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 dating of zircons from the Caledonian Xiongdian eclogite, western Dabie Mountains, China

This note reports the SHRIMP U-Pb data of zircons from the Caledonian Xiongdian eclogite, western Dabie Mountains. Zircons from the rock occur mainly in garnet and other metamorphic minerals with sharp boundaries and exhibit textures growing under metamorphic conditions. Analyses of 7 grains give ²⁰⁶Pb/²³⁸U ages ranging from 335 to 424 Ma, showing a certain degree of radiogenic lead loss. This suggests a minimum age of (424±5) Ma for the metamorphic zircons, as well as the high-pressure metamorphic event. The outer peripheral zone of a zircon gives ²⁰⁶Pb/²³⁸U age of about 300 Ma. Combined with Sm-Nd, ⁴⁰Ar-³⁹Ar, U-Pb and ²⁰⁷Pb/²⁰⁶Pb ages, the peak metamorphism of the Xiongdian eclogite is documented between 424—480 Ma.     

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.     

The age of the base of the paleoproterozoic Hutuo Group in the Wutai Mountains area,North China Craton: SHRIMP zircon U-Pb dating of basaltic andesite

Basalt and basaltic andesite metamorphosed at greenschist facies occurs with conglomerate layers at the bottom of the Paleopro-terozoic Hutuo Group in the Wutai Mountains area, North China Craton. Detailed geological surveying confirms that these volcanic rocks are conformable within the neighboring sedimentary rocks. The SHRIMP results on basaltic andesite are divided into two groups. In one group the ²⁰⁷Pb/²⁰⁶Pb ages are from 2433 to 2558 Ma, which is consistent with the basement crustal age in Fuping and Wutai areas. In the other group, 13 grains yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2140±14 Ma. The latter is interpreted as the eruption age of the basaltic andesite, and gives the age of the base of the Hutuo Group. This result further suggests that the Hutuo Group formed in the middle Paleoproterozoic, not the early Palaeoproterozoic or late Archaean, as thought before, and is related to a 2.2–2.1 Ga rifting event in the Central North China Craton.     

Late Paleozoic magmatism in South China:Oceanic subduction or intracontinental orogeny?

A gneissic granite with an U-Pb age of 313±4 Ma was found in northeastern Fujian Province,South China.It is an S-type granite characterized by high K2O,Al2O3 and low SiO2,Na2O contents with high A/CNK ratio of 1.22 for the whole rock.Zircons with stubby morphology from the gneissic granite yield 206 Pb/238 U ages ranging from 326 to 301 Ma with a weighted average age of 313±4 Ma,and negative εHf(t) values from -8.35 to -1.74 with Hf model ages (TCDM) of 1.43 to 1.84 Ga.This S-type granite probably originated from late Paleoproterozoic crust in intracontinental orogeny.Integrated with previous results on paleogeographic reconstruction of South China,the nature of Paleozoic basins,Early Permian volcanism and U-Pb-Hf isotope of detrital zircons from the late Paleozoic to early Mesozoic sedimentary rocks,we suggest the occurrence of a late Paleozoic orogeny in the eastern Cathaysia Block,South China.This orogenic cycle includes Late Carboniferous (340-310 Ma) orogeny (compression) episode and Early Permian (287-270 Ma) post-orogenic or intraplate extension episode.Therefore,the late Paleozoic magmatism in the southeastern South China probably occurred during the intraplate orogeny rather than the arc-related process.     

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.     

Indosinian isotope ages of plutons and deposits in southwestern Miaoershan-Yuechengling,northeastern Guangxi and implications on Indosinian mineralization in South China

The Miaoershan-Yuechengling complex pluton is the largest granitoid complex in the western Nanling metallogenic belt with a surface exposure of >3000 km2.The complex pluton is composed of an early stage granitoid batholith and late stage small intrusions.The early stage batholith contains mainly medium-grained porphyritic mica granite and porphyritic monzonite granite,whereas the late stage intrusions are composed of muscovite granite porphyry and muscovite monzonitic granite.There are many W-Sn-Mo-Pb-Zn-Cu ores in the contact zone between the batholith and strata,forming an ore-rich belt around the batholith.Based on zircon LA-ICP-MS U-Pb ages,the southwestern part of the early stage batholith formed at 228.7 ± 4.1 Ma(MSWD = 2.49),with slightly earlier magmatic activity at 243.0 ± 5.8 Ma(MSWD = 2.62).The Yuntoujie muscovite granite was associated with W-Mo mineralization and has a zircon LA-ICP-MS U-Pb age of 216.8 ± 4.9 Ma(MSWD = 1.44).The Re-Os isochron age of molybdenite from the Yuntoujie W-Mo ore was 216.8 ± 7.5 Ma(MSWD = 11.3).Our new isotope data suggest that the late stage intrusive stocks from the southwestern Miaoershan-Yuechengling batholith were closely associated with W-Mo mineralization from the Indosinian period.These new results together with previous isotope data,suggest that South China underwent not only the well-known Yanshanian mineralization event,but also a widespread Indosinian metallogenic event during the Mesozoic period.Therefore,South China has a greater potential for Indosinian mineralization than previously thought such that more attention should be given to the Indosinian ore prospecting in South China.     

SHRIMP U-Pb zircon age of tuff at the bottom of the Lower Cambrian Niutitang Formation,Zunyi, South China

A SHRIMP U-Pb zircon geochronology study of the tuff at the bottom of the Lower Cambrian Niutitang Formation at Songlin, Zunyi, South China yielded a mean ^206Pb/^238U age of 518±5 Ma. It is significantly younger than the Re-Os ages of 537--542 Ma for the overlying polymetallic Ni-Mo-PGE-rich layer in the basal Niutitang Formation at Songlin, suggesting that the Re-Os ages might represent that of the original magma chamber for the Ni-Mo-PGE-rich layer rather than the formation age of the strata. The intra-basinal stratigraphic correlation also implies that the absolute age of the Chengjiang biota should be younger than 518±5 Ma. Our new result, together with the SHRIMP U-Pb zircon age for the K-bentonite in the Zhongyicun member of the Zhujiaqing Formation at the Meishucun section, Yunnan Province, provides a temporal constraint for the Lower Cambrian of the Yangtze Platform in South China.     

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…     

北祁连西段熬油沟——-二只哈拉达坂蛇绿岩的形成环境和时代

熬油沟蛇绿岩是北祁连造山带西段最具代表性的蛇绿岩残片之一。运用SHRIMPⅡ锆石U-Pb法, 对北祁连造山带西段的熬油沟和二只哈拉达坂两地出露的细粒辉长岩和粒玄岩进行了精确同位素测年。采自熬油沟剖面的细粒辉长岩样品 ²⁰⁶Pb/²³⁸U年龄数据分布在一个很小的范围(490~508 Ma), 其加权平均年龄为501±4 Ma (MSWD=1.09); 采自二只哈拉剖面的粒玄岩样品年龄值稍小, ²⁰⁶Pb/²³⁸U加权平均年龄为495±4 Ma (MSWD=0.98)。结合详细的野外调查、矿物化学和地球化学研究, 认为熬油沟?二只哈拉达坂蛇绿岩构造破坏严重, 其中基性岩浆岩代表早?中寒武世的北祁连洋壳。与新元古界朱龙关群碳酸盐岩和砂泥岩整合产出 的玄武岩不能作为蛇绿岩的喷出岩系列, 而有可能是震旦纪的大陆溢流玄武岩, 代表新元古代全球Rodinia大陆裂解的岩浆产物。     

新疆东准噶尔铜华岭中酸性侵入体锆石U-Pb年代学研究

伊吾县铜华岭中酸性侵入体位于新疆北部东准噶尔阿尔曼太断裂和克拉麦里断裂之间的野马泉岛弧区, 岩性主要为闪长玢岩、英云闪长岩和二长花岗岩等。通过锆石LA-ICP-MS U-Pb年代学方法, 对这3种岩性进行精确定年, 其²⁰⁶Pb/₂₃₈U的加权平均年龄分别为427.4±3.2,422.8±3.2和418.5±2.6Ma,说明铜华岭岩体是早古生代岩浆活动的产物, 为东准噶尔早古生代造山带演化提供了花岗岩证据。     

西藏日土白垩纪酸性侵入岩年代学及地球化学特征——对班公湖-怒江中特提斯洋盆闭合时限的制约

以西藏日土县北东地区的小型酸性侵入体为研究对象,结合岩石地球化学分析及锆石年龄测试,探讨岩石的成因类型及构造环境,确定班公湖-怒江中特提斯洋盆闭合的时限。研究区酸性岩体的岩性为黑云二长花岗岩,其主要组分的质量分数分别是:SiO_2为63.89%~77.49%,全碱(K_2O+Na_2O)为8.06%~8.95%,全铁(FeOT)为0.48%~4.37%,Al_2O_3为12.43%~16.17%;大离子亲石元素中Rb、Th、U、K明显富集,但Ba、Sr明显亏损;高场强元素中Nb、Ta、P、Ti明显亏损,而Zr、Hf相对富集;轻稀土元素相对富集、重稀土元素相对亏损,负Eu异常明显(δEu=0.09~0.67,平均为0.47),球粒原始标准化分布模式为向右缓倾的V型,属于A型花岗岩。其LA-ICP-MS锆石206Pb/238 U年龄加权平均值为(117.8±1.2)Ma,属于中生代早白垩世中期。从岩石碰撞后A1型花岗岩特征推测,班公湖-怒江中特提斯洋盆在日土地区的闭合时间应在早白垩世中期。     

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…     

In situ U-Pb dating of titanite by LA-ICPMS

Titanite is an ideal mineral for U-Pb isotopic dating because of its relatively high U,Th and Pb contents.Here,we developed a technique for U-Pb dating of titanite using the 193 nm ArF laser-ablation system and Agilent 7500a Q-ICP-MS.Standards of titanite (BLR-1 and OLT-1) and zircon (91500 and GJ-1) were dated using single spot and line raster scan analytical methods.To check the matrix effect,titanite (BLR-1) and zircon (91500) standards were analyzed as the external standards.The weighted mean 206 Pb/238 U ages of OLT-1 titanite are 1015±5 Ma (2,n=24) and 1017±6 Ma (2,n=24) by single spot and line raster scan analyses,respectively,using BLR-1 titanite as the external standard.These ages are consistent with its reference age of about 1014 Ma.However,using 91500 zircon as the external standard,the weighted mean 206 Pb/238 U ages are 917±4 Ma (2,n=24) and 927±5 Ma (2,n=24) for BLR-1 titanite,and 891±4 Ma (2,n=24) and 901±5 Ma (2,n=24) for OLT-1 titanite by single spot and line raster scan analyses,respectively.It is evident that these ages are ～12% younger than their reference values.Our results reveal that significant matrix effect does exist between different kinds of minerals during LA-ICPMS U-Pb age determination,whereas it is insignificant between same minerals.Therefore,same mineral must be used as the external standard for fractionation corrections during in situ LA-ICPMS U-Pb age analysis.In addition,we determined U-Pb ages for titanites from the Early Cretaceous Fangshan pluton,which indicates a rapid cooling history of this pluton.     

Zircon U-Pb ages of granites at Changba and Huangzhuguan in western Qinling and implications for source nature

Mesozoic granitoids are widespread in the Qinling-Dabie-Sulu orogenic belt. Precise U-Pb dating on these granitoids can reveal the evolution of the continental collision orogen and thus provide information on the nature of magma sources. This study presents zircon LA-ICP-MS U-Pb dating and whole-rock geochemical analyses for two intrusions at Changba and Huangzhuguan in western Qinling. Zircon U-Pb ages for central and marginal phases of the Huangzhuguang intrusion are 214±1 Ma and 213±3 Ma, respectively. Zircons from the Changba intrusion yield a dominant cluster with an U-Pb age of 213±2 Ma. Collectively, these ages are younger than ages of 220 to 240 Ma for ultrahigh-pressure metamorphism due to the continental collision between the South China Block and the North China Block, corresponding to syn-exhumation magmatism. Some inherited zircons occur in the Changba intrusion, yielding a weighted mean of ²⁰⁶Pb/²³⁸U ages at 757±14 Ma. This indicates that the Changba intrusion has the crustal source of mid-Neoproterozoic ages and a tectonic affinity to the South China Block. Geochemically, the two intrusuons are both rich in LILE and LREE but depleted in HFSE and HREE, similar to arc-type igneous rocks. The Huangzhuguang intrusion exhibits linear correlations between SiO₂ and the other major oxides, implying chemical evolution from a cognate magma source. It contains mafic enclaves, suggesting possible mixing of felsic-mafic magmas. The Changba granite is rich in Si and K but poor in Fe and Mg as well as has a high value of Fe*, suggesting strong differentiation of granitic magma. Therefore, the two intrusions were derived from the Late Triassic anatexis of the continental crust of different compositions in the northern margin of South China Block. This process may be coupled with exhumation of the subducted continental crust in the stage of late collision.     

Permian volcanisms in eastern and southeastern margins of the Jiamusi Massif, northeastern China: zircon U-Pb chronology, geochemistry and its tectonic implications

LA-ICP-MS zircon U-Pb dating and geochemical data for the Late Paleozoic volcanic rocks from eastern and southeastern margins of the Jiamusi Massif are presented to understand the regional tectonic evolution. Zircons from eight representative volcanic rocks are euhedral-subhedral in shape and display striped absorption and fine-scale oscillatory growth zoning as well as high Th/U ratios （0.33-2.37）, implying a magmatic origin. The dating results show that the Late Paleozoic volcanic rocks in the study area can be divided into two stages, i.e., the Early Permian （a weighted mean 205^Pb/238^U age of 288 Ma） and the Middle Permian volcanisms （a weighted mean 205^Pb/238^U age of 268 Ma）. The former is composed mainly of basalt, basaltic-andesite, andesite and minor dacite. They are characterized by low SiO2 contents, high Mg^# （0.40-0.59）, enrichment in Na （Na2O/K2O = 1.26-4.25） and light rare earth elements （LREEs）, relative depletion in heavy rare earth elements （HREEs） and high field strength elements （HFSEs）, indicating that an active continental margin setting could exist in the eastern margin of the Jiamusi Massif in the Early Permian. The latter consists mainly of rhyolite and minor dacite with high SiO2 （77.23%-77.52%）, low MgO （0.11%-0.14%）, enrichment in 1（20 （Na2O/K2O ratios 〈 0.80） and Rb, Th, U and depletion in Eu, Sr, P and Ti, implying a crust-derived origin. Therefore, it is proposed that the Middle Permian volcanic rocks could have formed under the collision of the Jiamusi and the Khanka Massifs.     

Zircon U-Pb dating for the earliest Neoproterozoic mafic magmatism in the southern margin of the North China Block

Geological records of Neoproterozoic magmaticevents have recently been identified in the central Phanerozoic orogenic belts of China[1]. In regions of east Qinling orogen[2], Dabie-Sulu orogen[3], north Qaidam Basin orogen[4―6], and southwest Tarim Basin…     

Opening of the Tethys in southwest China and its significance to the breakup of East Gondwanaland in late Paleozoic:Evidence from SHRIMP U-Pb zircon analyses for the Garzêophiolite block

SHRIMP U-Pb zircon analyses for a gabbro sample from the Garzê ophiolite block yielded a mean²⁰⁶Pb/²³⁸U age of 292 ±4 Ma, which indicated that the spreading time of the Garzê-Litang Tethys was most likely at the earliest Permian. Combined with previous studies, we suggest that the opening of the Tethys in southwest China was derived from breakup of the East Gondwanaland in the late Paleozoic.     

Identification of 3.5 Ga detrital zircons from Yangtze craton in south China and the implication for Archean crust evolution

The LA-ICP-MS U-Pb dating of hundreds of detrital zircon grains from the Sinian sandstones of Liantuo formation and tillites of Nantuo formation at Sanxia area in Yichang identified 3319?3508 Ma zircon grains. Their ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁶Pb/²³⁸U ages show excellent agreement (concordia degree 99%?100%). Their CL images exhibit well-developed oscillatory zoning and the Th/U ratios are within 0.46?0.76, implying that they are igneous zircons which formed during middle-early Archean. These zircons are the oldest ones discovered in Yangtze craton until now. However, the detrital zircons with ages older than 3.3 Ga in the metamorphic rocks of Kongling group were not found by further investigation, which suggests the presence of crust older than high-grade metamorphic Kongling terrain in Yangtze craton.     

U-Pb zircon dating constraints on formation time of Qilian high-grade metamorphic rock and its tectonic implications

In order to constrain the formation time of high-grade metamorphic rocks in the Qilian Mountains, U-Pb zircon dating was carried out by using LA-ICPMS technique for a paragneiss of the Hualong Group in the Qilian Mountains basement series and a weakly foliated granite that intruds into the Hualong Group. Zircons from the paragneiss consist dominantly of detrital magma zircons with round or sub-round shape. They have 207Pb/206Pb ages mostly ranging from 880 to 900 Ma, with a weighted mean age of 891 ±9 Ma, which is interpreted as the magma crystallization age of its igneous provenance and can be taken as a lower age limit for the Hualong Group. Magma crystallization age for the weak-foliated granite is 875±8 Ma, which can be taken as an upper age limit for the Hualong Group. Accordingly, the formation time of the Hualong Group is constrained at sometime between 875 and 891 Ma. A few zir- cons from both paragneiss and weak-foliated granite display old inherited ages of 1000 to 1700 Ma and young metamorphic ages of Early Paleozoic. The zircon age distribution pattern confirms that the Qilian Mountains and the northern margin of Qaidam Basin had a united basement, with geotectonic affinity to the Yangtze Block. The results also reveal that sediments of the Hualong Group formed by rapid accumulation due to rapid crustal uplift-erosion. This process may result from intensive Neoproterozoic orogenesis due to assembly of the suppercontinent Rodinia.