New Ar–Ar ages of southern Indian kimberlites and a lamproite and their geochemical evolution

Abstract

The kimberlites and lamproites of southern India are thought to have formed in the most prolific known period of Precambrian ultramafic/ultrapotassic magmatism at around 1100 Ma. This study reports new age data for southern Indian ultrapotassic rocks (kimberlites and lamproites), a controversial topic due to the wide range of published age data and disagreements over the reliability of previously published ages. In this study we obtained new high-precision Ar–Ar data that better constrain the ages of southern Indian ultrapotassic rocks. Dates from three samples are presented, including two kimberlites from Wajrakarur kimberlite field and one lamproite from the Krishna lamproite field. These age data are then combined with bulk-rock geochemical and Nd isotopic data to provide further constraints on the source region and primary magma composition of southern Indian kimberlites and lamproites. Previously, the Chelima lamproite (ca. 1400 Ma) was considered to be one of the oldest lamproites in the world. However, our age data suggest that at least one lamproite (Pochampalle) was generated in the same region 100 Ma before the Chelima event. The Pochampalle lamproite was emplaced around ∼1500 Ma as shown by the Ar–Ar data in this study, roughly 250 Ma before the other Krishna lamproites. It would seem that the Pochampalle lamproite was also derived from an isotopically distinct source region with a lower 143Nd/ 144Nd ratio than other lamproites in the Krishna field. These findings not only have implications for regional ultramafic/ultrapotassic magmatism, but also demonstrate that the mantle processes for producing lamproitic melts existed earlier than previously thought.