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The allochthonous Blåhø Nappe in the Nordøyane ultra high pressure (UHP) domain, Western Gneiss Region in Norway, acts as a window to examine geological processes occurring in continent–continent collisional zones, but many aspects regarding its tectonometamorphic evolution remain debated and elusive. In this contribution, an integrated study including major‐ and trace‐element zoning in garnet, phase equilibrium modelling and the simulation of cation diffusion in garnet was conducted on two high‐pressure (HP) granulite facies rocks from the Blåhø Nappe on the island of Fjørtoft. The results shed new light on the complex geodynamic processes that act in continent–continent collisional zones and finally shape collisional orogens. Phengite, biotite, amphibole, zoisite‐allanite and low‐Zr rutile enclosed in garnet likely attest to a prograde eclogite facies metamorphism for the studied rocks. Pressure–temperature (P–T) conditions of ~1.5–1.6 GPa and 615–670°C were retrieved for this stage. An extensive re‐equilibration under peak HP granulite facies conditions of ~1.5 GPa and 925 ± 50°C followed. Subsequently, the rocks were cooled and reburied to eclogite facies conditions of ~1.8–1.9 GPa and 805–825°C. This was followed by a final stage of decompression and cooling to amphibolite facies conditions of ~650–780°C and 0.5–1.0 GPa. Cooling and exhumation rates of >400°C/Ma and >75 km/Ma, respectively, indicating an ultrafast temperature and pressure decrease are estimated for this stage from simulations of cation diffusion in garnet. The anticlockwise P–T path obtained here is relatively complete and compatible with a repeated burial history during the Caledonian orogeny but not with UHP conditions proposed for the Blåhø Nappe. Our model proposes that the rocks later forming the Blåhø Nappe were buried to lower crustal depths of approximately 55 km equating to a geothermal gradient of ~13°C/km during the early Caledonian orogeny. Subsequent heating of these rocks to HP granulite facies conditions was likely driven by slab break‐off and hot mantle upwelling. Baltica underthrusting during the Scandian continent–continent collision cooled and transported the Blåhø Nappe to greater depths. The obtained cooling and exhumation rates indicate ultrafast exhumation, presumably in an exhumation channel. [ABSTRACT FROM AUTHOR]

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In this study, data from garnet‐kyanite metapelites in ultrahigh‐pressure (UHP) domains of the Western Gneiss Region (WGR), Norway, are presented. U–Pb geochronology and trace element compositions in zircon, monazite, apatite, rutile and garnet were acquired, and pressure–temperature (P–T) conditions were calculated using mineral equilibria forward modelling and Zr‐in‐rutile thermometry. Garnet‐kyanite gneiss from Ulsteinvik record a prograde evolution passing through ~690–710°C and ~9–11 kbar. Zircon and rutile age and thermometry data indicate these prograde conditions significantly pre‐date Silurian UHP subduction in the WGR and are interpreted to record early Caledonian subduction of continental‐derived allochthons. Minimum peak conditions in the Ulsteinvik metapelite occur at ~28 kbar, constrained by an inferred garnet+kyanite+omphacite+muscovite+rutile+coesite+H2O assemblage. The retrograde evolution passed through ~740°C and ~7 kbar, first recorded by the destruction of omphacite and followed by the partial replacement of kyanite and garnet by cordierite and spinel. Garnet‐kyanite metapelite from the diamond‐bearing Fjørtoft outcrop documents a polymetamorphic history, with garnet forming during the late Mesoproterozoic and limited preservation of high‐pressure Caledonian assemblages. Similar to the Ulsteinvik metapelite, zircon and rutile age data from the Fjørtoft metapelite also record pre‐Scandian Caledonian ages. Two potential tectonic scenarios are possible: (1) The samples were exhumed at different times during pre‐Scandian subduction of the Blåhø nappe, or (2) the samples do not share a history in the same nappe complex, instead the Ulsteinvik metapelite is a constituent of the Seve‐Blåhø Nappe, whilst the Fjørtoft metapelite shares its history within a separate nappe complex. [ABSTRACT FROM AUTHOR]

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Copyright of Canadian Journal of Earth Sciences is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The Grenville, Sveconorwegian, and Sunsas orogens are typically inferred to reflect collision between Laurentia, Baltica, and Amazonia at ca. 1.0 Ga, forming a central portion of the Rodinia supercontinent. This triple‐junction configuration is often nearly identical in otherwise diverse Rodinia reconstructions. However, available geological data suggest that although the Grenville and Sveconorwegian provinces shared a similar tectonic evolution from pre‐1.8 to ca. 1.5 Ga, they record distinctly different tectonic histories leading up to, during, and possibly following Grenville–Sveconorwegian orogenesis. Moreover, palaeomagnetic data suggest the two continents were separated at peak orogenesis, further invalidating any direct correlation. A number of possible interpretations are permissible with available geological and palaeomagnetic data, of which a "classic" triple‐junction configuration appears least likely. In contrast to the commonly inferred intertwined Proterozoic evolution of Baltica and Laurentia, the possibility remains that they were unrelated for a billion years between 1.5 and 0.45 Ga. [ABSTRACT FROM AUTHOR]

The Central Indian Tectonic Zone, because of its critical position in the tectonic framework of East Gondwanaland, holds insights into our understanding of the assembly and dispersal of Columbia and Rodinia supercontinents through the growth of the Greater Indian Landmass. The tectonic zone, which is now a collage of different supracrustal and granulite‐gneiss belts, bears testimony to a multistage orogenic evolution from Late Palaeoproterozoic to Early Neoproterozoic time. In this contribution, the current status of orogenic evolution in the Central Indian Tectonic Zone has been reviewed using petrological and geochronological constraints from the Sausar Mobile Belt, its southernmost constituent. It reveals a three‐stage evolution leading to the growth of the Greater Indian Landmass: (a) c. 1.6–1.5‐Ga accretionary orogenesis produced the first nucleus of Greater India through the assembly of arc, back arc, and continental margins; (b) a phase of Middle Proterzoic extension led to the disintegration of the Proto‐Greater India; and (c) classical Himalayan‐style continental collision between 1.06 and 0.93 Ga finally stitched the North and South India blocks to produce the final configuration of the Greater Indian Landmass. It is argued that the Late Palaeoproterozoic to Early Mesoproterozoic orogenesis in India and former Gondwanaland and Laurentian fragments is a globally significant amalgamation event separate from the Columbia Supercontinent assembly. [ABSTRACT FROM AUTHOR]

The Makkovik Province of eastern Labrador represents part of an accretionary orogen active during an early stage in the development of the Palaeoproterozoic southern Laurentian continental margin. New Nd isotope data for the eastern Makkovik Province suggest that accreted juvenile Makkovik crust was generated in the Cape Harrison domain during a single crust-forming event at c. 2.0 Ga. Pb isotope data support this model, and show a strong similarity to radiogenic crustal signatures in the juvenile Palaeoproterozoic crust of the Ketilidian mobile belt of southern Greenland. As previously proposed, an arc accretion event at c. 1.9 Ga triggered subduction-zone reversal and the development of an ensialic arc on the composite margin. After the subduction flip, a temporary release of compressive stress at c. 1.87 Ga led to the development of a retro-arc foreland basin on the downloaded Archean continental edge, forming the Aillik Group. Unlike previous models, a second arc is not envisaged. Instead, a compressive regime at c. 1.82 Ga is attributed to continued ensialic arc plutonism on the existing margin. The tectonic model for the Makkovikian orogeny proposed here is similar to that for the Ketilidian orogeny. Major- and trace-element analyses suggest that much of the magmatism in the Makkovik orogen results from post-accretionary ensialic arc activity, and that few vestiges remain of the original accreted volcanic arc. This pattern of arc accretion and intense post-accretion reworking is common to many accretionary orogens, such as the South American Andes and North American Cordillera. [ABSTRACT FROM AUTHOR]

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Copyright of Canadian Journal of Earth Sciences is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The hydrogeology of the municipalized territory (13,210 km2) of the Saguenay-Lac-Saint-Jean (SLSJ) region has been studied as part of the groundwater knowledge acquisition program entitled Programme d’acquisition de connaissances sur les eaux souterraines (PACES) launched in 2008 by the Government of Quebec, Canada. This study involved the collaboration of numerous contributors (municipal policymakers, government agencies, watershed organizations and universities) to meet the program’s multiple and wide-ranging requirements. The key deliverables included a numerical geodatabase, 38 regional-scale maps and a scientific report, all elaborated after 4 years of data gathering and compilation, fieldwork and information synthesis. In addition, numerous collateral research projects were undertaken by undergraduate and graduate students. The results of the SLSJ-PACES project provided new insights into regional groundwater resources and led to a generalized conceptual model of regional hydrostratigraphic features and groundwater quality. This paper summarizes the particular aspects of the major aquifers in the SLSJ region as uncovered by PACES, and presents the emerging challenges for updating and improving the region’s hydrogeological knowledge and ensuring the sustainable management of regional groundwater resources. [ABSTRACT FROM AUTHOR]

We studied the 3-D shear wave velocity ( V s) structure in the Gulf of St. Lawrence (GSL) and adjacent onshore areas to 20 km depth by inverting Rayleigh wave dispersion extracted from the vertical components of continuous ambient seismic noise waveforms. The region is divided into three broad zones based on their V s characteristics. In the northwest, the Grenville Province (i.e., the exposed edge of predominantly Middle-Proterozoic Laurentian crust) is dominated by high V s, except for well-known anorthosite sites, which are characterized by relatively lower V s. In contrast, the central segment of the GSL region corresponds to a belt with generally low V s at upper crustal levels. In the southeastern part of the GSL, prominent low V s in the uppermost crust are found to coincide with locations of subsidiary basins of the Canadian Maritime Basin, while higher V s characterize the accreted Appalachian terranes where they are exposed on land. The Grenville Province is wedged out at depth by the Red Indian Line, which is the suture between composite Laurentia and peri-Godwanan Ganderia in the Canadian Appalachians. The geometry and V s characteristics of the south-easternmost peri-Gondwanan terranes of Avalonia and Meguma suggest that they may be fully or partially structurally overlying a basement with distinct seismic characteristics, which could be a vestige of the West African craton that was underthrust beneath composite Laurentia during the terminal Alleghenian continent-continent collision. In the middle of the GSL, the 3-D geometry of the Canadian Maritimes sedimentary basins overlying the Appalachian terranes shows that the depth to the top of basement is in excess of 8 km. [ABSTRACT FROM AUTHOR]

The Grenville Province forms the exhumed remnants of a 1.1 Ga collisional orogeny that telescoped an older continental margin. Terranes with distinct crustal formation ages can be mapped using Nd isotopes, revealing a ramp-flat thrust structure. The ramp is identified by the presence of retrogressed eclogites, and its trajectory is refined using Nd model ages. The main allochthon is locally overlain by the Parry Sound klippe, but is also underlain by a tectonic duplex. Northwest-directed nappes represent remnants of a corrugated thrust sheet, but a ring-shaped remnant was also preserved where the thrust sheet was down-buckled under the dense rocks of Parry Sound domain. [ABSTRACT FROM AUTHOR]

Fluorapatite-monazite-xenotime-allanite mineralogy, petrology, and textures are described for a suite of Kiruna-type apatite-iron oxide ore bodies from the Grängesberg Mining District in the Bergslagen ore province, south central Sweden. Fluorapatite occurs in three main lithological assemblages. These include: (1) the apatite-iron oxide ore bodies, (2) breccias associated with the ore bodies, which contain fragmented fluorapatite crystals, and (3) the variably altered host rocks, which contain sporadic, isolated fluorapatite grains or aggregates that are occasionally associated with magnetite in the silicate mineral matrix. Fluorapatite associated with the ore bodies is often zoned, with the outer rim enriched in Y+REE compared to the inner core. It contains sparse monazite inclusions. In the breccia, fluorapatite is rich in monazite-(Ce) ± xenotime-(Y) inclusions, especially in its cores, along with reworked, larger monazite grains along fluorapatite and other mineral grain rims. In the host rocks, a small subset of the fluorapatite grains contain monazite ± xenotime inclusions, while the large majority are devoid of inclusions. Overall, these monazites are relatively poor in Th and U. Allanite-(Ce) is found as inclusions and crack fillings in the fluorapatite from all three assemblage types as well as in the form of independent grains in the surrounding silicate mineral matrix in the host rocks. The apatite-iron oxide ore bodies are proposed to have an igneous, sub-volcanic origin, potentially accompanied by explosive eruptions, which were responsible for the accompanying fluorapatite-rich breccias. Metasomatic alteration of the ore bodies probably began during the later stages of crystallization from residual, magmatically derived HCl- and H2SO4-bearing fluids present along grain boundaries. This was most likely followed by fluid exchange between the ore and its host rocks, both immediately after emplacement of the apatite-iron oxide body, and during subsequent phases of regional metamorphism and deformation. [ABSTRACT FROM AUTHOR]

Nearly 70 new Nd isotope analyses are presented for plutonic orthogneisses from the Grenvillian Central Metasedimentary Belt (CMB) in order to test a back-arc aulacogen model for its origin. Nd isotope signatures of metaplutonic rocks are used as probes of the formation age of the crust at depth, revealing sharp boundaries between old crustal blocks and juvenile (1.2–1.35 Ga) Elzevirian-age crust. Firstly, a hidden block of old crustal basement is revealed between areas of juvenile crust south of Douglas, Ontario. Secondly, TDM ages refine the boundary between juvenile crust and old basement (1.35–1.55 Ga) within the Weslemkoon batholith, showing this pluton to be a polygenetic stitching pluton that straddles a hidden crustal boundary. Finally, the CMB boundary zone is shown to form a sharp age boundary between juvenile and old crustal domains, and is interpreted as a reactivated rift-bounding normal fault. When the distribution of rift-related alkaline rocks is compared with these crustal boundaries, the Bancroft nepheline syenite suite is centrally located in a juvenile ensimatic zone between blocks of old basement. Such a location, near the axis of a juvenile crustal segment, implies emplacement late in the rifting process. Similarly, the Blue Mountain nepheline syenite appears to post-date an earlier rifting event to the southeast. Hence, a multi-stage model is proposed for the evolution of a back-arc aulacogen, which is consistent with the distribution of marble and volcanic/plutonic units in the CMB. The model places the Bancroft nepheline syenites in a precise plate tectonic context for the first time. [ABSTRACT FROM AUTHOR]

During the Proterozoic, several mafic dykes with variable trends and mineralogies intruded the Archaean basement of Southeast Greenland. Some of the younger ENE-trending dykes are interpreted to represent a prolongation of the Mesoproterozoic Gardar Province, and have been termed Timmiarmiit dykes. Extrapolations of their trends across the inland ice sheet coincide with the northernmost so-called brown dykes (BD’s) which are part of the Gardar Province. Baddeleyite U–Pb ID-TIMS analyses for three ENE-trending Timmiarmiit dykes give ages of 1277 ± 4, 1275 ± 3 and 1268 ± 4 Ma, which are slightly younger than the oldest (BD0 = 1284–1279 ± 3 Ma, Upton 2013) and only dated generation of dykes in the Gardar Province, and thereby indirectly provide a possible age for the two younger dyke generations (BD1and BD2). The Timmiarmiit–Gardar correlation is strengthened by a rigorous multivariate statistical analysis, on the basis of all major and trace elements. Thus, a coherent ENE-trending trans-Greenlandic dyke swarm is constituted. The major and trace element data of the Timmiarmiit dykes show that they crystallised from comparably evolved mantle-derived magma with minor crustal contamination and indicate a strong contribution of a metasomatised subcontinental lithospheric mantle component in the evolution of melts. This component was probably influenced by supra-subduction zone metasomatism during the Palaeoproterozoic Ketilidian orogeny. The data presented here, in addition to recent plate reconstruction models, give new evidence for a petrogenetic link between rift-related Mesoproterozoic magmatism in North America, South Greenland and Central Scandinavia which possibly formed in response to back-arc basin formation. [ABSTRACT FROM PUBLISHER]

Caledonian nappes of southern Norway comprise basement slices and sedimentary cover, and generally have close affinity with autochthonous terranes in the Fennoscandian foreland. The nappes can be distinguished in those having Gothian (1700–1600 Ma) character and those of Telemarkian (1550–1480 Ma) type. There were also several periods of variously intense magmatism between these early accretionary events and the end of the Sveconorwegian Orogeny. Most prominent features are the anorthositic-noritic massifs, which formed mainly during the Telemarkian period and in the late stages of the Sveconorwegian Orogeny between 980 and 920 Ma. In this paper, we review the main events affecting the crystalline parts of the nappes and report new results from the Eikefjord Nappe. An augen gneiss yields a crystallization age of 1191 ± 2 Ma, which is an event rarely seen in nappes or the autochthon. An amphibolitic gneiss formed at about 1500 Ma and underwent high-grade metamorphism at 986 ± 3 Ma, followed by lower grade overprints as reflected by titanite dates ranging to 915 Ma. The Sveconorwegian metamorphism was ubiquitous in the nappes, although its intensity was locally highly variable. The association of metamorphism with the second main anorthosite generation event points to intense subcrustal magmatism, heating and deformation of the lithosphere. The various crustal forming events have different geneses, some are clearly related to subduction processes, others more consistent with localized mantle melting. Those closer in character to classical Large Igneous Provinces are the events at 1470–1420, 1280–1240 and 1220–1180 Ma. [ABSTRACT FROM PUBLISHER]

Nearly 50 new Nd isotope analyses are presented for the Shawanaga region of Georgian Bay, Ontario, to study crustal evolution in the Grenvillian Central Gneiss Belt. Depleted mantle (TDM) Nd model ages are used to map a major Grenvillian tectonic boundary, the Allochthon Boundary Thrust (ABT), which in the Shawanaga area separates gneisses with TDM ages above and below 1.65 Ga. This is lower than the 1.8 Ga age cut-off observed further north, and is attributed to a southward increase in Mesoproterozoic magmatic reworking of an original Palaeoproterozoic continental margin, causing a progressive southward decrease in Nd model ages. Between Shawanaga Island and Franklin Island, Nd isotope mapping yields an ABT trajectory that closely matches published geological mapping, and passes within 100 m of four retrogressed eclogite bodies typically associated with the thrust boundary. This validation of the method gives confidence in the mapped trajectory south of Snake Island, where sparse outcrop inhibits lithological mapping. The new results suggest that published 1.7–1.9 Ga TDM ages in the Lower Go Home domain of the Central Gneiss Belt further south are also indicative of parautochthonous crust. Hence, we propose that the main ramp of the ABT is located in the immediate hangingwall of the Go Home domain, much further south than generally recognized. This has important implications for the large-scale crustal structure of the SW Grenville Province, suggesting that the ABT ramp has a similar curved trajectory to the Grenville Front and the Central Metasedimentary Belt boundary thrust. [ABSTRACT FROM PUBLISHER]

The anorthositic members of the Mealy Mountains Intrusive Suite (MMIS; Labrador, Canada) are host to 0·5-5m diameter pegmatitic, pod-like segregations, originally described as graphic granite pods. U-Pb zircon geochronology confirms that the pods are coeval with the 1650-1630Ma emplacement age range for the MMIS, yielding ages of 165468 to 1628635 Ma. Petrographic and geochemical analysis of five pods from anorthositic rocks of the MMIS reveals that the pods have a diverse compositional range from monzodiorite to granite, varying from Fe-rich and Si-poor, to Fepoor and Si-rich compositions. Fe-rich, Si-poor pods in the MMIS and other massifs (e.g. Laramie Anorthosite Complex) tend to be hosted by olivine-bearing anorthosites, whereas Si-rich, Fe-poor pods are hosted by pyroxene-bearing anorthosites. Each pod shows a range of graphic, myrmekitic and symplectitic textures, along with distinctive mineral assemblages (e.g. apatite and zircon) and highly enriched trace-element compositions. Evolved mineral assemblages, high concentrations of Fe, Ti and P (and in some cases SiO2), and 10-1000× chondrite enrichment in light rare earth elements, U, Th and Rb indicate that many of the pods are highly fractionated. The array of textural intergrowths provides clues about the final stages of crystallization in the pods and, by extension, the anorthosites. Macroscopic quartz-K-feldspar graphic intergrowths indicate high-viscosity, fluid-bearing and significantly undercooled magmatic conditions, whereas microscopic myrmekitic (plagioclase-quartz) and symplectitic (plagioclase-orthopyroxene) intergrowths on primary grain boundaries indicate replacement of phases in the presence of reactive fluids. In assessing the nature of these pegmatitic pods based on field, petrographic and geochemical evidence, we conclude that they represent the fluid-bearing, late-stage crystallization products of a residual liquid in the massif anorthosite system. The Fe and Si compositional variations observed in these late-stage pods can be linked to a fundamental olivine-pyroxene dichotomy observed in most Proterozoic anorthosite massifs, suggesting that pulses of magma experience variable contamination (in amount and/or composition) leading to varying differentiation paths. A range of lithologies (monzonites, monzonorites, ferrodiorites and jotunites) observed in similar pod-like structures, as well as dykes and plutons, has been observed in other Proterozoic anorthosite massifs and all have, at one time or another, been interpreted as the residual liquids of anorthosite crystallization. Our observation of in situ pods with similar compositions to all of the aforementioned lithologies, and displaying textures indicative of late-stage crystallization, supports the notion that all of these associated lithologies can be interpreted as comagmatic with, but variably contaminated and isolated residual liquids of, anorthosite crystallization. However, using isotopic evidence we cannot support the notion that the far larger granitic plutons associated with Proterozoic anorthosites are also residual liquids of anorthositic magma fractionation. [ABSTRACT FROM AUTHOR]

The Neoarchean and Paleoproterozoic granulite-gneiss complexes of the northern East European Craton, including their tectonic setting, accompanying igneous and sedimentary rocks, character of metamorphism and deep structure are considered in this paper. A similar approach was applied to the description and interpretation of the granulite-gneiss complexes of the Grenville-Sveconorwegian Orogen localized in the present-day structure of the European and North American continents. One of the most important results of this study is the recognition of a new type of tectonic structures called intracontinental oval orogens. These orogens were formed in the inner domain of the Neoarchean-Proterozoic supercontinent (2.8-0.85 Ga), which is called Lauroscandia. The extensional setting, which was initiated by a series of mantle plumes, locally passing to spreading but did not lead to the eventual breakup of the supercontinent and was followed by intracontinental collisional compression accompanied by closure of short-lived local oceans, subduction, and obduction. The Neoarchean and Proterozoic granulite-gneiss belts and areas of the East European Craton are components of the Karelian-Belomorian, Kola, Volgo-Uralia, Lauro-Russian, and Grenville-Sveconorwegian intracontinental oval orogens formed in the inner domain of Lauroscandia as a result of mantle-plume and plate-tectonic processes initiated by mantle superplumes. The geodynamic settings and tectonic processes in the Neoarchean-Proterozoic supercontinent differed markedly from both the Archean tectonics of miniplates and the Phanerozoic plate tectonics. [ABSTRACT FROM AUTHOR]

To assess the age and origin of the metasedimentary migmatites in southernmost Sweden and their relationships with the Mesoproterozoic granitoid magmatism in the area, we have dated migmatite zircon using the secondary ion mass spectrometry U–Pb method. The studied metasedimentary migmatites, here called the Nöteboda migmatites, occur along the southwestern boundary of the 1442 Ma Tåghusa granitoid intrusion in southeastern Skåne. They contain the mineral assemblage garnet+biotite ± muscovite+cordierite+sillimanite+quartz+plagioclase+K-feldspar and were formed during a retrograde evolution fromc. 750–720°C and 6 kbar (peak conditions) toc. 675°C and 4 kbar. Zircon is characterized by detrital cores surrounded by U-rich rims and overgrowths. Separate rounded metamorphic grains also exist. The age probability–density distribution peaks for detrital zircon are atc. 1700, 1670, 1650, 1610, 1570 and 1530 Ma. These ages suggest Gothian orogenic rocks in the present west as the most probable principal source. Sedimentation occurred afterc. 1530 Ma, the age of the youngest detrital zircon, indicating the existence of a previously unknown period of Mesoproterozoic sedimentation in southernmost Sweden. A homogeneous zircon overgrowth yielded a concordant207Pb/206Pb age of 1439 ± 5 Ma, which dates the migmatization and is close to the age of the Tåghusa intrusion. We conclude that the burial of the sediments down toc. 20 km, their metamorphism and progressive migmatization took place concurrently with granitic magmatism, NE–SW compression, folding and shearing of the crust between 1460 and 1440 Ma during the Danopolonian orogeny. [ABSTRACT FROM PUBLISHER]

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Endogenic events in the form of intrusive activity and regional metamorphism developed asynchronously in various parts in the Svecofennian Orogen of Fennoscandia. The Early and Late Svecofennian stages of regional high-temperature metamorphism and related plutonism are distinguished from isotopic evidence. The composition, structural features, and asynchronous peaks of endogenic activity within the orogen indicate that at least two zones (inner and outer) should be distinguished in the Svecofennides. The lateral heterogeneity of the orogen in present-day coordinates is traced southward from the margin of the Archean craton. The conjugation zone of the Svecofennian Orogen and the Archean Karelian Craton is characterized by transition from negative to positive εNd (1.9 Ga) values as evidence for a decreased contribution of Archean crustal material to the source of Proterozoic granitoids from the north toward the Proterozoic domain in the south. With allowance for lateral compositional and isotopic heterogeneity of the Svecofennian Orogen and asynchronous culmination of endogenic events in different parts of this orogen, a new scheme of tectonic regionalization has been proposed. [ABSTRACT FROM AUTHOR]

The Neoproterozoic Hedmark Group (Lower Allochthon), Valdres Group (Middle Allochthon) and Engerdalen Group (Middle Allochthon) contain allochthonous basement slices with tectonic and depositional contacts to sedimentary rocks. The basement slices were transported from the western margin of the continent Baltica during the Caledonian Orogeny. Zircon U-Pb and Lu-Hf isotopes reveal that allochthonous basement rocks in the Osen-Roa Nappe Complex, forming the depositional substrate of the Hedmark Group, are related to the Transscandinavian Igneous Belt (TIB) in terms of age and petrogenetic history. The Kvitvola Nappe Complex of the Middle Allochthon, carrying the sedimentary Engerdalen Group, contains c.1620 Ma and 1180 Ma augen gneisses. The oldest augen gneiss has TIB-like Hf characteristics, while the younger gneiss is more juvenile. These basement rocks may originally have been separated by a major lithotectonic boundary. A sample from the basement slice, located stratigraphically beneath the Valdres Group in the Valdres Nappe Complex, is dated at c.1480 Ma and also has juvenile Hf composition. The apparent younging of basement ages from Lower to Middle Allochthon suggests age zonation at the SW Baltican margin. The juvenile 1180 Ma and 1480 Ma samples are probably derived from the same crust as what is today known as SW Norway or "Telemarkia". Crustal rocks of present-day SW Norway must have continued further to the northwest (according to present geography) and were probably bordered by TIB rocks in the NE. We also present new U-Pb and Lu-Hf data from the autochthonous basement in front of and tectonostratigraphically beneath the Osen-Roa Nappe Complex, sampled along a transect from the Romerike Complex across the Mylonite Zone to Lake Storsjoen. Hf isotopes show the juvenile character of the gneisses on the western side of the Mylonite Zone and typical TIB 3 compositions on the eastern side. Hafnium isotopes from the Hustad Igneous Complex in the Western Gneiss Region (WGR) show strong similarity with the TIB, which supports a genetic connection between the TIB and WGR. Hafnium isotopes from the late Sveconorwegian Fla granite show crustal contamination, which supports models where the TIB continues underneath the Gothian domain. [ABSTRACT FROM AUTHOR]

The article reports on the abundance deposit of colored gemstones in Canada. It is said that high-quality transparent sapphire has never been found in Canada, but in 2002 the gem was found in Baffin Island, Nunavut. In eastern Ontario, it is noted that opaque is common at several localities. It is cited that star sapphire is abundant in many localities in Passmore, south-central British Columbia.

The presence of major crystalline basement provinces at depth in NW Ireland is inferred from in situ Hf isotope analysis of zircons from granitoid rocks that cut structurally overlying metasedimentary rocks. Granitoids in two of these units, the Slishwood Division and the Tyrone Central Inlier, contain complex zircons with core and rim structures. In both cases, cores have average ϵHf values that differ from the average ϵHf values of the rims at 470 Ma (the time of granitoid intrusion). The Hf data and similarity in U-Pb age between the inherited cores and detrital zircons from the host metasedimentary rocks suggests local contamination during intrusion rather than transport of the grains from the source region at depth. Rims from the Slishwood Division intrusions have average ϵHf470 values of -7.7, consistent with a derivation from juvenile Palaeoproterozoic crust, such as the Annagh Gneiss Complex or Rhinns Complex of NW Ireland, implying that the deep crust underlying the Slishwood Division is made of similarmaterial. Rims from the Tyrone Central Inlier have extremely negative ϵHf470 values of approximately -39. This isotopic signature requires an Archaean source, suggesting rocks similar to the Lewisian Complex of Scotland, or sediment derived wholly from it, occurs at depth in NW Ireland. [ABSTRACT FROM AUTHOR]

A compilation of literature concerning U-Pb geochronological data recording magmatic events is presented for the Sveconorwegian and Caledonian belts of Baltica. It illustrates the episodic nature of magmatism along the western margin of Baltica from Archaean to Permian and helps identify primary zircon sources for detrital zircon provenance studies in North Atlantic regions. Archaean orthogneisses ranging from 2885 to 2635 Ma occur in basement windows in the northern Caledonides. Voluminous Palaeoproterozoic, mainly felsic, magmatism is regarded as part of the Transcandinavian Igneous Belt: it is dated at 1805-1770 Ma in the Nordland, Lofoten and Rombak windows, 1686-1621 Ma in the West Norway Gneiss Complex, and mainly 1797-1640 Ma in the Eastern Segment of the Sveconorwegian belt. As a first approximation, crystalline sheets in the Middle Allochthon mirror basement windows at equivalent latitudes. Voluminous Mesoproterozoic magmatism ranges from 1659 to 1517 Ma in the Idefjorden Terrane, 1572 to 1460 Ma in the Bamble and Kongsberg Terranes and 1555 to 1459 Ma in the Telemarkia Terrane. The Sveconorwegian belt hosts a variety of local to regional magmatic suites covering the entire time spans between 1473 and 1130 Ma and between 1060 and 914 Ma. Sveconorwegian post-collisional suites between 971 and 914 Ma are voluminous in the west of the belt and include anorthosites. Pre-Sveconorwegian (1257-1237 and 1190 Ma) and Sveconorwegian (976 to 891 Ma) intrusions are common in the southern part of the Caledonides, while, in the Northern Caledonides, minor 981-973 Ma granite plutonism is reported in the Kalak Nappes. The Caledonian and Sveconorwegian belts are poor in Cryogenian to Ediacaran magmatism, the only significant exceptions being the Seve and Kalak Nappes showing low volume suites at 876-825 Ma and 711-680 Ma and the 571-523 Ma Seiland Igneous Province. Cambro-Silurian magmatism ranging from 497 to 424 Ma and including Iapetan ophiolites and volcanic arcs is specific for the Upper and Uppermost Allochthons of the Caledonides. Scandian to late-Scandian pegmatites and minor intrusions range from 436 to 390 Ma in the basement windows and the Middle Allochthon. Dated magmatism in the Oslo rift ranges from 300 to 277 Ma. The compilation highlights the Mesoproterozoic signature of the Sveconorwegian belt. Erosion of primary magmatic sources in the Sveconorwegian belt can generate a whole range of Palaeoproterozoic to Mesoproterozoic detrital zircons ranging from ca. 1800 to 900 Ma, with only one clear gap between 1130 and 1060 Ma corresponding to the onset of the Sveconorwegian orogeny. Erosion of magmatic rocks in the Caledonides can produce a discontinuous distribution of detrital zircons characterized by a number of discrete modes ranging from Archaean to Devonian, and large gaps in the early Palaeoproterozoic (2500-1940 Ma), and in the Mesoproterozoic and Neoproterozoic. Primary magmatic source rocks contribute only marginally to the present day discharge from the Caledonides. This implies, not unsurprisingly, major sourcing in metasediments exposed in Caledonian allochthons and widespread sediment polycyclism. [ABSTRACT FROM AUTHOR]

Rapakivi granites characteristic practically of all old platforms are greatly variable in age and irregularly distributed over the globe. Four types of magmatic associations, which include rapakivi granites, are represented by anorthosite-mangerite-charnockite-rapakivi granite, anorthosite-mangerite-rapakivi-peralkaline granite, gabbro-rapakivi granite-foidite, and rapakivi granite-shoshonite rock series. Granitoids of these associations used to be divided into the following three groups: (1) classical rapakivi granites from magmatic associations of the first three types, which correspond to subalkaline high-K and high-Fe reduced A2-type granites exemplifying the plumasitic trend of evolution; (2) peralkaline granites of the second magmatic association representing the highly differentiated A1-type reduced granites of Na-series, which are extremely enriched in incompatible elements and show the agpaitic trend of evolution; and (3) subalkaline oxidized granites of the fourth magmatic association ranging in composition from potassic A2-type granites to S-granites. Magmatic complexes including rapakivi granites originated during the geochronological interval that spanned three supercontinental cycles 2.7−1.8, 1.8−1.0 and 1.0−0.55 Ga ago. The onset and end of each cycle constrained the assembly periods of supercontinents and the formation epochs of predominantly anorthosite-charnockite complexes of the anorthosite-mangerite-charnockite-rapakivi granite magmatic association. Peak of the respective magmatism at the time of Grenvillian Orogeny signified the transition from the tectonics of small lithospheric plates to the subsequent plate tectonics of the current type. The outburst of rapakivi granite magmatism was typical of the second cycle exclusively. The anorthosite-mangerite-charnockite-rapakivi granite magmatic series associated with this magmatism originated in back-arc settings, if we consider the latter in a broad sense as corresponding to the rear parts of peripheral orogens whose evolution lasted from ∼1.9 to 1.0 Ga. Magmatism of this kind was most active 1.8−1.3 Ga ago and represented the distal effect of subduction or collisional events along the convergent boundaries of lithospheric plates. An important factor that favored the emplacement of rapakivi granites and anorthosites in a huge volume was the thermal and rheologic state of the lithosphere inherited from antedating orogenic events, first of all from the event ∼1.9 Ga ago, which was unique in terms of heat capacity transferred into the lithosphere. Anorthosite-mangerite-rapakivi granite-peralkaline granite magmatism is connected with activity of the mantle plums only. Degradation of the rapakivi granite magmatism toward the terminal Proterozoic was controlled by the general cooling of the Earth in the course of the steady dissipation of its endogenic energy, as these processes became accelerated since the Late Riphean [ABSTRACT FROM AUTHOR]

The principal results of project 440 “Assembly and Breakup of Rodinia” of the International Geological Correlation Programme (IGCP) are reviewed in this work. A map of that supercontinent compiled using geological and paleomagnetic data describes global paleogeography 900 Ma ago. The assembly of Rodinia, which comprised most of Precambrian continental blocks, lasted ca. 400 m.y. (from 1300 to 900 Ma). Its breakup presumably triggered by mantle superplume took place between 830 and 650 Ma. The correlation between tectonic events in different continental blocks is considered. Some problems concerning the Rodinia reconstruction and history, e.g., the slow growth of juvenile crust and effects of mantle-plume events during the amalgamation period and of glaciations at the breakup time, are discussed. The latter caused changes in the biosphere and climate, whereas postglacial periods stimulated progress in biota evolution. [ABSTRACT FROM AUTHOR]

Evolution of a metamorphic fold-thrust belt in Archean and Proterozoic rocks in the Gagnon terrane, northwestern Grenville province, involved underthrusting beneath a crustal-scale, orogenic wedge and metamorphism ranging from upper greenschist facies (∼600 ± 100 MPa/450 ± 50°C) to upper amphibolite facies (∼1100 ± 100 MPa/750 ± 50°C) during the terminal stages of the Grenvillian Orogeny. Structural and metamorphic data indicate that D1 structures formed at peak pressures during basal accretion to the overriding orogenic wedge, whereas D2 structures developed within the wedge during its displacement toward the foreland in a sinistral transpressive setting. The fold-thrust belt developed sequentially on two levels: a thin-skinned, cover-dominated thrust system preceded and overlies a thick-skinned, basement-dominated system. The D3 cross-folds postdated normal faulting at the top of the wedge and formed during gravitational collapse. On the basis of this study, characteristic features of midcrustal, metamorphic, thrust wedges include growth by basal accretion, polyphase ductile deformation and noncoaxial strain, dual-level development, extensive basement involvement, and terminal gravitational collapse associated with cross-folding. [ABSTRACT FROM AUTHOR]

We present results from numerical models for a convergent orogen with laterally variable lower crustal strength, representing a simplified orogenic system in which a strong craton, flanked by progressively weaker terranes, collides with another continent. With progressive convergence, crustal thickening, and thermal relaxation, lower crust becomes decoupled from upper and middle crust, forming a ductile orogenic infrastructure beneath a stronger superstructure. Collision with strong external crust results in uplift and expulsion of ductile nappes from the orogenic core, creating allochthonous terranes overlying a lower crustal indentor. The extent of transport and exhumation of lower crustal nappes over the indentor reflects the amount of convergence and the erosion rate. The western Grenville orogen displays across-strike variations in age, tectonic history, and protolith association, suggesting a systematic variation in precollision crustal strength. The Laurentian craton, margin, and accreted terranes were variably reworked at synorogenic depths of 25-35 km during the Ottawan orogeny. Deformation propagated from younger monocyclic rocks in the southeast into older polycyclic rocks flanking the craton on the northwest. A comparison between numerical model results and crustal-scale cross sections from the Georgian Bay and Montreal-Val d'Or transects shows close correspondence between crustal structure and model geometry. This indicates that the models produce geologically realistic results and provides a context for interpreting the tectonic evolution of the western Grenville orogen. Contrasts between the results of homogeneous channel flow models and the present ductile nappe models suggest that the effects of different styles of ductile flow can be distinguished in the geologic record. [ABSTRACT FROM AUTHOR]

The Hengshan-Wutai-Fuping belt is located in the middle segment of the Trans-North China Orogen, a Palaeoproterozoic continental collisional belt along which the Eastern and Western blocks amalgamated to form the North China Craton. The belt consists of the medium- to high-grade Hengshan and Fuping gneiss complexes and the intervening low- to medium-grade Wutai granite-greenstone terrane, and most igneous rocks in the belt are calc-alkaline and have affinities to magmatic arcs. Previous tectonic models assumed that the Hengshan and Fuping gneiss assemblages were an older basement to the Wutai supracrustal rocks, but recent studies indicate that the three complexes constitute a single, long-lived Neoarchaean to Palaeoproterozoic magmatic arc where the Wutai Complex represents an upper crustal domain, whereas the Hengshan and Fuping gneisses represent the lower crustal components forming the root of the arc. The earliest arc-related magmatism in the belt occurred at 2560-2520 Ma, marked by the emplacement of the Wutai granitoids, which was followed by arc volcanism at 2530-2515 Ma, forming the Wutai greenstones. Extension driven by widespread arc volcanism led to the development of a back-arc basin or a marginal sea, which divided the belt into the Hengshan-Wutai island arc (Japan-type) and the Fuping relict arc. At 2520-2480 Ma, subduction beneath the Hengshan-Wutai island arc caused partial melting of the lower crust to form the Hengshan tonalitic-trondhjemitic-granodioritic (TTG) suites, whereas eastward-directed subduction of the marginal sea led to the reactivation of the Fuping relict arc, where the Fuping tonalitic-trondhjemitic-granodioritic suite was emplaced. In the period 2360-2000 Ma, sporadic phases of isolated granitoid magmatism occurred in the Hengshan-Wutai-Fuping region, forming 2360 Ma, c. 2250 Ma and 2000-2100 Ma granitoids in the Hengshan Complex, the c. 2100 Ma Wangjiahui and Dawaliang granites in the Wutai Complex, and the 2100-2000 Ma Nanying granitoids in the Fuping Complex. At c. 1920 Ma, the Hengshan-Wutai island arc underwent an extensional event, possibly due to the subduction of an oceanic ridge, leading to the emplacement of pre-tectonic gabbroic dykes that were subsequently metamorphosed, together with their host rocks, to form medium- to high-pressure granulites. At 1880-1820 Ma, the Hengshan-Wutai-Fuping arc system was juxtaposed, intensely deformed and metamorphosed during a major and regionally extensive orogenic event, the Lüliang Orogeny, which generated the Trans-North China Orogen through collision of the Eastern and Western blocks. The Hengshan-Wutai-Fuping belt was finally stabilized after emplacement of a mafic dyke swarm at 1780-1750 Ma. [ABSTRACT FROM AUTHOR]

Granitic dikes intruding the upper thrust sheets of the Jotun Nappe Complex, southwest Norway, previously assigned a Sveconorwegian age based on Rb-Sr data, are redated to 427 ± 1 Ma by U-Pb. The erroneous Rb-Sr age can be explained by mixing of magmas with different isotopic compositions. Similar geochemical signatures, evidence of Sr mixing, inherited Proterozoic zircons and synkinematic modes of emplacement are also seen in Silurian granites in the Lifjorden Complex as well as in the Lindås Nappe and the Totland unit of the Bergen Arc Complex. We propose that these granites are related and derived from anatexis of Rb-poor sediments of Baltica affinity overridden by the nappes during Scandian thrusting. This interpretation is compatible with the presence of the granites in nappes of both oceanic and of continental affinities and their absence in the autochthonous basement. [ABSTRACT FROM AUTHOR]

Evolution of tectono-magmatic processes in the Paleoproterozoic is divisible into four stages each controlled by its own type of endogenic activity. The critical moment in geological history was at the third stage onset 2.05 Ga ago, when the Archean style of evolution gave way to subsequent one. Magmas of siliceous high-Mg (boninite-like) series (SHMS) dominant during the first stage (2.5–2.3 Ga) had the mixed mantle-crust genesis. They originated in highly depleted ultramafic reservoir of asthenospheric mantle being contaminated by the Archean crustal material during ascent to the surface. Typical of the SHMS magmas were the high content of silica, Al, Mg, Cr and elevated concentrations of Ni, Co, Cu, V, PGE and incompatible elements, LREE included, while concentrations of Fe, Ti, Nb and alkalies, especially K, were at a relatively low level. Magmatism of K-granitoid type was of a limited significance. The second stage, especially its second substage (2.3–2.05 Ga), was distinct because of mass eruptions of picritic and basaltic magmas enriched in Fe, Ti, Mn, P and incompatible elements, LREE in particular, which also had elevated Cr, Ni, Co, Cu and Ba concentration being relatively depleted in Mg and Al. This change in geochemistry of magmatism was independent of tectonic processes, which retained the former style. The third stage (2.05–1.8 Ga) was marked by opening of first oceans (Jormua, Purtuniq and other ophiolites) and by formation of orogens comparable with Phanerozoic orogens that was associated with development of subduction zones and back-arc basins with relevant magmatism. High Mg, Fe, Ti, Cr, Ni, Cu, V but low Th concentrations were typical of intense picrite-basaltic volcanism marking onset of the third stage. Magmas of suprasubduction genesis had considerably higher Si, Al, P, Zn, Th concentrations and very low CaO/Al2O3 ratios. Development of orogens came to the end 1.82–1.80 Ga ago. Appearance of giant belts of intraplate, usually silicic high-K volcanism juxtaposed on stabilized Paleoproterozoic orogens with abnormally thick crust was confined to the fourth stage spanning besides the initial Mesoproterozoic (1.8–1.5 Ga). Anorthosite-rapakivi granitoid batholiths, which originated above mantle plume heads, corresponded to intermediate magma chambers of relevant magmatic systems. As is suggested, juvenile basaltic melts of this stage retained within the crust provoking a large-scale melting of sialic material. The high K, Ti, Zn, Pb, Zr and elevated Be, Sn, Y, Nb, Rb, F, W, Mo, Li and U contents characterized geochemistry of magmas, which originated at the fourth stage [ABSTRACT FROM AUTHOR]

The paper is devoted to the history of the discovery, petrological and mineralogical characterization, and U-Pb dating of baddeleyite ZrO2, which was separated from various Archean, Proterozoic, and Paleozoic rocks from Fennoscandia. The morphology of this mineral and its U-Pb age values were examined in the Archean carbonatites (2613 ± 18 Ma) of Siilinjarvi, Finland, and gabbronorite dikes (2738 ± 6 Ma) at the Kirovogorskoe deposit. U-Pb isochrons are reported for the baddeleyite-zircon pair obtained from the gabbronorites and anorthosites of the Proterozoic pyroxenite-gabbronorite-anorthosite association. The U-Pb baddeleyite dates for the early gabbronorite phase (2.5 Ga) and for a gabbronorite dike (late phase, 2.4 Ga) suggest that the basic magmatism evolved over a long time period (100 m.y.) in the Proterozoic. U-Pb dates are also reported for baddeleyite from the Paleozoic carbonatites of Kovdor, Sebljarv, and Vuorijarvi. [ABSTRACT FROM AUTHOR]

The article presents information on the Calcite vein-Dikes of the Grenville Geological Province in Ontario. Calcite vein-Dikes, which are composed primarily of calcite, have been significantly mobile at some point of their formation, due to their contact with the wall rock. The province features a number of mineral mines, including the Dwyer mines and the McLaren mines.

The Nova Brasilândia metasedimentary belt (NBMB) of western Brazil marks a fundamental crustal boundary in the Amazon craton. The metasedimentary rocks of the NBMB (calc-silicates, metapelites, quartzites, metabasites) contrast strongly with the older, polycyclic granitoid rocks of the adjacent Amazon craton. Aeromagnetic anomalies indicate that the belt is continuous for at least 1000 km in an E-W direction, although the easternmost extent of the NBMB is covered by the Cretaceous sediments of the Parecis Formation. Additional geologic evidence suggests that the belt extends along an E-W trend for ∼2000 km. The northern portion of the NBMB preserves vestiges of an early high pressure-temperature (P-T) assemblage (kyanite + staurolite) overprinted by sillimanite during prograde metamorphism. A higher metamorphic grade is observed in the southern portion of the belt, with peak conditions calculated to be 800 MPa and 800°C for granulitic assemblages. The combined P-T path demonstrates that the competing processes of imbrication (northern domain) and magma generation (southern domain) are responsible for regional metamorphism and crustal thickening. Cooling from peak metamorphic conditions is recorded by U-Pb monazite ages of 1090 Ma and titanite ages of ∼1060 Ma. Integrated cooling rates of 2°-3°C/Myr from regional metamorphism are calculated from these U/Pb ages combined with 40Ar/39Ar ages of hornblende (∼970 Ma) and biotite (∼910 Ma). The NBMB marks the Mesoproterozoic limit of the SW Amazon craton. The discordance of the NBMB to the NNW structural trend of the younger Aguapeí belt (200 km SE of NBMB), together with marked differences between the two belts in sedimentary environment, metamorphic grade, and timing of deformation, signify that these two belts are not geologically continuous. The 'Grenvillian' deformation recorded by the NBMB belt marks the final docking of the Amazon craton and Paragua craton within the Rodinia framework. The Aguapeí belt, in contrast, seems to record only limited deformation internal to the Paragua craton. [ABSTRACT FROM AUTHOR]

The Åker metabasite occupies a key position in a major tectonic lineament in southernmost Sweden, the Protogine Zone, which coincides closely with the eastern boundary of the late Mesoproterozoic Sveconorwegian orogen of southwest Scandinavia. Metamorphic reactions, associated with the transformation from isotropic gabbro to foliated garnet amphibolite, were identified from disequilibrium textures of which some involved release of zirconium (Zr) and growth of metamorphic zircon. Ion microprobe dating of igneous zircon gave 1562 ± 6 Ma, whereas metamorphic zircons yielded ages of 1437 ± 21, 1217 ± 75, and 1006 ± 68 Ma. The presence of baddeleyite pseudomorphs made up of saccharoidal zircon and a higher abundance of older rather than younger metamorphic zircons suggest redistribution of Zr into new zircon, first by the breakdown of baddeleyite (ZrO2) and later by the consumption of igneous phases containing trace amounts of Zr. Several generations of metamorphic zircon and the presence of 1.56 and 1.22 Ga mafic intrusions along the Protogine Zone call for a complex tectonic history probably reaching back to at least ∼1.56 Ga. Growth of metamorphic zircon at ∼1.44 Ga may relate to a regional, compressional event. The WNW trending deformational structures on both sides of the Protogine Zone may possibly relate to that event. The ∼1.22 Ga metamorphic zircons are coeval with the emplacement of numerous granitic, syenitic, and mafic intrusions along and parallel to the Protogine Zone. The age around 1.0 Ga, finally, marks Sveconorwegian metamorphism for which thermobarometry of the Åker garnet-amphibolite suggests 1000-1200 MPa at 600°C-630°C. Thereafter, significant relative uplift of the rocks to the west of the Protogine Zone occurred on nearly vertical, north-south trending deformation zones. [ABSTRACT FROM AUTHOR]

Well-preserved eclogite-facies structural features in metamorphosed gabbro and diorite gneiss in Baltica crust, formed at a minimum depth of 60-70 km during the Scandian orogeny, allow direct inferences to be made regarding the geometry of folds, kinematics, and original structural orientations related to production and exhumation of high-pressure rocks. Folds associated with eclogite-facies fabrics are isoclinal to tubular with axes parallel to the trend of a stretching lineation. Strain estimates and L > S or L ≫ S fabrics indicate that these structures were formed in a constrictional strain field. Locally, they are well preserved in eclogite-facies mylonite zones at least 40 m thick that cut Proterozoic gabbro and adjacent gneiss. Where steeply dipping, they show a north-side-up shear sense across a younger, northeast trending, shallowly plunging, amphibolite-facies anticlinorium formed in a constrictional, noncoaxial strain field. Where original structural facing direction can be inferred, small-circle rotation of the eclogite-facies lineation about the anticline axis indicates that the relative motion vector of Baltica with respect to overlying imbricated crust and Laurentia was oriented 320° in a present-day reference frame. This result is identical to the orientation of the relative motion vector estimated from paleomagnetic plate reconstructions and consistent with Late Silurian to Early Devonian oblique sinistral convergence between Baltica and Laurentia. Results of this study indicate that coherent Baltica crust at 60-70 km depth experienced subvertical and horizontal shortening while extending toward the foreland during tops-southeast thrusting parallel to plate motion. High-pressure rocks were progressively imbricated and stacked along eclogite-facies mylonite zones, with the deforming zone separating coherent Baltica crust from imbricated basement above. The constrictional strains under eclogite-facies conditions are interpreted to have resulted from stretching associated with sinking of the cool dense mantle lithosphere. [ABSTRACT FROM AUTHOR]

Commercial seismic reflection profiles from SW Scandinavia image Sveconorwegian orogenic structures in the late Precambrian basement beneath the thin Phanerozoic cover. We interpret (1) pronounced west to WNW dipping reflections as Sveconorwegian localized ductile shear which overprint; (2) weaker east dipping reflections, interpreted as Gothian or early Sveconorwegian compressional structures. Folded and laminar seismic fabrics may represent F and C type crystalline thrust sheets emplaced during the Gothian and Sveconorwegian events. Some of the dipping reflections are observed to sole into (3) an interval of subhorizontal reflections at 2.5-4.5 s two-way travel time, which we interpret as a 'fossil' ductile to brittle transition that was unroofed during the Gothian Orogeny. (4) NW-SE striking thrusts cutting the Sveconorwegian structures may indicate a shift in the late Sveconorwegian stress field. They may represent an early stage in the subsequent Phanerozoic basin evolution in the Danish area. [ABSTRACT FROM AUTHOR]

Delamination of the lower crust and lithospheric mantle has occurred in several convergent orogens, most notably the Alps and Pyrenees. Some workers suggest that the process is mechanically similar to tectonic wedging and triangle-zone development, and dependent on the pattern of mechanical layering. A search of over 35 orogens that have been explored by geophysical techniques showed that no correlation exists between crustal composition and delamination. Delamination of passive subducting plate margins is more common than that of tectonically and/or volcanically active overriding margins. Estimates show that the lithospheric mantle of cool passive margins may contain greater net negatively buoyant mass than warmer active margins. The top of the lower lithospheric layer containing the greatest net negatively buoyant mass is aligned with a weak zone near the Moho. Passive margin lithosphere thus has a greater tendency to delaminate during a collisional event than active margin lithosphere. For delamination to initiate above the Moho, the lower crust must be subducted or depressed to allow for exposure to eclogite grade conditions. Transformation of the mafic lower crust to mafic eclogite also creates a likely delamination horizon above a lower lithospheric layer containing net negatively buoyant mass aligned with a weak zone in the lithosphere. In contrast to the stratigraphic delamination that is key to the development of small-scale wedge structures, syncollisional delamination is likely to occur in response to an increase in the lateral tectonic stress on a weak horizon already stressed by net negatively buoyant mass in the lower lithosphere. [ABSTRACT FROM AUTHOR]

Geologic mapping and structural analysis of Baltica basement and overlying thrust nappes have yielded a record of the late exhumation history of high-pressure rocks, where strain partitioning has preserved evidence for interpreting the evolution of late structural features. The earliest of these were extensional detachments juxtaposing eclogite-facies rocks against overlying amphibolite-facies rocks that show no evidence for eclogite-facies metamorphism. These early detachments are strongly overprinted and complexly folded, and they represent a phase of upper crustal extension that was active during continued convergence at deeper levels. Younger more localized mylonite zones formed synchronously with tubular, sheath, isoclinal, tight and open folding that shows a progression from WNW to ENE trends. The earliest mylonite zones, interpreted as originally subhorizontal, range in strike through a 20° angle from 110° to 90°. Later steeply dipping mylonite zones, formed under lower amphibolite-facies conditions, strike 75° and locally truncate earlier structures. The youngest mylonite zones, formed under lowest amphibolite conditions, strike 50° and truncate all earlier structures. Folds developed during this progression show the range in orientation from WNW to ENE reflected in the orientations of the mylonite zones that is interpreted to represent progressive evolution during top west shearing. These changes in orientation of the late structural features are interpreted to have been caused by changes in boundary conditions related to transtensional deformation during exhumation. L > S fabrics, absence of axial planar foliation, and chaotic orientations of axial surfaces of granulite to amphibolite-facies folds indicate formation in a constrictional strain field. This is also supported by estimates of the finite strain accumulated at ∼780°C and 45 km and similar observations by previous workers. Assuming a simple monoclinic deformation for transtension, strain estimates and structural measurements indicate apparent transtensional angles of 9-11° that increased to greater than 20° and then decreased to less than 20°. These changes in transtensional angle agree with changes in the orientation of the X-Y plane of strain during exhumation from 45 km to less than ∼20 km depth and appear to reflect changes in the boundary conditions of deformation. This upper crustal transtension is interpreted to have occurred during continued sinistral oblique convergence and provides a mechanism for syncollisional exhumation of HP and UHP metamorphic rocks. [ABSTRACT FROM AUTHOR]

Presents several articles which tackles broader questions related to the formation of different sediment-hosted deposits. Determination of the composition of seawater in the Palaeoproterozoic; Implications of thermo chemical sulphate reduction reaction kinetics on genesis of massive sulphide deposits in mid-ocean ridge environments.