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Publication Details
AFRICAN RESEARCH NEXUS
SHINING A SPOTLIGHT ON AFRICAN RESEARCH
earth and planetary sciences
A role for lower continental crust in flood basalt genesis? Isotopic and incompatible element study of the lower six formations of the western Deccan Traps
Geochimica et Cosmochimica Acta, Volume 58, No. 1, Year 1994
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Description
Flows of the lower six formations of the western Deccan Traps (Jawhar through Khandala) cover a range of ε{lunate}Nd(T) from 0 to -20, (87Sr 86Sr)T 0.7062 to 0.7128 and 206Pb 204Pb from 16.72 to 22.43. Oxygen isotopic data for fresh clinopyroxene and plagioclase separates indicate magmatic δ18O values between +6.6 and +7.4%.. Previous isotopic studies of the upper four formations (Bushe through Mahabaleshwar) have revealed two major trends that, to a first approximation, correspond to variable contamination of ε{lunate}Nd(T) ≥ +7 source magmas by two very different negative ε{lunate}Nd lithospheric endmembers. Isotopic data for the lower six formations describe completely different arrays from those of the upper formations and the fields for the individual lower formations are also distinct from one another. Significantly, the lower formation arrays overlap at or converge toward a common range of isotopic signatures, with ε{lunate}Nd(T) ≈ 0.0 to -5.5, (87Sr 86SrT ≈ 0.7067 to 0.7085, and 206Pb 204Pb ≈ 19.2 to 20.9. These values are unlike those of oceanic mantle and intermediate between the extremes defined by the littlecontaminated Ambenali and highly crustally contaminated Bushe formations in the upper part of the stratigraphic sequence. One explanation for this common signature is that it represents a mantle source located in the continental lithosphere and quite distinct from the Ambenali-like source dominating the upper formations. However, the incompatible element patterns of the common-signature (and other lower formation) samples do not resemble those of typical Proterozoic or Phanerozoic continental mantle xenoliths and, unlike the Ambenali basalts, all of the lower formation samples analyzed to date have significantly higher δ18O values than oceanic lavas or the great majority of continental lithospheric mantle xenoliths. An alternative possibility is that the common signature magmas could be the products of a large-scale, open-system, lower crustal contamination process similar to that postulated for the thick mafic complex in the Ivrea Zone of northern Italy. If so, then the isotopic arrays emanating from the common signature would represent secondary contamination episodes involving at least three different crustal endmembers. In the upper formations, a two-stage mixing process also appears necessary to account for the Pb-Nd and Pb-Sr isotopic relationships displayed by data for the Bushe and Poladpur formations. Model calculations indicate that incompatible element patterns and isotopic ratios similar to those of the common-signature samples can be produced, while still maintaining a basaltic major and compatible trace element composition, by mixing a large-degree partial melt (~40%) of Indian Archean basic amphibolite into Ambenalitype or Réunion-type primitive magma. With the particular amphibolite composition used, the proportion of contamination required is large: roughly 10-30%, comparable to the amounts proposed for the mafic complex in the Ivrea Zone. More siliceous contaminants permit smaller amounts of contamination but generally yield poorer trace element fits. © 1994.
Authors & Co-Authors
Peng, Z. X.
United States, Honolulu
School of Ocean and Earth Science and Technology
Mahoney, J.
United States, Honolulu
School of Ocean and Earth Science and Technology
Hooper, P.
United States, Pullman
Washington State University Pullman
Harris, C.
South Africa, Cape Town
University of Cape Town
Beane, J.
United States, Pullman
Washington State University Pullman
Statistics
Citations: 224
Authors: 5
Affiliations: 3
Identifiers
Doi:
10.1016/0016-7037(94)90464-2
ISSN:
00167037