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Evolution of the lithospheric mantle beneath the East African Rift in Tanzania and its potential signatures in rift magmas

Sonja Aulbach, Roberta L. Rudnick and William F. McDonough
Evolution of the lithospheric mantle beneath the East African Rift in Tanzania and its potential signatures in rift magmas (in Volcanism and evolution of the African lithosphere, Luigi Beccaluva (editor), Gianluca Bianchini (editor) and Marjorie Wilson (editor))
Special Paper - Geological Society of America (June 2011) 478: 105-125

Abstract

New and published whole-rock major-element contents of xenolithic peridotites, combined with mineral trace-element, (super 87) Sr/ (super 86) Sr, (super 143) Nd/ (super 144) Nd, and (super 3) He/ (super 4) He isotopic compositions, are used to unravel the metasomatic history of lithospheric mantle sampled by volcanic pipes in the Tanzanian section of the East African Rift. The deepest portion of the mantle beneath Labait (craton margin) exhibits high-mu (HIMU)-like (super 87) Sr/ (super 86) Sr (0.7029), (super 143) Nd/ (super 144) Nd (0.51286), and (super 3) He/ (super 4) He (5.9), which may reflect the plume in this region. Within the Mozambique belt, recent calcio-carbonatite melt metasomatism has overprinted the mantle lithosphere signature beneath Olmani, leading to high whole-rock Ca/Al and low SiO (sub 2) , and remarkably homogeneous (super 87) Sr/ (super 86) Sr (0.7034-0.7035) and (super 143) Nd/ (super 144) Nd (0.51281-0.51283) of clinopyroxenes. Identical Sr and Nd isotope values are also reported for clinopyroxenes from peridotite xenoliths from the northern portion of the Gregory Rift in Tanzania (Pello Hill and Eledoi), which have a strong rift magma overprint. The silicate and carbonatite metasomatic melts are likely to be related to recent plume-derived magmatism of the East African Rift, and thus (super 87) Sr/ (super 86) Sr and (super 143) Nd/ (super 144) Nd values of the clinopyroxene from these samples can be used to define the rift isotopic signature beneath northern Tanzania. Some mantle regions beneath Lashaine and Labait escaped the recent rift-related overprint and have highly variable Sr-Nd isotope systematics. Labait clinopyroxenes show a near-vertical array on a (super 87) Sr/ (super 86) Sr versus (super 143) Nd/ (super 144) Nd plot, indicating highly variable time-integrated rare earth element (REE) patterns and low time-integrated Rb/Sr. Lashaine peridotites range to much higher (super 87) Sr/ (super 86) Sr at a given (super 143) Nd/ (super 144) Nd, and several plot in the right quadrants in the (super 87) Sr/ (super 86) Sr versus (super 143) Nd/ (super 144) Nd diagram, suggesting the influence of a (subducted?) Archean upper continental crust component on the lithospheric mantle beneath Lashaine. Their variable whole-rock SiO (sub 2) and high Na (sub 2) O contents, and clinopyroxene with high Sr/Y, low Sm/Nd, and variable Zr/Sm are consistent with this interpretation. Silicate lavas from the eastern branch of the East African Rift show increasingly evolved Sr and Nd isotope composition from north to south and hence increasing input of ancient metasomatized lithosphere ("EM1" and "EM2" components), similar to that beneath Lashaine and Labait, and well outside the suggested range in isotope compositions of the heterogeneous Kenya plume ( (super 87) Sr/ (super 86) Sr = 0.7029-0.7036; (super 143) Nd/ (super 144) Nd = 0.51275-0.51286). In the western branch, the "anomalous" Sr signature identified in Lashaine peridotites is prominent in silicate lavas and may indicate that the lithospheric mantle beneath that area was similarly enriched during ancient subduction. By contrast, the Sr-Nd isotope systematics of carbonatites reflect EM1 but not EM2 inputs, suggesting that such melts in the East African Rift neither derive from nor have interacted with subduction-modified mantle regions.


ISSN: 0072-1077
EISSN: 2331-219X
Coden: GSAPAZ
Serial Title: Special Paper - Geological Society of America
Serial Volume: 478
Title: Evolution of the lithospheric mantle beneath the East African Rift in Tanzania and its potential signatures in rift magmas
Title: Volcanism and evolution of the African lithosphere
Author(s): Aulbach, SonjaRudnick, Roberta L.McDonough, William F.
Author(s): Beccaluva, Luigieditor
Author(s): Bianchini, Gianlucaeditor
Author(s): Wilson, Marjorieeditor
Affiliation: University of Maryland, Department of Geology, College Park, MD, United States
Affiliation: Universita di Ferrara, Dipartimento di Scienze della Terra, Ferrara, Italy
Pages: 105-125
Published: 201106
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
ISBN: 978-0-8137-2478-2
References: 111
Accession Number: 2014-002969
Categories: Igneous and metamorphic petrologySolid-earth geophysics
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables, sketch maps
S11°40'00" - S01°00'00", E29°30'00" - E40°30'00"
Secondary Affiliation: University of Leeds, GBR, United Kingdom
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201404
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