New Evidence for Genesis of the Zoige Carbonate-Siliceous-Pelitic Rock Type Uranium Deposit in Southern Qinling: Discovery and Significance of the 64 Ma Intrusions

The carbonaceous-siliceous-argillitic rock type uranium deposit in the Zoige area is located in the northeastern margin of the Tibetan Plateau, and has gained much attention of many geologists and ore deposit experts due to its scale, high grade and abundant associated ores. Because of the insufficient reliable dating of intrusive rocks, the relationship between mineralization and the magmatic activities is still unknown. In order to study this key scientific issue and the ore-forming processes of the Zoige uranium ore field, the LA-ICP-MS zircon U-Pb dating of magmatic rocks was obtained:64.08±0.59 Ma for the granite-prophyry and ~200 Ma for the dolerite. U-Pb dating results of uraninite from the Zoige uranium ore field are mainly concentrated on ~90 Ma and ~60 Ma. According to LA-ICP-MS U-Pb zircon dating, the ages for the dolerite, porphyry granite and granodiorite are 200 Ma, 64.08 Ma approximately and 226.5-200.88 Ma, respectively. This indicates that the mineralization has close relationship with activities of the intermediate-acidic magma. The ages of the granite porphyry are consistent with those uraninite U-Pb dating results achieved by previous studies, which reflects the magmatic and ore-forming event during the later Yanshanian. Based on the data from previous researches, the ore bodies in the Zoige uranium ore field can be divided into two categories:the single uranium type and the uranium with polymetal mineralization type. The former formed at late Cretaceous(about 90 Ma), while the latter, closely related to the granite porphyry, formed at early Paleogene(about 60 Ma). And apart from ore forming elemental uranium, the latter is often associated with polymetallic elements, such as molybdenum, nickel, zinc, etc.     

LA-ICP-MS Zircon U-Pb Geochronology of the Fine-grained Granite and Molybdenite?Re-Os?Dating in the Wurinitu Molybdenum Deposit, Inner Mongolia, China

The Wurinitu molybdenum deposit, located in Honggor, Sonid Left Banner of Inner Mongolia, China, is recently discovered and is considered to be associated with a concealed fine-grained granite impregnated with molybdenite.?The wall rocks are composed of Variscan porphyritic-like biotite granite and the Lower Ordovician Wubin’aobao Formation.?LA-ICP-MS zircon U-Pb dating of the fine-grained granite reveals two stages of zircons,?one were formed at 181.7±7.?4 Ma and?the other at 133.6±3.3 Ma. The latter age is believed to be the formation age of the fine-grained granite, while the former may reflect the age of inherited zircons, based on the morphological study of the zircon and regional geological setting. The Re-Os model age of molybdenite is 142.2±2.5?Ma, which is older than the diagenetic age of the fine-grained granite.?Therefore the authors believe that the metallogenic age of the Wurinitu molybdenum deposit should be?nearly 133.6±3.3 Ma or slightly later, i.e., Early Cretaceous.?Combined with regional geological background research, it is speculated that the molybdenum deposits were formed at the late Yanshanian orogenic cycle in the Hingganling-Mongolian orogenic belt, belonging to the relaxation epoch posterior to the compression and was associated with the closure of the Mongolia-Okhotsk?Sea.     

Lead in K－Feldspar and Its Relations to Rb Metallogenesis

In intermediate-acid magmatic rocks,alkaline magmatic rocks,gneisses and migmatitic rocks K-feldspar is a rock-forming rock in which the contents of Pb are highest,just 2-10 times those of the whole rock,3-16 times those of mica minerals and 6-32 times those of quartz.The lowest contents of K-feldspar are recognized in Early Proterozoic and Achaean rocks,with Pb in the K-feldspar accounting for less than 10% of that in the whole rock;in post Middle Proterozoic alkaine magmatic rocks,K-feldspar-rich granites and metamorphic rocks the contents of K-feldspar tend to increase,with the proportion granites and metamorphic rocks the contents of K-feldspar tend to increase,with the proportion of Pb over that in the whole rock being obviously increased.In the alkaline rocks in which K-feldspar accounts for 50%-70% of the total in the whole rock,the contents of Pb in K-feldspar account approximately for 70%-95% of the total lead in the whole rock.Being accessible to hydrothermal alteration in the late periods,K-feldspar was conversed to sericite,calcite,quartz,etc.In the process of such conversion the lead would be leached out and then find its way into fluid phase.This kind of trans-formation can provide sufficient ore-forming material for later Pb metallogenesis.     

SHRIMP U-Pb Zircon Age of the Ka''erjiao Intrusion in the Sawur Region in West Junggar,Xinjiang

Acid intrusions are widespread in the Sawur region, Xinjiang. The Ka'erjiao intrusion is mainly composed of albite granite porphyry, K-feldspar granite porphyry, ivernite and granite porphyry. Being a transitional product between magma intrusion and eruption in the Sawur region, the Ka'erjiao intrusion was formed at the telophase of the late Carboniferous to the begining of early Permian as determined by the SHRIMP U-Pb zircon dating, with an age of 302.6±7.6 Ma (1σ). The intrusion consists of alkali-enriched rock, whose REE distribution patterns are of the LREE enrichment type, theδEu value is low and Nd, Sr, Pb isotopes reflect its mantle source characteristics. Theδ18O value of intrusion is low as a result of isotope exchange with meteoric water. The geochemical characteristics show that it was formed in a post-collisional tectonic setting. Taking combined considerations of current studies of A-type granites and Permian volcanic rocks, we think that in the telophase of the late Carboniferous to the beginning of the early Permian, the Sawur region was within the extension or compressional to extensional period of a post-collisional stage. The Ka'erjiao intrusion from mantle sources can confirm the vertical continental crust growth in the late Paleozoic. The Sawur region in west Junggar is consistent with east Junggar in post-collisional tectonic evolution process.     

SHRIMP U-Pb Zircon Age of the Ka''erjiao Intrusion in the Sawur Region in West Junggar,Xinjiang

Acid intrusions are widespread in the Sawur region, Xinjiang. The Ka'erjiao intrusion is mainly composed of albite granite porphyry, K-feldspar granite porphyry, ivernite and granite porphyry. Being a transitional product between magma intrusion and eruption in the Sawur region, the Ka'erjiao intrusion was formed at the telophase of the late Carboniferous to the begining of early Permian as determined by the SHRIMP U-Pb zircon dating, with an age of 302.6±7.6 Ma (1σ). The intrusion consists of alkali-enriched rock, whose REE distribution patterns are of the LREE enrichment type, theδEu value is low and Nd, Sr, Pb isotopes reflect its mantle source characteristics. Theδ18O value of intrusion is low as a result of isotope exchange with meteoric water. The geochemical characteristics show that it was formed in a post-collisional tectonic setting. Taking combined considerations of current studies of A-type granites and Permian volcanic rocks, we think that in the telophase of the late Carboniferous to the beginning of the early Permian, the Sawur region was within the extension or compressional to extensional period of a post-collisional stage. The Ka'erjiao intrusion from mantle sources can confirm the vertical continental crust growth in the late Paleozoic. The Sawur region in west Junggar is consistent with east Junggar in post-collisional tectonic evolution process.     

SHRIMP U-Pb Zircon Age of the Ka''''erjiao Intrusion in the Sawur Region in West Junggar,Xinjiang

Acid intrusions are widespread in the Sawur region, Xinjiang. The Ka'erjiao intrusion is mainly composed of albite granite porphyry, K-feldspar granite porphyry, ivernite and granite porphyry. Being a transitional product between magma intrusion and eruption in the Sawur region, the Ka'erjiao intrusion was formed at the telophase of the late Carboniferous to the begining of early Permian as determined by the SHRIMP U-Pb zircon dating, with an age of 302.6±7.6 Ma (1σ). The intrusion consists of alkali-enriched rock, whose REE distribution patterns are of the LREE enrichment type, theδEu value is low and Nd, Sr, Pb isotopes reflect its mantle source characteristics. Theδ18O value of intrusion is low as a result of isotope exchange with meteoric water. The geochemical characteristics show that it was formed in a post-collisional tectonic setting. Taking combined considerations of current studies of A-type granites and Permian volcanic rocks, we think that in the telophase of the late Carboniferous to the beginning of the early Permian, the Sawur region was within the extension or compressional to extensional period of a post-collisional stage. The Ka'erjiao intrusion from mantle sources can confirm the vertical continental crust growth in the late Paleozoic. The Sawur region in west Junggar is consistent with east Junggar in post-collisional tectonic evolution process.     

Zircon U-Pb gcochronology and geochemical characteristics of the reshui monzogranite in the eastern kunlun and their tectonic significances北大核心CSCD

In order to probe the genesis and tectonic significance of the granite in the Reshui area, geoehronologicai and geochemical studies of the Reshui monzogranite have been carried out in this paper. The results show that the Reshui monzogranite has LA-1CP-MS zircon U-Pb age of 231 ±1 Ma, indicating that it was formed in late Triassic. It has high contents of Si()2(68. 74%-7l.69%) , Al,(),( 14. 78%-15. 18%). and k,()( 3. 78%-4. 32% ) , with k,()/Na,() ratios ranging from 1. 05 to 1. 32 ,r Rttmann indexes ranging from 1. 87 to 2. 21 . and A/CNk values varying from 1. 03 to 1. 07, respectively. Thus, the Reshui monzogranite belongs to the high k calc-Alkaline weak peraluminous granite. In addition, it has total REE contents(ΣREE) of I 18. 28x 10-6 - 148. 1 x 10-6, with right declined REE distribution patterns showing strong enrichment of LREE and relative depletion of HREE. Its large ion lithophile elements( L1LE) , including Rb. Ra, Th, and l.REE are enriched, while its high field strength elements( HESE) , including Nh, Ta, P. and Ti, are relatively depleted. It has characteristics of the 1-Type granite. It could be mainly sourced from the partial melting of the subducted slab or basic rocks of the lower crust, mixed partly with mantle materials. Combining with regional geological background, it is believed that the Reshui monzogranite was formed by the interaction of the crust and mantle under the tectonic environment of collision-post collision.     

Geological and Geochemical Constraints on the Newly Discovered Yangchongli Gold Deposit in Tongling Region, Lower Yangtze Metallogenic Belt

The newly discovered Yangchongli gold deposit is a unique independent gold deposit in the Tongling ore-cluster region controlled by the tectonic alteration firstly discovered in the Lower Yangtze Metallogenic Belt(LYMB). The host magmatic rocks mainly consist of monzodiorite and K-feldspar granite. The LA-ICP-MS U-Pb zircons dating yielded weighted mean ~(206)Pb/~(238)U ages of 140.7 ± 1.8 Ma and 126.4 ± 1.2 Ma for the monzodiorite and K-feldspar granite, respectively. Monzodiorites are enriched in Sr, Ba, Rb, and depleted in Y, Yb with high Sr/Y and La/Yb ratios, similar to the geochemical features of adakite, considered as products of differentiation of mafic magmas originating from lithospheric mantle melt/fluids caused by metasomatism during paleo-Pacific Plate subduction in the Mesozic. In contrast, the compositions of K-feldspar granites are A-type granites, indicating an extensional tectonic background. Gold ores hosted in the fracture zone occurred as quartz vein within cataclastic rock. Sulfur and lead isotopes from pyrites show crust-mantle mixing characteristics. Metal components from strata also took part in the gold mineralization, and resulted from two episodes of magmatism that were probably related to tectonic transition from a compressive to an extensional setting between 140–126 Ma, which led to the Mesozoic large-scale polymetallic mineralization events in eastern China.     

Enrichment of Mantle-derived Fluids in the Formation Process of Granitoids: Evidence from the Himalayan Granitoids around Kunjirap in the Western Qinghai-Tibet Plateau

Taking the Himalayan granitoids around Kunjirap in the western Qinghai-Tibet plateau as an example, the authors present in this paper the characteristics of the granitoids rich in mantle-derived fluid components and discuss their rock-forming mechanism. The research results indicate that the rock assemblage of the studied granitoids involves diopside syenite-diopside granite-biotite (monzonitic) granite, consisting mainly of K-feldspar, oligoclase, quartz, iron-phlogolite, diopside and edenite. The rocks are rich in mantle-derived fluid components of volatiles including F, alkali metal elements such as K, Na, Rb, Sr and Ba, and radiogenic heat-producing elements such as U and Th. They were generated by the influx of mantle-derived fluids into the lower crust to give rise to partial melting during the lithosphere thinning in the Qinghai-Tibet plateau.     

Rb—Sr Geochronology and Magma Source of the Mesozoic Fault—Granite Belt,East Shandong

Developed in the southeast coast of te East Shandong Peninsula,the Mesozoic fault-magma belt consists of five rock series:the syenite series;the monzonite series;the megaporphyritic monzogranite series;the biotite-granite series;and the alkali granite seres.Based on their Rb-Sr isochron ages(122-220Ma),these rock series may be divided into three magma subcycles dated at Triassic,Late Jurassic and Early Cretaceous.The initial ^87Sr/^86Sr ration in these rock series range from 0.70436 to 0.7155.The starting points of the Rb-Sr isochrons exhibit four different distribution trends on the(^87Sr/^86Sr)i-^87Rb/^86Sr diagram.These characteristics show that the multiple granitic rock series are different in genesis and derivation.The syenite series might be derived from the combination of mantle-derived magma and crustal material,and the others could be derived from granulite-facies and amphibilite-facies rocks in the deep crust.     

华南某花岗岩型铀矿床成矿机理的铀铅同位素体系研究

The uranium deposit studied in this paper occurs in a Precambrian (Xuefengian)granite batholith. The emplacement age of the granite is 760 m.y. The average content of uranium in the granite is 7 ppm, but the yield of leachsd U is higher With an average of about 33.8%. The granitic rocks, contain; Some uraninite. Uranium mineralization mainly takes the form of siliceous veins With an age of 47 m.y. K-feldspar. present in the granite and pyrite associated with pitchblende were massspectrometrically analyzed for Pb isotopic compositious, characterized by anomalous lead. A reasonably linear array can be seen on the ^207Pb/^206Pb--^205Pb/^204Pb plot, implying a genetic connection between the rock and the ore, and a derivation of rock-forming materials from. a uranium-rich source. According to the continuous-growth model for anomalous lead, evolution, two isochrons give the. ages-of about 764 m.y. and 708 m.y.for ore-forming materials, close to the emplacement age of .the granite batholith. This indicates that the ore-forming materials came from the granite batholith. Eight granite samples collected from the mining .area were analyzed, but no uranium-lead ages could be worked out because of the pregressive destruction of the closed U-Pb isotopic system since granite emplacement, which resulted in the loss of uranium(78%). It can be imagined that in the period of continental weathering the surface water would infiltrate downwards, leaching out large amounts of uranium from the granitic rocks to form infiltration solution. The solution was heated at great depth and then found its way upwards, on which it dissolved constantly U from the rocks to form the uranium deposit.     

Discriminating characters of ore-forming intrusions in the super-large Chalukou porphyry Mo deposit,NE China

The Chalukou porphyry Mo deposit, located in the Great Hinggan Range, is the largest Mo deposit in northeast China, although the age and genesis of the associated magmatic intrusions remain debated.Here we report zircon U-Pb ages and trace elements, whole rock geochemistry and Sre Nd isotope data with a view to understand the relationship between the magmatism and molybdenum mineralization.Zircon U-Pb analysis yield an age of 475 Ma for rhyolite in the older strata, 168 Ma for the premineralization monzogranite, and 154 Ma for the syn-mineralization granite porphyry. The granite porphyry and quartz porphyry are considered as the ore-forming intrusions. These rocks are peraluminous, alkali-calcic, and belong to high-K to shoshonitic series with a strong depletion of Eu. They also display characteristics of I-type granites. The rocks exhibit wide variations of(87 Sr/86 Sr)iin the range of 0.705426 -0.707363, and ε_(Nd)(t) of -3.7 to 0.93. Zircon REE distribution patterns show characteristics between crust and the mantle, implying magma genesis through crust-mantle interaction. The Fe_2O_3/FeO values(average 1) for the whole rock and EuN/Eu*Nvalues(average 0.45), Ce~(4+)/Ce~(3+) values(average 301)for zircon grains from the granite porphyry are higher than those from other lithologies. These features suggest that the ore-forming intrusions(syn-mineralization porphyry) had higher oxygen fugacity conditions than those of the pre-mineralization and post-mineralization rocks. The Chalukou Mo deposit formed in relation to the southward subduction of the Mongol-Okhotsk Ocean. Our study suggests that the subduction-related setting, crust-mantle interaction, and the large-scale magmatic intrusion were favorable factors to generate the super-large Mo deposits in this area.     

Geochronology and geochemistry of Yanchangbeishan biotite Adamellite of Lenghu area in Qinghai and their geological significanceEI北大核心CSCD

The Yanchangbeishan biotite adamellite in the Lenghu area of Qinghai is located in the west segment of the north margin of Qaidam. The rocks have high SiO2 (73.92%-75.98%) and K2O contents (3.71%-4.78%), but low MgO and CaO contents. The alumina saturation index (A/CNK) varies from 1.03 to 1.11, indicating the biotite adamellite is peraluminous. The chondrite-normalized REE patterns of the rocks are right dipping and display weak Eu anomalies. The rock contains the Al-rich mineral like garnet and possesses normative corundum contents of 1%. It has low Th and Y contents which are negatively correlated with Rb, suggesting the source rock was sedimentary rock. Thus, the biotite adamellite belongs to S-type granite. The granite was produced by partial melting of the crust material due to the underplating of mantle-derived magma. LA-ICP-MS zircon U-Pb dating shows that the formation age of the rock is 254±4 Ma. Based on the regional geological background and the geochemical characteristics of the rocks, we consider that the formation of the Yanchangbeishan biotite adamellite was related to subduction, which is illustrated by the characteristics similar to those of the island-arc igneous rocks. Therefore, the results reveal that the north margin of Qaidam was in active continental margin tectonic environment during Late Permian epoch. ©, 2015, Science Press. All right reserved.     

http://www.sciencedirect.com/science/article/pii/S1674987112000394

High-grade dehydration of amphibolite-facies rocks to granulite-facies is a process that can involve partial melting,fluid-aided solid-state dehydration,or varying degrees of both.On the localized meter scale,solid-state dehydration,due to CO₂-rich fluids traveling along some fissure or crack and subsequently outwards along the mineral grain boundaries of the surrounding rock,normally is the means by which the breakdown of biotite and amphibole to orthopyroxene and clinopyroxene occur.Various mineral textures and changes in mineral chemistry seen in these rocks are also seen in more regional orthopyroxene-clinopyroxene-bearing rocks which,along with accompanying amphibolite-facies rocks, form traverses of lower crust.This suggests that solid-state dehydration during high-grade metamorphism could occur on a more regional scale.The more prominent of these fluid-induced textures in the granulitefacies portion of the traverse take the form of micro-veins of K-feldspar along quartz grain boundaries and the formation of monazite inclusions in fluorapatite.The fluids believed responsible take the form of concentrated NaCl- and KC1- brines from a basement ultramafic magma heat source traveling upwards along grain boundaries.Additional experimental work involving CaSO₄ dissolution in NaCl-brines. coupled with natural observation of oxide and sulfide mineral associations in granulite-facies rocks,have demonstrated the possibility that NaCl-brines,with a CaSO₄ component,could impose the oxygen fugacity on these rocks as opposed to the oxygen fugacity being inherent in their protoliths.These results, taken together,lend credence to the idea that regional chemical modification of the lower crust is an evolutionary process controlled by fluids migrating upwards from the lithospheric mantle along grain boundaries into and through the lower crust where they both modify the rock and are modified by it. Their presence allows for rapid mass and heat transport and subsequent mineral genesis and mineral reequilibration in the rocks through which they pass.     

Felsic Igneous Rocks in the Hua’aobaote Pb-Zn-Ag Polymetallic Orefield, Southern Great Xing’an Range: Genesis, Metallogenetic and Tectonic Significance

The Hua’aobaote Pb-Zn-Ag Polymetallic orefield is situated in the southern section of the Great Xing’an Range(GXAR),which has experienced extensive magmatism.Since the Paleozoic,there are two stages of magmatism in Hua’aobaote orefield occurred in the Paleozoic and Mesozoic.The Mesozoic magmatism is of great significance for the PbZn-Ag Polymetallic mineralization in Hua’aobaote orefield.In this study,new geochemical data was obtained to discuss the timing and petrogenesis of the magmatic rocks and its geodynamic and metallogenic significance.Zircon U-Pb ages reveal that the felsic igneous rocks from the Hua’aobaote orefield were formed in the Early Permian(294.8±3.2 Ma)and Early Cretaceous(132.6±1.4 Ma).Geochemically,the Early Permian granodiorite porphyrite is characterized by high Sr/Y(42-63)ratios and Mg^#(62.24-70.74)values and low heavy rare earth element(HREE)(5.09-6.79 ppm)contents.The granodiorite porphyrite is also characterized by depleted Sr-Nd initial isotopic signatures[ε_Nd(t)=5.91-7.59,(⁸⁷Sr/⁸⁶Sr)i=0.7029-0.7030],exhibiting adakitic characteristics.The Early Cretaceous granite porphyry and rhyolite are A-type felsic igneous rocks,and demonstrate high SiO₂,Na₂O+K₂O and rare earth element(REE)contents,low CaO and MgO contents,low(⁸⁷Sr/⁸⁶Sr)i ratios(0.7044-0.7058),and positive ε_Nd(t)values(2.57-4.65).Whole-rock Pb isotopic compositions in granodiorite porphyrite are:206Pb/204Pb=17.631-18.149,²⁰⁷Pb/²⁰⁴Pb=15.422-15.450,and ²⁰⁸Pb/²⁰⁴Pb=37.325-37.729.The granite porphyry and rhyolite have initial ²⁰⁶Pb/²⁰⁴Pb,²⁰⁷Pb/²⁰⁴Pb,and ²⁰⁸Pb/²⁰⁴Pb ratios of 18.106-19.309,15.489-15.539,and 37.821-38.05,respectively.Sr-Nd-Pb isotopic evidence suggests that the Early Permian granodiorite porphyrite is likely to derive from slab melts and modified by peridotitic mantle wedge in the subduction tectonic setting of the Paleo-Asian Ocean.The Early Cretaceous A-type felsic igneous rocks were derived from juvenile lower crust,accompanied by limited crustal contamination and various degree of fractional crystallisation during magma emplacement.The Early Cretaceous magmatism and related mineralization were formed in a post-orogenic tectonic setting that attributed to the closure of the Mongol-Okhotsk Ocean.Pb isotopic data for the various rock units in the study area indicate that the Mesozoic magma source contributed substantial Pb,Zn,and Ag to the Hua’aobaote deposit.     

Geochemistry of Alkali-rich Igneous Rocks of Northern Xinjiang and Its Implications for Geodynamics

Five nearly E-W-trending alkali-rich igneous rock belts are distributed from north to south in northern Xinjiang, and they are composed mainly of riebeckite, K-feldspar granite and high-K and medium-K calc-alkaline volcanic rocks and shoshonite. They were mainly emplaced or erupted between the Carboniferous and Permian. The compositions of Sr, Nd, Pb, and O isotopes imply that their principal resource materials are derived from the upper mantle or juvenile crust, indicating obvious continental growth in the Phanerozoic. The trace element association implies that their tectonic settings are within plate and volcanic arc for alkali-rich granites, and post-collisional arc, late oceanic arc and continental arc for alkali-rich volcanic rocks. An archipelago model was suggested for the tectonic evolution in northern Xinjiang. It can be named the central-Asia-type orogeny, which is different from the so called circum-Pacific ocean-continent-type tectonics or the Alpine-Himalayan continent-continent-collisional     

The Bulong Gold Deposit-a Quartz-Barite Vein Type Gold Deposit in Xinjiang:Geological Characteristics and S, He and Ar Isotopic Compositions

The Bulong gold deposit, located in the southwest Tianshan in China, occurs in the Upper Devonian finegrained clastic rocks. The gold orebodies are controlled by an gently inclined interlayer fractured zone. They are hosted only in quartz-barite veins though there are barite veins and quartz veins in the ore district. The δ34S values of pyrite in the ores range from 14.6‰ to 19.2‰ and those of barite from 35.0‰ to 39.6‰, indicating that the sulfur was derived from the strata. 3He/4He ratios of fluid inclusions in pyrite are 0.24-0.82 R/Ra, approximating to that of the crust. The 40Ar/39Ar ratios range from 338 to 471, slightly higher than that of the atmosphere. 40Ar /4He ratios of ore fluids range from 0.015 to 0.412 with a mean of 0.153. Helium and argon isotope compositions of fluid inclusions show that the ore fluids of the Bulong gold deposit were mainly derived from the crust.     

中国云南哀牢山含海蓝宝石伟晶岩的起源和成岩模拟实验研究

The Ailaoshan aquamarine-bearing pegmatites are associated with Proterozoic metamorphic rocks in the southern portion of the Ailaoshan fault-folded complex.The gem-bearing pegmatite mineralization zones of the region occur in areas generally consistent with the regional tectonic trend.The pegmatites are found in metamorphic rocks,migmatites and in the inner/outer contact zones of gneissoid granites. The Rb-Sr isochron drawn for the pegmatites is 26～31 Ma,(i.e.in Himalayan).The homogenization temperatures of melt and liquid inclusions in minerals vary from 185 to 920℃,which are comparable to the inclusions observed in banded migmatites and ptygmatic quartz veins in the surrounding metamorphic rocks. The mineralization fluids of the pegmatite were rich in HCO_3 and CO_2,and their compositional assemblages are comparable to metamorphic fluids.Results of H,O,C,Si etc.isotopic analyses and REE,and Be analyses indicates that the sources of mineralization components that formed the pegmatites are closely associated with metamorphic fluids and the enclosing metamorphic rocks. A pegmatite structure simulation experiment was conducted at high temperature and pressure(840℃and 1,500×105Pa.),with various metamorphic rock samples in a water-rich and volatile-rich environment.When the liquidus was reached,the temperature was gradually decreased at the rate of 5～10℃/day over a time period of three months.SEM energy-dispersive spectrum analyses were performed on the experimental products.A series of pegmatoid textures were observed including zonal texture,megacryst texture,drusy cavities,crystal druses,and vesicular texture along with more than ten types of minerals including plagioclase,microcline,quartz and biotite.Different metamorphic rock melts generated different mineral assemblages.Experiment results revealed that the partial melting of metamorphic rocks could form melts similar to pegmatite magmas. Based upon the geological characteristics,geochemistry,and pegmatite texture simulation experimental results,it is concluded that the mineralization components of Ailaoshan aquamarine-bearing pegmatites came from metamorphic rocks.The petrogenetic model for the origin of pegmatites is related to ultrametamorphism and metamorphic anatexis.     

The Distribution of Trace Elements in Granitoids in the Nanling Region of China

Granitoids are widely spread in the Nanling Region of China.Four rockbodies in the region the been studied for their REE,Rb,Sr,Ba and Sc distributions.The four rockbodies occurred in different locations and are characterized as being different in age and type.The rock types are presented as follows:Qinghu monzonite,Guangxi;Fuxi granodiorite,Guangdong;Jiufeng monzonitic granite,Hunan;Zudong K-feldspar granite,Jiangxi.From the major and trace element distributions in these granitoids it is clearly shown that Rb/Sr ratios in the rocks tend to increase with increasing SiO2 content and differentiation index(DI),but LREE/HREE,La/Yb and K/Rb ratios tend to decrease,suggesting a correlation between trace element distribution and major element composition for the granitoids.The distribution characteristics of trace elements in each of the rockbodies are described in detail.From the comparative analysis of the Qinghu monzonite and Fuxi granodiorite it is evidenced that the REE distribution is closely related to the sequence of crystallization for the minerals,and also to the petrochemical types of these granitoids in addition to their crystal chemistry.     

Mineralkogical and Petrochemical Characterisitics and Genesis of Laoniugou Geneiss in Jiapigou Gold Mine,Jilin Province

Detailed mineralogical and petrochemical studies show that the Laoniugou gneiss of the Jiapigou gold mine is composed mainly of plagioclase gneiss and irregular to lentiform plagioclase amphibolite melanic enclaves.The major element contents show an obvious bimodal and trondhjemitic series evolutional trend.This situation is significantly different from that encountered in bimodal calc-alkalic volcanic rocks in the rift-type Archaean greenstone belt.The contents of Rb,Sr and Ba are 7-21 ppm,153-363ppm and 201-1451 ppm respectively ,close to those of common Archaean grey gneisses.All the samples of plagioclase gneisses show positive Eu anomalies (even up to 4.6).The protoliths of the plagioclase gneiss are high-Al2O3 trondhjemitic series rocks,belonging to typical TTG of Archaean high-grade metamorphic terrain .The gneiss is quite similar to the B-type Amitsoq gneiss of W.Greenland .The authors believe that the plagioclase amphibolite enclaves are the relics of ancient oceanic crust while the plagioclase gneiss is the TTG ancient intrusive rock resulting from partial melting of the oceanic crust.