Basalts of the northeastern Deccan Traps, India: Isotopic and elemental geochemistry and relation to southwestern Deccan stratigraphy

Abstract

The northeastern Deccan Traps include flows with strong isotopic and elemental affinities to lavas in the southwest as far as 900 km away. However, many of the northeastern flows show a negative correlation of 206 Pb/ 204 Pb with ε Nd ( t ) and positive correlation of 206 Pb/ 204 Pb with ( 87 Sr/ 86 Sr) t , distinct from any of the isotopic fields for the southwestern formations; this array (from ε Nd ( t )=+5, ( 87 Sr/ 86 Sr) t =0.7041, 206 Pb/ 204 Pb=17.9 to ε Nd ( t )=0, ( 87 Sr/ 86 Sr),=0.7068, 206 Pb/ 204 Pb=20.5) overlaps the Ambenali Formation (Fm.) field and runs toward that of the “common signature” [ Peng et al ., 1994] that appears to have been an important magma type in several of the lower formations of the southwest. This array appears to represent mixing between an Ambenali‐type and (1) a common‐signature end‐member or (2) an end‐member with even higher 206 Pb/ 204 Pb and 87 Sr/ 86 Sr and lower ε Nd than the common signature; such mixing has been postulated, but not documented previously, for the Deccan. Lavas isotopically and chemically indistinguishable from the Ambenali Fm. form the tops of sections near Jabalpur and Chikaldara. Beneath them are flows that chemically resemble the Poladpur Fm. (which sits immediately below the Ambenali in the southwest) and several lavas similar to the Khandala Fm. (beneath the Poladpur and intervening Bushe Fm. in the southwest). Given the general stratigraphic correspondence with the southwestern sequence, many of these northeastern flows may be petrogenetically related to the respective southwestern formations. Lavas with broadly Poladpurand Khandala‐like elemental compositions are also abundant in a thick section south of Mhow; several other Mhow‐area flows resemble the distinctive Bushe Fm. However, nearly all of the Poladpur‐ and Khandala‐like northeastern Deccan lavas analyzed have higher 206 Pb/ 204 Pb man any southwestern Poladpur or Khandala Fm. basalts. If they indeed are related petrogenetically to these formations, many of the northeastern magmas interacted less with lOw‐ 206 Pb/ 204 Pb continental material and thus must have erupted from different feeder systems.