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Pillow lava basalts with back-arc MORB affinity from the Usagaran Belt, Tanzania; relics of Orosirian ophiolites

Nelson Boniface and Tatsuki Tsujimori
Pillow lava basalts with back-arc MORB affinity from the Usagaran Belt, Tanzania; relics of Orosirian ophiolites
Journal of the Geological Society of London (April 2019) 176 (5): 1007-1021

Abstract

The Paleoproterozoic Usagaran Orogenic Belt in East-Africa preserves relics of Paleoproterozoic volcanic-arc magmas, and subducted and displaced oceanic-crust. This article describes the geochemical characteristics of displaced pillow basalts from the Usagaran Belt (Konse Group). Our data indicate that the Konse pillow basalts have tholeiitic composition and are overprinted by greenschist facies metamorphic conditions but their primary geochemical signatures are preserved by REE and fluid immobile elements. The (La/Sm)N ratios (0.62 - 1.09) and REE patterns point to Normal-MORBs and Transitional-MORBs mantle source. The analysis of high-valency elements and trace element patterns points to a mixed signature of MORB tholeiites and island-arc tholeiites with elevation of Ba, Th, U, Eu and Sr. This composition is similar to that of Phanerozoic back-arc suprasubduction-zone ophiolites. Therefore, Usagaran Belt pillow basalts make analogue of the Tethyan-type suprasubduction-zone ophiolite evolution and emplacement in the Precambrian. The emplacement of the Konse pillow basalts is older than the neighbouring 1920 - 1870 Ma volcanic-arc magmas and is probably coeval with the formation of the 2000 Ma Yalumba eclogites with MORB affinity. Therefore, the Konse Konse pillow basalts might have been displaced from its suprasubduction-zone tectonic setting of igneous construction within the Yalumba ocean basin around 2000 Ma.


ISSN: 0016-7649
EISSN: 2041-479X
Coden: JGSLAS
Serial Title: Journal of the Geological Society of London
Serial Volume: 176
Serial Issue: 5
Title: Pillow lava basalts with back-arc MORB affinity from the Usagaran Belt, Tanzania; relics of Orosirian ophiolites
Affiliation: University of Dar es Salaam, Dar es Salaam, Tanzania
Pages: 1007-1021
Published: 20190426
Text Language: English
Publisher: Geological Society of London, London, United Kingdom
Accession Number: 2019-044659
Categories: Igneous and metamorphic petrologyGeochemistry of rocks, soils, and sediments
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. strat. col., 1 table, geol. sketch maps
S07°10'00" - S07°00'00", E36°00'00" - E36°19'60"
Secondary Affiliation: Tohoku University, JPN, Japan
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data from The Geological Society, London, London, United Kingdom
Update Code: 201923
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