华北克拉通南部太古宙大陆地壳的生长和演化

太古宙约占地球已有演化历史的三分之一强,这一时期涉及到大陆地壳起源、陆壳的巨量生长和稳定以及板块构造作用的启动、建立等诸多最根本的全球性重大地质事件。太古宙岩石在华北克拉通南部的涑水、登封、太华、霍邱和五河等杂岩中广泛出露,这为解析上述重大科学问题提供难得的素材。近十年来,在华北克拉通南部古生代-中生代火山岩或早前寒武纪变沉积岩中陆续发现冥古宙-古太古代的捕获/碎屑锆石,暗示南部地块依然尚存地球形成最初期的陆壳物质。根据华北克拉通南部太古宙岩石年龄统计结果显示有2850~2700Ma和2580~2480Ma两个突出年龄区间,对应的峰值年龄分别为~2.76Ga和~2.52Ga。其中~2.76Ga的岩石主要出露于南部的鲁山、霍邱、五河和中条山地区。此外,在华北克拉通诸多地区,诸如怀安、阜平、五台、中条等地区的花岗质片麻岩和变质沉积岩中也均发现年龄为~2.76Ga的碎屑锆石或者继承锆石,暗示华北克拉通2.85~2.70Ga岩石的分布似乎比现今出露范围更为广泛。与整个华北克拉通类似,2.58~2.48Ga岩石亦在克拉通南部广泛分布,尤其是嵩箕地区的登封杂岩几乎全部是由新太古代晚期的岩石组成。~2.52Ga是华北克拉通南部,乃至整个克拉通太古宙地壳演化最突出、最重要的岩浆-构造事件,明显有别于全球其它诸多典型克拉通。已有的同位素资料研究表明华北克拉通南部,乃至整个克拉通在太古宙经历了两期明显的地壳生长事件:一期发生在2.85~2.70Ga左右,以形成于此时期的涑水杂岩中花岗质岩石和鲁山太华片麻岩系中深成侵入岩和斜长角闪岩为代表;另一期发生在2.58~2.48Ga,以登封杂岩、涑水杂岩以及小秦岭地区太华杂岩中~2.52Ga各类花岗质岩石和变基性岩为代表。华北克拉通正是经过这两期陆壳巨量生长事件之后完成初始的克拉通化。我们在登封杂岩中识别出形成于俯冲汇聚环境的TTG质片麻岩、类似于赞岐岩的变闪长岩和具有N-MORB地球化学特征的变基性火山岩,提出其构成"新太古代构造混杂岩",标志着新太古代末期具有现代体制的板块构造在华北克拉通南部已经开始启动。最近,在登封杂岩中识别出的新太古代双变质带也支持上述观点。     

华北克拉通中东部新太古代晚期变质火山岩及动力学体制

热状态和壳幔岩浆作用是理解早期地壳形成演化动力学机制的关键。华北克拉通是世界范围内为数不多的保存有大量新太古代晚期(约26~25亿年)变质火山岩记录的克拉通之一,对揭示全球新太古代晚期壳-幔动力学演化过程具有重要的指示意义。在我们研究组近期关于华北克拉通中东部中新太古代热状态和地壳厚度研究基础上,本文收集并整理了726个华北克拉通中东部(包括中部带)新太古代晚期变质火山岩样品的有效地球化学资料。按照现代通用岩石地球化学标准来分类,这些样品主要包括超铁镁质岩石(其中含苦橄岩、苦橄质玄武岩和科马提岩,~7%)、稀土未分异型玄武岩(~14%)、稀土分异型玄武岩(~27%)、玻安岩(~4%)、高镁安山岩(~12%)、低镁安山岩(~26%)和英安岩-流纹岩(~10%)。然而不同区块之间火山岩岩石组合及其量比存在较大差异,其中吉林南部和赞皇等地区以大量稀土分异型玄武岩、高镁和低镁安山岩为主,含有少量的长英质火山岩;胶东、登封和阜新等地区以稀土未分异和稀土分异型玄武岩占有绝对优势,存在少量安山岩和长英质火山岩;冀北、冀东北部、冀东南部(迁安-滦县)、五台-云中山、辽北、辽南和鲁西等地区岩石组合比较复杂,最突出的特点是出现不同比例的玻安岩,组合有稀土未分异和大量稀土分异型玄武岩、高镁和低镁安山岩,出现少量超铁镁质岩石和长英质火山岩。岩石成因研究揭示稀土未分异型和分异型玄武岩、高镁安山岩和玻安岩主要形成于俯冲板片流体、熔体和沉积物熔体交代地幔的部分熔融,而低镁安山岩、英安岩和其它长英质火山岩则大都经历了上述俯冲相关初始岩浆的结晶分异或地壳物质熔融和地壳混染等过程。新太古代晚期胶东地区表现为相对较薄的地壳厚度和较高的地热梯度(18℃/km),而冀东地区表现为厚的地壳厚度和低的地热梯度(最低8.7℃/km),满足现代俯冲地热梯度需求,其它区域的地温梯度介于热俯冲和现代冷俯冲之间。综合以上资料,我们认为新太古代晚期板块构造体制已经是最主要的壳-幔动力学体制,地幔柱构造体制和板块构造-地幔柱联合作用体制可能仍然在局部地区存在,但其作用范围和强度已经明显减小。因此,随着地幔温度的下降,中太古代到新太古代晚期地幔柱和板片俯冲的转化可能是相互关联、此消彼长的动力学过程,而不是一个突变过程。     

Late Neoarchean geodynamic evolution: Evidence from the metavolcanic rocks of the Western Shandong Terrane,North China Craton

Late Neoarchean metavolcanic rocks are widely distributed in the Western Shandong Terrane (WST). They are classified as ~2590–2580 Ma tholeiites (Group MB-1), ~2550–2530 Ma tholeiites (Group MB-2), calc-alkaline basalts (Group MB-3), high-Si adakites (Group MAD) and ~2520–2500 Ma tholeiites (Group MB-4) based on zircon U-Pb chronological and geochemical data. Their parental magmas have complex origins and were derived from a depleted mantle wedge enriched by slab-derived melts or fluids (Group MB-1); an unaltered depleted mantle (Group MB-2); the delaminated lower crustal materials (Group MAD); a strongly melt- and fluid-metasomatized depleted mantle (Group MB-3); and a fluid- and sediment-metasomatized asthenospheric mantle (Group MB-4). The late Neoarchean geodynamic evolution of the WST revealed by these multi-genetic volcanic rocks can be summarized as follows: (1) an ~2.62–2.53 Ga eastward subduction operated along the ancient continental margin, followed by delamination of unstable continental lithosphere in the back-arc region during ~2.60–2.53 Ga; and (2) delamination-derived mantle magmas ascended and caused the regional extension, further inducing the asthenosphere to passively rise and the back-arc basin to open during ~2.52–2.50 Ga. The above intense mantle magmatism and crust-mantle interactions have consumed abundant mantle energy and facilitated the continental stratification and final cratonization of the WST.     

华北陆块基底构造格局及早期大陆克拉通化过程

依据区域构造分析及同位素年代娄数据库,华北克拉通普质基底主要可以区划为以处构造单元：１）鄂尔多斯陆块新太古代被动边缘沉积;２）恒山－－承德太古代末期构造带;３）太古代末期五台－－登封岛弧带杂岩及构造缝合带;４）鲁西－－冀东－辽吉新太古代活动大陆边缘岩浆杂岩带;５）胶辽陆块;６）冀北－－固阳古元代初造山带及内蒙－＝东再造麻粒岩要带;７）吕梁－－中条古元古代裂谷带;８）辽南古元古代裂谷带。华北克拉通早     

http://www.sciencedirect.com/science/article/pii/S1674987114000206

The North China Craton（NCC） has a complicated evolutionary history with multi-stage crustal growth,recording nearly all important geological events in the early geotectonic history of the Earth.Our studies propose that the NCC can be divided into six micro-blocks with ＆gt;～3.0-3.8 Ga old continental nuclei that are surrounded by Neoarchean greenstone belts（CRB）.The micro-blocks are also termed as highgrade regions（HGR） and are mainly composed of orthogneisses with minor gabbros and BIF-bearing supracrustal beds or lenses,all of which underwent strong deformation and metamorphism of granulite- to high-grade amphibolite-facies.The micro-blocks are,in turn,from east to west,the Jiaoliao（JL）,Qianhuai（QH）,Ordos（ODS）,Ji’ning（JN） and Alashan（ALS） blocks,and Xuchang（XCH） in the south.Recent studies led to a consensus that the basement of the NCC was composed of different blocks/terranes that were finally amalgamated to form a coherent craton at the end of Neoarchean.Zircon U-Pb data show that TTG gneisses in the HGRs have two prominent age peaks at ca.2.9-2.7 and2.6-2.5 Ga which may correspond to the earliest events of major crustal growth in the NCC.Hafnium isotopic model ages range from ca.3.8 to 2.5 Ga and mostly are in the range of 3.0-2.6 Ga with a peak at2.82 Ga.Recent studies revealed a much larger volume of TTG gneisses in the NCC than previously considered,with a dominant ca.2.7 Ga magmatic zircon ages.Most of the ca.2.7 Ga TTG gneisses underwent metamorphism in 2.6-2.5 Ga as indicated by ubiquitous metamorphic rims around the cores of magmatic zircon in these rocks.Abundant ca.2.6-2.5 Ga orthogneisses have Hf-in-zircon and Nd wholerock model ages mostly around 2.9-2.7 Ga and some around 2.6-2.5 Ga,indicating the timing of protolith formation or extraction of the protolith magma was from the mantle.Therefore,it is suggested that the 2.6-2.5 Ga TTGs probably represent a coherent event of continental accretion and major reworking（crustal melting）.As a distinct characte     

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。     

http://www.sciencedirect.com/science/article/pii/S1674987111001125

<正>Greenstone belts of the eastern Dharwar Craton,India are reinterpreted as composite tectonostratigraphic terranes of accreted plume-derived and convergent margin-derived magmatic sequences based on new high-precision elemental data.The former are dominated by a komatiile plus Mg-tholeiitic basalt volcanic association,with deep water siliciclastic and banded iron formation(BIF) sedimentary rocks.Plumes melted at90 km under thin rifted continental lithosphere to preserve inlraoceanic and continental margin aspects.Associated alkaline basalts record subduction-recycling of Mesoarchean oceanic crust,incubated in the asthenosphere.and erupted coevally with Mg basalts from a heterogeneous mantle plume.Together.komaliites-Mg basalts-alkaline basalts plot along the Phanerozoic mantle array in Th/Yb versus Nb/Yb coordinate space,representing zoned plumes,establishing that these reservoirs were present in the Neoarchean mantle. Convergent margin magmatic associations are dominated by tholeiitic to calc-alkaline basalts eompositionally similar to recent intraoceanic arcs.As well,boninitic flows sourced in extremely depleted mantle are present,and the association of arc basalts with Mg-andesites-Nb enriched basalts-adakites documented from Cenozoic arcs characterized by subduction of young(20 Ma),hot,oceanic lithosphere. Consequently.Cenozoic style "hot" subduction was operating in the Neoarchean.These diverse volcanic associations were assembled to give composite terranes in a subduction-accretion orogen at～2.1 Ga,coevally with a global accretionary orogen at ~2.7 Ga,and associated orogenic gold mineralization. Archean lithospheric mantle,distinctive in being thick,refractory,and buoyant,formed complementary to the accreted plume and convergent margin terranes.as migrating arcs captured thick plumeplateaus. and the refractory,low density.residue of plume melting coupled with accreted imbricated plume-arc crust.     

The Bastar craton,central India: A window to Archean – Paleoproterozoic crustal evolution

The Bastar craton in central India, surrounded by cratonic blocks and Paleoproterozoic to Neoproterozoic orogenic belts, is a window to investigate the Archean-Paleoproterozoic crustal evolution and tectonic processes. Here we propose a new tectonic classification of the craton into the Western Bastar Craton (WBC), Eastern Bastar Craton (EBC), and the intervening Central Bastar Orogen (CBO). We present petrologic, geochemical and zircon U-Pb, REE and Lu-Hf data from a suite of rocks from the CBO and along the eastern margin of the WBC Including: (1) volcanic successions comprising meta-andesite and fine-grained amphibolite, representing arc-related volcanics along a convergent margin; (2) ferruginous sandstone, in association with rhyolite, representing a volcano-sedimentary succession, deposited in an active trench; and (3) metamorphosed mafic-ultramafic suite including gabbro, pyroxenite and dunite invaded by trondhjemite representing the section of sub-arc mantle and arc root adjacent to a long-lasting subduction system. Petrologic studies indicate that the mafic-ultramafic suite crystallized from an island arc tholeiitic parental magma in a suprasubduction zone environment. The chondrite-normalized and primitive mantle normalized diagrams of the mafic and ultramafic rocks suggest derivation from MORB magma. The mixed characters from N-MORB to E-MORB of the studied samples are consistent with subduction modification of a MORB related magma, involving partial melting of the metasomatized mantle wedge. Our zircon U-Pb age data suggest that the cratonic nuclei was constructed as early as Paleoarchean. We present evidence for active subduction and arc magmatism through Mesoarchean to Neoarchean and early Paleoproterozoic, with the trench remaining open until at least 2.3 Ga. Two major crust building events are recognized in the Bastar craton: during Mesoarchean (recycled Paleoarchean subduction-related as well as juvenile/depleted mantle components) and Neoarchean (accretion of juvenile oceanic crust, arc magmatism including granite batholiths and related porphyry mineralization). The final cratonization occurred during latest Paleoproterozoic, followed by collisional assembly of the craton and its incorporation within the Peninsular Indian mosaic during Mesoproterozoic. In the global supercontinent context, the craton preserves the history of Ur, the earliest supercontinent, followed by the Paleo-Mesoproterozoic Columbia, as well as minor thermal imprints of the Neoproterozoic Rodinia and associated Grenvillian orogeny.     

Ancient deep roots for Mesozoic world-class gold deposits in the north China craton:An integrated genetic perspective

The North China Craton(NCC)hosts some of the world-class gold deposits that formed more than 2 billion years after the major orogenic cycles and cratonization.The diverse models for the genesis of these deposits remain equivocal,and mostly focused on the craton margin examples,although synchronous deposits formed in the interior domains.Here we adopt an integrated geological and geophysical perspective to evaluate the possible factors that contributed to the formation of the major gold deposits in the NCC.In the Archean tectonic framework of the NCC,the locations of the major gold deposits fall within or adjacent to greenstone belts or the margins of micro-continents.In the Paleoproterozoic framework,they are markedly aligned along two major collisional sutures-the Trans North China Orogen and the Jiao-Liao-Ji Belt.Since the Mesozoic intrusions hosting these deposits do not carry adequate signals for the source of gold,we explore the deep roots based on available geophysical data.We show that the gold deposits are preferentially distributed above zones of uplifted MOHO and shallow LAB corresponding to thinned crust and eroded sub-lithospheric mantle,and that the mineralization is located above regions of high heat flow representing mantle upwelling.The NCC was at the center of a multi-convergent regime during the Mesozoic which intensely churned the mantle and significantly en riched it.The geophysical data on Moho and LAB upwarp from the centre towards east of the craton is more consistent with paleo-Pacific slab subduction from the east exerting the dominant control on lithospheric thinning.Based on these results,and together with an evaluation of the geochemical and isotopic features of the Mesozoic magmatic intrusions hosting the gold mineralization,we propose a genetic model that invokes reworking of ancient Au archives preserved in the lower crust and metasomatised upper mantle and which were generated through multiple subduction,underplating and cumulation events associated with cratonization of the NCC as well as the subduction-collision of Yangtze Craton with the NCC.The heat and material input along zones of heterogeneously thinned lithosphere from a rising turbulent mantle triggered by Mesozoic convergent margins surrounding the craton aided in reworking the deep roots of the ancient Au reservoirs,leading to the major gold metallogeny along craton margins as well as in the interior of the NCC.     

锦州-迁安太古宙赞岐岩类片麻岩成因及其动力学意义

详细的野外地质调查和综合研究表明冀东-辽西南部地区太古宙变质基底主要由富钾花岗质岩石组成,由锦州至迁安构成一条NEE向延伸200余千米的富钾花岗质岩石带。这些富钾花岗质岩石主要由似斑状/中粒石英二长闪长质-花岗闪长质-二长花岗质片麻岩和中粒二长花岗岩-正长花岗岩构成。全岩地球化学分析表明这些石英二长闪长质-花岗闪长质-二长花岗质片麻岩具有高FeO~T、MgO、K_2O和Mg~#值的地球化学特征,与全球范围内中-新太古宙赞歧岩类相似。LA-ICP-MS锆石U-Pb同位素定年结果表明这些岩石形成于2546~2543Ma。岩石成因研究表明这些赞歧岩类片麻岩形成于俯冲板片及其拖曳的洋壳沉积物、增生楔物质的熔体和受俯冲流体、熔体交代的地幔楔之间相互作用引发的一系列的岩浆作用。这一多样化的赞岐岩类岩浆作用形成了一条新太古代赞岐岩类带,该赞岐岩类带反映了冀东-辽西南部地区新太古代从NNW向SSE向板片热俯冲的动力学体制。     

Petrogenesis and geological implications of the ca.2520Ma gabbroic intrusions in Wutai Mountain of the North China CratoSCIEI北大核心CSCD

本次研究从华北克拉通五台地区基底中识别出一套-25亿年的基性侵入体(主要为岩株)。其中,龙王堂岩株出露面积约5km2,侵入新太古界五台群台怀亚群,岩石类型主要为粗粒辉长岩。一条北东向基性岩墙侵入该岩株,岩墙边部发育明显的冷凝边,岩性为中粒辉长岩。LA-ICP-MS锆石U-Pb同位素年代学分析显示龙王堂粗粒辉长岩岩株形成时代为新太古代晚期(-2520Ma),中粒辉长岩岩墙形成时代为晚三叠世(-230Ma)。锆石原位Lu-Hf同位素分析显示龙王堂岩株具有正的εHf(t)值(4.75-7.95,峰值为6.3),其亏损地幔Hf模式年龄与岩浆年龄接近(tDM=2.64-2.53Ga),指示龙王堂粗粒辉长岩的原始岩浆来自于亏损地幔端元。全岩主微量元素地球化学分析显示龙王堂粗粒辉长岩表现出与洋岛玄武岩(OIB)相似的地球化学特征,并经历了橄榄石和单斜辉石等的结晶分异。结合前人研究成果,本文认为龙王堂新太古代晚期辉长岩岩株与区域上绿岩带(五台群)及同时期花岗侵入体形成时代接近,可能共同指示早期地幔直接分异形成陆壳物质的过程。这可能与俯冲构造体制不同,而与地幔柱作用产生的岩浆特征更为相似。     

华北克拉通太古宙TTG岩石的时空分布、组成特征及形成演化:综述

华北克拉通具有3.8Ga以上的演化历史,TTG是其地质记录的最重要载体。华北克拉通太古宙(特别是中太古代以前)地质演化在很大程度上与TTG岩石密切相关。在华北克拉通,始太古代(3.6~4.0Ga)TTG岩石仅在鞍本地区被发现,但冀东地区已在多种变质碎屑沉积岩中发现大量3.6~3.88Ga碎屑锆石;古太古代(3.2~3.6Ga)TTG岩石在鞍本、冀东、信阳地区被识别出来;中太古代(2.8~3.2Ga)TTG岩石在鞍本、冀东、胶东、鲁山等地存在;可把新太古代(2.5~2.8Ga)进一步划分为早期和晚期两个阶段:新太古代早期(2.6~2.8Ga)TTG岩石已在10余个地区被发现,新太古代晚期(2.5~2.6Ga)TTG岩石几乎在每一个太古宙基底岩石出露区都存在。野外地质、锆石定年、元素地球化学和Nd-Hf同位素组成研究表明,中太古代以前TTG岩石局部存在,主要分布于Wan et al.(2015)所划分的三个古陆块中;新太古代TTG岩石广泛分布,是陆壳增生最重要时期岩浆作用的产物。TTG岩石类型随时代变化,3.1~3.8Ga和2.7~2.9Ga TTG岩石分别主要为奥长花岗岩和英云闪长岩;2.5~2.6Ga期间花岗闪长岩大规模出现,并有壳源花岗岩广泛分布,表明这时陆壳已有相当的成熟度。奥长花岗岩轻重稀土分异程度从弱到强的时间出现在~3.3Ga;2.5~3.3Ga的TTG岩石轻重稀土分异程度变化很大,表明其形成条件存在很大差异。TTG岩石主要为新生地壳,但也有相当部分为壳内再循环产物或形成过程中受到陆壳物质影响。华北克拉通中太古代以前的主要构造机制是板底垫托或地幔翻转作用,新太古代晚期板块构造体制可能已起作用。     

Early Neoarchean Magmatic and Paleoproterozoic Metamorphic Events in the Northern North China Craton: SHRIMP Zircon Dating and Hf Isotopes of Archean Rocks from the Miyun Area, Beijing

The Miyun area of Beijing is located in the northern part of the North China Craton(NCC)and includes a variety of Archean granitoids and metamorphic rocks.Magmatic domains in zircon from a tonalite reveal Early Neoarchean(2752±7 Ma) ages show a small range in ε_(Hf)(t) from 3.1 to 7.4and t_(DM1)(Hf) from 2742 to 2823 Ma,similar to their U-Pb ages,indicating derivation from a depleted mantle source only a short time prior to crystallization.SHRIMP zircon ages of granite,gneiss,amphibolite and hornblendite in the Miyun area reveal restricted emplacement ages from 2594 to2496 Ma.They also record metamorphic events at ca.2.50 Ga,2.44 Ga and 1.82 Ga,showing a similar evolutionary history to the widely distributed Late Neoarchean rocks in the NCC.Positive ε_(Hf)(t) values of 1.5 to 5.9,with model ages younger than 3.0 Ga for magmatic zircon domains from these Late Neoarchean intrusive rocks indicate that they are predominantly derived from juvenile crustal sources and suggest that significant crustal growth occurred in the northern NCC during the Neoarchean.Late Paleoproterozoic metamorphism developed widely in the NCC,not only in the Trans-North China Orogen,but also in areas of Eastern and Western Blocks,which suggest that the late Paleoproterozoic was the assembly of different micro-continents,which resulted in the final consolidation to form the NCC,and related to the development of the Paleo-Mesoproterozoic Columbia or Nuna supercontinent.     

鲁西徂徕山地区新太古代岩浆作用——锆石SHRIMP U-Pb定年证据

徂徕山是鲁西新太古代岩石的重要出露区。在野外地质基础上,本文对3个岩石样品进行了锆石SHRIMP U-Pb定年研究。英云闪长质片麻岩遭受一定重结晶影响的岩浆锆石年龄为(2711±11)Ma,变质作用时代可能为~2.6 Ga。变质石英闪长岩遭受强烈重结晶影响的岩浆锆石年龄为2.51 Ga,被解释为更接近构造热事件的时代,而岩浆锆石年龄可能在2.53~2.55 Ga之间。二长花岗岩锆石普遍存在强烈铅丢失,根据数据点在谐和线上的分布特征,二长花岗岩的形成时代应为~2.5 Ga。结合地球化学研究和前人工作,确定了徂徕山地区新太古代地质演化历史及新太古代早期古老岩石带(B带)和新太古代晚期新生岩石带(C带)之间的界线。     

Neoarchean (2.5-2.8 Ga) crustal growth of the North China Craton revealed by zircon Hf isotope: A synthesis

The crustal growth of the North China Craton(NCC) during the Neoarchean time(2.5—2.8 Ga) is a hotly controversial topic,with some proposing thai the main crustal growth occurred in the late Neoarchean (2.5—2.6 Ga),in agreement with the time of the magmatism,whereas others suggest that the main crustal accretion took place during early Neoarchean time(2.7—2.8 Ga),consistent with the time of crustalformation of other cratons in the world.Zircon U-Pb ages and Hf isotope compositions can provide rigorous constraints on the time of crustal growth and the evolution and tectonic division of the NCC.In this contribution, we make a comprehensive review of zircon Hf isotope data in combination with zircon U-Pb geochronology and some geochemistry data from various divisions of the NCC with an aim to constrain the Neoarchean crustal growth of the NCC.The results suggest that both 2.7—2.8 Ga and 2.5—2.6 Ga crustal growth are distributed over the NCC and the former is much wider than previously suggested.The Eastern block is characterized by the main 2.7—2.8 Ga crustal growth with local new crustal-formation at 2.5—2.6 Ga,and the Yinshan block is characterized by～2.7 Ga crustal accretion as revealed by Hf-isotope data of detrital zircons from the Zhaertai Group.Detrital zircon data of the Khondalite Belt indicate that the main crustal growth period of the Western block is Paleoproterozoic involving some～2.6 Ga and minor Early- to Middle-Archean crustal components,and the crustal accretion in the Trans-North China Orogen(TNCO) has a wide age range from 2.5 Ga to 2.9 Ga with a notable regional discrepancy.Zircon Hf isotope compositions,coupled with zircon ages and other geochemical data suggest that the southern margin may not be an extension of the TNCO,and the evolution and tectonic division of the NCC is more complex than previously proposed,probably involving multi-stage crustal growth and subduction processes.However, there is no doubt that 2.7—2.8 Ga magmatism and crustal-formation are more widely distributed than previously considered,which is further supported by the data of zircons from Precambrian lower crustal rocks, overlying sedimentary cover,modern river sediments and Late Neoarchean syenogranites.     

Neoarchean crust-mantle interactions in the Yishui Terrane,south-eastern margin of the North China Craton: Constraints from geochemistry and zircon U-Pb-Hf isotopes of metavolcanic rocks and high-K granitoids

The Neoarchean Yishui Terrane (YST) is situated in the east of Western Shandong Province (WSP), south-eastern margin of the North China Craton (NCC). The metavolcanic rocks of the YST are fine-grained hornblende plagioclase gneisses (Group #1) and fine-grained amphibolites (Group #2) in the Yangzhuangzhen area and fine- to medium-grained amphibolites (Group #3) in the Leigushan area. The high-K granitoids associated with Groups #1 and 2 are dominated by fine- to medium-grained monzogranitic gneisses. Zircon LA-ICP-MS U-Pb dating reveals that the magmatic precursors of Groups #1 and #2 were formed at 2641 Ma and the magmatic precursors of concomitant monzogranitic gneisses were emplaced from 2615 to 2575 Ma, whereas Group #3 represents a later 2500 Ma volcanic eruption, and all these metamorphic volcanic rocks and monzogranitic gneisses were subjected to subsequent 2470–2460 Ma metamorphism.The metamorphic volcanic rock samples in Group #1 exhibit the chemical compositions of calc-alkaline andesites, showing fractionated chondrite-normalized REE patterns ((La/Yb)_N = 10.48–19.30) and negative Nb, Ta and Ti anomalies ((Nb/La)_PM = 0.13–0.22), which are akin to those of typical high-Mg andesites (HMAs) in the subduction-related settings. The magmatic precursors of the Group #1 samples were derived from partial melting of a fluid- or melt-metasomatized depleted mantle wedge at deep levels in the upper mantle. Samples in Group #2 show calc-alkaline chemical compositions with less fractionated chondrite-normalized REE patterns ((La/Yb)_N = 2.24–3.34) and negative Nb, Ta and Ti anomalies ((Nb/La)_PM = 0.47–0.76), which are consistent with those of the volcanic rocks in the Aleutian island arc. The magmatic precursors of Group #2 were generated by partial melting of a fluid-metasomatized depleted mantle wedge at shallow levels in the upper mantle. The monzogranitic gneisses exhibit high SiO₂ and K₂O contents with high-K calc-alkaline affinities and peraluminous characteristics. Based on their distinct HREE contents and chondrite-normalized REE patterns, these granitoid samples are subdivided into low-Yb monzogranitic gneisses (LYMGs) and high-Yb monzogranitic gneisses (HYMGs). The LYMG magma was derived from partial melting of a mixed source of juvenile two-mica pelites and minor basic-intermediate igneous rocks at lower crustal levels with pyroxene + amphibole + garnet as the main residual phases, and the HYMG magma was derived from partial melting of multi-sourced juvenile two-mica pelites at middle to lower crustal levels with pyroxene + amphibole and subordinate plagioclase and garnet as the main residual phases. In addition, Group #3 resembles tholeiitic back-arc basalts in the Okinawa Trough and displays flat chondrite-normalized REE patterns ((La/Yb)_N = 1.22–2.08) and slightly negative Nb and Ta anomalies ((Nb/La)_PM = 0.35–0.59). This group was most likely derived from partial melting of a depleted mantle source that had been modified by the addition of subducted slab-derived fluids at shallow levels in the upper mantle. These metavolcanic rocks and concomitant high-K granitoids record important Neoarchean crust-mantle interactions involving the first modification and partial melting of the lithospheric mantle induced by oceanic crust subduction; then, upwelling and underplating of mantle-derived magmas triggered partial melting of the middle to lower crust and mixing between crust- and mantle-derived magmas. These processes imply that Neoarchean crust-mantle interaction played a crucial role in the evolution of the southeastern margin of the NCC.Available whole-rock Sm-Nd and zircon Lu-Hf isotopic data from metamorphic volcanic rocks and plutonic granitoids from this study and previous studies reveal that YST experienced three crucial juvenile crustal growth events from ~2.78–2.69 Ga, ~2.64–2.56 Ga and ~2.54–2.50 Ga.     

Late Neoarchean volcanic rocks in the southern Liaoning Terrane and their tectonic implications for the formation of the eastern North China Craton

The late Neoarchean metamorphosed volcanic rocks in the southern Liaoning Terrane (SLT) of the eastern North China Craton (NCC) are mainly composed of amphibolites and felsic gneisses and can be chemically classified as basalt (Group#1), basaltic andesite (Group#2), dacite (Group#3) and rhyodacite (Group#4). LA-ICP-MS zircon U–Th–Pb dating reveals that they formed at ~2.53–2.51 ​Ga. Group#1 samples are characterized by approximately flat chondrite-normalized rare earth element (REE) patterns with low (La/Yb)_N ratios and a narrow range of (Hf/Sm)_N ratios, and their magmatic precursors were generated by partial melting of a depleted mantle wedge weakly metasomatized by subducted slab fluids. Compared to Group#1 samples, Group#2 samples display strongly fractionated REE patterns with higher (La/Yb)_N ratios and more scattered (Hf/Sm)_N ratios, indicative of a depleted mantle wedge that had been intensely metasomatized by slab-derived melts and fluids. Group#3 samples are characterized by high MgO and transition trace element concentrations and fractionated REE patterns, which resemble typical high-Si adakites, and the magmatic precursors were derived from partial melting of a subducted oceanic slab. Group#4 samples have the highest SiO₂ and the lowest MgO and transition trace element contents, and were derived from partial melting of basaltic rocks at lower crust levels. Integrating these tholeiitic to calc-alkaline volcanic rocks with the mass of contemporaneous dioritic-tonalitic-trondhjemitic-granodioritic gneisses, the late Neoarchean volcanic rocks in the SLT were most likely produced in an active continental margin. Furthermore, the affinities in lithological assemblages, metamorphism and tectonic regime among SLT, eastern Hebei to western Liaoning Terrane (EH–WLT), northern Liaoning to southern Jilin Terrane (NL–SJT), Anshan-Benxi continental nucleus (ABN) and Yishui complex (YSC) collectively indicate that an integral and much larger continental block had been formed in the late Neoarchean and the craton-scale lateral accretion was a dominantly geodynamic mechanism in the eastern NCC.     

华北克拉通的形成以及早期板块构造

地球上最早的地壳岩石是高钠的花岗质(TTG)岩石,但是否有更老的洋壳存在过、以及陆壳是怎样形成的,涉及到地球动力学几乎所有的问题。其中板块构造是在什么时候开始的,就是个延续了数十年热度不减的前沿科学问题。流行的说法是板块构造始于新元古代,也有一些学者认为在新太古代就已经开始,或者认为自从地球上有了水的记录,就开始有板块构造。在众多的判别板块构造的标志中,蛇绿岩残片和古老的高压变质岩无疑是两个最具影响力的问题。前者可以确定有远古的古老洋壳存在过并成为缝合带中的残片,后者可以指示曾有地表的岩石单元被俯冲到深部,是俯冲、消减与碰撞的岩石学证据。本文在讨论和比较了太古宙绿岩带与蛇绿岩,以及早前寒武纪高温高压(HTHP)麻粒岩/高温—超高温(HT-UHT)麻粒岩与造山带高压变质带之后,认为尚不能作为板块构造的证据。本文还对华北的新太古代末的稳定大陆形成以及古元古代活动带的裂谷-俯冲-碰撞进行了论述。提出华北克拉通在新太古代末的绿岩带-高级区格局可能标志着热体制下有限的横向活动构造,微陆块被火山-沉积岩系焊接,随后发生变质作用和花岗岩化,完成稳定大陆的克拉通化过程。其构造机制可能是适度规模且多发的地幔柱构造控制下小尺度的横向构造运动的机制。华北克拉通的古元古代活动带有与绿岩带-高级区不同的构造样式,表壳岩带状分布,经受了强烈的变形以及中级变质作用,伴随花岗岩的侵入,虽然没有蛇绿岩和高压变质带,但已表现出板块构造的雏形特征。     

The Archean of the Baltic Shield: Geology, Geochronology, and Geodynamic settings

The Archean provinces and lithotectonic complexes of the Baltic (Fennoscandian) Shield are considered. The supracrustal complexes are classified by age: <3.2, 3.10–2.90, 2.90–2.82, 2.82–2.75, and 2.75–2.65 Ga. The data on Archean granitoid complexes and metamorphic events are mentioned briefly, whereas the recently found fragments of the Archean ophiolitic and eclogite-bearing associations are discussed in more detail. The Paleoarchean rocks and sporadic detrital grains of Paleoarchean zircons have been found in the Baltic Shield; however, the relatively large fragments of the continental crust likely began to form only in the Mesoarchean (3.2–3.1 Ga ago), when the first microcontinents, e.g., Vodlozero and Iisalmi, were created. The main body of the continental crust was formed 2.90–2.65 Ga ago. The available information on the Paleoarchean complexes of the Baltic Shield is thus far too scanty for judgment on their formation conditions. The geologic, petrologic, isotopic, and geochronological data on the Meso-and Neoarchean lithotectonic complexes testify to their formation in the geodynamic settings comparable with those known in Phanerozoic: subduction-related (ensialic and ensimatic), collisional, spreading-related, continental rifting, and the setting related to mantle plumes.     

What Happened in the Trans-North China Orogen in the Period 2560-1850 Ma？

The Trans-North China Orogen （TNCO） was a Paleoproterozic continent-continent collisional belt along which the Eastern and Western Blocks amalgamated to form a coherent North China Craton （NCC）. Recent geological, structural, geochemical and isotopic data show that the orogen was a continental margin or Japan-type arc along the western margin of the Eastern Block, which was separated from the Western Block by an old ocean, with eastward-directed subduction of the oceanic lithosphere beneath the western margin of the Eastern Block. At 2550-2520 Ma, the deep subduction caused partial melting of the medium-lower crust, producing copious granitoid magma that was intruded into the upper levels of the crust to form granitoid plutons in the low- to medium-grade granite-greeustone terranes. At 2530-2520 Ma, subduction of the oceanic lithosphere caused partial melting of the mantle wedge, which led to underplating of mafic magma in the lower crust and widespread mafic and minor felsic volcanism in the arc, forming part of the greenstone assemblages. Extension driven by widespread mafic to felsic volcanism led to the development of back-arc and/or intra-arc basins in the orogen. At 2520-2475 Ma, the subduction caused further partial melting of the lower crust to form large amounts of tonalitic-trondhjemitic-granodioritic （TTG） magmatism. At this time following further extension of back-arc basins, episodic granitoid magmatism occurred, resulting in the emplacement of 2360 Ma, -2250 Ma 2110-21760 Ma and -2050 Ma granites in the orogen. Contemporary volcano-sedimentary rocks developed in the back-arc or intra-are basins. At 2150-1920 Ma, the orogen underwent several extensional events, possibly due to subduction of an oceanic ridge, leading to emplacement of mafic dykes that were subsequently metamorphosed to amphibolites and medium- to high-pressure mafic granulites. At 1880-1820 Ma, the ocean between the Eastern and Western Blocks was completely consumed by subduction, and the dosing of the ocean led to the continent-arc-continent collision, which caused large-scale thrusting and isoclinal folds and transported some of the rocks into the lower crustal levels or upper mantle to form granulites or eclogites. Peak metamorphism was followed by exhumation/uplift, resulting in widespread development of asymmetric folds and symplectic textures in the rocks.