Magmatic rifting in the Main Ethiopian Rift began in thick continental lithosphere; the case of the Galema range
File(s)
Accepted version
Author(s)
Type
Journal Article
Abstract
The northern Main Ethiopian Rift (MER) in East Africa is considered a region of incipient oceanic spreading, with Miocene border faulting now largely abandoned at the expense of magmatic extension in the Wonji Fault Belt (WFB). However, whether magmatic extension began when the Ethiopian lithosphere was still-thick, or heavily stretched, is unknown. The
Galema range, a linear Pliocene dike swarm parallel to the eastern margin of the present-day central MER, is an ideal study locale to constrain melting depths, and by inference the thickness of the lithosphere, during early magmatic rifting. To address this issue, we present whole-rock, trace element data on 77 samples of Galema range magmas. We interpret contrasting results between two modeling approaches as evidence for magma ponding subsequent to melt generation. Trace element models of melt generation reveal melting conditions of TP=1418-1450°C at 2.9-3.2
GPa, some ~68-100°C above ambient. In contrast, Si/Mg activity thermobarometry, which probes the point at which these magmas last re equilibrated with the mantle, yielded broadly similar temperatures (1435-1474°C) but at lower pressures (2.1-2.6 ± 0.2 GPa: 78-89 km depth); these results are broadly parallel to contemporaneous magmatism on the western rift margin in the Akaki Magmatic Zone. We interpret these results as evidence for magma stalling at a thermo-mechanical boundary to ascent: the lithosphere-asthenosphere boundary. The Ethiopian continental lithosphere has therefore remained relatively thick late into the rifting process, with important potential implications for late-stage decompression melting prior to the onset of seafloor spreading.
Galema range, a linear Pliocene dike swarm parallel to the eastern margin of the present-day central MER, is an ideal study locale to constrain melting depths, and by inference the thickness of the lithosphere, during early magmatic rifting. To address this issue, we present whole-rock, trace element data on 77 samples of Galema range magmas. We interpret contrasting results between two modeling approaches as evidence for magma ponding subsequent to melt generation. Trace element models of melt generation reveal melting conditions of TP=1418-1450°C at 2.9-3.2
GPa, some ~68-100°C above ambient. In contrast, Si/Mg activity thermobarometry, which probes the point at which these magmas last re equilibrated with the mantle, yielded broadly similar temperatures (1435-1474°C) but at lower pressures (2.1-2.6 ± 0.2 GPa: 78-89 km depth); these results are broadly parallel to contemporaneous magmatism on the western rift margin in the Akaki Magmatic Zone. We interpret these results as evidence for magma stalling at a thermo-mechanical boundary to ascent: the lithosphere-asthenosphere boundary. The Ethiopian continental lithosphere has therefore remained relatively thick late into the rifting process, with important potential implications for late-stage decompression melting prior to the onset of seafloor spreading.
Date Issued
2021-12-15
Date Acceptance
2021-09-30
Citation
Lithos, 2021, 406-407, pp.1-16
ISSN
0024-4937
Publisher
Elsevier
Start Page
1
End Page
16
Journal / Book Title
Lithos
Volume
406-407
Copyright Statement
© 2021 Elsevier B.V. All rights reserved.
Sponsor
Natural Environment Research Council (NERC)
Identifier
https://www.sciencedirect.com/science/article/pii/S0024493721005375?via%3Dihub
Grant Number
NE/S014136/1
Subjects
Science & Technology
Physical Sciences
Geochemistry & Geophysics
Mineralogy
Continental rifting
Main Ethiopian Rift
East African Rift system
Mantle potential temperature
Lithosphere-asthenosphere boundary
UPPER-MANTLE
CRUSTAL STRUCTURE
FLOOD BASALTS
AFAR PLUME
PROTRACTED DEVELOPMENT
GEOCHEMICAL EVIDENCE
VELOCITY STRUCTURE
VOLCANISM
BENEATH
EVOLUTION
0402 Geochemistry
0403 Geology
0404 Geophysics
Geochemistry & Geophysics
Publication Status
Published
Date Publish Online
2021-10-14