A Pb isotope investigation of the Lovozero Agpaitic Nepheline Syenite,Kola Peninsula,Russia

For the first time Pb isotope composition was established in Lovozero rocks and raremetal ores, which is important for identifying their sources. The world’s largest layered intrusion of agpaitic nepheline syenite-the Lovozero alkaline massif—is located near the center of the Kola Peninsula in Russia. This superlarge complex plutonic body hosts the economically important loparite and eudiallyte deposits [1]. These deposits contain immense resources of REE, Nb, Ta, Zr, and constitute a world class mineral district. The Lovozero massif belongs to the Kola ultramafic alkaline and carbonatitic province (KACP) of Devonian age. Previous bulk rock studies have shown that the initial Sr and Nd isotope ratios of Lovozero rocks plot in the depleted mantle quadrant of Sr-Nd diagrams [2]. More recently, Hf isotope data obtained by Kogarko et al. (3) confirm that the Lovozero and Khibina massifs with ?_Hf between 6 and 8 are derived predominantly from a depleted mantle source. It was shown that Sr, Nd, and Hf abundances are significantly elevated in the Kola alkaline rocks, and thus their isotopic compositions are relatively insensitive to minor contamination by the overlying crustal rocks. By contrast, Pb in the KACP rocks is a much more sensitive indicator of a crustal component. In this paper we investigate the lead isotopic signature of all resentative types of Lovozero rocks (Table 1) in order to further characterize their mantle sources. The Lovozero massif consists of four intrusive phases. Rocks of phase I (mostly nepheline syenites) comprise about 5% of the total volume, phase II (urtites, foyaite, lujavrites) forms the main portion of the massif comprising 77% in volume, and phase III (eudialyte lujavrites) contributes about 18%. Country rocks are represented by Devonian effusive rocks and Archean gneisses.     

In situ U–Pb,Sr and Nd isotopic analysis of loparite by LA-(MC)-ICP-MS

This work describes the in situ analysis of loparite [(Na,REE)Ti₂O₆], a perovskite group mineral with extremely low Rb/Sr ratios and high rare earth contents, by LA-(MC)-ICP-MS for the determination of U–Pb ages together with Sr and Nd isotopic composition. The reliability of these data were validated by analysis of a loparite standard by TIMS solution methods. Data are given for loparite from the Lovozero and Khibiny peralkaline complexes of the Kola Alkaline Province (Russia). For Lovozero loparite the Tera–Wasserburg intercept age for 15 loparites analysed is 373 ± 11 Ma, and the weighted ²⁰⁷Pb corrected ²⁰⁶Pb/²³⁸U age is 373 ± 2 Ma. For Khibiny loparite, the intercept age for 5 loparites analysed is 375 ± 10 Ma, and the weighted ²⁰⁷Pb corrected ²⁰⁶Pb/²³⁸U age is 374 ± 3 Ma. The common Pb compositions for Lovozero and Khibiny loparites are identical i.e. ²⁰⁷Pb/²⁰⁶Pb = 0.898 ± 0.009 and 0.898 ± 0.007, respectively. The ⁸⁷Sr/⁸⁶Sr initial ratios of Lovozero loparite range from 0.703552 to 0.703682 (av. 0.703611), and εNd (t₃₇₀) from + 3.8 to + 4.4 (av. + 4.0). The ⁸⁷Sr/⁸⁶Sr initial ratios of Khibiny loparite range from 0.703560 to 0.703871, and εNd (t₇₃₀) from + 4.0 to + 4.8. Our data indicate that in situ LA-(MC)-ICP-MS analysis of loparite provides accurate and precise estimates of the intrusion ages and isotopic composition of peralkaline rocks.     

Lead isotopic evidence for interaction between plume and lower crust during emplacement of peralkaline Lovozero rocks and related rare-metal deposits,East Fennoscandia,Kola Peninsula,Russia

The Lovozero alkaline massif—an agpaitic nepheline syenite layered intrusion—is located in the central part of the Kola Peninsula, Russia, and belongs to the Kola ultramafic alkaline and carbonatitic province (KACP) of Devonian age. Associated loparite and eudialyte deposits, which contain immense resources of REE, Nb, Ta, and Zr, constitute a world class mineral district. Previous Sr, Nd, and Hf isotope investigations demonstrated that these rocks and mineral deposits were derived from a depleted mantle source. However, because the Sr, Nd, and Hf abundances in the Kola alkaline rocks are significantly elevated, their isotopic compositions were relatively insensitive to contamination by the underlying crustal rocks through which the intruding magmas passed. Pb occurring in relatively lower abundance in the KACP rocks, by contrast, would have been a more sensitive indicator of an acquired crustal component. Here, we investigate the lead isotopic signature of representative types of Lovozero rocks in order to further characterize their sources. The measured Pb isotopic composition was corrected using the determined U and Th concentrations to the age of the crystallization of the intrusion (376?±?28 Ma, based on a ²⁰⁶Pb/²⁰⁴Pb versus ²³⁸U/²⁰⁴Pb isochron and 373?±?9 Ma, from a ²⁰⁸Pb/²⁰⁴Pb versus ²³²Th/²⁰⁴Pb isochron). Unlike the previously investigated Sr, Nd, and Hf isotopes, the lead isotopic composition plot was well outside the FOZO field. The ²⁰⁶Pb/²⁰⁴Pb values fall within the depleted MORB field, with some rocks having lower ²⁰⁷Pb/²⁰⁴Pb but higher ²⁰⁸Pb/²⁰⁴Pb values. Together with other related carbonatites having both lower and higher ²⁰⁶Pb/²⁰⁴Pb values, the combined KACP rocks form an extended linear array defining either a?~2.5-Ga secondary isochron or a mixing line. The projection of this isotopic array toward the very unradiogenic composition of underlying 2.4–2.5-Ga basaltic rocks of the Matachewan superplume and associated Archean granulite facies country rock provides strong evidence that this old lower crust was the contaminant responsible for the deviation of the Lovozero rocks from a presumed original FOZO lead isotopic composition. Evaluating the presence of such a lower crustal component in the Lovozero rock samples suggests a 5–10% contamination by such rocks. Contamination by upper crustal rock is limited to only a negligible amount.     

Chemical evolution and petrogenetic implications of loparite in the layered,agpaitic Lovozero complex,Kola Peninsula,Russia

Summary Lovozero, the largest of the world’s layered peralkaline intrusions, includes gigantic deposits of Nb + REE-loparite ore. Loparite, (Na,Ce,Ca)₂(Ti,Nb)₂O₆, became a cumulus phase after crystallisation of about 35% of the ‘Differentiated Complex’, and its compositional evolution has been investigated through a 2.35 km section of the intrusion. The composition of the cumulus loparite changes systematically upwards through the intrusion with an increase in Na, Sr, Nb and Th and decrease in REE and Ti. This main trend of loparite evolution records differentiation of the peralkaline magma through crystallisation of 1600 m of the intrusion. The formation of the loparite ores was the result of several factors including the chemical evolution of the highly alkaline magma and mechanical accumulation of loparite at the base of a convecting unit. At later stages of evolution, when concentrations of alkalis and volatiles reached very high levels, loparite reacted with the residual melt to form a variety of minerals including barytolamprophyllite, lomonosovite, steenstrupine-(Ce), vuonnemite, nordite, nenadkevichite, REE, Sr-rich apatite, vitusite-(Ce), mosandrite, monazite-(Ce), cerite and Ba, Si-rich belovite. The absence of loparite ore in the “Eudialyte complex” is likely to be a result of the wide crystallisation field of lamprophyllite, which here became a cumulus phase. Received November 6, 2000; revised version accepted January 18, 2001     

Petrology,geochemistry and source characteristics of the Burpala alkaline massif,North Baikal

The Burpala alkaline massif contains rocks with more than 50 minerals rich in Zr,Nb,Ti,Th,Be and rare earth elements(REE).The rocks vary in composition from shonkinite,melanocratic syenite,nepheline and alkali syenites to alaskite and alkali granite and contain up to 10%LILE and HSFE,3.6%of REE and varying amounts of other trace elements(4%Zr,0.5%Y,0.5%Nb,0.5%Th and 0.1%U).Geological and geochemical data suggest that all the rocks in the Burpala massif were derived from alkaline magma enriched in rare earth elements.The extreme products of magma fractionation are REE rich pegmatites,apatite-fiuorite bearing rocks and carbonatites.The Sr and Nd isotope data suggest that the source of primary melt is enriched mantle(EM-Ⅱ).We correlate the massif to mantle plume impact on the active margin of the Siberian continent.     

Peculiarities of the Formation of Loparite Ores: The Lovozero Rare Metal Deposit,East Fennoscandia

Doklady Earth Sciences - A giant deposit of loparite ores, a source of Nb, Ta, and rare earth metals, is related to the world’s largest layered Lovozero pluton in the Kola Peninsula. The Sr...     

Geochemistry of the Adamello massif (northern Italy)

The Tertiary Adamello massif, outcropping over an area of more than 550 km² in the southern Alps (northern Italy) is composed mainly of granitoid rocks (granodiorite, tonalite, quartz diorite) with minor amounts of diorite and gabbro. The major and trace element composition of these rocks is comparable to calc-alkaline volcanic rocks of continental margins. The granitoid rocks display spatial and temporal variations in their composition, particularly in Na, P, Sr, La, Nb and Y contents and ⁸⁷Sr/⁸⁶Sr ratios. The variations were probably produced by concurrent contamination/wall-rock assimilation and fractional crystallization of high-alumina basaltic magma.     

Loparite-(Ce) from the Khibiny Alkaline Pluton,Kola Peninsula,Russia

Data on the occurrence, morphology, anatomy, composition, and formation conditions of loparite-(Ce) in the Khibiny alkaline pluton are given. Loparite-(Ce), (Na,Ce,Sr)(Ce,Th)(Ti,Nb)2O6, resulted from metasomatic alteration and assimilation of metamorphic host rocks at the contact with foyaite as well as foyaite on the contact with foidolite. This alteration was the highest in pegmatite, and albitite developed there. A decrease in temperature resulted in enrichment of the perovskite and tausonite endmembers in loparite-(Ce) owing to a decrease in the loparite and lueshite endmembers. La and Ce sharply predominate among rare earth elements in the composition of loparite-(Ce).     

Helium and argon isotopes in rocks and minerals of the Lovozero Alkaline Massif

The contents and ratios of helium and argon isotopes were studied in rocks of the Lovozero Massif and related rare-metal (loparite) deposits. The gases were extracted by melting (from whole-rock and mineral samples) and crushing (mainly from fluid inclusions) methods. The wide variations in the He and Ar isotopic compositions can be explained by the fact that the trapped fluid represents a mixture of variable proportions of mantle, crustal, and atmogenic components and radiogenic in situ produced gas. The obtained gas-geochemical data reflect the complex evolution of the considered ore-magmatic system and the similar trends of melt evolution and complementary fluid phase in the magmatic chamber, in general, in three-rock (urtite-foyaite-lujavrite) units and, in each individual layers, the relative closeness of the system during magmatic crystallization and initial epimagmatic processes. It was also found that the earliest magmatic mineral was loparite and that ore units and mineralization could be partially transformed during a comparatively late postmagmatic stage. An important role of paleometeoric waters in the low-temperature mineral formation was shown.     

Experimental study of liquid immiscibility in the fluid-magmatic silicate systems containing Ti,Nb, Sr,REE, and Zr

Aluminosilicate alkaline systems containing Ti, REE (La, Ce), Y, Sr, and Nb were studied experimentally at T = 1200 and 1250°C and P = 2 kbar in the presence of aqueous and alkaline fluid. In the fluid-absent experiments with systems containing the same elements, loparite crystals in a silicate matrix were obtained. These systems in the presence of aqueous and alkaline fluid pressure conditions demonstrate an immiscible splitting into two liquids: (1) aluminosilicate matrix and (2) droplets enriched in Ti, REE (La, Ce), Y, Sr, and Nb, which contain silicate admixture and are compositionally close to loparites. According to approximate estimates, the partition coefficients between the melts of the droplets and aluminosilicate matrix (K = C _dr/C _sil) are more than 5 for TiO₂, < 0.35 for SiO₂, 10–20 for Nb₂O₅, > 15 for REE, and from 2.3 to 7.6 for SrO. Obtained unmixing may be of great significance for explaining the genesis of REE-Nb (loparite) deposits. In addition, the experiments demonstrate that Zr may be accumulated together with Ti and REE due to liquid immiscibility of this type. The partition coefficient of ZrO₂ between the melts of droplets and aluminosilicate matrix (K = C _dr/C _sil) according to approximate estimates varies from ∼ 3.5 to 9.     

Small-scale transport of trace elements Nb and Cr during growth of titanite: an experimental study at 600?°C, 0.4 GPa

Trace element distribution in titanite overgrowths on rutile has been investigated experimentally at 600?°C, 400?MPa and f_O2 near NiNiO buffer. Compositionally homogenous Cr- or Nb-doped synthetic rutile single crystals or Nb-containing natural rutile crystals were the source of Cr, Nb and Ti to synthesize titanite using the double-capsule technique. All element exchange with the source of Si, Ca and Al occurred via a NaCl–H₂O fluid. Titanite forms quickly and exclusively around the rutile crystals. The titanite overgrowth separates rutile from the bulk fluid, and all elements from rutile dissolution have to pass through the titanite rim. Trace element concentrations in titanite show a considerable scatter in experiments with and without Al, although the average concentrations of Cr or Nb of titanite around compositionally homogeneous synthetic rutile approach the expected values for closed system conditions. Variability of Al with Cr or Nb in the titanite is not correlated. The Al zoning is irregular and patchy, and also the distribution of trace elements does not show systematic trends in the spatial distribution. In experiments using zoned natural rutile, the concentrations of Nb in titanite are related to the Nb zoning in rutile, but the contents also vary unsystematically. Under the controlled conditions of the experiment, the explanation for the strongly irregular spatial distribution is most likely due to variations in elemental concentrations during transport from the rutile along the titanite grain boundaries. The transport pathway is complex because grain boundary migration is important during titanite growth. Such irregular element distribution is also found in a natural sample of titanite overgrowth on rutile from an eclogite with retrograde overprint in the amphibolite facies. Transport of Ti and trace elements was focused on grain boundaries and shielded from the rutile as a source of these elements. We conclude that this type of zoning is not related to changes in P–T or composition in an open system, but solely controlled by transport in and through the titanite rim.     

The Riphean Meteshikha island-arc peridotite-gabbro massif (western Transbaikalia)

New data are presented on the geologic structure, age, petrogeochemical composition, and conditions of formation of the Late Proterozoic Meteshikha ultramafic-mafic pluton of the Ikat complex. Mafic rocks are the main rocks of the massif, whereas ultramafic rocks are secondary; both of them correspond to two intrusive phases. The first phase includes a layered rock series enriched in intercumulus amphibole, which varies in composition from olivine gabbro to leucocratic gabbro-anorthosite; the second is composed of wehrlite, plagiowehrlite, and olivine clinopyroxenite. Mineralogical, petrographic, geochemical, and isotope studies show that the rocks of both phases crystallized from the same mantle melt; note that the PT-conditions of their formation were considerably different. We suppose that they were separated in the intermediate chamber during fractional crystallization and the accumulation of early minerals (olivine and, probably, clinopyroxene) in the lower part of the chamber. Using the COMAGMAT software, we have found the composition of the parental melt for the rocks of the first phase—normal tholeiitic basalt with 0.2-0.5 wt.% water, which might have crystallized at 3.0-3.5 kbar and the oxygen activity controlled by the QFM buffer. The differentiated series is characterized by gradual depletion with Cr and Ni and enrichment with Sr, Ti, Cu, and REE during the evolution of melt. The REE patterns for the massif rocks have a similar low-fractionation trend with domination of light lanthanides over heavy ones and (La/Yb)_N = 1.25-2.75. Multielement spectra are characterized by negative anomalies of K, Th, Nb, and Zr and positive anomalies of Ba, U, Sm, and Sr. The geochemical characteristics of the rocks are similar to those of the tholeiitic basalts of present-day island arcs. Studies show that the Meteshikha massif formed in the subduction setting of the active margin of the Siberian continent in the Late Riphean (809 Ma). © 2015, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.     

Typochemistry of rinkite and products of its alteration in the Khibiny Alkaline pluton,Kola Peninsula

The occurrence, morphology, and composition of rinkite are considered against the background of zoning in the Khibiny pluton. Accessory rinkite is mostly characteristic of foyaite in the outer part of pluton, occurs somewhat less frequently in foyaite and rischorrite in the central part of pluton, even more sparsely in foidolites and apatite–nepheline rocks, and sporadically in fenitized xenoliths of the Lovozero Formation. The largest, up to economic, accumulations of rinkite are related to the pegmatite and hydrothermal veins, which occur in nepheline syenite on both sides of the Main foidolite ring. The composition of rinkite varies throughout the pluton. The Ca, Na, and F contents in accessory rinkite and amorphous products of its alteration progressively increase from foyaite and fenitized basalt of the Lovozero Formation to foidolite, rischorrite, apatite–nepheline rocks, and pegmatite–hydrothermal veins.     

Late Permian to Early Triassic crustal evolution of the Kontum massif,central Vietnam: zircon U–Pb ages and geochemical and Nd–Hf isotopic composition of the Hai Van granitoid complex

The Hai Van granitoid complex constitutes an important part of the Indosinian batholith of the northern Kontum massif in central Vietnam. The Kontum massif is a key region for understanding the palaeo-Tethyan accretionary history and the crustal evolution of the Indochina terrane. In this study, zircon U–Pb ages and geochemical data on the Hai Van complex are reported to shed more light on these geological processes. Zircon U–Pb isotopic analyses of six representative samples by the LA-ICP-MS technique show that granitoids from this complex crystallized between 242 and 224 Ma, indicating a prolonged period of magmatism in a late orogenic stage. Rocks from the Hai Van complex have S-type geochemical characteristics with high SiO₂, Na₂O + K₂O, and Zr contents as well as typical S-type minerals such as cordierite and muscovite. Trace elements of the samples are depleted in Ba, Nb, P, Sr, and Ti and enriched in Rb, K, Th, U, and Pb. Negative Eu-anomalies indicate feldspar fractionation during magma crystallization. The Hai Van granitoids exhibit Nd–Hf isotopic features similar to Proterozoic rocks exposed in South China, with low initial ?_Hf and initial ?_Nd values and two-stage Hf model ages of 1.9–1.7 Ga. It is suggested that during the palaeo-Tethyan orogeny, central Vietnam experienced strong early Mesozoic magmatism that originated from partial melting of crustal material similar in composition to the basement of the South China block.     

Structure,age, and ore potential of the Burpala rare-metal alkaline massif,northern Baikal region

The Burpala alkaline massif is a unique geological object. More than 50 Zr, Nb, Ti, Th, Be, and REE minerals have been identified in rare-metal syenite of this massif. Their contents often reach tens of percent, and concentrations of rare elements in rocks are as high as 3.6% REE, 4% Zr, 0.5% Y, 0.5% Nb, 0.5% Th, and 0.1% U. Geological and geochemical data show that all rocks in the Burpala massif are derivatives of alkaline magma initially enriched in rare elements. These rocks vary in composition from shonkinite, melanocratic syenite, nepheline and alkali syenites to alaskite and alkali granite. The extreme products of magma fractionation are rare-metal pegmatites, apatite-fluorite rocks, and carbonatites. The primary melts were related to the enriched EM-2 mantle source. The U-Pb zircon ages of pulaskite (main intrusive phase) and rare-metal syenite (vein phase) are estimated at 294 ± 1 and 283 ± 8 Ma, respectively. The massif was formed as a result of impact of the mantle plume on the active continental margin of the Siberian paleocontinent.     

New data on carbonatites of the Il’mensky-Vishnevogorsky alkaline complex,the southern Urals,Russia

Carbonatites that are hosted in metamorphosed ultramafic massifs in the roof of miaskite intrusions of the Il’mensky-Vishnevogorsky alkaline complex are considered. Carbonatites have been revealed in the Buldym, Khaldikha, Spirikha, and Kagan massifs. The geological setting, structure of carbonatite bodies, distribution of accessory rare-metal mineralization, typomorphism of rock-forming minerals, geochemistry, and Sr and Nd isotopic compositions are discussed. Dolomite-calcite carbonatites hosted in ultramafic rocks contain tetraferriphlogopite, richterite, accessory zircon, apatite, magnetite, ilmenite, pyrrhotite, pyrite, and pyrochlore. According to geothermometric data and the composition of rock-forming minerals, the dolomite-calcite carbonatites were formed under K-feldspar-calcite, albite-calcite, and amphibole-dolomite-calcite facies conditions at 575–300°C. The Buldym pyrochlore deposit is related to carbonatites of these facies. In addition, dolomite carbonatites with accessory Nb and REE mineralization (monazite, aeschynite, allanite, REE-pyrochlore, and columbite) are hosted in ultramafic massifs. The dolomite carbonatites were formed under chlorite-sericite-ankerite facies conditions at 300–200°C. The Spirikha REE deposit is related to dolomite carbonatite and alkaline metasomatic rocks. It has been established that carbonatites hosted in ultramafic rocks are characterized by high Sr, Ba, and LREE contents and variable Nb, Zr, Ti, V, and Th contents similar to the geochemical attributes of calcio-and magnesiocarbonatites. The low initial ⁸⁷Sr/⁸⁶Sr = 0.7044?0.7045 and εNd ranging from 0.65 to ?3.3 testify to their derivation from a deep mantle source of EM₁ type.     

周口店岩体矿物学、年代学、地球化学特征及其岩浆起源与演化

周口店岩体由三次侵入的中酸性岩石组成, 本次测得石英闪长岩锆石U-Pb年龄为131.6±2.1 Ma, 闪长玢岩锆石U-Pb年龄为128.1±1.4 Ma.周口店岩体各种类型岩石属高钾钙碱性系列、偏铝质, Mg#较高, 重稀土元素和Ta、Nb、P以及Ti明显亏损, 轻稀土元素和Ba、K以及Sr相对富集, Eu没有异常, Yb元素含量小于2×10-6, (La/Yb)_N和Sr/Y比值较高.斜长石复杂环带能谱线扫描表明, 花岗闪长岩中的斜长石核部牌号高, 完整的幔部由内向外由反环带和正环带组成, 微粒包体中的斜长石核部牌号低, 幔部以尘状环带开始, 然后演变为正环带, 这揭示存在多期基性岩浆的注入作用, 结合暗色微粒包体的形态、大小、数量、反向脉、矿物含量统计、矿物成分、地球化学和各类环带包体、岩墙状包体群等特征, 说明暗色微粒包体是在花岗闪长岩浆冷凝过程的不同阶段, 多期幔源基性岩浆注入并与酸性岩浆在围绕包体周缘的局部范围内发生不均一机械混合作用的结果.周口店中酸性岩石体现埃达克质岩的地球化学特征, 岩浆成分主要受源区控制, 形成于加厚下地壳环境.由石英闪长岩-花岗闪长岩至中酸性岩脉, 岩石(Er/Lu)_N和Nb/Ta比值升高, 说明源区残留相矿物组合由角闪石+石榴石向石榴石+金红石变化, 岩浆源区不断变深.      

The crystal structure of loparite: a new acentric variety

Mineralogy and Petrology - The crystal structure of a new structural variety of loparite (Na0.56Ce0.21La0.14Ca0.06Sr0.03Nd0.02Pr0.01)Σ=1.03(Ti0.83Nb0.15)Σ=0.98O3 from the Khibiny alkaline...     

U-Pb SHRIMP-II ages of titanite and timing constraints on apatite-nepheline mineralization in the Khibiny and Lovozero alkaline massifs (Kola Peninsula)

Results of this study of titanite samples collected from silicate rocks and apatite-nepheline-(sphene) ores from Paleozoic polyphase alkaline nepheline syenite complexes of the Khibiny and Lovozero massifs revealed the possibility of their in-situ U-Pb dating using sensitive high-resolution ion microprobe SHRIMP-II with an accuracy of 1.0-1.5%, which is comparable with that of U-Pb zircon analysis. Employing different approaches to age determination of the formation of the U-Pb system of titanites, the combined isochrons and mixing lines were plotted from the data obtained from the differentiated complex samples (121 analyses of five Khibiny samples and 52 analyses of one Lovozero sample) and apatite-nepheline ores (120 analyses of five Khibiny samples and 88 analyses of three Lovozero samples). They indicate synchronous crystallization of titanite in silicate rocks throughout the complexes: 374.1 ± 3.7 Ma for the Khibiny massif and 380.9 ± 4.5 Ma for the Lovozero massif, and attest to the later formation of phosphate-rare-metal ores: 371.0 ± 4.2 and 361.4 ± 3.2 Ma, respectively. The relatively delayed ore mineralization specific to the Lovozero massif can be accounted for the significantly lower volumes of magmatic melt and ore fluid involved, different thermal conditions, and the pattern of the investigated mineralization. As such, the obtained U-Pb data from titanite make it possible to limit significantly the time interval (most likely, not exceeding 15-20 Ma) comprising the evolution and activity of the ore-magmatic system of major agpaitic complexes, which is probably associated with plume magmatism.     

张广才岭五道岭组火山岩的锆石U——Pb 年代学及地球化学特征

对张广才岭五道岭组火山岩进行锆石U--Pb LA--ICP--MS 年代学和岩石地球化学研究,以确定该组火山岩形成的时代和构造背景。选取样品中的锆石均呈自形--半自形晶,具有清晰的振荡生长环带和较高的Th /U 比值( 0. 52 ～ 2. 28) ,暗示其岩浆成因。测年结果显示,五道岭组火山岩形成于289 ± 3 Ma,是早二叠世岩浆活动的产物。火山岩的岩石组合为玄武岩--流纹岩,显示双峰式火山岩的特点,属于高钾钙碱性--钾玄岩系列,玄武岩具有富集Ｒb、K 等大离子亲石元素( LILE) ,亏损Nb、Ta 等高场强元素( HFSE) 的特征; 流纹岩呈负铕异常,富集Ｒb、Th、U 和K 等大离子亲石元素( LILE) ,亏损Nb、Ta 和Ti 等高场强元素( HFSE) 及Sr、P 元素,并显示A 型流纹岩的特点。结合该时期区域构造演化史,火山岩地球化学特征揭示了五道岭组火山岩的形成与岛弧环境下的局部伸展作用有关,这种构造背景是古亚洲洋板块俯冲于松嫩-张广才岭-佳木斯陆块之下造成的。