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Bells Corners, Canada

Lubnina N.,Moscow State University | Ernst R.,Ernst Geosciences | Ernst R.,Carleton University | Klausen M.,Stellenbosch University | Soderlund U.,Lund University
Precambrian Research | Year: 2010

431 oriented samples were collected from 27 dolerite dykes at 17 sites, belonging to 2.95, 2.65, and 1.90. Ga swarms, that trend SE, E and NE, respectively from the Bushveld Igneous Complex into the eastern Kaapvaal Craton (ages determined by Olsson et al., 2010; Olsson in Söderlund et al., 2010). Samples were analyzed for paleomagnetism and also anisotropy of magnetic susceptibility (AMS). For the 2.95. Ga SE-trending dykes high temperature/coercivity 'P' component has unblocking temperatures up to 590°C and coercivity 40-90. mT and demonstrate SSW declination and intermediate positive inclination. Based on positive contact and conglomerate tests we argue for a primary origin of this component. The paleopole (BAD), calculated from '. P' component, does not correspond to any of the previously obtained Archean-Paleoproterozoic paleopoles for the Kaapvaal Craton, and represents a new key pole for 2.95. Ga. The high-coercivity '. H' component for the 2.65. Ga-old E-trending dykes has a SSW declination and steep positive inclination. Paleomagnetic pole (RYK), recalculated from this component, is close to the paleopoles, obtained by Wingate (1998) and Strik et al. (2007) for 2.78. Ga Ventersdorp volcanics. The third group, NE-trending dykes of the 1.90. Ga Black Hill swarm demonstrate an '. M' component with dual polarity high-coercivity component with SSE-declination and negative intermediate inclination. The paleopole (BHD), calculated from this component is close to the 1.87. Ga pole of the Kaapvaal Craton obtained by Hanson et al. (2004). Overprint directions include a very well developed thermo-chemical overprint (Dec. =329° Inc. =-36°), which is believed to be associated with a ∼0.18. Ga regional 'Karoo' thermal event. © 2010 Elsevier B.V.

Ernst R.,Ernst Geosciences | Bleeker W.,Geological Survey of Canada
Canadian Journal of Earth Sciences | Year: 2010

Large igneous provinces (LIPs) are high volume, short duration pulses of intraplate magmatism consisting mainly of flood basalts and their associated plumbing system, but also may include silicic components and carbonatites. Many LIPs have an associated radiating diabase dyke swarm, which typically converges on a cratonic margin, identifies a mantle plume centre, and is linked to breakup or attempted breakup to form that cratonic margin. We hypothesize that every major breakup margin in Canada can be associated with a LIP, and we attempt to identify this LIP. To this end, we focus mainly on high-precision age determinations and the distribution of diabase dyke swarms, which are uniquely valued for preserving the record of magmatic events. The analysis extends from the Phanerozoic to the Neoarchean, but our most complete information is for the Superior craton. There, events at 2.50-2.45, 2.22-2.17, and 2.12-2.08 Ga (LIP and plume) are linked with rifting and breakup or attempted breakup of the south-southeastern, northeastern, and southern margins, respectively. Events at 2.00-1.97 Ga are probably linked with the northern margin (Ungava promontory), while the Circum-Superior event at ca. 1.88 Ga is linked to the north to northwestern margins during a time of Manikewan Ocean closure. Similar linkages for other cratons of North America improve understanding of the breakup history to help identify which blocks were nearest neighbours to Canadian crustal blocks in Precambrian supercontinents. Such interpretations provide a framework for interpreting other geological features of these margins to further test models for the timing and location of breakup.

Bejgarn T.,Lulea University of Technology | Soderlund U.,Lund University | Weihed P.,Lulea University of Technology | Areback H.,Boliden Mineral AB | And 2 more authors.
Lithos | Year: 2013

The Skellefte district, northern Sweden, is known for the occurrence of 1.89Ga Palaeoproterozoic volcanogenic massive sulphide (VMS) deposits. The deposits are hosted by the older part of a volcanosedimentary succession, which was intruded at 1.88-1.86Ga by multiple phases of the syn-volcanic, early orogenic Jörn intrusive complex (JIC). The oldest phase of the JIC hosts different styles of mineralisation, among them porphyry Cu-Mo-Au, intrusion-related Au, and mafic-hosted Fe and Cu-Ni deposits. To discriminate between the different intrusive and ore related events, U-Pb ages of zircons have been obtained for nine intrusive phases and from Na-Ca alteration spatially related to mineralisation, while U-Pb ages of baddeleyite (ZrO2) have been used to constrain intrusive ages of three mineralised and barren mafic-ultramafic intrusive rocks.The two main JIC intrusive phases of a granodioritic-tonalitic composition in the southern study area intruded at 1887±3Ma and 1886±3Ma, respectively, and were succeeded by the intrusion of layered mafic-ultramafic intrusive rocks in the northern and southern study area at 1879±1Ma and 1884±2Ma, respectively. Emplacement of porphyry dykes took place at ca. 1877Ma in the southern, western and northern JIC. The dykes are spatially and temporally associated with formation of porphyry style mineralisation, alteration and Au-mineralisation, as inferred from 1879±5Ma zircons in adjacent Na-Ca alteration zones. High SiO2 and Al2O3 contents together with high Sr/Y ratios, mingling structures, mafic xenoliths and hornblende phenocrysts in the porphyry dykes suggest that the magma originated from hydrated partial melts, possibly from the base of the crust at a mature stage of subduction. Local extension resulted in intrusion of mafic-ultramafic rocks around 1.88Ga prior to and after, the porphyry dykes and associated mineralisation, approximately 10Ma after the formation of the spatially related 1.89Ga VMS deposits in the Skellefte district. This 1.88Ga event correlates with other 1.88Ga mafic-ultramafic units widespread around the world, and could possibly be interpreted as a large scale response to supercontinent formation. © 2012 Elsevier B.V.

Olsson J.R.,Lund University | Soderlund U.,Lund University | Klausen M.B.,Stellenbosch University | Ernst R.E.,Ernst Geosciences
Precambrian Research | Year: 2010

The Archean basement in the northeastern part of the Kaapvaal craton is intruded by a large number of mafic dykes, defining three major dyke swarms, which collectively appear to fan out from the Bushveld Complex. Herein we present U-Pb baddeleyite ages for two of these dyke swarms, the northwest trending Badplaas Dyke Swarm and the east-west trending Rykoppies Dyke Swarm, and infer their correlation with tectonic events in the Kaapvaal craton. We also present a U-Pb baddeleyite age for a noritic phase of the Marginal Zone of the Rustenburg Layered Suite (Bushveld Complex).The age of the Badplaas swarm is determined from two dolerites dated at 2965.9 ± 0.7. Ma and 2967.0 ± 1.1. Ma. These ages coincide with units of the Nsuze Group lavas (2967-2985. Ma), which constitute the world's oldest preserved rift basin, and suggest that dykes of this swarm are feeders to basaltic units of this group. Similarly, the E-W trending Rykoppies swarm has earlier been interpreted as a potential feeder system to the Bushveld Complex. However, the emplacement ages of six dolerites fall in the range 2.66-2.68. Ga, thus ∼600. Myr earlier than the intrusion of the Bushveld Complex (herein dated at 2057.7 ± 1.6. Ma). Rather, these ages coincide with the Allanridge Formation at the uppermost part of the Ventersdorp Supergroup as well as volcanic rocks of the " protobasinal" sequences preserved at the base of the overlying Transvaal Basin. The Rykoppies Dyke Swarm probably marks the initial stages of rifting of the Transvaal Basin and reflects a major shift from a NW-SE to an E-W trending tectonic setting. The origin of the Rykoppies Dyke Swarm can be linked either to prolonged mantle plume activity or to the onset of back-arc extension associated with south-directed subduction of oceanic lithosphere in a compressional setting along the northern margin of the Kaapvaal craton. © 2010 Elsevier B.V.

Kulikov V.S.,Russian Academy of Sciences | Bychkova Y.,RAS Institute of Chemistry | Kulikova V.,Russian Academy of Sciences | Ernst R.,Ernst Geosciences
Precambrian Research | Year: 2010

Ca. 2.5-2.4Ga Sumian magmatism is widespread in the Karelia and Kola cratons of Fennoscandia and probably represents at least two intermixed large igneous provinces (LIPs). It is distinct from other Paleoproterozoic LIPs (Jatulian 2.22-2.1Ga and Ludicovian 2.06-1.96Ga) elsewhere in the Fennoscandian Shield. A poorly understood portion of Sumian magmatism is the Vetreny Poyas (Windy Belt) subprovince, which covers ∼75,000km2 in southeastern Fennoscandia. This subprovince consists of four genetically related complexes which developed at different levels in the crust: a volcanic complex (komatiitic basaltic lava flows on Golets, Levgora and Myandukha hills, and Victoria lava lake on Levgora hill), a subvolcanic complex (mafic-ultramafic sills and lopoliths including Ruiga, Kirichgora, Kozhozero and Undozero), plutonic complexes (Burakovsky and Vyzhiga) and a dyke complex (gabbronoritic Avdeyevo and Shala dykes and peridotitic Vinela and Koppalozero dykes). Similar patterns are present in other Sumian belts elsewhere in Karelia, for instance in southern Lapland and the Kola Peninsula.Negative or near-zero e{open}Nd values for intrusive rocks from the Ruiga massif (-2.0±0.8) and volcanics from Golets Hill (-0.4±0.9) suggest that the mantle source for their melts was enriched or that their parental magmas were contaminated by crustal rocks during intrusion and crystallization. Overall, the petrologic-geochemical study of rocks from the above-mentioned complexes indicates that they were all derived from the same parental primary komatiitic melt (associated with a mantle plume) that experienced crustal contamination. This parental magma for the province is a komatiitic basalt magma type with key petrochemical parameters (9-18% MgO, 0.5-1.0% TiO2, SiO2 <53%), and is contrasted with other high-Mg magmatic types, boninites, komatiites, picrites and meimechites. © 2010 Elsevier B.V.

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