has shown that the main greisen assemblages are stable under the conditions of regional metamorphism, as long as they do not attain the sillimanite-almandine-orthoclase subfacies, when muscovite disappears. The ore minerals are also stable. In the mineralogy of the greisens, the regional metamorphism manifests itself only by the presence of subordinate minerals typical of regional metamorphism, such as sillimanite or spinel.  相似文献   

7.
Laser Raman spectroscopy and cathodoluminescence (CL) images show that most zircon crystals separated from paragneiss in the main drill hole of the Chinese Continental Scientific Drilling Project (CCSD-MH) at Maobei, southwestern Sulu terrane, contain low-pressure mineral-bearing detrital cores, coesite-bearing mantles and quartz-bearing or mineral inclusion-free rims. SHRIMP U Pb dating on these zoned zircons yield three discrete and meaningful age groups. The detrital cores yield a large age span from 659 to 313 Ma, indicating the protolith age for the analyzed paragneiss is Paleozoic rather than Proterozoic. The coesite-bearing mantles yield a weighted mean age of 228 ± 5 Ma for the UHP event. The quartz-bearing outmost rims yield a weighted mean age of 213 ± 6 Ma for the retrogressive event related to the regional amphibolite facies metamorphism in the Sulu UHP terrane. Combined with previous SHRIMP U-Pb dating results from orthogneiss in CCSD-MH, it is suggested that both Neoproterozoic granitic protolith and Paleozoic sedimentary rocks were subducted to mantle depths in the Late Triassic. About 15 million years later, the Sulu UHP metamorphic rocks were exhumed to mid-crustal levels and overprinted by an amphibolite-facies retrogressive metamorphism. The exhumation rate deduced from the SHRIMP data and metamorphic P-T conditions is about 6.7 km/Ma. Such a fast exhumation suggests that the Sulu UHP paragneiss and orthogneiss returned towards the surface as a dominant part of a buoyant sliver, caused as a consequence of slab breakoff.  相似文献   

8.
The metamorphic rocks of Mesozoic age, found sandwiched between the Pennine nappes of Simplon area, were subjected to a detailed structural-petrological investigation with a view to clarify the nature of the Alpine metamorphism.Preliminary structural works demonstrated the existence of two B-axes — B1 and B2 — both being younger than the regional folding and thrusting movements. Of these two, B1 is restricted to the northernmost belt of the present area and has been imprinted uniformly on the older crystalline rocks of the Aar-Gotthard massif as well as on their Mesozoic sedimentary cover. The B1-axis, with its usually steeply plunging habit, can be traced southwards upto the northernmost fringe of the Pennine Mesozoic metamorphites. The subhorizontal to weakly plunging B2-lineation overprints B1 in the northernmost sector of the region under consideration. Further southwards, it gains rapidly in prominence and becomes the sole linear fabric element. For the most part, B2 coincides roughly with the axes of the regional large scale folds B0; but at places the two may deviate from each other appreciably. The overall behaviour of the B-axes is to be seen in the structural map, in which only the statistical maximum of numerous measurements from each locality has been plotted.Zones of progressive regional metamorphism could be roughly delineated over the whole region. The metamorphic grade rises from low greenschist facies in the NW to middle almandine amphibolite facies to the SE. Although the delineation of the metamorphic facies boundaries is only tentative and it is to be expected that future work would refine them, it is firmly believed that the outline of the pattern would not be necessarily modified thereby. By correlating the individual zones of metamorphic facies of underground exposures of Simplon tunnel with the corresponding ones on the facies map, it was possible to determine approximately the lie of the facies boundary surfaces. They transgress cleanly the regional large scale structures in three dimensions in space, proving thereby that the metamorphism took place after the regional folding and thrusting movements died off. Ancillary evidences on this point were derived through a systematic study of oriented thin sections and intensive petrofabric analysis. It was established that the metamorphic recrystallisation and the post-thrusting penetrative movements leading to the origin of the secondary planar and linear fabric elements went on hand in hand, with the former process ultimately outlasting the latter. In fact, the recrystallisation continued long afterwards under a completely static condition and almost all the porphyroblasts were found to have grown at this stage. The regional metamorphism of this area is therefore independent of the penetrative movements due to Alpine orogenesis and owes its origin to the late plutonic actions.Comparison with other areas shows that the Simplon area could be regarded simply as a prototype of the usual case of regional metamorphism. It is concluded that the regional metamorphism in its ideal form is a complex overprinting of an area of previous dislocation metamorphism by a subsequent phase of plutonic metamorphism. Although deformation does not appear to have played an essential rÔle in bringing about appreciable amount of recrystallisation in regional metamorphic rocks, it ought to be regarded as a conditio sine qua non for the same. Indeed, dislocation metamorphism seems to have paved the path to be trodden over later by plutonic metamorphism — the result being crystalline schists, the products of the so called regional metamorphism. The time-honoured term regional metamorphism has got today the sole advantage of being universally understood.  相似文献   

9.
This study deals with the oxygen isotope composition of hematite-rich ore bodies in the Iron Quadrangle, Brazil. The area studied can be divided into two different regions: a western (W) zone of greenschist facies assemblages and an eastern (E) zone of amphibolite facies with transitions into granulite facies assemblages.The 18O values of 136 quartz-iron oxide pairs have been determined and temperatures of formation have been calculated. The 18O values of quartz vary between +6 and 20 except one value near +23, whereas the iron oxides fall between –4 and +10, with nearly 80% of the iron oxide values between –0.5 and 4.0. The regional distribution of the 18O values is as follows: in the W-region 85% of the quartz are >12, whereas in the E-region only 46% fall in this range, In contrast to quartz the iron oxides do not show any regional differences.The variation of oxygen isotope fractionations between quartz and iron oxides is obviously related to the complex deformation history of the iron ores. Samples with a primary schistosity (S1) only represent peak metamorphic conditions. In the E-region the (S1) high temperatures >700° C seem to correspond to orogenic events in the Archaen basement 2,700 m.y. ago. In the W-region S1-temperatures between 460° and 560° C seem to represent peak metamorphic conditions of the Proterozoic Minas metamorphism 2,000 m.y. ago. Iron ores which have been overprinted by later deformation events are selectively reset to lower isotopic temperatures. The more closely spaced the schistosity planes the larger the extent of a temperature lowering.The genetic processes associated with these hematite-rich ore bodies appear to be sedimentary-metamorphic rather than metasomatic processes. Furthermore, there is no evidence for secondary leaching by weathering solutions.  相似文献   

10.
The Sanshenjiang gold deposit in southeastern Guizhou Province, China, is hosted by the Neoproterozoic metasedimentary rocks which experienced low-grade greenschist facies metamorphism. Gold mineralization occurs mainly in the ribbon chiltern slate of the first member of the Longli Formation and is controlled by both strata and faults. Ore bodies are characterized by abundant quartz-arsenopyrite-gold-pyrite-bedding veins, veinlets and small lenses within the shear zone. In this study, trace element and REE geochemistry was analyzed to constrain the origin and genesis of this deposit. The trace element signatures of wall rocks and veins display a basically similar tendency in the spider diagram, showing the genetic relationship. The values of Co/Ni, Y/Ho, Hf/Sm, Nb/La and Th/La reflect that the hydrothermal fluids of this deposit were derived from the mixture of multiple sources with marked enrichment of Cl and moderate to high temperature. There is a broad similarity in the chondrite-normalized patterns and REE fractionation between wall rocks and ore bodies, possibly reflecting their similar origin. Based on the difference in δCe and δEu, quartz veins and lenses can be subdivided into weakly negative Ce-anomalies (δCe=0.81 to 1.06) with slight Eu anomalies (δEu=0.81 to 1.06) type and the significant positive Ce-anomalies (δCe=1.13 to 1.97) with moderate negative Eu-anomalies type, probably suggesting physical-chemical changes in the evolution process of ore-forming fluids from the early to late stage. It can be concluded that the ore-forming process may have experienced three stages: formation of the original ore source bed, regional metamorphism and gold mineralization, on the basis of trace element and REE analysis and field observation.  相似文献   

11.
Ion microprobe U-Th-Pb analyses of zircons from a granulite-grade orthogneiss from Mount Sones, Enderby Land, Antarctica, record the ages of four principal events in the history of the gneiss, three of which already have been recognized through previous isotopic dating of other samples. The structure of the zircons indicates at least four different stages of growth. The several zircon ages were obtained by grouping the analyses according to the stage they represented in the observed stratigraphic succession of growth and thereby defining separate U-Pb discordance patterns for each stage. The stratigraphically oldest zircon (rare discrete cores) is indistinguishable in age from the most common, euhedrally zoned zircon. Both crystallized when the tonalitic precursor of the orthogneiss was emplaced into the crust 3927±10 Ma ago, making the orthogneiss currently the oldest known terrestrial rock. The outer parts of most grains and some whole grains recrystallized at 2948±31/–17 Ma, during or immediately after possibly 100 Ma of high granulite grade metamorphism. The recrystallized zircon was isotopically disturbed by tectonism associated with reactivation of the southern margin of the Napier Complex at 1000 Ma. In the intervening time, at 2479±23 Ma, the cores and zoned zircon suffered a major isotopic disturbance involving movement of radiogenic Pb which left most of the crystals with radiogenic Pb deficiencies, but produced local radiogenic Pb excesses in others. A new generation of zircon, characterized by very high Th/U and low U, grew at that time. That event — deformation and possibly a minor rise in temperature — produced widespread perturbations of other isotopic systems throughout the Napier Complex.  相似文献   

12.
Polymict cristalline breccias are typical impact products of the Ries crater. They occur within the Ries crater (Appetshofen, Lierheim, Leopold Meyers Keller), on its rim (Maihingen-Klostermühle) and within the immediate vicinity of the crater (Itzing). Apart from very rare admixtures of sedimentary rock fragments the polymict cristalline breccias consist almost exclusively of fragments of various cristalline rocks, namely granites, gneisses and amphibolites. The petrographical and statistical investigations have shown that breccias from different localities have different composition. This reflects a possible difference in local compositions of the cristalline basement. The rocks in the breccias have been affected to various degress by shock metamorphism. The amphibolites could thus be shown to belong predominantly to stage I (diaplectic quartz and feldspar, 100–300 kb) and stage II (diaplectic quartz and feldspar glasses, 350–500 kb) whereas the granites and geisses can be attributed mostly to stage 0 (fractured quartz and feldspar, <100 kb) and stage I. This is in part the result of the bulk shock wave impedance of the rocks in question.Deformation structures resulting from shock metamorphism have been observed for the first time in sphene as well as in various planes of apatite.A large part of breccia rocks, which contain diaplectic quartz and feldspar glasses were altered into montmorillonite at a later date. The authigenic minerals were examined by x-rays and chemically by microprobe analysis.

Herrn Prof. Dr. W. von Engelhardt danke ich für Diskussion und Beratung. Dem Bundesministerium für Bildung und Wissenschaft sei für die finanzielle Unterstützung dieser Arbeit gedankt.  相似文献   

13.
Abstract Granitic orthogneiss is widespread throughout the metamorphic core of the Brooks Range in both the ductilely deformed blueschist/greenschist facies Schist Belt and the lower grade Central Belt (= Skajit allochthon) to the north. Orthogneiss occurs as large metaplutonic massifs and in small bodies enclosed within metasedimentary rocks. Crystallization ages for the granitic protoliths range from Proterozoic through Devonian (U-Pb zircon); the K-Ar system was reset during Cretaceous metamorphism. Mineral assemblages of the orthogneisses reflect nearly complete re-equilibration during Jurassic-Cretaceous collisional orogenesis in northern Alaska. The most common metamorphic paragenesis in orthogneiss is: Qtz + Kfs + Ab + Phe + Bt ± Ep, Ttn, Rt, Ap, Chl, Cal. Constituent minerals from 16 Brooks Range orthogneiss samples were analysed with the electron microprobe. Phengite from the Schist Belt samples is highly enriched in Al-celadonite, with Si values up to 3.50 per formula unit (on an 11-oxygen basis). Central Belt samples contain phengite with lower Si content (±3.38 p.f.u.). In nearly all samples, Si content of phengite varies considerably, reflecting partial re-equilibration to lower pressure and/or higher temperature conditions. Metamorphic conditions were estimated using the Phe-Bt-Kfs-Qtz barometer and the two-feldspar solvus thermometer. The results indicate that the Schist belt underwent high-pressure/low-temperature metamorphism (generally 9-12 kbar at 375-430° C), consistent with the widespread development of glaucophane + epidote/clinozoisite and lawsonite pseudomorphs in other rock types. The Central Belt also experienced a relatively high P-T metamorphism, with most samples yielding pressure estimates in the range 5-8 kbar (at 325-415° C). These results confirm the existence of two metamorphic belts in the core of the Brooks Range that differ in metamorphic conditions by up to 5 kbar. The range in Si content in phengite from Schist Belt samples is consistent with isothermal decompression of up to 5 kbar.  相似文献   

14.
Stable isotope analyses of quartz, sulphides, and magnetite were conducted to provide information on thermal history and source of hydrothermal fluids in the Palaeoproterozoic Enåsen gold deposit. Reequilibration and homogenization of oxygen isotopes throughout the rock have apparently not occurred despite the upper amphibolite to granulite facies regional metamorphism that has affected the rocks. However, oxygen isotope geothermometry on a coexisting quartz-magnetite pair gave a minimum temperature for peak metamorphism of around 650 °C which agrees with Fe-Mg geothermometry. This suggests that grain-scale equilibrium is achieved. The variation in oxygen isotope ratios (18O = 7.3 – 10.5) on quartz from the metamorphosed acid sulphate alteration zone is suggested to represent a cooling trend in the fossil hydrothermal system with higher 18O-values in more superficial parts. Temperatures of alteration and silicification and isotopic composition of hydrothermal fluids could not be defined from the present data but it was recognized that the data is compatible with a epithermal genesis for the deposit. It is suggested that alteration, silicification, and mineralization at the Enåsen gold deposit took place in a high sulphidation epithermal environment at temperatures of around 200–250 °C and that the hydrothermal fluids consisted of meteoric and magmatic water. A tentative reconstruction of the fossil hydrothermal system is presented. Sulphur isotope ratios of sulphides from the fold-bearing quartz-sillimanite gneiss gave 34S-values close to zero indicating a magmatic source of the sulphur.  相似文献   

15.
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16.
The massive Zn-(Pb) sulfide ore body at Rampura-Agucha in Bhilwara district, Rajasthan, occurs within graphitic metapelites surrounded by garnet-biotite-sillimanite gneiss containing concordant bodies of amphibolite. These rocks and the sulfide ores have been studied to estimate the pressure, temperature and fluid composition associated with upper amphibolite facies metamorphism. Geothermobarometric calculations involving garnet-biotite and garnet-hornblende pairs, as well as sphalerite-hexagonal pyrrhotite-pyrite and garnet-plagioclase-sillimanite-quartz assemblages indicate that the most pervasive P-T condition during peak of regional metamorphism was 650°C and 6 kb, and was attained between the first and second deformations in the region. Some temperature-pressure estimates also cluster around 500°C–5.1 kb which probably represent retrograde cooling during unloading. Consideration of devolatilization equilibria in the C-O-H-S system at the pervasive metamorphic conditions mentioned above shows that the metamorphic fluid was H2O-rich ( ) but also had a substantial component of . and were the other important phases in the fluid. CO (XCO = 0.002) and were the minor phases in the fluid. It is probable that a part of this aqueous fluid was consumed by re-/neocrystallization of hydrous silicate phases like chlorite during the retrogressive metamorphic path, so that fluid entrapped in quartz below 450°C was rendered CO2-rich (Holleret al 1996).  相似文献   

17.
北天山温泉群的地质特征、时代和构造意义   总被引：1，自引：0，他引：1  
北天山温泉群长期以来被认为是古元古代的变质岩。最新的野外调查和锆石SHRIMP U-Pb 测年结果表明,温泉群可 以划分为三个岩石构造单元：（1）前早新元古代变质火山岩和变质沉积岩,主要包括斜长角闪岩、云母片岩、石英片岩、 黑云母片麻岩、大理岩等;（2）早新元古代混合岩和正片麻岩;（3）早古生代未变质变形的辉长岩和闪长岩。上述三种岩 石组合类型均被后期二云母花岗岩（脉）所侵入。温泉县以南的混合岩和花岗片麻岩中锆石的SHRIMP U-Pb 年龄分别为 926±12 Ma 和907±11 Ma,与天山地区出露的新元古代花岗岩类的时代基本一致。结合前人对花岗片麻岩Nd 同位素组成 的研究,花岗片麻岩应为古老地壳物质部分熔融的产物,而同期的混合岩化作用则是新元古代地壳加厚和部分熔融的直接 地质证据。辉长岩和闪长岩侵入到温泉群花岗片麻岩和斜长角闪岩中,其中闪长岩的锆石SHRIMP U-Pb 年龄为452±7 Ma, 并含有1.1 Ga 和1.4 Ga 的继承锆石。根据前人的研究成果,本区早古生代辉长岩和闪长岩具有岛弧岩浆岩的地球化学特征, 可能与准噶尔-巴尔喀什洋的俯冲作用有关,这一俯冲增生作用最终导致伊犁北部与哈萨克斯坦陆块在志留纪拼贴造山, 并使温泉群前寒武纪变质岩与侵入岩发生变质变形作用。  相似文献   

18.
Two Neoarchean alkaline feldspar-rich granites sourced from partially melted granulite-facies granodioritic orthogneiss have been here recognised in the eastern part of the North China Block (NCB). These poorly foliated granites have previously been assumed to be Mesozoic in age and never dated, and so their significance has not been recognised until now. The first granite (AG1) is a porphyritic syenogranite with megacrystic K-feldspar, and the second (AG2) is a quartz syenite with perthitic megacryst. Zircons from the granites yield LA-ICP-MS U-Pb ages of 2499 ± 10 Ma (AG1), and 2492 ± 28 Ma (AG2), which are slightly younger than the granodioritic orthogneiss that they intrude with a crystallisation U-Pb age of 2537 ± 34 Ma. The younger granites have higher assays for SiO2 (71.91% for AG1 and 73.22% for AG2) and K2O (7.52% for AG1 and 8.37% for AG2), and much lower assays for their other major element than the granodioritic orthogneiss. All of the granodioritic orthogneiss and granite samples have similar trace element patterns, with depletion in Th, U, Nb, and Ti and enrichment in Rb, Ba, K, La, Ce, and P. This indicates that the granites are derived from the orthogneiss as partial melts. Although they exhibit a similar REE pattern, the granites have much lower total REE contents (30.97×10−6 for AG1, and 25.93×10−6 for AG2), but pronounced positive Eu anomalies (Eu/Eu* = 8.57 for AG1 and 27.04 for AG2). The granodioritic orthogneiss has an initial 87Sr/86Sr ratio of 0.70144, εNd(t) value of 3.5, and εHf(t) values ranging from −3.2 to +2.9. The orthogneiss is a product of fractional crystallisation from a dioritic magma, which was derived from a mantle source contaminated by melts derived from a felsic slab. By contrast, the AG1 sample has an initial 87Sr/86Sr ratio of 0.6926 that is considered too low in value, εNd(t) value of 0.3, and εHf(t) values between +0.57 and +3.82; whereas the AG2 sample has an initial 87Sr/86Sr ratio of 0.70152, εNd(t) value of 1.3, and εHf(t) values between +0.5 and +14.08. These assays indicate that a Sr-Nd-Hf isotopic disequilibrium exists between the granite and granodioritic orthogneiss. The elevated εHf(t) values of the granites can be explained by the involvement of Hf-bearing minerals, such as orthopyroxene, amphibole, and biotite, in anatectic reactions in the granodioritic orthogneiss. Based on the transitional relationship between the granites and granodioritic orthogneiss and the geochemical characteristics mentioned above, it is concluded that the granites are the product of rapid partial-melting of the granodioritic orthogneiss after granulite-facies metamorphism, and their crystallisation age of about 2500 Ma provides the minimum age of the metamorphism. This about 2500 Ma tectonic-metamorphic event in NCB is similar to the other cratons in India, Antarctica, northern and southern Australia, indicating a possible connection between these cratons during the Neoarchean.  相似文献   

19.
U-Pb isotope analyses of zircon and titanite extracted from different rocks of the Felbertal scheelite deposit yield the following information: (1) An age of 593±22 Ma (2) is obtained for zircon crystallization in the scheelite-bearing matrix of an eruption breccia in the western ore field. (2) Discordant zircons from an elongated, up to 8 m thick scheelite-rich quartzite body in the eastern ore field give an upper intercept age of 544±5 Ma. This quartzite contains a laminated, fine-grained scheelite mineralization. (3) Zircons from a small granitoid intrusion of the western ore field reveal an age of 336±16 Ma, and concordant titanites document an age of 282±2 Ma for Variscan amphibolite facies metamorphism. Both events, granitoid intrusion and later metamorphism caused ore re-mobilization, including the formation of yellowish fluorescent (molybdo-) scheelite porphyroblasts. (4) For a narrow lamprop-1hyric dike in the western ore field, a concordant titanite age of 283±7 Ma is obtained. This age is identical with the titanites from the amphibolite facies metamorphic intrusion. Tiny scheelite grains were tapped by the dike from pre-existing scheelite mineralizations in the truncated host rocks. (5) Alpine metamorphism at 31±4 Ma did not exceed lowermost amphibolite facies conditions, and it caused scheelite re-mobilization on a minor scale only, producing bluish fluorescent porphyroblasts in quartz veinlets and veins, as well as bluish fluorescent scheelite rims around older scheelite grains. Moreover, crosscutting Alpine fissure fillings show bluish fluorescent, inclusion-free scheelite. (6) The preservation of Variscan titanites, the absence of Alpine titanite growth, and the large degree of Variscan scheelite re-mobilization demonstrate that amphibolite facies metamorphism in the Felbertal area has a Variscan age. This result clearly documents Variscan tectono-metamorphism to be the dominant event, instead of the hitherto surmised Alpine metamorphism. This multi-stage evolution of the Felbertal ore bodies corroborates the view that tungsten deposits are conditioned by several succeeding thermal events, leading to a series of stages that ultimately produce high-grade scheelite concentrations. These high-grade ores predominately occur along shear zones of different age, accompanied by the formation of large volumes of low-grade scheelite mineralizations along host rock foliations and quartz veinlets and veins.  相似文献   

20.
弓长岭铁矿床磁铁富矿形成机制探讨   总被引：9，自引：0，他引：9  
弓长岭沉积变质型铁矿床是我国鞍山式铁矿床的典型代表,矿床中的磁铁富矿体的形成一直都是矿床研究的重点问题。研究表明,弓长岭富铁矿体赋存于反S型褶皱中,局部矿体出现褶曲现象。磁铁富矿体产于主要是由斜长角闪石、磁铁石英岩构成的含铁岩系中。磁铁富矿的围岩主要是绿泥片岩、石榴绿泥片岩、绿泥石榴岩和少量黑云母绿泥片岩。对绿泥石片岩、石榴绿泥片岩和绿泥石榴岩的常量、微量与稀土元素的研究和对绿泥石、石榴石、黑云母铁含量与磁铁矿品位的研究表明,含铁岩系在沉积间歇中出现的含铝的泥质-粉砂质物质在富铁矿形成过程中起对磁铁石英岩中带出的硅消耗（带入）形成石榴石、绿泥石、黑云母等的作用。区域变质作用为磁铁富矿的形成提供了储矿空间,同时区域变质作用产生变质热液,变质热液淋滤途经的磁铁石英岩可从中带出FeO,变质热液是磁铁富矿形成过程中重要的物质载体。在进变质过程中,区域变质作用为矿物转换提供所需的温压条件。可见,区域变质作用是磁铁富矿形成的重要条件。  相似文献   

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