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Deep crustal cumulates reflect patterns of continental rift volcanism beneath Tanzania

  • Emily J. Chin
Original Paper

Abstract

Magmatism on Earth is most abundantly expressed by surface volcanic activity, but all volcanism has roots deep in the crust, lithosphere, and mantle. Intraplate magmatism, in particular, has remained enigmatic as the plate tectonic paradigm cannot easily explain phenomena such as large flood basalt provinces and lithospheric rupture within continental interiors. Here, I explore the role of deep crustal magmatic processes and their connection to continental rift volcanism as recorded in deep crustal xenoliths from northern Tanzania. The xenoliths are interpreted as magmatic cumulates related to Cenozoic rift volcanism, based on their undeformed, cumulate textures and whole-rock compositions distinct from melt-reacted peridotites. The cumulates define linear trends in terms of whole-rock major elements and mineralogically, can be represented as mixtures of olivine + clinopyroxene. AlphaMELTS modeling of geologically plausible parental melts shows that the end-member cumulates, clinopyroxenite and Fe-rich dunite, require fractionation from two distinct melts: a strongly diopside-normative melt and a fractionated picritic melt, respectively. The former can be linked to the earliest, strongly silica-undersaturated rift lavas sourced from melting of metasomatized lithosphere, whereas the latter is linked to the increasing contribution from the upwelling asthenospheric plume beneath East Africa. Thus, deep crustal cumulate systematics reflect temporal and compositional trends in rift volcanism, and show that mixing, required by the geochemistry of many rift lava suites, is also mirrored in the lavas’ cumulates.

Keywords

Cumulates Continental rift Mixing Deep crust 

Notes

Acknowledgements

I thank Cin-Ty A. Lee for providing the xenolith samples analyzed in this study. Joseph Boesenberg and Soumen Mallick are thanked for their assistance on the electron microprobe and the laser-ablation ICP-MS, respectively. A Brown University Postdoctoral Fellowship to E. J. Chin provided support for analytical costs for LA-ICPMS and EPMA analyses and a GSA Grant-in-Aid provided funds for XRF analyses. I thank Martijn Klaver and Roberta Rudnick for their insightful and constructive reviews, and Executive Editor Othmar Muntener for his efficient handling of the manuscript. This manuscript benefited from informal discussions with Grant Bybee and Kei Shimizu.

Supplementary material

410_2018_1512_MOESM1_ESM.xlsx (82 kb)
Supplementary material 1 (XLSX 82 KB)

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Authors and Affiliations

  1. 1.Scripps Institution of Oceanography—University of California San DiegoLa JollaUnited States

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