Geochemistry, U-Pb, Hf isotopic characteristics and geological significance of Zhalaxiageyong trachydacite in Tuotuohe area, Qinghai

The Zhalaxiageyong lead-zinc-copper polymetallic deposit is a typical porphyry deposit of the Tuotuohe area. Whole-rock geochemical analyses,Zircon U-Pb dating and Hf isotope analysis are undertaken for the ore host trachydacite with the aim of constraining its petrogenesis,magma source and regional tectonic setting.LA-ICP-MS zircon U-Pb dating indicates that the trachydacite was formed in 32. 68 ± 0. 50 Ma( MSWD =1. 6),i. e.,Oligocene. The trachydacite is rich in potassium and poor in Mg#( 5. 10-9. 70),belonging to the peraluminous shoshonite series. The rocks are enriched in LILE( large ion lithophile elements) Rb,Ba,K and LREE,depleted in HFSE( high field strength elements) Nb,Ta,P,Ti,with high Sr and low Y and Yb,having the characteristics of the C type adakite. It is calculated that the initial εHf( t) of the zircons range from-0. 92 to 2. 07 and their two-stage Hf model ages T_(DM2) range from 978 Ma to 1 169 Ma. The magma source should be mainly the partially melt mafic rocks of the thickened Middle Neoproterozoic lower crust of the Northern Qiangtang massif with the addition of ancient aluminosilica material in the melting process. The rocks formed in the tectonic setting of delamination of lithosphere and extension of the thickened crust. During the period of 40-32 Ma,large-scale potassium rich alkaline magmatism occurred in this area. The porphyry metallogenesis is related to the magmatic activities in this period.

K-Ar age, geochemical, and Sr-Pb Isotopic compositions of keban magmatics, elazig, EasternAnatolia, Turkey

Keban magmatics consist of plutonic rocks of acidic and intermediate compositions with diffe rent phases. They are the equivalent of surface rocks. In the current study on plutonic rocks, general petrographic features, disequilibrium textures such as skeletal formation in minerals, poikilitic texture, oscillatory zoning, and mineral fragmentation, and growth states are observed. Besides these microscopic properties, the existence of rounded mafic enclaves of various sizes, petrographic synplutonic dykes, and field data support the idea that mafic and felsic magmas are mixed. Keban magmatics have I-type, metaluminous-peraluminous characteristics. Diorites and quartz diorites have low-K tholeiitic features, whereas tonalites have low-K calcalkaline features. Compared with diorites, tonalites are richer in terms of LREE (Rock/ Chondrite);Rb, Sr, and Ba (LILE);and Hf, Zr, Th, and U (HFSE) elements. LILE enrichment, which signals the crustal contamination of mantle- originated magmas, is particularly observable in tonalites. In both rock groups, the negative anomaly of Nb is a sign of similarity of pluton to the subduction zone magma series. Based on the K-Ar geochronology dating of amphibole minerals, the ages of these rocks are found to be 75.65 ± 1.5 and 59.77 ± 1.2 Ma in tonalites and 84.76 ± 1.8 and 84.35 ± 1.7 Ma in diorite and quartz diorites. The 87Sr/86Sr isotope ratios in tonalites are 0.705405 and 0.706053, whereas these ratios are 0.704828 and 0.704754 in dioritic rocks. Pb isotope ratios are similar in both rock types.

SHRIMP Age and Geochemistry of the Bikou Volcanic Terrane:Implications for Neoproterozoic Tectonics on the Northern Margin of the Yangtze Craton

The Bikou volcanic terrane is predominated by subalkaline tholeiitic lavas. Rock samples display lower initial ratios of Sr and Nd, 0.701248-0.704413 and 0.511080-0.512341 respectively. 207Pb and 208Pb are significantly enriched in the lavas. Most samples have positive εNd, which indicates that the magma was derived from EM-type mantle source, while a few samples with negative εNd indicate that there was contamination in the magma evolution. Magma differentiation is demonstrated by variations of LREE and LILE from depletion to enrichment. Additionally, normalized REE patterns and trace elements showed that lavas from the Bikou volcanic terrane have similar characteristics to those of basalts in arc settings caused by subduction and collision. Analyses showed that the Bikou volcanic terrane is a volcanic arc. New evidence proved that the Hengdan Group, north of the Bikou arc, is a turbidite terrane filling a forearc basin. Consequently, the Bikou volcanic terrane and the Hengdan turbidite terrane construct an arc-basin system. New SHRIMP ages showed that this arc-basin system developed on the northern margin of the Yangtze craton in the Neoproterozoic (846-776 Ma), and this arc-basin system is in agreement with the tectonic processes of Rodinia in the Neoproterzoic.

Re-Os and U-Pb Geochronology of the Erlihe Pb-Zn Deposit,Qinling Orogenic Belt,Central China,and Constraints on Its Deposit Genesis

The Erlihe Pb-Zn deposit is an important mine of the Pb-Zn metallogenic zone in the South Qinling Orogen.It has been considered a sedimentary exhalative deposit in previous investigations because the ore body occurs concordantly at the transitional location of an upright fold.Re and Os isotopic analyses for paragenetic pyrites with sphalerite and galena from the ore body have been used to determine the timing of mineralization and to trace the source of metallogenic materials.The Re-Os isotopic data of four pyrite samples construct an isochron,yielding a weighted average age of 226±17 Ma（mean square weighted deviation=1.7）,which is considered the main mineralization age.A dioritic porphyrite vein sample,showing weaker mineralization,was also dated using the SHRIMP zircon UPb isotopic method to constrain the youngest metallogenic age of the ore deposit,because it distributes along a group of tensional joints cutting not only the upright fold in the deposit field,but also the main ore bodies.The dioritic porphyrite sample yields a weighted mean ^（206）Pb/^（238）U age of 221±3 Ma,which is slightly younger than the Re-Os isotopic isochron age of the pyrites,considered as the upper age limit of the mineralization,namely the ending age of the mineralization.The Os isotopic compositions of sulfide minerals distribute within a range between Os isotopic compositions of the crust and the mantle, indicating that the ore deposit can be derived from magma-related fluid,and the metallogenic materials are most likely derived from the mixing source of the crust and the mantle.The Erlihe Pb-Zn deposit and associated dioritic porphyrite vein,important records of Qinling tectonic-magmatism-mineralization activities,were formed during the Triassic collisional orogeny processes.

Geochronology,Geochemistry,and Implications of Aplite Dyke in the Giant Jiama Porphyry Copper System,Tibet

Jiama is a giant,high-grade porphyry copper system in the Gangdese metallogenic belt,Tibet.Multistage intermediate-felsic porphyries intruded in this deposit,some of which are strongly associated with copper-polymetallic mineralization.These ore-bearing porphyries include monzogranite,granodiorite,and quartz diorite porphyries.A new granite aplite dyke was found in the south of Jiama.Its age,genesis,and relationship with ore-related magmatism are obscure.Here,its emplacement age and petrogenesis were determined using mineralogy,zircon U-Pb dating,geochemistry,and Sr-Nd-Pb isotope studies.The zircon LA-ICP-MS U-Pb age of the aplite dyke is 16.66±0.21 Ma(n=14,MSWD=0.66),earlier than that of the ore-bearing porphyries(~15 Ma)in Jiama.Furthermore,the aplite exhibits high amounts of silicon(SiO_(2)=73.39%-74.74%),potassium(K_(2)O=5.12%-6.61%),aluminum(Al_(2)O_(3)=14.25%-14.69%),and light/heavy rare earth elements(LREE/HREE=12.12-16.19)as well as negative europium(δEu=0.47-0.72)and weak negative cerium anomalies(δCe=0.84-0.93).The aplite dyke is characteristic of metaluminous-peraluminous I-type granite,which is rich in large-ion lithophile elements(Rb,Ba,Th and U)and depleted in high-field-strength elements(Nb,P and Ti).The aplite dyke and ore-bearing porphyries in the Jiama deposit are the results of a partial melting of the juvenile lower crust,according to whole-rock geochemistry and Sr-Nd-Pb isotope data,but the dyke and ore-bearing porphyries were emplaced from the same magma chamber at different times.Thus,the aplite dyke shows the composition of the early evolution stage of shallow magma in the Jiama deposit and is the product of rapid condensation and crystallization.

Geochemistry, Monazite U–Pb Dating, and Li–Nd Isotopes of the Madi Rare Metal Granite in the Northeastern Part of the North China Craton

The Madi rare metal granite is a complex massif,which contains a variety of rare metals,such as Nb,Ta,Li,and Be.In this paper,the geochemical characteristics of the granite were obtained by multi-collector inductively coupled mass spectrometry(MC-ICP-MS).The precise crystalline age of the granite was obtained from monazite U-Pb dating,and the source of the granite was determined using Li-Nd isotopes.The Madi rare metal granite is a high-K(calc-alkaline),peraluminous,S-type granite.The U-Pb monazite age indicates that the crystalline age of the granite is 175.6 Ma,which is Early Jurassic.The granite is characterized by a relatively wide range ofδ7 Li values(+2.99‰to+5.83‰)and high lithium concentrations(181 ppm to 1022 ppm).The lithium isotopic composition of the granite does not significantly correlate with the degree of magmatic differentiation.An insignificant amount of lithium isotope fractionation occurred during the granitic differentiation.The lithium isotopic composition of the granite significantly differs from that of the wall rock,but it is very similar to that of a primitive mantle peridotite xenolith(meanδ7 Li value+3.5‰).The plot of Li concentration versusδ7 Li indicates that the Li isotopic composition of the granite is similar to that of island arc lavas.Based on the above-described evidence,the granite was mainly derived from the crust,but it was contaminated by a deep granitic magma.

Re-Os isotopic system and formation age of subcontinental lithosphere mantle

The determination of the formation age of subcontinental Lithosphere Mantle (SCLM) is a widely concerned issue in mantle geochemistry. it is difficult to obtain the formation age of SCLM using lithophile isotopic systems such as Rb-Sr, Sm-Nd, U-Th-Pb, ete., but as siderophile elements, the Re-Os isotopic system provides a powerful tool for that work. Here a comprehensive review on the recent development in Re-Os dating for SCLM has been given.

Early Cretaceous to Middle Eocene Magmatic Evolution of Eastern Pontides:Zircon U-Pb Ages and Hf Isotopes,and Geochemical and Sr-Nd Isotopic Constraints from Multiphase Granitoids,NE Turkey

The Eastern Pontides orogenic belt(EPOB)represents a significant segment of the AlpineHimalayan orogenic belt that evolved from the Paleozoic to Cenozoic periods.Here we report new zircon U-Pb ages,together with Lu-Hf isotopes,and whole-rock geochemical and Sr-Nd isotopic analyses of plutonic rocks from EPOB,northeastern Turkey.Our aim is to interpret magmatic evolution in which the granitoids formed.Zircon U-Pb dating of six samples yielded crystallization ages of~134,~82,~39 Ma,respectively.They show a wide range of87Sr/86Sr((i))(0.7039–0.7109),andεNd(t)values varying from-9 to+4.6,yielding model ages(TDM)from 520 to 1623 Ma,suggesting a heterogeneous magma source.Dated zircons show exlusively positiveεHf(t)values(+12.4 to+1.4),yielding model ages(TDM)from 352 to 1059 Ma,implying that they are most likely derived from a juvenile lower crust rather than the mature continental crust.In this study,we suggested that the northward subduction of the Neo-Tethyan oceanic slab began from the Early Cretaceous and resulted in the Late Cretaceous magmatism.Moreover,the Middle Eocene magmatism in the EPOB was related to the collision of the Anatolide Taurid Platform(ATP)with the Pontides.

Geochronology and Geochemistry of the Late Cretaceous to Paleocene Intrusions in East Gangdese,Lhasa,Tibet and Their Tectonic Significances

In this study, we determined the granite ages in the middle to east Gangdese batholith. Zircon ages from these granites are 57.6-68.7 Ma, indicating that intrusions were formed in the Late Cretaceous to early Paleocene. The large-ion lithophile elements are highly enriched, whereas some high-field-strength elements are depleted. The Sr-Nd-Pb isotopic characteristics are similar to those of the Dianzhong volcanics in the Linzhou Basin, indicating the same origin and tectonic environment. The samples show positive εHf（t） values that are slightly lower than the values for the Linzizong volcanics, the Quxu intrusion, and other intrusions in middle Gangdese. We conclude that our samples, the Linzizong volcanics, and most main-collisional intrusions are derived from the same source with different ratios of crust and mantle input. On the basis of geological, geochemical, geochronological, and isotopic information, we conclude that the Late Cretaceous to Paleogene evolution of Gangdese can be divided into three stages. First, the collision began at 70-60 Ma, the same time as rollback of the Tethys Ocean slab. Second, during 60-50 Ma, slab breakoff triggered upwelling of the asthenosphere. Third, after 50 Ma, the Tethys Ocean slab＇s effect disappeared and the interaction between Indian and Asian crusts began influencing magmatism in Gangdese.

Indosinian Tectonic Setting of the Southern Yidun Arc: Constraints from SHRIMP Zircon Chronology and Geochemistry of Dioritic Porphyries and Granites

A mass of granitoid and dioritic intrusions are distributed in the southern Yidun Arc, among which the representative Indosinian intrusions include the Dongco and Maxionggou granitoid intrusions in Daocheng County and hypabyssal intrusions intruding into arc volcanic rocks near the Xiangcheng town. The Dongco and Maxionggou granitoid intrusions consist mainly of porphyraceous monzogranites, megacryst monzogranites and aplite granites. The Xiangcheng hypabyssal intrusions are composed dominantly of dioritic porphyries. SHRIMP zircon ages of 224±3 Ma and 222±3 Ma have been obtained for the Dongco granitoid intrusion and the Xiangcheng dioritic porphyries, respectively. The Xiongcheng dioritic porphyries show a calc-alkaline geochemical feature, and are characterized by higher Sr/Y ratios, depletive Nb, Ta, P and Ti, enriched LILEs, and lower εNd （t） （=-3.27）, suggesting that they might be derived from mantle source magmas that were obviously contaminated by continent crustal materials. However, the Dongco and Maxionggou granitoids belong to high-potassium calcalkaline series with a per-metaluminous feature, and are characterized by higher CaO/（∑FeO＋MgO） and Al2O3/（∑FeO＋ MgO） ratios, lower （La/Yb）n and Sr/Y ratios, depletive Nb, Ta, Sr, P and Ti, enriched LILEs, and very low εNd （t） （=-8.10）, indicating that the granitoids might be derived from partial melting of continental crust materials mainly of graywacke. Petrogenesis of Dongco and Maxionggou granitoids implies that there was an oceanic crust between the Zongza continental block （ZCB） and western margin of the Yangtze Craton （WMYZC）. And the oceanic crust slab subducted westward during the Indosinian Epoch, producing an Andes-type continent marginal arc and a backarc basin at the WMSCC. Then the oceanic basin closed and a sinistrally lateral collision occurred at ca. 224 Ma-222 Ma between the ZCB and the WMYZC, causing partial melting of sediments in the back-arc basin to generate granitoid magmas of the Dongco and Maxionggou intrusions.

Genesis of the Huoshenmiao Mo deposit in the Luanchuan ore district, China: Constraints from geochronology, ?uid inclusion, and H-O-Sisotopes

The Huoshenmiao δeposit is Mo skarn δeposit, located in the western part of the Luanchuan ore δistrict.Mineralization process can be δivided into a skarn and a quartz-sulfide episodes with six stages: prograde(I), retrograde(II), quartz-K-feldspar(III), quartz-molybdenite(IV), quartz-pyrite(V), and quartzcalcite(VI). A combined study of geochronology, fluid inclusion(FI), and stable isotopes was conducted to constrain the mineralization age, source of ore materials, as well as the origin and evolution of the ore-forming fluids. Molybdenite Ree Os δating indicates that the δeposit was formed in the Late Jurassic(~145 Ma). The δ^(34)S values of sulfides range from 3.0‰ to 7.1‰, implying that the ore materials in the δeposit are magmatic in origin. Three types and six subtypes of FIs are δistinguished, namely, aqueous two-phase(W_1-and W_2-type), δaughter mineral-bearing multiphase(S_1-and S_2-type), and CO_2-bearing three-phase(C_1-and C_2-type). In stages I and II, the W_1-type FIs δisplay homogenization temperatures(Th) from 496°C to >600°C, with salinities of 14.9-18.3 wt.% NaCl eqv. The FIs in stages III, IV and early stage V composed of coeval S-, C-and W-types, respectively homogenize at similar Th, suggesting the occurrence of boiling. The W1-type FIs in late stage V and stage VI, yield Th of 102-406°C and salinities of 0-4.7 wt.% NaCl eqv. The δD_(H_2O)and δ^(18) O(H_2O)values of the ore-forming fluids in quartz-sulfide episode vary from-112‰ to-76‰, and 11.0‰ to 1.0‰, respectively. All these above observations reveal that the early ore-forming fluids are magmatic in origin, and characterized by high temperature and moderate to high salinity, and gradually evolve to low temperature, low salinity meteoric water. The Huoshenmiao Mo δeposit is associated with the magmatism event induced by the protracted subduction of the Izanagi plate beneath the eastern China continent. The δecrease in temperature, salinity and f(O_2), as well as change of p H δue to boiling and fluid-rock interaction, are the main factors controlling Mo δeposition.

Metallogenesis and ore-forming time of the Changtuxili Mn-Ag-Pb-Zn deposit in Inner Mongolia:Evidence from C-O-S isotopes and U-Pb geochronology

This paper reports new geochronological(U-Pb) and isotope(C,O,and S) data to investigate the timing of mineralization and mode of ore genesis for the recently discovered Changtuxili Mn-Ag-Pb-Zn deposit,located on the western slopes of the southern Great Hinggan Range in NE China.The mineralization is hosted by intermediate-acidic lavas and pyroclastic rocks of the Baiyingaolao Formation.Three stages of mineralization are identified:quartz-pyrite(Stage I),galena-sphalerite-tetrahedrite-rhodochrosite(Stage II),and quartz-pyrite(Stage Ⅲ).δ13C and δ18O values for carbonate from the ore vary from-8.51‰ to-4.96‰ and 3.97‰ to 15.90‰,respectively,which are indicative of a low-temperature alteration environment.δ34SV-CDT values of sulfides range from-1.77‰ to 4.16‰ and show a trend of equilibrium fractionation(δ34SPy>δ34SSp>δ34SGn).These features indicate that pyrite,sphalerite,and galena precipitated during the period of mineralization.The alteration mineral assemblage and isotope data indicate that the weakly acidic to weakly alkaline ore-forming fluid was derived largely from meteoric water and the ore-forming elements C and S originated from magma.During the mineralization,a geochemical barrier was formed by changes in the pH of the ore-forming fluid,leading to the precipitation of rhodochrosite.On the basis of the mineralization characteristics,new isotope data,and comparison with adjacent deposits,we propose that the Changtuxili Mn-Ag-Pb-Zn deposit is an intermediate-to lowsulfidation epithermal deposit whose formation was controlled by fractures and variability in the pH of the oreforming fluid.The surrounding volcanic rocks yield zircon U-Pb ages of 160-146 Ma(Late Jurassic),indicating that the mineralization is younger than 146 Ma.

SHRIMP U-Pb zircon age,geochemistry and Nd isotope of the Guandaoshan pluton in SW Sichuan:Petrogenesis and tectonic significance

SHRIMP U-Pb zircon age, geochemical and Nd isotopic data are reported for the Neoproterozoic Guandaoshan pluton in the Yanbian region, SW Sichuan. This pluton is of typical I-type granite and emplaced at (857±13) Ma. Geochemical and Nd isotopic characters suggest that the pluton was generated by partial melting of pre-existing, young (late Mesoproterozoic to early Neoproterozoic) low-K tholeiitic protolith within an intraplate anorogenic setting. The Guandaoshan pluton probably records the earliest magmatism induced by the proposed ca. 860-750 Ma mantle superplume beneath the supercontinent Rodinia.

Xiba Granitic Pluton in the Qinling Orogenic Belt, Central China: Its Petrogenesis and Tectonic Implications

Xiba granitic pluton is located in South Qinling tectonic domain of the Qinling orogenic belt and consists mainly of granodiorite and monzogranite with significant number of microgranular quartz dioritic enclaves. SHRIMP zircon U-Pb isotopic dating reveals that the quartz dioritic enclaves formed at 214±3 Ma, which is similar to the age of their host monzogranite （218±1 Ma）. The granitoids belong to high-K calc-alkaline series, and are characterized by enriched LILEs relative to HFSEs with negative Nb, Ta and Ti anomalies, and right-declined REE patterns with （La/Yb） N ratios ranging from 15.83 to 26.47 and δEu values from 0.78 to 1.22 （mean= 0.97）. Most of these samples from Xiba granitic pluton exhibit εNd（t） values of 8.79 to 5.38, depleted mantle Nd model ages （T DM ） between 1.1 Ga and 1.7 Ga, and initial Sr isotopic ratios （ 87 Sr/ 86 Sr） i from 0.7061 to 0.7082, indicating a possible Meso-to Paleoproterozoic lower crust source region, with exception of samples XB01-2-1 and XB10-1 displaying higher （ 87 Sr/ 86 Sr） i values of 0.779 and 0.735, respectively, which suggests a contamination of the upper crustal materials. Quartz dioritic enclaves are interpreted as the result of rapid crystallization fractionation during the parent magmatic emplacement, as evidenced by similar age, texture, geochemical, and Sr-Nd isotopic features with their host rocks. Characteristics of the petrological and geochemical data reveal that the parent magma of Xiba granitoids was produced by a magma mingling process. The upwelling asthenosphere caused a high heat flow and the mafic magma was underplated into the bottom of the lower continent crust, which caused the partial melting of the lower continent crustal materials. This geodynamic process generated the mixing parent magma between mafic magma from depleted mantle and felsic magma derived from the lower continent crust. Integrated petrogenesis and tectonic discrimination with regional tectonic evolution of the Qinling orogen, it is suggested that the granitoids are most likely products in a post-collision tectonic setting.

Re-Os Dating of the Suoerkuduke Cu (Mo) Deposit,Fuyun County,Xinjiang,and Its Geodynamic Implications

The Suoerkuduke Cu （Mo） deposit, in the same metallogenic belt with Xilekuduke Cu-Mo deposit, is located in the Armantai island arc belt on the northern margin of East Junggar, Northwest China. Rhenium and osmium isotopic analysis of seven molybdenite samples from the deposit was used to determine the age of mineralization. A seven-point isochron age of 317.7±7.6 Ma, which is consistent, within analytical error, with the average model age of 323.3±1.9 Ma indicates that this deposit was formed at transitional period between the Early and Late Carboniferous. This age is obviously later than that of the Lower Devonian Tuoranggekuduke Formation acting as the wall rock but contemporaneous with the early stage of plutonism （330-268 Ma） in East Junggar. Based on the characteristics of mineralization age and tectonic setting of many typical deposits on the northern margin of East Junggar, we proposed that the mineralization age of Suoerkuduke deposit is a key period of East Junggar for the tectonic regime transforming from compression to extension and also the time when granitoids and deposits widely spread in this area. Mineralization and formation of skarn in this deposit are closely related to felsic-intermediate magmatism. The distinct characteristic from typical skarn-type deposits is that the metallogenic parent intrusion is a huge batholith in the depth. The Cu （Mo） mineralization, moyitc, and granite porphyry are all derived products of the batholith.

REE and Sr-Nd Isotope Geochemistry for Yixian Fluorite Deposit,Western Liaoning Province,China,and Its Geological Implications

Up to now, there were no systematic studies of geochemistry and isotopic age for the Yixian （义县） fluorite deposit, western Liaoning （辽宁） Province, China. Based on the analysis of metallogenic geological setting, we studied the REE, Rb-Sr and Sm-Nd isotopes. The chondrite-normalized REE patterns of fluorite are characterized by moderate LREE depletion （LREE/HREE=0.95-3.57, （La/Yb）N=0.08-2.84） and enrichment of Sr （146×10^-6-596×10^-6） and moderately positive Eu anomalies （δEu=1.10-1.34）, which are similar to those of the host Mesoproterozoic carbonate rocks. The fluorite display （^87Sr/^86Sr）t=-0.708 5, （^143Nd/^144Nd）t=-0.511 785, and δNd（t）=-12.8, which are similar to those of the host Mesoproterozoic carbonate rocks and volcanic rocks of Middle Jurassic Lanqi （蓝旗） Formation. The REE and Sr-Nd isotope geochemistry suggest that the source of the ore-forming material may be the volcanic rocks of Lanqi Formation and host carbonate rocks. The Sm-Nd isochron age of 154±14 Ma （MSWD=0.23） indicates that the Yixian fluorite mineralization nearly corresponds to the period of Lanqi Formation. Based on the integrated geological and geochemical studies, coupled with previous studies, we suggest that Yixian fluorite deposit formed in the extension setting of postcoilisional stage and may be attributed to the partial melting of ancient basaltic rocks in the lower crust induced by underplating of basic magma and to the reaction between the F-rich ore-forming fluids and the host carbonate rocks.

Piaoac Granites Related W-Sn Mineralization, Northern Vietnam: Evidences from Geochemistry, Zircon Geochronology and Hf Isotopes

Piaoac granites exposed in the Cao Bang region, northern Vietnam, are S-type granite, which are associated with W-Sn-Mo-Be-F mineralization. Zircon U-Pb ages, major and trace elements, mineral chemical and Hf isotopic compositions of the W-Sn-bearing granites from the Piaoac District have been investigated in detail. LA-ICP-MS U-Pb dating of zircon grains from these granites yielded ages of 82.5±2.3 and 82±1.8 Ma, representing an episode of Late Cretaceous magmatic event. These granites are characterized by high peraluminous and have typical S-type geochemical signatures with high SiO_2(72.37 wt.%–73.07 wt.%), high A/CNK values(1.61–1.65) and Al_2O_3(14.4 wt.%–15 wt.%). They are enriched in Rb, U, K, Th, Ta and Pb and display pronounced negative Ba, Sr, Nb, Ti and Eu(Eu/Eu*=0.19–0.24) anomalies. The high degree of fractional crystallization is characterized by low Rb, Sr, Ba and Eu concentrations with high ratios of La/Sm and Eu/Eu*. Zircon grains show εHf(t) values from-9.69 to-0.9 and the corresponding TDM2 range from 1.2 to 1.7 Ga, indicating that these granites could be derived from the Proterozoic basement rocks with minor input from mantle-derived magmas. The calculation of Fe^(3+) and Fe^(2+) of biotites indicates a low oxygen fugacity condition(log fO_2 ranging from 10-17 to 10-18 bars, below MH), which is favorable for the W-Sn mineralization. Tungsten and tin have been enriched in granitic magmas through fractionation, and low oxygen fugacity conditions have promoted the accumulation and transportation of W-Sn in the hydrothermal fluids, leading to deposition of mineral phases. The geochemical data suggest that Piaoac granites formed in an extensional setting related with the Late Cretaceous magmatism occurring large-scale lithospheric extensional in South China Block.

Re-Os Geochronology of the Liuchapo Formation across the Ediacaran-Cambrian Boundary of the Yangtze Block(South China)

The Ediacaran-Cambrian(E-C) succession in South China records remarkable oceanic,biological and geochemical variations,but it was not well defined geochronologically,which hinders the interpretation of the spatio-temporal seawater chemical architecture during the time E-C interval.This study presents two Re-Os isochron ages of 520.2±6.1 and 561.7±8.5 Ma for the barite-rich black shales from the top Liuchapo and Doushantuo formations respectively in Tianzhu County,Guizhou Province.In combination with existing age data,the two new Re-Os isochron ages suggest that the Liuchapo Formation was deposited between 550 and 520 Ma.Moreover,like the polymetallic Ni-MoPGE layers of shelf margin(or platform) facies and V-rich horizons of transitional(or shelf slope) to deep-water facies,the barite deposits were likely formed due to differential mineralization.The timing offset likely resulted from differential elemental concentration related to certain local factors(i.e.,hydrothermal fluids,seawater redox and biological activity).The isochron-derived initial ^(187)Os/^(188)Os ratios of the top Liuchapo Formation(0.902 ± 0.048) and the Doushantuo Formation(0.740 ± 0.042) fall within the range of continental weathering flux(1.54) and oceanic crust(0.126),implying the involvement of marine hydrothermal fluids.Moreover,their difference of initial ^(187)Os/^(188)Os ratios may reflect variations of continental weathering intensity and uplift magnitude.

Origin of the Oligocene Tuolangla porphyry-skarn Cu-W-Mo deposit in Lhasa terrane,southern Tibet

Although some porphyry-skarn deposits occur in post-collisional extensional settings,the post-collisional deposits remain poorly understood.Here the authors describe the igneous geology,and mineralization history of Tuolangla,a newly-discovered porphyry-skarn Cu-W-Mo deposit in southern Tibet that belongs to the post-collisional class.The deposit is associated with Lower Cretaceous Bima Formation.It was intruded by granodiorite porphyry intrusions at about 23.1 Ma.Field investigation indicated that mineralization is spatially and temporally associated with granodiorite porphyry.Molybdenite yielded a Re-Os weighted mean age of 23.5±0.3 Ma and is considered to represent the age of skarn mineralization at the deposit.Theδ^34S values of sulfides,concentrated in a range between 0.6‰to 3.4‰,show that the sulfur has a homogeneous source with characteristics of magmatic sulfur.The Pb isotopic compositions of sulfides indicate that ore-forming metal materials were derived from the mantle and ancient crust.The granodiorite porphyry displays high SiO2(68.78%–69.75%)and K2O(3.40%–3.56%)contents,and relatively lower Cr(2.4×10^-6–4.09×10^-6),Ni(2.79×10^-6–3.58×10^-6)contents,and positiveεHf(t)values(7.7–12.9)indicating that the mineralization porphyry was derived from the partial melting of juvenile lower crust.The Tuolangla deposit is located in the central part of Zedang terrane.This terrane was once considered an ancient terrane.This terrane is in tectonic contact with Cretaceous ophiolitic rocks to its south and Mesozoic continental margin arc volcanics and intrusions of the Gangdese batholith of the Lhasa terrane to its north.Thus,the authors proposed that the Oligocene porphyry skarn Cu-W-Mo mineralization is probably associated with the Zedang terrane.This finding may clarify why the Oligocene(about 23 Ma)deposits are found only in the Zedang area and why mineralization types of the Oligocene mineralization are considerably different from those of the Miocene(17–14 Ma)mineralization.

Contrasting Peridotite Types in the Dabie UHP Belt, Eastern China: the Raobazhai and Bixiling Ultramafic-mafic Complex

The Dabie UHP metamorphic belt, central China,contains two contrasting types of mafic-ultramafic complex. The Bixiling peridotite in the southern Dabie terrane contains abundant garnet (21.1-32.2 vol% )and thus has high CaO + Al2O3 (9.81-15.9 wt% ).……