Crystallization processes in the Bjerkreim-Sokndal layered intrusion, south Norway: evidence from the boundary between two macrocyclic units
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Layered intrusion
Magma chamber
Anorthosite
Phenocryst
Base (topology)
The Bushveld Complex, a layered mafic intrusion in South Africa, shows extreme vertical differentiation in terms of mineral compositions and modal proportions from dunite to ferrodiorite. In a continuous borehole core drilled through the uppermost 2·8 km of the intrusion, typical rocks range upwards from troctolite, through gabbronorite and ferrogabbronorite to ferrodiorite, with extreme examples of anorthosite, magnetitite and feldspathic pyroxenite. The An content of plagioclase has previously been determined for 420 samples and decreases upward from An78 to An36, with six minor, slow reversals. Variations in modal proportions of plagioclase have been calculated based on 2200 density determinations on whole-rocks. Forty-five anorthosite layers have been identified, ranging from 1 to 23 m thick. None of these layers is associated with the above-mentioned reversals in An content in plagioclase and nearly all have leucocratic rocks below and above, with more than the likely cotectic proportions of plagioclase. These observations argue against an origin for anorthosite related to magma addition or to supersaturation and oscillatory nucleation. Rhythmically pulsed crystallization, possibly associated with pressure changes, followed by crystal settling and sorting of minerals of different densities is a hypothesis consistent with all the observations. Twenty layers of magnetitite have been identified. There is a significant reversal in An content in the overlying plagioclase compared with the underlying sample across only one such layer. Again, this observation challenges hypotheses that such layers result from magma addition, but is consistent with a pressure-change hypothesis for triggering magnetite crystallization. The upper contacts of magnetitite layers that grade into anorthosite over many centimetres possibly also reflect settling and sorting. Rocks forming the uppermost 100 m of the intrusion contain the most sodic plagioclase compositions, demonstrating that there is no downward crystallizing roof facies. Furthermore, this uppermost 100 m section is depleted in plagioclase relative to its cotectic proportions. Hence, we find no evidence supporting flotation or prolonged suspension of plagioclase.
Anorthosite
Layered intrusion
Fractional crystallization (geology)
Magma chamber
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Anorthosite
Norite
Layered intrusion
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Intermediate-composition plagioclase (An40–60) is typically less dense than the relatively evolved basaltic magmas from which it crystallizes and the crystallization of plagioclase produces a dense residual liquid, thus plagioclase should have a tendency to float in these magmatic systems. There is, however, little direct evidence for plagioclase flotation cumulates either in layered intrusions or in Proterozoic anorthosite complexes. The layered series of the Poe Mountain anorthosite, southeast Wyoming, contains numerous anorthosite–leucogabbro blocks that constrain density relations during differentiation. All blocks are more mafic than their hosting anorthositic cumulates, their plagioclase compositions are more calcic, and each block is in strong Sr isotopic disequilibrium with its host cumulate. Associated structures—disrupted and deformed layering—indicate that (1) a floor was present during crystallization and that plagioclase was accumulating and/or crystallizing on the floor, (2) compositional layering and plagioclase lamination formed directly at the magma–crystal pile interface, and (3) the upper portions of the crystal pile contained significant amounts of interstitial melt. Liquid densities are calculated for proposed high-Al olivine gabbroic parental magmas and Fe-enriched ferrodioritic and monzodioritic residual magmas of the anorthosites taking into account pressure, oxygen fugacity, P2O5, estimated volatile contents, and variable temperatures of crystallization. For all reasonable conditions, calculated block densities are greater than those of the associated melt. The liquid densities, however, are greater than those for An40–60 plagioclase, which cannot have settled to the floor. Plagioclase must either have been carried to the floor in relatively dense packets of cooled liquid plus crystals or have crystallized in situ. A sloping floor, possibly produced by diapiric ascent of relatively light plagioclase-rich cumulates, is required to allow for draining and removal of the dense interstitial liquid produced in the crystal pile and may be a characteristic feature during the crystallization of many Proterozoic anorthosites and layered intrusions.
Anorthosite
Layered intrusion
Incompatible element
Fractional crystallization (geology)
Mineral redox buffer
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Fractional crystallization (geology)
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Abstract Bulk analyses of plagioclase megacrysts in massif anorthosites contain enough information on the 11 major element oxides of common igneous rocks to retrieve their complete parent magma compositions. The necessary partition coefficients for this inversion process are generated from a core-drilled trapped liquid and a pure anorthosite autolith in the Anorthosite–Norite–Troctolite (ANT) Nain Plutonic Suite (NPS) of Labrador. When applied to plagioclase megacrysts from five anorthositic intrusions in the NPS, the CIPW norms of the derived liquids correctly classify the parent liquids of dark plagioclase as olivine-normative and those of pale plagioclase as quartz-normative. Dark plagioclase megacrysts have higher An, lower K and some tetrahedral ferrous iron, compared to pale megacrysts. Experimental melts considered parental to the Lower Zone troctolites of the Kiglapait Layered Intrusion plot closely among the calculated olivine-normative liquids from the Nain anorthosites, lending further credibility to the inversions. The coupled nature of silica activity and oxygen fugacity for these and other anorthosite magma types permits a classification of massif anorthosites from melatroctolite to andesine norite based on silication and redox state, with potential for quantitative treatment. This treatment may also scale with the degree of crustal contamination experienced by ponded anorthositic magmas. Experimentally determined linear partitioning of plagioclase confirms a narrowing of the binary loop with high pressure, and allows retrieval of one unknown among pressure, temperature and liquid composition. These principles can be used for modeling the plagioclase component of liquids at their source, their intermediate storage sites, during ascent and at emplacement conditions. Experiments on a model Kiglapait bulk composition at high pressure suggest the separation of magma at 11 kbar from a spinel harzburgite source, with crystallization of olivine on ascent to saturation with plagioclase. Ponding of such high-temperature liquids in the lower or middle crust allows assimilation and silication leading to the noritic trend. Felsic suspensions leaving mafic crystals behind can account for anorthosite intrusions at upper levels. Lithospheric extension permits uplift of mantle melting regions to shallower depths, not to be confused with crust, as inferred for the Central North American Rift. Crustal melting appears to be neither necessary nor viable as a source of ANT (perhaps excluding monzonite) magmas. Origins and ideas: The array of anorthosite massifs in eastern North America is of a length scale equal to the volcanic Cameroon Line, which also has a heterogeneous distribution of ages on a very much shorter time scale. Precursor anorthosites far older than the commonly recognized Mesoproterozoic massifs must reflect a deep and revisited root cause of the anorthosite epoch, which may have begun near the time of origin of the Earth's inner core. A tectonically related superplume origin of massif anorthosite magmatism is appealing and has some credibility in paleogeographic reconstructions of Laurentia. It can also explain the apparent Al–Fe-rich nature of the ANT mantle source. The Mg#–An trends of two vastly different layered mafic intrusions, Bushveld and Kiglapait, are uncannily parallel, suggesting a fundamental, scale-independent, physicochemical behavior of large magma systems.
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Abstract A sequence of eight poikilitic anorthosite layers (labeled 1 to 8), within the Upper Main Zone in the eastern lobe of the Bushveld Complex, are exposed along a road-cut, 5.3 km northeast of the town of Apel, Limpopo Province. The anorthosite layers are meter-scale in thickness (0.4 to 10 m), have sharp contacts and are defined on the size and shape of pyroxene oikocrysts they contain. The anorthosite sequence is bounded by typical Main Zone gabbronorites. Euhedral, zoned primocrystic laths of plagioclase (An62.5-80.6; 0.2 to 4 mm long) are morphologically identical throughout the anorthosite sequence and define a moderate to strong foliation that is typically aligned parallel to the plane of layering. Interstitial clinopyroxene and orthopyroxene typically occur as large (0.8 to 80 cm) oikocrysts enclosing numerous partly rounded plagioclase chadacrysts. Rarely, orthopyroxene appears as subophitic crystals enclosing few and significantly smaller (0.08 to 0.4 mm), equant plagioclase inclusions. Detailed plagioclase and pyroxene mineral compositions for layers 2 to 5 show minimal variations within layers (0.1 to 2.3 mol% An and 0.7 mol% Mg#), whereas compositional breaks occur between layers (0.5 to 3.8 mol% An and 1.3 mol% Mg#). In layers 2 to 5, the An-content of plagioclase cores and the Mg# of both clinopyroxene and orthopyroxene crystals decrease by 2.5 mol%, 8.6 mol% and 13.0 mol% upwards, respectively. Bulk-rock incompatible trace element concentrations and patterns are similar for all analyzed anorthosite layers indicating that they are related to the same parental magma. However, bulk-rock major element oxides (e.g. Al2O3, TiO2, K2O) and atomic Mg# become more evolved upwards, consistent with magmatic differentiation. Based on the consistent plagioclase crystal morphologies and relatively constant chemistries within each anorthosite layer, we propose that each layer was formed by the intrusion of a plagioclase slurry. The upwards-evolving mineral chemistries, bulk-rock major element oxides and atomic Mg# suggests that each plagioclase slurry injection, that yielded an anorthosite layer, was derived from a slightly more fractionated parental magma prior to emplacement.
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