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.

Discussion on Rb-Sr Isochron Ages of Granitoids in the Hengduan Mountains

Based on the Rb-Sr isotopic study of the granitoids in the Hengduan Mountains, the classification andgeologic significance of whole-rock Rb-Sr isochrons as well as the factors influencing homogenization of theisotopic systems are discussed. Usually. there is no good linear correlation of isochrons for diorites andalkali-rich intrusive rocks (including alkaline rock and alkalic granite). But by means of the external isochron ofmonominerals from the rocks. isochrons with good linear correlation as well as ages and Sr initial ratios with ahigh confidence can be obtained. In order to obtain a satisfactory isochron, the discriminant parameters shouldbe calculated from the Rb/Sr ratios and estimated ages of samples. Only those that meet the requirements canbe used as the Rb-Sr isochrons. The quality of an isochron should be judged from three factors, i.e. correlationcoefficient. MSWD and homogenization degree.

Dating Model of Illite Grain Rb-Sr Isochron and Its Application to Precambrian Stratigraphy

This paper discusses the Rb-Sr geochemical behaviour of illites in the sedimentary process, and points out that illites of the lmd and lm types could reach Sr-isotopic equilibrium with its environmental medium during the sedimentary process and initial stage of diagenesis, and illite grains of different sizes have various Rb contents. A new dating method for sedimentary rocks----a dating model of illite grain Rb-Sr isochron----is established, in consideration of the geochemical process of Sr-isotopic homogenization and differentiation in the course of their formation and evolution. In addition, the paper concerns the application of the dating model to the Precambrian unmetamorphosed sedimentary rocks in western Henan Province, which gives satisfactory results coincident with the geological facts.

Quantification of radiation damage in natural and synthetic zircon by Raman spectroscopy:application to low-temperature thermochronology

Due to its ubiquitous occurrence in igneous,metamorphic,and sedimentary rocks and its wide application in geochronology and geochemistry,zircon has become the most widely used accessory mineral in the geological community.Nevertheless,the decay of U and Th causes radiation damage to the zircon structure,resulting in various degrees of metamictization,which can affect the accuracy of U–Pb dates and Hf and O isotope results.If the degree of zircon radiation damage can be quantified,the influence on geochemical analyses can be evaluated,and the results can be corrected more precisely.In this paper,synthetic and natural zircon crystals with different crystallization ages were selected for Raman spectroscopy analysis,cathodoluminescence imaging,and determination of the U and Th concentrations.The results show that Raman FWHM(full width at half bandmaximum)and Raman shift correlate with alpha dose(Da)ofzirconsfollowingtheseequations,FWHM=44.36(±2.32)×[1-exp(-2.74×Da)]-+1.7(±0.19),Raman Shift=-6.53×Da+1007.69.Analysis of synthetic zircon crystals shows that doped REEs(rare earth elements and P)can also lead to an increase in the FWHM.However,this effect can be ignored for natural zircon samples with REE contents at a normal level of hundreds to a few thousand ppm.The FWHM and Raman shift can be used as proxies to measure the degree of zircon radiation damage.Using the updated equations to calculate the latest age when zircon began to accumulate radiation damage,a more accurate and more meaningful“radiation damage age”can be obtained.

Age and Tectonic Significance of the Mafic Dyke Swarm in the Kuruktag Region, Xinjiang

The mafic dykes are composed of a series of northwest-trending diabase in Kuokesu of the Kuruktag region, Xinjiang. Four whole-rock samples of the mafic dykes in the region give an 40Ar/36Ar–40K/36Ar isochron age of 282 Ma with the initial 40Ar/36Ar = 507. The characteristics of the major, trace and rare earth elements of the dyke rocks indicate that they belong to the calc-alkaline series and were emplaced in extensional structures at an intermediate depth of the crust. This shows that there was a large-scale extension in the Permian Period, which might be associated with the post-collisional extension of the ancient Tianshan Mountains.

Re–Os geochronology of the Cambrian stage-2 and-3 boundary in Zhijin County, Guizhou Province, China

The black shale series that formed in the Edi- acaran-Cambrian transition are important stratigraphic records of the co-evolution of the paleo-ocean, -climate, and -biology. In this study, we measured Re-Os isotopic compositions of the black shale in the Niutitang Formation from the Gezhongwu section in Zhijin, Guizhou Province. The samples had high Re and Os contents, with Re ranging from 21.27 to 312.78 ng/g and Os ranging from 0.455 to 7.789 ng/g. The Re-Os isotope isochron age of 522.9 ± 8.6 Ma implies deposition of the Niutitang black shale predated the Chengjiang Fauna, providing an age constraint for the expansion of oceanic anoxia in the study area. The initial ^187Os/^188Os ratio of 0.826 ± 0.026 indi- cates that enhanced continental weathering might have triggered the expansion of the oceanic anoxia.

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.

Rb-Sr isochron ages and significance of bimodal volcanic association in Baimianshi uranium ore-field, South China

1 Geologic background THE volcanic U-deposits are generally associated with felsic to intermediate volcanic rock and their sedimentary derivatives. However, uranium mineralization has been found to be spatially associated with thebasalt of bimodal volcanic associations in Baimianshi uranium ore field. Baimianshi uranium ore-field islocated in Baimianshi volcanic-sedimentary basin of south Jiangxi Province. The cover rocks of the basinconsist of a suite of bimodal volcanic-sedimentary sequences. Its basement rocks are two-mica granites ofBaimianshi rock body. The basalt of cover rocks is interbedded with arkoses in lower sequence, and rhyolite or rhyolitic tuff in upper sequence. Uranium minerals mainly occur as disseminations in the

The Rb-Sr isochron of ore and pyrite sub-samples from Linglong gold deposit, Jiaodong Peninsula, eastern China and their geological significance

Here we report the first direct Rb-Sr dating of pyrites and ores using sub-sampling from lode gold deposits in Linglong, Jiaodong Peninsula, which is a supra-large lode gold deposit and propose this as a useful geochronological technique for gold mineralization with poor age constraint. The Rb-Sr data of pyrites yield an isochron age of (121.6±8.1) Ma, whereas those of ore and ore-pyrite spread in two ranges from 120.0 to 121.8 Ma and from 110.0 to 111.7 Ma. Studies of characteristic of gold deposit and microscopy of pyrite and quartz indicate that the apparent ages of ore and ore-pyrite are not isochron ages, only mixed by two end members, i.e. the primitive hydrothermal fluids and wall rocks, whereas the isochron age of pyrite sub-samples constrains the age of gold mineralization (121.6±8.1) Ma, i.e. early Cretaceous, which is in good agreement with the published SHRIMP zircon U-Pb ages.

^(40)Ar/^(39)Ar and Rb-Sr isochron dating of the gold deposits on northern margin of the Jiaolai Basin,Shandong,China

The Pengjiakuang, Dazhuangzi and Fayunkuang gold deposits, located on the northern margin of the Mesozoic Jiaolai Basin, east of Shandong Province, are controlled by a low-angle normal fault. Gold ores are typically brecciated, veinlet and disseminated. The Ar-Ar and Rb-Sr isochron dating methods were adopted to date ores and lamprophyre dike. The results indicate that the age of the Pengjiakuang gold deposit is 117.33—118.42 Ma, that of the Dazhuangzi gold deposit is 117.39 Ma, and that of the Fayunkuang gold deposit is (128.49±7.2) Ma. The consistency in metallogenic age between the gold deposits on the margin of the Jiaolai Basin and the gold deposits (115—126 Ma) of the northern uplift area suggests that both were formed in the same metallogenic period. That is to say, the large-scale metallogeny of the Jiaodong region took place in late-Yanshannian ((120±10) Ma).

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.

^(40)Ar/^(39)Ar isochron ages of lawsonite blueschists from Jiuquan in the northern Qilian Mountain,NW China,and their tectonic implications

Using laser 40Ar/39Ar dating method,we have gotten the metamorphic ages of lawsonite blueschist and epidote blueschist from Jiuquan,northern Qilian Mountain,NW China.The high quality laser 40Ar/39Ar dating of glaucophane from lawsonite blueschist gives an isochron age of 413±5 Ma.The isochron age obtained from phengite in epidote blueschist is 415±7 Ma.These data,combining with peak metamorphic P-T conditions and regional geological setting,allow us to infer that the lower limit of the ages of the prograde subduction metamorphism from lawsonite blueschist facies to epidote blueschist facies occurred at ca.413-415 Ma,which also suggests that the formation of lawsonite blueschist in the northern Qilian Mountain maybe resulted from the corner flow in the cold subduction zone.This study shows that the final closing time of the northern Qilian remnant oceanic basin is about 413-415 Ma,which also represents the convergent age between the North China Craton and the Qaidam block.

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).

Redefinition of Early Mesoproterozoic (1800-1600Ma) stratigraphy in the northern Kongling area,China:The nucleus of Yangtze Craton and its tectonic significance

The Wujiatai Formation, which is well exposed in Huangjiatai-Xichahe region of the northern Kongling area of central Yangtze Craton, is a suite of epimetamorphic conglomerates to pebbly sandstones to fine sandstone-dolostones deposited in littoral-carbonate platform facies. The formation has angular unconformity contacts with both the overlying Neoproterozoic Nantuo Formation and the underlying Paleoproterozoic Huanglianghe Formation complex. Detrital zircons from metafine sandstones of the lower Wujiatai Formation have ages ranging from 3377 –1828 Ma, with the youngest zircons dating to about 1828 Ma. In addition, whole-rock Pb-Pb isochron ages from dolostones in the upper Wujiatai Formation yield an age of 1718±230 Ma. These dates constrain the depositional age of the Wujiatai Formation between 1800 Ma and 1600 Ma. These are the earliest Mesoproterozoic sedimentary records reported in the Kongling region, and fill the gaps in Early Mesoproterozoic stratigraphy in Yangtze Craton. Histograms of detrital zircon ages for the Wujiatai Formation reveal four major peaks at 2039 Ma, 2691 Ma, 2966 Ma and 3377 Ma, which is consistent with the ages of the basement rocks that underlie the center of Yangtze Craton, indicating that sediment provenance is mainly from the Kongling complex. The lower Wujiatai Formation mainly consists of clastic rocks, whereas the upper Wujiatai Formation consists of dolostones. This stratigraphic change implies a deepening sequence in an expanding basin with an initial cratonic rifting tectonic setting, corresponding to the initial breakup of the Columbia supercontinent in Yangtze Craton.

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.

Effect of isotope diffusion in solids on isochron ages under -pper mantle conditions

DODSON presented the concepts of closure temperature (T_c), transitional temperature range (T_h—T_l) and T_l【T_c【T_h in geochronology when he developed the theory of cooling ages. For the mineralogical system in the earth crust, geochronometers work usually under cooling conditions of T【T_c. But in the earth mantle at T】T_l, the effect of matter transfer by diffusion on radiometric dating is not negligible in the transitional temperature range

The Rb-Sr age and geological implication of alkaline syenitic complex from Liangtun-Kuangdonggou at Gaixian,eastern Liaoning Province

Liangtun-Kuangdonggou complex, 30 km southeast of Gaixian Town, Liaoning Province, is composed of aegirine-augite hornblende syenite, aegirine-augite syenite, hornblende syenite, pyroxene syenite and syeno-aegirinolite. Rb-Sr isotopic data to the complex are reported and used for discussion of chronology and geological implication. They show that the complex occurred in early Proterozoic and is a tectonic sign of extension setting. Moreover, the date indicates that the Liaohe Group, which is metamorphic wall rock of the complex, formed over 1 900 Ma ago.

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.

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