Bilateral geochemical asymmetry in the Karoo large igneous province

Arto V Luttinen

doi:10.1038/s41598-018-23661-3

Arto V Luttinen

Open Access

https://doi.org/10.1038/s41598-018-23661-3

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Abstract

In the Karoo large igneous province, the geochemical assessment of mantle source variability and structure is hampered by probable crustal contamination overprinting of compositionally diverse flood basalts. Mantle source characteristics have been defined only for exceptional, primitive rock types. Here I use a compiled dataset for over 800 samples to demonstrate that the abundance of Nb relative to Zr, Ti, and Y provides a useful geochemical tracer of mantle sources for variably contaminated rock types of the Karoo province. Variations in the relative abundance of Nb reveal emplacement of distinctive, Nb-undepleted and Nb-depleted magmas in the North Karoo and South Karoo sub-provinces, respectively, and clarify correlation between flood basalts and previously proposed mantle source components. Judging from plate tectonic reconstructions and the compositions of plausible mantle source components, the geochemical bilateral asymmetry in Karoo may reflect tapping of contrasting plume and upper mantle reservoirs in the two sub-provinces.

Highlights

Large igneous provinces (LIPs) are expressions of very large-scale mantle melting events that frequently relate to continental breakup, but are not explained by the plate-tectonic concept of magmatic activity
Generalizing, normal mid-ocean ridge basalts (MORBs) have relatively Nb-depleted compositions, whereas ocean island basalts (OIBs and MORBs generated at enriched ridge segments exhibit Nb-undepleted compositions (Fig. 2) and have been associated with mantle plume-related magmatism[31,32,33]
From the viewpoint of Continental flood basalt (CFB) mantle sources, it is crucial to note that ΔNb is an isotope-like parameter, i.e. it is only weakly affected by variable degrees of fractional crystallisation or mantle melting[33] and that the continental crust is Nb-depleted

Summary

Introduction

Large igneous provinces (LIPs) are expressions of very large-scale mantle melting events that frequently relate to continental breakup, but are not explained by the plate-tectonic concept of magmatic activity. While some continental LIPs can be geochemically associated with plume sources based on abundant ocean island basalt (OIB) -like rock types (e.g. Ethiopian Traps)[3], many CFB provinces are characterised by isotopic and incompatible element ratios typical of continental crust Whether or not this geochemical ‘crustal signature’ results entirely from crustal contamination of plume-sourced magmas[4] or represents a primary feature derived from non-plume sources in subcontinental lithospheric mantle (SCLM)[5] or subduction-modified convective upper mantle[6], or from exotic plume components[7] has been a long-standing controversy. Characteristics of primary magmas and mantle sources in the Karoo LIP have been constrained in studies of picritic rock types that preserve primary compositional features These are compositionally notably variable and point to melting of SCLM20,21 and different recycled lithospheric[22,23,24,25] and subduction fluid-influenced components[23] in the convective upper mantle

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