SHRIMP zircon U-Pb geochronology of Indosinian granites in Hunan Province and its petrogenetic implications

The structure, magmatism and sedimentation within the South China Block (SCB) related to the Indosinian Orogeny had attracted considerable attention since De-prat[1] and Fromagat[2] proposed the “Indosinian move-ment” based on two unconformities between Pre-Norian and Pre-Rhaetian during Triassic in Vietnam. However, Indosinian tectonic evolution of the SCB has been long debated[3―6]. Some researchers believed that the compli-cated structure-magmatism-sedimentation within the SCB w…     

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.     

SHRIMP U-Pb zircon geochronology of Yangfang aegiriteaugite syenite in Wuyi Mountains of South China and its tectonic implications

In addition to the wide spread peraluminous granites, some alkaline or alkali-rich intrusive rocks were recently observed in the Wuyi Mountains. The Yangfang aegiriteaugite syenites in the Wuyi Mountains are strongly enriched in light rare earth elements (LREE), K, Ba, Ga, Zr and Y. What is more, differentiations of REE are obvious. In addition, the Yangfang aegiriteaugite syenites have also some characteristics similar to A-type granite. Zircons are selected from the Yangfang aegiriteaugite syenites for Sensitive High-Resolution Ion Microprobe (SHRIMP-Ⅱ) U-Pb geochronology study. 15 analyses give concordant 206Pb/238U ages in a narrow range of 233—249 Ma, which correspond to a single age population with a weighted mean 206Pb/238U age of 242 4 Ma, which records the crystallization time of the aegiriteaugite syenites. It is suggested that: (1) there were early Triassic peralkaline intrusive rocks in South China; (2) the early Indosinian extension event was probably active in local area, South China. This discovery provides a new insight for farther understanding of 揑ndosinian orogeny in South China.     

Zircon SHRIMP geochronology and geochemistry of TTG rocks in Sushui Complex from Zhongtiao Mountains with its geological implications

The tonalite-trondhjemite-granodiorite (TTG) rocks in Sushui Complex of Zhongtiao Block can be divided into two series according to their zircon U-Pb ages and geochemical characteristics: one is subduction-related (SR) and the other collision-related (CR). The SR TTG rocks, together with other Late Archean island arc magmatism, were developed as a result of oceanic subduction between the Eastern and Western blocks in Late Archean; while the CR TTG rocks formed in a thickened crustal environment, which was responding to the collision between the Eastern and Western blocks in Paleoproterozoic. All these features support a model that the Zhongtiao Block is a part of the Trans-North China Orogen in the middle of North China Craton.     

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…     

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 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.     

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.     

Mixing events between the crust- and mantle-derived magmas in Eastern Kunlun: Evidence from zircon SHRIMP Ⅱ chronology

Granitoidswidelyoccurincontinentalcrustandareusuallygeneratedinresponsetocontinentalgrowthandassociatedcrust-mantleinteraction.Granitoidscanbepro-ducedthroughdifferentways,ofwhichmagmamixinghasdrawnspecialattentionbecauseofitsprofoundbear-ingsoncrust-mantleinteractionandcontinentalgeody-namics[1].Themostdistinctfeatureforgranitoidsformedbymagmamixingisprobablytheoccurrenceofmaficmicrogranularenclaves(MMEs).Sofar,originforMMEsisstillhotlydebated,andavailablehypothesesconsidertheMMEsas(1)ref…     

Emplacement age of the Tahe granite and its constraints on the tectonic nature of the Ergun block in the northern part of the Da Hinggan Range

Large-scale granitic plutons are exposed in the Ergun block in the northern part of the Da Hinggan Range, but their emplacement age and petrogenesis remain unknown. Of these plutons, the Tahe pluton is composed mainly of porphyritic syenogranite and monzogranite, with minor hornblende alkali feldspar granite and gabbro, which have affinities to post-orogenic granitoids. Laser-ablation Inductively Coupled Plasma Mass-spectroscope （LA-ICP-MS） analysis shows that the major rock types of the Tahe pluton formed at 494- 480 Ma ago, indicating its emplacement in the Early Paleozoic. It is concluded, therefore, that the collision between the Ergun and the Xing＇an blocks ended in Early Paleozoic. Considering the geochronological data of the plutons in the adjacent areas, the Early Paleozoic evolutionary history of the Ergun block is similar to the central Mongolia and the Tuvino blocks in Mongolia, and the Ergun block should be a part of the accretionary continental margin in the southern Siberian Craton.     

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 geochronology and petrogenesis of the plagiogranites from the Lagkor Lake ophiolite, Gerze, Tibet, China

Field relationship, along with petrographical, petrological, and geochemical characteristics, and zircon populations, indicates that the plagiogranites from the Lagkor Lake ophiolite in the Bangong-Nujiang suture zone (~ 20 km south of Gerze County, central Tibet, China), derived from anatexis of hydrated gabbros by ductile shearing during transport of the oceanic crust. Therefore, this kind of plagiogranite is coeval to or little later than the spreading of the oceanic crust. Zircon SHRIMP age of 166 ± 2.5 Ma for the plagiogranites indicates that an oceanic basin existed in the western part of the Bangong-Nujiang zone in the Middle Jurassic.     

SHRIMP U-Pb zircon dating for the dacite of the Sangxiu Formation in the central segment of Tethyan Himalaya and its implications

The Tethyan Himalaya is located between the In-dus-Yarlung Zangbo suture zone to the north and HigherHimalayan crystallines to the south, paleogeographicallybelonging to the northern margin of “Greater India”. TheSangxiu Formation is used to define a series of specialstrata with rich volcanic interbeds, and is only distributedto the southeast of Yangzuoyong Co within 200 km×50 km along the eastern part of central Tethyan Hima-laya[1]. Based on overlapped relationships of strata andfos…     

Yttrium zoning in garnet from the Xihuashan granitic complex and its petrological implications

The Xihuashan granitic complex is characterized by enrichment in rare-earth elements (REE). In particular, the second-stage granite (G-b) is markedly enriched in yttrium, and therefore contains complex associations of Y-bearing minerals. In this granite, garnet displays specific yttrium zoning with an Y-rich core and a “clean“ rim. Besides minute inclusions of Y-bearing minerals, garnet involves a striking amount of Y and HREE in its central area.It is suggested that enrichment in Y in the garnet core accords with that in the melt as a result of REE magmatic fractionation. However, the “clean“ rim may be the direct result of accumulation of fluid phases in the magma, which is virtually unfavorable for the entrance of REE in the garnet structure.     

SHRIMP U-Pb zircon geochronology of granulites at Rimana （Southern Tibet） in the central segment of Himalayan Orogen

TheCenozoicHimalayanorogenisimportantinun-derstandingcontinentaldynamicprocesses.Lowercrustal,high-grademetamorphicrockshaverecentlybeendiscov-eredinNamcheBarwaandParmir,i.e.therespectivere-gionsofeasternandwesternsyntaxisoftheorogen[1—4].Eclogitesthatunderwentgranulite-faciesmetamorphismhaverecentlybeenreportedintheCadaareatotheeastofMountEverestinthecentralsegmentoftheHimalayanorogen[5].WiththehelpofthegeologicalsurveyofDing-gyeCountyandChentangSheets(scale1︰250000),high-pressuremafic…     

LA-ICP-MS U–Pb geochronology of detrital zircons from the Zhaochigou Formation-complex in the Helan Mountain and its tectonic significance

This paper carries out studies about LA-ICPMS U–Pb dating and Hf isotopic compositional analysis for detrital zircons in two metasedimentary samples that were taken from the Zhaochigou Formation-complex in the Helan Mountain.The results show that the Zhaochigou Formation-complex formed*1.96 Ga,and it is a part of the Khondalite Series that is widespread in the North China Craton(NCC).The age spectrum of the detrital zircons indicates that the Zhaochigou Formation-complex shared the same provenance with the eastern edge of the Khondalite Series.Products of ancient magmatic activities within the Khondalite Belt may be the major source for the sedimentary materials that formed the Khondalite Series.Our age-dating results suggest that the Khondalite Series in the NCC may have experienced two stages of metamorphism,as the early stage occurred during 1.96–1.95 Ga,and the later stage occurred at about 1.87 Ga.The Hfisotopic data indicate that the Western Block of the NCC endured a stage of crustal growth at 2.5–2.3 Ga.     

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 …     

Zircon U-Pb SHRIMP ages of high-pressure granulite in Yushugou ophiolitic terrane in southern Tianshan and their tectonic implications

Zircons from two high-pressure granulite samples from the Yushugou ophiolitic terrane, southern Tian shan have been investigated by cathodoluminescence, LA ICPMS and ion microprobe (SHRIMP) for their internal textures, trace elemental compositions and in situ dating. The weighted mean ages of these two samples are 392±7 and 390±11 Ma, respectively, representing the granulite-facies meta morphic age of the ophiolitic terrane, and indicating that the southern Tianshan ocean initiated its northward subduction since the early Devonian.     

The zircon SHRIMP chronology and trace element geochemistry of the Carboniferous volcanic rocks in western Tianshan Mountains

The genesis of the Carboniferous volcanic rocks in the Dahalajunshan group, mainly consisting of trachytes and trachy-andesites and distributing widely in western Tianshan Mountains, remains to be controversial. It has been proposed to be relevant to “rift” or “plume”. Detailed petrology and geochemical data presented in this paper show that these volcanic rocks represent typical continental arc magmatism. The volcanic rocks are mainly trachy-andesitic, and the magma source is enriched in LILE, Th and Pb, and depleted in HFSE and Ce. Trace element geochemical study suggests that the basalts could be modeled by 7% -11% partial melt of garnet Iherzolite. The volcanic rocks in the Dahalajunshan group are neither the products of “rift” nor so-called “plume”but represent the continental island arc of the Paleo-Southern Tianshan Ocean. The mantle wedge had been modified by the melt generating in subduction zone during a long evolution history of this island arc. The continental crust materials （i.e. mainly sediment on ocean floor） had been added into island arc through melt in subduction zone. Volcanic rocks occurring in different regions might represent magma eruption in different time. The zircon SHRIMP dating indicates that the ages of the basalt varies between 334.0 Ma and 394.9 Ma. The 13 analyses give an average age of 353.7±4.5 Ma （MSWD = 1.7）. The apparent ages of zircons in trachy-andesite vary between 293.0 Ma and 465.4 Ma. All analyses fall on the U-Pb concordant line and are divided into two groups. 8 analyses produce an average age of 312.8±4.2 Ma （MSWD = 1.7）, which represents the crystallizing age of zircon rims in trachy-andesite. The acquired two ages （i.e. 354 and 313 Ma） belong to the Early Carboniferous and Late Carboniferous epochs, respectively. Thus, the Dahalajunshan group would be separated into several groups with the accumulation of high-quality age dating and data of trace element and isotopic geochemistry, in our opinion.     

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…