Depleted Mantle-sourced CFB Magmatism in the Jurassic Africa–Antarctica Rift: Petrology and 40Ar/39Ar and U/Pb Chronology of the Vestfjella Dyke Swarm, Dronning Maud Land, Antarctica

Bohrson

2015

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and partial melting of crustal wall rock and (2) assimilationfractional crystallization equations for lithospheric mantle contamination by using highly alkaline continental volcanic rocks (i.e., partial melts of mantle lithosphere) as contaminants. Calculations indicate that the different magma types can be produced by just minor (1-15 wt%) contamination of asthenospheric parental magmas by melts from variable lithospheric reservoirs. Our models imply that the role of continental lithosphere as a CFB source component or contaminant may have been overestimated in many cases. Thus, CFBs may represent major juvenile crustal growth events rather than just recycling of old lithospheric materials.

Section: Geological Settingmentioning

confidence: 91%

“…Comparison of incompatible element and isotopic compositions ( Fig. 3b) of these two sub-provinces indicates development in distinct magmatic systems (Luttinen et al , 2015.…”

Section: Geological Settingmentioning

confidence: 99%

Section: Geological Settingmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Enriched continental flood basalts from depleted mantle melts: modeling the lithospheric contamination of Karoo lavas from Antarctica

Bohrson

2015

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Enrichment of 18O in the mantle sources of the Antarctic portion of the Karoo large igneous province

Whitehouse

2018

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Karoo continental flood basalt (CFB) province is known for its highly variable trace element and isotopic composition, often attributed to the involvement of continental lithospheric sources. Here we report oxygen isotopic compositions measured with secondary ion mass spectrometry for handpicked olivine phenocrysts from ~190-180 Ma CFBs and intrusive rocks from Vestfjella, western Dronning Maud Land, that forms an Antarctic extension of the Karoo province. The Vestfjella lavas exhibit heterogeneous trace element and radiogenic isotope compositions (e.g., εNd from-16 to +2 at 180 Ma) and the involvement of continental lithospheric mantle and/or crust in their petrogenesis has previously been suggested. Importantly, our sample set also includes rare primitive dikes that have been derived from depleted asthenospheric mantle sources (εNd up to +8 at 180 Ma). The majority of the oxygen isotopic compositions of the olivines from these dike rocks (δ 18 O = 4.4-5.2 ‰; Fo = 78-92 mol. %) are also compatible with such sources. The olivine phenocrysts in the lavas, however, are characterized by notably higher δ 18 O (6.2-7.5 ‰; Fo = 70-88 mol. %) and one of the dike samples gives intermediate compositions (5.2-6.1 ‰, Fo = 83-87 mol. %) between the other dikes and the CFBs. The oxygen isotopic compositions do not correlate with radiogenic isotope compositions susceptible to crustal assimilation (Sr, Nd, and Pb) or with geochemical indicators of pyroxene-rich mantle sources. Instead, δ 18 O correlates positively with enrichments in large-ion lithophile elements (especially K) and 187 Os. We suggest that the oxygen isotopic compositions of the Vestfjella CFB olivines primarily record large-scale subduction-related metasomatism of the sub-Gondwanan mantle (base of the lithosphere or deeper) prior to Karoo magmatism. The overall influence of such sources to Karoo magmatism is not known, but, in addition to continental lithosphere, they may be responsible for some of the geochemical heterogeneity observed in the CFBs.

Deep open storage and shallow closed transport system for a continental flood basalt sequence revealed with Magma Chamber Simulator

Spera

et al. 2019

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The Magma Chamber Simulator (MCS) quantitatively models the phase equilibria, mineral chemistry, major and trace elements, and radiogenic isotopes in a multicomponent-multiphase magma + wallrock + recharge system by minimization or maximization of the appropriate thermodynamic potential for the given process. In this study, we utilize MCS to decipher the differentiation history of a continental flood basalt sequence from the Antarctic portion of the ~ 180 Ma Karoo large igneous province. Typical of many flood basalts, this suite exhibits geochemical evidence (e.g., negative initial ε Nd) of interaction with crustal materials. We show that isobaric assimilation-fractional crystallization models fail to produce the observed lava compositions. Instead, we propose two main stages of differentiation: (1) the primitive magmas assimilated Archean crust at depths of ~ 10-30 km (pressures of 300-700 MPa), while crystallizing olivine and orthopyroxene; (2) subsequent fractional crystallization of olivine, clinopyroxene, and plagioclase took place at lower pressures in upper crustal feeder systems without significant additional assimilation. Such a scenario is corroborated with additional thermophysical considerations of magma transport via a crack network. The proposed two-stage model may be widely applicable to flood basalt plumbing systems: assimilation is more probable in magmas pooled in hotter crust at depth where the formation of wallrock partial melts is more likely compared to rapid passage of magma through shallower fractures next to colder wallrock.