Geochronological studies on whole-rock basalts, Deccan Traps, India: evaluation of the timing of volcanism relative to the K-T boundary

Abstract

KAr and 40 Ar 39 Ar incremental heating studies have been carried out on whole-rock basalts spanning different sections of the Deccan Traps. KAr dates typically fall in the range ∼ 55–65 Ma and, for an individual section, are commonly out of stratigraphic order; indicating post-crystallization loss of 40Ar ∗ from some samples. Specimens from six lava flows spanning a composite Western Ghats section and another six from sections in the southern, western, and northern areas of the Deccan, were selected for 40 Ar 39 Ar step-heating studies. Many of these specimens yield low temperature steps with apparent ages < 60 Ma, indicating post-crystallization loss of 40Ar ∗. The following (low-intermediate temperature) steps commonly yield apparent ages (> 70 Ma) older than crystallization values. Such spectra are characteristic of fine-grained basalts exhibiting 39Ar recoil loss and could involve: (1) 39Ar internal redistribution, out of K-rich sites into K-poor phases, and/or (2) 39Ar recoil loss out of the sample. Comparison of total gas ages with KAr dates permitted constraints to be placed on these two mechanisms. For some rocks, plateau ages were recovered from the intermediate-high temperature steps in the range 64–66 Ma, with 1σ errors of 0.5–1.0 m.y. A dyke specimen from the western Deccan, with a KAr date of ∼ 97 Ma, shows a descending staircase type of age spectrum and its estimated crystallization age is ∼ 65 Ma. Two lava flows from the central Deccan with low KAr dates ( ∼ 50–55 Ma) yield crystallization ages of ∼ 65 Ma; KAr dates deviating significantly from ∼ 65 Ma, do not reflect crystallization ages. Integration of the age data with the magnetic polarity of the different sections of lavas suggests the main reversed polarity epoch trapped in the Deccan volcanic province is chron 29R, which includes the Cretaceous-Tertiary boundary (estimated age 64.5 Ma). The plateau ages presented herein tend to be slightly older ( ∼ 1%) than the age of chron 29R; this probably results from minute amounts of 39Ar recoil loss out of the sites degassed in the intermediate-high temperature range. The Western Ghats section of the Deccan Traps, representing > 80% of the exposed material, was extruded in ∼ m.y.