Fragments of Paleoproterozoic Large Igneous Provinces in Northern Fennoscandia: Baddeleyite U–Pb Age Data for Mafic Dykes and Sills
Е. Б. СальниковаА. В. СамсоновA. V. StepanovaR. V. VeselovskiyС. В. ЕгороваА. А. АрзамасцевK. G. Erofeeva
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Baddeleyite
Sill
Dike
Baltic Shield
Large igneous province
The St. Charles sill is located in the Grenville Province and consists of rocks of the anorthosite suite. The sill is a northwesterly trending body, 11 km long and as much as 0.8 km wide, and with a steep dip to the northeast. The sill is characterized by interlayered massive and gneissic rocks metamorphosed under conditions of the amphibolite facies. In the massive rocks plagioclase occurs as strongly twinned laths that range in size from fine-grained crystals to megacrysts. Hornblende, biotite, and garnet occur as subophitic masses and apparently replace original pyroxene. In the gneissic rocks the plagioclase ranges in size from fine to coarse grained and the primary grains are partially replaced by elongate, weakly twinned, anhedral plagioclase. The gneissosity is defined by a dimensional preferred orientation of biotite, hornblende, and secondary plagioclase. The formation of the secondary plagioclase is attributed largely to growth by grain boundary diffusion and, to a lesser extent, by replacement of primary plagioclase by grain boundary migration. In the diffusion mechanism strain rate is inversely proportional to grain size and it is interpreted that the tectonic fabric developed in the finer grained layers of the sill while the coarser grained layers remained essentially undeformed.
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Hornblende
Anorthosite
Pyroxene
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Sill
Large igneous province
Mantle plume
Primitive mantle
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Late Miocene basaltic sills and dikes in the Paiute Ridge area of southern nevada show evidence that their emplacement was structurally controlled. Basaltic dikes in this area formed by dilating pre-existing vertical to steeply E-dipping normal faults. Magma propagation along these faults must have required less energy than the creation of a self-propagated fracture at dike tips and the magma pressure must have been greater than the compressive stress perpendicular to the fault surface. N- to NE-trending en echelon dikes formed locally and are not obviously attached to the three main dikes in the area. The en echelon segments are probably pieces of deeper dikes, which are segmented perhaps as a result of a documented rotation of the regional stresses. Alternatively, changes in orientation of principal stresses in the vicinity of each en echelon dike could have resulted from local loads associated with paleotopographic highs or nearby structures. Sills locally branched off some dikes within 300 m of the paleosurface. These subhorizontal bodies occur consistently in the hanging wall block of the dike-injected faults, and intrude Tertiary tuffs near the Paleozoic-Tertiary contact. The authors suggest that the change in stresses near the earth`s surface, the material strength of the tuff and paleozoic rocks, and the Paleozoic bedding dip direction probably controlled the location of sill formation and direction of sill propagation. The two largest sills deflected the overlying tuffs to form lopoliths, indicating that the magma pressure exceeded vertical stresses at that location and that the shallow level and large size of the sills allowed interaction with the free (earth`s) surface. 32 refs., 4 figs., 1 tab.
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Echelon formation
Magma chamber
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The Plains Sill is a thick diabase-granophyre body that intruded the wet sediments of the Middle Proterozoic Prichard Formation of the Belt-Purcell Supergroup. The diabase is a high-iron tholeiite geochemically compatible with large-volume mantle melting in an intracratonic rift environment. Evidence of emplacement into wet sediments includes thick zones of homogenized granosediments adjacent to the sill, soft-sediment deformation at sill contacts, and sedimentary ovoid structures possibly formed by local fluidization of sediments. Utilizing sediment pore water and driven by heat from the sill, the diabase was metamorphosed during crystallization and cooling, leaving hornblende as the dominant mafic phase. Continued retrograde alteration resulted in overgrowths of secondary hornblende and variable alteration of plagioclase to epidote. A miarolitic granophyre layer, up to 150 m thick, caps the diabase and appears igneous in origin. Locally the granophyre is anomalously thick, perhaps reflecting updip migration of granophyric fluid where the Plains Sill cuts upsection through the Prichard Formation stratigraphy.
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Abstract Field studies in the Eucalyptus area, northeastern Yilgarn Block have shown intrusive and extrusive rocks in an Archaean greenstone sequence to be comagma‐tic, and have suggested the sequence of subsequent granitoid intrusion and gold mineralisation. Andesitic volcanic rocks and related subvolcanic granodiorite porphyry and epiclastic sediments were followed by tholeiitic basalt with gabbro/dolerite sills and dykes, which were in turn succeeded by high‐Mg basalt with associated peridotite intrusions. Large, irregular gabbro and peridotite intrusions, which are inferred to represent subvolcanic magma chambers, occur in lower stratigraphic levels, whereas comformable subvolcanic sills occur in higher stratigraphic levels. Granodiorite plutons were followed by adamellite plutons; at least some gold mineralisation was contemporaneous with granitoid emplacement.
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Magma chamber
Flood basalt
Peridotite
Greenstone belt
Layered intrusion
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Large igneous province
Flood basalt
Petrogenesis
Continental arc
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Baddeleyite
Geochronology
Sill
Metamictization
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Dike
Baltic Shield
Large igneous province
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The Nipigon Embayment is underlain by Archean rocks of the English River, Wabigoon, and Quetico subprovinces, and intruded along the west side by late- to post-tectonic mafic to ultramafic intrusions. The early Mesoproterozoic ultramafic to felsic Badwater intrusion and felsic English Bay Complex are located in the northwest corner of the Nipigon Embayment. Three mafic to ultramafic intrusions, the Disraeli, Seagull, and Hele intrusions, are located south of Lake Nipigon, and the Kitto intrusion is located east of the lake. A number of mafic to ultramafic bodies (Jackfish (Island), Shillabeer, Kama Hill, Nipigon Bay) have only limited outcrops. The gabbroic Nipigon diabase sills intrude all other rocks in the Nipigon Embayment and generally have a consistent mineralogy and geochemistry, except for the Inspiration sill(s) and the McIntyre Sill. Geological and geophysical data suggest emplacement of the ultramafic intrusions by mechanisms similar to those controlling emplacement of the saucer-shaped diabase sills. These mechanisms are partially dependent on a series of pre-existing north-, northwest-, and northeast-trending faults formed prior to Keweenawan magmatism. The presence of sills, rather than dykes, indicates that the Nipigon Embayment was not extensional during the Keweenawan Midcontinent Rift, suggesting that the Nipigon Embayment is not a classic failed arm.
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Felsic
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Sill
Large igneous province
Felsic
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