Ages and Hf isotopes of detrital zircons from the Permian strata in the engbatu area(Inner Mongolia) and tectonic implications

The Central Asian Orogenic Belt(CAOB) was built up through protracted accretion and collision of a variety of terranes/micro-continents during Neoproterozoice Mesozoic time. To understand potential links among Paleozoic subduction and accretionary processes that were operative during the development of the southeastern CAOB, we conducted a combined U-Pb and Hf-isotope analysis of detrital zircons from previously defined Devonian, Carboniferous and Early Permian strata in the Bengbatu area,Inner Mongolia. Detrital zircons from(meta-) sandstones in these strata commonly yield major Paleozoic age populations at ca. 300-261 Ma, 351-300 Ma and 517-419 Ma, and also give several Precambrian ages that range from 2687 Ma to 544 Ma. The youngest ages redefine the deposition of all these strata to be in the Middle Permian(Wordiane Capitanian) or later, much younger than previously considered.These ages, coupled with regional magmatic records, support an interpretation of most surrounding areas as possible detritus sources, including the Mongolian arcs to the north, the Northern Accretionary Orogen to the south, and the intervening Erenhote Hegenshan Ophiolite Belt. Zircons with magmatic ages of ca. 500-350 Ma and ca. 300-261 Ma display a large range of εHf(t) values(-13.97 to +15.31),whereas ca. 350-300 Ma zircons are dominated by positive εHf(t) values(+0.14 to +16.00). These results support the occurrence of two significant shifts of the zircon εHf(t) values, which has tectonic implications for the understanding of the Carboniferouse Permian evolution of the southeastern CAOB. A marked shift from mixed to positive zircon εHf(t) values at 350 -330 Ma likely manifests the incipient opening of the Hegenshan Ocean, due to the slab rollback of the subducting Paleo-Asian Oceanic lithosphere. Another shift from positive to mixed zircon εHf(t) values at ca. 300 Ma likely corresponds to a tectonic switch from syn-orogenic subduction-related to post-orogenic extensional setting, genetically related to the tectonic collapse of a formerly overthickened crust.

U-Pb Age and Hf Isotope Study of Detrital Zircons from the Wanzi Supracrustals:Constraints on the Tectonic Setting and Evolution of the Fuping Complex,Trans-North China Orogen

Located in the middle segment of the Trans-North China Orogen, the Fuping Complex is considered as a critical area in understanding the evolution history of the North China Craton （NCC）. The complex is composed of various high-grade and multiply deformed rocks, including gray gneiss, basic granulite, amphibolite, fine-grained gneiss and marble, metamorphosed to upper amphibolite or granulite facies. It can be divided into four rock units： the Fuping TTG gneisses, Longquanguan augen gneisses, Wanzi supracrustals, and Nanying granitic gneisses. U-Pb age and Hf isotope compositions of about 200 detrital zircons from the Wanzi supracrustals of the Fuping Complex have been analyzed. The data on metamorphic zircon rims give ages of 1.82-1.84 Ga, corresponding to the final amalgamation event of the NCC, whereas the data for igneous zircon cores yield two age populations at -2.10 and -2.51 Ga, with some inherited ages scattering between 2.5 and 2.9 Ga. These results suggest that the Wanzi supracrustals were derived from the Fuping TTG gneisses （-2.5 Ga） and the Nanying granitic gneisses （2.0-2.1 Ga） and deposited between 2.10 and 1.84 Ga. All zircons with -2.51 Ga age have positive initial εHf values from ＋1.4 to ＋10.9, suggesting an important crustal growth event at -2.5 Ga through the addition of juvenile materials from the mantle. The Hf isotope data for the detrital zircons further imply that the 2.8 Ga rocks are important components in the lower crust, which is consistent with a suggestion from Nd isotope data for the Eastern Block. The zircons of 2.10 Ga population have initial εHf values of-4.9 to ＋6.1, interpreted as mixing of crustal re-melt with minor juvenile material contribution at 2.1 Ga. These results are distinct from that for the Western Block, supporting that the Fuping Complex was emplaced in a tectonic active environment at the western margin of the Eastern Block.

Geochemistry and detrital zircon records of the Ruyang-Luoyu groups,southern North China Craton:Provenance,crustal evolution and Paleo-Mesoproterozoic tectonic implications

Paleo-to Mesoproterozoic sedimentary rocks in the southern margin of the North China Craton(NCC)are represented by the Ruyang and Luoyu groups.We studied the sedimentary rocks from the Yunmengshan and Beidajian formations of the Ruyang Group and the Cuizhuang and Sanjiaotang formations of the Luoyu Group.Detrital zircon grains from these formations have U–Pb age populations of 3.64–3.31 Ga,2.96–2.86 Ga,2.72–2.59 Ga,2.56–2.47 Ga,2.45–2.0 Ga,1.99–1.85 Ga and 1.84–1.65 Ga.The geochemical features of the sedimentary rocks suggest that some of the sediments were sourced from intermediate to felsic magmatic rocks.The age groups of the detrital zircon are roughly consistent with the tectono-thermal events in the southern margin of the NCC.The Hf isotopic compositions of detrital zircon from the sedimentary rocks in Ruyang and Luoyu groups suggest that significant crustal growth and reworking of the NCC took place during the Neoarchean and early-to mid-Paleoproterozoic,while crustal reworking at the Paleoarchean and late-Paleoproterozoic,and crustal growth at the Mesoarchean.We suggest the depositional times of the Ruyang Group and Luoyu Group are constrained to no older than 1.75–1.7 Ga and 1.7–1.65 Ga,respectively.Formation of late-Paleoproterozoic basins related to the strike slip and extrusion tectonics that cross-cut the NCC during the late Paleoproterozoic(<1.75 Ga),and the late Paleoproterozoic sedimentation once isochronous developed in the southern margin of the NCC through the Taihang region of the interior NCC and linked the Yanshan–Liaoxi regions of the northern NCC.

Detrital zircon geochronology and provenance of sediments within the Mesozoic basins:New insights into tectonic evolution of the Qinling Orogen

Mesozoic sedimentary units within the Nanzhao and Mashiping basins record the paleogeographic and tectonic evolution of the Qinling Orogen(QO).This study uses new detrital zircon U-Pb ages and Hf isotopic data to constrain the timing of deposition and provenance of the Taishanmiao,Taizishan,Nanzhao,and Mashiping formations of the North Qinling Orogen(NQO).The detrital zircons can be split by age peaks into five major groups,including Early Cretaceous,Late Triassic,Early Paleozoic,Neoproterozoic and Paleoproterozoic age peaks.On the basis of the youngest zircon ages and age-diagnostic fossils,we conclude that the Taishanmiao(youngest age of 234 Ma)and Taizishan(216 Ma)formations were deposited during the Late Triassic,whereas the Nanzhao Formation(110 Ma)was deposited during the Early Cretaceous rather than the Late Jurassic as previously thought.The Mashiping Formation(110 Ma)was also deposited during the Early Cretaceous.Combined with zircon Hf isotopic compositions,zircons from Late Triassic units were generally derived from the NQO,South Qinling Orogen(SQO),and North China Craton(NCC),with minor amounts derived from the Xing-Meng Orogenic Belt.The sediments within the Nanzhao Formation were mainly derived from the QO,with a minor contribution from the NCC.The Early Cretaceous conglomerates of the Mashiping Formation were generally derived from recycled earlier detritus.This implies that Late Triassic deposition was related to the final closure of the Mianlue Ocean,whereas Early Cretaceous deposition was correlated to the continued intercontinental subduction of the Yangtze Craton beneath the QO.The change in provenance within these Mesozoic sediments suggests the QO underwent two periods of significant uplift,which was a process generated sediments that were deposited in a series of basins of this area.

Paleozoic tectonic evolution of the proto-Korean Peninsula along the East Asian continental margin from detrital zircon U-Pb geochronology and Hf isotope geochemistry

Detrital zircon geochronology and Hf isotope analysis can be used for inferring provenance characteristics,and to evaluate the tectonic evolution of sedimentary basins and their link with regional orogenesis.The Paleozoic sequences of the Okcheon Belt consist of the Lower Paleozoic Joseon and the Upper Paleozoic Pyeongan supergroups with Middle Paleozoic hiatus locally on top of the Neoproterozoic bimodal volcanic rocks,reflecting an intracontinental rift setting between the two basements(viz.Gyeonggi and Yeongnam massifs)at southern part of the Korean Peninsula.Our detrital zircon U-Pb ages and Lu-Hf isotope results show that all these Paleozoic strata commonly have Paleoproterozoic and Paleozoic zircon ages with rare Meso-to Neoproterozoic ages.The individual zircon populations display following features,allowing estimation of their sedimentary provenances:(i)The Paleoproterozoic zircons(ca.1.85 Ga and 2.50 Ga)with similar ranges ofεHf(t)values are most common in the basement rocks of the Korean Peninsula,and were sourced from both the Gyeonggi and Yeongnam massifs.(ii)The Meso-to Neoproterozoic zircons,preserved only in the Middle to Late Cambrian clastic sedimentary rocks within the carbonate sequences probably reflect proximal provenance.(iii)The youngest Paleozoic zircons of each formation,almost coincident with their deposition ages,suggest presence of syndepositional magmatism,indicating proximal magmatic sources during their deposition.(iv)The Cambrian-Ordovician zircons,from the Lower Paleozoic sequences,but rare in the successive Upper Paleozoic sequences,suggest a provenance change after the hiatus between the two sedimentary successions.(v)The Permian zircons showing differentεHf(t)values indicate that detrital sources were varied at that time.The integrated results in our study suggest provenance variability linked to diverse tectonic environments,reflecting prolonged subduction-related crustal evolution of the proto-Korean Peninsula during the Paleozoic.

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.

Provenance of Late Mesozoic Strata and Tectonic Implications for the Southwestern Ordos Basin,North China:Evidence from Detrital Zircon U-Pb Geochronology and Hf Isotopes

In order to determine the provenance and variation characteristics of sandstone-type uranium deposits located in the southwest Ordos Basin,U-Pb geochronology and Hf isotope analyses were conducted on detrital zircons from the Late Mesozoic strata of the SD01 well in the Zhenyuan area.The detrital zircon U-Pb ages of four samples exhibited four main peaks at 250–330,420–500,1720–2000,and 2340–2580 Ma,with a small number of zircons dated at 770–1100 Ma.The detrital zircon age spectrum and further restriction provided by the in-situ Hf isotopic data suggest that the provenance of each stratum was mainly derived from the crystalline basement rock series(Khondalites,intermediate-acidic intrusive rocks,and metamorphic rocks)of the Alxa Block to the northwest and the Yinshan Block to the north,with minor amounts of Caledonian magmatic rocks and Jingning Period rocks from the western part of the northern Qilian orogenic belt to the west and the northern Qinling orogenic belt to the south.The provenance of the sandbody has not changed significantly and is of the Middle Jurassic–Early Cretaceous.The clear variations in the zircon ages of the samples from the Zhiluo and Anding formations were likely influenced by climate change during the Middle–Late Jurassic.The Triassic zircon age(<250 Ma)first appeared in Early Cretaceous strata,suggesting that tectonic activity was relatively strong in the northern Qinling orogenic belt during the Late Jurassic and produced extensive outcrops of Indo-Chinese granite,which were a source of basin sediments.

东准噶尔三塘湖盆地中侏罗统砂岩碎屑锆石U-Pb年龄、Hf同位素特征及对铀成矿的意义

三塘湖盆地汉水泉地区是东准噶尔地区砂岩型铀矿找矿的有利地段,为揭示该地区中侏罗统西山窑组和头屯河组沉积物物源体系,分析地层铀源条件,对西山窑组和头屯河组砂岩开展碎屑锆石LA-ICP-MS U-Pb测年和Hf同位素分析。分析结果显示西山窑组砂岩的碎屑锆石年龄集中于晚古生代,2个样品年龄峰值分别为328 Ma和340 Ma,碎屑锆石ε_(Hf)(t)为8.9~14.7,二阶段模式年龄介于690~415 Ma之间;头屯河组砂岩的碎屑锆石年龄可划分出238~163 Ma、505~259 Ma和1950~744 Ma 3个区间段,碎屑锆石ε_(Hf)(t)为-6.5~12.1,二阶段模式年龄介于1709~575 Ma之间。物源分析表明西山窑组砂岩物源单一,主要来自于研究区西部卡拉麦里—麦钦乌拉和野马泉地区;头屯河组砂岩物源相对复杂,除了有来自于卡拉麦里地区的碎屑物质外,还接受了其他区域的物源供给。二者之间碎屑锆石年龄特征的突变,代表了东准噶尔地区中侏罗世一次剧烈的构造变形事件,物源特征指示汉水泉地区中侏罗统地层铀含量相对有限,铀源是限制该地区砂岩型铀矿成矿的重要因素。

赞皇杂岩翁城地区长城系常州沟组砂岩碎屑锆石U-Pb-Hf同位素研究

常州沟组是华北克拉通太古宙-古元古代变质基底上最早的沉积盖层之一,其碎屑锆石U-Pb年代学与Hf同位素特征分析对探讨长城系形成时代、源区特征以及基底演化均具有重要的研究意义。本文对华北克拉通中部赞皇杂岩瓮城地区长城系常州沟组底部砂岩样品进行了LA-ICP-MS碎屑锆石U-Pb定年和LA-MC-ICP-MS碎屑锆石Lu-Hf同位素分析。砂岩样品中碎屑锆石主要年龄峰值约为2 500 Ma,推断其碎屑物质主要来自于华北克拉通新太古代晚期变质基底,而最年轻的碎屑锆石年龄为1 822 Ma,结合赞皇杂岩变质基底普遍经历了1 850~1 800 Ma变质作用以及区域上上覆大红峪组火山岩中1 635 Ma结晶锆石年龄,限定瓮城地区长城系常州沟组沉积时代为1 800~1 635 Ma。砂岩样品中碎屑锆石的εHf(t)值变化于-6.8~+5.2之间,相应的两阶段模式年龄峰值约为2 820 Ma,结合赞皇杂岩已发表的2 900~2 700 Ma岩浆锆石和碎屑锆石Hf同位素结果,进一步表明2 900~2 700 Ma为赞皇杂岩地壳生长最主要的时期。综合太行山中南部地区已发表的长城系常州沟组的沉积学与新的年代学数据,推测赞皇杂岩瓮城地区长城系常州沟组沉积于陆内裂谷盆地。

海南岛雷鸣盆地鹿母湾组碎屑岩地球化学特征、锆石U-Pb年龄和Hf同位素组成:对物源及构造环境的启示

海南岛雷鸣盆地鹿母湾组的确切沉积时限和沉积物来源是研究的空白区域。本文选取雷鸣盆地鹿母湾组作为研究对象,通过全岩地球化学、碎屑锆石U-Pb年代学和Lu-Hf同位素分析,旨在限定其沉积时限及物质来源,并探讨其沉积构造环境。鹿母湾组碎屑沉积岩的最年轻碎屑锆石年龄峰(113.3 Ma)与侵入其中的花岗闪长岩岩体年龄(~107 Ma)限定鹿母湾组沉积时限为109.9~113.3 Ma。全岩地球化学研究表明,鹿母湾组碎屑沉积岩的分异指数(ICV)为0.78~1.52(平均值0.96)、化学风化指数(CIA)为51.11~73.45(平均值65.27),反映其源区岩石具有中等成熟度并经历了弱的化学风化作用。结合微量元素、稀土元素含量及相关比值(如La、Th、Sc、Zr、Hf、∑REE及δEu)、源区判别图和构造环境判别图解,本文认为鹿母湾组碎屑沉积岩可能沉积于活动大陆边缘相关的盆地,主要来源于同时代的长英质火山岩或火山物质,少量碎屑来自华南内陆和海南岛早古生代岩浆岩。

共和盆地河流碎屑锆石U-Pb年龄特征、Hf同位素组成及其对构造和地壳演化的启示

为揭示青海共和盆地沉积源区构造活动和地壳演化历史,应用LA-MC-ICP-MS技术,对共和盆地曲什安河与西河河漫滩沉积物中的碎屑锆石进行了U-Pb定年和Hf同位素分析。得出碎屑锆石U-Pb年龄可分为5组:309~172 Ma、471~368 Ma、943~737 Ma、2368~1698 Ma、2543~2453 Ma。结合研究区构造-岩浆事件得出,曲什安河与西河河漫滩碎屑沉积物是西秦岭地块在不同时期提供的结果。碎屑锆石U-Pb年龄存在2个主要峰值,约236 Ma和约403 Ma,反映了古特提斯洋和原特提斯洋在不同部分的俯冲导致陆块碰撞,形成强烈岩浆事件的地质记录,碰撞后伸展阶段出现裂谷和新生洋盆。Hf同位素组成特征揭示,源区地壳在2.7~1.3 Ga经历了主要增生期,形成现今地壳的84%。ε_(Hf)(t)值为-14.7~+15.0,Hf亏损模式年龄在3.6~0.24Ga范围内,表明源区基底存在古太古代古老地壳物质。约0.5 Ga以来,地壳增长以古老地壳物质再循环为主,并在0.28 Ga左右地壳停止了生长。依据前人公式计算了曲什安河与西河河漫滩碎屑沉积物物源区各时期的重熔/增生比例。

贵州北部马鬃岭铝土矿床中富锂铝质粘土岩碎屑锆石U-Pb年龄、Lu-Hf同位素特征及其对锂源的制约

为研究贵州北部马鬃岭铝土矿床中富锂铝质粘土岩的物源体系及其锂的来源,对出露于大竹园组顶部的该类岩石进行了锆石U-Pb定年、Lu-Hf同位素及锆石微量元素分析。分析结果表明富锂铝质粘土岩中碎屑锆石普遍发育岩浆振荡环带,为造山作用/弧作用相关的岩浆岩类锆石。碎屑锆石U-Pb年龄从早泥盆世到太古宙(411~3001 Ma)均有分布,具627~879 Ma和925~1018 Ma两个主峰及539~602 Ma一个次峰。ε_(Hf)(t)值为-27.61~7.03之间,二阶段模式年龄(t_(DM2))分布在1242~3387 Ma之间。结合碎屑锆石U-Pb-Hf同位素、微量元素特征及区域地质对比,认为富锂铝质粘土岩中碎屑锆石最主要的初始物源区为南岭-云开地体、江南造山带西段新元古代岩浆岩,其锂可能来源于江南造山带西段桂北-黔东地区860~750 Ma的基性岩。

吉林敦化地区万宝岩组碎屑锆石U-Pb-Hf同位素组成:对区域构造演化的制约

为制约古亚洲洋在吉林东部地区的最终闭合时间,本文选取吉林省敦化地区万宝岩组变质粉砂岩为研究对象,开展系统的碎屑锆石微量元素组成和U-Pb-Hf同位素研究,确定了万宝岩组的沉积时限和物源区特征,并通过碎屑锆石微量元素获得晚古生代地壳厚度的变化规律,探讨了华北板块北缘东段晚古生代的构造演化历史。万宝岩组由互层的大理岩、变质粉砂岩和变质细砂岩组成,碎屑锆石定年结果显示,万宝岩组最年轻峰值年龄约为316 Ma,此外还存在355、398、1 842和2 360 Ma的峰值年龄。其中,古生代碎屑锆石(409~312 Ma)以具有负的εHf(t)值(-15.32~-1.60)及新太古代—古元古代的TDM2年龄(2 293~1 480 Ma)为特征,侵入万宝岩组闪长岩体的时期为276 Ma。综合研究认为,敦化地区万宝岩组沉积时限为312~276 Ma,即早二叠世时期,其沉积时限和岩石组合特征可与延边地区的庙岭组相对比。万宝岩组中的古生代碎屑锆石来自华北板块太古宙—古元古代结晶基底物质的部分熔融。结合其中古元古代碎屑锆石(62.9%)的大量出现,暗示敦化地区或/和附近地区存在华北板块前寒武纪的结晶基底。通过地壳厚度计算表明,敦化地区和内蒙古地区晚古生代的地壳厚度变化趋势一致,说明二者晚古生代的演化历史趋于同步。约245 Ma地壳厚度达到最大(80 km),暗示了古亚洲洋的最终闭合。

新疆乌拉斯台河沉积物碎屑锆石U-Pb和Hf同位素研究及其地质意义

本文利用LA-MC-ICP技术,对乌拉斯台河沉积物碎屑锆石进行了U-Pb定年和Hf同位素组成分析研究。U-Pb测年结果显示,样品中碎屑锆石年龄主要分布在470~390 Ma、380~320 Ma和310~260 Ma。通过与南天山南部河流碎屑锆石U-Pb年龄对比分析,并结合其他相关证据,探讨了古生代中天山南缘的地质演化过程。本文样品中年龄为470~390 Ma的碎屑锆石及南天山南部河流中460~390 Ma的碎屑锆石,揭示了南天山洋在中奥陶—晚泥盆世期间发生的双向俯冲作用。380~320 Ma锆石记录了晚泥盆世—晚石炭世南天山洋北向俯冲到中天山—伊犁地块之下。310~260 Ma的碎屑锆石记录了石炭世晚期南天山洋的闭合和之后发生的同碰撞和碰撞后的岩浆作用。结合Lu-Hf同位素分析结果表明,巴伦台地区古生代期间有大量新生物质逐渐加入到岩浆生成过程中,这可能与南天山洋北向俯冲和该洋盆闭合等过程有关。

沙巴地区新生代碎屑岩物源分析及构造意义:锆石年代学和Lu-Hf同位素组成约束

沙巴地区位于印度‒澳大利亚、太平洋和菲律宾海板块交汇区,其发育的新生代沉积对理解婆罗洲北部的沉积‒构造演化具有重要意义。本文对沙巴北部始新世‒中新世碎屑岩开展岩石学、锆石U-Pb年代学及Lu-Hf同位素研究,结果表明:该套碎屑岩包括岩屑石英砂岩和长石岩屑石英砂岩,其中始新统Trusmadi组碎屑锆石年龄主要峰值为160 Ma和120 Ma,对应年龄群(185~145 Ma和140~100 Ma)的εHf(t)值分别为–15.6~+14.8和+0.6~+12.2,表明源区新生地壳物质贡献明显。始新统‒中新统Crocker组碎屑锆石年龄主要峰值为248 Ma,次要峰值为1860 Ma、438 Ma、280 Ma和120 Ma,主要年龄群(265~220 Ma)的εHf(t)值为–11.9~+9.9。中新统Kudat组的碎屑锆石年龄集中在120 Ma、εHf(t)值(+1.8~+9.0)均为正值,次要峰值174 Ma。本次研究认为Trusmadi组砂岩物源主要受控于越南大叻带和婆罗洲西部中生代花岗质岩及古晋带再循环沉积物,Crocker组主要源自婆罗洲西部和马来半岛东部中生代火成岩,以及沙巴本身。Kudat组砂岩物源主要来自沙巴蛇绿岩及部分巴拉望陆块的再循环沉积物。此外,少量始新世岩浆成因碎屑锆石暗示沙巴地区存在始新世与古南海板块俯冲相关的岩浆作用。

Early Precambrian tectonothermal events of the North China Craton:Constraints from in situ detrital zircon U-Pb,Hf and O isotopic compositions in Tietonggou Formation

Clastic sediments and sedimentary rocks are widely used for understanding the formation and evolution of the continental crust.The Tietonggou Formation outcrops in the Xiaoqinling region at the southern margin of the North China Craton(NCC)and has unconformable contacts with the underlying Taihua Complex and overlying Paleoproterozoic Xiong’er Group.It mainly consists of quartzite and its protoliths are mature terrigenous clastic rocks.On the basis of the ages of the youngest detrital zircons from the quartzites and the ages of the Xiong’er Group,the depositional ages of the protoliths of the Tietonggou Formation were well constrained to 1.91–1.80 Ga.The U-Pb isotopic data of detrital zircons from the Formation show a major age peak at^2.1 Ga,which is consistent with 2.2–2.0 Ga magmatism in the Trans-North China Orogen of the NCC.Taking into account the texctural and compositional maturity of the Tietonggou Formation quartzite,the^2.1 Ga lithologic units in the Trans-North China Orogen are interpreted as the major source of the Tietonggou Formation.The majority of these^2.1 Ga detrital zircons mostly have highδ18O values(>6.5‰)and negativeεHf(t)values(–7.8–0.0),with corresponding Hf model ages significantly older than their crystallization ages,indicating that these zircons formed from the partial melting of ancient continental crust.The majority of the2.8–2.7 Ga and^2.5 Ga detrital zircons from the Tietonggou Formation had positiveεHf(t)values,and mantle-likeδ18O values,suggesting that the NCC has experienced two stages of significant crustal growth in the Neoarchaean at 2.7 and 2.5 Ga,respectively.The Hf isotopic data of detrital zircons from Paleoproterozoic metasedimentary rocks in the Trans-North China Orogen varied mainly toward the reduction of the radiogenic Hf isotope and gradually show a similar trend of the isotope trajectories of crustal evolution.This reveals that the NCC probably has not developed a long-lived subduction to complete the final assembly of the NCC.Alternatively,these maybe imply that the tectonic setting of the NCC substantially changed at^2.1 Ga,the reduction of the radiogenic Hf isotope could be attributed to the rollback of the subducting slab.

Detrital Zircon U-Pb Geochronology and Lu-Hf Isotopic Compositions of the Wuliangshan Metasediment Rocks in SW Yunnan(China)and Its Provenance Implications

The Wuliangshan Group occurs to the east of the Lancang giant igneous zone in SW Yunnan, and is mainly composed of low-grade metamorphosed sedimentary rocks. The group has been considered as the syn-orogenic product of the Baoshan with Simao-Indochina blocks. However, its depositional time and provenance remain to be poorly constrained. This paper presents zircon U-Pb dating and Lu-Hf-isotopic data for five representative sandstone samples from the Wuliangshan Group. The detrital grains yield a major age-peak at ~259 Ma, and four subordinary age-peaks at ~1 859, ~941, ~788, and ~447 Ma, respectively. Our results suggest that the Wuliangshan metasedimentary sequence was deposited after Middle Triassic rather than previously-thought Cambrian. The detrital zircon age spectrum, along with in-situ Lu-Hf isotopic data suggest that the Wuliangshan Group might be a syncollisional sedimentary product related to the collision of Baoshan with Simao-Indochina blocks. It is inferred that the provenance of the Wuliangshan Group is mainly from the Simao/Yangtze blocks to the east rather than the Baoshan Block or Lancang igneous zone to the west.

胶东昌邑地区新太古代石英岩的锆石U-Pb年代学和Hf同位素特征及其地质意义

基底古老地层的研究可以为探索地球早期陆壳形成与演化历史提供重要支持,胶东地区是华北克拉通变质基底重要的出露区之一,在胶北地体的西缘有一套高成熟度的古老石英岩-山阳村石英岩,前人将其划归古元古代粉子山群的小宋组,但缺乏明确的年代学限定。本文对山阳村石英岩的碎屑锆石进行U-Pb年代学和Hf同位素分析,获得年龄谱的最大峰值为2501±16Ma,且是最小峰值年龄。与古元古的粉子山群和中元古的芝罘群的碎屑锆石年龄谱均不同。综合年龄谱特征、野外产状以及岩石组合,本文认为山阳村石英岩是新太古代的岩石建造,建议为其单独建立新太古代岩石地层单位。结合本次研究及前人工作本文认为应将原粉子山群小宋组划到太古代归属胶东岩群。山阳村石英岩中,~2.5Ga锆石Hf同位素εHf(t)>0且从下方靠近亏损地幔演化线并在~2.7Ga地壳演化线附近分布,显示其原岩来自~2.7Ga新生地壳的重熔;ca.2.7~3.0Ga的锆石Hf同位素εHf(t)>0且在~3.1Ga地壳演化线附近,意味着其母岩的源区为~3.1Ga的新生地壳,且在ca.2.7~3.0Ga发生了持续的地壳再熔融;>3.0Ga碎屑锆石的Hf同位素εHf(t)<0,源区岩石形成于更古老陆壳物质的再循环。已知的胶北地块>3.0Ga锆石的Hf同位素特征暗示华北克拉通在4.1Ga前就存在地壳,而且在~4.1Ga发生了重熔。

华北克拉通北缘白云鄂博群和腮林忽洞群底部碎屑锆石U-Pb定年、Hf同位素分析及其地质意义

白云鄂博群位于华北克拉通北缘,由于赋存超大型REE-Nb-Fe矿而受到广泛关注。白云鄂博群形成时代有中元古代、古生代等不同认识。腮林忽洞群位于白云鄂博群的南部,与白云鄂博群的关系也有不同认识。本文报道了白云鄂博群和腮林忽洞群底部中-粗粒砂岩的碎屑锆石SHRIMP U-Pb年龄和LA-ICPMS Hf同位素组成。两个岩群碎屑沉积岩的碎屑锆石在形态和内部结构上类似,按结构特征可分为继承或捕获锆石、岩浆锆石、变质锆石和重结晶锆石等不同类型。年龄都主要分布在1.8～2.1Ga之间和2.4～2.7Ga之间,尽管年龄峰值存在一定区别。它们的Hf同位素组成也类似,εHf（t）和tDM2（Hf）变化范围分别主要在约-6.0～6.0之间和2550～2950Ma之间。结合前人研究,可得出如下结论：白云鄂博群和腮林忽洞群的物源区类似,主要由新太古代晚期和古元古代晚期岩浆岩组成,为新太古代早期陆壳物质再循环产物;部分岩石遭受新太古代晚期和古元古代晚期高级变质作用改造;碎屑物质都来自华北克拉通北缘早前寒武纪变质基底,与其形成时代相同的认识不矛盾。

华北克拉通地壳生长和演化:来自现代河流碎屑锆石Hf同位素组成的启示

定量地给出大陆地壳生长速率以及生长量随时间的演化是研究大陆地壳形成与演化的核心问题之一。近年来,利用沉积岩和沉积物中碎屑锆石的U-Pb年龄和Hf同位素组成已成为目前研究大陆地壳生长和演化最为简捷、有效的工具。本文对来自华北克拉通西部泾河和洛河河沙中的187颗碎屑锆石进行Hf同位素组成分析,并结合已有的资料来探讨华北克拉通地壳的生长和演化规律。结果显示:随着地质历史变化,华北克拉通地壳生长呈阶段性特点。如在中太古代中期-新太古代末期(3.0～2.5Ga)地壳生长速率较快,大约已有60%现今大陆地壳形成。此后,陆壳呈较稳定速率增长,到新元古代晚期(600Ma)基本己形成现存大陆地壳。表明现今的大陆主要生长于太古宙和元古宙,而显生宙陆壳的增生量可以忽略不计。根据河流碎屑锆石和前寒武纪岩石中锆石的U-Pb年龄、两阶段模式年龄(tDM2C和tNC2C)和εHf(t)所获得华北克拉通早前寒武纪地壳演化曲线,本文提出～2.7Ga和～2.5Ga分别曾经为华北克拉通太古宙岩浆作用最活跃时期,也是地壳快速生长时期,表明华北克拉通在新太古代曾发生过两期明显的地壳生长。