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 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.     

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.     

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.     

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 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.     

Zircon U-Pb and Hf isotopes of volcanic rocks from the Batamayineishan Formation in the eastern Junggar Basin

An internal structural study was conducted to investigate U-Pb age, trace elements and Hf isotopes of basaltic zircons from the Batamayineishan Formation. The basalt was obtained from drill well San-Can 1 on the eastern Luliang uplift within the Junggar Basin. Trace element data of zircons show that all samples are magmatic, with similar REE patterns, including positive Ce (δCe=5.06–134), but negative Eu (δEu=0.06⦒0.55) anomalies and enrichment in heavy rare earth elements. Among 25 grains, the concordant ages were subdivided into three groups; ages of 300.4±1.3 Ma (n=11), 339.2±2.7 Ma (n=3) and 392.0±1.7 Ma (n=8). Three remaining grains were nearly concordant, with ²⁰⁶Pb/²³⁸U ages of 510±7, 488±6 and 453±6 Ma, respectively. The youngest concordant age (i.e., 300.4±1.3 Ma) could be interpreted as the formation age of the studied basaltic rock; this is consistent with the sampling position at the upper part of the Batamayineishan Formation. On the other hand, ages such as Ordovician and early Devonian are consistent with the ages of island-arc volcanic rocks (enrichment in Pb) or ophiolites around the basin. Moreover, the positive ɛ _Hf(t) values of the early and middle Paleozoic zircons (+3.6–+10.5) may suggest that the basement traversed by the studied volcanic rocks may be Paleozoic in age, formed from the residual oceanic crust and island-arc complex. The ɛ _Hf(t) values (+4.2–+17.1) of the late Paleozoic (∼300.4 Ma) zircons suggest that the basaltic magmas were derived from partial melting of the asthenospheric mantle or depleted lithospheric mantle. These magmas were slightly contaminated by the existence of early-middle Paleozoic materials. The late Carboniferous basalts represent direct eruption of mantle-derived magmas at the upper crustal level during a post-collisional tectonic setting. We therefore consider that extensive vertical growth of the continental crust to have occurred before the late Carboniferous.     

Discovery of ∼4.0 Ga detrital zircons in the Changdu Block,North Qiangtang,Tibetan Plateau

Using in situ zircon dating by LA-ICP-MS and MC-ICP-MS, detrital zircon of 3981±9 Ma age was found in metamorphic rocks of the Ningduo Rock Group, Changdu Block of Northern Qiangtang. This is the oldest age record that has been found in the Qiangtang area. This finding also constitutes the third zircon locality in China with an age older than 3.9 Ga. Thus, the discovery provides new information for the study of Hadean crust. In addition, we found 3.51–3.13 Ga, ∼2440 Ma, ∼1532 Ma, ∼982 Ma and ∼618 Ma age peaks from 100 test spots. The younger ages of ∼982 Ma and ∼618 Ma correspond to the formation of the Rodinian super-continent and the Pan-African event, respectively. These findings suggest a close relationship between these zircons and the Gondwanan super-continent. The age of ∼618 Ma defines the lower limit on the deposit time of the protolith for the garnet-mica-quartz schist in the Ningduo Rock Group. Zircons with an age of ∼982 Ma generally display a negative ɛ_Hf(t) and a two-stage Hf model with concentrated ages around 1933–2553 Ma. This pattern indicates that the source area of the Ningduo Rock Group underwent a significant separation of depleted mantle into the crust during the Paleoproterozoic Era. However, zircons with ages of 2854–3505 Ma also show a negative ɛ_Hf(t) and a two-stage Hf model with a concentration of ages around 3784–4316 Ma. These results demonstrate that the source area of the Ningduo Rock Group contains a residual amount of ancient (Hadean) crustal materials. This paper provides new information on the relationship between the basement of the Qiangtang area and the Paleoproterozoic basements of the Gangdese and Himalayan regions, which constrains the northern boundary of Gondwana.     

Zircon SHRIMP geochronology of the Xinkailing-Kele complex in the northwestern Lesser Xing＇an Range, and its geological implications

Located in the eastern portion of the Xing'an-Mongolian Orogenic Belt (XMOB), the Xinkailing-Kele complex has previously been considered to be Precambrian metamorphic rocks, mainly according to its relatively high metamorphic grade. Our filed observation, however, revealed that the complex is composed mainly of metamorphic rocks (Kele complex), tectono-schists ("Xinkailing Group"), and granitoids (Xinkailing granitic complex). Dating on these rocks using advanced SHRIMP zircon U-Pb technique indicates that: (1) Biotite-plagioclase gneiss from the Kele complex has a protolith age of 337±7 Ma (2σ) and a metamorphic age of 216±3 Ma (2σ); (2) the tectono-schist of the "Xinkailing Group" gave a magmatic age of 292±6 Ma (2σ), indicative of felsic volcanic protolith of the schist formed in late Paleozoic time; and (3) the Menluhedingzi and Lengchuan granites of the Xinkailing granitic complex were emplaced at 167±4 (2σ) and 164±4 Ma (2σ), respectively. These results suggest that the Xinkailing-Kele complex is not Precambrian metamorphic rocks and the so-called Precambrian "Nenji-ang Block" does essentially not exist. In combination with regional geological data, we propose that the Kele metamorphic complex is likely related to a collisional tectonism that took place in Triassic time, as indicted by its metamorphic age of 216±3 Ma. The Xinkailing granitic complex was em-placed along the collisional zone during Mid-Jurassic time, likely in a post-orogenic or anorogenic setting.     

Sm-Nd age of syncollisional mafic-ultramafic intrusions in the Dabie Mountains

A Sm_Nd age of (228±42) Ma with initial ε Nd =-16.4 for the Renjiawan pyroxenite intrusion in the North Dabie terrane is reported. This age with another Sm_Nd age of (230±44) Ma for the Zhujiapu pyroxenite in the same terrane documents that the pyroxenite in the North Dabie terrane are formed during continental subduction time of the Yangtze craton in the Triassic.     

Simultaneous in-situ determination of U-Pb age and trace elements in zircon by LA-ICP-MS in 20 μm spot size

Simultaneous in-situ determination of U-Pb ages and 20 trace elements of three international zircon standards （91500, GJ1 and TEMORA 1） and one laboratory zircon standard （SK10-2） separated from Cenozoic fine-grained gabbro was carried out on quadrupole ICP-MS equipped with 193 nm excimer laser in 20 μm spot size. The weighted mean ^206Pb/^238U ages of 91500, GJ1 and TEMORA 1 are 1064.4±4.8 Ma （2σ）, 603.2±2.4 Ma （2σ） and 418.2±2.4 Ma （2σ）, respectively. The relative standard deviations （RSDs） of ^206Pb/^238U ages （2σ） are less than 2.2% for single measurements and 0.6% for weighted means. The obtained weighted mean ^206Pb/^238U ages of three standard zircons agree with the recommended values within 2σrerror. The weighted mean ^206Pb/^238U age of SK10-2 is 31.42=0.25 Ma （2σ） and the RSDs of ^206Pb/^238U ages （2σ） are between 2.4% and 5.7% for single measurements and less than 0.8% for weighted mean. The obtained weighted mean ^206Pb/^238U age is in good agreement with the age obtained by Yuan （2004）. Trace element concentrations of NIST612 and NIST614 obtained under the same LA-ICP-MS operating conditions agree with the recommended values within analytical error. The results indicate that it is possible to measure U-Pb age and trace elements simultaneously by LA-ICP-MS in a small spot size of 20μm.     

In situ simultaneous determination of trace elements, U-Pb and Lu-Hf isotopes in zircon and baddeleyite

This paper describes a combined method of simultaneously measuring U-Pb and Lu-Hf isotopes as well as trace elements in Phalaborwa baddeleyite and 91500, GJ-1, TEMORA-1 and SK10-2 zircons by means of Neptune MC-ICPMS and Agilent Q-ICPMS connected to a 193 nm excimer laser ablation system. Material ablated by laser was carried in different proportions into Q-ICPMS for U-Pb isotopic and trace elemental and MC-ICPMS for Lu-Hf isotopic compositions. Experiments indicate that different proportions of ablated material for the Q-ICPMS and MC-ICPMS （6：4, 5：5 and 4：6 respectively） do not show any bias for the zircon/baddeleyite U-Pb age, Lu-Hf isotope and trace elemental compositions within analytical errors. Using 40-60 μm spot size, the obtained U-Pb ages of Phalaborwa baddeleyite, 91500, GJ-1, TEMORA and SK10-2 zircons are 2065±15 （2σ, n=20）, 1063±6 （2σ, n=-19）, 613±6 （2σ, n=20）, 416±5 （2σ, n=20） and 32.6±0.5 （2σ, n=20） Ma, respectively. The ^176Hf/^177Hf ratios are 0.281231±24 （2SD, n=20）, 0.282310±35 （2SD, n=19）, 0.282028±34 （2SD, n=20）, 0.282687±34 （2SD, n=20） and 0.282752±53 （2SD, n=20）, respectively. The obtained trace elemental compositions are identical to the reference values. Therefore, this kind of technique makes it possible to simultaneously obtain the U-Pb age, Lu-Hf isotopes and trace elemental compositions of zircon and baddeleyite, which could be an important tool in solving problems in earth sciences.     

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.     

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.     

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.     

SHRIMP zircon U-Pb ages of Kalatongke No. 1 and Huangshandong Cu-Ni-bearing mafic-ultramafic complexes,North Xinjiang,and geological implications

In Xinjiang, northwestern China, there are some veryimportant magmatic Cu-Ni sulfide deposits hosted by ma-fic-ultramafic complexes. In addition to the Jingbulake inwestern Tianshan and the Xingdi in northeastern Tarimblock, the others are concentrated in the Kalatongke andHuangshan-Jingerquan mafic-ultramafic complex beltsand two largest Cu-Ni sulfide deposits occur in the Kala-tongke No.1 and Huangshandong complexes. In the pastdecade, a great number of researches have been conductedfo…     

In situ U-Pb dating of xenotime by laser ablation （LA）-ICP-MS

Xenotime is an ideal mineral for U-Th-Pb isotopic dating because of its relatively high U and Th contents, but typically low concentration of common Pb. These characteristics, and the fact that it is widespread throughout various types of rocks, suggest that the U-Th-Pb dating of xenotime has broad applications. Studies of U-Pb dating on xenotime by ion microprobe (such as SHRIMP) have increased in recent years, whereas studies by laser ablation (LA)-ICP-MS are still rare. In this study, we developed a technique for U-Pb dating of xenotime using the 193 nm ArF laser-ablation system and Agilent 7500a Q-ICP-MS. To evaluate the reliability of our method, a xenotime standard, BS-1, was analyzed and calibrated against another xenotime standard, MG-1. The weighted mean 206 Pb/ 238 U ages of 510.1 ± 5.2 Ma (2 n = 21), 509.8 ± 4.3 Ma (2 n = 21) and 510.0 ± 4.6 Ma (2 n = 21) were obtained using beam diameters of 16, 24 and 32 m, respectively. These ages are identical to those determined by ID-TIMS method (weighted mean 206 Pb/ 238 U age of 508.8 ± 1.4 Ma), which supports the reliability of our LA-ICP-MS method. We also analyzed xenotimes in leucogranites from South Tibet and granites from Xihuashan in southern China, and obtained accurate and precise ages. Nevertheless, we observed systematic differences in Pb/U fractionation among xenotime, monazite and zircon. The matrix-effect resulted in either under-correction or over-correction of fractionation, and thus led to inaccurate ages. Thus, a matrix-matched material is required for U-Pb dating of xenotime by LA-ICP-MS.     

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.     

Paleoproterozoic mafic dyke from Kongling terrain in the Yangtze Craton and its implication

Mafic dyke is a sign of regional extension, and thus has important tectonic significance. A great amount of mafic dykes occur in the Kongling terrain of the Yangtze Craton, which have great bearing on the early evolution of the Yangtze Craton. Their ages, however, have not been well constrained. In this paper we report an integrated study of zircon U-Pb age and Hf isotope compositions for a mafic dyke in the Kongling terrain. The zircons yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 1852±11Ma, which represents its intrusion age. They have ɛ _Hf(t) values of −6.3 to 0.5, with a weighted mean of −3.06±0.88, suggesting that the mafic dyke came from metasomatic mantle. The results indicate that the Yangtze Block had transformed into post-collisional extensional regime at ca. 1850 Ma. In the same period, the Yangtze Craton shows enough rigidity to produce brittle rupture, and thus has the characteristics of a craton. Supported by National Natural Science Foundation of China (Grant Nos. 40772042, 90714010 and 40521001), the Ministry of Education of China (Grant Nos. IRT0441, B07039 and NCET-06-0659), and Foundation of the State Key Laboratory of Continental Dynamics, Northwest University     

SHRIMP U-Pb zircon ages for the UHP metamorphosed granitoid gneiss in Altyn Tagh and their geological significance

Different types of UHP metamorphic rocks havebeen recently discovered in the Altyn Tagh[1—4], the north-ern margin of Qadam Basin[5—7], the southwestern Tian-shan Mountains[8,9] and the northern Qinling Moun-tains[10,11] in Central and Western China. And these areashave attracted focus attention of geologists at home andabroad to the studying of UHP metamorphism and conti-nental deep subduction. However, as newly discoveredUHP metamorphic terranes, some questions have beenarisen abou…