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Michael B. Stephens - One of the best experts on this subject based on the ideXlab platform.
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biotite and muscovite 40ar 39ar geochronological constraints on the post Svecofennian tectonothermal evolution forsmark site central sweden
International Journal of Earth Sciences, 2009Co-Authors: Pia Soderlund, Tobias Hermansson, Laurence Page, Michael B. StephensAbstract:In order to characterize the post-Svecofennian tectonothermal evolution of the Fennoscandian Shield, 40Ar-39Ar biotite and some 40Ar-39Ar muscovite geochronological data are reported from a total of 30 surface outcrop and 1000 m long borehole samples at Forsmark, central Sweden. The thirteen surface samples were collected across three branches of a major WNW to NW trending system of deformation zones, whereas the boreholes were drilled within a tectonic lens, in between two of these zones. The 40Ar-39Ar biotite ages indicate that the present erosion surface, in central Sweden, cooled below 300 °C at 1.73-1.66 Ga, and that the rocks could respond to stress by brittle deformation between 1.8 and 1.7 Ga. The variation in surface ages is suggested to be due to faulting along the large WNW to NW trending deformation zones, following the establishment of a sub-Cambrian peneplain. The minor variation of ages within a single crustal block may be due to disturbance along ENE to NNE trending fracture zones. Possible cooling paths, derived from 40Ar-39Ar hornblende, muscovite and biotite ages, were calculated for the time interval from 1.80 to 1.66 Ga, when the area cooled from 500 °C to 300 °C. Between 1.70 and 1.64 Ga, uplift rates of c. 22 m/m.y. were calculated from borehole 40Ar-39Ar biotite data. Tectonothermal histories, inferred from the combined cooling and uplift rates, are related to simple slow cooling after the Svecofennian Orogeny, to crustal movement in response to far-field effects of c. 1.7 Ga orogenic activity further to the west or to a combination of these possibilities. (Less)
Pia Soderlund - One of the best experts on this subject based on the ideXlab platform.
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biotite and muscovite 40ar 39ar geochronological constraints on the post Svecofennian tectonothermal evolution forsmark site central sweden
International Journal of Earth Sciences, 2009Co-Authors: Pia Soderlund, Tobias Hermansson, Laurence Page, Michael B. StephensAbstract:In order to characterize the post-Svecofennian tectonothermal evolution of the Fennoscandian Shield, 40Ar-39Ar biotite and some 40Ar-39Ar muscovite geochronological data are reported from a total of 30 surface outcrop and 1000 m long borehole samples at Forsmark, central Sweden. The thirteen surface samples were collected across three branches of a major WNW to NW trending system of deformation zones, whereas the boreholes were drilled within a tectonic lens, in between two of these zones. The 40Ar-39Ar biotite ages indicate that the present erosion surface, in central Sweden, cooled below 300 °C at 1.73-1.66 Ga, and that the rocks could respond to stress by brittle deformation between 1.8 and 1.7 Ga. The variation in surface ages is suggested to be due to faulting along the large WNW to NW trending deformation zones, following the establishment of a sub-Cambrian peneplain. The minor variation of ages within a single crustal block may be due to disturbance along ENE to NNE trending fracture zones. Possible cooling paths, derived from 40Ar-39Ar hornblende, muscovite and biotite ages, were calculated for the time interval from 1.80 to 1.66 Ga, when the area cooled from 500 °C to 300 °C. Between 1.70 and 1.64 Ga, uplift rates of c. 22 m/m.y. were calculated from borehole 40Ar-39Ar biotite data. Tectonothermal histories, inferred from the combined cooling and uplift rates, are related to simple slow cooling after the Svecofennian Orogeny, to crustal movement in response to far-field effects of c. 1.7 Ga orogenic activity further to the west or to a combination of these possibilities. (Less)
Pekka Heikkinen - One of the best experts on this subject based on the ideXlab platform.
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the accretionary Svecofennian orogen insight from the babel profiles
Precambrian Research, 2005Co-Authors: A Korja, Pekka HeikkinenAbstract:Abstract The BABEL profiles B, C, 1–7 form a 1200 km long nearly continuous cross-section through the Svecofennides. The near-vertical marine reflection profiles display a wide range of crustal structures that can be associated with both the accretionary Svecofennian Orogeny (1.96–1.75 Ga) and the following Subjotnian and Jotnian rift-stages (1.65–1.11 Ga). The Svecofennian accretionary Orogeny took place when a number of micro-plates with island arc affinities and surface expression of a large archipelago accreted to the continental Karelian plate. Some of the accreting terranes seem to have had older cores that have acted as crustal indentors during the collision. BABEL profiles 3 and 4 image a series of collisional terrane boundaries between Karelian continental margin, Savo arc (SA) and Central-Finland arc (CFA). In the north, the Karelian margin has been both over- and underthrust by the Savo arc. CFA comprises a folded thrust package on top of a continental nucleus and the Savo arc. The associated subduction zone and accretionary prism are interpreted to lie to the south, underneath the Bothnian basin area, where prominent NE-dipping, lower to middle crustal reflections are found along BABEL profiles 1 and 4. An oblique collision of the Central Finland arc and the continent resulted in the development of the strike-slip fault on the young, hot Savo arc. BABEL profiles 1, 6, 7 and C image the internal architecture of the Southern and Central Finland arcs. The unusually thick crust (55–60 km) hosts unreflective, high density, mafic intrusions acting as a crustal indentor. A highly reflective antiform structure developed on the southern side of it. Southern Finland arc complex (SFA) is an imbrication structure comprising stacked slices of arc-related crust on an older continental nucleus, Bergslagen nucleus. Prior to the collision, the SFA suffered from gravitational collapse during which the crust was thinned. Profile B images the architecture of the Central and Southern Swedish Svecofennides. The Sormland terrane (SoB) is interpreted as the accretionary prism of the Southern Finland arc squeezed between the Svecofennian collage in the north and the advancing continent to the south. The southern continent/island arc is characterised by NE dipping crustal reflections and Moho offsets as well as step-wise increasing thickness of the crust. After the final collision, large volumes of mantle-derived material intruded the crust as large mushroom-shaped plutons. They are interpreted as the heat source for the TIB magmatism in southern and western Scandinavia.
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seismic and geoelectric evidence for collisional and extensional events in the fennoscandian shield implications for precambrian crustal evolution
Tectonophysics, 1993Co-Authors: A Korja, Toivo Korja, U Luosto, Pekka HeikkinenAbstract:Extensive deep seismic and electromagnetic sounding programs have resulted in tens of deep refraction seismic profiles, normal-incidence reflection profiles, several hundred magnetotelluric soundings, and several magnetovariational array observations. Results are compiled into four maps: (1) Moho depth; (2) thickness of the lower crustal high-velocity layer (Vp > 7 km/s); (3) depth to the upper interface of the high-velocity layer; and (4) distribution of upper and middle crustal electrical conductors. The Moho depth is approximately 40 km in the Archaean Domain and varies between 40 and 65 km in the Svecofennian Domain. Most of the thickness variation (0–24 km) is concentrated in the lower crustal high-velocity layer. Electrically, the crust is formed of several rather homogeneous, either resistive or conductive, blocks that are bounded by upper and middle crustal conductive zones. The cratonized Archaean crust experienced an extensional phase during which a thin, mafic, lower crustal high-velocity layer was produced in association with mafic dykes (2.2–2.1 Ga) and well-conducting volcanogenic and metasedimentary belts (2.5–2.0 Ga). The Moho of the Archaean Domain was, consequently, reformed in the Early Proterozoic. Major thickening of the crust (50–65 km) occurred when a Svecofennian island arc and the Archaean craton collided. The Svecofennian Orogeny (1.76–1.9 Ga) resulted in a collage of metasedimentary rocks squeezed between crustal blocks of the island arc. The internal terrane boundaries are seen as good reflectors and/or inclined or vertical conductors that extend at least to the middle crust. The thick high-velocity (> 7 km/s) lower crust is a combination of mafic lower crust of the island arc crustal blocks and mafic additions from either delamination of the lithosphere after the collision or from a period of Late Svecofennian subduction. The collisional boundary is also preserved as a discontinuous conductive zone between the Archaean and Svecofennian Domains. The thickened crust was subsequently thinned to 42 km along E—W-striking zones during an extensional period characterized by the intrusion of anorogenic rapakivi granitoid batholiths, coeval mafic dykes, and gabbro-anorthosite bodies (1.6–1.5 Ga). The crust of the central part of the shield has since remained virtually intact.
Holly J Stein - One of the best experts on this subject based on the ideXlab platform.
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repeated syn and post orogenic gold mineralization events between 1 92 and 1 76 ga along the kiistala shear zone in the central lapland greenstone belt northern finland
Ore Geology Reviews, 2018Co-Authors: Ferenc Molnar, Lassi Pakkanen, Holly J Stein, Alexander Middleton, Hugh O Brien, Yann Lahaye, Hannu Huhma, Bo JohansonAbstract:Abstract The Central Lapland Greenstone Belt (CLGB) is well endowed with orogenic gold deposits along major shear zones and structural lineaments. In the northern part of the belt, the Kiistala Shear Zone (KiSZ) hosts the Suurikuusikko gold deposit (Kittila Mine), currently the largest gold producer in Europe. The Iso-Kuotko deposit is located 10 km to the north of Suurikuusikko along the same shear zone. Gold mineralization at Iso-Kuotko formed in two major stages: an early stage refractory gold mineralization with auriferous arsenopyrite with similarities to the ore at Suurikuusikko and a late, main stage mineralization with free gold in carbonate-quartz veins with abundant pyrrhotite, native bismuth and other sulphide minerals. This latter ore is absent at Suurikuusikko. Based on U-Pb dating, as well as Re-Os and Pb-isotope systematics of rock-forming and hydrothermal minerals, we evaluate the relationships between tectonic evolution and formation of gold deposits throughout the late Palaeoproterozoic in the northern part of the CLGB. Results from in situ U-Pb dating of zircon and petrographically well constrained hydrothermal monazite and xenotime by LA-ICPMS, and model age calculations using Re-Os isotopic data for arsenopyrite and in situ Pb-isotope data for galena from the Iso-Kuotko deposit provide evidence for multiple episodes of hydrothermal activity along the KiSZ. Data suggest coincident felsic magmatism and structurally controlled fluid flow events during wrench fault-dominated N-S shearing associated with early, micro-continent accretion ushering in the Svecofennian Orogeny. This includes the formation of refractory gold ores at Suurikuusikko (1916 ± 19 Ma) and Iso-Kuotko (>1868 ± 15 Ma). Subordinate barren veining took place at ∼1830 Ma during transient extension between the accretion and final collision stage. Xenotime in late stage veins with visible gold at Iso-Kuotko yield U-Pb ages between 1761 ± 9 and 1770 ± 7 Ma. This time-interval is concurrent with widespread granitoid emplacement at the close of the Svecofennian Orogeny. Disturbance of the Re-Os system in the early auriferous arsenopyrite can be connected to the late stage mineralizing processes. Pb isotope data suggest a mixed mantle and lower crust origin for fluids in the post-orogenic hydrothermal system. In situ U-Pb dating in polished thin sections in tandem with electron microprobe analyses of mineral compositions and detailed textural observations, highlights the utility of xenotime and monazite as robust geochronometers capable of recording repeated hydrothermal events in Paleoproterozoic orogenic systems. Results provide precise temporal constraints for orogenic gold mineralization and vital information to refine the tectonic evolution of the Central Lapland Greenstone Belt.
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geochronology of hydrothermal processes leading to the formation of the au u mineralization at the rompas prospect perapohja belt northern finland application of paired u pb dating of uraninite and re os dating of molybdenite to the identification of
Minerals, 2017Co-Authors: Ferenc Molnar, Hugh Obrien, Holly J Stein, Nick D J CookAbstract:The Perapohja belt comprises a greenschist to amphibolite facies; multiply-folded supracrustal sequence of quartzites; mafic volcanics; carbonate rocks; black shales; mica schists and greywackes deposited from ca. 2.44 Ga to 1.92 Ga; during protracted rifting of the Archaean basement. Metamorphism and multiple folding of the basin fill occurred during the Svecofennian Orogeny (1.92–1.80 Ga). The Rompas Au–U mineralization is hosted within deformed and metamorphosed calcsilicate veins in mafic volcanics. Textural evidence suggests that deposition and periods of uraninite re-mobilization were followed by localized hydrocarbon-bearing fluid flow which produced pyrobitumen crusts around grains of uraninite. Gold precipitated during the latest hydrothermal event at around 1.75 Ga. In situ U–Pb dating of uraninite by laser ablation inductively coupled mass spectroscopy (LA-ICP-MS), and Re–Os dating of molybdenite, indicate that primary hydrothermal uranium mineralization forms two age clusters; about 2.03–2.01 and 1.95–1.94 Ga. Resetting of the U–Pb system and precipitation of new generations of uraninite are associated with major deformation and metamorphic stages of the Svecofennian Orogeny at 1.91–1.89 Ga, 1.85 Ga, and 1.80 Ga. Gold deposition was synchronous with the emplacement of the 1.75–1.78 Ga late/post-orogenic granitoids. The gold-producing hydrothermal event is also recorded by Re–Os dating of molybdenite from the gold-bearing Mg-metasomatized metasedimentary and metavolcanic units at the Palokas prospect; a few kilometres from Rompas. Results of this study confirm that some domains in the structure of uraninite may preserve the original crystallization age, despite an overprinting amphibolite facies metamorphic and other hydrothermal events. The study supports the utility of in situ U–Pb dating of uraninite and the ability of Re–Os dating to assist in sorting out different hydrothermal events in areas with complex tectonic; magmatic and metamorphic histories.
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low rhenium molybdenite by metamorphism in northern sweden recognition genesis and global implications
Lithos, 2006Co-Authors: Holly J SteinAbstract:Abstract Re–Os dating of molybdenite is an accurate means to date intrusions and intrusion-related ore deposits using the model age or isochron approach. But, molybdenite has a new niche in the greenschist- to granulite-facies metamorphic environment. Re–Os ages for metamorphic molybdenite may be used to construct regional metamorphic histories. Age significance and accuracy are established by analyzing multiple molybdenite separates extracted from single, petrographically-characterized molybdenite occurrences. In this study, twelve geologically distinct molybdenite-bearing samples from two small Mo districts in northern Sweden trace a 150 m.y. Paleoproterozoic Svecofennian metamorphic history from ∼1900 to 1750 Ma. These data reveal a little-known, widespread and protracted, Late Svecofennian anatexis in northern Sweden. The Kataberget Mo–(Cu, F) deposit is located in the Moskosel granite batholith north of the economically-renown Skellefte district. Four different molybdenite samples from outcrop at Kataberget indicate an intrusion age of 1895 ± 6 Ma with the formation of later pegmatite–aplite at 1875 ± 6 Ma. The Allebuoda (Bjorntjarn) and Munka Mo–(W) deposits in the Rappen district are represented by three outcrop and five drill core samples of molybdenite-bearing aplite–pegmatite–granite. These two deposits were previously described as intrusion-related Climax-type Mo mineralization. Re–Os ages for molybdenites from these deposits range from 1865 to 1750 Ma and, significantly, Re concentrations are markedly low, extending to the sub-ppm level. Age agreement within the deposits is conspicuously lacking, whereas, with one exception, age agreement within any single sample (geologic occurrence), as established by analysis of additional molybdenite separates, is very good. These data, together with fundamental geologic observations discussed in this paper, suggest that Mo–(W) mineralization in northern Sweden is not intrusion-related, but the local product of episodic melting of Archean–Paleoproterozoic supracrustal gneisses related to the Svecofennian Orogeny. Petrographic traverses across the boundary between widespread, foliation-parallel units of aplitic to pegmatitic pink granite and hosting biotite gneiss directly capture the process of ore formation. Dehydration breakdown of zircon-rich biotite aligned with the foliation in the gneiss is accompanied by formation of new pristine, post-deformational biotite plus sulfides, oxides, hydrothermal zircon and fluorite, all associated with microcline-dominant leucosomes. This process has profound implication for the traditional leucogranite, intrusion-related genesis attributed to the broad classification of Mo–W–Sn–base and precious metal mineralization (e.g., South Mountain Batholith, Nova Scotia; Okiep, Namaqualand, South Africa; Mactung, Yukon; Pogo–Liese, Tintina, Alaska; Carajas and Goias–Rio Tocantins, Brazil; New England Batholith, NSW, Australia; Bergslagen, Sweden; Nevoria, Western Australia; Alpeinerscharte, Austria; Erzgebirge, Germany; Sardinia–Corsica Batholith). In addition to biotite, metallogenic contributions (e.g., Mo, W, Sn, U, Bi, Cu, Pb, Zn, Fe, Ni, Co, Au, Ag, Te, As, Sb, REE) in various combinations may also be controlled by breakdown of amphibole. In effect, the trace element composition of dehydrating or recrystallizing components in a gneissic rock essentially defines the local and district metallogenic suite. In the absence of focusing structures (e.g., shear zones, sheeted vein development), this process will generally form small and disconnected subeconomic deposits with erratic and unpredictable grades. Low Re content in associated molybdenite is a key indicator for a subeconomic origin by local melting of biotite gneiss (Mo–W) or muscovite schist (Sn–W).
Laurence Page - One of the best experts on this subject based on the ideXlab platform.
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biotite and muscovite 40ar 39ar geochronological constraints on the post Svecofennian tectonothermal evolution forsmark site central sweden
International Journal of Earth Sciences, 2009Co-Authors: Pia Soderlund, Tobias Hermansson, Laurence Page, Michael B. StephensAbstract:In order to characterize the post-Svecofennian tectonothermal evolution of the Fennoscandian Shield, 40Ar-39Ar biotite and some 40Ar-39Ar muscovite geochronological data are reported from a total of 30 surface outcrop and 1000 m long borehole samples at Forsmark, central Sweden. The thirteen surface samples were collected across three branches of a major WNW to NW trending system of deformation zones, whereas the boreholes were drilled within a tectonic lens, in between two of these zones. The 40Ar-39Ar biotite ages indicate that the present erosion surface, in central Sweden, cooled below 300 °C at 1.73-1.66 Ga, and that the rocks could respond to stress by brittle deformation between 1.8 and 1.7 Ga. The variation in surface ages is suggested to be due to faulting along the large WNW to NW trending deformation zones, following the establishment of a sub-Cambrian peneplain. The minor variation of ages within a single crustal block may be due to disturbance along ENE to NNE trending fracture zones. Possible cooling paths, derived from 40Ar-39Ar hornblende, muscovite and biotite ages, were calculated for the time interval from 1.80 to 1.66 Ga, when the area cooled from 500 °C to 300 °C. Between 1.70 and 1.64 Ga, uplift rates of c. 22 m/m.y. were calculated from borehole 40Ar-39Ar biotite data. Tectonothermal histories, inferred from the combined cooling and uplift rates, are related to simple slow cooling after the Svecofennian Orogeny, to crustal movement in response to far-field effects of c. 1.7 Ga orogenic activity further to the west or to a combination of these possibilities. (Less)
