A ROLE FOR LOWER CONTINENTAL-CRUST IN FLOOD-BASALT GENESIS - ISOTOPIC AND INCOMPATIBLE ELEMENT STUDY OF THE LOWER 6 FORMATIONS OF THE WESTERN DECCAN TRAPS

Flows of the lower six formations of the western Deccan Traps (Jawhar through Khandala) cover a range epsilon(Nd)(T) from 0 to -20, (Sr-87/Sr-86)T from 0.7062 to 0.7128, and Pb-206/Pb-204 from 16.72 to 22.43. Oxygen isotopic data for fresh clinopyroxene and plagioclase separates indicate magmatic deltaO-18 values between +6.6 and +7.4 parts per thousand. 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 epsilon(Nd)(T) greater-than-or-equal-to +7 source magmas by two very different negative epsilon(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 epsilon(Nd)(T) almost-equal-to 0.0 to -5.5, (Sr-87/Sr-86)T almost-equal-to 0.7067 to 0.7085, and Pb-206/Pb-204 almost-equal-to 19.2 to 20.9. These values are unlike those of oceanic mantle, and intermediate between the extremes defined by the little-contaminated 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 delta O-18 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 (approximately 40%) of Indian Archean basic amphibolite into Ambenali-type or Reunion-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.