Rb-Sr chronology and cooling history of the Renjiawan pyroxenite intrusion in North Dabie Terrane

A group of Rb-Sr ages, i.e. plagioclase-whole rock (WR) isochron age of (205 ± 7) Ma, hornblende-WR isochron age of (164 ± 10) Ma and biotite-WR isochron age of (123.4 ± 1.2) Ma, for one pyroxenite sample from Renjiawan in North Dabie Terrane (NDT) are reported. According to the closure temperature of 600℃ for plageoclase and 300℃ for biotite, the Renjiawan pyroxenite should intrude into the NDT before 205 Ma, and the younger Rb-Sr ages may reflect the cooling history of the intrusion or reset by the very strong Cretaceous thermal event in NDT.     

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) (2σ) Ma and age by Rb-Sr isochronal method (t) = (358.02±0.30) (2σ) Ma.     

Sm-Nd dating and Nd-Sr isotopic characteristics of the Shimian ophiolite suite, Sichuan Province

By measuring the Sm-Nd and Rb-Sr isotopic compositions of harzburgite and gabbro from Shimian ophiolite suite, we got the whole rock Sm-Nd isochron age of (938±30) Ma (2?), and the ??Nd of 7.6±0.8 (2?), which shows that the ophiolite was formed at the Early Neoproterozoic. The obvious change (0.70209-0.70708) of ISr values of the ophiolite is caused by the meteoric hydrothermal alteration. The high ? Nd values indicate that the primitive magma was derived from the intense depleted mantle reservoir. It is suggested that this area was in a back-arc basin environment during the Early Neoproterozic.     

On emplacement ages of lamproites 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 magmatism-tectonism probably was at 442.64-435.54 Ma, as dated by the whole rock K-Ar method.     

Sm-Nd isotopic age and genesis of gabbro xenoliths in Daoxian County, Hunan Province

Conclusions The gabbro xenoliths are inferred to form at (224 ± 524) Ma by means of Sm-Nd dating, which reflected the intrusive age or crystallization age of gabbric melts. This event indicates that the region transformed from stable platform stage into Mesozoic intensive tectonomagmatic activity stage. The dynamics originated from the upwelling of asthenosphere and lithospheric thinning. Additionally, according to ε_Nd(T) = + 8.8 ± 0.6 of the gabbro xenoliths, within ε_Nd variation of the MORB at 224 Ma, we know that the gabbro xenoliths are the outcrops of partial melting of the asthenosphere.     

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 ε_Nd(t) = +8), which provides a significant bound on the time of formation of the Luobusha ophiolite. The Nd and Pb isotopic characteristics 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.     

Garnet Sm-Nd and U-Pb systems: A case study of a granulite from the European Variscan belt

This study presents zircon and garnet ages of a mafic granulite from the high-grade Variscan basement of the Black Forest, Germany and discuss isotope closure temperature of garnet Sm-Nd and U-Pb systems. Zircon grains yield 207Pb/206Pb ages between ～340 and ～414 Ma by the U-Pb and evaporation methods. In contract, garnet dating gives Sm-Nd and Pb-Pb isochron ages of (398±3) Ma and (411±14) Ma, respectively, which are older than most of zircon ages. These data imply that most of zircons lost radiogenic Pb, probably due to metamictization or recrystallisation during the granulite-facies metamorphism (～800℃) at ～340 Ma. Garnet Sm-Nd and U-Pb systems preserve chronological information of pro-grade metamorphism, probably profiting from a fluid-absence metamorphic environment. These results demonstrate that garnet mineral can be a better candidate than zircon mineral to date high-grade metamorphism by the U-Pb and Sm-Nd methods in some cases.     

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),ε_Nd(t) =8.40,and its model age (T _DM ^Nd ) 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 and associated chromites orebodies, it is believed that the Honguleleng ophiolitic magma derived from the LREE-enriched, self-depleted mantle source and formed at paleo-ocean ridge. This ophiolite is a relic of an oceanic crust within the collision zone.     

Zircon U-Pb ages of olivine pyroxenite xenolith from Hannuoba: Links between the 97—158 Ma basaltic underplating and granulite-facies metamorphism

U-Pb zircon dating by LA-ICP-MS andSHRIMP for one olivine pyroxenite yields complex agepopulations including Mesozoic ages of 97-158 Ma and 228±8.7 Ma, Early Paleozoic ages of 418--427 Ma, Paleoprotero-zoic age of 1844±13 Ma, Neoarchean age of 2541±54 Ma andmiddle Archean age of 3123±4.4 Ma. The 97--158 Ma and228±8.7 Ma zircons show typical igneous oscillatory zona-tion in CL images, suggesting two episodes of magmaticevents. Overlapping of the 97-158 Ma ages with that ofgranulite xenoliths indicates that the Mesozoic granu-lite-facies metamorphism was induced by heating from thebasaltic underplating at the base of the lower crust. Bothprocesses lastcd at least from about 158 to 97 Ma. Ages of 418--427 Ma could be records of the subduction of Mongoliaoceanic crust under the North China craton. Ages of 1.84 Ga,2.54 Ga and 3.12 Ga correspond to the three importantcrust-mantle evolutionary events in the North China craton,and imply preservation of Precambrian lower crust in thepresent-day lower crust.     

Rutile U-Pb age for the ultrahigh pressure eclogite from the Dabie Mountains,central China: Evidence for rapid cooling

The U-Pb isotope compositions of rutile, omphacite and garnet in the eclogite from the Jinheqiao area in the Southern Dabie ultrahigh-pressure metamorphic zone were analyzed. The consistent high precision U-Pb age (218 ± 1.2) Ma of rutile in eclogite from the Dabie Mountains was obtained by two ways of isochron and common Pb correction based on the composition of omphacite. This proves that the omphacite in eclogite has a U/Pb ratio (m = 2.8) low enough to be used for common Pb correction in the analyses of rutile. Under the rapid cooling condition (40℃/Ma) the closure temperature for U-Pb diffusion in rutile is about 470℃. Thus, this U-Pb age of rutile proves that 218 Ma should be the cooling age of eclogite at 470℃ instead of the peak metamorphic age.     

Electron microprobe monazite Th-Pb dating and its constraints on multi-stage metamorphism of low-pressure pelitic granulite from the Jingshan Group in the Jiaobei terrane

U-Th-Pb chemical dating of monazite with the electron microprobe is a new method developed in the last dec-ade[1,2]. It is attracting more and more attention for its ad-vantages of quickness, cheapness and relative high reli-ability[3]. Although its precision (±10?30 Ma)[4,5] is not as high as the ion-probe (SHRIMP), it has very high spatial resolution (1?2 μm), more superior than other in-situ techniques for analyzing compositions of zoned monazite crystals of less than 50 μm in size. …     

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.     

Fluid inclusions hidden in coesite-bearing zircons in ultrahigh-pressure metamorphic rocks from southwestern Sulu terrane in eastern China

Fluidinclusionstudiesarepowerfultoolsfordeci-pheringthefluidevolutionandfluid-rockinteractionin-volvingultrahigh-pressure(UHP)metamorphismofcrustalrocksatmantledepths.Thecompositionandprop-ertiesoffluidinclusionscanreflectthephysico-chemicalconditionsofthefluidsduringplatesubduction,UHPmetamorphismandexhumation.However,UHPmeta-morphicrockscommonlyexperiencedconsiderablede-compressionrelatedtotherapidexhumationprocess.Thishascausedtheinternalpressureoftheinclusionstrappedinthemetamorphicmin…     

Multi-stage evolution of gneiss from North Dabie： evidence from zircon U-Pb chronology

TTG gneiss is a common rock to outcrop in the northern part of the Dabie orogen, a few of which are closely associated with eclogites that experienced the Triassic ultrahigh pressure metamorphism. Although they were thermally metamorphosed by a large-scale magma activity in this region at the Early Cretaceous, it is unclear whether or not they are also affected by the Triassic metamorphism during continental subduction and exhumation. In order to resolve this issue, SHRIMP zircon U-Pb dating was carried out for the host gneiss of eclogites in North Dabie. The results show that cores from the gneiss have an age of 746~.31 Ma, consistent with the protolith ages of granitic gneisses in the Dabie orogen. Zircon overgrowing with different U and Th concentrations give concordant ages of 212~21 and 120~11 Ma, respectively. Th/U ratios of overgrown zircons are both lower than 0.1, suggesting a metamorphic genesis. The present resuits suggest that the gneiss in North Dabie has the similar protolith ages of Neoproterozoic to those granitic gneisses elsewhere in the Dabie orogen, and experienced not only the Triassic metamorphism but also the thermal metamorphism due to the Early Cretaceous magmatism. This provides an important insight into the geodynamic evolution of gneissic rocks in the Dabie orogen.     

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 Qaidarn (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 age dating of high-pressure granulites and amphibolite from Sanggan area, North China craton

The high pressure (HP) metamorphic age has been dated to HP rocks from the Sanggan area, North China craton. We have got garnet+whole rock isochron ages of (1 842±38) Ma for HP granulite, and (1 856 ± 26) Ma for HP amphibolite. The Sm-Nd whole rock isochron of HP granulites give out an age of (1 870±150) Ma with Nd depleted mantle model age of (2 402—2 482) Ma. Considering the Nd isotope homogenization during the peak metamorphism of the HP granulite, Sm-Nd closure temperature and the retention of Nd isotopic memory in garnets partially broken down during decompression, all these isochron ages are thought to be HP metamorphic age. Furthermore, we proposed that the HP metamorphism took place at the end of Paleoproterozoic during the large-scale collision and assembly of the North China craton.     

Isotope chronological trace of granite gravel in Hefei Basin

The discovery of granite gravel in Hefei Jurassic backland basin, which came from the Dabie Shan or North Huaiyang, has a great significance for probing into uplift and exhumation of the Dabie orogenic belt. Lithochemical and isotope chronological analyses on the granite gravel in the Sanjianpu Formation and the Fenghuangtai Formation in Jinzhai County prove that post-orogenic granite was developed in the major provenance of the Hefei Basin; that is, Dabie Shan or North Huaiyang. Discordant lower intersection point age of zircon U-Pb of granite gravel is about 214 Ma, and⁴⁰Ar/³⁹Ar age of muscovite about 196 Ma, K-Ar age of K-spar about 181 Ma. The former is the diagenetic age of the sample, and the latter two represent the sealed ages of muscovite and K-spar respectively. The age of sedimentary rocks in which a lot of granite gravels appear in the sedimentary section may represent the newest age of the exhumation of the granite body, so we assume that the age is 166 Ma. Therefore, we calculated the uplift rates of the granite body from Late Triassic to early Middle Jurassic; that is about 0.08 km/Ma and 0.4–0.3 km/Ma in the early slowly uplifting stage and later fast uplifting stage respectively, after the formation of the body at 214 Ma. These results are basically identical to the uplifting ages of the Dabie ultrahigh pressure metamorphic (UHPM) belt and the North Huaiyang thrust belt.     

Sm-Nd and Rb-Sr isotopical ages of magnetite-chlorite formation gold deposit in the volcanic rock area of Late Paleozoic Era, East Tianshan

Through studying geo-geochemical characteristics and determining Sm-Nd and Rb-Sr isotopical ages in the Kanggur magnetite-chlorite formation gold deposit, some conclusions can be achieved: Sm-Nd isochron age is (290.4 ± 7)Ma, Rb-Sr isochron age is (282.3 ± 5)Ma, mineralization is Late Hercynian and the formation of this gold deposit is related to the macroscopic Huangshan-Qiugemingtashi ductile shear zone which is produced as a result of collision between the Tarim plate and the Junggar plate.     

The ages of U-Pb and Sm-Nd for eclogite from the western segment of Altyn Tagh tectonic belt

The eclogites of Altyn Tagh tectonic belt occur as lens within gneisses characterized by amphibolite-facies mineral parageneses. The well-preserved eclogite is selected for Sm-Nd and U-Pb isotopic dating. The Sm-Nd isotopic data yield a whole rock-garnet-omphacite isochron of (500 ± 10) Ma. The U-Pb isotopic measurements of zircons show that the four grain populations are nearly concordant and well plotted on concordia curves, giving a weighted mean age (503.9 ± 5.3) Ma. Two kinds of methods obtain a similar age which reveals the peak metamorphic age of eclogites, and reflect the existence of orogenic root related to Caledonian subduction and continent-continent collision.     

The ages of U-Pb and Sm-Nd for eclogite from the western segment of Altyn Tagh tectonic belt——Evidence for existence of Caledonian orogenic root

The eclogites of Altyn Tagh tectonic belt occur as lens within gneisses characterized by amphibolite-facies mineral parageneses. The well-preserved eclogite is selected for Sm-Nd and U-Pb isotopic dating. The Sm-Nd isotopic data yield a whole rock-garnet-omphacite isochron of (500 ± 10) Ma. The U-Pb isotopic measurements of zircons show that the four grain populations are nearly concordant and well plotted on concordia curves, giving a weighted mean age (503.9 ± 5.3) Ma. Two kinds of methods obtain a similar age which reveals the peak meta-morphic age of eclogites, and reflect the existence of orogenic root related to Caledonian sub-duction and continent-continent collision.