Paleoproterozoic mafic dyke swarms of the Belomorian Province,eastern Fennoscandian Shield

Paleoproterozoic mafic igneous rocks (2450–1970 Ma) are exposed in the form of layered intrusions, dykes, and volcanic rocks in the Karelian, Kola and Murmansk provinces and in the form of dykes and small intrusions in the Belomorian Province, Eastern Fennoscandian Shield. The age and sequence of mafic dyke emplacement during the Paleoproterozoic are very similar in these regions. Further comparisons of geochemical characteristics of mafic dyke swarms in the Belomorian Province and neighboring cratons show considerable similarities.     

Postglacial chironomid assemblage succession in northernmost Ungava Peninsula,Canada

Sediment cores spanning the postglacial period were recovered from two coastal freshwater basins located 300 km apart in northernmost Ungava (Nunavik, Canada). A basal date from the sequence recovered near Wakeham Bay suggests that the region was ice‐free by as early as 9600 cal. a BP. The initial development of both investigated lacustrine ecosystems was characterised by chironomid assemblages strongly dominated by the Subtribe Tanytarsina. These early postglacial assemblages showed greatly reduced diversity but much greater abundance than during subsequent periods of lake evolution. An abrupt and distinct turnover in sedimentary chironomid assemblage composition and productivity, marked by a switch to a dominance of Orthocladiinae, occurred around 6 ka BP. This turnover likely corresponds with a significant disturbance in the regional environment, which could be linked to the final disappearance of the last remnants of the Laurentide Ice Sheet. These findings, based on the first palaeolimnological investigations undertaken in northernmost Ungava, provide new insights into lateglacial and postglacial faunal recolonisation patterns in aquatic ecosystems of the eastern Canadian Arctic. Copyright © 2009 John Wiley & Sons, Ltd.     

Magma flow in dyke swarms of the Karoo LIP: Implications for the mantle plume hypothesis

The ~ 183 Ma old Karoo Large Igneous Province extends across southern Africa and is related to magmatism in Antarctica (west Dronning Maud Land and Transantarctic Mountains) and parts of Australasia. Intrusive events, including the emplacement of at least ten dyke swarms, occurred between ~ 183 Ma and ~ 174 Ma. We review here the field evidence, structure and geochronology of the dyke swarms and related magmatism as it relates to melt sources and the mantle plume hypothesis for the Karoo LIP. Specifically, the magma flow-related fabric(s) in 90 dykes from five of these swarms is reviewed, paying particular attention to those that converge on triple junctions in southern Africa and Antarctica. The northern Lebombo and Rooi Rand dyke swarms form an integral part of the Lebombo monocline, which converges upon the Karoo triple junction at Mwenezi, southern Zimbabwe. Dykes of the Northern Lebombo dyke swarm (182–178 Ma) appear to have initially intruded vertically, followed later by lateral flow in the youngest dykes. In dykes of the Okavango dyke swarm (178 Ma) there is evidence of steep magma flow proximal to the triple junction, and lateral flow from the southeast to the northwest in the distal regions. This is consistent with the Karoo triple junction and the shallow mantle being a viable magma source for both these dyke swarms. In the Rooi Rand dyke swarm (174 Ma) there is also evidence of vertical and inclined magma flow from north to south. This flow direction cannot be reconciled with the Karoo triple junction, as the northern termination of the Rooi Rand dyke swarm is in east-central Swaziland. The Jutulrøra and Straumsvola dyke swarms of Dronning Maud Land display evidence of sub-vertical magma flow in the north and lateral flow further south. The regional pattern of magma flow is therefore not compatible with direction expected from the Weddell Sea triple junction. The overall flow pattern in Karoo dykes is consistent with the triple junction being an important magma source. However, the Limpopo Belt and Kaapvaal Craton have significantly controlled the structure and distribution of the Lebombo and Save–Limpopo monoclines and the Okavango dyke swarm. The locus of magma flow in dykes of Dronning Maud Land is at least 500 km from the Karoo triple junction, as is the apparent locus for the Rooi Rand dyke swarm. In comparison with recent modelling of continental assembly, the structure and flow of the dyke swarms, linked with geochronology and geochemistry, suggests that thermal incubation during Gondwana assembly led to Karoo magmatism. A plate tectonic, rather than a fluid dynamic plume explanation, is most reasonably applicable to the development of the Karoo LIP which does not bear evidence of a deep-seated, plume source.     

Configuration and timing of Ungava Bay ice streams, Labrador–Ungava, Canada

The prominent Ungava Bay landform swarm (UBLS), covering an area of ˜260000 km² south of Ungava Bay, Canada, is defined by drumlins, crag-and-tails, horned crag-and-tails and flutes indicating convergent ice flow towards Ungava Bay. The UBLS has been difficult to interpret in terms of ice-sheet configuration, dynamics and age. Aerial photograph and satellite image interpretations of the Labrador-Ungava region reveal a previously unrecognized level of complexity within the UBLS consisting of several well-defined segments, most interpreted as representing discrete stream-flow events. Each of the segments is characterized by one or more of the criteria (convergent flow patterns at their heads, attenuated till lineations and abrupt lateral margins) previously suggested as diagnostic for formation by fast-flowing ice (ice streams). The UBLS reflects the most direct and probably fastest contact (in terms of sediment transport) between the Laurentide Ice Sheet interior and the ocean. It is therefore a prime candidate for abrupt changes in glacial-age northwest Atlantic seafloor sedimentation.     

Neotectonic crustal uplifts as a consequence of mantle fluid infiltration into the lithosphere

As evidenced by plentiful data, most of the large recent positive topographic features formed as a result of a dramatically accelerated crustal uplift in the Pliocene–Quaternary after a relatively stable period (～100 Myr in most of the regions). The methods used are illustrated by the well-studied large neotectonic crustal uplifts on the Tibetan Plateau and in the Himalayas. Farther north, neotectonic uplifts with amplitudes of several hundred meters to several kilometers spread over a vast area from Central and Northeast China in the south to the Taimyr Peninsula and Northeastern Asia in the north. They are often attributed to the India–Asia plate collision which began ～50 Ma.Most of the uplifts in these regions have formed only during the last few Myr, unaccompanied by significant crustal shortening. Therefore, the large neotectonic crustal uplifts can be explained by a decrease in the lithospheric density. One of the causes was the rapid convective replacement of the lower part of the denser mantle lithosphere by the asthenosphere or mantle plume. This became possible owing to a drastic weakening of the mantle lithosphere under the influence of asthenospheric fluids. In some areas, a considerable asthenospheric top uplift is evidenced by seismic tomography data.The lower mantle lithosphere (～50–100 km thick) was replaced by the asthenosphere underneath the neotectonic crustal uplifts of ～1.0 km in Central Asia. Areas with a thick lithosphere were affected by relatively small neotectonic uplifts, strongly nonuniform in space. They point to metamorphism with mafic-rock expansion in the lower crust upon the infiltration of an asthenospheric fluid. The large crustal uplifts which formed on the continents in the Pliocene and Pleistocene indicate large-scale quasi-synchronic supply of the mantle fluid to their lithosphere.     

峨眉山大火成岩省基性墙群几何学研究及对地幔柱中心的指示意义

本文以1:20万和1:5万区调资料为基础,结合野外观察研究了峨眉山大火成岩省主要分布区内的基性(辉绿岩)岩墙群的几何学特征。结果表明,岩墙群的几何学特征表现为中心放射状,由6条巨型岩墙群组成,辐射角度近200°。其中心收敛于永仁一带,与地层学指示的最大隆起位置吻合,指示了二叠纪峨眉山玄武岩事件的地幔柱中心位置。岩墙群分布呈中东部较多而西部少的特点,这可能与不同的剥蚀程度有关。     

Cumberland batholith,Trans-Hudson Orogen,Canada: Petrogenesis and implications for Paleoproterozoic crustal and orogenic processes

Large volume, plutonic belts, such as the ～ 221,000 km², ca. 1.865–1.845 Ga Cumberland batholith (CB) of the Trans-Hudson Orogen in Canada, are major components of Paleoproterozoic orogenic belts. In many cases, they have been interpreted as continental arc batholiths. The petrogenesis and tectonic context of the CB and implications for crustal growth and recycling are interpreted herein based on a 900 km geochemical-isotopic (Nd–O) transect across it and into granitoid plutons within bounding Archean cratons in central and southern Baffin Island.The mainly granulite grade CB, emplaced over an age span of between 14 and 24 Ma, consists mainly of high-K to shoshonitic monzogranite and granodiorite, but also includes low- and medium-K granitoid rocks. Metaluminous to slightly peraluminous compositions and δ¹⁸O (VSMOW) values (+ 6 to + 10‰) indicate derivation from infracrustal (I-type) sources. ε_Nd 1.85 Ga signatures (? 12 to ? 2) of both mafic and felsic units suggest a dominance of evolved sources. Isotopic signatures in the interior of the CB (? 2 to ? 7) are more radiogenic than those within Archean domains in central (? 8 to ? 15) and southern (? 5 to ? 19) Baffin Island. The isotopic transect is interpreted as ‘imaging’ an accreted microcontinental block (Meta Incognita) and bounding Archean cratons. The CB includes granites of arc, within-plate (A-type) and post-collisional affinity and volumetrically minor mafic rocks with both arc and non-arc features. (La/Yb)_CN and Sr/Y values range from < 1 to 225 and < 1 to 611, respectively. In these respects, some CB granitoid rocks resemble Paleozoic adakitic granites, interpreted as partial melts of greatly thickened crust within post-collisional settings, such as Tibet. Thus, the CB likely encompasses various non-consanguineous magmatic suites generated at deep- to mid-crustal depths. Although CB granitoid rocks undoubtedly had important crustal sources, it is hard to assess the relative contribution of mantle-derived magmas.The CB is best interpreted as a post-accretion batholith resulting from large-scale lithospheric mantle delamination followed by the upwelling of hot asthenospheric mantle leading to voluminous crustal partial melting. Contributors to crustal instability which may have facilitated such delamination included: (a) a collage of recently assembled small cratons underlain by hot, weak lithosphere with mantle-depth structural breaks within this segment of the Trans-Hudson Orogen; (b) the gabbro-eclogite phase transformation, and (c) a greatly thickened crustal section (> 60 km), as evidenced by adakitic granites.     

Evidence for crustal components in the mantle and constraints on crustal recycling mechanisms: pyroxenite xenoliths from Hannuoba,North China

Spinel and garnet pyroxenite xenoliths in Cenozoic basalts from Hannuoba, North China show extremely heterogeneous chemical and isotopic compositions (ε_Nd=−27 to +34). Most of these pyroxenites are relatively young, probably late Mesozoic in age, although a few Al-pyroxenites could be very old (∼2 Ga). While their texture and major element compositions suggest an origin of high pressure cumulates, the trace element and isotopic compositions of the Hannuoba pyroxenites require multiple segregation processes from different parental magmas. Strong LREE enrichment, ubiquitous HFSE depletion and some Eu anomalies of the Al- and Cr-pyroxenites indicate the involvement of crust components in their source. Their Sr–Nd isotopic ratios are negatively correlated and plot below the MORB–OIB–IAB–sediment trend, suggesting that the parental melts of the Cr- and Al-pyroxenites may have been derived from a mixture of asthenospheric melts and a long-term evolved continental crust. The garnet pyroxenites significantly deviate from the isotopic array defined by the Al-pyroxenites, due to their relatively high ⁸⁷Sr/⁸⁶Sr at given ε_Nd. They thus more likely represent segregates from melts derived from partial melting of hydrothermally altered oceanic crust (basalts+marine sediment). If the crustal component involved in the Al-pyroxenites is subducted terrigenous sediments or other continental materials from the Archean Sino-Korean Craton, the Al-pyroxenites and garnet pyroxenites may have formed contemporaneously at a palaeo-convergent plate margin. This may be related to the subduction of the Mongol–Okhotsk plate beneath North China during the late Jurassic. Alternatively, if the delaminated lower crust was involved, it implies that most of the Al-pyroxenites are younger than the garnet pyroxenites, and their formation may be temporally correlated with lithospheric thinning during the Cretaceous. This model is attractive because the inferred tectonic evolution from a convergent setting to an extensional environment is consistent with the geologic record in the area.     

华北北部中生代岩墙群

岩墙群是深源岩浆浅侵位的产物，它可提供有关壳-幔演化的重要信息。通过华北北部大同早中生代碳酸岩-煌斑岩岩墙群、赤峰早中生代闪长玢岩岩墙群、林西中生代辉绿岩岩墙群、京北晚中生代双峰式岩墙群的对比研究，揭示了华北中生代存在的两次底侵作用及其相关的伸展作用。华北早、晚中生代岩墙群与晋北地区元古代岩墙群的初步对比，从一个侧面反映了华北壳幔的演变。     

Lateral displacement of crustal units relative to underlying mantle lithosphere: Example from the Bohemian Massif

We propose a mechanical model of deformation of the entire lithosphere of the Bohemian Massif (BM), whose core is formed by an asymmetric block of the Teplá-Barrandian (TB) unit in between the Saxothuringian (ST) and Moldanubian (MD) units. For the modelling, we have re-processed P-wave travel times recorded during the last two decades at dense networks of seismic stations installed in the BM during several passive seismic experiments. We also use previous results of anisotropic studies based on splitting of teleseismic shear waves. This allows us to refine estimates of the lithosphere thickness and delimit deep margins of the individual mantle lithosphere domains. The domains are rigid enough to preserve pre-orogenic olivine fabrics differently oriented in each of the units. Shapes and dips of the mantle boundaries, representing major zones of weakness inherited from the Variscan amalgamation of independent microplates, indicate that north-westward subductions beneath the TB unit dominated tectonic development of the core of the BM. Two mantle lithosphere domains with different fabric orientations, separated by a WSW-ENE striking shear zone, underlie the TB crust. The NW domain is the TB mantle lithosphere, while the SE domain is the MD mantle lithosphere thrust under the TB crust. Lithosphere of the north-western TB domain, compressed between early Variscan subductions of the ST continental lithosphere from the northwest and the MD continental lithosphere from the southeast, was pushed south-westward by about 50 km. Though the crust of the south-westerly TB promontory is commonly attributed to the MD unit, apparently it preserves the TB mantle lithosphere. The shifted TB lithosphere provides compelling evidence in support of older views suggesting that the Zone Erbendorf-Vohenstrauss (ZEV) originally belonged to the tilted western rim of the TB unit. During the final phase of the assemblage of the BM, the rigid TB lithosphere was disrupted by the southward pushing ST lithosphere along the newly formed NW-SE striking Jáchymov Fault Zone (JFZ). This lithosphere-scale process most likely changed the tectonic regime, released subduction-related forces and started the gravity-dominated tectonics.     

Geochemistry of mafic magmas from the Ungava orogen, Québec, Canada, and implications for mantle reservoir compositions at 2.0 Ga

The Ungava orogen of northern Québec is one of the best preserved Proterozoic mobile belts of the world, recording > 200 Ma of plate divergence and convergence. Voluminous magmatism associated with rifting of the Superior Province basement ≈2.04 Ga resulted in the development of a volcanic rift margin sequence and an ocean basin. Four distinct mafic magma suites were erupted: (1) continental basalts (Eskimo Formation, western and central Povungnituk Group) with moderate to high Zr/Nb and negative Nb anomalies which have interacted with the continental crust (ε_Nd(2.0 Ga)) from −7.4); (2) mafic lavas from the Flaherty Formation, eastern Povungnituk Group and some Watts Group lavas associated with passive margin rifting, having slightly enriched isotopic signatures (ε_{Nd(2.0 Ga)} = +2.7 to +4.4) compared to the contemporaneous depleted mantle, high (Nb/Y)_n and low Zr/Nb ratios (≈4.4 and ≈8.9, respectively); (3) a highly alkaline OIB-like suite (ε_{Nd(2.0 Ga)} = +2.3 to +3.2, (Nb/Y)_n> 12) within the Povungnituk Group composed of nephelinites, basanites and phonolites; and (4) depleted Mg-rich basalts and komatiitic basalts (ε_{Nd(2.0 Ga)} ≈ + 4.5 to + 5.5) with trace-element characteristics of N-MORB, but with higher Fe and lower Al than primitive MORB (Chukotat Group, Ottawa Islands and some Watts Group samples). The ocean basin into which these lavas were erupted was subsequently destroyed during subduction between ≈1.90 and ≈1.83 Ga, resulting in the development a magmatic arc (Narsajuaq terrane and Parent Group).

The Ungava magmas provide a unique window into the mantle at 2.0 Ga. The chemical and isotopic similarity of these Proterozoic magmas to modern-day magmas provides strong evidence that the interplay between depleted mantle, OIB mantle and sub-continental mantle during the Proterozoic was comparable to that of the modern Earth.     

岩墙群的岩石学、地球化学和几何学特征及其大地构造意义

对近几十年来岩墙群在构造环境、侵位模式、形成机制、岩相学、地球化学、同位素地质年代学、几何学和比较行星学等方面的研究成果进行了总结,为岩墙群的深入研究提供重要信息。岩墙群主要与伸展构造环境有关,是岩浆侵位事件(如地幔柱)造成上覆地壳形成裂隙系统,岩浆随后灌入而形成的。岩墙群的侵位模式主要有垂向和侧向两种,也可能是两种模式共同作用的结果。岩墙群的岩性多样,但主要是超基性-基性岩,说明其与深部岩浆作用密切相关。对岩墙群的地球化学研究可揭示岩浆事件的构造环境、岩浆演化过程以及源区特征。放射性岩墙群的几何学特征有重要的指示意义,其收敛中心被认为是地幔柱的中心位置。岩墙群是古陆重建十分重要的"契合点",对古陆重建研究十分有帮助。类地行星(金星)巨型放射状系统很可能是岩墙群系统,暗示了地球上也存在过这种完整的巨型放射状岩墙群系统。岩墙群对于古陆重建、地幔柱中心的指示、地幔源区示踪、火山机构以及区域岩浆演化的研究都有十分重要的意义,并将对地球科学的发展产生重要影响。     

A window on West Antarctic crustal boundaries: the junction between the Antarctic Peninsula, the Filchner Block, and Weddell Sea oceanic lithosphere

A new airborne magnetic survey of the southeastern Antarctic Peninsula and adjacent Weddell Sea embayment (WSE) region suggests a continuity of geological structure between the eastern Antarctic Peninsula and the attenuated continental crust of the Filchner Block. This has implications for the reconstructed position of the Ellsworth–Whitmore Mountains block in Gondwana, which is currently uncertain. Palaeomagnetic data indicate that it has migrated from a Palaeozoic position between South Africa and Coats Land to its current position as a microplate embedded in central West Antarctica. The most obvious route for migration is between the Antarctic Peninsula and the Weddell Sea embayment. Evidence that geological structures are continuous across the boundary places constraints on the timing and pathway of migration. Magnetic textures suggest the presence of shallow features extending from the Beaumont Glacier Zone (BGZ) in the west for at least 200 km into the Weddell Sea embayment. These data suggest that the Eastern Domain of the Antarctic Peninsula and the stretched continental crust of the Filchner Block share a common recent, probably post-Early Jurassic, history. However, examination of deep anomalies indicates differences in the magnetic characteristics of the two blocks. The boundary may mark either the edge of extended continental crust, or a discontinuity between two, once separated, blocks. This discontinuity, or pre-Late Jurassic Antarctic Peninsula terrane boundaries to the west, may have allowed the passage of the Ellsworth–Whitmore Mountains block to its present location.     

新疆库鲁克塔格地区基性岩墙群几何学与岩浆超压估算

新疆库鲁克塔格地区发育大量密集的岩墙群,反映了该区所经历的岩浆活动及地壳伸展事件。本文基于多源遥感影像(Landsat 7 ETM+、SPOT、CORONA KH-4B、QuickBird2)共提取了区内2284条岩墙,在此基础上进行几何学分析和岩浆超压估算。这些岩墙主要侵入新元古代侵入岩及前寒武系沉积岩和变质岩。走向以NW向(300°~320°)为主,同时存在NE向(40°~70°)和近东西向岩墙。岩墙单体长90m~22.8km,平均4.3km,呈对数正态分布。垂直岩墙走向,沿NE60°方向取4条剖面计算地壳伸展率,由北至南分别为4.22%、1.83%、2.60%和7.40%。根据岩墙纵横比(长度/厚度),计算获得岩墙侵位时岩浆超压为7.9MPa(E=10GPa)~23.7MPa(E=30GPa)。该区同时出露新元古代和二叠纪岩墙群,其空间分布与花岗岩有密切关联,为多种构造背景的产物,且经历了多期构造叠加和改造。     

华北克拉通中生代以来基性岩墙群的分布及研究意义

围绕岩石圈破坏和减薄这个主题.华北克拉通成为近10年来国内外研究的热点地区.华北克拉通中生代以来基性岩墙群分布广泛,然而对此仍缺乏系统的研究.通过对基性岩墙群的系统总结,在详尽的资料收集和已有研究的基础上,重点对华北克拉通中生代基性岩墙群的研究现状、分布和意义进行了汇总和讨论.随后,结合近年来的研究经验,提出了基性岩墙群定年中应注意的问题.     

Analysis of the ore-controlling role of Guojiadian mantle branch structure, Northwest Shandong Peninsula, China

Metallogenesis in the gold ore-concentrated zone of Northwest Shandong Peninsula is closely related to deep processes.The region in the eastern part of North China entered into the stage of mantle plume evolution during the Yanshanian movement,following the long-time stage of stable platform evolution during Paleozoic time.At that time,the ore-concentrated zone of Northwest Shandong Peninsula just entered into the development-evolution stage of the Laiyang sub-mantle plume and the Guojiadian mantle branch structure in its periphery.The core-mantle-source gold was present in the gas-liquid form,and it migrated through mantle plume→sub-mantle plume→mantle branch structure→favorable tectonic expansion zone to the favorable loci of the mantle branch structure,where gold was deposited as ores,thereafter constituting a series of large-to medium-sized gold deposits distributed around the Guojiadian mantle branch structure.This study also dealt with the Jiaojia fault as the main detachment(fault altered rock) belt on the northwestern margin of the mantle branch structure and also presented a basic cognition about the fact that the Sanshandao fault as the listric fault on the hanging wall of the detachment belt.Furthermore,on this basis,this study also pointed out the orientation for further ore prospecting in this region.     

晋北地区中元古代岩墙群的地球化学特征和大地构造背景

晋北地区发育大规模的中元古代北北西向岩墙群，未变形和未变质，是华北克拉通前寒武纪构造演化研究的最显著标志，根据本区岩墙群的岩石学和地球化学研究，这些岩墙群的岩石属于板内大陆裂谷玄武岩系列，形成于大陆裂谷拉张的构造环境，与东侧的燕辽－中条拗拉槽内的火山岩岩石化学对比分析，本区岩墙群的基性岩浆可能来自拗拉槽深部，这些基性岩浆从拗拉槽的深部向北西方向侵位于晋北地区先存的构造裂隙中形成了基性岩墙群。