Grenvillian orogeny in the Southern Cathaysia Block: Constraints from U-Pb ages and Lu-Hf isotopes in zircon from metamorphic basement

Metamorphic basement rocks in the Cathaysia Block are composed mainly of meta-sediments with different ages. New zircon U-Pb geochronological results from the meta-sedimentary rocks exposed in the Zengcheng and Hezi areas, southern Cathaysia Block, show that they consist dominantly of early Neoproterozoic （1.0-0.9 Ga） materials with minor Paleo- to Mesoproterozoic and late Neoproterozoic （0.8-0.6 Ga） components, suggesting that the detritus mostly come from a Grenvillian orogen. The youngest detrital zircon ages place a constraint on the deposition time of these sediments in Late Neoproterozoic. Zircon Hf isotopic compositions indicate that the Grenvillian zircons were derived from the reworking of Mesoproterozoic arc magmatic rocks and Paleoproterozoic continental crust, implying an arc-continent collisional setting. Single-peak age spectra and the presence of abundant euhedral Grenvillian zircons suggest that the sedimentary provenance is not far away from the sample location. Thus, the Grenvillian orogen probably preexisted along the southern margin of the Cathaysia Block, or very close to the south. Similarity in the ages of Grenvillian orogeny and the influence of the assembly of Gondwana in South China with India and East Antarctic are discussed, with suggestion that South China was more likely linked with the India-East Antarctica continents in Early Neoproterozoic rather than between western Laurentia and eastern Australia.     

Chronological record of the Early Mesozoic underplating in the northern margin of North China—U-Pb chronometry of zircons in the Late Mesozoic andesite from Western Liaoning

Cathodoluminescence spectroscopic analysis of zircons in the uppermost sanukites of the early Cretaceous Yixian Forma- tion in Western Liaoning reveals zoning structures of the zircons in the volcanic rocks. LA-ICP-MS chronometry shows that the central part of the zircons is remnant zircon with a U-Pb dating of 254 Ma,which is consistent with the age of the Early Mesozoic underplating granulites,and that the periphery is zircon crystallizing out of the host magma with a U-Pb dating of 116 Ma,which is highly consistent with the Ar-Ar dating of the host volcanic rocks.Compositions of the clinopyroxenes xenocrystals in the sanukites are similar to those of the clinopyroxenes in the underplating granulites,indicating that the granulites accreted to the lower crust in the Early Mesozoic have been reformed and disrupted by the upwelling diapers of the asthenosphere and taken part in formation of the sanukites in the Late Mesozoic.     

Zircon U-Pb SHRIMP dating for the volcanic rocks of the Xiong’er Group: Constraints on the initial formation age of the cover of the North China Craton

The volcanic rocks of the Xiong‘er Group occur widely in the southern part of the North China Craton, which mark the beginning of the cover in the southern part of the North China Craton. The age of the volcanic rocks is thus crucial to understand the tectonic regime and evolutionary history of the North China Craton in the Proterozoic age. Zircons from five volcanic rocks and intrusions were dated by U-Pb SHRIMP method. The results indicate that the Xiong‘er Group formed in 1.80--1.75 Ga of Paleo-Proterozoic. Since the Xiong‘er Group formed earlier than the Changcheng System, the earliest rocks in the Changcheng System is therefore assumed to be formed in 1.75 Ga. A thermal-tectonic event of ca. 1.84 Ga is indicated by new zircon U-Pb SHRIMP ages in the southern part of the North China Craton. The volcanic rocks of the Xiong‘er Group thus represent the initial magmatism of the Paleo-Proterozoic breakup of the North China Craton. Numerous inherited zircons in the volcanic rocks mainly formed in ～2.20 Ga, indicating that the source magma of the volcanic rocks may be derived from the ～2.20 Ga crust, or from a mantle magma with significant contamination of the ～2.20 Ga crust.     

Zircon U-Pb geochronology of the volcanic rocks from Fanchang-Ningwu volcanic basins in the Lower Yangtze region and its geological implications

The latest eruptions in two important Mesozoic volcanic basins of Fanchang and Ningwu located in the middle-lower reaches of the Yangtze River formed the bimodal volcanic rocks of the Kedoushan Formation and ultrapotassic volcanic rocks of the Niangniangshan Formation, respectively. The representative volcanic rocks of the two Formations were selected for LA-ICPMS zircon U-Pb dating. The results indicate that there exist a large amount of magmatic zircons as indicated by high Th/U ratios in these volcanic rocks. The weighted mean age of 21 analyses is 130.7±1.1 Ma for the Kedoushan Formation, and that of 20 analyses is 130.6±1.1 Ma for the Niangniangshan Formation. These U-Pb ages are interpreted to represent the formation times of the volcanic rocks. In combination with other known geochronological data for Mesozoic volcanic rocks from the Lower Yangtze region, it is proposed that the latest volcanic activations in the Jinniu, Luzong, Fanchang and Ningwu volcanic basins probably came to end prior to ca. 128 Ma. There is no significant time interval between the early and later volcanic activities in the Luzong and Ningwu basins, suggesting a short duration of volcanic activities and thus implying the onset of an extensional tectonic setting at about 130 Ma in the Lower Yangtze region. Integrated studies reveal that the Early Cretaceous magmatic activities and their geochronological framework in the Lower Yangtze region are a response to progressively dynamic deep processes that started with the transformation of tectonic setting from compression to extension, followed by delaminating of the lower part of the thickened lithosphere, lithospheric thinning, asthenosphere upwelling, and crust-mantle interaction.     

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.     

SHRIMP zircon U-Pb dating for volcanic rocks of the Dasi Formation in southeast Hubei Province, middle-lower reaches of the Yangtze River and its implications

The Jinniu Basin in southeast Hubei,located at the westernmost part of middle-lower val-ley of the Yangtze River,is one of the important vol-canic basins in East China. Volcanic rocks in the Jin-niu Basin are distributed mainly in the Majiashan Formation,the Lingxiang Formation and the Dasi Formation,consisting of rhyolite,basalt and basaltic andesite,(trachy)-basalt and basaltic trachy-andesite and (trachy)-andesite and (trachy)-dacite and rhyolite respectively,in which the Dasi volcanism is volumet-rically dominant and widespread. The Dasi volcanic rocks were selected for SHRIMP zircon U-Pb dating to confirm the timing of volcanism. The results indi-cate that there exist a large amount of magmatic zir-cons characterized by high U and Th contents in the volcanic rocks. The concordia ages for 13 points are 128 ± 1Ma (MSWD = 3.0). On account of the shape of zircons and Th/U ratios,this age is considered to represent the crystallization time of the Dasi volcan-ism. The volcanic rocks in the Dasi,Majiashan and Lingxiang Formations share similar trace elementand REE partition patterns as well as Sr-Nd isotopic compositions. In combination with the regional geol-ogy,it is proposed that the southeast Hubei volcanic rocks were formed mainly during the Early Creta-ceous,just like other volcanic basins in middle-lower Yangtze valley. A lithospheric extension is also sug-gested for tectonic regime in this region in the Cre-taceous Period.     

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.     

Discovery of ∼4.0 Ga detrital zircons in the Aermantai ophiolitic mélange, East Junggar, northwest China

The East Junggar is an important part of the Central Asian Orogenic Belt(CAOB).Using in situ zircon dating and Hf isotopic analysis by LA-ICP-MS and MC-ICP-MS,respectively,a detrital zircon of 4040 Ma age was found in sedimentary sequences from the Aermantai ophiolitic mélange,East Junggar.This is the oldest age record in the East Junggar terrane,and also marks the first zircon locality in the CAOB with an age older than 4.0 Ga,which is attributed to the Hadean crust.The 4040 Ma detrital zircon has anεHf(t)value of–5.2 and a two-stage Hf modal age of 4474 Ma,suggesting the presence of very old(Hadean)crustal material in the source area.Beside peak ages of 446 Ma,we found four age groups of 3.6–3.1 Ga,2.53–2.37 Ga,1.14–0.89 Ga and 0.47–0.42 Ga from 141 effective measuring points.The age of 426±4 Ma for the five youngest detrital zircons defines the lower limit of the deposition time of sedimentary sequencess in the Aermantai ophiolitic mélange.The 0.47–0.42 Ga zircons exhibit176Hf/177Hf ratios of 0.282156 to 0.282850,corresponding to variableεHf(t)values from–9.3 to 12.0 and Hf model ages from2011 to 646 Ma.These characteristics are similar to those of the early Paleozoic igneous and gneissic zircons from the Altai,but significantly different from those of the East Junggar.Based on the material structures of felspathic greywacke,the morphology,internal texture and age distributions of dated detrital zircons,in combination with a study of the regional geological data,it is suggested that the sedimentary sequences in the Aermantai ophiolitic mélange was deposited in the Late Silurian,with the main provenance from the Altai Orogen in the north.This indicates that the early Paleozoic ocean represented by the Aermantai ophiolitic mélange was readily closed during the Late Silurian,and the northern edge of the East Junggar terrane was accreted to the Altai Orogen.The joint of them then served as a marginal orogen in the southern edge of the Siberia Paleocontinent.     

～4.1 Ga xenocrystal zircon from Ordovician volcanic rocks in western part of North Qinling Orogenic Belt

High-precision U-Pb dating by in situ LA-ICP-MS yields an age of 4079±5 Ma for a xenocrystal zircon from Ordovician volcanics of the Caotangou Group in western part of the North Qinling Orogenic Belt. As a result, the North Qinling Orogenic Belt becomes one of a few localities in the world that contain Hadean age records (4276±6 Ma and 4404±8 Ma detrital zircons from Jack Hill of the Yilgarn craton, 4016 Ma Acasta gneisses of the Wopmay Orogeny and Burang quartzite with detrital zircon of 4103 Ma in Tibet). It is also the first report of the Hadean age in Phanerozoic volcanics. The finding of the 4.1 Ga xenocrystal zircon provides not only the geochronological record of the oldest crustal materials in China, but also the condition for further search for rocks forming in the region during the early time of the Earth’s evolution. Thirty-six zircon U-Pb dates from the Ordovician volcanic rocks are subgrouped into seven generations that represent different tectono-magmatic events in the North Qinling Orogenic Belt. Among them, two periods of 0.9-1.5 Ga and 0.4-0.5 Ga are consistent with Mesoproterozoic and Early Paleozoic orogenies, respectively.     

U–Pb ages and Hf isotopes for detrital zircons from quartzite in the Paleoproterozoic Songshan Group on the southwestern margin of the North China Craton

In situ U-Pb dating and Lu-Hf isotopic analysis were carried out for detrital zircons from quartzite in the Paleoproterozoic Songshan Group on the southern margin of the North China Craton (NCC). The re- sults provide further constraints on the crustal formation and evolution history of NCC. Four 207Pb/206Pb age populations were obtained from 99 analyses, with clusters at ~3.40 Ga, 2.77―2.80 Ga, ~2.50 Ga and 2.34 Ga, respectively. The 3.40 Ga old zircons have similar Hf isotopic compositions to those from Ar- chean rocks in the Jidong and Anshan areas of NCC. However, crustal remnants older than 3.6 Ga have been identified in the southern margin of NCC, the South China Craton, the northwestern part of the Qinling Orogen and its adjacent area. Thus, it is not easy to trace the source rock from which the 3.40 Ga detrital zircons were derived. It can be inferred that the crustal remnants older than 3.40 Ga might have been widely distributed in the North China Craton. The 2.77―2.80 Ga zircons make up a relatively small proportion and have the highest εHf (t) values (up to 6.1±1.6), consistent with the Hf isotopic composition of the depleted mantle at 2.83 Ga. Their single-stage Hf model age of 2.83 Ga is close to their crystallized age, suggesting that their source rocks were extracted from the contemporaneous depleted mantle. The ~2.50 Ga zircon grains constitute about 85% of the total grain population and their Hf isotopic compositions indicate major growth of juvenile crust at ~2.50 Ga but minor reworking of ancient crust. The youngest zircon dated in this study gave an U-Pb age of 2337±23 Ma, which can be considered the maximum depositional age of the formation of the Songshan Group.     

～4.1 Ga xenocrystal zircon from Ordovician volcanic rocks in western part of North Qinlin80rosenic Belt

High-precision U-Pb dating by in situ LA-ICP-MS yields an age of 4079±5 Ma for a xenocrystal zircon from Ordovician volcanics of the Caotangou Group in western part of the North Qinling Orogenic Belt. As a result, the North Qinling Orogenic Belt becomes one of a few localities in the world that contain Hadean age records （4276±6 Ma and 4404±8 Ma detrital zircons from Jack Hill of the Yilgarn craton, 4016 Ma Acasta gneisses of the Wopmay Orogeny and Burang quartzite with detrital zircon of 4103 Ma in Tibet）. It is also the first report of the Hadean age in Phanerozoic volcanics. The finding of the 4.1 Ga xenocrystal zircon provides not only the geochronological record of the oldest crustal materials in China, but also the condition for further search for rocks forming in the region during the early time of the Earth＇s evolution. Thirty-six zircon U-Pb dates from the Ordovician volcanic rocks are subgrouped into seven generations that represent different tectono-magmatic events in the North Qinling Orogenic Belt. Among them, two periods of 0.9--1.5 Ga and 0.4--0.5 Ga are consistent with Mesoproterozoic and Early Paleozoic orogenies, respectively.     

Chronological record of the Early Mesozoic underplating in the northern margin of North China─U-Pb chronometry of zircons in the Late Mesozoic andesite from Western Liaoning

Cathodoluminescence spectroscopic analysis of zircons in the uppermost sanukites of the early Cretaceous Yixian Formation in Western Liaoning reveals zoning structures of the zircons in the volcanic rocks. LA-ICP-MS chronometry shows that the central part of the zircons is remnant zircon with a U-Pb dating of 254 Ma, which is consistent with the age of the Early Mesozoic underplating granulites, and that the periphery is zircon crystallizing out of the host magma with a U-Pb dating of 116 Ma, which is highly consistent with the Ar-Ar dating of the host volcanic rocks. Compositions of the clinopyroxenes xenocrystals in the sanukites are similar to those of the clinopyroxenes in the underplating granulites, indicating that the granulites accreted to the lower crust in the Early Mesozoic have been reformed and disrupted by the upwelling diapers of the asthenosphere and taken part in formation of the sanukites in the Late Mesozoic.     

Chronological record of the Early Mesozoic underplating in the northern margin of North China─U-Pb chronometry of zircons in the Late Mesozoic andesite from Western Liaoning

Cathodoluminescence spectroscopic analysis of zircons in the uppermost sanukites of the early Cretaceous Yixian Formation in Western Liaoning reveals zoning structures of the zircons in the volcanic rocks. LA-ICP-MS chronometry shows that the central part of the zircons is remnant zircon with a U-Pb dating of 254 Ma, which is consistent with the age of the Early Mesozoic underplating granulites, and that the periphery is zircon crystallizing out of the host magma with a U-Pb dating of 116 Ma, which is highly consistent with the Ar-Ar dating of the host volcanic rocks. Compositions of the clinopyroxenes xenocrystals in the sanukites are similar to those of the clinopyroxenes in the underplating granulites, indicating that the granulites accreted to the lower crust in the Early Mesozoic have been reformed and disrupted by the upwelling diapers of the asthenosphere and taken part in formation of the sanukites in the Late Mesozoic.     

Geochronolosic constraints on masmatic intrusions and mineralization of the Zhunuo porphyry copper deposit in Gansdese, Tibet

In situ zircon U-Pb ages for the recently discovered Zhunuo porphyry copper deposit in the western part of the Gangdese metallogenic belt in Tibet were determined by sensitive high-resolution ion microprobe （SHRIMP）. The ages can be divided into two separate groups, reflecting more than four major tectono-magmatic events in the area. The 62.5±2.5 Ma age of inherited zircons may be related to the volcanic eruption of the Linzizong Group formed shortly after the India-Asia continental collision. The 50.1±3.6 Ma age most likely corresponds to the time of underplating of mantle-derived mafic magma in Gangdese. The 15.6±0.6 Ma age obtained from magmatic zircons is interpreted as the age of crystallization of the Zhunuo ore-forming porphyry. Finally, a molybdenite Re-Os isochron age of 13.72±0.62 Ma is consistent with another zircon U-Pb age of 13.3±0.2 Ma, representing the time of copper mineralization. These ages, in combination with available literature data, indicate that magmatic crystallization and copper mineralization in the Gangdese metallogenic belt became gradually younger westward, and further suggest that the Zhunuo porphyry copper deposit was formed in the same tectonic stage as other porphyry copper deposits in the eastern and central Gangdese belt. This conclusion provides critical information for future exploration of porphyry copper deposits in western Gangdese.     

Metallogenetic epoch and mechanism of the tungsten ore in Yushan,Beishan orogenic belt,Gansu

Using SHRIMP zircon U-Pb dating methods and cathodoluminescence （CL） technique, we have precisely dated ore samples from tungsten ore deposit, ore-bearing volcanic-sedimentary strata and granite in the Yushan region within the east Tianshan-Beishan W-bearing zone. This study reveals Yushan tungsten ore deposit zone formed in 250 ± 9 Ma, and it is closely associated with the Permian granites and felsic volcanic rocks. This new type of tungsten ore deposit in the east Tianshan-Beishang multiple-metallogenetic province may probably be associated with the regionally right-literal shearing after the main Carboniferous collisional orogeny, and the formation of pull-apart basin and the felsic magmatism.     

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…