Late Neoarchean thick-skinned thrusting and Paleoproterozoic reworking in the MacQuoid supracrustal belt and Cross Bay plutonic complex,western Churchill Province,Nunavut, Canada

In the western Churchill Province, Canadian Shield, Neoarchean supracrustal and plutonic rocks, intruded by Paleoproterozoic mafic dykes and granitic rocks, comprise the MacQuoid supracrustal belt and the structurally overlying Cross Bay plutonic complex. They form part of the northwestern Hearne subdomain that occupies an intermediate position between the continental Rae domain to the north and west, and the oceanic central Hearne subdomain to the south and east. New geological mapping and supporting geoscience are compatible with the presence of 2550–2500 Ma, southeast-directed, mid-crustal, thick-skinned thrusting that juxtaposed the plutonic complex over the supracrustal belt. The structural contact between the MacQuoid supracrustal belt and the Cross Bay plutonic complex potentially represents a fundamental boundary between isotopically distinct crustal blocks.The ∼2190 Ma MacQuoid mafic dyke swarm cuts across Neoarchean deformation fabrics, but records ∼1.9 Ga, deep-crustal, regional metamorphism that affected both the supracrustal belt and the plutonic complex. Other Paleoproterozoic deformation events that occurred at ∼1850–1810 Ma are of local extent and appear to be relatively minor manifestations of more important events elsewhere, related to the Trans-Hudson orogen.     

Complete metamorphic cycle and long-lived anatexis in the c. 2.1 Ga Mistinibi Complex,Canada

Secular changes in the architecture, thermal state, and metamorphic style of global orogens are thought to have occurred since the Archean; however, despite widespread research, the driving mechanisms for such changes remain unclear. The Paleoproterozoic may prove to be a key era for investigating secular changes in global orogens, as it marks the earliest stage of an eon that saw the onset of modern-style global tectonics. The 2.1 Ga granulite-facies Mistinibi-Raude Domain (MRD), located in the Southeastern Churchill Province, Canada, offers a rare exposure of Paleoproterozoic high metamorphic grade supracrustal sequences (Mistinibi Complex, MC). Rocks from this domain were subjected to petrochronological investigations to establish P–T–t–X evolutions and to provide first order thermal state, burial and exhumation rates, and metamorphic gradients for the transient Paleoproterozoic times. To obtain comprehensive insight into the P–T–t–X evolution of the MRD, we used multi-method geochronology—Lu–Hf on garnet and U–Pb on zircon and monazite—integrated with detailed petrography, trace element chemistry, and phase equilibria modelling. Despite the extensive use of zircon and monazite as geochronometers, their behaviour in anatectic conditions is complex, leading to substantial ambiguity in interpreting the timing of prograde metamorphism. Our results indicate a clockwise metamorphic path involving significant melt extraction from the metasedimentary rocks, followed by cooling from >815°C to ~770°C at ~0.8 GPa. The timing of prograde burial and cooling from supra- to subsolidus conditions is constrained through garnet, monazite, and zircon petrochronology at 2,150–2,120 Ma and at 2,070–2,080 Ma, respectively. These results highlight long-lived residence of the rocks at mid-crustal supra-solidus conditions (55–70 Ma), with preserved prograde and retrograde supra-solidus monazite and zircon. The rocks record extremely slow burial rates (0.25–0.30 km/Ma) along a high metamorphic gradient (900–1,000°C/GPa), which appears symptomatic of Paleoproterozoic orogens. The MC did not record any significant metamorphism after 2,067 Ma, despite having collided with terranes that record high-grade metamorphism during the major 1.9–1.8 Ga Trans-Hudson orogeny. The MC would therefore represent a remnant of a local early Paleoproterozoic metamorphic infrastructure, later preserved as superstructure in the large hot Trans-Hudson orogen.     

内蒙古狼山地区叠布斯格岩群锆石U-Pb年龄、Hf同位素特征及其对阿拉善地块构造演化的制约

阿拉善地块位于华北克拉通西端,其早前寒武纪地质演化一直存在争议。选取阿拉善地块东北缘的叠布斯格岩群为研究对象,通过锆石U-Pb定年和Lu-Hf同位素分析,探讨阿拉善地块的性质和归属及华北克拉通早期地质演化。叠布斯格岩群主要由变质沉积岩和变质火山岩组成,包括高级变质的片麻岩、大理岩、石英岩、角闪岩,并被古元古代钾质花岗岩侵入。来自叠布斯格岩群3个副片麻岩样品的LA-ICP-MS锆石U-Pb定年结果显示,叠布斯格岩群副片麻岩中碎屑锆石的207Pb/206Pb年龄范围为2.60~2.10 Ga,变质锆石的年龄显示叠布斯格岩群经历了1.98~1.90 Ga和1.90~1.79 Ga的两组峰期变质作用,指示叠布斯格岩群原岩形成于2.10~1.98 Ga。研究区未发现有太古宙岩石出露,但不排除其下部地壳中存在太古宙基底岩石的可能。碎屑锆石的Lu-Hf同位素数据结果显示,叠布斯格岩群副片麻岩中碎屑锆石的εHf(t)均为负值,指示其成岩物质源于古—中太古代(3.42~2.79 Ga)古老地壳在新太古代晚期—古元古代早期的重熔或再造。通过对比叠布斯格岩群和阴山地块、孔兹岩带的岩浆-变质事件及Hf同位素特征,推测阿拉善地块前寒武纪基底在古元古代可能是孔兹岩带的一部分。     

冀西北怀安杂岩的年代学、地球化学、Nd-Hf-O同位素组成及其地质意义

冀西北地区怀安杂岩由变质表壳岩和变质深成岩组成,其中变质表壳岩的形成时代、怀安杂岩的构造背景以及其与孔兹岩带间的关系一直存在较大争议.本文对怀安杂岩的几处代表性露头进行了详细野外考察,对4件样品进行了岩石学、锆石SHRIMP U-Pb定年、同位素和元素地球化学分析.所有样品都给出了1.86~1.81 Ga的变质锆石年龄,进一步支持怀安杂岩广泛遭受到古元古代晚期变质作用改造的认识.侵入/包裹含BIF表壳岩组合的变质辉长岩（HB1425）和片麻状英云闪长岩（HB1426）分别给出了~2.5 Ga和2.55 Ga的形成年龄,限制表壳岩形成时代老于2.55 Ga,推测为新太古代表壳岩.浅粒岩（HB1431）和紫苏石榴黑云斜长片麻岩（HB1435）中最老的碎屑锆石分别为2.46 Ga和2.51 Ga,可能还存在古元古代的碎屑锆石,表明它们都为古元古代表壳岩.上述结果进一步确定了怀安杂岩中发育两期表壳岩组合.变质辉长岩和片麻状英云闪长岩的全岩ε_Nd（t）、T_DM1和T_DM2分别为+2.19~+3.06、2.67~2.75 Ga和2.67~2.69 Ga,表明其物源区不存在较大规模的古老陆壳物质,新太古代是怀安地区主要陆壳生长期.变质辉长岩中~1.82 Ga变质锆石中较多具有正的ε_Hf（t）值,最高可达11.1,最可能的解释是古元古代变质过程存在地幔添加作用.锆石的O同位素分析显示区域上可能存在低δ18O的岩石,在古元古代变质过程中,可能存在低δ18O流体对锆石的改造.怀安杂岩和西部孔兹岩带中不同类型岩石的比例明显不同,但两者都同样发育新太古代和古元古代的双层地壳结构,怀安杂岩或许代表孔兹岩带剥蚀更深而出露的深部地壳部分.      

Eclogites of the Snowbird tectonic zone: petrological and U-Pb geochronological evidence for Paleoproterozoic high-pressure metamorphism in the western Canadian Shield

Eclogite occurs within the southern domain of the East Athabasca mylonite triangle in northern Saskatchewan. Situated at the boundary between the Archean Rae and Hearne Provinces of the western Canadian Shield, the East Athabasca mylonite triangle is a fundamental exposure of the ~3,000-km-long Snowbird tectonic zone. The eclogite occurs in association with a variety of lower crustal high-pressure granulites that record a complex metamorphic history from 2.6 to 1.9 Ga. Temperatures of the eclogite facies metamorphism are constrained by garnet-clinopyroxene exchange thermometry at 920–1,000 °C. Minimum pressure conditions are recorded by the jadeite+quartz=albite geobarometer at 1.8–2.0 GPa. A near-isothermal decompression path to granulite facies conditions is inferred from retrograde reaction textures involving the formation of granulite facies assemblages such as orthopyroxene-plagioclase and pargasite-plagioclase. U-Pb IDTIMS zircon geochronology of the eclogite yields a weighted mean ²⁰⁷Pb/²⁰⁶Pb date of 1,904.0±0.3 Ma, which we interpret as the time of peak eclogite facies metamorphism. SHRIMP in situ analyses of metamorphic zircons included within omphacitic clinopyroxene support this interpretation with a weighted mean ²⁰⁷Pb/²⁰⁶Pb date of 1,905±19 Ma. Inclusion suites of high-pressure phases and the petrographic setting of zircon are a direct link between zircon growth and eclogite facies metamorphism. Zircon from one eclogite sample has older cores that are 2.54 Ga, which is a minimum age for the emplacement or earliest metamorphism of the gabbroic protolith. U-Pb rutile data indicate slow cooling at ~1°C/Ma below ~500 °C from 1.88 to 1.85 Ga. The formation and exhumation of the eclogites at ca.1.9 Ga has important implications for the tectonic significance of the Snowbird tectonic zone during the Paleoproterozoic. The eclogites described here are consistent with transport of continental crust to mantle depths during the Paleoproterozoic, followed by rapid buoyancy-driven exhumation to normal lower crustal depths.Editorial responsibility: T.L. Grove     

Protolith and metamorphic ages of the Haiyangsuo Complex, eastern China: a non-UHP exotic tectonic slab in the Sulu ultrahigh-pressure terrane

Summary The Haiyangsuo Complex in the NE Sulu ultrahigh-pressure (UHP) terrane has discontinuous, coastal exposures of Late Archean gneiss with amphibolitized granulite, amphibolite, Paleoproterozoic metagabbroic intrusives, and Cretaceous granitic dikes over an area of about 15 km². The U–Pb SHRIMP dating of zircons indicates that theprotolith age of a garnet-biotite gneiss is >2500 Ma, whereas the granulite-facie metamorphism occurred at around 1800 Ma. A gabbroic intrusion was dated at ∼1730 Ma, and the formation of amphibolite-facies assemblages in both metagabbro and granulite occurred at ∼340–460 Ma. Petrologic and geochronological data indicate that these various rocks show no evidence of Triassic eclogite-facies metamorphism and Neoproterozoic protolith ages that are characteristics of Sulu-Dabie HP-UHP rocks, except Neoproterozoic inherited ages from post-collisional Jurassic granitic dikes. Haiyangsuo retrograde granulites with amphibolite-facies assemblages within the gneiss preserve relict garnet formed during granulite-facies metamorphism at ∼1.85 Ga. The Paleoproterozoic metamorphic events are almost coeval with gabbroic intrusions. The granulite-bearing gneiss unit and gabbro-dominated unit of the Haiyangsuo Complex were intruded by thin granitic dikes at about 160 Ma, which is coeval with post-collisional granitic intrusions in the Sulu terrane. We suggest that the Haiyangsuo Complex may represent a fragment of the Jiao-Liao-Ji Paleoproterozoic terrane developed at the eastern margin of the Sino-Korean basement, which was juxtaposed with the Sulu terrane prior to Jurassic granitic activity and regional deformation.     

冀西北怀安地体高级变质表壳岩的锆石年代学研究

位于华北克拉通中部造山带中北段的怀安地体与内蒙孔兹岩带相接,出露高压麻粒岩和退变榴辉岩等多种高级变质岩,是洞悉华北克拉通古元古代构造演化历史的重要窗口。研究区变质表壳岩包括夕线石榴长英质片麻岩、石榴长英质粒状岩石以及紫苏黑云二长片麻岩。阴极发光图像特征显示研究区样品的锆石主要包括碎屑锆石和变质锆石,其中碎屑锆石具有岩浆结晶环带,而变质锆石为单颗粒或围绕着继承性碎屑锆石边部生长,内部结构均匀,Th/U比值较低。锆石LAICP-MS U-Pb定年结果与前人研究结果综合表明该区变质表壳岩石的碎屑锆石的207Pb/206Pb年龄主要集中在~2040Ma,其原岩形成时代与孔兹岩带变泥质岩石相近,均为~2.0Ga。变质锆石记录其变质时代为1957~1804Ma,结合前人对怀安地区变泥质岩和变基性岩变质作用和年代学研究结果,推测得出1.95~1.92Ga代表了峰期(高压)麻粒岩相变质时代,1.90~1.85Ga代表峰后减压阶段变质时代,而1.85~1.80Ga代表退变质晚期的时代。怀安地区变质岩石可能卷入了阴山陆块、鄂尔多斯陆块以及东部陆块间的先后碰撞造山过程,并持续较长时间(1.95~1.80Ga),最终拼贴为统一的整体。     

胶北地体多期变质事件的P-T-t轨迹及其对胶-辽-吉带形成与演化的制约

胶北地体位于华北克拉通东部陆块胶-辽-吉带南端,主要由闪长质-TTG-花岗质片麻岩、变质表壳岩系和变质镁铁-超镁铁质岩所组成。本文通过对胶北早前寒武纪变质岩系的岩石学、矿物化学、变质反应结构和序列、变质温度和压力估算与同位素年代学资料的综合研究和总结,得出以下重要结论:(1)与华北克拉通东部陆块其它地区太古宙变质基底类似,本区也存在~2500Ma区域性新太古代变质事件,且与本区2550~2500Ma岩浆作用在时间上非常接近,其变质作用发生的时间比岩浆作用要晚10~50Myr,指示本区~2500Ma区域性变质事件可能与大规模的幔源岩浆底侵作用存在密切的成因关系。(2)胶北还存在1950~1850Ma区域性古元古代变质事件,并导致了大量高压基性和泥质麻粒岩的形成,高压基性麻粒岩主要以不规则透镜体、变形岩墙群或岩脉群的形式赋存于闪长质-TTG-花岗质片麻岩之中,并集中分布在安丘-平度-莱西-莱阳-栖霞一带,大致沿北东-南西向断续带状分布,构成了一条长约300km的古元古代高压麻粒岩相变质带。(3)本区古元古代高压麻粒岩以记录近等温减压(ITD)及随后近等压降温(IBC)的顺时针P-T-t轨迹为特征,指示本区变质杂岩在古元古代晚期曾强烈地卷入了与俯冲-拼贴-碰撞造山有关的构造过程,并可能经历了如下复杂的构造演化:(I)在古元古代晚期2000~1950Ma,随着有限大洋地壳的持续俯冲作用,本区各类变质岩的原岩开始经历一次构造增厚事件,并导致了它们的原岩经历了早期绿片岩相-角闪岩相进变质作用;(II)1950~1870Ma,大洋地壳俯冲作用结束,本区开始发生弧-陆拼贴和陆-陆碰撞作用,大陆地壳持续缩短和加厚,在加厚下地壳或岛弧根部带约50km的深度,发生了区域性高压麻粒岩相变质作用,并导致了本区变基性岩和变泥质岩分别形成了石榴石+单斜辉石+斜长石±角闪石±石英±铁-钛氧化物和石榴石+蓝晶石+钾长石+斜长石+黑云母+石英+铁-钛氧化物+熔体的高压麻粒岩相矿物组合。(III)1870~1800Ma,在同碰撞峰期变质结束之后,本区造山作用进入了后碰撞构造折返-伸展演化阶段,先后经历了早期快速构造折返和晚期缓慢冷却降温两个构造热演化阶段。其中,在早期快速构造折返阶段,高压麻粒岩经历了峰后近等温或略微增温减压退变质作用的叠加,高压基性麻粒岩表现为沿石榴石边部形成了含斜方辉石的后成合晶。与此同时,早期快速构造折返阶段还伴随着热松弛和伸展作用,出现一系列的幔源基性岩浆活动,不仅导致了本区大量未经历高压麻粒岩相变质的变基性岩群的形成,同时也诱发了区内大规模的地壳深熔作用的发生。自温度高峰期之后,本区地壳岩石还经历了一个近等压冷却降温过程,并发生了区域性角闪岩相退变质作用,高压基性麻粒岩表现为石榴石和斜方辉石边部常出现含角闪石的退变边或后成合晶。最终,在1800Ma左右,本区含电气石花岗伟晶质岩脉的大量出现,则标志着胶北地体古元古代晚期(2000~1800Ma)俯冲-拼贴-碰撞造山作用的最终结束。     

扬子陆核古元古代变质事件——来自孔兹岩系变质锆石U-Pb同位素年龄的证据

崆岭杂岩除太古宙结晶基底外,还出露一套以含石墨和富铝矿物为特征的孔兹岩系。相对于结晶基底,目前对于该套表壳岩系的同位素年代学研究有限。对该套孔兹岩系中代表性岩石类型榴线英岩开展了变质锆石U-Pb同位素年龄测定。研究结果表明,榴线英岩变质年龄为1964±12Ma。结合前人在相近地层岩石组合中报道的锆石U-Pb年龄数据,推测崆岭杂岩孔兹岩系原岩可能沉积于2.1~2.0Ga。扬子陆核的古元古代变质-岩浆作用可能与全球广泛存在的同时期(2.1~1.8Ga)碰撞造山事件有关,暗示其很可能是Columbia超大陆的重要组成部分。     

华北克拉通南缘太华杂岩组成及演化

太华杂岩位于华北克拉通南部,其组成复杂,记录了几乎所有早前寒武纪各阶段重要的地质事件;此外,由于其所处特殊地理位置,研究太华杂岩对于华北克拉通早前寒武纪地壳形成和演化、构造单元划分和基底拼合等都具有举足轻重的科学价值。本文综合已有的岩石学、变质作用、地球化学以及同位素年代学等诸多研究工作,得到以下阶段性结论和认识:1)将鲁山地区太华划分为以深成侵入岩为主的片麻岩系和以变质沉积-火山岩为主的表壳岩系;前者形成于中太古代晚期-新太古代早期,后者形成于古元古代晚期。而小秦岭地区太华杂岩中变质深成侵入岩形成时间跨度较大,为中太古代晚期-古元古代早期;而其上覆的火山-沉积岩可与鲁山太华杂岩的表壳岩类比,形成时间亦为古元古代晚期。2)中太古代-新太古代(2.91～2.50Ga)为华北克拉通南部大陆最主要的地壳形成时期。提出太华杂岩在太古宙经历了两期明显的地壳生长时间,一期发生在2.85～2.70Ga,以鲁山太华片麻岩系中的深成侵入岩和斜长角闪岩为代表;另一期发生在～2.50Ga,以小秦岭华山和崤山地区太华杂岩中各类花岗质岩石为代表。3)太华杂岩在所谓的全球陆壳生长"沉寂期(2.45～2.20Ga)"岩浆活动异常发育,推测这一时期的岩石形成于古元古代俯冲-汇聚环境,可能是与华北克拉通南部太古宙陆块和其他陆块汇聚-碰撞相关。4)太华杂岩在古元古代晚期普遍遭受了强烈的变质和变形,其变质程度主体为高角闪岩相,局部可达麻粒岩相,且记录了包含近等温降压退变质片段的顺时针变质作用P-T轨迹,经历了一个漫长的变质演化过程(1.97～1.80Ga),变质作用的时限跨度可达150Myr。5)提出华北克拉通南部曾经为一个统一基底,称之为"南部太古宙地块",此地块形成时间为新太古代末期(～2.5Ga)。该古老陆块经历了如下5个构造-演化阶段:(1)冥古宙-始太古代初始陆核形成;(2)中太古代-新太古代陆壳快速生长;(3)古元古代早期(～2.3Ga)岩浆活动异常活跃;(4)古元古代(2.30～1.97Ga)陆内拉伸-破裂;和(5)古元古代末期(1.97～1.80Ga)陆块最终拼合。     

华北克拉通怀安杂岩中早前寒武纪两期变质表壳岩的重新厘定:岩石学及锆石U-Pb年代学证据

华北克拉通怀安杂岩中早前寒武纪变质表壳岩具有变质程度深（麻粒岩相）、连续性差、变形及构造置换强烈等特征,地层划分及形成时代一直存在较大争议.通过详细的地质调查、岩石学及年代学研究,新厘定出新太古代桑干岩群铁英岩岩组、石榴黑云片麻岩岩组以及古元古代集宁岩群黄土窑岩组石榴高压基性麻粒岩-大理岩岩段和含石墨矽线石榴片麻岩-变粒岩-大理岩岩段4套变质表壳岩单元,并建立了相应的岩石学识别标志.新太古代变质表壳岩一般呈透镜体或带状体产出于新太古代变质深成岩中,且锆石年龄存在~2.5 Ga和~1.8 Ga两个峰值.铁英岩岩组原岩为一套基性火山岩夹磁铁石英岩建造,产出阿尔戈马型BIF铁矿,形成年龄为2 489±19 Ma;石榴黑云片麻岩岩组主体岩石为条带状石榴黑云斜长片麻岩,原岩为杂砂岩建造,物源来自新太古代TTG岩石.古元古代石榴高压基性麻粒岩-大理岩岩段原岩为一套基性火山岩夹大理岩建造,形成年龄早于~2.03 Ga;含石墨矽线石榴片麻岩-变粒岩-大理岩岩段野外呈带状构造岩片体产出,连续性较好,为一套"有层无序"的孔兹岩组合,碎屑锆石年龄介于2.30~1.99 Ga,与区域上孔兹岩年龄结构一致,形成时代为古元古代晚期（1.99~1.95 Ga）.新太古代和古元古代变质表壳岩均卷入古元古代末期的造山过程（1.95~1.80 Ga）,遭受麻粒岩相变质与变形作用.区内孔兹岩可能是造山过程卷入的构造岩片.本研究进一步明确了晋冀蒙交界地区存在与孔兹岩不同的新太古代陆源碎屑岩和古元古代洋壳残片,这对重新认识本区早前寒武纪地质演化具有重要意义.      

Evidence of 1.7- to 1.8-Ga Collisional Arc in the Kaoko Belt, NW Namibia

Detailed structural and isotopic analyses in the Hoanib and Ugab River Valleys indicate the existence of an exotic 1.7- to 1.8-Ga terrane in the Pan-African Kaoko Belt. This crustal block, called as Mudorib Complex, is imbricated between autochthonous and para-autochthonous rocks of Congo Craton, Kaoko Basin, and Western Kaoko Batholith units during the main tectono-thermal phase of Kaoko Belt collision around 580?Ma, involving the Rio de La Plata, Congo and Kahalari paleoplates. This terrain is positioned between the 1.9-Ga Pruwes Complex units of SW edge of the Congo Craton and the 0.58- to 0.55-Ga Amspoort Suite granitoids of the Western Kaoko Batholith. It is coincident with a regional positive aeromagnetic anomaly trending from NNW in the Ugab region to the Namibia-Angola border. Internally, Mudorib Complex consists in 1.73- to 1.81-Ga tonalitic?Ctrondhjemitic?Cdioritic?Cgranodioritic sequence of gneisses associated with cogenetic gabbroic and anothositic-gneisses in the core zone of this Pan-African structure. Field relationship and U?CPb zircon and Sm?CNd whole-rock isotope data combined with geochemical information suggest the existence of two rock associations in the Mudorib Complex, namely late Paleoproterozoic tonalitic?Ctrondhjemitic?Cdioritic-gneisses with island-arc affinity and tholeiitic metabasites of juvenile origin, showing Nd model age of 1.73?C2.17?Ga and ??Nd(t) of ?2.05?C+4.3. This 1.8- to 1.7-Ga complex is also intruded by granitic dykes formed at 1.49?C1.50?Ga with Nd model age of 1.75?C2.34?Ga during stable tectonic conditions. In addition to widespread Pan-African tectono-metamorphic events, a secondary metamorphic event of ~1.3?Ga is also recognized in the Mudorib rocks, which may be associated with accretion process of the complex to the Paleoproterozoic to Archean nucleus of the Kaoko Belt in the Hoanib River Valley.     

The Late Paleoproterozoic Orogeny in the North China Craton

Granulite TTG and khondalite formations are the two typical types of Precambrian metamorphic basement in the North China Craton. The former belongs to the basement of the Archean craton, while the latter is a Paleoproterozoic metasedimentary rock series. The formation of the TTG rock series took place during the period of 2500 to 2450 Ma, near the end of the Archean, and it has undergone multiple times of metamorphism. The cooling rate of the TTG lower crust is quite low (less than 0.3°/Ma), developed during slow crustal cooling in a rather stable tectonic environment. The khondalite rock series underwent amphibolite- to granulite-facies metamorphism that is isotopically constrained to 2000 to 1800 Ma followed by fast crustal cooling from 2.4° to 3.4°/Ma. Based on the thermal and dynamic features of different types of metamorphic rocks, the tectonic state of the crust, and the obvious change of crustal cooling rate from Neo-Archean to Paleoproterozoic, it is clear that continental orogeny existed in the North China craton between 2000 Ma and 1800 Ma.     

冀西北尚义黄土窑地区尚义杂岩和红旗营子群的锆石SHRIMP U-Pb定年及地质意义

尚义杂岩是冀西北地区有代表性的变质地质体,由变质表壳岩和侵入岩组成。其中的变质表壳岩曾认为是新太古代晚期蛇绿岩洋壳残片。该区也有红旗营子群存在。我们对尚义黄土窑地区尚义杂岩和红旗营子群不同类型岩石进行了锆石SHRIMP U-Pb定年。含石榴黑云斜长片麻岩(HB1410)核部锆石年龄为310~2500Ma,边部变质锆石年龄为255Ma左右。黑云片麻岩(HB1411)和石英岩(HB1415)碎屑锆石年龄都为2.5Ga左右。辉长岩(HB1518)和石英闪长岩(HB1519)岩浆锆石年龄为278~279Ma。根据这些资料和前人研究,该地区的原红旗营子群和尚义杂岩变质表壳岩主体可能不是新太古代地质作用产物,而是形成于晚古生代,由不同时代地质体组成的构造杂岩。不存在所谓的太古宙蛇绿岩残片。晚古生代时期,中亚造山带对华北克拉通北缘的影响十分强烈。     

内蒙古白云鄂博地区基底岩石锆石年代学及对构造背景的指示

白云鄂博地区的克拉通基底岩石主要由糜棱岩化花岗片麻岩(2588±15Ma),正长岩、花岗闪长岩(2018±15Ma)和黑云母花岗片麻岩、含石榴石蓝晶石花岗片麻岩(～1890Ma)等组成,白云鄂博超大型稀土矿床就位于华北克拉通太古代基底之上。太古代克拉通基底在早元古代中晚期(2.0Ga)又经历了一次强烈的碰撞造山运动,造成了闪长质与花岗质岩浆侵位,以及1.9Ga片麻岩相变质事件。     

辽东半岛南辽河群锆石U-Pb年代学及其地质意义

本文报道了辽东半岛古元古代胶-辽-吉活动带南辽河群中变质火山岩和沉积岩的锆石U-Pb年代学数据。变质流纹岩的锆石具典型的岩浆振荡环带结构和较高的Th/U比值(0.3),锆石U-Pb年龄为~2.2Ga,该年龄可代表其原岩形成年龄,在误差范围内与古元古代辽吉花岗岩年龄一致,表明辽吉花岗岩并不是辽河群的基底。变质玄武岩的锆石阴极发光强度较弱、弱分带或无分带,同时具较低的Th/U比值(0.1),为典型的变质成因锆石,锆石U-Pb年龄为~1.9Ga,代表其变质时代。变质沉积岩的碎屑锆石年龄主要介于1981~3520Ma之间:峰期年龄为2033Ma和2092Ma的锆石年龄信息暗示辽东半岛至少存在一期2000~2100Ma的岩浆事件,并且该时期的中酸性岩浆岩是南辽河群沉积岩的一个重要物源;峰期年龄为2155Ma、2446Ma、2509Ma、2594Ma、2668Ma、2790Ma、3356Ma、3467Ma和3520Ma的锆石年龄信息,区域上与古元古代辽吉花岗岩、辽河群火山岩及太古宙基底年龄相吻合,暗示它们为南辽河群沉积岩提供了重要物源。沉积岩中最年轻的碎屑锆石U-Pb年龄为~2.0Ga,可代表其沉积时的最大年龄。所以,辽河群火山-沉积-变质的时限为2.2~1.9Ga,其演化时间约300Myr。结合前人有关辽东半岛前寒武纪岩石的研究成果,本文研究认为胶-辽-吉活动带的形成演化与弧-陆碰撞有关,而不是许多人坚持的裂谷环境。     

朝鲜北部狼林地块构造归属与地壳形成时代

朝鲜半岛北部的狼林地块一直被认为是中朝克拉通的重要组成部分。传统认为,它向南与我国辽东半岛的辽南太古宙地体相接,与其北部的鞍山-辽北-吉南太古宙地体(龙岗地块)具有基本类似的物质组成。两大太古宙地体之间是著名的以辽河群、集安群和老岭群为代表的辽吉古元古代岩系。辽吉岩系目前最主要的学术争论是,它是原本一体的太古宙地体的裂解产物,还是两个性质不同的太古宙地体拼合的结果。无论采用何种模型,学术界都普遍接受狼林地块主要由太古代岩石组成这一基本假定。为准确厘定狼林地块的地壳性质与形成时代,本文选择大同江、清川江、城川江、长津江、厚州川、厚昌江和秃鲁江中的河沙样品作为研究对象。这些河流均发源于狼林山脉,是狼林地块的核心区域,因而这些河流沉积物能够较好地全面反映狼林地块的物质组成情况。上述河流不同部位8件样品的分析结果显示,狼林地块主要由18~19亿年的古元古代岩石组成,太古宙岩石比例极为有限。但锆石Hf同位素模式年龄集中在28亿年左右,与华北克拉通全岩样品的Nd同位素模式年龄基本一致。结合狼林地块大量麻粒岩相变质表壳岩系和古元古宙花岗岩的发育,本文认为狼林地块是与辽吉岩系基本类似的古元古代地体,它可能是华北克拉通在古元古代期间东南大陆边缘的巨型造山带,我们可将其简称为辽-吉-朝古元古代造山带。因此,先前认为狼林地块主要由太古宙岩石组成的观点需要重新检查和认识。     

SHRIMP U–Pb zircon dating of the Neoproterozoic Penglai Group and Archean gneisses from the Jiaobei Terrane, North China, and their tectonic implications

Archean basement gneisses and supracrustal rocks, together with Neoproterozoic (Sinian) metasedimentary rocks (the Penglai Group) occur in the Jiaobei Terrane at the southeastern margin of the North China Craton. SHRIMP U–Pb zircon dating of an Archean TTG gneiss gave an age of 2541 ± 5 Ma, whereas metasedimentary rocks from the Neoproterozoic Penglai Group yielded a range in zircon ages from 2.9 to 1.8 Ga. The zircons can be broadly divided into three age populations, at: 2.0–1.8 Ga, 2.45–2.1 Ga and >2.5 Ga. Detrital zircon grains with ages >2.6 Ga are few in number and there are none with ages <1.8 Ga. These results indicate that most of the detrital material comes from a Paleoproterozoic source, most likely from the Jianshan and Fenzishan groups, with some material coming from Archean gneisses in the Jiaobei Terrane. An age of 1866 ± 4 Ma for amphibolite-facies hornblende–plagioclase gneiss, forming part of a supracrustal sequence within the Archean TTG gneiss, indicates Late Paleoproterozoic metamorphism. Both the Archean gneiss complex and Penglai metasedimentary rocks resemble previously described components of the Jiao-Liao-Ji orogenic belt and suggest that the Jiaobei Terrane has a North China Craton affinity; they also suggest that the time of collision along the Jiao-Liao-Ji Belt was at 1865 Ma.     

甘肃敦煌水峡口地区前寒武纪岩石的锆石U-Pb年龄、Hf同位素组成及其地质意义

敦煌杂岩位于塔里木克拉通的东部,探寻和研究其中的早前寒武纪地质体对于探讨敦煌地块早前寒武纪地壳的形成和演化及其构造归属等问题具有重要的意义.甘肃敦煌水峡口地区的敦煌杂岩主要由英云闪长质片麻岩、花岗闪长质片麻岩以及表壳岩石组成.利用LA-ICP-MS锆石U-Pb定年方法测得水峡口英云闪长片麻岩和花岗片麻岩原岩的形成年龄分别为2561±16Ma和2510±22Ma,确证了在敦煌杂岩中存在太古宙岩石.此外,还获得斜长角闪岩的变质年龄为1806±14Ma,推测其原岩岩浆可能来自古老岩石圈地幔的部分熔融.根据已有的资料提出在古元古代晚期(1.80 ～ 1.85Ga)敦煌杂岩经历了一期较广泛的变质作用.锆石Hf同位素显示～2.5Ga的岩石年龄在敦煌地块代表新太古代晚期重要地壳生长时期,而～1.8Ga的构造-热事件则是以古老地壳物质循环再造为主.这些资料显示敦煌地块和华北克拉通在早前寒武纪经历了类似的地壳形成和演化过程,且共同记录了全球性的Columbia碰撞造山事件信息.     

Crustal architecture above the high-pressure belt of the Grenville Province in the Manicouagan area: new structural, petrologic and U–Pb age constraints

Recent U–Pb age determinations and P–T estimates allow us to characterize the different levels of a formerly thickened crust, and provide further constraints on the make up and tectono-thermal evolution of the Grenville Province in the Manicouagan area. An important tectonic element, the Manicouagan Imbricate zone (MIZ), consists of mainly 1.65, 1.48 and 1.17 Ga igneous rocks metamorphosed under 1400–1800 MPa and 800–900 °C at 1.05–1.03 Ga, during the Ottawan episode of the Grenvillian orogenic cycle, coevally with intrusion of gabbro dykes in shear zones. The MIZ has been interpreted as representing thermally weakened deep levels of thickened crust extruded towards the NW over a parautochthonous crustal-scale ramp. Mantle-derived melts are considered as in part responsible for the high metamorphic temperatures that were registered.New data show that mid-crustal levels structurally above the MIZ are represented by the Gabriel Complex of the Berthé terrane, that consists of migmatite with boudins of 1136±15 Ma gabbro and rafts of anatectic metapelite with an inherited monazite age at 1478±30 Ma. These rocks were metamorphosed at about the same time as the MIZ (metamorphic zircon in gabbro: 1046±2 Ma; single grains of monazite in anatectic metapelite: 1053±2 Ma) and under the same T range (800–900 °C) but at lower P conditions (1000–1100 MPa). They are mainly exposed in an antiformal culmination above a high-strain zone, which has tectonic lenses of high P–T rocks from the MIZ and is intruded by synmetamorphic gabbroic rocks. This zone is interpreted as part of the hangingwall of the MIZ during extrusion. A gap of 400 MPa in metamorphic pressures between the tectonic lenses and the country rocks, together with the broad similarity in metamorphic ages, are consistent with rapid tectonic transport of the high P–T rocks over a ramp prior to the incorporation of the mafic lenses in the hangingwall.Between the antiformal culmination of the Gabriel Complex and the MIZ 1.48 Ga old granulites of the Hart Jaune terrane are exposed. They are intruded by unmetamorphosed 1228±3 Ma gabbro sills and 1166±1 Ma anorthosite. Hart Jaune Terrane represents relatively high crustal levels that truncate the MIZ-Gabriel Complex contact and are preserved in a synformal structure.Farther south, the Gabriel Complex is overlain by the Banded Complex, a composite unit including 1403+32/−25 Ma granodiorite and 1238+16/−13−1202+40/−25 Ma granite. This unit has been metamorphosed under relatively low-P (800 MPa) granulite-facies conditions. Metamorphic U–Pb data, limited to zircon lower intercept ages (971±38 Ma and 996±27 Ma) and a titanite (990±5 Ma) age, are interpreted to postdate the metamorphic peak.The general configuration of units along the section is consistent with extrusion of the MIZ during shortening and, finally, normal displacement along discrete shear zones.