Geochemistry and petrogenesis of the Tsirub nephelinite intrusions, southern Namibia

Abstract The widespread Emeishan igneous province in southwestern China comprises the Emeishan continental flood basalts (ECFB) and associated mafie-ultramafic intrusions. The ECFB have variable SiO2, ranging from 43.6 to 52.1 wt%, Al2O3 from 5.0 to 12.6 wt%, and total alkali (K2O + Na2O) from 0.7 to 6.5 wt%. These oxides exhibit negative correlations with MgO (5.4 - 23.1 wt%), implying fractional crystallization of olivine and clinopyroxene, which occur as phenocrysts in the rocks. Linear correlations between Zr, Nb, and La suggest that crustal contamination is not important. The primitive-mantle-normalized trace-element patterns show that the ECFB are enriched in high-field-strength trace elements, large-ion-lithophile elements, and light-rare-earth elements, similar to ocean-island basalt. Incompatible element ratios of the ECFB, such as Zr/Nb (7-10), Th/La (0.1-0.15), and Rb/Nb (0.9-1.7), differ from those of primitive mantle, N-MORB, and continental crust, but are similar to ocean-island basalts from an enriched mantle source (EM-1). However, the ECFB have isotopic ratios (143Nd/144Nd = 0.51229 -0.51276 and 87Sr/86Sr = 0.70480-0.70647) that imply that the ECFB were derived from a homogeneous, primitive lower mantle carried upward by a mantle plume. We propose that the original melts derived from the mantle plume were contaminated through interaction at shallower depth with an enriched lithospheric mantle. This model suggests that the lithospheric mantle beneath the ECFB was modified by subduction of an oceanic slab.

Flood basalt

Mantle plume

Large igneous province

Crustal recycling

Primitive mantle

Fractional crystallization (geology)

Incompatible element

10.1080/00206810109465009

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Citations (165)

Geochemistry of lavas from the Ahu and Tupa volcanic fields, Easter Hotspot, southeast Pacific: Implications for intraplate magma genesis near a spreading axis

Earth and Planetary Science Letters (1996)

Karsten M. Haase Colin W. Devey

Hotspot (geology)

Mantle plume

Incompatible element

Primitive mantle

10.1016/0012-821x(95)00217-z

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Citations (63)

Intracontinental anorogenic alkaline magmatism and carbonatites, associated mineral systems and the mantle plume connection

Gondwana Research (2014)

Franco Pirajno

Carbonatite

Mantle plume

Large igneous province

10.1016/j.gr.2014.09.008

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Citations (124)

Trace element constraints on mantle sources during mid-Proterozoic magmatism: evidence for a link between the Gardar (South Greenland) and Abitibi (Canadian Shield) mafic rocks

Canadian Journal of Earth Sciences (2007)

Ralf Halama Jean‐Louis Joron B. Villemant Gregor Markl M. Treuil

Trace and major element compositions of mid-Proterozoic (1.20–1.16 Ga) basaltic lava flows and dikes from the Gardar Province (South Greenland) provide evidence for two geochemically distinct magma sources. Based on distinct features of incompatible trace element ratios, such as Th/Ta, Th/Tb, or Th/Hf, they differ by the composition of their mantle source and by their partial melting trends. One mantle source is compositionally transitional between mid-ocean ridge basalt (MORB)-type and ocean-island basalt (OIB)-type sources with relatively low Ta/Hf ratios (~0.2), moderate enrichment in light rare-earth elements (LREE), and slightly positive initial ε Nd values (+2). It can be attributed to either a lithospheric mantle source or a depleted astenospheric mantle plume component that has been enriched shortly prior to eruption. The other mantle source is characterized by high Ta/Hf ratios (~0.6), a more pronounced LREE enrichment, and initial ε Nd values around 0. Elevated Ce N /Yb N (7.0–9.8) and Tb N /Yb N ratios (1.6–1.8) of the rocks derived from this source indicate the presence of garnet during melting, suggesting melt generation at depths > 70 km. This mantle source has the geochemical characteristics of an OIB-type source and is interpreted as originating from a mantle plume. Samples from the slightly younger (1.14 Ga) Abitibi dike swarm (Superior Province, Canada), spatially connected to the Gardar Province, show very similar trace element characteristics and the same two distinct magma sources can be identified. The geochemical similarities between the magma sources in South Greenland and Canada support the idea of a genetic link between the two magmatic provinces. This link strengthens the idea that the system was a long-lived major intracontinental rift zone.

Mantle plume

Primitive mantle

Trace element

Incompatible element

10.1139/e06-108

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Citations (11)

Genesis and crystallization of ultramafic alkaline carbonatite magmas of Siberia: ore potential, mantle sources, and relationship with plume activity

Russian Geology and Geophysics (2016)

N. V. Vladykin

Carbonatite

Massif