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1.
A RbSr whole-rock isochron gives an age of 1168±21 m.y. for the agpaitic units of Ilímaussaq, showing that this complex belongs to the main phase of Gardar igneous activity in south Greenland and is not, as previously supposed, a significantly younger intrusion. Moreover, the agpaites must have intruded very soon after the earlier augite syenite phase of Ilímaussaq. The initial 87Sr/86Sr ratio of 0.7096±0.0022 for the agpaites is in marked contrast to the low (～0.703) ratio obtained for the augite syenites and suggests that selective enrichment of 87Sr occurred by preferential leaching of radiogenic strontium from unstable positions in Rb lattice sites in older crustal material by the highly reactive agpaitic magma. 相似文献
2.
Kent Brooks 《Geology Today》2012,28(1):13-19
Two contrasting intrusions in Greenland are described which both have extreme compositions and mineralogy. Ilímaussaq is part of the Mezoproterozoic Gardar Province of southern West Greenland. It developed by extreme fractionation in the crust of non‐remarkable weakly alkaline basic magmas leading to extraordinarily high levels of many rare elements. Indeed, in this intrusion the zirconium mineral, eudialyte, attains rock‐forming status. Gardiner in the East Greenland sector of the Palaeogene North Atlantic Igneous Province, was formed from nephelinitic magmas formed at great depth in the mantle by very low degrees of partial melting. Here, the magmas were so silica‐poor that there are large amounts of rocks composed of the melilite group of minerals, generally with perovskite (CaTiO4) as a major phase. Thus, extreme compositions at Ilímaussaq were caused by fractionation to very small amounts of melt while at Gardiner it was caused by very small degrees of mantle melting. Both intrusions are known for fine specimens of rare minerals; in the case of Ilímaussaq over 200 have been described. Rocks such as those found here require their own names as they cannot be accommodated in the usual petrological nomenclature. 相似文献
3.
South Greenland has been the site of historic mining of cryolite, copper, graphite and gold, hosts mineral deposits with gold, uranium, zinc, niobium, tantalum, zirconium, hafnium, REE, iron, titanium, vanadium, fluorite and graphite, and has additional potential for lithium, beryllium, phosphorus, gallium and thorium. Data from stream sediment geochemical surveys document that South Greenland is enriched in a range of these elements relative to the rest of Greenland and to estimates of the upper crust composition. Distribution patterns for individual elements within south Greenland exhibit enriched regions that are spatially related to lithological units, crustal structure and known mineralisation.The Northern Domain of South Greenland includes the southernmost part of the orthogneiss-dominated North Atlantic craton. Orogenic gold mineralisation is hosted by quartz veins and hydrothermally altered rocks associated with shear zones intersecting the Mesoarchaean Tartoq Group of mafic metavolcanic rocks. Geochemical exploration indicates that additional potential for gold mineralisation exists within Palaeoproterozoic supracrustal rocks overlying the Archaean basement.Rocks formed during the Palaeoproterozoic Ketilidian orogeny occupy a major part of South Greenland and has been divided into two domains. The Central Domain is underlain by the Julianehåb igneous complex forming a 100 km wide ENE–WSW zone centrally across South Greenland. Intrusive and extrusive, mostly felsic magmatic rocks were emplaced in two main stages (1850–1830 and 1800–1780 Ma) in a continental arc setting. Positive anomalies in aeromagnetic data indicate that mafic plutons are common in the late igneous complex. Intra-arc mafic metavolcanic rocks contain syngenetic stratabound copper sulphide and epigenetic shear zone-hosted copper–silver–gold mineralisation at Kobberminebugt and Kangerluluk, whereas metasedimentary and metapyroclastic rocks contain stratabound uraninite mineralisation. Orthomagmatic iron–titanium–vanadium mineralisation is hosted by a gabbro. A potential for porphyry-type mineralisation related to the late intrusive stages of the Julianehåb igneous complex is suggested by showings with copper, molybdenum and gold together with stream sediment anomalies for these elements. Vein-type uranium mineralisation occurs in fault zones in the Julianehåb igneous complex related to Mesoproterozoic rifting.The Southern Domain contains an assemblage of Palaeoproterozoic metasedimentary and metavolcanic rocks that underwent moderate to strong deformation, peak HT–LP metamorphism and partial melting with subsequent retrograde exhumation at 1790–1765 Ma. The supracrustal rocks contain syngenetic Au, As, Sb, U, and Zn mineralisation in volcanic or graphite- and sulphide-rich sedimentary environments; graphite was mined historically at two sites. Many stream sediment gold anomalies are located in a NE-trending belt along the boundary between the early Julianehåb complex and the supracrustal rocks to the south. They reflect a number of auriferous quartz vein occurrences, including the Nalunaq gold deposit, hosted in a system of shear zones and probably generated as orogenic gold during Ketilidian accretion. The 1755–1730 Ma, A-type Ilua plutonic suite is the latest magmatic event in the Ketilidian orogen.The 1300–1140 Ma Gardar period involved continental rifting, sedimentation and alkaline magmatism. Numerous dykes and 10 ring-shaped intrusion complexes were formed across South Greenland. An orthomagmatic iron–titanium–vanadium deposit is hosted by troctolitic gabbro. Residual magmas and fluids resulting from extreme magmatic differentiation, possibly combined with assimilation of older crust, created mineral deposits including cryolite that was mined at Ivigtut, large low-grade deposits of uranium–rare earth elements–zinc at Kvanefjeld and tantalum–niobium–rare earth element–zirconium at Kringlerne, in the Ilímaussaq complex, as well as tantalum–niobium–rare earth elements at Motzfeldt Sø in the Igaliko complex.The South Greenland crustal evolution records effects of mantle processes, such as lithospheric extension, subduction and underplating, which resulted in recurrent magma emplacement in tectonically active environments. As such, the geology of South Greenland reflects events and circumstances that are favourable to the generation and preservation of hydrothermal ore-forming fluid systems during the Ketilidian orogeny as well as to the development of extreme rock compositions within the Gardar alkaline igneous province. 相似文献
4.
Ronny Schoenberg Michael A.W. Marks Jan A. Schuessler Friedhelm von Blanckenburg Gregor Markl 《Chemical Geology》2009,258(1-2):65-77
The Ilímaussaq intrusion, South Greenland, provides an exceptional test case for investigating the changes of stable Fe isotope fractionation of solidus phases with changes in the Fe3+/∑Fe ratio of an evolving melt. The intrusion comprises a sequence of four melt batches that were fed from the same parental alkali basaltic magma. Differentiation produced cumulate rocks that range from augite syenite (phase I) over peralkaline granite (phase II) to agpaitic syenites (phases IIIa and IIIb). Fe3+/∑Fe ratios in amphiboles increase substantially from phase I to phase II and III rocks and mark a major change in the parental magma composition from augite syenites to peralkaline granites and agpaitic syenites. Before this transition, olivine, clinopyroxene, and amphibole in augite syenite, the most primitive rock type in the Ilímaussaq Complex, have a uniform Fe isotope composition that is identical to that of the bulk of igneous crustal rocks and approximated by the average isotopic composition of basalts (δ56/54FeIRMM-014 = 0.072 ± 0.046‰). After the transition, amphiboles in the peralkaline granites and agpaitic syenites yield significantly heavier Fe isotope compositions with δ56/54FeIRMM-014 values ranging from 0.123 to 0.237‰. Contamination of the Ilímaussaq magma by ongoing crustal assimilation as cause for this increase can be excluded on the grounds of Nd isotope data. Large-scale metasomatic overprint with an external fluid can also be dismissed based on amphibole O and Li isotope systematics. Rather, the increase towards heavy Fe isotope compositions most likely reflects the change in chemical compositions of amphiboles (calcic in augite syenite to sodic in the agpaitic syenites) and their Fe3+/ΣFe ratios that mirror changes in the chemical composition of the melt and its oxygen fugacity. A sensitive adjustment of equilibrium Fe isotope fractionation factors to amphibole ferric/ferrous ratios is also supported by beta-factors calculated from Mössbauer spetroscopy data. Comparison of the measured isotope fractionation between clinopyroxene and amphibole with that predicted from Mössbauer data reveal Fe isotope systematics close to equilibrium in augite syenites but Fe isotopic disequilibrium between these two phases in phase IIIa agpaitic syenites. These results are in agreement with O and Li isotope systematics. While amphiboles in all Ilímaussaq lithologies crystallized at temperatures between 650 and 850 °C, textural evidence reveals later clinopyroxene crystallization at temperatures as low as 300–400 °C. Therefore, isotopic equilibrium at crystallization conditions between these two phases can not be expected, but importantly, subsolidus reequilibration can also be dismissed. 相似文献
5.
The origin and mode of emplacement of lujavrites in the Ilímaussaq alkaline complex, South Greenland
Lujavrites are rare meso- to melanocratic agpaitic nepheline syenites that are characterized by elevated contents of elements such as Li, Be, Zr, REE, Nb, Th and U. They are the most evolved members of the three large composite agpaitic complexes – Lovozero, Kola Peninsula, Russia; Pilansberg, South Africa; and Ilímaussaq, South Greenland – and are inferred to stem from the same deep fractionating magma sources that fed the earlier members of the complexes. The composition of the melts that evolved into lujavrites is, however, not well known. The agpaitic part of the Ilímaussaq complex is divided into a roof series, a floor series of cumulates and an intermediate series of lujavrites sandwiched between the two. In the traditional view, the lujavrites formed from residual melts left between the downward crystallizing roof series and the floor cumulates. New field observations and geochemical data suggest that the floor cumulates and the main mass of lujavrites constituted a separate intrusive phase which was emplaced into the already consolidated roof series rocks largely by piecemeal stoping. Studies of the contact facies of the floor cumulates indicate that the initial magma of the floor cumulate–lujavrite sequence was peralkaline nepheline syenitic with enhanced contents of Zr, Hf, HREE, Y, Nb, Ta, F, Ba and Sr. Subsequent crystallization in a closed system resulted in the formation of the floor cumulates and lujavrites. Chemical analyses of dykes within and outside the complex represent stages in the magmatic evolution of the agpaitic rocks. 相似文献
6.
阜新萤石成矿区稀土元素地球化学特征及指示意义 总被引：1，自引：0，他引：1
为了研究阜新萤石成矿机制,对其稀土元素地球化学特征进行了分析。阜新地区萤石矿赋存于早二叠世、晚三叠世和晚侏罗世花岗岩中。地球化学分析结果显示,所有萤石均具有弱的Ce负异常,其稀土配分模式存在3种类型:Eu明显亏损型、Eu弱亏损型和Eu富集型。萤石中稀土元素的含量并不随围岩中的稀土元素含量的增加而增加,晚期侵入的花岗岩富集轻稀土元素。从成矿早期到成矿晚期,萤石的稀土元素配分型式从Eu明显亏损型向富集型演化,稀土元素总量逐渐降低。赋存于早二叠世和晚三叠世花岗岩中萤石矿流体包裹体中SO42-含量及液相成分还原参数指标指示,成矿流体由还原条件向氧化条件转变,成矿物质主要来源于赋矿花岗岩。 相似文献
7.
四川牦牛坪稀土矿床萤石Sr、Nd同位素对地幔成矿流体的指示意义 总被引：14，自引：0，他引：14
萤石是四川牦牛坪稀土矿床主要的脉石矿物之一，其形成贯穿了整个稀土成矿过程，因此同位素的研究对探讨萤石和稀土成矿流体的来源具有重要的价值。矿区6件萤石样品的Sr、Nd同位素组成没有明显差异，结合围岩（碳酸岩－正长岩，花岗岩）同位素组成特征研究表明，不同颜色、来自不同矿石类型、具有不同REE类型的萤石为同源产物，稀土成矿流体来源于富集地幔，与区内碳酸岩－正长岩岩浆活动密切相关。 相似文献
8.
REE and fluid inclusions in zoned fluorites from Eastern Transbaikalia: Distribution and geochemical significance 总被引：1，自引：0，他引：1
S. F. Vinokurov V. N. Golubev T. L. Krylova V. Yu. Prokof’ev 《Geochemistry International》2014,52(8):654-669
Special methodology was used to study the distribution of REE and some other elements in zoned fluorites from the different deposits of Eastern Transbaikalia. Fluorites from the uranium and polymetallic ore fields sharply differ in their REE distribution pattern and the composition of fluid inclusions, which reflects the geochemical specifics and indicates the possible sources of parental solutions. A gradual change in REE distribution patterns established in the successive growth zones of fluorites clearly coincides with the gradual decrease of temperature and mineralization of fluid inclusions. It is suggested that a change in the REE distribution pattern was provoked by the crystallochemical differentiation related to the formation of nano-sized mineral admixtures of REE phosphates and/or fluorcarbonates, which possess an ability to the selective accumulation of different REE groups. It was found that the zoned fluorites from the Streltsovka and Garsonui deposits show an opposite trends in the change of REE pattern with zonation. With a general decrease in total REE contents, fluorite from the Streltsovka deposit shows a change from positive parabolic to subchondritic pattern, while that from the Garsonui deposit, varies from the negative via subchondritic to the positive patterns. 相似文献
9.
Experimental data indicate that high F concentrations in leucocratic aluminosilicate melts (of granite and nepheline syenite composition) bring about the crystallization of F-rich minerals (topaz, villiaumite, and cryolite) on the liquidus. The crystallization of the minerals is controlled by the silicity, agpaitic coefficient, and proportions of alkalis in the system SiO2-Al2O3-Na2O-K2O-F-H2O. Our earlier experimental data on this system are compared with petrographic and petrochemical data on granites and nepheline syenites containing accessory topaz, cryolite, and villiaumite. The composition of topaz- and cryolite-bearing rocks is proved to correspond to the experimentally established equilibrium fields of F-rich aluminosilicate melt with these minerals. It is proved that the high-F minerals can crystallize from melt. The partial substitution of K and Na for Li modifies phase relations in the system, first of all, significantly expands the equilibrium field of aluminosilicate melt and alkaline aluminofluoride melts. The two melts are proved to be immiscible within broad compositional ranges in the SiO2-Al2O3-Na2O-Li2O-F-H2O system at 800–650°C and 1 kbar. Experimental data indicate that fluoride brine can coexist with aluminosilicate melts in nature. This finds support data on melt inclusions in granites and alkaline rocks whose contents of major components, water and fluorine are close to those in the experimental glasses. Our data lend support to the hypothesis that large cryolite bodies at the Ivigtut, Pitinga, Ulog-Tanzek, and other deposits were formed by fluoride salt melts that separated from F-rich aluminosilicate magmas late in the course of their differentiation. It is experimentally established that fluoride salt melts are able to concentrate valuable trace elements, such as Li, W, Nb, Hf, Sc, U, Th, and REE, which suggests that such melts can play an important role in the origin of rare-metal deposits genetically related to rocks that crystallize from magmas rich in F. 相似文献
10.
Mansour Rezaei Azizi Ali Abedini Samad Alipour Hashem Bagheri 《Arabian Journal of Geosciences》2018,11(22):719
The Laal-Kan fluorite deposit (west of Zanjan city, NW Iran) mainly occurred as some open-space filling and vein/veinlet in the schist of the Paleozoic age. Mineralogically, calcite, fluorite types (white, smoky, and violet), and quartz are the principal constituents accompanied by a number of minor accessory minerals such as hemimorphite, hematite, barite, and clays. Based on chemical analyses, fluorites of various colors were found to have low rare earth element (REE) concentrations (4.16–25.67 ppm). The chondrite-normalized REE patterns indicated that early fluorites were enriched in LREE, relative to HREE, whereas late fluorites were enriched in HREE relative to LREE. This study, therefore, indicated that fugacity of oxygen likely played a significant role in the occurrence of positive Ce and negative anomaly in the late fluorite. Furthermore, the Gd behavior of the fluorite samples could be attributed to the Gd-F complex in ore-forming fluids. On the other hand, low pH hydrothermal fluids under alkaline conditions were probably the main mechanism responsible for the deposition of the early fluorites in this district. Fluorite-hosted fluid inclusion analyses also indicated that fluorite-forming fluids consisted of NaCl, MgCl2, CaCl2, and LiCl with a narrow TH (118–151 °C) and high salinities (18.96–23.47 wt.% NaCl equiv.). Further, the diagram of Tb/La-Tb/Ca ratios revealed that fluorites were predominantly deposited in the hydrothermal environment and the late stage fluorites could be considered as the product of the secondary mineralization of the early fluorites due to the interaction of the fluid with the early fluorites. 相似文献
11.
Nasser Mourad Mahdy Mahmoud Hany Shalaby Hassan Mohamed Helmy Ali Farage Osman El Sayed H. El Sawey Ehab Korany Abu Zeid 《Arabian Journal of Geosciences》2014,7(7):2573-2589
Uranium mineralizations occur and form in a broad range of geologic setting and age, including magmatic to surfacial conditions, and there are numerous controls on their transportation and deposition, such as redox, pH, ligand concentration, complexation, and temperature. These temporal and spatial variations have caused a range of ore deposit mineral assemblages. Consequently, understanding their conditions of formation is still in its infancy. This research reports rare earth elements (REE) and trace elements of fluorite associated with hexavalent uranium mineralizations and tests of genetic models for the deposits. These data contribute to a better understanding of the variables controlling fluorite formation and uranium ore composition through understanding the evolution of these ore-forming hydrothermal systems. Fluorite in Gabal Gattar granite occurs as disseminations and/or thin veinlets and encrustations filling some uranium mineralized fissures and fractures along the northern margin of host granite mass. In the U-poor samples, fluorite forms well-developed large crystals that are commonly zoned. The zones are represented by alternating colorless and violet zones, and the outer zones are frequently dark violet. In the U-rich samples, fluorite is usually anhedral, unzoned, and has a dark violet color. The results of analysis of REE and trace element contents of fluorites using laser ablation inductively coupled plasma mass spectrometry indicate that total REE in the anhedral unzoned fluorite are elevated compared to the well developed zoned fluorite, and also total REE in dark violet zones of zoned fluorite are elevated with respect to the colorless zones. The fluorites and host granite are generally characterized by strongly negative Eu anomalies and slightly negative or chondritic Ce anomalies. Accordingly, REE patterns of the fluorite and host granite are roughly alike, indicating that the source of REE and trace elements of hydrothermal fluids is the host granite leached by fluids. Y/Y*, Ce/Ce,* and Eu/Eu* patterns show that fluorite clearly records the compositional evolution of the hydrothermal solutions that have transferred trace and REE from host granite during the fluid–wall rocks interactions. The high uranium contents of fluorite in Gabal Gattar granite suggest that parent fluids bearing fluorine have interacted with host granite to leach uranium from the accessory minerals of granite and tetravalent uranium minerals in reduced or weakly oxidized zones. 相似文献
12.
13.
Data on processes that occurred at contacts of large agpaitic syenite intrusions and basement gneisses obtained by the authors
by studying and sampling profiles across the contacts and involve the composition of minerals, analysis of mineral assemblages,
isotopic dating of the processes, and analysis of the behavior of major, volatile, and trace elements in rocks near the contact.
The contact zones of the massifs were determined to consist of products of contact interaction during the early and late magmatic
stages and provide a record of successive stages of a continuous process of gneiss transformations, starting with the filling
of the magmatic reservoir with melt and ending with late- and postmagmatic processes related to the development of a system
of alkaline veins and pegmatite bodies in the gneisses. Early alkaline metasomatic processes in the Khibina Massif were local,
controlled by diffusion, and were induced by the immediate thermal and chemical effect of alkaline melts on the gneisses.
In the Lovozero Massif, metasomatism was related predominantly to the development of postmagmatic veins at 359 ± 5 Ma, was
controlled by infiltration, and proceeded immediately after the consolidation of the main intrusive series. The metasomatic
transformations during the early and late magmatic stages under the effect of agpaitic melts on gneisses predetermined different
closure conditions and, correspondingly, different behaviors of the Rb-Sr and Sm-Nd isotopic systems during the contact processes:
while the interaction of agpaitic melts with gneisses has modified the (87Sr/86Sr)(T = 370 Ma) ratio via the enrichment of radiogenic Sr in the host Archean rocks, the Sm-Nd isotopic characteristics of the syenites in
the inner contact zone and veins preserved their mantle values, which corresponded to the average ones for rocks in the central
parts of the intrusion. Experimental data, model simulations, and natural observations testify that Nb, Ta, Zr, Hf, and REE
were mobile in the contact interaction zone with agpaitic melts. With regard for data on the fluid regime of the agpaitic
melts and the concentrations of volatile components in the contact zones, we believe that the main role in the transfer of
REE and HFSE during contact metasomatism could be played by their ligands with F−, Cl−, and SO42−. 相似文献
14.
YUAN Shunda ;PENG Jiantang ;HU Ruizhong ;BI Xianwu ;QI Liang ;LI Zhaoli ;LI Xiaomin ;SHUANG Yan 《中国地球化学学报》2008,27(4):342-350
In this paper the authors present the REE concentrations and Sr and Nd isotopic compositions of fluorites from the Bailashui tin deposit of the Furong ore field, southern Hunan Province. The results showed that the total amount of REE in fluorites is usually low, ranging from 0.705 to 8.785 μg/g with the chondrite-normalized REE distribution patterns similar to those of the Qitianling granites in the study area, characterized by LREE-enrichment patterns with pronounced negative Eu anomalies. The fluorites vary in Sr isotopic composition within the range of 0.7083-0.7091, the values are lower than those of the granites and higher than those of the host carbonate rocks in this area. The εNd（t） values of fluorites vary between -9.4 and ＋10.3, revealing that both the crust- and mantle-source materials were involved in the ore-forming hydrothermal fluids. Combined with previous studies on this ore deposit, the Bailashui tin deposit is temporally and spatially closely related with granitic magmatism in this area. The hydrothermal fluorites are the product of fluid/rock interactions between granitic magmatic hydrothermal fluid and marine carbonate rocks. The REE and F in the ore-forming fluid were derived from the granites, whereas Sr in the ore-forming fluid came mainly from the granitic magmatic hydrothermal fluid and marine carbonate rocks, although variations in Sr isotopic composition cannot be explained by a simple mixture of these two end-members. Evidence demonstrated that the ore-forming fluids are of crustal-mantle mixing origin, but that the fluids were probably incompletely homogenized and this may be caused by inhomogeneous mixing of the fluids of different sources. 相似文献
15.
The use of vein fluorite as probe for paleofluid REE and Sr-Nd isotope geochemistry: The Santa Catarina Fluorite District, Southern Brazil 总被引：1，自引：0，他引：1
Fluorite can be used as a probe for the source of Sr and REE, as well as for the Sr and Nd isotope systematics of mineralizing solutions, allowing characterization of the composition, oxidation state and sources of the fluids. The 87Sr / 86Sr ratios in vein fluorite from the Santa Catarina Fluorite District, southern Brazil, are low (0.720 to 0.745) relative to those of the majority of host granites at the time of mineralization (90 Ma), but are similar to those of less abundant and less evolved Sr- and Ca-rich granites and plagioclases of the heterogeneous Pedras Grandes granite association. Major contributions of Sr from the unradiogenic Parana Basin rocks (87Sr / 86Sr90 Ma = 0.705 to 0.718) are unlikely, considering the radiogenic character of the lower 87Sr / 86Sr end-member in fluorite mixing lines. Estimated fluorite fluid partition coefficients (KdSr-Ca = 0.019 and DSr ≈ 600) indicate a Sr / Ca ratio in the fluorite-forming solution of 0.012, and Sr contents of 0.05 to 0.25 ppm, which are similar to those of present-day granitic geothermal waters. Initial Nd isotopic compositions of the vein fluorites (0.5120 to 0.512) are similar to those of the Pedras Grandes granites. The 143Nd / 144Nd90 Ma of the evolved granites of the Tabuleiro granite association, their accessory fluorites and the Parana Basin rocks are considerably more radiogenic (0.5120 to 0.5127) and these are thus considered to be unlikely sources of the fluids. The REE patterns of vein fluorites, normalized to upper continental crust, show a range of LREE-depleted patterns, with highly variable positive and negative Eu anomalies. The host Pedras Grandes granites show flat to slightly depleted UCC normalized LREE patterns with strong negative Eu anomalies. Depletion of the LREE in fluorites resulted from the mobility of HREE fluoride complexes during fluid migration. A REE fractionation model based on ionic potential ratios indicates that Eu3+ was stable during fluid migration and fluorite precipitation. The coexistence of pyrite and Eu3+ in the mineralizing fluids is consistent with low pH and oxygen fugacities near the hematite-magnetite buffer. 相似文献
16.
Sm-Nd isotopic compositions were determined for the peralkaline Ilímaussaq Complex of the Gardar Province of southern Greenland. The majority of the samples in the agpaitic and augite syenitic units have near chondritic initial Nd(≈ 0), whereas a few samples trend towards Nd values as low as − 6 at the time of intrusion (1143 Ma). This latter value, from a sample taken from the margin of the complex, lying on the evolutionary trend for Ketilidian country-rock granitoids, suggests that large-scale contamination took place only at the margins of the complex. The similarity of the Nd isotopic compositions of the augite syenite and agpaitic units suggests that their parental magmas were derived from the same reservoir. A comparison of the Nd with existing Sr and Hf isotopic data for the complex suggests an origin by combined assimilation fractionation processes. Assimilation-fractional crystallization modeling of the isotopic compositions indicates that the Ilímaussaq magmas could have formed through fractional crystallization of a basaltic melt while assimilating granitic crust. The model requires initially higher assimilation rates from basalt to augite syenite composition with subsequent decreasing assimilation rates from augite syenite to agpaitic compositions. Alkali granites, which formed after the intrusion of the augite syenites, have isotopic compositions intermediate between those of the augite syenites and the surrounding Ketilidian basement. This implies even greater amounts of assimilation and is interpreted as evidence for an origin through fractionation of a basaltic or augite syenite magma with concurrent assimilation of Ketilidian crust. 相似文献
17.
Uranium analyses by the fission-track method on eudialytes from the undersaturated rocks of the Ilímaussaq intrusion demonstrate that uranium enters eudialyte in isomorphous substitution. The content of uranium in the eudialytes varies with the crystallization of the magma in two ways.In the downwards-crystallizing roof rocks, eudialyte is interstitial and the content of uranium in eudialyte decreases with proceeding crystallization, whereas in the bottom rocks, formed by upwards accumulation of liquidus minerals including eudialyte, the uranium content in eudialyte increases with crystallization. The reason for the abnormal trend in the roof rocks is discussed and compared with similar trends elsewhere. 相似文献
18.
This work describes rare accessory minerals in volcanic and subvolcanic silica-undersaturated peralkaline and agpaitic rocks from the Permo-Triassic Cerro Boggiani complex (Eastern Paraguay) in the Alto Paraguay Alkaline Province. These accessory phases consist of various minerals including Th-U oxides/silicates, Nb-oxide, REE-Sr-Ba bearing carbonates-fluorcarbonates-phosphates-silicates and Zr-Na rich silicates. They form a late-stage magmatic to deuteric/metasomatic assemblage in agpaitic nepheline syenites and phonolite dykes/lava flows made of sodalite, analcime, albite, fluorite, calcite, ilmenite-pyrophanite, titanite and zircon. It is inferred that carbonatitic fluids rich in F, Na and REE percolated into the subvolcanic system and metasomatically interacted with the Cerro Boggiani peralkaline and agpaitic silicate melts at the thermal boundary layers of the magma chamber, during and shortly after their late-stage magmatic crystallization and hydrothermal deuteric alteration. 相似文献
19.
《International Geology Review》2012,54(12):1166-1181
Geological and isotope-geochemical studies of acid volcanics in the Verkhovtsevo greenstone belt and surrounding tonalite-trondhjemite plutons within the central Dnieper gneiss-green- stone terrain were conducted in the search for genetic relationships and increased understanding of the petrogenesis of acid melts. The acid volcanic and plutonic rocks are similar in mineral composition and form a unified calc-alkaline-like trend from dacite/tonalite to rhyolite/ trondhjemite. Dacites and tonalites have the same rare-earth element (REE) patterns with moderately fractionated light and heavy REE as well as small negative Eu anomalies. Rhyolite and trondhjemites have less-fractionated REE patterns with larger negative Eu anomalies. Whole-rock data for the acid volcanic and plutonic rocks yielded a single isochron of 3117 ± 204 Ma, εNd = +1.14 ± 0.80. The data suggest a temporal and genetic relationship between the acid volcanics of the greenstone sequences and the surrounding plutonic rocks; both appear to belong to a single suite. The positive eNd value tends to suggest that a source of their melts can be traced to mafic materials rather than to older sialic crust. Petrochemical data and REE-model calculations suggest that dacite/tonalite liquids might have formed during partial melting of a mafic source, such as Archaean tholeiite TH-1 in equilibrium with hornblende-pyroxene-plagioclase restite. Subsequent differentiation of these melts in equilibrium with titanoilmenite-pyroxene-plagioclase cumulate may have given rise to the trondhjemites and rhyolites. Such a mineralogy of the restite and cumulate phases suggests that felsic melts containing little water in the Verkhovtsevo greenstone belt were generated at depths up to 30 km, probably in the greenstone belt's mafic basement. 相似文献
20.
萤石是我国战略性矿种之一,提高其资源储备对国民经济发展具有重要意义。本研究对河南省萤石矿床的地质特征、时空分布、成矿物质来源等进行分析,系统总结萤石矿床的成矿规律,并提出河南省萤石找矿方向。结果表明：（1）河南省萤石矿床类型以热液充填型为主,全省可划分为三个萤石成矿带;（2）河南省萤石矿床主要分布于中生代花岗质侵入岩体的内部或其周边地层的北西向、北东向断裂构造带中,方城地区萤石矿床主要赋存于岩体附近的地层中;（3）萤石矿床的成矿年龄在120 Ma左右,方城地区萤石矿床可能形成于新元古代;（4）中生代花岗质岩浆作用或其期后热液活动为萤石成矿提供重要的物质和驱动热;地层岩石为萤石成矿提供重要的氟,其丰富钙源很可能是巨量萤石富集的关键;（5）萤石主成矿阶段的成矿流体应属低温、低盐度、低密度的流体系统,大气降水对萤石成矿具有重要作用;（6）河南省萤石找矿重点地区应在太华群地层岩石和中生代花岗质侵入岩体广泛发育的北带和中带,特别是合峪、太山庙、天目山、铜山等岩体的内部或其周围。 相似文献