Shock metamorphic features in mafic and ultramafic inclusions in the Sudbury Igneous Complex: Implications for their origin and impact excavation
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Abstract:
The lowermost, discontinuous parts of the impact-generated Sudbury Igneous Complex (Canada), comprising the Sublayer and Offset Dikes, are distinguished from overlying Main Mass norite rocks by the presence of abundant inclusions and Ni-Cu-PGE (PGE—platinum group element) sulfide mineralization. The majority of the felsic to mafic inclusions appear to be derived from the exposed country rocks, but the volumetrically important olivine-bearing mafic and ultramafic inclusions have only very rare equivalents in the surrounding country rocks. We record the discovery of abundant shock metamorphic features (e.g., mosaicism in olivine; strong fracturing and partial isotropization of plagioclase) in the olivine-bearing mafic and ultramafic inclusions consistent with a shock pressure of 20–30 GPa. Olivine compositional data are inconsistent with a local country rock or mantle origin for these inclusions. Abundant plagioclase, the absence of garnet or Mg-spinel, and calculated low pressures (<500 MPa) provide evidence for derivation of the inclusions from unexposed mafic-ultramafic intrusions in the upper to middle crust that were disrupted during formation of the transient crater, incorporated into the impact melt sheet, and preserved because of their relatively refractory compositions. These observations support models involving intermediate, rather than very deep or very shallow, excavation for the Sudbury impact event.Keywords:
Ultramafic rock
Norite
Felsic
Layered intrusion
The Journal of the Japanese Association of Mineralogists Petrologists and Economic Geologists (1961)
The facies variation of the gabbro which is altered country rock of the nickeliferous pyrrhotite deposits, are reported in the present paper. The gabbro which is a member of the igneous rocks of the Hidaka metamorphic zone is distributed throughout the Hidaka Mountain range. The Satsunai-Dake area is situated in its northern part. The whole nickeliferous pyrrhotite deposits of the Hidaka metamorphic zone usually occur in the gabbro bodies. They are the coarsened gabbros owing to a kind of petroblastase. The olivine gabbro series has been distinguished as follows, (1) medium grained ophitic olivine gabbro, (2) coarse grained granoblastic olivine gabbro. Also in the norite series, (1) fine grained norite, (2) medium grained norite, (3) coarse grained granoblastic norite. The original rock of the olivine gabbro series is estimated the medium grained ophitic olivine gabbro (An70-75) and of the latter is the fine grained norite (An 33). These two series are used to be converged into coarse grained granoblastic olivine gabbro and norite with An60 plagioclase. In present investigation the plagioclase compositions are presented in the form of the An composition frequency diagrams. (Figures 4-10)
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Anorthosite
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Layered intrusion
Norite
Massif
Magma chamber
Anorthite
Ultramafic rock
Fractional crystallization (geology)
Anorthosite
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Abstract The Upper Critical Zone of the Rustenburg Layered Suite (RLS) in the Swartklip Sector, north-western Bushveld Complex, is considerably attenuated relative to other parts of the Complex. The interval between the UG2 chromitite and the Merensky Reef amounts to as little as 25 m in places. Within this interval, the aggregate thickness of orthopyroxenite-dominated ultramafic layers hosting the UG1 and UG2 chromitites and the Merensky and Bastard reefs does not differ significantly from the area around Rustenburg, to the south. The total thickness of ultramafic lithologies is, in fact, increased by the presence of the 3 to 5 m thick olivine-rich Pseudo Reef Unit, which is developed between the UG2 and Merensky Reef units in the Swartklip Sector, but does not occur in any significant form elsewhere in the Bushveld intrusion. The substantial thinning of the succession is due almost entirely to the fact that plagioclase-rich rocks (norite and anorthosite) between the ultramafic layers are radically thinned in the Swartklip Sector relative to virtually all other parts of the Bushveld Complex. The ultramafic layers, although dominated by orthopyroxenite, are characterized by higher proportions of olivine than in other parts of the Bushveld Complex. In our logging of the substantial number of exploration drill cores that form the basis of this study, we have found it expedient to define stratigraphic units that are either exclusively plagioclase-rich (norite and anorthosite) or plagioclase-poor (consisting of varying proportions of orthopyroxenite, harzburgite and chromitite). This effectively binary system of lithological classification has no overt genetic connotations. Our nomenclature has, in fact, enabled us to rigorously document the nature of contacts between ultramafic and plagioclase-rich units, and thus to identify unconformities between the ultramafic units (orthopyroxenite and harzburgite) and intervening noritic and anorthositic units, which have in the past been ascribed to localized thermo-chemical erosion of pre-existing plagioclase-rich cumulates. Apart from the well-documented evidence of erosional unconformities at the basal contacts of ultramafic units, we also provide evidence for unconformities at the tops of these units.
Chromitite
Ultramafic rock
Anorthosite
Norite
Layered intrusion
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Pyroxene
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The well-preserved ?lower Proterozoic McIntosh intrusion consists of 96 macro-layers with a total stratigraphic thickness of about 6 km. The lowermost rocks in this possible cone-shaped intrusion are hidden, and the roof and the upper layers were removed by erosion. The layered sequence is dominated by 40 bimodal cyclic units of troctolite and olivine gabbro. Minor gabbronorite layers occur throughout the sequence, and are more abundant and more fractionated higher in the sequence. Six imperfect megacycic units are developed in the upper 2700 m, each unit consisting of several troctolite-olivine gabbro cyclic units followed by a Fe-Ti oxide-bearing gabbronorite. The overall cumulus crystallization order in each megacyclic unit was plagioclase first, closely followed by olivine, then augite, orthopyroxene, and magnetite successively. Cryptic composition data for troctolites and olivine gabbros show a slight overall decrease of 10 mol per cent An and Fo from the base to the top of the layered sequence (approximate ranges An80–70 and Fo78–68). Several major fluctuations occur however, and are generally associated with the oxide gabbronorites, which are significantly more fractionated than the adjacent layers (plagioclase An53–60, orthopyroxene Mg52–69 Each fluctuation comprises a marked progressive discontinuity (rapid normal fractionation) followed by a gradual to rapid regressive discontinuity (or reversal) in the overlying troctolites and olivine gabbros. Apparently, such marked progressive discontinuities have not been described in layered intrusions. A chilled margin and the overall composition of the intrusion suggest an olivine tholeiite parent magma, inferred to have crystallized at P ≤ 6 kb, relatively low PH2O and high fO2 (> NNO buffer). The troctolite-olivine gabbro cyclic units are inferred to have formed by fractional crystallization of periodic additions of new magma. However, the oxide gabbronorites seem too fractionated relative to the underlying layers to have formed by conventional crystal fractionation mechanisms, and they could have resulted from a 'liquid fractionation' process in which fractionated residual magma, instead of rising, periodically became denser and ponded on the temporary floor (a density crossover). Gradual, reversed cryptic trends in the cyclic units above the oxide gabbronorite layers may reflect mixing of this fractionated magma with successive magma additions.
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Anorthosite
Norite
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Norite
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Pentlandite
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Norite
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