The Cretaceous Ofuku Pluton and Its Relation to Mineralization in the Western Akiyoshi Plateau,Yamaguchi Prefecture,Japan

The relationship between the magmatism of the Cretaceous Ofuku pluton and mineralization in and around the Akiyoshi Plateau, Yamaguchi Prefecture, Japan was investigated using a combination of field observation, petrographic and geochemical analyses, K–Ar geochronology, and fluid inclusion data. The Ofuku pluton has a surface area of 1.5 × 1.0 km, and was intruded into the Paleozoic accretionary complexes of the Akiyoshi Limestone, Ota Group and Tsunemori Formation in the western part of the Akiyoshi Plateau. The pluton belongs to the ilmenite‐series and is zoned, consisting mainly of early tonalite and granodiorite that share a gradational contact, and later granite and aplite that intruded the tonalite and granodiorite. Harker diagrams show that the Ofuku pluton has intermediate to silicic compositions ranging from 60.4 to 77.9 wt.% SiO₂, but a compositional gap exists between 70.5 to 73.4 wt.% SiO₂ (anhydrous basis). Modal and chemical variations indicate that the assumed parental magma is tonalitic. Quantitative models of fractional crystallization based on mass balance calculations and the Rayleigh fractionation model using major and trace element data for all crystalline phases indicate that magmatic fractionation was controlled mainly by crystal fractionation of plagioclase, hornblende, clinopyroxene and orthopyroxene at the early stage, and quartz, plagioclase, biotite, hornblende, apatite, ilmenite and zircon at the later stage. The residual melt extracted from the granodiorite mush was subsequently intruded into the northern and western parts of the Ofuku pluton as melt lens to form the granite and aplite. The age of the pluton was estimated at 99–97 Ma and 101–98 Ma based on K–Ar dating of hornblende and biotite, respectively. Both ages are consistent within analytical error, indicating that the Ofuku pluton and the associated Yamato mine belong to the Tungsten Province of the San‐yo Belt, which is genetically related to the ilmenite‐series granitoids of the Kanmon to Shunan stages. The aplite contains Cl‐rich apatite and REE‐rich monazite‐(Ce), allanite‐(Ce), xenotime and bastnäsite‐(Ce), indicating that the residual melt was rich in halogens and REEs. The tonalite–granodiorite of the Ofuku pluton contains many three‐phase fluid inclusions, along with daughter minerals such as NaCl and KCl, and vapor/liquid (V/L) volume ratios range from 0.2 to 0.9, suggesting that the fluid was boiling. In contrast, the granite and aplite contain low salinity two‐phase inclusions with low V/L ratios. The granodiorite occupies a large part of the pluton, and the inclusions with various V/L ratios with chloride daughter minerals suggest the boiling fluids might be related to the mineralization. This fluid could have carried base metals such as Cu and Zn, forming Cu ore deposits in and around the Ofuku pluton. The occurrence and composition of fluid inclusions in the igneous rocks from the Akiyoshi Plateau are directly linked to Cu mineralization in the area, demonstrating that fluid inclusions are useful indicators of mineralization.     

Uranium mineralization in the muscovite-rich granites of the Shalatin region, Southeastern Desert, Egypt

1INTRODUCTION THELATEPRECAMBRIANGRANITOIDSOFTHEARABONU BIANSHIELDINEGYPTWEREEXPOSEDBYEARLYTOMIDDLE TERTIARYUPLIFTANDENSUINGEROSIONDURINGTHEREDSEA RIFTINGEVENT(GREENBERG,1981).THEREAREANUMBER OFEFFECTIVEANDRELATIVELYSUCCESSFULSCHEMESFORTHE CLASSIFICATIONOF…     

花岗岩中原生与次生白云母的鉴别特征及其地质意义——以赣南富城强过铝质花岗岩体为例

通过显微镜下观察和电子探针成分分析,发现赣南富城强过铝质花岗岩中存在3种类型白云母,即原生白云母、交代型白云母和次生白云母,其平均晶体化学式分别为:K1.62Na0.06Fe0.32Mg0.39Ti0.02Al4.89Si6.54O10(OH)4(原生白云母)、K1.55Na0.07Fe0.43Mg0.24Ti0.03Al4.96Si6.50O10(OH)4(交代型白云母)、K1.51Na0.07Fe0.27Mg0.21Ti0.00Al4.98Si6.65O10(OH)4(次生白云母)。根据富城花岗岩主要造岩矿物的结晶顺序(斜长石→钾长石→黑云母→白云母→石英),结合白云母、黑云母稳定曲线及合成花岗岩初融曲线对比分析,富城强过铝质花岗岩中交代型白云母是在花岗岩结晶过程中交代较早晶出的黑云母形成的,其形成温度高于花岗岩熔体的固相线温度(～650℃),故应归属于原生白云母。本文提出根据岩石学宏观特征、岩石化学特征及岩相学微观特征区分花岗岩中原生白云母与次生白云母的综合鉴别方法。     

含铀副矿物的热液蚀变：来自华南鹿井矿田碎裂蚀变岩型铀矿床的矿物学研究

花岗岩型铀矿中铀的来源问题,长期以来是铀矿床学研究的热点问题之一。大多数学者认为其成矿物质主要来源于花岗岩本身的含铀副矿物,然而对于含铀副矿物热液蚀变行为研究较少。鹿井铀矿田位于诸广山复式岩体的中部,是华南最主要花岗岩型铀矿田之一,碎裂蚀变岩型铀矿化在该矿田内占主导地位。小山铀矿床位于鹿井矿田中部,是近些年新发现的碎裂蚀变岩型矿床。本文以钻孔ZK1-1为研究对象,对热液蚀变带开展了精细矿物学研究。研究表明：蚀变带中发育有晶质铀矿、铀石—钍石、独居石、磷钇矿、锆石、磷灰石、金红石等含铀副矿物。晶质铀矿、铀石—钍石中铀含量高,热液蚀变条件不稳定,铀容易释放;独居石蚀变为直氟碳钙铈矿和磷钇矿蚀变为次生磷灰石过程中容易释放出铀;锆石因结构稳定,铀难以释放;磷灰石、金红石中铀含量较低,供铀能力差。综合分析认为花岗岩中晶质铀矿、铀石—钍石是主要铀源矿物,独居石、磷钇矿为次要铀源矿物。     

Residence of REE, Y, Th and U in Granites and Crustal Protoliths; Implications for the Chemistry of Crustal Melts

A systematic study with laser ablation—ICP-MS, scanningelectron microscopy and electron microprobe revealed that 70–95wt% of REE (except Eu), Y, Th and U in granite rocks and crustalprotoliths reside within REEYThU-rich accessories whose nature,composition and associations change with the rock aluminosity.The accessory assemblage of peraluminous granites, migmatitesand high-grade rocks is composed of monazite, xenotime (in low-Cavarieties), apatite, zircon, Thorthosilicate, uraninite andbetafite-pyrochlore. Metaluminous granites have allanite, sphene,apatite, zircon, monazite and Thorthosilicaie. Peralkaline graniteshave aeschinite, fergusonite, samarskite, bastnaesite, fluocerite,allanite, sphene, zircon, monazite, xenotime and Th-orthosilicate.Granulite-grade garnets are enriched in Nd and Sm by no lessthan one order of magnitude with respect to amphibolite-gradegarnets. Granulitegrade feldspars are also enriched in LREEwith respect to amphibolite-grade feldspars. Accessories causenon-Henrian behaviour of REE, Y, Th and U during melt—solidpartitioning. Because elevated fractions of monazite, xenotimeand zircon in common migmatites are included within major minerals,their behaviour during anatexis is controlled by that of theirhost. Settling curves calculated for a convecting magma showthat accessories are too small to settle appreciably, beingseparated from the melt as inclusions within larger minerals.Biotite has the greatest tendency to include accessories, therebyindirectly controlling the geochemistry of REE, Y, Th and U.We conclude that REE, Y, Th and U are unsuitable for petrogeneticalmodelling of granitoids through equilibrium-based trace-elementfractionation equations. KEY WORDS: accessory minerals; geochemical modelling; granitoids; REE, Y, Th, U     

诸广南部产铀花岗岩长江岩体中的绿泥石和铀源矿物研究

长江岩体是诸广南部地区重要的产铀花岗岩体之一,此次研究运用电子探针和扫描电镜对长江岩体新鲜花岗岩和 蚀变花岗岩中的绿泥石和有关含铀矿物进行了精细对比,揭示花岗岩中铀的活化与成矿前期或早期致使花岗岩发生绿泥 石化的还原性热液蚀变作用关系密切,黑云母等的绿泥石化蚀变,使其中包裹的一些含铀副矿物也发生蚀变,导致原来 以类质同象形式存在于副矿物中的惰性铀转变成活性铀,并在绿泥石附近沉淀成铀石等铀含量高且在成矿期低度氧化性 热液作用下容易释放铀的矿物。长江岩体中的副矿物有锆石、磷灰石、褐帘石、铀石-钍石、晶质铀矿、独居石等,其 中,晶质铀矿、铀石、铀钍石中铀含量高且铀容易释放,是长江岩体的主要铀源矿物;独居石中铀含量较高,当其周围 矿物绿泥石化时,独居石蚀变形成直氟碳钙铈矿并释放铀,因而也是长江岩体的潜在铀源矿物;锆石中铀含量虽高,但 因其结构稳定,铀难以释放,因此它不是长江岩体中重要的铀源矿物;磷灰石、褐帘石中铀含量均低于检测限,作为铀 源矿物的可能性很小。     

豆乍山地区产铀与非产铀岩体黑云母成分及赋存副矿物特征

豆乍山岩体和香草坪岩体成岩时代相近,空间关系密切,为苗儿山中段豆乍山地区产铀与非产铀岩体。文章选择豆乍山和香草坪新鲜花岗岩中黑云母为研究对象,结合诸广、贵东地区产铀与非产铀岩体中黑云母的电子探针数据开展综合分析,并在此基础上进行了黑云母中包裹副矿物种类的对比研究。结果表明:产铀岩体黑云母基本以铁叶云母为主,非产铀岩体黑云母为铁质黑云母,且前者具有相对富铝、铁、锰,贫镁、钛特征;产铀豆乍山岩体黑云母中副矿物为锆石-磷钇矿-独居石-钛铁矿-金红石-方钍石-晶质铀矿,非产铀香草坪岩体黑云母中副矿物为锆石-磷灰石-褐帘石-独居石,缺少钍石和晶质铀矿。不同亚种黑云母具有不同的成岩成矿环境,与各矿化类型往往具有直接对应关系,铁叶云母的出现有可能可以作为产铀与非产铀岩体(铀成矿潜力)的间接判别标志,对于后期找矿具有较大指导意义。     

The Genesis of the Cornubian Batholith (South-West England): the example of the Carnmenellis Pluton

The origin of the Carnmenellis granite is discussed in the lightof new petrographic and chemical data which suggest that thepresent mineral assemblage comprises refractory, magmatic, subsolidusand hydrothermal phases. The present homogeneity of the plutonresults from subsolidus to pervasive hydrothermal reworkinginvolving selective leaching of alkalis and Fe-Mg and an increasein Li, F, B, and Rb, which were introduced, or redistributed,through high temperature hydrothermal circulation. The compositionsof the primary minerals were re-equilibrated with respect tothese water-rock interactions. Rb-Sr systematics demonstratethat whole rock samples show lower ages than the minerals (260and 285 Ma, respectively). The perturbation in the former isotopedistribution is in agreement with the chemical re-equilibrationof the mica with external Rb-rich and Sr-depleted solutions.The distribution of rare-earths reveal that most reside in afew accessory phases, viz. monazite, zircon, and apatite; theirnear-liquidus fractionation is responsible for the loweringof the bulk REE content compared to pelitic source materialand for the present REE pattern of the pluton. Conditions of magma generation by partial melting of cordierite-sillimanite-spinelpelitic gneisses were estimated to be around 800?C and 5 kbwith water content in the magma of about 4 wt. per cent. H₂Osaturation was reached late during the ascent of the magma,making possible the crystallization of muscovite from the residualmelt. The highly evolved peraluminous composition of the plutonis not explained by a simple magmatic differentiation. Comparedto the Carnmenellis pluton, the Land's End massif of similarmineralogy appears less evolved, either because of less subsolidusreworking or deeper structural emplacement.     

Petrochemistry,Age and Petrogenesis of the Shexian Pluton in Southern Anhui

The Shexian gneissic granodiorite in southern Anhui trends NE 55° from Shexian in the west to Guitoujian in the east with a length of 22 km and an outcrop area of 32 km. It was considered formerly to be Caledonian on the basis of a biotite K-Ar age of 474 Ma (1982). However, new evidence indicates that it may be Early Jinning in age as shown by: (1) it is found intruding into the Mid-Proterozoic Shangxi Group and is unconformably overlain by the Sinian Xiuning Formation, and (2) a zircon U-Th-Pb age of 928 Ma is obtained for the pluton. The pluton is composed of plagioclase (An=27.37%), K-feldspar(14%), biotite(16%) and quartz(32%). Accessory minerals are ilmenite (150g/T), xenotime (15g/T). garnet(25g/T), monazite(10g/T), zircon (20g/T) and apatite (104g/T). Petrochemical characteristics of the intrusion are:(l) Al-enrichment (A/NKC=1.30); (2) H₂O enrichment (H₂O= 1.74%); and (3) low oxidation index (f ’=0.10). It belongs to the continental crust transformation type as evidenced by: (1) MF and Mg/Y values of biotite are 0.41 and 0.31 respectively; (2) (⁸⁷Sr/⁸⁶Sr)=0.71119; (3) δ Eu=0.52; and (4) A/NKC=130. The Shexian pluton is therefore considered as a product of melting of phyllite at depth in the light of similarities in trace element and REE contents with the phyllite of the Banxi Group. Calculations of REE batch partial melting indicate that it may have resulted from 75% melting of the Banxi phyllite.     

西藏琼嘉岗伟晶岩型锂矿矿物包裹体形成过程及其对熔-流体特征的指示

西藏琼嘉岗伟晶岩型锂矿是喜马拉雅地区发现的首例具有工业价值的伟晶岩型锂矿床, 该矿床的发现证实了喜马拉雅地区具有成为我国稀有金属战略基地的潜力, 也为喜马拉雅地区寻找伟晶岩型锂矿床提供了指示意义。本研究主要针对琼嘉岗锂矿花岗岩和伟晶岩中三种主要副矿物独居石、磷灰石和锆石中的矿物包裹体进行扫描电镜分析, 确定矿物包裹体种类、频率以及产状(与裂隙关系), 结合磷灰石中长石包裹体电子探针分析, 综合指示琼嘉岗锂矿熔-流体性质及演化过程。研究表明: (1)琼嘉岗锂矿独居石、磷灰石和锆石中主要发育硅酸盐、氧化物、磷酸盐以及少量硫化物包裹体, 其中填充裂隙和穿切裂隙的富稀土矿物独居石和磷灰石包裹体均由热液蚀变形成, 而锆石放射性损伤强烈的区域中发育的远离裂隙的晶质铀矿和方钍石包裹体的形成也和流体作用相关; (2)电气石白云母花岗岩的磷灰石中发育钶钽铁矿以及烧绿石包裹体表明早期花岗岩岩浆富集铌和钽, 稀有金属包裹体的数量以及类型也对高演化的花岗岩和伟晶岩是否具有稀有金属成矿潜力以及矿化类型具有一定指示意义; (3)锂辉石伟晶岩磷灰石中发育异常高且变化大的An值的斜长石包裹体, 它们分别记录了磷灰石在结晶时对早期富钙熔体的捕获以及熔体的分异过程。本文的研究结果还表明副矿物中矿物包裹体的种类以及元素组成可以为高演化花岗岩-伟晶岩体系的熔-流体特征及演化提供指示意义。

     

On “Birthmarks” in rare-metal granites of the Losevka pluton,northern Kazakhstan

The Losevka pluton of rare-metal albite granite, which was explored as a possible source of columbite-zircon-malacon ore, is composed of quartz, sodic plagioclase, potassium feldspar, annite, protolithionite, lepidomelane, and Li-muscovite. The average chemical composition of this rock is as follows, wt %: 74.14 SiO₂, 0.04 TiO₂, 14.07 Al₂O₃, 1.05 Fe₂O₃, 0.78 FeO, 0.15 MnO, 0.09 MgO, 0.47 CaO, 4.65 Na₂O, 4.11 K₂O, and 0.03 P₂O₅. The accessory minerals are zircon, malacon, and cyrtolite (874 ppm); apatite (18 ppm); ilmenite (114 ppm); xenotime and monazite (119 ppm); and Nb-columbite (463 ppm). The black inclusions up to 15 cm in size, which are observed in this granite and called “birthmarks” by local geologists, consist of the same rock-forming minerals as the surrounding granite, but are enriched in MnO, MgO, CaO, TiO₂, and F and depleted in SiO₂ relative to the light granite. The black granite is also distinguished by much higher Sr and Ba contents and lower La, Rb, Y, Nb, REE, Cs, Ta, Th, and U contents. The black color is caused by enrichment in manganese oxides, manganoilmenite, and Mn-annite. All rock-forming minerals are pervaded by thin veinlets of Mn-oxides. In addition, bastnaesite, Y-and Th-fluorides, zircon, and malacon have been identified. Aggregates of black-colored minerals are not the products of the fractionation of the initial magma or immiscibility effects, because the structure of the albite-potassium feldspar-quartz-mica matrix is the same both in black and light granites. The percolation of a deep-sourced fluid enriched in Mn and F into a granitic melt might be a more probable origin.     

电子探针技术研究粤北龙华山岩体中独居石蚀变晕圈的结构与成分特征

独居石是华南产铀花岗岩中常见的含铀副矿物.龙华山岩体是粤北诸广山复式岩体中一个重要的产铀花岗岩,该岩体的独居石具有蚀变晕圈现象.但是,该岩体中独居石蚀变晕圈的结构和成分特征以及对铀成矿的指示意义尚未开展研究.本文利用电子探针(EPMA)对龙华山岩体的独居石蚀变晕圈开展结构和成分研究.测试结果表明:独居石蚀变晕圈是从内到...     

内蒙古维拉斯托稀有金属-锡多金属矿床的成矿作用:来自花岗质岩浆结晶分异的启示

内蒙古维拉斯托稀有金属-锡多金属矿床位于大兴安岭南段西坡,是一个以锡为主,共伴生锌、钨、铜、钼、铌、钽、锂和铷的大型矿床.该矿床以发育铌、钽、铷、锂矿化有别于大兴安岭南段的其他锡多金属矿床.本文对维拉斯托地区的北大山岩体和维拉斯托岩体开展了 LA-ICP-MS锆石U-Pb定年、全岩地球化学研究和锆石Lu-Hf同位素分析...     

Geochemistry of the Sweetwater Wash Pluton,California: Implications for “anomalous” trace element behavior during differentiation of felsic magmas

Field relations, mineralogy and major and trace element data for the very felsic, peraluminous Sweetwater Wash pluton establish a differentiation sequence dominantly controlled by fractional crystallization processes. Elements Ba and Sr show depletion by factors of 50–60X from the earliest granite unit analyzed to the late-stage pegmatites and aplites. The strong Ba depletion is largely due to the partitioning behavior of this element in K-feldspar, while the Sr depletion is due to the combined effects of the two feldspars. The 4-fold increase in Rb during crystallization is also predictable from mineral/ melt partition coefficients.Coefficients for the light rare-earth elements (LREE) in major mineral species predict that these elements should behave incompatibly during crystallization and increase with fractionation. In fact, the LREE abundances decrease by a factor of 10–20X during crystallization. This anomalous behavior is commonly observed in felsic plutonic and volcanic sequences. In the Sweetwater Wash pluton monazite occurs in minute quantities, but it is sufficiently abundant to govern the partitioning of LREE and Th during crystallization. Petrographic observations indicate that monazite was on the liquidus throughout most of the crystallization. Analyses of silicate mineral separates suggest that the monazite contains more than 75% of the LREE in the whole rocks.Fractionation of REE-rich accessories (in particular monazite) from felsic magmas may be the general cause of REE depletion during differentiation of these melts. Monazite can easily be mistaken for zircon and, because it typically contains 50% LREE, extremely minute and easily overlooked quantities of monazite can control LREE abundances.     

The Montecristo monzogranite (Northern Tyrrhenian Sea,Italy): a collisional pluton in an extensional setting

The Montecristo monzogranite (MM) is a near-circular peraluminous monzogranite pluton occupying the entire 10 km² of Montecristo Island. Outcrops of country rock are scarce, and are mainly roof pendants of metagabbros and calcsilicate hornfels of the Apenninic ophiolite sequence. Emplacement of the pluton (Rb–Sr age=7·1±0·2 Ma), following the early Miocene onset of continental collision, occurred during an extensional phase which migrated eastward via a combined process of subduction–delamination. The MM rocks are strongly porphyritic, the assemblage being composed of alkali-feldspar, quartz, plagioclase (all occurring as mega- or phenocrysts), biotite and minor cordierite. Accessory minerals include tourmaline, apatite, zircon, ilmenite, allanite, monazite, rutile and hellandite. Reconstructed crystallization histories for the mineral phases reveal a polybaric crystallization starting at about 5 kb. Textural variations of MM occur in sharp contact with each other; darker types often form globular masses containing fewer megacrysts and more abundant mafic microgranular enclaves. Geochemical, isotopic and petrographic data indicate that the MM magma was produced by anatectic melting of an intermediate to deep pelitic crustal source. On the basis of the geochemical and mineralogical characteristics of the enclaves, modification of their parent magma occurred by crystal fractionation coupled with mixing and mingling of components from the MM magma. The limited geochemical variation in MM is interpreted as due to crystal fractionation processes during the magma's ascent. Younger porphyritic dykes with more potassic and alkaline affinities cut the pluton; these dykes are concentrated in a major fracture zone and are associated with contemporaneous pseudotachylites. © 1997 John Wiley & Sons, Ltd.     

Evolution of feldspars at the amphibolite-granulite-facies transition in augen gneisses (SW Norway): geochemistry and Sr isotopes

The Sveconorwegian Augen Orthogneisses of Rogaland — Vest-Agder (SW Norway) were emplaced as amphibole- and biotite-bearing granodiorites at 1040 Ma (concordant Rb/Sr and zircon U/Pb ages). They underwent prograde metamorphism which increased from lower amphibolite-facies in the eastern zone to granulite-facies in the western zone, close to the Rogaland anorthosite complex. K-feldspar megacrysts initially crystallised as phenocrysts and were chemically equilibrated during metamorphism, as shown by the flat Ba concentration profiles and the increase of the anorthite content from An_1.1 in the amphibolitefacies to An_2.6 in the granulite-facies. This increase of the An content suggests an increase in metamorphic temperature. The REE content of the megacrysts is related to the associated accessory minerals which depend upon the metamorphic grade: sphene + allanite + apatite + zircon and rarely thorite in amphibolite-facies, and apatite + zircon + monazite ± thorite in lower amphibolite-and granulite-facies. Amphibole and biotite inclusions in megacrysts were also equilibrated during metamorphism. Groundmass K-feldspar and plagioclase experienced late-metamorphic changes during uplift. An internal Rb/Sr mineral isochron (plagioclase, apatite, K-feldspar) defines an age of 870 Ma, which represents the closure of the Rb/Sr isotopic system in minerals of the augen gneisses. This age also represents a K-feldspar cooling age in regionally distributed augen gneiss samples. The K-feldspar cooling age appears to be similar to or slightly older than the biotite cooling age.     

REE Abundance and REE Minerals in Granitic Rocks in the Nanling Range,Jiangxi Province,Southern China,and Generation of the REE‐rich Weathered Crust Deposits

Studies of Mesozoic granites associated with rare earth element (REE)‐rich weathered crust deposits in southernmost Jiangxi Province indicate that they have high‐K to shoshonite compositions and belong to ilmenite‐series I‐type granites. Of the studied rocks at 59–292 ppm of bulk REE content, the highest are seen in the biotite granites of Dingnan (358, 429 ppm) and mafic biotite granite of the Wuliting Granite (344 ppm) near the Dajishan tungsten mine, both areas where weathered‐crust REE deposits occur. REE‐bearing accessory minerals in these granites are mainly zircon, apatite and allanite, and REE‐fluorocarbonates are common. REE enrichment occurs in the rims of apatite crystals, and in fluorocarbonates that occur along grain boundaries of and cracks in major silicate minerals, and in fluorocarbonates that replaced altered biotite. It is therefore thought that a major part of the REE content of these granites was concentrated during deuteric activity, rather than during magmatic crystallization. The crack‐filling REE‐fluorocarbonates could subsequently have been easily leached out and deposited in weathered crust developed during a long period of exposure.     

陕西勉县地区关帝坪闪长岩体的LA—ICP—MS锆石U—Pb年龄及其地质意义

秦岭造山带印支期花岗岩的成因研究是当前秦岭造山带研究的热点问题。通过对出露于陕西勉县地区勉略缝合带中光头山岩体西端的关帝坪黑云母闪长岩锆石LA—ICPMSU-Pb年代学研究,探讨其地质意义。岩石主要由斜长石、角闪石和黑云母组成,蚀变较轻。LA—ICP—MS锆石U—Pb测年得到的加权平均年龄为220．5士3．1Ma（MSWD=0．66,2σ）,代表该闪长岩体的结晶年龄。该年龄与附近的光头山黑云母斜长花岗岩的年龄216±2Ma很接近,由此推断两者应产出于相同的构造背景。结合前人研究结果,推断该黑云母闪长岩体可能为勉略洋闭合后的碰撞后期产物。     

Behavior of accessory phases and redistribution of Zr,REE, Y,Th, and U during metamorphism and partial melting of metapelites in the lower crust: an example from the Kinzigite Formation of Ivrea-Verbano,NW Italy

This study is aimed at understanding the behavior of monazite, xenotime, apatite and zircon, and the redistribution of Zr, REE, Y, Th, and U among melt, rock-forming and accessory phases in a prograde metamorphic sequence, the Kinzigite Formation of Ivrea-Verbano, NW Italy, that may represent a section from the middle to lower continental crust. Metamorphism ranges from middle amphibolite to granulite facies and metapelites show evidence of intense partial melting and melt extraction. The appearance of melt controls the grain size, fraction of inclusions and redistribution of REE, Y, Th, and U among accessories and major minerals. The textural evolution of zircon and monazite follows, in general, the model of Watson et al. (1989). Apatite is extracted from the system dissolved into partial melts. Xenotime is consumed in garnet-forming reactions and is the first source for the elevated Y and HREE contents of garnet. Once xenotime is exhausted, monazite, apatite, zircon, K-feldspar, and plagioclase are progressively depleted in Y, HREE, and MREE as the modal abundance of garnet increases. Monazite is severely affected by two retrograde reactions, which may have consequences for U-Pb dating of this mineral. Granulite-grade metapelites (stronalites) are significantly richer in Ti, Al, Fe, Mg, Sc, V, Cr, Zn, Y, and HREE, and poorer in Li, Na, K, Rb, Cs, Tl, U, and P, but have roughly the same average concentration of Cu, Sr, Pb, Zr, Ba, LREE, and Th as amphibolite-grade metapelites (kinzigites). The kinzigite-stronalite transition is marked by the sudden change of Th/U from 5–6 to 14–15, the progressive increase of Nb/Ta, and the decoupling of Ho from Y. Leucosomes were saturated in zircon, apatite, and (except at the lowest degree of partial melting) monazite. Their REE patterns, especially the magnitude of the Eu anomaly, depend on the relative proportion of feldspars and monazite incorporated into the melt. The presence of monazite in the source causes an excellent correlation of LREE and Th, with nearly constant Nd/Th ≈ 2.5–3. The U depletion and increase in Th/U characteristic of granulite facies only happens in monazite-bearing rocks. It is attributed to enhancement of the U partitioning in the melt due to elevated Cl activity followed by the release of a Cl-rich F-poor aqueous fluid at the end of the crystallization of leucosomes. Halide activity in partial melts was buffered by monazite and apatite. Since the U (and K) depletion does not substantially affect the heat-production of metapelites, and mafic granulites maintain similar Th/U and abundance of U and Th as their unmetamorphosed equivalents, it seems that geochemical changes associated to granulitization have only a minor influence on heat-production in the lower crust.     

REE Mineralization of Weathered Crust and Clay Sediment on Granitic Rocks in the Sanyo Belt,SW Japan and the Southern Jiangxi Province,China

Rare earth element (REE) geochemistry and mineralogy have been studied in the weathered crusts derived from the Early Yanshanian (Jurassic) biotite granites of Dabu and Dingnan, as well as in the Indosinian (Permian) muscovite–biotite granite of Aigao in southern Jiangxi province, China, and the weathered crusts and clay sediments on biotite granites in the Sanyo belt, SW Japan, that is, Okayama, Tanakami, and Naegi areas. In all of the weathered crusts, biotite and plagioclase commonly tend to decrease toward the upper part of the profile, whereas kaolinite and residual quartz and K‐feldspar increase. The weathered crusts of the Dingnan granites and some Naegi granites, which are characterized by the enrichment in light REE (LREE) in C horizons, have higher total REE (ΣREE) content than the parent REE‐enriched granites. Weathering of LREE‐bearing apatite and fluorocarbonates in the Dingnan granites and allanite and apatite in some Naegi granites may account for the leaching of LREE at the B horizons. The leached LREE must result in subsequent enrichment of LREE in the C horizons. The enrichment is probably associated with mainly adsorption onto kaolinite and partly formation of possible secondary LREE‐bearing minerals. In Japan it was found that REE mineralization occurs not in the weathered granitic crusts but in reworked clay sediments, especially kaolinite‐rich layers, derived mainly from the weathering materials of REE‐enriched granitic rocks. The clay sediments are more enriched in LREE, which likely adsorbed onto kaolinite. Concentration of heavy REE within almost all the weathered crusts and clay sediments, however, may reflect mainly residual REE‐bearing minerals such as zircon, which originated in the parent granitic rocks. The findings of the present study support the three processes for fractionation of the REE during weathering: (i) selective leaching of rocks containing both stable and unstable REE‐bearing minerals; (ii) adsorption onto clay minerals; and (iii) presence of possible secondary LREE‐bearing minerals.