New Insights into the Kimberlites and Lamproites of Southern India via Ar/Ar dating and Nd isotope analysis

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 a lack 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 kimberlites and lamproites. Dates from three samples are presented, including two kimberlites and one lamproite (Pochampalle) 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 the oldest recorded lamproite in the world. However, our age data suggest that at least one lamproite (Pochampalle) was generated in the same region 150 Ma before the Chelima event. The Pochampalle lamproite was emplaced around ~1560 Ma as shown by the Ar-Ar data in this study, roughly 300 Ma before the other Krishna lamproites. It would seem that Pochampalle was also derived from an isotopically distinct source region with a lower 143Nd /144Nd ratio. This not only has implications for regional ultramafic magmatism, but also demonstrates that the mantle processes for producing lamproitic melts existed considerably earlier than previously thought.