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 results 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 Archean 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 Ge 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±2.3 Ma, which can be considered the maximum depositional age of the formation of the Songshan Group.     

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.     

An in situ zircon Hf isotopic,U-Pb age and trace element study of banded granulite xenolith from Hannuoba basalt： Tracking the early evolution of the lower crust in the North China craton

Backscattered electron images, in situ Hf isotopes, U-Pb ages and trace elements of zircons in a banded granulite xenolith from Hannuoba basalt have been studied. The results show that the banded granulite is a sample derived from the early lower crust of the North China craton. It is difficult to explain the petrogenesis of the xenolith with a single process. Abundant information on several processes, however, is contained in the granulite. These processes in-clude the addition of mantle material, crustal remelting, metamorphic differentiation and the delamination of early lower crust. About 80% of zircons studied yield ages of 1842 ±40 Ma, except few ages of 3097-2824 Ma and 2489-2447 Ma. The zircons with ages older than 2447 Ma have high εHf (up to +18.3) and high Hf model age (2.5-2.6 Ga), indicating that the primitive materials of the granulite were derived mainly from a depleted mantle source in late Archean. Most εhf of the zircons with early Proterozoic U-Pb age vary around zero, but two have     

Zircon U–Pb and Hf isotopic study of Neoproterozoic granitic gneisses from the Alatage area,Xinjiang:constraints on the Precambrian crustal evolution in the Central Tianshan Block

The architecture and growth history of Precambrian crustal basements in the Central Tianshan Block play a key role in understanding the tectonic evolution of the Chinese Tianshan Orogenic Belt.In this study,we present precise LA-ICP-MS zircon U–Pb dating and LAMC-ICPMS zircon Hf isotopic data for two granitic gneisses from Alatage area in the Central Tianshan Block.The magmatic zircons from both samples yield similar protolith ages of 945±6 and 942±6 Ma,indicating that the early Neoproterozoic magmatism is prevailed in the Alatage area.These zircons have crustal Hf model ages of1.82–2.22 and 1.70–2.03 Ga,respectively,which are significantly older than their crystallization ages.It indicates that their parental magmas were derived from the reworking of ancient crust.However,we suggest that these Paleoproterozoic Hf model ages might result from mixing of continental materials with different ages in the Neoproterozoic crust.The inherited(detrital)zircon cores not only yield a wide age range of ca.989–1617 Ma,but also exhibit large Hf-isotope variations with Hf model ages of1.54–2.30 Ga.In particular,some 1.4–1.6 Ga zircons show high initial176Hf/177Hf ratios,consistent with those of depleted mantle,which indicates that the Mesoproterozoic event involved both reworking of older crust and generation of juvenile crust.The Central Tianshan Block has different Precambrian crustal growth history from the Tarim Craton.Therefore,it would not be a fragment of the Precambrian basement of the Tarim Craton.     

Formation and evolution of Precambrian continental crust in South China

The occurrence of zircons with U-Pb ages of ~3.8 Ga and Hf model ages of ~4.0 Ga in South China suggests the existence of the Hadean crustal remnants in South China. Furthermore, a detrital zircon with a U-Pb age as old as 4.1 Ga has been found in Tibet. This is the oldest zircon so far reported in China. These results imply that continental crust was more widespread than previously thought in the late Hadean, but its majority was efficiently reworked into Archean continental crust. On the basis of available zircon U-Pb age and Hf isotope data, it appears that the growth of continental crust in South China started since the early Archean, but a stable cratonic block through reworking did not occur until the Paleoproterozoic. Thus the operation of some form of plate tectonics may occur in China conti- nents since Eoarchean. The initial destruction of the South China craton was caused by intensive magmatic activity in association with the assembly and breakup of the supercontinent Rodinia during the Neoproterozoic. However, most of the Archean and Paleoproterozoic crustal materials in South China do not occur as surface rocks, but exist as sporadic crustal remnants. Nevertheless, the occur- rence of Neoproterozoic magmatism is still a signature to distinguish South China from North China.     

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.     

Ca. 2.5 Ga TTG rocks in the western Alxa Block and their implications

The Alxa Block is considered part of the North China Craton, but the unambiguous Archean basement has not been reported. In this study, we present the first evidence of the Neoarchean rocks in the Beidashan area of the western Alxa Block. The petrographic and geochemical data show that these rocks are granodiorite with TTG (tonalite-trondhjemite-granodiorite) characteristics. Zircon U-Pb dating gave an age of 2522±30 Ma for the magmatic core and 2496±11 Ma for the metamorphic recrystallized rim. The near-identical age between the Latest Neoarchean magmatism and the high-grade metamorphism shows that these features were related to the same Latest Neoarchean-Earliest Paleoproterozoic tectonothermal event. The age-corrected Hf (t) value is mainly between 0.4 and 4.9. The two-stage zircon Hf model age ranges from 2.7 to 3.0 Ga, suggesting that the Mesoarchean- Neoarchean (2.7-3.0 Ga) juvenile crust was reworked at the end of the Neoarchean in the western Alxa Block. These data suggest that the western Alxa Block experienced a Mesoarchean-Neoarchean crust growth and Latest Neoarchean-Earliest Paleoproterozoic tectonothermal event similar to the North China Craton.     

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.     

Palaeoproterozoic Khondalite Belt in the western North China Craton： New evidence from SHRIMP datin8 and Hf isotope composition of zircons from metamorphic rocks in the Bayan Ui-Helan Mountains area

Zircom U-Pb age and Hf isotope analyses were made on gneissic granite and garnet-mica two-feldspar gneiss from the Helanshan Group in the Bayan Ul-Helan Mountains area, the western block of the North China Craton (NCC). Zircons from the gneissic granite commonly show core-mantle-rim structures, with magmatic core, metamorphic mantle and rim having ages of 2323±20 Ma, 1923±28 Ma and 1856±12 Ma, respectively. The core, mantle and rim show similar Hf isotope compositions, with single-stage depleted mantle model ages (TDM1) of 2455 to 2655 Ma (19 analyses). Most of the detrital zircons from the garnet-mica two-feldspar paragneiss have a concentrated U-Pb age distribution, with a weighted mean 207Pb/206Pb age of 1978±17 Ma. A few detrital zircons are older (2871 to 2469 Ma). The age for metamorphic overgrown rim was not determined because of strong Pb loss due to their high U content. The zircons show large variation in Hf isotope composition, with TDM1 ages of 1999 to 3047 Ma. In com- bination with previous studies, the main conclusions are as follows: (1) protolith of the khondalite se- ries in the Helanshan Group formed during Palaeoproterozoic rather than the Archaean as previously considered; (2) The results lend support to the contention that there is a huge Palaeoproterozoic Khondalite (metasedimentary) Belt between the Yinshan Mountains Block and the Ordos Block in the Western Block of NCC; (3) The widely-distributed bodies of early Palaeoproterozoic orthogneisses in the Khondalite Belt might be one of the important sources for detritus material in the khondalite series; (4) Collision between the Yinshan Block, the Ordos Block and the Eastern Block occurred in the same tectonothermal event of late Palaeoproterozoic, resulting in the final assembly of the NCC.     

Zircon U-Pb age and Hf isotope of Quanyishang A-type granite in Yichang: signification for the Yangtze continental cratonization in Paleoproterozoic

Zircon U-Pb age and Hf isotope, and major and trace element compositions were reported for granite at Quanyishang, which intruded into the Kongling complex in Yichang, Hubei Province. The results show that the Quanyishang granite is rich in silicon and alkalis but poor in calcium and magnesium, and displays enrichment in Ga, Y, Zr, Nb but depletion in Sr and Ba, exhibiting the post-orogenic A-type affinity. 90% zircons from the granite are concordant, and give a middle Paleoproterozoic magmatic crystallization age (mean 1854 Ma). Initial Hf isotope ratios (176Hf/177Hf)i of the middle Paleoproterozoic zircons range from 0.280863 to 0.281134 and they have negative eHf(t) values with a minimum of -26.3. These zircons give the depleted mantle model ages (TDM) of 2.9―3.3 Ga (mean 3.0 Ga), and the average crustal model ages (Tcrust) of 3.6―4.2 Ga (mean 3.8 Ga). A Mesoarchean grain with 207Pb/206Pb age of 2859 Ma has a slightly high TDM (3.4 Ga) but similar Tcrust (3.8 Ga) to the Paleoproterozoic zircons. All these data suggest that the source materials of the Quanyishang A-type granite are unusually old, at least ≥2.9 Ga (even Eoarchean). The event of crustal remelting, which resulted in the formation of the Quanyishang granite in the middle Paleoproterozoic, recorded the cratonization of the Yangtze conti-nent. The process may have relation to the extension and collapse of the deep crust with Archean ages, in response to the transition stage of the assembly and breakup of the Columbia supercontinent.     

吕梁地区近周营组蚀变火山岩 U-Pb-Hf同位素特征及其地质意义

选择吕梁群中原岔上群北部地层的蚀变火山岩进行锆石U-Pb年代学和Hf同位素研究。锆石U-Pb测试获得两组年龄结果, 较年轻的谐和年龄为1813±6 Ma (n=7), 较老的²⁰⁷Pb/²⁰⁶Pb加权平均年龄为2516±31 Ma(n=2), 前者为火山岩喷发时代, 后者代表捕获锆石年龄。年轻锆石的ε_Hf(t)值为-10.8~-2.3, T_DM1值为2308~2655 Ma; 捕获锆石的ε_Hf(t)值为+10.0~+13.1。年龄约为2.5 Ga锆石的ε_Hf(t)值高于亏损地幔演化线, 考虑到UPb同位素和Hf同位素测点位置不完全相同, 说明所获得的Hf同位素组成为无地质意义的混合数值; 年龄约为1.8 Ga锆石的Hf同位素特征反映其可能源于富集地幔或受地壳物质混染的亏损地幔。结合前人的研究成果, 推断岩浆作用事件发生在约1.81 Ga 的碰撞后阶段。     

内蒙古中部宝力高庙组长英质火山岩U-Pb-Hf同位素特征及其地质意义

锆石SHRIMP U-Pb定年结果显示: 内蒙古中部白音乌拉地区原宝力高庙组的流纹岩形成时代为300.0±2.9 Ma, 属晚石炭世; 青格勒宝拉格地区原宝力高庙组的凝灰岩结晶年龄为159.6±1.4 Ma, 并获得 3颗捕获锆石的年龄分别为291.8±3.4, 304.0±3.3和734.7±9.2 Ma, 应属于晚侏罗世满克头鄂博组。锆石LA-MC-ICP-MS Hf同位素分析显示: 流纹岩锆石ε_Hf(t) 值为+10.5~+12.9, T_DM^C值为493~645 Ma; 凝灰岩岩浆锆石ε_Hf(t)值为+10.1~+13.1, T_DM^C值为369~563 Ma。研究结果表明, 流纹岩源于晚古生代新生地壳的重熔并混入少量老地壳物质, 凝灰岩源于晚古生代地壳的熔融。Hf同位素特征显示晚古生代流纹岩和中生代凝灰岩源于相似的源区, 揭示了晚古生代的一次重要的增生事件, 并且在约160 Ma时期发生过地壳的再造。结合前人的研究成果表明, 兴蒙造山带在约300 Ma时处于古亚洲洋演化过程中岛弧向碰撞后伸展环境的转换时期, 在约160 Ma受到蒙古?鄂霍茨克构造域的影响。     

Crustal evolution of the Shiwandashan area in South China: Zircon U-Pb-Hf isotopic records from granulite enclaves in Indo-Sinian granites

U-Pb dating coupled with Hf isotope analyses on zircon from metasedimentary granulite enclaves in the Jiuzhou peraluminous granite from the Shiwandashan area in southeastern Guangxi Province, South China are presented in this paper. The results show that the protoliths of these granulite enclaves were mainly composed of Neo-Mesoproterozoic (564–1061 Ma) clastic materials with a peak age at ~822 Ma. These materials were probably derived from the igneous rocks that were emplaced during the Neoproterozoic breakup of Rodinian Supercontinent. Subordinate sediments include the Paleoproterozoic (1778–2227 Ma) and even the Meso-Paleoarchean materials with the oldest U-Pb age at 3551±8 Ma, indicating the existence of ancient crustal rocks in the area and/or its surrounding regions. Younger grains include the early Mesozoic (234±2 Ma) magmatic zircon populations and the late Permian (253±3 Ma) metamorphic zircon populations. Further zircon Hf isotope analyses reveal that their protoliths were complex, containing both recycled crustal rocks and juvenile materials. Combined zircon U-Pb ages and Hf isotope compositions indicate that at ~253 Ma, the Shiwandashan area experienced an intensive thermal event that resulted in the granulite-facies metamorphism; and that crustal remelting occurred at ~234 Ma to form the S-type granitoids during the uplifting stage. The metasedimentary granulite enclaves are resitites of these granitoids.     

甘肃北山桥湾北花岗岩体的年代学、地球化学及其地质意义

为加深对北山晚古生代构造演化的认识, 选取位于甘肃北山南带、敦煌地块北缘的桥湾北花岗岩体进行年代学、地球化学研究。桥湾北角闪石花岗岩体锆石LA-ICP MS年龄为303.7±2.4 Ma, ε_Hf(t)为-1.2~5.8, ε_Nd(t)相对较高(-0.40~-0.06), (⁸⁷Sr/⁸⁶Sr)_i =(0.704524~0.705062)。 桥湾北花岗岩体K₂O含量为4.09%~5.58%, 属高钾钙碱性岩石, A/CNK值在0.92~1.04之间, 为I型花岗岩; 微量元素较富集LILE, 相对亏损HFSE, 有明显的Nb, Ta负异常。岩体具相对富集的微弱分馏的轻稀土元素, 重稀土元素分馏不明显, 基本没有Eu异常。由Hf, Nd同位素及地球化学特征判断, 桥湾北花岗岩体为壳幔混合成因, 其形式可以是底侵的幔源岩浆再分异, 上升过程中遭受地壳物质混染。根据区域构造背景及岩石学、地球化学特征, 可以认为桥湾北岩体是北山南带后碰撞过程岩浆活动的产物, 反映出在晚石炭世北山地区的碰撞拼贴已经完成。     

东昆仑尕林格铁矿床成因年代学及Hf同位素制约

尕林格矿床发育大量与铁多金属成矿关系密切的花岗闪长岩、石英二长岩、石英二长闪长岩和石英闪长岩体.LA-ICP-MS锆石U-Pb年代学表明石英二长闪长岩的年龄为228.3土0.5 Ma,石英二长岩的年龄为234.4±0.6 Ma.地球化学特征也较为相似,SiO2质量分数中等(58.4％～64.3％),w(FeOT)/w(MgO)偏低(0.62～1.38),A/CNK值(0.52～0.86)＜1,轻稀土富集,轻重稀土分馏明显,中等—强烈的负Eu异常(δEu为0.33～0.82),富集Rb,Th等大离子亲石元素,明显亏损高场强元素(Sr,P,Nb,Ta,Ti等),w(Rb)/w(Sr)为0.12～0.39,具有壳幔混合Ⅰ型花岗岩的特征.锆石Hf同位素组成表明,石英二长闪长岩的εHf(t)为-4.15～3.44,T2DM为1042～1523 Ma;石英二长岩εHf(t)为-5.06～1.7,T2DM为1157～1587 Ma,二者具有相同的岩浆来源,可能为壳幔混合岩浆作用的结果.综合研究认为尕林格铁矿床中三叠世岩体为东昆仑218～242 Ma俯冲—碰撞转换阶段,地幔底侵古老陆壳,幔源基性岩浆与壳源花岗质岩浆发生不同程度混合作用的产物,中高温岩浆流体与围岩发生水—岩反应是矽卡岩型矿体形成的主要因素.