Metamorphism and geochronology of garnet amphibolite from the Beishan Orogen,southern Central Asian Orogenic Belt:Constraints from P-T path and zircon U-Pb dating

Numerous lenses of garnet amphibolite occur in the garnet-bearing biotite-plagioclase gneiss belt in the Baishan area of the Beishan Orogen,which connects the Tianshan Orogen to the west and the Mongolia-Xing'anling Orogen to the east.The study of metamorphism in Beishan area is of great significance to explain the tectonic evolution of Beishan orogen.According to the microstructures,mineral relationships,and geothermobarometry,we identified four stages of mineral assemblages from the garnet amphibolite sample:(1) a pre-peak stage,which is recorded by the cores of garnet together with core-inclusions of plagioclase(Pl_1);(2) a peak stage,which is recorded by the mantles of garnet together with mantle-inclusions of plagioclase(Pl_2)+amphibole(Amp_1)+Ilmenite(Ilm_1)+biotite(Bt_1),developed at temperature-pressure(P-T) conditions of 818.9-836.5℃ and7.3-9.2 kbar;(3) a retrograde stage,which is recorded by garnet rims + plagioclase(Pl_3)+amphibole(Amp_2)+orthopyroxene(Opx_1)+biotite(Bt_2)+Ilmenite(Ilm_2),developed at P-T conditions of 796.1-836.9℃ and5.6-7.5 kbar;(4) a symplectitic stage,which is recorded by plagioclase(Pl_4)+orthopyroxene(Opx_2)+amphibole(Amp_3)+biotite(Bt_3) symplectites,developed at P-T conditions of 732 ±59.6℃ and 6.1 ±0.6 kbar.Moreover,the U-Pb dating of the Beishan garnet amphibolite indicates an age of 301.9 ±4.7 Ma for the protolith and 281.4±8.5 Ma for the peak metamorphic age.Therefore,the mineral assemblage,P-T conditions,and zircon U-Pb ages of the Beishan garnet amphibolite define a near-isothermal decompression of a clockwise P-T-t(Pressure-Temperature-time) path,indicating the presence of over thickened continental crust in the Huaniushan arc until the Early Permian,then the southern Beishan area underwent a process of thinning of the continental crust.     

The giant tin polymetallic mineralization in southwest China:Integrated geochemical and isotopic constraints and implications for Cretaceous tectonomagmatic event

The Gejiu-Bozushan-Laojunshan W-Sn polymetallic metallogenic belt(GBLB) in southeast Yunnan Province is an important part of the southwestern Yangtze Block in South China.Tin polymetallic mineralization in this belt includes the Niusipo,Malage,Songshujiao,Laochang and Kafang ore fields in the Gejiu area which are spatially and temporally associated with the Kafang-Laochang and Songshujiao granite plutons.These granites are characterized by variable A/CNK values(mostly 1.1,except for two samples with 1.09),high contents of SiO_2(74.38-76.84 wt.%) and Al_2 O_3(12.46-14.05 wt.%) and variable CaO/Na_2 O ratios(0.2-0.65) as well as high zircon δ~(18)O values(7.74‰-9.86‰),indicative of S-type affinities.These rocks are depleted in Rb,Th,U,Ti,LREE[(La/Yb)N=1.4-20.51],Ba,Nb,Sr,and Ti and display strong negative Eu and Ba anomalies.The rocks possess high Rb/Sr and Rb/Ba ratios,relatively low initial ~(87)Sr/~(86)Sr ratios(0.6917-0.7101),and less radiogenic εNd(t)values(-8.0 to-9.1).The zircon grains from these rocks show negative ε_(Hf)(t) values in the range of-3.7 to-9.9 with mean T_(DM2)(Nd) and T_(DM2)(Hf) values of 1.57 Ga and 1.55 Ga.They show initial ~(207)Pb/~(204)Pb ranging from15.69 to 15.71 and ~(206)Pb/~(204)Pb from 18.36 to 18.70.Monazite from Songshujiao granites exhibits higher U and lower Th/U ratios,lower δ~(18)O values and higher ε_(Hf)(t) values than those of the zircon grains in the KafangLaochang granites.The geochemical and isotopic features indicate that the Laochang-Kafang granites originated by partial melting of Mesoproterozoic crustal components including biotite-rich metapelite and metagraywacke,whereas the Songshujiao granites were derived from Mesoproterozoic muscovite-rich metapelite crustal source.Most zircon grains from the Songshujiao,Laochang and Kafang granites have high-U concentrations and their SIMS U-Pb ages show age scatter from 81.6 Ma to 88.6 Ma,80.7 Ma to 86.1 Ma and 82.3 Ma to 87.0 Ma,suggesting formation earlier than the monazite and cassiterite.Monazite SIMS U-Pb ages and Th-Pb ages of three same granite samples are consistent and show yielded 206 Pb/~(238)U ages of 83.7 ± 0.6 Ma,83.7±0.6 Ma,and 83.4±0.6 Ma,and ~(208)Pb/~(232)Th ages of 83.2 ± 0.5 Ma,83.8 ± 0.4 Ma,and 83.5±0.9 Ma,which are within the range of the SIMS zircon U-Pb ages from these rocks.The data constrain the crystallization of the granites at ca.83 Ma.In situ U-Pb dating of two cassiterite samples from the cassiterite-sulfide ore in the Songshujiao ore field and Kafang ore field,and two from the cassiterite-oxide+cassiterite bearing dolomite in the Laochang ore field yielded weighted mean 206 Pb/~(238)U ages of 83.5±0.4 Ma(MSWD=0.6),83.5 ± 0.4 Ma(MSWD=0.5),83.6 ±0.4 Ma(MSWD=0.6) and 83.2 ±0.7 Ma(MSWD=0.6),respectively.Combined with geological characteristics,the new geochronological data indicate that the formation of the granites and Sn polymetallic deposits are coeval.We correlate the magmatic and metallogenic event with lithospheric thinning and asthenosphere upwelling in continental extension setting in relation to the eastward subduction of the Neo-Tethys beneath the Sanjiang tectonic domain during Late Cretaceous.     

Paleoproterozoic juvenile magmatism within the northeastern sector of the São Francisco paleocontinent: Insights from the shoshonitic high Ba–Sr Montezuma granitoids

New,integrated petrographic,mineral chemistry,whole rock geochemical,zircon and titanite UPb geochronology,and zircon Hf isotopic data from the Montezuma granitoids,as well as new geochemical results for its host rocks represented by the Corrego Tingui Complex,provides new insights into the late-to post-collisional evolution of the northeastern Sao Francisco paleocontinent.U-Pb zircon dates from the Montezuma granitoids spread along the Concordia between ca.2.2 Ga to 1.8 Ga and comprise distinct groups.Group I have crystallization ages between ca.2.15 Ga and 2.05 Ga and are interpreted as inherited grains.Group II zircon dates vary from 2.04 Ga to1.9 Ga and corresponds to the crystallization of the Montezuma granitoids,which were constrained at ca.2.03 Ga by the titanite U-Pb age.Inverse age zoning is common within the ca.1.8 Ga Group III zircon ages,being related to fluid isotopic re-setting during the Espinhaco rifiting event.Zircon ε_(Hf)(t) analysis show dominantly positive values for both Group I(-4 to+9) and Ⅱ(-3 to+8) zircons and T_(DM2) model ages of 2.7-2.1 Ga and 2.5-1.95 Ga,respectively.Geochemically,the Montezuma granitoids are weakly peraluminous to metaluminous magnesian granitoids,enriched in LILES and LREE,with high to moderate Mg#and depleted in some of the HFSE.Their lithochemical signature,added to the juvenile signature of both inherited and crystallized zircons,allowed its classification as a shoshonitic high Ba-Sr granitoid related to a late-to post-collisional lithosphere delamination followed by asthenospheric upwelling.In this scenario,the partial melting of the lithospheric mantle interacted with the roots of an accreted juvenile intra-oceanic arc,being these hybrid magma interpreted as the source of the Montezuma granitoids.The Corrego Tingui Complex host rocks are akin to a syn-to late-collisional volcanic arc granitoids originated from the partial melting of ancient crustal rocks.The results presented in this study have revealed the occurrence of juvenile rocks,probably related to an island arc environment,that are exotic in relation to the Paleo-to Neoarchean crust from the Sao Francisco paleocontinent's core.     

Genesis of the supergiant Huayangchuan carbonatite-hosted uranium-polymetallic deposit in the Qinling Orogen,Central China

The newly-discovered supergiant Huayangchuan uranium (U)-polymetallic deposit is situated in the Qinling Orogen, Central China. The deposit contains economic endowments of U, Nb, Pb, Se, Sr, Ba and REEs, some of which (e.g., U, Se, and Sr) reaching super-large scale. Pyrochlore, allanite, monazite, barite-celestite and galena are the major ore minerals at Huayangchuan. Uranium is mainly hosted in the primary mineral of pyrochlore, and the mineralization is mainly hosted in or associated with carbonatite dikes.According to the mineral assemblages and crosscutting relationships, the alteration/mineralization at Huayangchuan comprises four stages, i.e., pegmatite REE mineralization (I), main mineralization (II), skarn mineralization (III) and post-ore alteration (IV). Coarse-grained euhedral allanite is the main Stage I REE mineral, and the pegmatite-hosted REE mineralization (ca. 1.8 Ga) occurs mostly in the shallow-level of northwestern Huayangchuan, corresponding to the Paleoproterozoic Xiong'er Group volcanic rocks (1.80–1.75 Ga) in the southern margin of North China Block. Carbonatite-hosted Stage II mineralization contributes to the majority of U-Nb-REE-Ba-Sr resources, and is controlled by the Huayangchuan Fault. Stage II mineralization can be further divided into the sulfate mineralization (barite-celestite) (II-A), alkali-rich U mineralization (aegirine-augite + pyrochlore + uraninite + uranothorite) (II-B) and REE (allanite + monazite + chevkinite)-U (pyrochlore + uraninite) mineralization (II-C) substages. Stage II mineralization may have occurred during the Late Triassic Mianlue Ocean closure. Skarn mineralization contributed to the majority of Pb and minor U-REE (uraninite-allanite) resources at Huayangchuan, and is spatially associated with the Late Cretaceous-Early Jurassic (Yanshanian) Huashan and Laoniushan granites. We suggested that hydrothermal fluids derived from the Laoniushan and Huashan granites may have reacted with the Triassic carbonatites, and formed the Huayangchuan Pb skarn mineralization.The mantle-derived Triassic carbonatites may have been fertilized by the U-rich subducting oceanic sediments, and were further enriched through reacting with the Proterozoic U-REE-rich pegmatite wallrocks at Huayangchuan. Ore-forming elements were likely transported in metal complexes (F⁻, ${\mathrm{PO}}_4^{3–}$, ${\mathrm{CO}}_3^{2–}$ and ${\mathrm{SO}}_4^{2–}$), and deposited with the dilution of the complex concentration. This may have formed the distinct vertical mineralization zoning, i.e., sodic fenite-related alkali-U mineralization at depths and potassic fenite-related REE-U mineralization at shallow level.     

陕西省华阳川铀铌铅矿床碳酸岩岩石学及地球化学特征

陕西省华阳川铀铌铅矿床是小秦岭成矿带中成矿特征最为独特的矿床,碳酸岩脉的破碎带是重要的成矿空间。未矿化的碳酸岩中矿物以方解石为主,其他矿物很少;发育铀矿化的碳酸岩脉中矿物种类繁多,大部分为方解石,其次为角闪石、金云母、榍石、褐帘石、铌钛铀矿、重晶石、磷灰石、石英、磁铁矿、碱性长石等矿物。碳酸岩的LREE含量异常高,δ13CV-PDB和δ18OV-SMOW值显示典型的火成碳酸岩特征。基于碳酸岩脉的Sr、Nd、Pb同位素比值(87Sr/86Sr-206Pb/204Pb、207Pb/204Pb-206Pb/204Pb-143Nd/144Nd-87Sr/86Sr)的关系图,初步判断华阳川铀铌铅碳酸岩脉是源于EMI的碱性硅酸盐-碳酸盐熔体-溶液结晶分异的产物。     

Geochemistry and Sr–Nd–Pb–Hf isotopic composition of the Donggou Mo-bearing granite porphyry,Qinling orogenic belt,central China

The eastern Qinling belt is characterized by widespread Mesozoic post-orogenic magmatism and abundant Mo–(Au–Ag) polymetallic mineralization. Most Mo deposits in this belt are genetically related to Mesozoic granitoids. The tectonic context of this close spatial and temporal relationship is still debated. This study reports U–Pb ages and Hf isotopic composition of zircons, major and trace element and Sr–Nd–Pb isotopic composition of the Donggou granite porphyry, host rock to one of the important Mesozoic Mo deposits in this orogen. Based on geochemical results, the Donggou granite porphyry is a silica-supersaturated, high-K metaluminous A-type granite showing enrichment in light REEs, depletion in middle REEs and significant negative Eu, Ba, Nb, Sr, P, and Ti anomalies. Negative initial ?_Nd values of??17.0 to??13.2 for whole-rock and negative initial ?_Hf values of??19.9 to??7.8 for zircon suggest that the magma was derived from a mixture of Archaean/Proterozoic crustal rocks and mantle-derived or newly added crust. Its Pb isotopic composition is similar to the lower crust of the North China block, but different from superjacent country rocks (Xiong'er and Taihua Groups). Zircon U–Pb dating yields a late Mesozoic emplacement age of 118–117 Ma, identical with the third episode of Mo mineralization in the eastern Qinling–Dabie belt. We postulate that the Donggou Mo-related porphyry granite formed by reworking of North China lower crust with significant input of juvenile material. The magmas formed in an extensional tectonic setting, induced by lithospheric thinning and asthenospheric upwelling beneath eastern China during Cretaceous time.     

华阳川铀多金属矿床碳酸岩脉矿化特征

华阳川铀多金属矿床是小秦岭成矿带中成矿特征最为独特的矿床,其碳酸岩脉破碎带是重要的成矿空间。区内碳酸岩分布很广泛,沿NW向张性和张扭性断裂及其伴生的次级裂隙充填,呈单脉状和网脉状穿插于变质岩、伟晶岩中。矿化碳酸岩脉中,脉石矿物大部分为方解石,少量为角闪石、石英、磁铁矿、碱性长石等;矿石矿物为铌钛铀矿、独居石、褐帘石、重晶石、天青石。铌钛铀矿是最主要的赋铀矿物,矿化元素铀、铌、钡、锶、稀土等平均含量分别为2554.20×10-6、1836.80×10-6、4145.78×10-6、4412.80×10-6、7348.44×10-6,可以综合开发利用。     

江西朱溪超大型钨矿床成矿年代学、矿物学及成矿过程研究

位于江南造山带东部的江西朱溪钨矿,是近年发现的一个超大型钨矿床,其矿体主要由矽卡岩型白钨矿组成,产于 燕山期侵入岩与碳酸盐岩接触带的矽卡岩或矽卡岩化大理岩中。为了更好地认识朱溪钨矿的特征和成因,文章采集了花岗 岩和矽卡岩的钻孔样品,进行了岩石学、矿物学、岩石地球化学和同位素年代学的分析。研究表明,朱溪矿区的黑云母花 岗岩具有高硅、富碱、高分异的特征,属于钙碱性、过铝质花岗岩,微量元素中Rb,U,Ta等元素富集,Ba,Nb,Sr和Ti 等元素亏损。稀土元素总量偏低,轻稀土相对富集。矽卡岩矿物的电子探针成分结果表明,其中石榴子石主要为钙铝榴 石-钙铁榴石端元组分;单斜辉石以透辉石-钙铁辉石系列为主。与白钨矿密切共生的矽卡岩矿物中,萤石、符山石、磷灰 石和榍石等富氟的矿物大量出现,表明朱溪钨矿成矿流体为富氟体系,这有利于钨的运移和沉淀。白钨矿REE配分曲线及 Mo含量变化所反映的流体性质表明,朱溪钨矿在矽卡岩阶段,总的矿化环境则由氧化向还原环境变化。利用朱溪含矿矽卡 岩中榍石进行了原位LA-ICP-MS U-Pb定年,206Pb/238U加权平均年龄为153±2 Ma,结果显示朱溪钨矿的成矿时代为晚侏罗 世,属燕山期岩浆活动后的产物。     

陕西南秦岭柞水岩体锆石U——Pb 年龄、岩石地球化学特征及其地质意义

锆石U --Pb 定年; 柞水花岗岩; 南秦岭     

Geochemical characteristics of the Bayan Obo giant REE–Nb–Fe deposit: Constraints on its genesis

Geochemical characteristics of different dolomites in the Bayan Obo giant REE–Nb–Fe deposit in Inner Mongolia have been studied. Intensively REE-mineralized dolomites (total REE over 800 ppm) show similar geochemical characteristics to associated carbonatite dykes, with Ba, Th, REE enrichments and Sr, Nb, Ti, Cu depletions, which is different from those of dolomites in the deposit with low REE contents (total REE less than 800 ppm). The low REE dolomites display some transitional characteristics between carbonatite dyke and sedimentary carbonate, with La depletion and Nb enrichment. This indicates that the genesis of the REE-mineralized dolomites might be related to both carbonatite magma and sedimentary carbonates. Sulfur isotope data indicates two sulfur sources, a mantle source (δ³⁴S c.a. 0‰) and seawater (δ³⁴S c.a. +25‰). It is proposed that mineralized dolomites in the Bayan Obo giant REE–Nb–Fe deposit are the product of sedimentary carbonate hydrothermally metasomatised by carbonatite magma and/or associated fluids. These dolomites formed the large-scale rare earth mineralization in the unique Bayan Obo REE–Nb–Fe deposit.     

Late Mesozoic magmatism in the Jiaodong Peninsula,East China:Implications for crust-mantle interactions and lithospheric thinning of the eastern North China Craton

A section from the Linglong gold deposit on the northwestern Jiaodong Peninsula,East China,containing Late Mesozoic magmatic rocks from mafic and intermediate dikes and felsic intrusions,was chosen to investigate the lithospheric evolution of the eastern North China Craton(NCC).Zircon U-Pb data showed that low-Mg adakitic monzogranites and granodiorite intrusions were emplaced during the Late Jurassic(～145 Ma) and late Early Cretaceous(112-107 Ma),respectively;high-Mg adakitic diorite and mafic dikes were also emplaced during the Early Cretaceous at～139 Ma and ～118 Ma,and 125-145 Ma and 115-120 Ma,respectively.The geochemical data,including whole-rock major and trace element compositions and Sr-Nd-Pb isotopes,imply that the mafic dikes originated from the partial melting of a lithospheric mantle metasomatised through hydrous fluids from a subducted oceanic slab.Low-Mg adakitic monzogranites and granodiorite intrusions originated from the partial melting of the thickened lower crust of the NCC,while high-Mg adakitic diorite dikes originated from the mixing of mafic and felsic melts.Late Mesozoic magmatism showed that lithosphere-derived melts showed a similar source depth and that crust-derived felsic melts originated from the continuously thickened lower crust of the Jiaodong Peninsula from the Late Jurassic to Early Cretaceous.We infer that the lower crust of the eastern NCC was thickened through compression and subduction of the Palaeo-Pacific plate beneath the NCC during the Middle Jurassic.Slab rollback of the plate from ～160 Ma resulted in lithospheric thinning and accompanied Late Mesozoic magmatism.     

Petrogenesis and tectonic implications of Indosinian granitoids from Western Qinling Orogen,China:Products of magma-mixing and fractionation

The Western Qinling Orogen(WQO) is characterized by voluminous distribution of Indosinian granitoids,the formation of which provides an important window to unravel the geochemical and geodynamic evolution and associated metallogeny.Here we investigate a group of intrusions termed "Five Golden Flowers" based on petrological,geochemical,zircon U-Pb geochronological and Lu-Hf isotopic studies on the granitoids and their mafic microgranular enclaves(MMEs).Our results show that these intrusions are genetically divided into two types,namely,magma-mixing and highly fractionated.The Jiaochangba,Lujing,Zhongchuan,and Luchuba granitoids are biotite monzogranites(220±0.8 Ma to 217±2.6 Ma) with abundant coeval MMEs(220±.1 Ma to 217±2.7 Ma).The rocks contain moderate to high SiO_2,high MgO,Rb,Sr,Ba,and Th contents,but low TiO_2,P_2 O_5,and Sc values,A/CNK of 1.1,and a range of ε_(Hf)(t) values of-11.7 to +2.23 with corresponding T_(DM2)values of 1967-1228 Ma.The MMEs possess K-feldspar megacrysts,abundant acicular apatites,and show lopsided textures.They have lower SiO_2,Al_2 O_3,and Th contents,but higher MgO,TiO_2,and Sc,with ε_(Hf)(t) values of-18.0 to +3.18 and T_(DM1) of 849-720 Ma.The data indicate that the MMEs were derived from a magma sourced from the enriched lithospheric mantle.We suggest that these host granitoids were produced by partial melting of latePaleoproterozoic to early-Mesoproterozoic lower crust with the involvement of Neoproterozoic SCLM-derived mafic magmas.The Baijiazhuang pluton is dominantly composed of leucogranite(muscovite granite and twomica monzogranite,216±1.5 Ma) without MMEs.The rocks are peraluminous with high A/CNK(1.06-1.27).Compared with the other four granitoids,the Baijiazhuang leucogranite shows higher SiO_2 content,markedly lower concentrations of TiO_2,MgO,Al_2 O_3,CaO,and Fe_2 O_3~T,and lower LREE/HREE and(La/Yb)N values.These leucogranites are also rich in Rb,Th,and U,and display marked depletions in Ba,Sr,Ti,and Eu,indicating that they experienced significant fractionation.Zircon ε_(Hf)(t) values(-10.2 to-3.27) and T_(DM2)(1868-1424 Ma),as well as the Nb/Ta and K_2 O/Na_2 O values are similar to the other four granitoids,indicating that they are likely to have been derived from a similar source;with sediments playing only a minor role in the magma generation.The low contents of Yb and Y suggest that their partial melting was controlled by garnets and micrographic texture of K-feldspar reflects high-temperature melting through undercooling.Based on the above features,we infer that the Baijiazhuang leucogranite likely represents the product of high degree fractionation of the I-type biotite monzogranite magma which generated the other four granitoids at relatively high temperatures,within magma chambers at mid-crust depths.We propose that the granitoid suite was formed in the transitional setting from synto post-collision during the collisional orogeny between the SCB and NCB,following break-off of the subducted South China Block lithosphere during 220-216 Ma.     

Fluid inclusions,C-H-O-S-Pb isotope systematics,geochronology and geochemistry of the Budunhua Cu deposit,northeast China:Implications for ore genesis

The Budunhua Cu deposit is located in the Tuquan ore-concentrated area of the southern Great Xing'an Range,NE China.This deposit includes the southern Jinjiling and northern Kongqueshan ore blocks,separated by the Budunhua granitic pluton.Cu mineralization occurs mainly as stockworks or veins in the outer contact zone between tonalite porphyry and Permian metasandstone.The ore-forming process can be divided into four stages involving stage Ⅰ quartz-pyrite-arsenopyrite;stage Ⅱ quartz-pyrite-chalcopyrite-pyrrhotite;stage Ⅲ quartz--polynetallic sulfides;and stage IV quartz-calcite.Three types of fluid inclusions(FIs) can be distinguished in the Budunhua deposit:liquid-rich two-phase aqueous FIs(L-type),vapour-rich aqueous FIs(V-type),and daughter mineral-bearing multi-phase FIs(S-type).Quartz of stages Ⅰ-Ⅲ contains all types of FIs,whereas only L-type FIs are evident in stage Ⅳ veins.The coexisting V-and S-type FIs of stages Ⅰ-Ⅲ have similar homogenization temperatures but contrasting salinities,which indicates that fluid boiling occurred.The FIs of stages Ⅰ,Ⅱ,Ⅲ,and Ⅳyield homogenization temperatures of 265-396℃,245-350℃,200-300℃,and 90-228℃ with salinities of3.4-44.3 wt.%,2.9-40.2 wt.%,1.4-38.2 wt.%,and 0.9-9.2 wt.% NaCl eqv.,respectively.Ore-forming fluids of the Budunhua deposit are characterized by high temperatures,moderate salinities,and relatively oxidizing conditions typical of an H_2 O-NaCl fluid system.Mineralization in the Budunhua deposit occurred at a depth of0.3-1.5 km,with fluid boiling and mixing likely being responsible for ore precipitation.C-H-O-S-Pb isotope studies indicate a predominantly magmatic origin for the ore-forming fluids and materials.LA-ICP-MS zircon U-Pb analyses indicate that ore-forming tonalite porphyry and post-ore dioritic porphyrite were formed at 151.1±1.1 Ma and 129.9±1.9 Ma,respectively.Geochemical data imply that the primary magma of the tonalite porphyry formed through partial melting of Neoproterozoic lower crust.On the basis of available evidence,we suggest that the Budunhua deposit is a porphyry ore system that is spatially,temporally,and genetically associated with tonalite porphyry and formed in a post-collision extensional setting following closure of the Mongol-Okhotsk Ocean.     

http://www.sciencedirect.com/science/article/pii/S1674987110000125

<正>Carbonatites are commonly related to the accumulation of economically valuable substances such as REE.Cu,and P.The debate over the origin of carbonatites and their relationship to associated silicate rocks has been ongoing for about 45 years.Worldwide,the rocks characteristically display more geochemical enrichments in Ba,Sr and REE than sedimentary carbonate rocks.However,carbonatite's geochemical features are disputed because of secondary mineral effects.Rock-forming carbonates from carbonatites at Qinling.Panxi region,and Bayan Obo in China show REE distribution patterns ranging from LREE enrichment to flat patterns.They are characterized by a Sr content more than 10 times higher than that of secondary carbonates.The coarse- and fine-grained dolomites from Bayan Obo H8 dolomite marbles also show similar high Sr abundance,indicating that they are of igneous origin.Some carbonates in Chinese carbonatites show REE(especially HREE) contents and distribution patterns similar to those of the whole rocks.These intrusive carbonatites display lower platinum group elements and stronger fractionation between Pt and Ir relative to high-Si extrusive carbonatite.This indicates that most intrusive carbonatites may be carbonate cumulates.Maoniuping and Daluxiang in Panxi region are large REE deposits.Hydrothermal fluorite ore veins occur outside of the carbonatite bodies and are emplaced in wallrock syenite.The fiuorite in Maoniuping has Sr and Nd isotopes similar to carbonatite.The Daluxiang fiuorite shows Sr and REE compositions different from those in Maoniuping.The difference is reflected by both the carbonatites and rock-forming carbonates,indicating that REE mineralization is related to carbonatites.The cumulate processes of carbonate minerals make fractionated fluids rich in volatiles and LREE as a result of low partition coefficients for REE between carbonate and carbonatite melt and an increase from LREE to HREE.The carbonatite-derived fluid has interacted with wallrock to form REE ore veins.The amount of carbonatite dykes occurring near the Bayan Obo orebodies may support the same mineralization model,i.e.that fluids evolved from the carbonatite dykes reacted with H8 dolomite marble,and thus the different REE and isotope compositions of coarse- and fine-grained dolomite may be related to reaction processes.     

A model for carbonatite hosted REE mineralisation — the Mianning–Dechang REE belt,Western Sichuan Province,China

The Mianning–Dechang (MD) REE belt of Sichuan, China is one of the most important REE belt in China, which includes Maoniuping, the third largest REE deposit in the world and a series of large to small REE deposits. Mineralization styles varied across the belt, as well as within different parts of the same deposit. Styles include vein-stockworks, pegmatites, breccias and disseminated REE mineralization. Based on geological, geochemical and inclusion studies, this paper proposes a new model for carbonatite hosted REE mineralization. The results show that ore-forming fluid is derived from carbonatite magma, which has high temperature, pressure and density, and is characterized by high K, Na, Ca, Sr, Ba, REE and SO₄ contents. The supercritical ore fluid underwent a distinctive evolution path including phase separation, exolution of sulfate melt and unmixing between aqueous fluid and liquid CO₂. Rapid geochemical evolution of a dense carbonatite fluid causes REE mineralization and associated alteration to occur within or proximal to the source carbonatite. Veins, pegmatites and carbonatite comprise a continuum of mineralization styles. Veins occur in the outer zone of the upper levels of the deposit. Pegmatites occur in the inner zone of upper levels, whereas disseminated REE ore occurs at the base of the carbonatite. High water solubility in the carbonatite magma and low water, high REE in the exsolved ore-forming fluids, imply that a giant carbonatite body and deep magma chamber are not necessary for the formation of giant REE deposits.     

High-pressure metamorphic rocks in the Borborema Province,Northeast Brazil: Reworking of Archean oceanic crust during proterozoic orogenies

We present the first evidence of Archean oceanic crust submitted to Proterozoic high-pressure (HP) metamorphism in the South American Platform. Sm–Nd and Lu–Hf isotopic data combined with U–Pb geochronological data from the Campo Grande area, Rio Grande do Norte domain, in the Northern Borborema Province, reflect a complex Archean (2.9 ​Ga and 2.6 ​Ga) and Paleoproterozoic (2.0 ​Ga) evolution, culminating in the Neoproterozoic Brasiliano/Pan-African orogeny (ca. 600 Ma). The preserved mafic rocks contain massive poikiloblastic garnet and granoblastic amphibole with variable proportions of plagioclase ​+ ​diopside in symplectitic texture, typical of high-pressure rocks. These clinopyroxene-garnet amphibolites and the more common garnet amphibolites from the Campo Grande area are exposed as rare lenses within an Archean migmatite complex. The amphibolite lenses represent 2.65 ​Ga juvenile tholeiitic magmatism derived from depleted mantle sources (positive ε_Hf(t) values of +3.81 to +30.66) later enriched by mantle metasomatism (negative ε_Nd(t) values of –7.97). Chondrite and Primitive Mantle-normalized REE of analyzed samples and discriminant diagrams define two different oceanic affinities, with E-MORB and OIB signature. Negative Eu anomalies (Eu/Eu1 ​= ​0.75–0.95) indicate depletion of plagioclase in the source. Inherited zircon cores of 3.0–2.9 ​Ga in analyzed samples indicate that the Neoarchean tholeiitic magmatism was emplaced into 2923 ​± ​14 ​Ma old Mesoarchean crust (ε_Nd(t) ​= ​–2.58 and Nd T_DM ​= ​3.2 ​Ga) of the Rio Grande do Norte domain. The age of retro-eclogite facies metamorphism is not yet completely understood. We suggest that two high-grade metamorphic events are recognized in the mafic rocks: the first at 2.0 ​Ga, recorded in some samples, and the second, at ca. 600 Ma, stronger and more pervasive and recorded in several of the mafic rock samples. The Neoproterozoic zircon grains are found in symplectite texture as inclusions in the garnet grains and represent the age of HP conditions in the area. These zircon grains show a younger cluster of concordant analyses between 623 ​± ​3 ​Ma and 592 ​± ​5 ​Ma with ε_Hf(t) values of +0.74 to –65.88. Thus, the Campo Grande rock assemblage is composed of Archean units that were amalgamated to West Gondwana during Neoproterozoic Brasiliano orogeny continent-continent collision and crustal reworking.     

Late Precambrian OIB magmatism in the Kuznetsk Alatau,Siberia: Geochemical features of the Kulbyurstyug Formation volcanics

Basaltoids of the Vendian–Cambrian Kulbyurstyug volcanic complex in the eastern part of Kuznetsk Alatau have a high content of titanium (TiO₂ 3–4 wt %). They are relatively enriched with LILE and HFSE (Ba 360–900, Zr 160–726, Nb 66–101, ΣREE up to 225–329 ppm), and demonstrate a fractionated spectrum of REEs (La/Yb ～ 13–17), high Nb/U (44–66), and low Th/Ta and Th/Nb. These features are comparable with derivatives of OIB (Ocean Island Basalts) magmatism. The rock geochemistry suggests the possible formation of an initial mafic melt in the garnet-bearing peridotite mantle with 2–4% degree of melting and the presence of a small amount of spinel. By the age and composition, the studied volcanics are correlated with the OIB magmatism products, occurring in the adjacent Gorny Altai and also considered to be derivatives of intraplate magmatism of Riphean–Early Cambrian age. This magmatism was caused by plume activity during formation of the Paleo-Asian Ocean.     

东秦岭地区碳酸岩型钼- 铀多金属矿床成矿时代：来自LA- ICP- MS独居石U- Pb和辉钼矿Re- Os年龄的证据

东秦岭地区碳酸岩型钼- 铀多金属矿床主要包括华阳川铀多金属矿、黄龙铺和黄水庵钼矿等。其中,华阳川矿床为近期取得勘查突破的一例以U、Nb、Pb为主并伴生稀土元素的超大型铀多金属矿床;黄龙铺钼矿为东秦岭钼矿带中成矿类型最为独特的大型钼矿床。为了精确获得东秦岭地区碳酸岩型钼- 铀多金属成矿时代,本研究采用辉钼矿Re- Os法和LA- ICP- MS独居石U- Pb法,分别对黄龙铺大石沟矿床的辉钼矿、秦岭沟矿床和华阳川矿床含矿碳酸岩脉中的独居石进行测定。结果表明,黄龙铺地区大石沟钼矿辉钼矿Re- Os等时线年龄为221. 3±8. 4Ma（MSWD=10. 9);秦岭沟钼矿碳酸岩中独居石LA- ICP- MS Tera- Wasserburg年龄为207±11Ma（MSWD=3. 7, n =38),华阳川铀多金属矿LA- ICP- MS独居石Tera- Wasserburg年龄为222. 5±6. 7Ma（MSWD=1. 8, n =37),表明该地区碳酸岩中的钼矿化和铀多金属矿化均形成于晚三叠世。综合分析认为,东秦岭地区发育于碳酸岩中的黄龙铺钼矿田、华阳川铀多金属矿是同一成矿系列的产物,碳酸岩型钼- 铀多金属的成矿金属可能来源于地幔,这类碳酸岩可能是秦岭地区印支期造山后伸展环境下的产物。     

The Paleoproterozoic Montviel carbonatite-hosted REE–Nb deposit,Abitibi, Canada: Geology,mineralogy, geochemistry and genesis

The Montviel 250 Mt carbonatite-hosted REE–Nb deposit is hosted in a Paleoproterozoic alkaline suite located in the Sub-Province of Abitibi, in the Archean Province of the Superior. The alkaline intrusion consists of biotite clinopyroxenites, melano- to leucosyenites, a melteigite–ijolite–urtite series, riebeckite granite, a series of carbonatites and a carbonatite polygenic breccia. The carbonatite series includes silicocarbonatites, calciocarbonatites, rare magnesiocarbonatites, ferrocarbonatites and mixed carbonatites and are cut by a late, high-energy carbonatite polygenic breccia. Diamond drill hole assays and microscope observations indicate that Nb is hosted in pyrochlore from silicocarbonatite whereas the REE mineralization is mainly hosted in ferrocarbonatite, late mixed carbonatites and polygenic breccia, in REE-bearing carbonates and fluorocarbonate minerals. Diamond drill hole underground mapping and systematic assays have shed light on zones enriched in Nd and LREE with preferential Ba and Sr hydrothermal precipitation and zones enriched in Dy, Y and HREE displaying preferential F and P bearing hydrothermal precipitation. Petrographic observations, electron microprobe analyses, LA-ICPMS and X-ray diffraction were used to study the mineralization processes and to identify and quantify the REE-bearing burbankite–(Ce), carbocernaite–(Ce), ewaldite–(Y), huanghoite–(Nd), cordylite–(Ce), cordylite–(Nd), kukharenkoite–(Ce) and synchysite–(Ce). Most minerals are enriched in total LREE with values around 19.3 wt.%, have total MREE values around 2.2 wt.% and extremely variable total HREE values, with very high contents of Dy and Y averaging around 0.3 wt.% and 1.0 wt.%, respectively, and with total HREE reaching up to 10.0 wt.%. A paragenetic sequence is proposed that consists of: (1) a silicocarbonatite Nb stage, and (2) a calciocarbonatite stage, dominated by magmatism but accompanied by hydrothermal fluids, (3) a main ferrocarbonatite stage, dominated by episodes of Ba- and Sr-hydrothermalism and LREE mineralization, F- and P-hydrothermalism and HREE mineralization and evolved ferrocarbonatitic magmatism, (4) a renewed, mixed carbonatite magmatic stage with minor but increasing hydrothermalism, and (5) a terminal stage of fluid pressure buildup and explosion, leading to the creation of a HREE-enriched polygenic breccia. Globular melt inclusions of Ba–Cl–F (± Si–O) may indicate the presence and contribution of barium-bearing chlorofluoride melts during hydrothermal activity and mineralization of the carbonatite.     

Record of Early Tonian mafic magmatism in the central Espinhaço (Brazil): New insights for break-up of the Neoproterozoic landmass ancestor of São Francisco-Congo paleocontinent

Petrological characterization, U–Pb geochronology, Lu–Hf analyses and major and trace element data from mafic intrusions in the Central Espinhaço (central portion of the Brazilian shield) are used here to investigate the geological significance of the Early Neoproterozoic magmatism in the context of the São Francisco-Congo paleocontinent. These mafic bodies are represented by medium to coarse-grained metagabbros with plagioclase, amphibole and clinopyroxene. Zircon U–Pb isotopic data from two samples yielded weighted mean ²⁰⁶Pb/²³⁸U ages of 895 ​± ​3.4 ​Ma (MSWD ​= ​1.7) and 896 ​± ​2.4 ​Ma (MSWD ​= ​0.64), regarded as the best estimates for the crystallization age of these mafic rocks. Major and trace element data (including REEs) show that the gabbros originated from a subalkaline tholeiitic magma, typical of intraplate magmatism. Such rocks are slightly enriched in LREEs and LILEs and depleted in HFSEs. Our new isotope and geochemical data, along with regional knowledge, indicate that these metagabbros mark the beginning of an important Tonian-age extensional tectonic event of the landmass of which the São Francisco-Congo paleocontinent was part (Rodinia supercontinent or Central African block?). We furthermore suggest that these rocks belong to a prominent suite of Tonian-age mafic rocks that mark a diachronic breakup attempt of this landmass which may have occurred from south to north along the Espinhaço mountain range.