Rb-Sr and Sm-Nd Isochron Ages of the Dongmozhazhua and Mohailaheng Pb-Zn Ore Deposits in the Yushu area, southern Qinghai and Their Geological Implications

Located on the northeast margin of the Qiangtang terrane between the Jinshajiang suture zone and Bangonghu-Nujiang suture zone,the Dongmozhazhua and Mohailaheng Pb-Zn deposits in the Yushu area of Qinghai Province are representative Pb-Zn deposits of the Pb-Zn-Cu polymetallic mineralization belt in the northern part of the Nujiang-Lancangjiang-Jinshajiang area,which are in the front belt of the Yushu thrust nappe system.The formed environments of these two deposits are different from those of sediment-hosted base metal deposits elsewhere in the world.The authors hold that they were formed during the Indian-Asian continental collision and developed within the fold-thrust belt combined with thrust and strike-slip-related Cenozoic basins in the interior of the collisional zone.Studying on the metallogenic epochs of these two deposits is helpful to the understanding of ore-forming regularity of the regional Pb-Zn-Cu mineralization belt and also to the search for new deposits in this region.The age of the Dongmozhazhua deposit has been determined by the Rb-Sr isochron method for sphalerite residues,whereas the age of the Mohailaheng deposit has been determined by the Rb-Sr isochron method for sphalerite residues and the Sm-Nd isochron method for fluorite.The age of the Dongmozhazhua deposit is 35.0-±0.0 Ma （（87Sr/86Sr）0=0.708807） for sphalerite residues.The age of the Mohailaheng deposit is 32.2±0.4 Ma （（87Sr/86Sr）o=0.708514） for sphalerite residues and 31.8±0.3 Ma （（143Nd/144Nd）o=0.512362） for fluorite with an average of 32.0 Ma.Together with the regional geological setting during mineralization,a possible tectonic model for metallogeny of the Dongmozhazhua and Mohailaheng Pb-Zn deposits has been established.These two ages are close to the ages of the Pb-Zn deposits in the Lanping and Tuotuohe basins,indicating that it is possible that the narrow 1000-kilometer-long belt controlled by a thrust nappe system on the eastern and northern margins of the Tibetan plateau could be a giant Pb-Zn mineralized belt.

Sm-Nd ISOTOPIC AGE OF LAMPRO PHYRES IN THE GEZHEN GOLD BEARING SHEAR ZONE ON HAINAN ISLAND AND ITS GEOLOGICAL IMPLICATIONS

Sm Nd isotopic compositions of eight lamprophyre samples, which come from the Gezhen gold bearing shear zone on western Hainan Island, are measured. The Sm Nd isochron age is 495.98±13.14 Ma, (143Nd/144Nd)0=0.512094, εNd(t) ranges from +1.80 to +2.00 and TDM from 982 Ma to 1196 Ma (average: 1060 Ma). The authors point out that the whole rock Sm Nd isochron age (495.98±13.14 Ma) really represents the petrogenetic age of lamprophyre and the time of magmatism during subsequent subduction.

The Sm-Nd Geochronology of the High Pressure Basic Granulite in Laixi, Eastern Shandong

A high pressure basic granulite ultramafic rock belt, which is about 200km long and trends toward NE, occurs in the late Archaean orthogneiss in Laixi Laiyang Qixia area, eastern Shangdong. This belt is located east to the Su Lu UHPM terrane and west to the Archaean rocks of the North China craton, therefore, its geological situation and implication for collision between the North China plate and the Yangtze plate is very important. The analyzed sample of high pressure basic granulite has decreasing pressure metamorphic texture. The mineral assemblage of retrograde metamorphism is of granulite facies. The sample yields an internal mineral whole rock Sm Nd isochron age of 1725Ma. The T(DM) age of whole rock is 2788Ma. These data are very similar to those of high pressure basic granulites in Northern China craton. Considering the petrological and geochemical characteristics and isotopic ages, the high pressure granulite ultramafic rock belt in eastern Shandong is suggested to belong to the North China craton and to be of the early Precambrian lower crust. Their lifting from the lower crustal level is related to collision between the North China plate and the Yangtze plate.

Sm-Nd isotope dating of hydrothermal calcites from the Xikuangshan antimony deposit,Central Hunan

The research on Samarium-Neodymium isotope systematics of hydrothermal calcites from the Xi-kuangshan antimony deposit, Central Hunan, places precise timing constraints on the Sb mineralization in this area. It is revealed that the Xikuangshan deposit formed during the late Jurassic-early Cretaceous Period, the early- and late-stage mineralization took place at (155.5±1.1) Ma and (124.1±3.7) Ma, respectively. The accurate age determination of mineralization is very crucial for revealing the super-enrichment mechanism of the element Sb at the Xikuangshan mine, and lays some foundations for the further understandings of its ore genesis and mineralization mechanism.

Sm-Nd Geochronology,REE and C and O Isotope Study of Calcites and Stibnites from Banian Antimony Deposit

The Banian sediment-hosted antimony deposit is a medium-scale deposit located in the Dushan County,South Guizhou,China.Calcite as the dominant gangue mineral can be divided into two types: white and pink,the later is more intimate with mineralization.It is interesting that the REE characteristics are

Sm-Nd isochron age of ophiolite along northern margin of Altun Tagh Mountain and its tectonic significance

An ophiolite belt is exposed to the northern edge of Altun Tagh Mountain. Geochronological researches were made on gabbro and basalt. Sm-Nd isochron age of gabbro is (829 ± 60) Ma, while the age of gabbro mixed basalt is (949 ± 62) Ma. The dating of Sm-Nd isochron proves that ophiolite formed in (829 ± 60) Ma, which implies that the northern half of Tarim (or north Tarim Block) had been separated by an ocean from the southern half of Tarim and Qaidam (or south Tarim Block) until (829 ± 60) Ma. The south Tarim Block could accrete to the north Tarim Block at the beginning of Sinian,thereby forming the north Altun Tagh suture. The Sinian system would be the first cover on the amalgamated Tarim craton.

Sm-Nd ISOCHRONOUS AGE OF FUCHUAN OPHIOLITE SUITE IN SHEXIAN COUNTY,ANHUI PROVINCE AND ITS GEOLOGICAL SIGNIFICANCE

The ophiolite suite in Shexian County, Anhui Province extends more than 40 km in NE orientation. It is called Fuchuan ophiolite because its outcrop is the most complete in Fuchuan. This ophiolite which upthrusts over the Precambrian Shexian granodiorite body is composed of dunite, harzburgite, cumulate pyroxenite, spilite, keratophyre and tuffaceous phyllite (Fig. 1).

Sm-Nd isochron age evidence for Precambrian metamorphic rocks from the eastern Junggar in northern Xinjiang, China

The characterstic of the basement of Junggar block has been a topic of long-standingdebate.Based on the data of gravity and magnetic anomaly,some researchers deduced thatthere were Precambrian basic-ultrabasic complex on the basement of Junggar block.However,others maintained that the basement of Junggar block was probably the Paleozoicoceanic crust because of the lack of isotopic evidence and older outcrops aroundthe Junggar Basin. In recent years,molecules of fossil community were discovered from Aleandao Groupat the Qingshuiquan and Kamuster of eastern Junggar.Most of the fossils,such asPylocorpus,Retincnlafopora,Asteropyla,were found at the boundary between Cambrianand Sinian in Qinling and South China.An isotopic age of 1908 Ma for leuco-pink graniticgneiss from the lower Huangcaopo Group was gained by evaporated Pb-isotope method forsingle zircon in Xiaoluigou of eastern Junggar by Zhang et al.But all of these results onlyindicate a possibility that the basement of Junggar block consists of

THE MINEROGENETIC AGES OF Sm-Nd AND Rb-Sr OF YULI SKARN POLYMETALLIC ORE DEPOSIT IN THE SUZHOU REGION AND ITS GEOLOGICAL SIGNIFICANCE

Using the Sm-Nd and Rb-Sr isotopic dating method, this note makes an analysis of Yuli Pb, Zn and Ag deposits that lie in the west of Suzhou region. The minerogenetic age of Yuli skarn polymetallic ore deposit and its source materials are studied on the basis of detailed geological and geochemical studies.

Sm-Nd isotopic ages of Tiqiushan eclogites in Xinyi region and its geological significance

GEOLOGICAL scientists have paid much attention to the suture zones formed by collision between the Sino-Korean and Yangtze blocks because of the discovery of the ultrahigh pressue metamorphic (UHPM) rocksfrom northern Jiangsu Province (Su), southern Shandong Province (Lu) and the Dabie Mountains.Although the age of 200-240 Ma is probably the best estimation for UHPM metamorphism in the DabieMountains, the primary intrusive and metamorphic ages of the complex from northern Jiangsu Provincehave not been determined before. Using the Sm-Nd isotopic dating method, our study is aimed at datingthe UHPM metanorphic event of northern Jiangsu Province and researching a probable relation between theUHPM rocks from the Dabie Mountains and the UHPM rocks from northern Jiangsu Province.

The evidence of Sm-Nd isochron age for the early Paleozoic ophiolite in Mangya area,Altun Mountains

The REE patterns of the basic volcanic rocks in Mangya area, Altun, are slight rich in LREE with (La/Ya)-N=1.69-3.20, (La/Sm)-N=1.37-1.87, other trace element ratios of the rocks are Th/Ta≈1 (for a few samples greater than 1.5), Nb/Y=0.34-0.62, Ti/Y=310-443 (on the average: 381), Ti/V=37-62, Zr/Nb=9.4-12.4, Sr/Rb=12-80 (on the average: 37), and Nb/Th=7.7-16.8. These features are similar to that of E-MORE or OIB. The ε Nd(t) value, being 3.95- 4.12, shows that the source of the volcanic rocks is derived from depleted asthenosphere mantle mixed with materials from enriched mantle. These, together with the information of geological setting and rock assemblages, indicate that the basic volcanic rocks are of ophiolite. The Sm-Nd isotope ages for the eight basic volcanic rock samples construct a straight line with good correlation, and the calculated isochron age is (481.3±53) Ma. Besides, the eight calculated ε Nd(t) and model ages are close to each other, which suggests that they are homologous, so the isochron is not a mixed line. In the meantime, the isochron age ((481.3±53) Ma) is lower than the model ages (T DM=1 004-1 534 Ma) of the samples, suggeting that the isochron age represents the formation age of the basic volcanic rocks and the ophiolite belt in Mangya area, Altun is formed in the early Paleozoic (Cambrian-Ordovician). In spite of the greater uncertainty of the age, it is still reliable because it is consistent with the age constrained by the regional strata.

Sm-Nd and zircon SHRIMP U-Pb dating of Huilanshan mafic granulite in the Dabie Mountains and its zircon trace element geochemistry

The mafic granulites from Huilanshan are outcropped on the center of the Luotian dome in the northern Dabie Mountains. The Sm-Nd isochron defined by granulite-facies meta-morphic minerals (garnet + clinopyroxene + hypersthene) yields an age of 136 ± 18 Ma indicating the early Cretaceous granulite-facies metamorphism. The cathodoluminescence (CL) images of zircons from the granulite show clearly core-mantle-rim structures. The zircon cores are charac-terized by typical oscillatory zoning and highly HREE enriched patterns, which suggests their magma origin. Some zircon cores among them with little Pb loss give SHRIMP U-Pb ages rang-ing from 753 to 780 Ma, which suggests that the protolith of Huilanshan granulite is Neopro-terozoic mafic rocks. The zircon mantles usually cut across the oscillatory zone of the zircon cores have 3―10 times lower REE, Th, U, Y, Nb and Ta contents than the igneous zircon cores but have high common Pb contents. These characteristics suggest that they were formed by hydrothermal alteration of the igneous zircons. The part of zircon mantles with little Pb loss give a similar SHRIMP U-Pb age (716―780 Ma) to the igneous zircon cores, which implies that the hydrothermal events occurred closely to the magmatic emplacement. In view of the strong early Cretaceous magmatism in the Luotian dome, consequently, the Huilanshan mafic granulite was formed by heating of the Neoproterozoic mafic rocks in mid-low crust, which caused the granu-lite-facies metamorphism underneath the Dabie Mountains. The similarity between the granulite metamorphic age (136 ± 18 Ma) defined by Sm-Nd isochron and K-Ar age of 123―127 Ma given by amphible from the gneiss in Luotian dome suggests a rapid uplifting of the Luotian dome, which may result in further exhumation of the ultrahigh pressure metamorphic rocks in the Dabie Mountains.

Diagenetic-metallogenic ages of pyritic cherts and their implications in Mojiang nickel-gold deposit in Yunnan Province,China

Diagenetic-metallogenic ages of pyritic cherts formed by the syn-sedimentation of hydrothermal vent and ages of the Jinchang Rock Formation in the Mojiang large nickel-gold deposit in the Ailaoshan gold metallogenic belt have been discussed on the basis of chronology of isotopic geochemistry. Nickel-gold-bearing pyritic cherts in the mining were formed by syn-sedimentation of hydrothermal vent in the Late Devonian, i.e. age by Sm-Nd isochronal method (t) = (358±8.6) (2σ) Ma and age by Rb-Sr isochronal method (t) = (354.7±0.72) (2σ) Ma. On the other hand, deep-water cherts from the Jinchang Rock Formation of the Upper Devonian in the area were initiated at the same time; that is, age by Sm-Nd isochronal method (t) = (359(+21) (2(7) Ma and age by Rb-Sr isochronal method (t) = (358.02±0.30) (2σ) Ma.

On emplacement ages of lam-proites in Zhenyuan County,Guizhou Province,China

Emplacement ages of lamproites that comprise lamproite and olivine lamproite in Zhenyuan County, Guizhou Province, China, have been constrained by the whole-rock Sm-Nd and Rb-Sr isochron methods and the whole rock K-Ar method. Intrusive activities of lamproites in the Sinantang area of Zhenyuan County, Guizhou Province, took place during the Late Cambrian, as indicated by the Sm-Nd isochron ages (t) = (503±17) (2σ) Ma and by the Rb-Sr isochron ages (t) = (501.2±4.6) (2σ) Ma. Intrusive activities of olivine lamproites at the Maping with Ⅱ -type diamond and Xitou in Zhenyuan County, Guizhou Province, took place from the Late Cambrian to the Early Ordovician, as evidenced by the Sm-Nd isochron ages (t) = (502±27) (2σ) Ma and by the Rb-Sr isochron age (t) = (502± 27) (2σ) Ma. This suggests that the upwelling hot materials derived from the deep mantle were emplaced from the Late Cambrian to the Early Ordovician (503-497 Ma), while the time terminal of cooling of the geothermal event of

Sm-Nd isotope study and dating on Honguleleng ophiolite in Xinjiang, China

A study of the Sm-Nd isotope composition of Honguleleng ophiolite in Xinjiang, China, isochron age ((626±25)Ma),εNd(t) = 8.40,and its model age (TNdDM) in the range of 1 180-1 208 Ma, indicates that the ophiolite was formed in the late stage of the Upper Sinian. In combination with its facies association, petrochemistry, REE patterns, trace elements

Sm-Nd DATING OF THE JINCHUAN ULTRAMAFIC ROCK BODY, GANSU, CHINA

The Jinchuan ultramafic rock body mainly consists of lherzolites and a small amount of dunites and websterites. The largest Cu-Ni sulphide ore in China occurs in the rock body. Its original age remains equivocal now because the available age data are in disorder. This note reports the dating result obtained with Sm-Nd internal isochron method.

Sm-Nd Isotopic Dating of Gabbro in Yejili, Neimenggu (Inner Mongolia)

The gabbro in Yejili, Neimenggu, is located about 100 km northeast of the largest Cu-Ni sulphide ore in Jinchuan, Gansu, which is on the upwarped district of margin of Alashan Platform southwest of Sino-Korean Block. The gabbro intruded into the Precambrian contact zone of plagiogranite, moyite and pladorite. Its strike on the surface is along the lines of 45° northeast, about 3.6 km in length and 2.1 km in width. Several small ore veins of ilmenite and magnetite have been discovered in

Geochronology and Nd and Pb isotope characteristics of gabbro dikes in the Luobusha ophiolite, Tibet

We report a combined internal and whole-rock Sm-Nd isochron age, and Nd and Pb isotopic data for gabbro dikes of the Luobusha ophiolite in Tibet. The Sm-Nd isochron of data for two whole rocks and plagioclase and clinopyroxene separates from one of the rocks yields a Middle Jurassic age of (177±31) Ma (with an initial εNd(t) = +8), which provides a significant bound on the time of formation of the Luobusha ophiolite. The Nd and Pb isotopic charac-teristics of the dike indicate an Indian-Ocean-type isotopic affinity, and we conclude that the Luobusha ophiolite formed in an oceanic setting during the Middle Jurassic.

DISCOVERY OF 3.3 Ga ARCHAEAN ROCKS IN NORTH TARiM BLOCK OF XINJIANG, WESTERN CHINA

Geologists have been paying much attention to the characteristics and age of the basement of the Tarim Block. Recently, some isotopic ages for the Precambrian rocks in the margin areas of the Tarim Block have been obtained. The oldest age presented by the above study is 2487±5.1 Ma determined with evaporated Pb-isotope method for zircons of the blue quartz-bearing granite from the Kuruktag area. However, the possibility of the Archaean basement of Tarim Block was only deduced. In this study the age of 3.3 Ga

Discovery of the Early Proterozoic Basement in the Middle-Lower Reaches of Yangtze River and Its Significance

The middle-lower reaches of the Yangtze River are a famous copper-ironmetallogenic belt, but many vital basic geologic problems about lithogenesis andmineralization are hard to be solved, since the metamorphic basement has not beendiscovered. Although Dongling Group metamorphic rock series outcrops in Huaining,Anhui Province, and Huangxu System metamorphic rock series outcrops in NingzhenMountain in Jiangsu Province, they are generally considered to be of the Sinian, dueto their low metamorphism and the lack of convincible isotopic age data.