华北克拉通古元古代构造事件

本文讨论了华北克拉通的古元古代表壳岩系、高压麻粒岩和孔兹岩系的若干问题,提出了(1)华北克拉通在约2500Ma太古宙结束时已基本形成.在2300Ma之前处于相对稳定的构造环境;(2)2300～1950Ma期间,华北克拉通经历了一次基底陆块的拉伸-破裂事件,在克拉通内部发育了晋豫、胶辽裂陷盆地和丰镇陆内凹陷盆地;(3)约1900Ma期间,有地幔上涌并伴随辉长岩浆的底侵作用,引起大青山-丰镇地区的超高温变质作用.底侵的辉长岩浆作为岩体和岩墙在下地壳就位,并发生高压麻粒岩相变质作用;(4)约1850Ma期间,华北克拉通经历了一次挤压构造事件,导致了裂陷盆地的闭合和焊接,形成晋豫和胶辽两个类似于现代陆.陆碰撞型的造山带;(5)在华北克拉通的北缘,华北克拉通可能与其北部的另一古老陆块或岛弧拼合,其拼合带应位于白云鄂博以北.现在已残缺不全.孔兹岩系可能代表了平行于北缘造山带的一条构造带.与北部造山带的俯冲碰撞相关的陆内深部逆掩造成了麻粒岩相岩石的形成和抬升;(6)白云鄂博群、化德群和渣尔泰(-狼山)群是与长城系相同时期发育的被动大陆边缘裂谷盆地;(7)1850～1700Ma期间,华北克拉通进入伸展构造体制,导致基底抬升,产生裂陷槽、基性岩墙群和非造山岩浆活动.     

Late Paleoproterozoic extension and a paleostress field reconstruction of the North China Craton

Mafic dyke swarms and aulacogens are major anorogenic extensional events in the Late Paleoproterozoic North China Craton (NCC). The N–NNW mafic dyke swarms are widespread in the NCC, whose ages span between 1.83 and 1.77 Ga. The similar ages and orientations of  1.8 Ga dyke swarms in the NCC demonstrate that the amalgamated NCC experienced widespread extension at this time.Based on the width statistics of dyke swarms on ten survey lines, an average crustal extension ratio of 0.35% was found for the NCC. The small magnitude of overall extension suggests that the mafic dyke swarms were emplaced into the elastic fractures, and indicates that the NCC had become a brittle plate prior to the emplacement of the mafic dyke swarms.Precisely dated mafic dyke swarms, when used as paleostress indicators, can be employed in the paleostress field reconstruction of Precambrian cratons. Two dimensional finite element modeling (2-D FEM) of the NCC, in which the various blocks were assigned densities and elastic constants, shows that north–south compression favors dyke intrusion along generally N–NW lines, and that deviations in dyke trends can be explained by the effects of boundary constraints and the physical properties of the crust. The best fitting model can be considered a plausible representation of the tectonic force acting on the NCC that produces the intraplate stress field that is most consistent with the observed orientation of dyke swarms. The results of modeling of the Late Paleoproterozoic stress field suggest a common tectonic setting for the emplacement of mafic dyke swarms in the Central Orogenic Zone, Western and East Blocks of the NCC. The results also show that the north–south tectonic forces play an important role in determining the paleostress field in the NCC. The widespread extension of the NCC resulted from the north–south tectonic forces which may be related to the break-up of the Late Paleoproterozoic supercontinent. The paleostress field modeling provides a possible approach to consider the supercontinent paleostress reconstruction and to reveal the mechanisms of the supercontinent break-up.     

华北克拉通晚太古代末-古元古代初的岩浆事件及构造热体制

华北克拉通变质基底中广泛出露岩浆成因的正片麻岩。本文将华北克拉通分为11个变质地区,对其中新太古代末-古元古代初的正片麻岩的产状、岩性、形成时代进行了讨论。通过对这11个变质地区的对比可以看出,华北克拉通新太古代末-古元古代初的正片麻岩具有以下特征:(1)新太古代末-古元古代初的岩浆事件分布广泛,该期岩浆事件所形成的正片麻岩不仅在东部陆块、西部陆块广泛分布,而且在中部造山带也有广泛出露,在各变质地区该期的正片麻岩通常占变质基底总面积的70%以上;(2)该期岩浆事件主要集中在从2540Ma到2490Ma的50Ma期间。目前的年龄数据统计该期间形成的正片麻岩占2600Ma到2450Ma所形成的正片麻岩的75%;(3)该阶段形成的钠质片麻岩和钾质片麻岩在形成时间上几乎一致,在各变质地区不同成分的正片麻岩不具分带性。但钾质花岗岩在辽宁鞍山、辽西绥中、冀东秦皇岛和鲁西傲徕山分布相对集中;(4)该期岩浆事件与变质事件几乎同时发生,变质事件通常较岩浆事件晚10～15Ma,在有些地区几乎同时,表明岩浆事件和变质事件是同一构造热事件的产物;(5)该期岩浆事件所产生片麻岩的εHf(t)值绝大多数为正值,表明它们是有地幔物质加入的新生地壳再造的产物。综合以上特征,我们认为在华北克拉通新太古代末-古元古代初广泛的岩浆事件是地幔柱底侵导致的,该期广泛的岩浆事件造就了华北克拉通的基本形态。     

Late Archean to Paleoproterozoic evolution of the North China Craton: key issues revisited

A recently proposed model for the evolution of the North China Craton envisages discrete Eastern and Western Blocks that developed independently during the Archean and collided along the Trans-North China Orogen during a Paleoproterozoic orogenic event. This model has been further refined and modified by new structural, petrological and geochronological data obtained over the past few years. These new data indicate that the Western Block formed by amalgamation of the Ordos Block in the south and the Yinshan Block in the north along the east-west-trending Khondalite Belt some time before the collision of the Western and Eastern Blocks. The data also suggest that the Eastern Block underwent Paleoproterozoic rifting along its eastern continental margin in the period 2.2–1.9 Ga, and was accompanied by deposition of the Fenzishan and Jingshan Groups in Eastern Shandong, South and North Liaohe Groups in Liaoning, Laoling and Ji’an Groups in Southern Jilin, and possibly the Macheonayeong Group in North Korea. The final closure of this rift system at ∼1.9 Ga led to the formation of the Jiao-Liao-Ji Belt. In the late Archean to early Paleoproterozoic, the western margin of the Eastern Block faced a major ocean, and the east-dipping subduction beneath the western margin of the Eastern Block led to the formation of magmatic arcs that were subsequently incorporated into the Trans-North China Orogen. Continued subduction resulted in a major continent-continent collision, leading to extensive thrusting and high-pressure metamorphism. The available age data for metamorphism and deformation in the Trans-North China Orogen indicate that this collisional event occurred at about 1.85 Ga ago, resulting in the formation of the Trans-North China Orogen and final amalgamation of the North China Craton.     

华北克拉通中部造山带早元古代盆地演化

华北克拉通中部造山带被认为是由东、西部陆块碰撞而产生的陆陆碰撞带。然而,关于两个陆块碰撞的时间以及方式还存在着争议。其中一个模式认为俯冲方向是向西的,两个陆块最终碰撞的时间是在2.5Ga左右,而另一个模式则认为俯冲方向是向东的,并且存在着2.1Ga和1.85Ga两期碰撞事件,第三个模式则认为俯冲方向是向东的,而最终的碰撞拼合发生在1.85Ga左右。近几年来,对于中部造山带中浅变质表壳岩系的研究取得了很大进展,为解决以上争议提供了进一步的资料。根据岩石组合和表壳岩中的不整合接触关系,位于中部造山带中段五台杂岩中的滹沱群和吕梁杂岩中的野鸡山群被分成了上下两个部分。其中,两个群的下部以及位于中部造山带南段中条杂岩中的绛县群和下中条群主要由类似于弧后盆地沉积的变质碎屑岩、碳酸盐岩和火山岩组成,而滹沱群和野鸡山群的上部以及上中条群和担山石群的岩石组合则是类似于山前磨拉石建造的变质砾岩和砂岩。来自于这些浅变质碎屑岩中碎屑锆石的年龄为我们提供了源区以及最大沉积年龄的信息。滹沱群和野鸡山群中的碎屑锆石具有两个峰值年龄,2.5Ga和2.15Ga,它们分别对应于中部造山带中段两期重要的岩浆事件。中条杂岩中的碎屑锆石年龄图谱则较为复杂,除了具有2.5Ga和2.15Ga这两个峰值,还具有2.7Ga这个较低和较老的峰值,这些较老的锆石可能来自中部造山带最南端太华杂岩中的古老岩石。弧后盆地沉积中最年轻的碎屑锆石年龄在2.1Ga左右,而山前磨拉石中最年轻的碎屑锆石在1.85Ga左右。结合前人的研究成果,我们给出了一个华北克拉通中部造山带早元古代沉积盆地的演化模式。从2.1Ga左右开始,一系列弧后盆地在"安第斯型"大陆边缘弧之后产生,在之后的东西陆块碰撞过程中他们成为了中部造山带的一部分。从1.85Ga左右开始,东西陆块沿着中部造山带碰撞,导致了陆壳加厚以及随后的快速抬升和剥蚀,从而形成了周缘前陆盆地中的山前磨拉石建造。发生在早元古代晚期的由弧后盆地向周缘前陆盆地的转化支持了华北的最终克拉通化发生在1.85Ga的构造演化模式。     

Paleoproterozoic Granitoids on Liaodong Peninsula,North China Craton

Paleoproterozoic granitoids are an important constituent of the Jiao–Liao–Ji Belt(JLJB). The spatial-temporal distribution and types of Paleoproterozoic granitoids are closely related to the evolution of the JLJB. In this paper, we review the field occurrence, petrography, geochronology, and geochemistry of Paleoproterozoic granitoids on Liaodong Peninsula, northeast China. The Paleoproterozoic granitoids can be divided into pre-tectonic(～2.15 Ga; peak age=2.18 Ga) and post-tectonic(～1.85 Ga) granitoids. The pre-tectonic granitoids are magnetite and hornblende–biotite monzogranites and granodiorites. Pre-tectonic monzogranites are widespread in the JLJB and have A_2-type affinities. In contrast, pretectonic granodiorites are only present in the Simenzi area and have adakitic affinities. The post-tectonic granitoids consist of porphyritic monzogranite, syenite, diorite, granodiorite, quartz monzonite, monzogranite, and granitic pegmatite, which are adakitic rocks and I-, S-, and A_2-type granitoids. The assemblage of pre-tectonic A_2-type granitoids and adakitic rocks indicates the initial tectonic setting of the JLJB was a continental back-arc basin. The assemblage of post-tectonic adakitic rocks and I-, S-, and A_2-type granitoids indicates a post-collisional setting. The 2.20–2.15 Ga A_2-type granitoids and adakitic rocks were associated with the initial stage of back-arc extension, and the peak of back-arc extension is inferred from the subsequent(2.15–2.10 Ga) mafic intrusive activity. The ～1.90 Ga adakitic rocks mark the beginning of the postcollisional stage, which was followed by the intrusion of low-temperature S-and I-type granitoids. High-to low-pressure granitoids(S-type) were generated during the peak of post-collisional lithospheric delamination and asthenospheric upwelling. The emplacement of later granitic pegmatites occurred during the waning of the orogeny.     

华北克拉通古元古代地质记录及其构造意义

华北克拉通上存在广泛的古元古代地质记录,尤其是构造岩石组合和同位素年龄资料,显示克拉通上可能存在多条古元古代的汇聚拼合带:如鄂尔多斯地块与华北东部-阴山地块之间的拼合造山带、华北东部地块与狼林地块之间的胶辽造山带以及华北克拉通北缘的安第斯型汇聚边界等.华北克拉通中部地区的古元古代锆石年龄数据统计,显示出2.3~2.4 Ga、2.0~2.2 Ga和1.8~1.95 Ga三组年龄值,暗示华北克拉通古元古代可能存在两个演化阶段:其中古元古代末期(1.8~1.95 Ga)的造山事件已得到了广泛认可,并认为与全球Columbia超大陆汇聚事件相关;而2.3~2.4 Ga、2.00~2.2 Ga所代表的构造热事件的性质和意义尚不明确.我国古元古代末期有关陆块汇聚的构造热事件以及其后的裂解事件群和地层学记录,与新的国际前寒武纪地质年代表建议的古-中元古代界线一致,这将促进我国特别是古中元古代的前寒武纪研究.     

Stratigraphy and Zircon Provenance of a Late Paleoproterozoic Terrestrial Sequence underlying the Xiong’er Volcanics in the Southern North China Craton

The late Paleoproterozoic Dagushi Formation comprises a fluvial-lacustrine succession and represents the initial fill of the Xiong’er Basin in the southern North China Craton. Employing integrated outcrop surveys and detrital zircon U-Pb-Hf dating, this study examines the provenance and depositional setting of the Dagushi Formation. Five major depositional facies, including braided channel, distributary channel, subaqueous stream/mouth bar, pro-delta and shallow lake, were identified, based on lithofacies and associations. They were interpreted as representing a braided river delta-lacustrine system. The ages of the last metamorphic event of the basement, covering volcanics and the youngest zircon together constrain a depositional age of ca. 1.79 Ga for the Dagushi Formation. Zircon age distributions reveal a provenance change from ca. 2.7–2.5 Ga rocks in the lower part, to ca. 2.3–1.9 Ga sources in the middle-upper part of the Dagushi Formation. Considering the vertical sedimentology, this provenance change could be induced by the rising water-level caused by a tectonic subsidence. The ca. 2.7–2.5 Ga zircons are suggested to be locally sourced from the late Neoarchean–Paleoproterozoic metamorphic basement. The northeast Zhongtiao Mts area (current co-ordinates) is supposed to have appeared as a paleo-uplift and served as a source area for the Paleoproterozoic grains. The Dagushi Formation records an early ‘underfilled’ stage of the Xiong’er Rift.     

天镇-怀安地区变质基性岩墙群:华北克拉通古元古代末期碰撞-伸展构造体制转换标志

古元古代是华北克拉通形成过程中重要的造山构造演化阶段,该阶段形成的基性岩墙群,为深入理解裂解-俯冲-碰撞-抬升的造山构造-岩浆过程提供了重要信息.本文报道了天镇-怀安地区广泛分布于新太古代-古元古代变质基底中的变质基性岩墙（二辉麻粒岩）,野外产状与区域主期构造面理协调一致,主要由单斜辉石、斜方辉石、斜长石和少量角闪石组成.LA-MC-ICPMS锆石U-Pb同位素定年获得变质基性岩墙的变质年龄为1 820~1 834 Ma,与区内麻粒岩相变质事件一致,结合区域基性岩墙年龄记录,推测其原岩形成年龄为1.95~1.91 Ga.根据岩石地球化学特征可将变质基性岩墙划分为高Mg低Ti型和低Mg高Ti型两类,两者经历了不同程度的橄榄石、单斜辉石和斜长石的分离结晶.两类基性岩墙均亏损高场强元素（如Nb、Ta、Ti、Zr和Hf）,结合锆石Hf同位素分析,研究表明基性岩墙来源于俯冲流体交代的岩石圈地幔或者受到过地壳物质的混染.华北克拉通古元古代存在2.16~2.04 Ga和1.97~1.83 Ga两期基性岩墙侵位事件:早期代表在初始克拉通基础上发生的板内裂解过程,晚期记录了由俯冲碰撞到伸展的转换过程,即碰撞造山构造体制由水平挤压转变为垂向抬升,构造转换时限大致介于1.95~1.91 Ga.      

华北克拉通孔兹岩带古元古代凉城石榴石花岗岩成因机制及其岩石学意义

在华北克拉通孔兹岩带东段凉城地区分布有大规模古元古代石榴石花岗岩。凉城石榴石花岗岩是产于麻粒岩相变沉积岩中的原地-半原地花岗岩,伴生有徐武家岩体为代表的辉长苏长岩小侵入体群。凉城石榴石花岗岩富含石榴子石（5%~15%,局部可达25%）,常见紫苏辉石,推测形成于高温（>850~900℃）、高压（8~10kbar）条件下,不同于典型的S型花岗岩。地球化学特征上,石榴石花岗岩具有强过铝的地球化学属性（A/CNK=1.1~1.6）,显示低SiO₂（55%~75%）和富MgO+FeO^T（5%~14%）的特征,已经偏离了正常S型花岗岩的成分范围。凉城石榴石花岗岩低硅富镁铁的成分特征很可能是由残留体（石榴石岩）带入和辉长苏长岩物质添加造成,残留体和辉长苏长岩的物质贡献比例约占20%~40%。年代学研究表明凉城石榴石花岗岩形成时代为1.93~1.92Ga,与辉长苏长岩形成时代（1.93Ga）和变质沉积岩记录的超高温变质作用时代（1.92Ga）一致,指示凉城石榴石花岗岩是幔源基性岩浆侵入麻粒岩相变质沉积岩中引起变沉积岩在高温-超高温条件下大规模部分熔融的产物。大规模出露的高温石榴石花岗岩及多点分布的超高温麻粒岩共同反映了集宁-凉城区域上超高温的特点。

     

Paleomagnetic Evidence for Tectonic Setting of Paleoproterozoic Dyke Swarms in the North China Craton,China

正In order to investigate the tectonic setting of 1.77-1.78Ga dyke swarms emplaced into the central North China Craton(NCC),we carried out a paleomagnetic and magnetic fabric study on the well geochronologically     

华北克拉通大青山地区古元古代晚期钾长伟晶岩脉SHRIMP锆石U-Pb定年和Hf同位素组成

报道了华北克拉通大青山地区哈德门沟钾长伟晶岩脉的锆石SHRIMP U-Pb年龄和Hf同位素组成。钾长伟晶岩脉广泛分布,不定向切割早期地质体。部分钾长石具出溶结构,利用三元长石温度计获得其形成温度为750~800℃。锆石粒度粗大,大多呈等轴状,锆石的阴极发图像中部具板状环带或扇形结构,边部出现振荡环带。SHRIMP锆石 U-Pb定年,数据点位于谐和线上,207Pb/206Pb年龄加权平均值为1841Ma±9Ma (MSWD=1.5)。锆石的εHf(t)和tDM1(Hf)分别为-10.4~-6.6和2498~2648Ma。结合其它资料,可得出如下结论：①大青山地区至少在1.84Ga已进入伸展体制;②钾长伟晶岩脉形成于高温条件;③钾长伟晶岩脉的物质来自该区早前寒武纪变质基底。     

华北克拉通大青山地区古元古代晚期钾长伟晶岩脉SHRIMP锆石U-Pb定年和Hf同位素组成

报道了华北克拉通大青山地区哈德门沟钾长伟晶岩脉的锆石SHRIMP U-Pb年龄和Hf同位素组成。钾长伟晶岩脉广泛分布,不定向切割早期地质体。部分钾长石具出溶结构,利用三元长石温度计获得其形成温度为750～800℃。锆石粒度粗大,大多呈等轴状,锆石的阴极发图像中部具板状环带或扇形结构,边部出现振荡环带。SHRIMP锆石U-Pb定年,数据点位于谐和线上,207Pb/206Pb年龄加权平均值为1841Ma±9Ma(MSWD=1.5)。锆石的εHf(t)和tDM1(Hf)分别为-10.4～-6.6和2498～2648Ma。结合其它资料,可得出如下结论:①大青山地区至少在1.84Ga已进入伸展体制;②钾长伟晶岩脉形成于高温条件;③钾长伟晶岩脉的物质来自该区早前寒武纪变质基底。     

Deformation history of the Paleoproterozoic Liaohe assemblage in the eastern block of the North China Craton

The Paleoproterozoic Liaohe assemblage and associated Liaoji granitoids represent the youngest basement in the Eastern Block of the North China Craton. Various structural elements and metamorphic reaction relations indicate that the Liaohe assemblage has experienced three distinct deformational events (D₁ to D₃) and four episodes of metamorphism (M₁ to M₄). The earliest greenschist facies event (M₁) is recognized in undeformed or weakly deformed domains wrapped by the S₁ schistosity, suggesting that M₁ occurred before D₁. The D₁ deformation produced small, mostly meter-scale, isoclinal and recumbent folds (F₁), an associated penetrative axial planar schistosity (S₁), a mineral stretching lineation (L₁) and regional-scale ductile shear zones. Concurrent with D₁ was M₂ metamorphism, which occurred before D₂ and produced low- to medium-pressure amphibolite facies assemblages. Regionally divergent motion senses reflected by the asymmetric F₁ folds and other sense-of-shear indicators, together with the radial distribution of the L₁ lineation surrounding the Liaoji granitoids, imply that D₁ represents an extensional event. The D₂ deformation produced open to tight F₂ folds of varying scales, S₂ axial crenulation cleavages and ENE-NE-striking thrust faults, involving broadly NW–SE compression. Following D₂ was M₃ metamorphism that led to the formation of sillimanite and cordierite in low-pressure type rocks and kyanite in medium-pressure rocks. The last deformational event (D₃) formed NW-WNW-trending folds (F₃), axial planar kink bands, spaced cleavages (S₃), and strike–slip and thrust faults, which deflect the earlier D₁ and D₂ structures. D₃ occurred at a shallow crustal level and was associated with, or followed by, a greenschist facies retrograde metamorphic event (M₄).The Liaohe assemblage and associated Liaoji granitoids are considered to have formed in a Paleoproterozoic rift, the late spreading of which led to the occurrence of the early extensional deformation (D₁) and the M₁ and M₂ metamorphism, and the final closing of which was associated with the D₂ and D₃ phases of deformation and M₃ and M₄ metamorphism.     

华北克拉通几个地区古元古代碳酸盐岩地层C-O同位素特征

对华北克拉通古元古代辽河群、中条群和湾子群的碳酸盐岩进行了C、O同位素研究。辽河群王家沟组条带状大理岩的δ13CPDB变化于-1.02‰~3.14‰之间,大部分在0.16‰~1.79‰之间,平均值为1.07‰,稍高于海相碳酸盐岩的δ13C平均值0.5‰,δ18OPDB变化于-17.2‰~-10.1‰之间,大部分在-13.4‰~-10.1‰之间,20组数据的均值为-11.9‰。中条群余家山组大理岩的δ13CPDB变化于-0.2‰~0.8‰之间,大部分在0~0.3‰之间,前14个样品的δ13C平均值为0.34‰,稍低于海相碳酸盐岩的δ13C平均值,后16个样品的δ13C平均值为0.54‰,与海相碳酸盐岩的δ13C平均值基本一致,其δ18OPDB变化于-7.9‰~-6.1‰之间,大部分在-7.3‰~-6.1‰之间,前14个样品的δ18O平均值为-6.80‰,后16个样品的δ18O平均值为-6.68‰,两个剖面上的δ18O值没有明显差别。阜平宋家口南湾子群大理岩δ13CPDB变化于1.0‰~3.8‰之间,平均值为2.65‰,明显高于海相碳酸盐岩的δ13C平均值0.5‰,其δ18OPDB变化于-8.8‰~-5.7‰之间,平均值为-6.97‰。研究结果表明所研究的大理岩均形成于一个比较稳定而又相对波动的气候环境,大理岩沉积期间存在海平面和气温旋回变化但没有突变事件。湾子群宋家口剖面大理岩对Jatulian事件有响应,中条群余家山组大理岩对Jatulian事件没有响应,辽河群王家沟组是否存在对Jatulian事件响应还不确切。     

华北地块西南缘古元古代花岗斑岩及其构造意义

在六盘山南段陇县固关镇东部出露多个原认为属于中-新生代的花岗斑岩岩体,其中的白家沟花岗斑岩显示古元古 代壳源岩浆活动的记录,属于钙碱性花岗岩系。该花岗斑岩高SiO2 （71.14%~73.33%）,高碱（Na2O+ K2O=7.61%~8.70%）, 富钾（K2O/Na2O=1.21~2.52,平均为2.03）,具有准铝-弱过铝质特点（铝饱和指数A/CNK=0.93~1.09,平均1.00）。稀土元 素配分曲线呈轻稀土强烈富集的典型“海鸥型”样式,具有明显的Eu负异常（δEu=0.44~0.47）。岩石富集大离子亲石元素 K,Rb,Ba,Th,亏损高场强元素Ti,Nb,Ta,具有壳源A型花岗岩的地球化学特征。对该花岗斑岩进行的锆石U-Pb LA-ICP-MS同位素年代测定获得了1 846±32 Ma的上交点年龄,说明花岗斑岩形成于古元古代而不是前人认为的中-新生 代,上奥陶统背锅山组砾屑灰岩实际上是沉积于古元古代花岗斑岩之上。结合区域地质资料研究结果表明华北地块西南缘 在古元古代时期处于强烈的伸展环境,这可能为古元古代贺兰坳拉槽向南延伸至六盘山南段提供了直接的岩石学证据。     

华北克拉通南缘A型花岗岩的年代学和Nd-Hf同位素组成:对古元古代晚期伸展事件的制约

河南上店和登封古元古代晚期A型花岗岩的锆石LA-ICP-MS U-Pb年代学、岩石地球化学和Nd-Hf同位素研究为讨论华北克拉通南缘中部古元古代晚期构造演化提供了依据。结果表明,上店花岗斑岩和登封正长花岗岩中锆石发育振荡生长环带,结合高的Th/U比值(0.36~1.64),表明其为岩浆成因,岩浆锆石的207Pb/206Pb加权平均年龄分别为1801Ma、1801Ma和1795Ma,即它们形成于古元古代晚期;上店花岗斑岩具有高SiO_2(70.30%~71.83%)、富碱(K2O+Na2O=8.76%~11.01%)、贫Ca O(0.18%~0.36%)、P2O5(0.12%~0.15%)和MgO(0.02%~0.20%)的特征,富集LILEs(Cs、Rb、Ba和K)、贫HFSEs(Nb、Ta、Zr和Hf)和强烈亏损Sr、P、Ti元素,具有负的Eu异常(δEu=0.44~0.59),结合高的FeOT/(FeOT+MgO)比值(0.94~1.00)和10000×Ga/Al比值(2.70~3.74),暗示其为A型花岗岩,具造山后A2型花岗岩特征;上店花岗斑岩的全岩εNd(t)值变化于-8.94~-6.87之间,tDM2=3243~3049Ma,锆石εHf(t)值介于-14.1~-6.5之间,tDM2=3030~2655Ma;登封正长花岗岩的锆石εHf(t)值介于-11.3~-8.4之间,tDM2=2887~2744Ma。上述结果暗示,上店和登封A型花岗岩均来源于幔源岩浆底侵作用下的中-新太古代基底物质的部分熔融,形成于华北克拉通东部和西部陆块碰撞造山后的伸展环境。     

华北克拉通南缘蚌埠隆起古元古代侵入体地质地球化学特征

华北陆块是世界上最古老、最大的克拉通地块之一。蚌埠隆起作为华北陆块的组成部分,其初始陆壳的形成时限为中太古代晚期,随着扬子陆块与华北陆块碰撞、拼接,经历了较为复杂的演化过程。为了深入探讨蚌埠隆起带古元古代岩浆活动,更新补充年代学及岩石地球化学分析数据,本次通过对区内古元古代侵入体的野外观察、年代学和岩石地球化学研究,在庄子里岩体和磨盘山岩体中分别获得锆石U-Pb年龄为(2089±44)Ma、(2133±27)Ma。地球化学研究显示两岩体明显富钠贫钾,总体铝碱比偏高,钙碱比偏低,属准铝—铝过饱和类型;轻稀土相对富集,具明显的铕负异常,铈异常不明显或无异常;K、Nb、Sr、P和Ti有较明显的亏损,Rb、Th、U、Nd、Zr和Sm相对富集,成因类型具备造山后A型花岗岩特征。ε_(Nd)(t)值介于-3.87～+3.20,暗示侵入体可能来源于同一源区,Nd两阶段模式年龄(2.37～2.84 Ga)与华北克拉通主体形成于太古宙,并以2.40～2.50 Ga为主体相一致。ε_(Hf)(t)值介于-9.77～+9.59,且差别较大,说明其具物质起源的复杂性,古老的两阶段Hf模式年龄(2.27～2.96 Ga)说明其物质起源主要为新太古宙,与蚌埠隆起拉张背景时间上具有一致性。构造环境判别属板内花岗岩范畴,表现为非造山构造环境,暗示其伸展构造背景。     

华北克拉通南缘古元古代晚期汝阳群微体化石及其古环境意义:来自微细构造和生物标志化合物的证据

华北克拉通南缘古元古代晚期汝阳群白草坪组页岩中产出大量的具刺和细网状纹饰的球形疑源类,其中以Shuiyousphaeridium macroreticulatum与Dictyosphaera macroreticulata化石标本最具代表性.利用扫描电子显微镜结合光学生物显微镜对其膜壳微细构造进行观察,结果显示:膜壳表面的刺饰形态具有多样性分叉;其膜壳壁可能是由多边形网脊及膜层构成,而不是由多角形板片及膜层构成.同时通过对比研究,认为两种最具代表性的特征分子可能是同一生物不同阶段的产物.此外,通过对赋存微体化石页岩进行生物标志化合物分析,检测到的姥鲛烷/植烷比(Pr/Ph)大于1、伽马蜡烷丰度中等及生物标志化合物(C₃₀藿烷、C₂₇甾烷和三环萜烷),结合前人关于汝阳群沉积环境的研究,不仅指示了汝阳群微体化石群沉积古环境应为正常的海洋环境——滨海-浅海相,也指示了元古代真核生物已经存在,为重建微体化石群和探讨早期生命演化提供了重要的理论依据.      

华北古元古代碳酸岩起源深度及现代板块构造启动

俯冲作用是改变地球内部物质演化的关键因素之一,而现代板块的启动时间一直存在争论.华北克拉通中部造山带内蒙古丰镇古元古代碳酸岩内超硅石榴石包体和榴辉岩捕虏体的发现,为研究岩浆起源深度和板块构造提供了窗口.矿物学和高温高压实验限定超硅石榴石（Si~3.18 pfu）富集三价铁（Fe3+/∑Fe~0.8）,来源于地幔约400 km的深度表明碳酸岩岩浆起源于地幔过渡带.矿物对温压计和P-T视剖面图确定榴辉岩捕虏体变质峰期温压分别为~660 ℃和~2.65 GPa,温压梯度为~250 ℃·GPa-1,与现代板块深俯冲产物相似,说明现代板块构造在古元古代已经启动.统计显示全球古元古代碳酸岩与造山带内高压变质岩石密切共生.全球古元古代的板块俯冲可能与哥伦比亚超大陆的拼合有关.大规模板片俯冲携带地壳沉积物质进入深部地幔,形成碳酸岩岩浆及氧化的富Fe3+超硅石榴石.地壳物质在地幔源区循环约20亿年,导致了地幔源区的不均一性以及长时间的碳循环.