Chelima dykes, Cuddapah Basin, southern India: A review of the age, petrology, geochemistry and petrogenesis of world's oldest lamproites

Chelima dykes represent an important phase of igneous activity in the Palaeo- to Meso- Proterozoic Cuddapah Basin of southern India. These dykes are also widely regarded as a source of nearby alluvial diamonds. Recently, questions have been raised regarding their petrological status as lamproites and on the possibility of crustal contamination influencing their geochemistry. One of the objectives of the present review is to address these issues. Petrology and geochemistry are indeed consistent with their nomenclature as lamproites. Crustal contamination is shown to be minimal in influencing their geochemistry. Available geochronological data make them the oldest yet recorded lamproites (ca. 1400 Ma) in the world. Chelima dykes were derived from an ancient and anomalously enriched melt source region with lower-time integrated Sm/Nd ratios than Bulk Earth and evolved in isolation from the convecting mantle. Petrogenetic modelling reveal that their source regions have been strongly depleted in the garnet stability field followed by enrichment by a metasomatic melt rich in LREE and other incompatible elements before final partial melting. Despite their geographical proximity the Chelima dykes (along with those at Zangamarajupalle) are considered to be temporally as well as genetically unrelated to the (i) other igneous activities in the Cuddapah sediments and (ii) adjoining kimberlites in the Wajrakarur area towards the western margin of the Cuddapah Basin. Chelima dykes occur in an altogether different geodynamic setting compared to the deformed and metamorphosed alkaline rocks of the Eastern Ghats Mobile Belt which are recently opined to have been located in an ancient suture zone.