Polymetamorphism of the ultrahigh-temperature granulites in the Rauer Group,East Antarctica:new evidence from zircon SHRIMP U-Pb ages

The Rauer Group is located on the eastern margin of the early Paleozoic Prydz Belt in East Antarctica,and the typical ultrahigh-temperature(UHT,>900℃)granulites outcrop on Mather Peninsula.However,the timing of UHT metamorphism and P–T path of the UHT granulites have long been debated,which is critical to understanding the tectonic nature and evolution history of the Prydz Belt.Thus,both a sapphirine-bearing UHT metapelitic granulite and a garnet-bearing UHT mafic granulite are selected for zircon SHRIMP U-Pb age dating.The results show that metamorphic zircon mantles yield weighted mean^(206)Pb/^(238)U ages of 918±29 Ma and 901±29 Ma for the metapelitic and mafic granulites,respectively,while zircon rims and newly grown zircons yield weighted mean^(206)Pb/^(238)U ages of 523±9 Ma and 532±11 Ma,respectively.These new zircon age data suggest that the UHT granulites may have experienced polymetamorphism,in which pre-peak prograde stage occurred in the early Neoproterozoic Grenvillian orogenesis(1000–900 Ma),whereas the UHT metamorphism occurred in the late Neoproterozoic to early Paleozoic Pan-African orogenesis(580–460 Ma).This implies that P–T path of the UHT granulites should consist of two separate high-grade metamorphic events including the Grenvillian and Pan-African events,which are supposed to be related to assembly of Rodinia and Gondwana supercontinents respectively,and hence the overprinting UHT metamorphic event may actually reflect an important intracontinental reworking.

Zircon SHRIMP U–Pb geochronology,geochemistry,and geological significance of dacite in the Zhesang gold district,southeast Yunnan Province,China

The Late Paleozoic and Mesozoic tectonic framework of the Nanpanjiang Basin has much been disputed.Herein,the middle-acid volcanic rock,dacite,exposed to the Zhesang gold district,southeast Yunnan Province,has been analyzed.The results show that the dacite belongs to a calc-alkaline series,SiO_(2)contents range from 62.79 to 76.66 wt%.Zircon SHRIMP U–Pb dating of dacite demonstrates that they were formed in the Early Triassic(247.8±1.7 Ma,MSWD=1.2).All samples exhibit enrichment in LILE(e.g.Rb,K,Th,and U),and depletion in HFSE(e.g.Nb,Ta,and Ti),which has the geochemical affinity of I-type granite.La–La/Sm and La–La/Yb discrimination diagrams show that the partial melting,mainly of the mafic lower crust,of rocks,plays a major role in the formation process.The dacite has low initial ^(87)Sr/^(86)Sr ratios(0.706954 to 0.708589)and negative ε_(Nd)(t)values(-11.77 to-10.88).Zircons in dacite have ε_(Hf)(t)values of-16.2 to-8.3,and the two-stage Hf model ages are 1799–2301 Ma,mostly concentrated between 1800 and 1900 Ma,indicating that the magma source area is the reconstructed ancient lower crust mixed with some mantle materials,and crystal fractionation process underwent in the late stage of magma migration.This study reveals that the arc-volcanic rocks of the Early Triassic in the southern margin of the Nanpanjiang Basin were formed by the subduction of the Late Paleozoic ocean basin within the border region between China and Vietnam.

Discovery of the Dagele Eclogite in East Kunlun,Western China and Its Zircon SHRIMP U-Pb Ages:New Constrains on the Central Kunlun Suture Zone

Objective Eclogites are important indicators of ancient plate boundaries or paleosuture zones. Despite their great geological significance, very few investigations have been carried out in the Kunlun region. The Central East Kunlun fault zone was believed to be an Early Paleozoic suture zone, but there has been no reliable evidence for this, though studies on ophiolite, granite, and basic granulite indicate that the Early Paleozoic orogeny occurred in the East Kunlun. This work focused on the Dagele eclogites in Central East Kunlun to provide new constraints for the Central East Kunlun suture zone.

Relationship between Granite and Eclogite on the Southern Margin of the Qilian Mountains: Evidence from Zircon SHRIMP Agesof the Aolaoshan Granite

Zircon SHRIMP ages of the Aolaoshan granite on the south margin of the QilianMts. range from 445 + - 15.3 to 496 + - 7.6 Ma (averaging 473 Ma), belonging to the EarlyOrdovician. Geochemically, the granite is similar to I-type granite and, tectonically, was formed inan island-arc environment based on relevant diagrams for structural discriminations. Consideringalso the regional geology, the authors suggest that the granite is part of an ultrahigh-pressurebelt on the south margin of the Qilian Mts. and that its formation bears a close relationship tothis belt.

New Zircon SHRIMP U-Pb Ages of the Langjiu Formation Volcanic Rocks in the Shiquanhe Area,Western Lhasa Terrane and their Implications

Objective The potassic and ultrapotassic rocks relating to the India-Eurasia collision and continual plate convergence are widely distributed in the Lhasa terrane. These rocks are very important to understand the deep processes of the India-Eurasia collision and the uplift and evolution of the Tibetan Plateau. Although high-potassic volcanic rocks are also exposed in the western Lhasa terrane, their formation time is still uncertain for the lack of reliable dating. We carried out zircon U-Pb geochronological study on the Langjiu Formation volcanic rocks, which are part of the Early Cretaceous Zenong group volcanic rocks based on 1：250000 scale Shiquanhe regional geological survey report, in the Shiquanhe area of the western Lhasa terrane. These new age data not only offer chronological basis for the regional stratigraphic correlation and classification, but also provide an essential opportunity for revealing signatures of magmatic pulses hidden in the deep crust of the Lhasa terrane.

Zircon SHRIMP U–Pb age of Late Jurassic OIB-type volcanic rocks from the Tethyan Himalaya: constraints on the initial activity time of the Kerguelen mantle plume

This work presents zircon U–Pb age and wholerock geochemical data for the volcanic rocks from the Lakang Formation in the southeastern Tethyan Himalaya and represents the initial activity of the Kerguelen mantle plume. SHRIMP U–Pb dating of zircons from the volcanic rocks yielded a ^(206) Pb/^(238) U age of 147 ± 2 Ma that reflects the time of Late Jurassic magmatism. Whole rock analyses of major and trace elements show that the volcanic rocks are characterized by high content of Ti O_2(2.62 wt%–4.25 wt%) and P_2O_5(0.38 wt%–0.68 wt%), highly fractionated in LREE/HREE [(La/Yb)N= 5.35–8.31] with no obvious anomaly of Eu, and HFSE enrichment with no obvious anomaly of Nb and Ta, which are similar to those of ocean island basalts and tholeiitic basaltic andesites indicating a mantle plume origin. The Kerguelen mantle plume produced a massive amount of magmatic rocks from Early Cretaceous to the present, which widely dispersed from their original localities of emplacement due to the changing motions of the Antarctic, Australian, and Indian plates. However, our new geochronological and geochemical results indicate that the Kerguelen mantle plume started from the Late Jurassic. Furthermore, we suggest that the Kerguelen mantle plume may played a significant role in the breakup of eastern Gondwanaland according to the available geochronological, geochemical and paleomagnetic data.

Zircon SHRIMP U-Pb dating for island arc volcanic rock of Fangniugou area in Yitong region of Jilin Province

Polymetallic iron ore sulphate deposits of marine volcanic rock have been developed in the Fangniugou area,Jilin Province,China,but the division of volcanic ore-bearing strata has not been specifically elucidated and there is disagreement about the division.The sampling and SHRIMP U-Pb zircon dating of volcanic rock for Daheishan in the Fangniugou area and the northeast slope of the Duanjiadian were described.The volcanic rock formation period and recorded the volcanic events in the Daheishan mountains were systematically researched.Two samples of high-precision U-Pb zircon dating were used to represent the volcanic rock fomation period of the Late Silurian.The measured data reflect that multiple volcanic activities occurred during the Middle Silurian,Early Silurian,Middle Ordovician and Silurian,and Late Ordovician,probably matching volcanic events in the Songnan Basin identified from zircon dating.At the same time,it is confirmed that a controversial "conglomerate of Daheishan" did in fact develop in the Late Silurian,and those sections of both the Dazigou and Xinlitun-Taoshan with graptolite had been reversed.

Zircon SHRIMP U-Pb Ages,Geochemistry and Nd-Hf Isotopes of~1.0 Ga A-Type Felsic Rocks in the Southwestern Yangtze Block,South China:Petrogenesis and Tectonic Implications

The petrogenesis and tectonic affinity of Late Mesoproterozoic igneous rocks in the Yangtze Block are important to understand its tectonic evolution within the context of the Rodinia supercontinent’s reconstruction.Here,we report the SHRIMP zircon U-Pb ages,geochemistry,and Nd-Hf isotopic data for dacite from the Zegu Formation of the Dengxiangying Group in southwestern Yangtze Block.The crystallization age of the Zegu dacite is 1037 Ma.These rocks have relatively high Ga,Zr,Hf,and HREE contents,as well as high(10000×Ga)/Al(2.7-2.9)and FeOt/MgO(3.8-10.5)ratios,showing a geochemical affinity of A-type granitoids.Moreover,they are characterized by negativeεNd(t)(-3.6 to-7.5)and zirconεHf(t)values(-0.3 to-14.4),indicating that they likely generated via partial melting of an ancient continental crust source.The tectonic discriminant diagrams showed that the Zegu dacites have high Y and Nb contents(plotted in the‘within-plate’magmatic rocks field).In conclusion,the geochemistry and isotopic data indicated that the 1037 Ma felsic volcanic rock in the Dengxiangying Group generated during an intra-plate rift basin along a passive margin.Our work thus argues against a Grenvillian orogen’s existence in the Yangtze Block.In view of the subsequent Neoproterozoic subduction-related magmatism in the region,we suggest that the Yangtze Block might have been along the periphery of the Rodinia.

华北克拉通栖霞地区~2.9Ga变质基性火山岩锆石SHRIMP U-Pb定年、Hf-O同位素特征及构造意义

胶东地区是华北克拉通重要的早前寒武纪研究区之一。中太古代唐家庄岩群在形成时代、岩石成因及构造环境等方面一直少有研究。作者在栖霞地区对出露的唐家庄岩群变质表壳岩进行系统地质调查研究基础上,开展了变质基性火山岩全岩地球化学、锆石SHRIMP U-Pb和Hf-O同位素等分析研究。结果显示,变质基性火山岩岩浆锆石年龄为2907.9±6.3Ma,代表了中太古代唐家庄岩群的形成时间,并遭受新太古代早、晚期变质作用。这是~2.9Ga唐家庄岩群变质基性火山岩在胶东地区首次报道。岩石地球化学特征表现为相对富集大离子亲石元素K、Rb、Sr等,Th、Nb、Zr、Hf等高场强元素相对亏损,具有中等分异的REE配分模式。岩石中岩浆锆石具有相似的Hf、O同位素组成,εHf(t)值介于-0.68~+4.21,δ18 O值介于1.38‰~6.73‰。结合前人的工作,我们得出的主要结论如下:(1)胶东地区经历~2.9Ga岩浆事件,局部地段仍保留下了同时期的变质表壳岩;(2)唐家庄岩群是中太古代初始的洋盆环境中所形成的一套火山-沉积建造;(3)华北克拉通中太古代晚期是亏损地幔新生地壳加积的主要时期,局部地壳再循环了较老的陆源物质;(4)华北克拉通新太古代晚期经历了伸展构造背景,岩浆底侵作用使岩石发生强烈变质作用。

贵东复式岩体中东部晚期花岗质小岩体的SHRIMP锆石U-Pb年龄及其地质意义

贵东复式岩体由多期次、多类型的岩体组成,且东部盛产铀矿床,因此精确厘定该复式岩体的时代及其时序,查明复式岩体东部与中西部岩浆活动的差异性及其与铀成矿的关系,不仅有利于深入研究区域岩浆活动,而且有利于深刻认识铀的成矿规律。现有研究认为,以龟尾山断裂带为界,贵东复式岩体中西部岩基属燕山早期、东部岩基属印支早期,东部广泛发育的燕山期中基性脉岩与铀成矿关系密切。但对复式岩体中东部呈岩枝、岩株广泛出露的晚期花岗质小岩体的成岩地质时代及其分布规律尚存在争议,也有待进一步查明花岗质小岩体与铀矿床的时空关系。文章选择贵东复式岩体中东部晚期花岗质小岩体为研究对象,采用SHRIMP锆石U-Pb法厘定其成岩时代,并分析它们的时空分布及其与铀成矿的关系。测试结果显示,贵东复式岩体中东部晚期花岗质小岩体普遍存在高铀锆石,这可能是对同一岩体不同学者所获得的^(206)Pb/^(238)U加权平均年龄差别较大的主要原因。剔除高铀锆石的年龄数据,龟尾山断裂带以西的隘子、司前、陈洞岩体锆石^(206)Pb/^(238)U加权平均年龄分别为163.0±1.6 Ma、163.1±1.6 Ma、160.7±1.9 Ma,属燕山早期岩浆活动的产物;而以东的帽峰、分水坳、龟尾山岩体锆石^(206)Pb/^(238)U加权平均年龄分别为230.0±2.7 Ma、225.9±2.9 Ma和232.6±3.1 Ma,属印支期岩浆活动的产物。所以,龟尾山断裂以西和以东晚期花岗质小岩体的时代不同。且龟尾山断裂带以东的印支期花岗质小岩体空间上与铀矿化、尤其是与燕山早期的绢英岩化型等中高温热液铀矿化关系密切,是主要的储矿围岩;结合元素地球化学研究结果,该区产出的印支期花岗质小岩体也可为铀成矿提供铀源。

燕山地区团山子组层凝灰岩SHRIMP锆石U-Pb年龄及其地质意义

燕辽地区长城群年代学工作一直是华北克拉通中元古代地质学研究的重点之一。由于团山子组与下伏串岭沟组界限年龄不明确,且此前的年龄都是来自于其中的火山熔岩锆石的年龄,亟待新的测年工作加以验证和完善。本文作者在北京市密云区、平谷区的团山子组下部发现了层凝灰岩,并对其开展了系统的SHRIMP锆石U-Pb测年,获得两个高精度年龄1634±9Ma和1637±8Ma,进一步精确限定了该组的形成年代。结合前人研究,燕辽地区团山子组的底界、也即下伏串岭沟组顶界年龄,可进一步限定为1638Ma。由此可对燕辽地区长城群地层格架进行更准确的年代学厘定,并进一步确认其整体处于持续拉张的裂谷-被动陆缘背景下的快速沉积特征、以及长城群团山子组-大红峪组与华北克拉通南缘汝阳群洛峪口组等相关沉积更为精确的对比关系。结合其他资料还可推断,在团山子组-大红峪组及洛峪口组等形成时,华北克拉通南北两侧都开始被碳酸盐岩浅海覆盖,且最终还可能在其中部出现了南北联通的碳酸盐岩浅海通道。这显示出华北克拉通与早先所拼合克拉通之间已产生明显的拉张性疏离及新生海洋,应标志着华北克拉通从哥伦比亚超大陆裂解的新阶段。

铀矿区花岗岩锆石定年的“高铀效应”:来自鲁溪和下庄花岗岩SHRIMP锆石U-Pb年龄的启示

下庄铀矿田是我国最重要的花岗岩型铀矿田之一,其中的复式花岗岩体已获得了一批锆石U-Pb年龄,基本上构筑了下庄铀矿田加里东、印支和燕山多期多阶段的岩浆演化系列。分析前人数据发现,不同学者获得的同一岩体的锆石U-Pb年龄差别较大,最大相差达17 Ma,主要原因之一可能是高U锆石的存在导致了加权平均年龄可靠程度的降低。针对这一问题,笔者以下庄铀矿田鲁溪粗粒黑云母花岗岩和下庄中粒黑云母花岗岩为研究对象,采用SHRIMP锆石U-Pb法测试U-Th-Pb含量,重点分析了锆石U含量对^(206)Pb/^(238)U年龄的影响,并厘定了这两个岩体的侵位年龄。测试结果显示,鲁溪粗粒黑云母花岗岩38颗锆石的U含量变化于152×10^(–6)~5687×10^(–6)之间,^(206)Pb/^(238)U年龄变化于234~1345 Ma之间,可分为五组,其中年龄最小一组由27颗锆石组成,变化于234~265 Ma之间,加权平均年龄为247.3±3.2 Ma,MSWD=3.9;下庄中粒黑云母花岗岩18颗锆石的U含量变化于243×10^(–6)~2071×10^(–6)之间,^(206)Pb/^(238)U年龄为227~871 Ma,可分为三组,其中年龄最小一组由15颗锆石组成,加权平均值236.5±3.3 Ma,MSWD=1.6。鲁溪粗粒黑云母花岗岩年龄最小一组的锆石U含量变化大,^(206)Pb/^(238)U年龄变化也比较大,且U含量>2000×10^(–6)的锆石^(206)Pb/^(238)U年龄存在不确定性,大部分呈明显偏大的现象,若剔除U含量>2000×10^(–6)的锆石^(206)Pb/^(238)U年龄数据,加权平均年龄为240.9±2.4 Ma(n=11,MSWD=0.38);下庄中粒黑云母花岗岩也有一颗锆石的U含量>2000×10^(–6),^(206)Pb/^(238)U年龄为245.4±4.7 Ma,存在年龄偏大的问题,若剔除这个数据,加权平均年龄为235.9±3.3 Ma(n=12,MSWD=1.4)。再结合前人的测试数据表明,铀矿区普遍存在锆石的“高铀效应”,锆石的U含量>2000×10^(–6)时,虽然也存在部分颗粒年龄变化不大的情况,但多数颗粒的^(206)Pb/^(238)U年龄明显偏大或偏小。因此,认为U含量>2000×10^(–6)的锆石,其^(206)Pb/^(238)U年龄不一定能代表寄主岩石的形成年龄,在加权平均年龄数据处理中应尽量剔除。

江西上饶蔡家坪铅锌矿床地质特征和成矿岩体SHRIMP锆石U-Pb定年

蔡家坪铅锌矿床是近年来在北武夷成矿带内铜钹山中生代陆相火山盆地中新发现的铅锌矿床。在野外地质调查的基础上,通过钻孔揭露和岩矿鉴定等方法对蔡家坪铅锌矿床的地质特征进行分析,并对成矿流纹斑岩进行同位素定年。蔡家坪铅锌矿矿化类型可分为次火山岩型和层间破碎带型,矿床成因均与流纹斑岩关系密切;流纹斑岩SHRIMP锆石U-Pb定年显示其^(206)Pb/^(238)U与^(207)Pb/^(235)U的谐和曲线年龄为133±3 Ma,而^(206)Pb/^(238)U加权平均年龄为133.3±2.7 Ma,表明成岩时代为早白垩世,属于燕山中晚期。综合区域地质分析认为,该矿床的成岩成矿时代、成矿条件等与冷水坑铅锌银矿相似,找矿潜力较大。

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 U-Pb Zircon Geochronology of the Xiaotongguanshan and Shatanjiao Intrusions and Its Petrological Implications in the Tongling Area, Anhui

This paper determines the crystallization ages of the Xiaotongguanshan quartz monzodiorite and Shatanjiao quartz monzonitc porphyry from the Tongling area using the SHRIMP zircon U-Pb method. The crystallization age of the former is 142.8±1.8 Ma; that of the latter is 151.8±2.6 Ma. These data indicate that they were formed during the Late Jurassic （142.8 to 151.8 Ma）. Zoned magma chamber was formed because of double diffusive convection. Therefore, the intrusive sequence of magma is generally from quartz monzonite through quartz monzodiorite to pyroxene monzodiorite, i.e. an inverted sequence.

The zircon SHRIMP chronology and trace elementgeochemistry of the Carboniferous volcanic rocksin 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 pe-trology 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 lherzolite. The volcanic rocks in the Dahala-junshan group are neither the products of “rift” nor so-called “plume” but represent the continental island arc of the Pa-leo-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 dat-ing 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 crystal-lizing 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 Da-halajunshan 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.

An Important Spreading Event of the Neo-Tethys Ocean during the Late Jurassic and Early Cretaceous: Evidence from Zircon U-Pb SHRIMP Dating on Diabase in Nagarze, Southern Tibet

Widely distributed in Gyangzê-Chigu area, southern Tibet, NW- and nearly E-W-trending diabase（gabbro）-gabbro diorite dykes are regarded as the product of the large-scale spreading of the late Neo-Tethys Ocean. In order to constrain the emplacement age of these dykes, zircons of two samples from diabases in Nagarzê were dated by using the U-Pb SHRIMP method. Two nearly the same weighted mean ^206pb/^23SU ages were obtained in this paper, which are 134.9±1.8 Ma （MSWD=0.65） and 135.5 ± 2.1 Ma （MSWD=1.40）, respectively. They not only represent the crystallization age of the diabase, but also documented an important spreading event of the Neo-Tethys Ocean during the late Jurassic and early Cretaceous. This dating result is of great significance to reconstruct the temporal framework of the late Neo-Tethys Ocean in the Qinghai-Tibet Plateau.

Zircon SHRIMP U-Pb geochronology and petrogenesis of the plagiogranites from the Lagkor Lake ophiolite,Gerze,Tibet,China

地关系与 petrographical 一起， petrological ，和 geochemicalcharacteristics ，和锆石人口，显示从在 Bangong-Nujiang 缝术地区的 Lagkor Lakeophiolite 的 plagiogranites (～ Gerze 县的 20 km 南方，中央西藏，中国)，由在海洋的外壳的运输期间的可锻的砍源于含水的斑粝岩的 anatexis 。因此，这种 plagiogranite 比传播海洋的外壳晚对或很少同时代。锆石虾 166 ±使 2.5 妈变老因为 plagiogranites 显示海洋的盆在中间的侏罗记在 Bangong-Nujiang 地区的西方的部分存在。

Zircon SHRIMP geochronology of the Xinkailing-Kele complex in the northwestern Lesser Xing'an Range, and its geological implications

Located in the eastern portion of the Xing'an-Mongolian Orogenic Belt (XMOB), the Xinkailing-Kele complex has previously been considered to be Precambrian metamorphic rocks, mainly according to its relatively high metamorphic grade. Our filed observation, however, revealed that the complex is composed mainly of metamorphic rocks (Kele complex), tectono-schists (“Xinkailing Group”),and granitoids (Xinkailing granitic complex). Dating on these rocks using advanced SHRIMP zircon U-Pb technique indicates that: (1) Biotite-plagioclase gneiss from the Kele complex has a protolith age of 337±7Ma (2σ) and a metamorphic age of 216±3Ma (2σ); (2) the tectono-schist of the “Xinkailing Group” gave a magmatic age of 292±6Ma (2σ), indicative of felsic volcanic protolith of the schist formed in Late Paleozoic time; and (3) the Menluhedingzi and Lengchuan granites of the Xinkailing granitic complex were emplaced at167±4 (20σ) and 164±4Ma (2σ), respectively. These results suggest that the Xinkailing-Kele complex is not Precambrian metamorphic rocks and the so-called Precambrian “Nenji-ang Block” does essentially not exist. In combination with regional geological data, we propose that the Kele metamorphic complex is likely related to a collisional tectonism that took place in Triassic lime, as indicted by its metamorphic age of 216±3Ma. The Xinkailing granitic complex was emplaced along the collisional zone during Mid-Jurassic time，likely in a post-orogenic or anorogenic setting.

Zircon SHRIMP U-Pb dating of meta-diorite from the basement of the Songliao Basin and its geological significance

The basement of the Songliao Basin mainly contains low-grade metamorphic rocks and granites. It has been long disputed whether the basin has Precambrian metamorphic basement. This is a report of zircon SHRIMP U-Pb dating results of a meta-diorite sample, which was taken from the Si-5 drilling hole in the southern portion of the Songliao Basin. The SHRIMP analyses indicate that the meta-diorite with a weighted mean 207Pb/206Pb age of 1839±7 Ma (2σ, n = 8) was emplaced during Paleo-Proterozoic time. Additionally, the meta-diorite has old Nd model ages (TDM1: 2999Ma; TDM2: 2849Ma). These data suggest that the south- ern part of the Songliao Basin do possess Precambrian base- ment.