Characteristics of rare-earth elements （REE）, strontium and neodymium isotopes in hydrothermal fluorites from the Bailashui tin deposit in the Furong ore field,southern Hunan Province,China

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.     

Errors Involved in the Taylor‘s Model for the Crustal Composition and Evolution—An Analysis of Their Causers

It is suggested in this paper that the famous Taylor‘s model for the composition and evolution of the continental crust consists of three closely related key links.These links include an assumption that the surfical environment has no effect on the sedimentary REE patterns and the REE patterns in sdhales can reflect the composition of their provenance,a discovery about the discrepancy of sedimentary REE patterns between Archean and Proteroxoic,and a deduction that there was a global scale K-granitoid event at the end of the Archean.Based on a detailed discussion,this paper substantially negates the rationality of the Taylor‘s model and argues that its three critical links be three great errors indeed.Moreover,some other deficiencies or problems it confronts are described in this paper.The authors suggest that what led to the errors invoved in the Taylor‘s model is its wrongly neglecting the effect of the sedimentary envronment on the chemical composition of sediments.and that the environment should be an important factor affecting the distribution patterns of trace elements in the sediments.     

The Hizeh-Jan Kaolin Deposit of NW Iran:the TetradEffect in REE Distribution Patterns

The Hizeh-Jan kaolin deposit(northwest of Varzeghan, East-Azarbaidjan Province, NW Iran) is a product of the alteration of Eocene andesitic rocks. Based on mineralogical examinations, kaolinite, quartz, smectite, pyrophyllite, muscovite-illite, alunite, calcite, diaspore, goethite and hematite are the most abundant mineral phases in this deposit. The geochemical indicators, such as Y/Ho and Zr/Hf, indicate the non-CHARAC(non-Charge-radius control) behavior of these pairs, which are likely to be due to the occurrence of the tetrad effect phenomenon in this deposit. Simultaneous concave and convex shapes in the chondrite-normalized REE distribution patterns are a remarkable feature of the kaolin samples. Bivariate diagrams of the size of the third tetrad effect(T_3) versus geochemical parameters such as Y/Ho, Nb/Ta and Zr/Hf ratios display two distinct populations for the kaolin samples. The first population is characterized by high T_3 values(>0.13), which are near or on the fault zone. The second population is characterized by low T_3 values(<0.13), and are farther from the fault zone. The obtained results from the geochemical data have furnished compelling evidence that fluidrock interaction, overprint of hypogene processes by supergene ones, and structural control, are key controlling factors for the occurrence of tetrad effects in REE distribution patterns in the Hizeh-Jan kaolin deposit.     

Distribution of Rare—earth Elements in the Shenzhen Bay and Dapeng Bay Modern Sediments

The aim of this paper is to study the REE geochemistry of the Shenzhen Bay (SZB) and Dapeng Bay(DPB) modern sediments, discuss their REE distribution patterns, reveal the REE geochemical difference between the two bays which share the same material source but are deposited in different sedimentary environments, and expound their dynamic changes.It can be concluded that the SZB and DPB sediments are essentially of continental source.Their REE distribution patterns are quite different from those of Pacific pelagic sediments, but are very similar to those of South Chi-na granites.Because of different sedimentary environments prevailing in the SZB and DPB, some REE fractionation would have taken place in the sediments of the two bays.     

Crustal Electrical Structure and Deep Metallogenic Potential in Northern Wuyi Area (South China), based on Magnetotelluric Data

The northern Wuyi area, which is located in the northern Wuyi metallogenic belt, has superior mineralization conditions. The Pingxiang–Guangfeng–Jiangshan–Shaoxing fault (PSF) extends across the whole region regardless of whether or how the PSF relates to the near-surface mineralization. We carried out an MT survey in the region and obtained a reliable 2D model of the crustal electrical structure to a depth of 30 km. In the resistivity model, we inferred that a continuous high conductivity belt that ranges from the shallow to deep crust is a part of the PSF. Then, we estimated the fluid content and pressure gradient to identify the deep sources of fluid as well as its pattern of motion pattern. Finally, we proposed a model for the deep metallogenic migration processes that combines geological data, fluid content data, pressure gradient data, and the subsurface resistivity model. The model analysis showed that the Jiangnan orogenic belt and the Cathaysia block formed the PSF during the process of com. The deep fluid migrated upward through the PSF to the shallow crust. Therefore, we believe that the PSF is an ore-forming fluid migration channel and that it laid the material basis for large-scale mineralization in the shallow crust.     

Zircon U-Pb Geochronology,Geochemistry and Petrogenesis of Early Cretaceous A-type Granites in the Mikengshan Area,Southern Jiangxi ProvinceEI北大核心CSCD

In the Mikengshan-Yanbei area, a special region for the tin mineralization in South China, there are some Yanshanian granites and porphyries and associated porphyry tin deposits (e.g., Yanbei). Although the tin mineralization is closely associated with the granitoids magmatism, the age and petrogenesis of the granites remain controversial. The Mikengshan pluton, which intrudes into the rhyolitic tuff and pyroclastic rocks of the Upper Jurassic Jilongzhang Formation, is the largest one in Mikengshan-Yanbei area and mainly consists of K-feldspar bearing granites. In this study, we report new age and geochemical data for the Mikengshan K-feldspar bearing granites. LA-ICP-MS zircon U-Pb age dating shows that they were generated in the Early Cretaceous (~138 Ma). They have high SiO2 (74.42% to 76.69%), low Al2O3 (12.39% to 13.49%) and Mg# (11 to 19), and negative Eu, Sr and Ba anomalies, and are rich in potassium (most K2O/Na2O=1.37 to 1.94) and high field strength elements (HFSEs), and are weakly peraluminous (A/CNK=1.03 to 1.1) with high 10000×Ga/Al ratios (3.46 to 4.96) and slightly high zircon saturation temperatures (807 ℃ to 817 ℃), showing the characteristics of the typical A-type granites. Moreover, they have high Rb contents 842×10-6 to 1295×10-6, Rb/Sr (90 to 255) ratios and show rare earth elements (REE) tetrad effect, which are similar to those of high fractionated granites. Thus, we suggested that the Mikengshan K-feldspar bearing granites are high fractionated and aluminous A-type granite. Their slightly high εNd(t) values (-3.4 to -4.6) indicate that the mantle components should have played a role during their formation. Taking into account regional igneous rocks and tectonic setting data, we suggest that the Mikengshan K-bearing granites were formed in an extensional setting, which was probably related to the roll-back of the subducted Paleo-Pacific plate. © 2018, Science Press. All right reserved.     

关于南岭花岗岩时代和分期讨论的倡议

Based upon the present research stratus on Nan Ling granites and in the view of the defferenees in the opinions on the ages and periodic division of Nan Ling granites existing among some research units concerned, a proposal has been snbmitted in the paper, pointing out that it is quite necessary to launch a discussion in the deeper-going way on the problem involving the ages of Nan Ling granites and their periodic division in the journal “GEOCHIMICA”. The main topics expected to be discussed are as follows : (1) The existanec of Hereynian granites and the possibility of the formation of non-orogenic granites; (2) The divisional boundary between Yeushanian and Indonsinian granites and their distributional characteristics; (3) The periodic sub-division of Yenshanian granites and their relatiouship with mineralization ; (4) The division of the ,ages of granites older than the Caledonian period.     

HEFEI BASIN IN EARLY CRETACEOUS -CHARACTERIZATION AND ANALYSIS OF PETROLEUM POTENTIAL

Comprehensive analyses were made based on seismic prospecting data, electrical prospecting data and basin simulation data as well as regional geological data and thorough discussions were conducted about the complicated structures, features and evolution of Hefei Basin in Early Cretaceous in this study, and it was derived that that Hefei Basin was a composite basin formed during the transformation of the stress field from compressive toward tensile in Early Cretaceous. In other words, this basin was a foreland basin of gliding-thrust type, which is mainly controlled by the Dabie orogenic belt in the south side in the early to middle period of Early Cretaceous, while being a strike-slip basin of pull-apart type, which is mainly controlled by the activity of Tanlu fracture in the east side in the middle to late period of Early Cretaceous. Moreover, the potential Lower Cretaceous oil and gas system in the pull-apart basin and the vista for its prospecting were explored in this study. Tectonism of the Tanlu fracture was further discussed based on the results of characterization of the basin, and it was pointed out that this is beneficial and instructive to the oil and gas prospecting in Hefei Basin.     

The Rapid Inversion of 3-D Potential Field and Program Design

The application of three-dimensional inversion of gravity and magnetic fields is very important not only in geophysical researches, but also in the study of geological structures. A formula of potential field in frequency-domain, developed by Parker in 1973, can be used as a rapid and effective algorithm in gravity and magnetic inversion. The technique has been improved then by Oldenburg, Sprenke, Feng and others.In addition to a brief introduction of Parker's algorithm and its applications, this paper includes the following five parts: basic computational techniques, inversion of single layer, convergence and constraints, simultaneous inversion for density and topography as well as inversion of multilayers. The authors present relevant practical iterative formulas and its varieties when density distribution varies with depth in linear or exponential relation. In order to maintain computation stability and speed up iteration convergence, some approaches are taken in the program design, for instance shifting lower interface of the studied layer, inverting corrections of topography, reducing grid boundary effects and utilizing low-pass filter. With the consideration of the nonuniqueness of the inversion, a method of using seismic data to constrain the range ofpossible models is discussed. It is pointed out that the density variation generates less effects than those of topography on the spectrum of gravity anomaly in second order. Therefore density contrast and topography can be inverted simultaneously by an alternative weighting iteration. By analogy, the inversion of multilayer model can be done in the above procedure. An approach of model decomposition is useful in the computation of multilayer model. The techniques discussed in the present paper for gravitational field are also valid for magnetic field.     

大厂花岗岩年龄及成矿时代

The Dachang tin ore fields is one of the largest in China, where eassiterite-sulphide-type ore deposits occur. The ages of Dachang granites which are closely related to the ore deposits are discussed in this paper. Based on sixteen K-At biotite and one U-Pb zircon ages, it is suggested that Dachang granites were formed at 72--102 m.y.The cooling rates of Dachang granites have been estimated at 18.4℃/m.y. in the arly, 13.3℃/m.y. in the middle, and 4.5℃/m.y. in the late period.     

Tetrad effects in the rare earth element patterns of granitoid rocks as an indicator of fluoride-silicate liquid immiscibility in magmatic systems

This paper focuses on reasons for the appearance of tetrad effects in chondrite-normalized REE distribution patterns of granitoids (Li-F granites, peralklaine granites, ongonites, fluorine-rich rhyolites, and granitic pegmatites). The analysis of published data showed that the alteration of such rocks by high- and/or low-temperature metasomatic processes does not result in most cases in the appearance or enhancement of M-type tetrad effects in REE patterns. These processes are accompanied by the removal or addition of lanthanides, a W-type sag appears between Gd and Ho, and negative or positive Ce anomalies develop sometimes in REE patterns. The formation conditions of peculiar rocks enriched in Ca and F from the Ary Bulak ongonite massif (eastern Transbaikalia) and the character of REE distribution in these rocks and melt inclusion glasses were discussed. Based on the obtained data and the analysis of numerous publications, it was concluded that REE tetrad effects in rare-metal granitoids are caused by fluoride-silicate liquid immiscibility and extensive melt differentiation in the accumulation chambers of fluorine-rich magmas. A considerable increase in fluorine content in a homogeneous granitoid melt can cause its heterogenization (liquation) and formation of fluoride melts of various compositions. The redistribution of lanthanides between the immiscible liquid phases of granitoid magma will result in the formation of M-type tetrad effects in the silicate melts, because the REE patterns of fluoride melts exhibit pronounced W-type tetrad effects. The maximum M-type tetrad effect between La and Nd, which is observed in many rare-metal granitoids, is related to the character of REE partitioning between fluoride and silicate melts and F- and Cl-rich magmatic fluids. The low non-chondritic Y/Ho ratio (<15) of many rare-metal granitoids may be indicative of a contribution of fluoride melts to the differentiation of F-rich silicic magmas, from which these rocks were formed. The influence of high-temperature F-Cl-bearing fluids on melts and/or granitoid rocks results in an increase in Y/Ho ratio owing to the elevated solubility of Ho in such fluids.     

Trace elements and evolution of granite melt as exemplified by the Raumid Pluton,Southern Pamirs

New trace element data were obtained by ICP-MS for 58 samples representing eight intrusive phases of the Raumid granite Pluton. All of the rocks, except for one sample that was deliberately taken from a greisenized zone, were not affected by postmagmatic fluid alteration. The sequential accumulation of incompatible trace elements (Rb, Ta, Nb, Pb, U, and others) in the Raumid Pluton from the early to late phases coupled with a decrease in incompatible element contents (Sr, Eu, Ba, and others) indicates a genetic link between the granites of all phases via fractional crystallization of a granite melt. The REE distribution patterns of final granite phases are typical of rare-metal granites. The Ta content in the granites of phase 8 is only slightly lower than that of typical rare-metal granites. Greisenization disturbed the systematic variations in trace element distribution formed during the magmatic stage. The ranges of trace element contents (Rb, Sr, Ta, Nb, and others) and ratios (Rb/Sr, La/Lu, Eu/Eu*, and others) in the Raumid granite overlap almost entirely the ranges of granitic rocks of various compositions, from the least differentiated with ordinary trace element contents to rare-metal granites. This indicates that the geochemical signature of rare-metal granites can develop at the magmatic stage owing to fractional crystallization of melts, which is the case for the melt of the Raumid granite.     

富氟花岗岩体系岩浆流体内稀土元素演化规律的实验研究

高温高压实验结果表明，随着富氟过铝花岗质岩浆分离结晶作用的进行，在与熔体相共存的流体相中，REE浓度呈有规律地变化：当温度从750℃下降至接近固相线（570℃）时，流体相中REE浓度逐渐降低，这一规律与REE在稀有金属花岗岩体上部岩相带中REE含量贫化的地质事实相一致。在富氟过铝质花岗岩体系中，REE易于分散进入某些造岩矿物（如黑云母等）和副矿物（如萤石和锡石等）中，从而不利于REE形成热液矿床。     

Mineral chemistry of columbite–tantalite from spodumene pegmatites of Kolmozero,Kola Peninsula (Russia)

Compositional variation (results of electron microprobe analyses and mass-spectrometry analyses) of columbite-group minerals (CGM) from fully differentiated albite–spodumene pegmatites at Kolmozero in the Kola Peninsula is evaluated. Concentric zoning, typical of rare-metal pegmatites, was not observed in the Kolmozero pegmatites. Columbite-group minerals occur in all main parageneses of the pegmatites and form four generations, reflecting the sequence of pegmatite formation. These minerals demonstrate wide variations in the content of major and trace elements. The composition of CGM ranges from columbite-(Fe) to tantalite-(Mn). Fractionation trends were observed in Mn/(Mn + Fe) versus Ta/(Ta + Nb) diagrams and trace-element abundances plotted versus XTa and XMn. The early CGM paragenesis is characterized by homogeneous, oscillatory and progressive oscillatory zoning and corresponds to a primary magmatic type. Late-generation CGM show patchy irregular internal textures replacing earlier regular patterns of zoning. The irregular zoning points to metasomatic replacement processes. For the first time, it is shown that distributions of rare earth elements (REE) in CGM reflect the evolution of a pegmatite-forming system. At Kolmozero, the main trend of REE variation from early to late generations of CGM involves decreasing total REE contents due to a decrease in heavy REE and Y, decreasing negative Eu anomaly and decreasing magnitude of M-shape tetrad effect between Gd and Ho. These changes are accompanied by gradual flattening of the “bird-like” patterns of chondrite-normalized REE distribution. All these features are typical for late differentiates of granitic volatile-rich magma. Late metasomatic tantalite-(Mn) is characterized by sharp changes in its REE distribution pattern: decreasing total REE contents, changing shape of the REE distribution pattern, the absence of Eu anomaly and tetrad effects, and the appearance of a negative Ce anomaly. The textural characteristics and mineral chemistry of CGM indicate that the pegmatite-forming system underwent several stages of evolution. The earliest magmatic stage can be divided into two sub-stages, involving direct crystallization and collective recrystallization, respectively, and was succeeded by a late hydrothermal–metasomatic post-magmatic stage. Variations in chemical composition among the different generations of CGM are explained by the interplay of several processes: fractional crystallization; competitive crystallization of main rock-forming (feldspar, muscovite, spodumene) and accessory (triphylyte–lithiophilite, spessartine, fluorapatite, zircon, microlite) minerals; and evolution of the mineral-forming environment from a melt to a hydrothermal–metasomatic fluid.     

Rare-earth elements as indicators of ore sources and the degree of differentiation and ore potential of rare-metal granite intrusions (eastern Transbaikalia)

The meteoritic-material-normalized REE patterns of rare-metal granite intrusions of the ore-bearing Kukul'bei complex (J2–J3), eastern Transbaikalia, were studied. It is shown that the intrusions were initially enriched in granitophile volatiles and trace elements (rare metals), i.e., this phenomenon is not related to the differentiation of their parental magma chambers. On the differentiation of the Kukul'bei rare-metal intrusions, the REE contents decrease in passing from granites of the main intrusive phase (MP) to late leucocratic differentiates (muscovite and amazonite granites), the differentiates become more enriched in granitophile elements, and their rare-metal contents drastically increase as compared with the MP granites. The ore-bearing bodies of muscovite and amazonite granites have extremely low REE contents and the highest contents of granitophile (including ore-forming) elements.The REE patterns of the Kukul'bei intrusive differentiates are not universal among rare-metal intrusions. By the example of highly ore-bearing rare-metal granite intrusions of the Erzgebirge, Central Europe, it has been established that their late deep-seated differentiates (ultrarare-metal lithionite-zinnwaldite Li-F-granites) accompanied by highly productive Sn-W mineralization concentrate both granitophile elements and REE (particularly HREE). Among the studied Transbaikalian rare-metal intrusions of the Kukul'bei complex, only the differentiates of the most ore-bearing Sherlovaya Gora intrusive system belong to the above type. The analysis of the REE patterns of the Kukul'bei granites confirmed the earlier conclusions on the low ore potential of the rare-metal mineralization of the studied intrusive complex.     

The lanthanide tetrad effect and its correlation with K/Rb,Eu/Eu1, Sr/Eu,Y/Ho,and Zr/Hf of evolving peraluminous granite suites

Lanthanide tetrad effects are often observed in REE patterns of more highly evolved Variscan peraluminous granites of mid-eastern Germany (Central Erzgebirge, Western Erzgebirge, Fichtelgebirge, and Northern Oberpfalz). The degree of the tetrad effect (TE_1,3) is estimated and plotted vs. K/Rb, Sr/Eu, Eu/Eu1, Y/Ho, and Zr/Hf. The diagrams reveal that the tetrad effect develops parallel to granite evolution, and significant tetrad effects are strictly confined to highly differentiated samples. Mineral fractionation as a cause for the tetrad effect is not supported by a calculated Rayleigh fractionation, which also could not explain the fractionation trends of Sr/Eu and Eu/Eu1. The strong decrease of Eu concentrations in highly evolved rocks suggests that Eu fractionates between the residual melt and a coexisting aqueous high-temperature fluid. Mineral fractionation as a reason for the tetrad effect is even more unlikely as REE patterns of accessory minerals display similar tetrad effects as the respective host rocks. The accessory minerals inherit the REE signature of the melt and do not contribute to the bulk-rock tetrad effect via mineral fractionation. These results point in summary to significant changes of element fractionation behavior in highly evolved granitic melts: ionic radius and charge, which commonly control the element distribution between mineral and melt, are no longer the exclusive control. The tetrad effect and the highly fractionated trace element ratios of Y/Ho and Zr/Hf indicate a trace element behavior that is similar to that in aqueous systems in which chemical complexation is of significant influence. This distinct trace element behavior and the common features of magmatic-hydrothermal alteration suggest the increasing importance of an aqueous-like fluid system during the final stages of granite crystallization. The positive correlation of TE_1,3 with bulk-rock fluorine contents hints at the importance of REE fluorine complexation in generating the tetrad effect. As the evolution of a REE pattern with tetrad effect (M-type) implies the removal of a respective mirroring REE pattern (W-type), the tetrad effect identifies open system conditions during granite crystallization.     

Tetrad effect in rare earth element distribution patterns: a method of quantification with application to rock and mineral samples from granite-related rare metal deposits

In some geological environments, the tetrad effect can be observed as a split of rare earth element (REE) patterns into four rounded segments. A new method is proposed to quantify the sizes of the individual segments, and for the first time, the significance of observed tetrad effects is evaluated by taking analytical errors into account. The outlined method was applied to lanthanide patterns of whole-rock and fluorite samples collected from granite-related rare metal deposits. The REE patterns of the granite and greisen samples investigated exhibit significant tetrad effects that may not be accounted for by analytical uncertainties. It is shown that the study of whole-rock samples is insufficient to determine whether this effect is developed during fractional crystallization or is due to other processes such as fluid-rock interaction. A concave tetrad effect mirroring the pattern of the whole-rock samples was not observed in the REE patterns of related vein fluorite samples. Therefore, it is unlikely that the convex tetrad effect in the samples from the magmatic environment can be explained by removal of a respective complementary REE pattern by a coexisting hydrothermal fluid, as previously suggested. It is proposed that the tetrad effect formed within the magma-fluid system before emplacement in the subvolcanic environment where phase separation caused a split of this system into fluid and magma subsystems. Alternatively, the tetrad effect may also be inherited from an external fluid influencing the system during or after the emplacement of the magma. On the basis of the fluorite data, it is shown that the behavior of Eu in the fluids is not related to the tetrad effect. Consequently, different physico-chemical factors control the occurrence of both phenomena. Y was found to be strongly enriched in samples precipitating from hydrothermal fluids that experienced prolonged interaction with the wall-rocks, whereas the tetrad effect in the fluids vanished with time and increasing distance from the ore-bearing granite. Thus, these different geochemical parameters can be used to reconstruct different aspects of the fluid evolution within this type of deposit.     

云母:花岗岩-伟晶岩稀有金属成矿作用的重要标志矿物

云母是花岗岩、伟晶岩中的重要造岩矿物,不仅是整个岩浆阶段的结晶产物,而且也是热液过程的参与者。作为层状硅酸盐矿物,层间或八面体位置上可容纳锂、铷铯、锡、铌钽等稀有金属。本文结合前人研究积累和作者近年来的研究成果,阐述了云母作为一个重要的稀有金属成矿标志矿物的矿物学特征。铁锂云母-锂云母是稀有金属成矿作用中重要的锂矿物,同时云母中锂含量可以反映花岗岩的分异程度。铷、铯可以置换云母层间钾,在高演化花岗岩、伟晶岩中可以形成铷、铯为主的云母(既可以是锂云母系列,也可以是黑云母系列)。黑云母是稀有金属花岗岩中一个特殊的矿物。准铝质含锡花岗岩中黑云母锡含量可达100×10~(-6),其锡含量可以指示其锡成矿能力。稀有金属花岗岩中,常见的是铌钽氧化物矿物。但是最近研究发现,黑云母中铌可以超常富集(超过1000×10~(-6)),成为稀有金属花岗岩中最重要、甚至唯一的铌矿物,形成一种以富铌黑云母为特色的新类型稀有金属花岗岩,并可能代表了一种新型的潜在铌资源。基于云母在花岗岩中的重要性和结构的特殊性,今后要利用微区成分和结构分析技术,加强对云母中稀有金属晶体化学的研究,以及进一步揭示云母对稀有金属成矿的特殊重要意义。     

Highly evolved juvenile granites with tetrad REE patterns: the Woduhe and Baerzhe granites from the Great Xing'an Mountains in NE China

In NE China, voluminous granitoids were emplaced in late Paleozoic and Mesozoic times. We report here Sr–Nd–O isotopic and elemental abundance data for two highly evolved granitic plutons, Woduhe and Baerzhe, from the Great Xing'an Mountains. They show a rather “juvenile” Sr–Nd isotopic signature and a spectacular tetrad effect in their REE distribution patterns as well as non-CHARAC (charge-and-radius-controlled) trace element behavior. The emplacement ages are constrained at 130±4 Ma for the Woduhe and 122±5 Ma for the Baerzhe granites by Rb–Sr and Sm–Nd isotope analyses. Both granites are also characterized by low but imprecise initial ⁸⁷Sr/⁸⁶Sr ratios of about 0.703. The Nd–Sr isotope data argue for their generation by melting of dominantly juvenile mantle component with subordinate recycled ancient crust. This is largely compatible with the general scenario for much of the Phanerozoic granitoids emplaced in the Central Asian Orogenic Belt. The parental magmas for both the Woduhe and Baerzhe granites have undergone extensive magmatic differentiation, during which intense interaction of the residual melts with aqueous hydrothermal fluids (probably rich in F and Cl) resulted in the non-CHARAC trace element behavior and the tetrad effect of REE distribution. Both the Woduhe and Baerzhe granites show the characteristic trace element patterns of rare-metal granites, but their absolute abundances differ by as much as two orders of magnitude. The oxygen isotope compositions of the two granites have been severely disturbed. Significant ¹⁸O depletion in feldspar, but not so much in quartz, suggests that the hydrothermal alteration took place in a temperature condition of 300–500 °C. This subsolidus hydrothermal alteration is decoupled from the late-stage magma–fluid interaction at higher temperatures. Despite the two distinct and intense events of “water–rock” interaction, the Rb–Sr and Sm–Nd geochronological systems seem to have maintained closed, hence, suggesting that the two events occurred shortly after the plutonic emplacements.     

The Zr/Hf ratio as a fractionation indicator of rare-metal granites

The Zr-Hf geochemical indicator, i.e., the Zr/Hf ratio (in wt %) in granitic rocks is proposed to be used as the most reliable indicator of the fractionation and ore potential of rare-metal granites. It was empirically determined that the fractional crystallization of granitic magma according to the scheme granodiorite → biotite granite → leucogranite → Li-F granite is associated with a decrease in the Zr/Hf ratio of the granites. The reason for this is the stronger affinity of Hf than Zr to granitic melt. This was confirmed by experiments on Zr and Hf distribution between granitic melt and crystals of Hf-bearing zircon (T = 800°C, P= 1 kbar). The application of the Zr/Hf indicator was tested at three classic territories of rare-metal granites: eastern Transbaikalia, central Kazakhstan, and the Erzgebirge in the Czech Republic and Germany. The reference Kukul’bei complex of rare-metal granites in eastern Transbaikalia (J₃) is characterized by a uniquely high degree of fractionation of the parental granitic melt, with the granites and their vein derivatives forming three intrusive phases. The biotite granites of phase 1 are barren, the leucogranites of phase 2 are accompanied by greisen Sn-W mineral deposits (Spokoininskoe and others), and the final dome-shaped stocks of amazonite Li-F granites of phase 3 host (in their upper parts) Ta deposits of the “apogranite” type: Orlovka, Etyka, and Achikan. The Kukul’bei Complex includes also dikes of ongonites, elvanes, amazonite granites, and miarolitic pegmatites. All granitic rocks of the complex are roughly coeval and have an age of 142±0.6 Ma. The Zr/Hf ratio of the rocks systematically decreases from intrusive phase 1 (40–25) to phases 2 (20–30) and 3 (10–2). Compared to other granite series, the granites of the Kukul’bei Complex are enriched in Rb, Li, Cs, Be, Sn, W, Mo, Ta, Nb, Bi, and F but are depleted in Mg, Ca, Fe, Ti, P, Sr, Ba, V, Co, Ni, Cr, Zr, REE, and Y. From earlier to later intrusive phases, the rocks become progressively more strongly enriched or depleted in these elements, and their Zr/Hf ratio systematically decreases from 40 to 2. This ratio serves as a reliable indicator of genetic links, degree of fractionation, and rare-metal potential of granites. Greisen Sn, W, Mo, and Be deposits are expected to accompany granites with Zr/Hf < 25, whereas granites related to Ta deposits should have Zr/Hf < 5.