Mineralogy and origin of the alkaline Nsungwe Formation tuffs of the Rukwa Rift Basin, southwestern Tanzania

Lawrence, L., Spandler, C., Roberts, E.M., and Hilbert-Wolf, H.L. (2021) Mineralogy and origin of the alkaline Nsungwe Formation tuffs of the Rukwa Rift Basin, southwestern Tanzania. Lithos, 380-381. 105885.

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The incipient phases of continental rift development remain enigmatic, as early rift sequences are rarely exposed, or poorly preserved in the geologic record. Here, we present petrographic and geochemical analyses of heavy mineral separates from pervasively weathered volcanic horizons intercalated within syn-rift sedimentary deposits from the Rukwa Rift Basin, southwestern Tanzania. These Oligocene (25–26 Ma) ashfalls represent the first magmatic products associated with the initiation of rifting in the western branch of the East African Rift System (EARS). In this work, we combine detailed single crystal petrography with in-situ geochemistry to characterise an alkaline volcanic mineral assemblage consisting of titanite, phlogopite, fluorapatite, clinopyroxene, Ti-rich andradite, sanidine, pyrochlore, nepheline, cancrinite, Fe-sulphide (pyrrhotite), calcite, Na-REE‑carbonate and peralkaline silicate melt. These minerals are significantly enriched in trace elements, particularly Ba, Sr and rare earth elements (REE), with REE + Y contents consistent with derivation from peralkaline phonolitic melts. Placed in context of EARS development, these tuffs display the hallmarks of explosive alkaline‑carbonatite volcanism in their resistant minerals and inclusions; the nature and occurrence of which is consistent with early rift progression elsewhere in the EARS. Importantly, we showcase the depth of extractable petrographic information preserved in extensively weathered volcanic ash horizons deposited into an active continental rift basins. Such an approach may have significant implications for recognising early volcanogenic sequences in continental rift zones worldwide, but particularly in other segments of the western branch of the EARS.

Item ID: 66968
Item Type: Article (Research - C1)
ISSN: 1872-6143
Keywords: Carbonatite, Mineral inclusions, Oligocene, Peralkaline tuff, Rukwa Rift, Titanite
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Copyright Information: © 2020 Elsevier B.V. All rights reserved.
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A version of this publication was included as Chapter 2 of the following PhD thesis: Lawrence, Leigh (2021) Geochemistry and origin of explosive alkaline volcanism in the early Rukwa Rift, southwestern Tanzania. PhD thesis, James Cook University, which is available Open Access in ResearchOnline@JCU. Please see the Related URLs for access.

Date Deposited: 03 Nov 2021 04:07
FoR Codes: 37 EARTH SCIENCES > 3705 Geology > 370505 Mineralogy and crystallography @ 50%
37 EARTH SCIENCES > 3705 Geology > 370512 Volcanology @ 50%
SEO Codes: 25 MINERAL RESOURCES (EXCL. ENERGY RESOURCES) > 2503 Mineral exploration > 250307 Titanium minerals, zircon, and rare earth metal ore (e.g. monazite) exploration @ 100%
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