Other
Scientific paper
Jan 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58..267p&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 1, pp.267-288
Other
27
Scientific paper
Flows of the lower six formations of the western Deccan Traps (Jawhar through Khandala) cover a range of Nd ( T ) from 0 to -20, ( 87 Sr / 86 Sr ) T 0.7062 to 0.7128 and 206 Pb / 204 Pb from 16.72 to 22.43. Oxygen isotopic data for fresh clinopyroxene and plagioclase separates indicate magmatic 18 O 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 Nd ( T ) +7 source magmas by two very different negative 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 Nd ( T ) 0.0 to -5.5, ( 87 Sr / 86 Sr T 0.7067 to 0.7085, and 206 Pb / 204 Pb 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 18 O 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.
Beane J.
Harris Caroline
Hooper Peter
Mahoney John J.
Peng Zhi-Xin
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