scholarly journals New zircon data supporting models of short-lived igneous activity at 1.89 Ga in the western Skellefte District, central Fennoscandian Shield

2011 ◽  
Vol 3 (1) ◽  
pp. 355-383 ◽  
Author(s):  
P. Skyttä ◽  
T. Hermansson ◽  
J. Andersson ◽  
P. Weihed

Abstract. New U-Th-Pb zircon data (SIMS) from three intrusive phases of the Palaeoproterozoic Viterliden intrusion in the western Skellefte District, central Fennoscandian Shield, dates igneous emplacement in a narrow time interval at about 1.89 Ga. A locally occurring quartz-plagioclase porphyritic tonalite, here dated at 1889 ± 3 Ma, is, based on the new age data and field evidence, considered the youngest of the intrusive units. This supports an existing interpretation of its fault-controlled emplacement after intrusion of the dominating hornblende-tonalite units, in this study dated at 1892 ± 3 Ma. The Viterliden magmatism was synchronous with the oldest units of the Jörn type early-orogenic intrusions in the eastern part of the district (1.89–1.88 Ga; cf. Gonzàles Roldán, 2010). A U-Pb zircon age for a felsic metavolcanic rock from the hanging-wall to the Kristineberg VMS deposit, immediately south of the Viterliden intrusion, is in this study constrained in the 1.89–1.88 Ga time interval. It provides a minimum age for the Kristineberg ore deposit and suggests contemporaneous igneous/volcanic activity throughout the Skellefte District. Furthermore, it supports the view that the Skellefte Group defines a laterally continuous belt throughout this "ore district". Tentative correlation of the 1889 ± 3 Ma quartz-plagioclase porphyritic tonalite with the Kristineberg "mine porphyry", which cuts the altered ore-hosting metavolcanic rocks, further constrain the minimum age for ore deposition at 1889 ± 3 Ma. Based on the new age determinations, the Viterliden intrusion may equally well have intruded into, or locally acted as a basement for the ore-hosting Skellefte Group volcanic rocks.

Solid Earth ◽  
2011 ◽  
Vol 2 (2) ◽  
pp. 205-217 ◽  
Author(s):  
P. Skyttä ◽  
T. Hermansson ◽  
J. Andersson ◽  
M. Whitehouse ◽  
P. Weihed

Abstract. New U-Th-Pb zircon data (SIMS) from three intrusive phases of the Palaeoproterozoic Viterliden intrusion in the western Skellefte District, central Fennoscandian Shield, dates igneous emplacement in a narrow time interval at about 1.89 Ga. A locally occurring quartz-plagioclase porphyritic tonalite, here dated at 1889 ± 3 Ma, is considered the youngest of the intrusive units, based on the new age data and field evidence. This supports an existing interpretation of its fault-controlled emplacement after intrusion of the dominating hornblende-tonalite units, in this study dated at 1892 ± 3 Ma. The Viterliden magmatism was synchronous with the oldest units of the Jörn type early-orogenic intrusions in the eastern part of the district (1.89–1.88 Ga; cf. Gonzàles Roldán, 2010). A U-Pb zircon age for a felsic metavolcanic rock from the hanging-wall to the Kristineberg VMS deposit, immediately south of the Viterliden intrusion, is constrained at 1883 ± 6 Ma in this study. It provides a minimum age for the Kristineberg ore deposit and suggests contemporaneous igneous/volcanic activity throughout the Skellefte District. Furthermore, it supports the view that the Skellefte Group defines a laterally continuous belt throughout this "ore district". Tentative correlation of the 1889 ± 3 Ma quartz-plagioclase porphyritic tonalite with the Kristineberg "mine porphyry" suggests that these units are coeval at about 1.89 Ga. Based on the new age determinations, the Viterliden intrusion may equally well have intruded into or locally acted as a basement for the ore-hosting Skellefte Group volcanic rocks.


1968 ◽  
Vol 5 (3) ◽  
pp. 737-747 ◽  
Author(s):  
J. D. Obradovich ◽  
Z. E. Peterman

This paper presents new radiometric data that permit some qualified statements to be made on the depositional history of the Belt sedimentary rocks. The period of deposition of sedimentary rocks of the Precambrian Belt Series has been placed within a broad time interval, for they rest on metamorphosed basement rock dated at ~ 1800 m.y. and are overlain by the Middle Cambrian Flathead Quartzite (circa 530 m.y.). Prior geochronometric data gathered during the last decade indicate most of the Belt Series to be older than ~ 1100 m.y.K–Ar and Rb–Sr techniques have been applied recently to a variety of samples selected from the whole gamut of the Belt Series. Glauconite from various formations in the sequence McNamara Formation down to the uppermost beds of the Empire Formation in the Sun River area has been dated at 1080 ± 27 m.y. by the K–Ar method and at 1095 ± 22 m.y. by the Rb–Sr mineral isochron method. A Rb–Sr whole-rock isochron based on argillaceous sedimentary rocks from this 5000-ft section gives an age of 1100 ± 53 m.y. The concordance of the preceding results and the K–Ar ages (1075 to 1110 m.y.) on Purcell sills and lava imply that this age represents the time of sedimentation of these units.A Rb–Sr isochron based on whole-rock samples stratigraphically far below the Umpire Formation— the Greyson Shale, Newland Limestone, Chamberlain Shale, and Neihart Quartzite in the Big Belt and Little Beit Mountains—yields an age of 1325 ± 15 m.y. This result is interpreted as indicating a substantial unconformity beneath the Belt Series, at least in central Montana; it also suggests a major hiatus, unsuspected from field evidence, between the uppermost part of the Empire Formation and the Greyson Shale.The results for the youngest of Belt rocks—the Pilcher Quartzite and the Garnet Range Formation, which are exposed in the Alberton region—are equivocal in that there is widespread dispersion. A large component of detrital muscovite in some of the samples could readily account for the magnitude and sense of this dispersion. A maximum age of ~930 m.y. based on an isochron of minimum slope through the various points may be inferred for this sequence. A K–Ar age of 760 m.y. obtained on biotite from a sill in the Garnet Range Formation provides a minimum age for these younger Belt rocks.Three distinct periods of sedimentation for Belt rocks sampled are suggested at ≥ 1300, 1100, and ≤ 900 m.y., with two substantial hiatuses of 200 m.y. or more. In addition the data for the sequence in the Big and Little Belt Mountains suggest that sedimentation may not have commenced for a period of possibly 400 m.y. after the metamorphism that affected basement rocks, while the data for the Garnet Range and Pilcher sequence suggest that sedimentation ceased some 200 to 400 m.y. prior to the deposition of the Middle Cambrian Flathead Quartzite.To suggest that the Belt sediments were deposited continuously over a period of 400 m.y. or more would imply an unusually low average rate of deposition of ≤ 0.1 ft/1000 yr, and this for the thickest part of the Belt Series. As a realistic expression of the depositional history of the Belt Series, both viewpoints are open to question, but the viewpoint that the Belt basin has been characterized by discontinuous sedimentation would be more in keeping with the principle of uniformity.


1966 ◽  
Vol 59 ◽  
pp. 1-102
Author(s):  
A Escher

The Nanortalik peninsula, situated between the fjords of Tasermiut and Sarqâ, is largely composed of Ketilidian schists, quartzites and volcanic rocks. All these rocks are more or less strongly folded. The folding took place probably in three successive phases during the Ketilidian period : A first deformation resulting in folds with NNE trending axes, was followed by a second major phase of folding with NW axes. This second folding was essentially plastic. A third deformation, acting probably on a more rigid mass, was characterised by the formation of fracturec1eavage. Third-period folds possess very long wavelengths; their axes are oriented NNE to NE. Migmatisation started probably during the second deformation period resulting in the formation of many dykes and veins of pegmatite and aplite. Four generations of Ketilidian pegmatites can be recognised. Most of them appear to have been formed by metasomatic replacement. It seems that during the Ketilidian orogeny, the evolution of the schists and gneissic schists tended to a granodioritic composition. Potassium metasomatism only became active at the end of the Ketilidian period. In the NE part of the Nanortalik peninsula, three Sanerutian granites can be observed. These granites are similar in composition (quartz-microline-biotite), but possess different ages and textures. The time interval between the last Ketilidian deformation and the emplacement of the first Sanerutian granite was marked by the intrusion of several metadoleritic dykes. The first and principal Sanerutian granite usually shows an indistinct foliation due to numerous oriented inc1usions. Field evidence indicates that this granite was formed mainly by replacement of volcanic rocks. Chemical analyses show that large amounts of K, Si and Na have been supplied to produce the granitisation of the volcanic rocks. The second Sanerutian granite is characterised by a coarse porphyroblastic texture and appears to have been emplaced partially by the intrusion of a melt and partially by a subsequent replacement of the host-rock. Finally, the last Sanerutian granite displays all the characteristics of a pure intrusive body. It is generally very fine-grained and forms many cross-cutting dykes.


2001 ◽  
Vol 38 (4) ◽  
pp. 619-637 ◽  
Author(s):  
R M Friedman ◽  
L J Diakow ◽  
R A Lane ◽  
J K Mortensen

New U–Pb ages and K–Ar dates, primarily for rocks proximal to mineral occurrences in the Fawnie Range of central British Columbia, document latest Cretaceous (ca. 74–66 Ma) continental-arc igneous activity and date associated base and precious metal mineralization. U–Pb ages of ca. 73–69 Ma for the Capoose pluton and hypabyssal to extrusive garnet rhyolites at the Capoose prospect demonstrate a latest Cretaceous age for mineralization and a likely plutonic source for mineralizing fluids. A U–Pb age of ca. 67 Ma for a late mineralized felsic dyke and two K–Ar dates (ca. 70 and 68 Ma) for hornfelsed Jurassic volcanic rocks at the Blackwater–Davidson prospect constrain a latest Cretaceous age for mineralization. A U–Pb age of ca. 74 Ma for a fine grained diorite sill that cuts a significant epithermal gold vein at the Tsacha prospect places a minimum age on mineralization at this probable Jura-Cretaceous deposit and documents latest Cretaceous magmatism. Latest Cretaceous K–Ar dates are reported for an andesite flow adjacent to the Eocene Holy Cross deposit (ca. 66 Ma), about 35 km north of the Fawnie Range, and a Kasalka Group rhyolite (ca. 68 Ma) exposed near the western margin of the Nechako Plateau. Latest Cretaceous magmatism and mineralization in the Fawnie Range represent the waning stages of Bulkley suite magmatism and porphyry-style mineralization, which was concentrated along the western margin of the Nechako Plateau at circa 88–70 Ma. The distribution of latest Cretaceous arc igneous rocks along the North American Cordilleran is reviewed and tectonic implications discussed.


1999 ◽  
Vol 90 (3) ◽  
pp. 221-254 ◽  
Author(s):  
Åke Johansson ◽  
Alexander N. Larionov ◽  
Alexander M. Tebenkov ◽  
David G. Gee ◽  
Martin J. Whitehouse ◽  
...  

AbstractThe basement of the island of Nordaustlandet, northeastern Svalbard, consists of a complex of metasedimentary and metavolcanic rocks, granites and augen gneisses, unconformably overlain by the Neoproterozoic Murchisonfjorden Supergroup. Earlier U-Pb dating of the Laponiafjellet and Kontaktberget granites has shown them to be late Grenvillian, with ages of c. 960 Ma and 940 Ma, respectively. Here, we present conventional U-Pb zircon and monazite data, single zircon Pb-evaporation data and ion microprobe data from the Kapp Hansteen Group and Svartrabbane Formation volcanic and subvolcanic rocks, and from the Fonndalen and Ringåsvatnet augen gneisses of central Nordaustlandet. The combined evidence suggests late Grenvillian magmatic ages of 940–970 Ma for all these rocks, with inherited zircons ranging in age from c. 1200 Ma to 2600 Ma. The investigated rocks vary in chemical composition from andesites to rhyolites and granites, and show generally similar trace and rare earth element patterns, with trace element compositions suggesting a volcanic arc or syn-collisional tectonic setting, and major element compositions suggesting a large sedimentary input to the magmas. Contributions from older crustal material are also supported by Nd isotope data and the presence of inherited zircons.The Grenvillian magmatic rocks thus originated in a series of magmatic events along a continental margin over a time span not longer than 30 Ma. The volcanic rocks were extruded onto folded strata of the Brennevinsfjorden Group–Helvetesflya Formation, which must have been deposited in the time interval 1200–960 Ma. Folding of the metasediments preceded the volcanism, but was synchronous with intrusion of the augen gneiss protolith, and was followed by crustal stabilisation, uplift and erosion. This newly formed Grenvillian crust then served as basement for the deposition of the Neoproterozoic Murchisonfjorden Supergroup. The present outcrop pattern of the Grenvillian rocks is largely the result of large-scale Caledonian folding and doming.


2020 ◽  
Author(s):  
Jonathan Rincon ◽  
Simon Johansson ◽  
Nils Jansson ◽  
Helen Thomas ◽  
Majka Christiane Kaiser ◽  
...  

<p>Remobilisation of sulphides in metamorphosed volcanic-hosted massive sulphide deposits has been investigated in many VMS districts with regards to scale, mineral assemblages, texture and relative competence of minerals under certain p-t conditions (Gilligan & Marshall, 1987; Marshall & Gilligan, 1987). Examples of syn-tectonic remobilisation can be found at the Rävliden Norra (RVN) volcanic-hosted massive sulphide in the Skellefte district. At Rävliden, polymetallic sulphide mineralization occurs at the transition from meta-volcanic rocks of the Skellefte group rocks to the overlying Vargfors group, comprising volcaniclastic metasedimentary rocks and graphitic shales. This contribution details features of mesoscale (0.1-50 cm) remobilisation of sulphides, such as sulphide-rich veins, tension gashes, ball-ore, massive sulphides with cataclastic texture, and micro-scale features such as infilling of pressure shadows, displaying evidences of brittle and ductile deformation. Sulphide-rich veins containing sphalerite, galena, and a relative high content of Ag-sulphosalts (e.g. freibergite, pyrargyrite, pyrostilpnite) are hosted in the hanging wall (HW) of the RVN mineralization. Brittle deformation is shown in accessory quartz and calcite as bulging recrystallization, grain boundary migration and deformation lamellae or twinning. Ductile expressions include ball-ore (i.e. “durchbewegung”) textures, typically made up of two components, one composed of clasts of graphite shale or tremolite-, actinolite-, talc-altered meta-volcanic rocks and the other comprising a matrix of massive sulphide mineralization. In the massive sulphide matrix of sphalerite, chalcopyrite or pyrrhotite, micro-scale tension gashes and/or pressure shadows occur around clasts infilled by pyrrhotite, chalcopyrite, galena, freibergite, boulangerite, or gudmundite. A similar mineralogy is observed in ore lenses in the ore zone, comprising sphalerite, galena and Ag-Sb-As sulphosalts, hosted structurally above chalcopyrite + pyrrhotite stringer zones in the footwall (FW). Sulphosalts and galena present a high silver content relative to other VMS deposits in the district. This is evidenced by SEM and EMPA analysis in both HW and FW ore lenses. Argentopyrite, sternbergite and stephanite are also locally present in the HW as minor silver species hosted in veins. Inclusions of freibergite in galena contain Ag with average values of 18.4 wt. % in the HW (n=5), 18 wt. % in the massive sphalerite and ball-ore (n= 15), and 20.2 wt. % in the chalcopyrite + pyrrhotite stringer zone (n= 5). Similarly, Pb is 0.2 wt. %, 0.3 wt. %, and 0.4 wt. %, respectively. For sphalerite, Fe is on average 8.0 wt. % (n=3), 7.4 wt. % (n = 11), and 8.3 wt. % (n=3), respectively. Our preliminary results suggest that mineralization in the HW is remobilized from the main ore and textural relationships support a hypothesis that remobilisation involved a relative silver-enrichment in paragenetically later assemblages. At least two stages of deformation in the deposit can be recognized. In the first stage, sphalerite- and chalcopyrite-rich mineralization was deformed along with tremolite and talc to form a S1 foliation. The second stage involved folding of S1, and remobilisation of galena, chalcopyrite and Ag-Sb-As sulphosalts as veins or breccia infill in the HW or filling tension gaps or ball-ore, in the FW. These are often parallel to S2 crenulation or axial planes.</p>


1992 ◽  
Vol 29 (1) ◽  
pp. 3-14 ◽  
Author(s):  
G. Beaudoin ◽  
J. C. Roddick ◽  
D. F. Sangster

The Ag–Pb–Zn–Au vein and replacement deposits of the Kokanee Range, southeastern British Columbia, are hosted by the Middle Jurassic Nelson batholith and surrounding Cambrian to Triassic metasedimentary rocks in the hanging wall of the transcrustal Slocan Lake Fault, Field relations indicate that mineralization is younger than the Nelson batholith and a Middle Jurassic foliation in the Ainsworth area but coeval or older than Eocene unroofing of the Valhalla metamorphic core complex in the footwall of the Slocan Lake Fault. Lamprophyre and gabbro dykes are broadly coeval with mineralization and have biotite and hornblende K–Ar ages defining a short-lived Middle Eocene alkaline magmatic event between 52 and 40 Ma. An older, Early Cretaceous alkaline magmatic event (141 – 129 Ma) is possible but incompletely documented.K–Ar and step-heating 40Ar/39Ar analyses on hydrothermal vein and alteration muscovite indicate that hydrothermal fluids were precipitating vein and replacement deposits 58–59 Ma ago. Crosscutting relationships with lamprophyre dykes indicate the Kokanee Range hydrothermal system lasted for more than 15 Ma. Eocene crustal extension resulted in a high heat flow and structures which were probably responsible for hydrothermal fluid movement and flow paths.A 100 Ma time interval is documented between batholith emplacement and spatially associated mineralization, ruling out any genetic link between the two. Similar large age differences between granite intrusion and peripheral mineralization have recently been documented for two world-sea le Ag–Pb–Zn vein districts, which suggest that spatial association between granite and Ag–Pb–Zn mineralization is not sufficient to infer a genetic link.


2018 ◽  
Vol 13 (4-5) ◽  
pp. 36-51
Author(s):  
J. V. Frolova ◽  
V. V. Ladygin ◽  
E. M. Spiridonov ◽  
G. N. Ovsyannikov

The article considers the petrogenetic features of the volcanogenic rocks of the Middle Jurassic age of the Mountain Crimea and analyzes their influence on physical (density, porosity, water absorption, and magnetic susceptibility) and physical-mechanical properties (strength, modulus of elasticity, and Poisson's ratio). Among volcanogenic strata there are subvolcanic, effusive and volcanogenic-clastic rocks. All volcanic rocks were altered under the influence of the regional low-grade metamorphism of the zeolite and prehnite-pumpellyite facies, which resulted in a greenstone appearance. Among the secondary mineral the most common are albite, chlorite, quartz, adularia, sericite, calcite, pumpellyite, prenite, zeolites, epidote, sphene, and clay minerals. It is shown that low-grade metamorphism is characterized by heterogenious transformations: there are both slightly modified, practically fresh differences, and fully altered rocks. Tuffs are usually altered to a greater extent than effusive and subvolcanic rocks. In general, effusive and volcanogenic-clastic rocks differ markedly in their physicalmechanical properties, which is due to the peculiarities of their formation: the former are substantially more dense and stronger, less porous and compressible. However, these differences are leveled as a result of intensive changes in mineral composition and porosity in the process of low-grade metamorphism. The most characteristic values of metavolcanite properties were revealed. It is shown that among all studied parameters, the magnetic susceptibility most clearly correlates with the degree of rocks alteration.


2019 ◽  
Vol 27 (2) ◽  
pp. 161-186
Author(s):  
A. B. Vrevskii

The paper reports new data on the composition and age of the Neoarchean calc-alkaline volcanic rocks of the Uraguba–Kolmozero–Voron’ya greenstone belt (UKV GB). Petrological-geochemical modeling indicates a polygenetic origin of primary melts of the basalt–andesite–dacite association and non-subduction geodynamic mechanisms for the crustal growth in the largest greenstone belt of the Kola–Norwegian Block of the Fennoscandian shield.


Author(s):  
Emmanuel Gabet

Hildreth et al. (2021) analyzed a set of table mountains near the San Joaquin River that are capped by a 9.3 Ma trachyandesite lava flow and concluded that, since the deposition of the volcanic rocks, the table mountains have been tilted 1.07° due to uplift of the central Sierra Nevada. While Gabet (2014) suggested that, under a limited set of conditions, the size of fluvial gravels under the table mountains would support the hypothesis of postdepositional uplift, the authors claimed that their evidence is more definitive. In addition, the authors proposed that the central Sierra Nevada tilted as a rigid block. However, their analyses rely on inferences and assumptions that are not supported by field evidence.


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