Zircon SHRIMP U-Pb ages of the “Xinghuadukou Group” in Hanjiayuanzi and Xinlin areas and the “Zhalantun Group” in Inner Mongolia, Da Hinggan Mountains

A report is presented of SHRIMP zircon U-Pb dating data of meta-igneous and meta-sedimentary rocks of the Xinghuadukou Group(Xinlin-Hanjiayuanzi area,Heilongjiang Province)and meta-volcanic rocks of the Zhalantun Group(Zhalantun district,Inner Mongolia).The SHRIMP analyses show that the meta-igneous rocks from the Xinghuadukou Group formed at 506±10―547±46 Ma,belonging to Early-Middle Precambrian,whereas the meta-sedimentary rocks yielded detrital zircons,with ages of 1.0―1.2,1.6―1.8 and 2.5―2.6 Ga,indicative of deposition age at least<1.0 Ga. Meta-basic volcanic rocks from the Zhalantun Group have a formation age of 506±3 Ma.These data suggest that both the Xinghuadukou and Zhalantun Groups formed during Cambrian and/or Neoproterozoic time,rather than Paleoproterozoic time as previously thought.Early Precambrian inherited zircons in the meta-igneous rocks and numerous Precambrian detrital zircons in the meta-sedimentary rocks imply that these rocks were formed proximal to older crust.It is inferred that the Xinghuadukou and Zhalantun Groups represent Cambrian and/or Neoproterozoic vol- cano-sedimentary sequences formed in an active continental margin setting.     

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.     

Zircon SHRIMP dating of sodium alkaline rocks from Maomaogou area of Huili County in Panxi, SW China and its geological implications

Alkalinerocksareoftenassociatedwithextensionaltectonicsandregardedasthecharacteristicproductsoc-curringincontinentalmarginsorriftzones[1].Theywereusuallygeneratedindeep-large-faultzoneandcloselyconnectedwithbasic/ultrabasicrocksinspace.Therefore,alkalinerocksareofsignificanceintectonicpetrology.Atthebeginningofthe1980s,geologicalandgeophysicalstudieswereperformedinPanzhihua-Xichangpaleo-riftzone(Panxiriftzone).However,thesestudiesonlyin-volvedgeologyandpetrochemistryoftheMaomaogouringalkalic…     

A ∼2.5 Ga magmatic event at the northern margin of the Yangtze craton: Evidence from U-Pb dating and Hf isotope analysis of zircons from the Douling Complex in the South Qinling orogen

The poorly studied Douling Complex is a crystalline basement that developed in the Neoproterozoic-Paleozoic weakly metamorphosed to non-metamorphosed strata at the South Qinling tectonic belt. Five banded dioritic-granitic gneiss samples from the Douling Complex were chosen for LA-MC-ICPMS U-Pb zircon dating, which yielded protolith emplacement ages of 2469 ± 22 Ma, 2479 ± 12 Ma, 2497 ± 21 Ma, 2501 ± 17 Ma and 2509 ± 14 Ma, respectively. An important peak age of ～2.48 Ga was also obtained for a metasedimentary rock in the same region. These discoveries suggest the occurrence of magmatic activity of 2.51–2.47 Ga at the northern margin of the Yangtze craton. The age-corrected ? _Hf(t) values obtained from in situ zircon Hf isotopic analysis are mainly between ?5.5 and +0.3, and the two-stage zircon Hf model ages range from 3.30 to 2.95 Ga. Considering two important periods of ～3.3–3.2 Ga and ～2.95–2.90 Ga for the continental crustal growth in the Yangtze craton, we infer that the dioritic-granitic gneisses from the Douling Complex are the products of reworking of Paleo- to Mesoarchean crust at the northern margin of the Yangtze craton at ～2.5 Ga. In addition, metamorphic ages of 837 ± 8 Ma and 818 ± 10 Ma were obtained for zircon overgrowth rims from a dioritic gneiss and a metasedimentary rock, indicating that the main phase amphibolite facies metamorphism of the Doulng Complex occurred during the Neoproterozoic, although its geological meaning remains ambiguous.     

Identification of Mesoproterozoic zircons in a Triassic dolerite from the Liaodong Peninsula,Northeast China

Evolution of mantle and crust in all blocks of China and their relationship with the surrounding blocks are re-lated to amalgamation and breakup of the supercontinents. Studies on the Mesoproterozoic Grenvillian orogeny and the configuration of Neoproterozoic Rodinia provide the most important advances to the ideas of continental crustal growth and configuration of plates[1,2]. Recent studies show that there are geological records of assembly and breakup of Rodinia in the Cathaysia and Yang…     

Zircon U-Pb geochronology of basement metamorphic rocks in the Songliao Basin

Zircon LA-ICP MS U-Pb dating of six metamorphic rocks and a metagranite (breccia) from southern basement of the Songliao Basin are reported in order to constrain the formation ages of basement. The basement metamorphic rocks in the Songliao Basin mainly consist of metagabbro (L45-1), amphibolite (SN117), metarhyolitical tuff (G190), sericite (Ser) schist (N103), chlorite (Chl) schist (T5-1), biotite (Bi)-actinolite (Act)-quartz (Q) schist (Y205), and metagranite (L44-1). The cathodoluminesence (CL) images of the zircons from metagabbro (L45-1) and metagranite (L44-1) indicate that they have cores of magmatic origin and rims of metamorphic overgrowths. Their U-Pb isotopic ages are 1808±21 Ma and 1873±13 Ma, respectively. The zircons with oscillatory zoning from amphibolite (SN117) and Chl schist (T5-1), being similar to those of mafic igneous rocks, yield ages of 274 ± 3.4 Ma and 264 ± 3.2 Ma, re-spectively. The zircons from metarhyolitical tuff (G190) and Ser schist (N103) display typical magmatic growth zoning and yield ages of 424 ± 4.5 Ma and 287 ± 5.1Ma, respectively. Most of zircons from Bi-Act-Q schist (Y205) are round in shape and different in absorption degree in the CL images, implying their sedimentary detritals. U-Pb dating yield concordant ages of 427 ± 3.1Ma, 455 ± 12 Ma, 696 ± 13 Ma, 1384±62 Ma, 1649±36 Ma, 1778±18 Ma, 2450±9 Ma, 2579±10 Ma, 2793±4 Ma and 2953±14 Ma. The above-mentioned results indicate that the Precambrian crystalline basement (1808―1873 Ma) exists in the southern Songliao Basin and could be related to tectonic thrust, and that the Early Paleozoic (424―490 Ma) and Late Paleozoic magmatisms (264―292 Ma) also occur in the basin basement, which are consistent with the ages of the detrital zircons from Bi-Act-Q schist in the basement.     

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.     

SHRIMP U-Pb zircon dating for Qiashikansayi granodiorite, the Northern Altyn Tagh Mountains and its geological implications

The Qiashikansayi granodiorite is foliated resulting in a granodioritic gneiss. Its geochemical features, such as alumina saturation index(A/CNK) of 0.81-0.99, Na2O/K2O values>1, TiO2 contents <1.0%, LREE enrichment with high fractionation factors, weakly negative or no Eu anomalies, and significant Ba and Ti negative anomalies, suggest that it is similar to a typical island arc pluton. The trace elements of the Qiashikansayi granodiorite are plotted in the island arc field in the tectonic setting discrimination diagrams as well. Cathodoluminescence images demonstrated that the zircons have clear rhythmic crystallized zoning, without any remnant core and new crystallized rim, suggesting the zircons be magmatic ones. Their U and Th contents vary in the range of 574-870 μg/g, and 279-556μg/g respectively, with the Th/U ratio in the range of 0.52-0.68. SHRIMP zircon U-Pb dating yielded the 481.5±5.3 Ma age for the intrusion of the granodiorite, which is coeval with the island arc volcanic rocks in the no     

Zircon SHRIMP U-Pb dating of meta-diorite from the basement of the Songliao Basin and its geological significance

Located in the east portion of the North Orogenic Belt, the Songliao Basin is bounded by the Da Hinggan Mountains in the west, the Xiao Hinggan Mountains in the north, the Zhangguangcai Range in the east, and the North China Craton (NCC) in the south (Fig…     

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…     

Crustal growth at ～2.5 Ga in the North China Craton：evidence from whole-rock Nd and zircon Hf isotopes in the Huai＇an gneiss terrane

The Huai’an gneiss terrane mainly consists of TTG gneisses and dioritic gneisses. Laser in situ U-Pb dating of magmatic zircon cores indicates that protolith of these gneisses was formed at ~2.5 Ga. The TTG gneisses have positive εNd（t） values of 2.7 to 4.3, and most of the magmatic zircons have positive εHf（t） values of 2.0 to 8.3. These positive εNd（t） and εHf（t） values are both similar to those of the contemporaneous depleted mantle at 2.5 Ga. Moreover, the young Hf model ages of 2.44 to 2.73 Ga for the magmatic zircons are close to the timing of the zircons growth. The whole-rock εNd（t） values are lower in the dioritic gneisses （0.8 to 1.7） than in the TTG gneisses due to the involvement of ancient crust in its source. However, many magmatic zircons from the dioritic gneisses have similar εHf（t） values （2.0 to 7.9） to that of the coeval depleted mantle; their Hf model ages of 2.49 to 2.75 Ga are close to the U-Pb ages of zircons. The highest εHf（t） values are close to the value of the depleted mantle, and the relatively high εHf（t） values corresponds to the relatively young Hf model age. These Nd and Hf isotope features suggest that these two types of gneisses of the Huai’an gneiss terrane originated from the juvenile crust at ca. 2.5 Ga.     

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.     

Finding of Neoproterozoic （～775 Ma） magmatism recorded in metamorphic complexes from the North Qilian orogen： Evidence from SHRIMP zircon U-Pb dating

The Qilian Mountain is considered a part of the Cen- tral Orogenic Belt of China[1]. The Central Orogenic Belt of China, extending for a distance of about 4000 km long in the E-W direction from the east coast west- ward through the mainland China all the …     

SHRIMP U-Pb geochronology of the zircons from the Precambrian basement of the Qilian Block and its geological significances

Origin and tectonic evolution of the Qilian Precambrian basement on NW China were investigated using zircon U-Pb ages with collaborating stratigraphic and paleontological evidence. Zircon grains were separated from two schists, two granitic gneisses and one mylonized gneiss and dated with SHRIMP. Seventy percent of sixty-one detrital zircon ages from two schists ranges from 0.88 Ga to 3.09 Ga, mostly within 1.0 Ga to 1.8 Ga with a peak at 1.6 Ga to 1.8 Ga, and twenty percent varies from 2.0 Ga to 2.5 Ga. A few falls in the Archean and Neoproterozoic periods. The two granitic gneisses were dated 930±8 Ma and 918±14 Ma, whereas the mylonized granitic gneiss was dated 790±12 Ma. These ages represent two periods of magmatisms, which can be correlated with the early and late stages of magmatisms associated with the Jinningian movement on the Yangtze Blocks. The results from this and previous studies indicate that the ages of the Precambrian detrital zircons from the Qilian Block are widely distributed in the Proterozoic era, distinct from the North China Block which was stable in the Neo-Mesoproterozoic era. By contrast, the age histograms of the detrital zircons from the Qilian Block is similar to those from Precambrian basement of the Yangtze Craton. Therefore, it is suggested that the Qilian Block had a strong affinity toward the Yangtze Craton and might belong to the supercontinent Gondwana in the Neoproterozoic time. This inference is supported by Nd model age (TDM), stratigraphic, and paleontological evidence. It is further considered that the Qilian Block was rifted from the supercontinent Gondwana during late Sinian to form an isolated continent in the Proto-Tethyan Ocean, moving towards the Alaxa Block in the North China Craton. The part of Proto-Tethyan Ocean between the Qilian and Alaxa Blocks should correspond to the so-called Paleo-Qilian Ocean. Following the closure of the Paleo-Qilian Ocean in the early Paleozoic, the Qilian Block collided with the Alaxa Block to form the North Qilian Orogenic Belt. Based on this tectonic explanation, the North Qilian ophiolites should represent parts of lithosphere from the Proto-Tethyan Ocean. Lithological and geochronological evidence also indicates that the Qilian Block underwent continental reactivation possibly induced by the deep northward subduction of the North Qaidam Block in early Paleozoic time.     

东海丽水凹陷晚古生代碎屑锆石的发现及其意义

通过对东海新生代陆架盆地丽水凹陷N6井月桂峰组和灵峰组下段地层碎屑锆石U-Pb定年,发现月桂峰组碎屑锆石主要为岩浆锆石,包括140—90 Ma和190—170 Ma两个年龄组分,并有零星的大于400 Ma的变质锆石;向上至灵峰组下段碎屑锆石中除上述锆石年龄组分外,新出现280—260 Ma的变质锆石组分,说明除140—90 Ma及190—170 Ma岩浆岩物源外,随着物源区持续剥蚀,至灵峰组下段物源区明显加入了280—260 Ma变质岩.这一与二叠纪相关的变质碎屑锆石和变质作用,可能与晚古生代古太平洋板块汇聚华南大陆有关.     

SHRIMP U-Pb geochronology of the detrital zircons from the Longshoushan Group and its tectonic significance

Sixty-two geologically meaningful U-Pb dates were obtained by using SHRIMP technique for the detrital zircons in three metasedimentary rocks from stratigraphically uppermost parts of the Longshoushan Group in the present study. Eighty percents of these dates range from 1.7 Ga to 2.2 Ga with a peak at 1.8-2.0 Ga and twenty percents from 2.3 Ga to 2.7 Ga. The youngest detrital zircon is dated at 1724±19 Ma which is interpreted as the maximum depositional age of the metasedimentary rocks. Therefore, the age for the diagenesis and lithification of the original sedimentary rocks of the Longshoushan Group before the metamorphism must be younger than 1724±19 Ma. Comparison of the age histograms of these detrital zircons with the ages of the igneous rocks on the surrounding older massifs suggests that the sediments of the Longshoushan Group were most likely derived from the Alaxa Block and Tarim Craton. This implies that the affinity between Alaxa Block and Tarim Craton was strong and that they might have been a unified craton during middle-early Proterozoic time.     

Preliminary study of microscale zircon oxygen isotopes for Dabie-Sulu metamorphic rocks： Ion probe in situ analyses

151 in situ analyses of oxygen isotopes were carried out by ion micro-probe for zircons from 8 localities of HP-UHP metamorphic rocks including eclogites in the Dabie-Sulu terrane. The results show significant heterogeneity in δ^18O values, with variation in different rocks from -8.5‰ to 9.7‰ and within one sample from 2‰ to 12‰. No measurable difference in δ^18O was observed between protolith magmatic (detrital) zircons and metamorphic recrystallized zircons within analytical uncertainties from the ion micro-probe measurements. This indicates that the metamorphic zircons have inherited the oxygen isotopic compositions of protolith zircons despite the HP to UHP metamorphism. According to their protolith ages from zircon U-Pb in situ dating by the same ion micro-probe, two groups of oxygen isotope composition are recognized, with one having δ^18O values of 6‰-7‰ for old protolith of 1.9-2.5 Ga ages and the other 0‰-2‰ for young protolith of 0.7-0.8 Ga ages. The latter anomalously low δ^18O values of zircons indicate that the magma has had the obvious involvement of meteoric water when forming the young protolith of high-grade metamorphic rocks. This may be correlated with the snowball Earth event occurring in South China and the world elsewhere during the Neoproterozoic.     

Zircon SHRIMP U-Pb dating for olivine gabbro at Wangmuguan in the Beihuaiyang zone and its geological significance

In the Neoproterozoic, a large-scale magmatic activ- ity took place in the northern margin of the South China Block, with ages in a range of 700―800 Ma[1―4]). A systematic zircon U-Pb dating for bimodal metaigneous rocks in the Dabie-Sulu orogen yields ages of 758 ± 15 Ma[5], typifying rift magmatism along the northern margin of the South China Block during the middle Neoproterozoic. In addition, there is a widespread oc- currence of volcanic tuff interlayers around 635 Ma with the se…     

Formation age and tectonic environment of the Gantaohe Group, North China Craton: Geology, geochemistry, SHRIMP zircon geochronology and Hf-Nd isotopic systematics

The Gantaohe Group is an important early Precambrian unit in the Trans-North China Orogen,North China Craton,and is mainly composed of greenschist-facies metabasalt,meta-sandstone and dolomitic marble.We report whole-rock geochemical compositions and SHRIMP zircon ages as well as LA-ICP-MS Hf-in-zircon isotopeic analyses for metabasalts from the Gantaohe Group.SHRIMP dating yielded a weighted mean 207Pb/206Pb age of 2087±16 Ma(MSWD=1.3) for magmatic zircons,but there are also abundant ca.2.5 Ga inherited zircon xenocrysts.The magmatic zircons shows a large Hf(t) variation in Hf(t) from 7.17 to +0.45,suggesting an isotopically highly heterogeneous source for the metabasalt.Chemically all samples show no distinct Zr or Hf anomalies,and some samples show no Nd or Ta anomalies in a primitive mantle-normalized trace element variation diagram,and their whole-rock Nd(t) values range from 4.0 to 0.8.We suggest that the basalt is formed by partial melting of a depleted mantle source,followed by significant crustal contamination.Field observations,the presence of abundant inherited zircon,as well as isotope and trace elements geochemistry support formation of the Gantaohe Group on top of a continental basement.These data and the regional geology lead us to conclude that the Trans-North China Orogen constituted an intracontinental rift during the Paleoproterozoic that was connected to the Eastern Block since the end of the Archean.     

Paleoproterozoic reworking of ancient crust in the Cathaysia Block, South China: Evidence from zircon trace elements, U-Pb and Lu-Hf isotopes

A combined study of zircon LA-ICP-MS U-Pb dating, trace elements and Hf isotope was carried out for gneissic granite from the Sanzhishu area in Jingning, SW Zhejiang Province. Nearly all the zircons separated from the granite exhibited oscillatory zoning and high Th/U ratios (>0.1). The REE profile showed a pronounced positive Ce anomaly, negative Eu anomaly and an enrichment of HREE, which are typical characteristics of magmatic zircon. Thirteen concordant or nearly concordant analytical data yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 1860±13 Ma (MSWD=0.084), representing the formation age of the granite. The magmatic zircons had negative εHf(t) values of −15.6 to −10.0 and two-stage Hf model ages of 3.1 to 3.4 Ga, indicating that the granites were formed by reworking of ancient crust. The major- and trace-element data indicate that the gneissic granites are metaluminous high-K calc-alkaline rocks and exhibit the same geochemical characteristics as aluminous A-type granites, implying the emplacement of the granite in a post-orogenic extensional tectonic setting. We conclude that the Paleoproterozoic crustal reworking event in the Cathaysia Block of South China marked the transition from assembly to break-up of the Columbia supercontinent. Supported by National Natural Science Foundation of China (Grant No. 40873004), Special Funds for National Scientific Research of Commonweal Industries, the Ministry of Land and Resources of China (Grant No. 2008110015), Opening Foundation of State Key Laboratory of Continental Dynamics, Northwest University (Grant No. 06LCD12) and the Project of Land and Resources Bureau of Zhejiang Province (Grant No. 2004005)