Petrogenesis of macrocrystic and aphanitic intrusions in Mesoproterozoic diamondiferous pipe 2 kimberlite, Wajrakarur kimberlite field, eastern Dharwar craton, southern India

Abstract

Mesoproterozoic Pipe 2 kimberlite intruding granitic gneisses in the Wajrakarur Kimberlite Field (WKF) of Eastern Dharwar craton, southern India, is one of the first discovered pipes from this field. The two different intrusions belonging to coherent facies are observed in this kimberlite which are texturally distinct i.e., macrocrystic (with abundant macrocrysts) as well as aphanitic (devoid of or very fewer macrocrysts). Petrographic observations give evidence of presence of veined metasomatic mantle in the source region. Distinct geochemical variation is observed between macrocrystic (Mg# 75.4-76.6, SiO₂: 34.3-35.4 wt%, MgO: 18.7-20.4 wt%) and aphanitic samples (Mg# 74.7-75.5, SiO₂: 30.4-31.8 wt%, MgO: ∼21 wt%), furthermore aphanitic samples are relatively more enriched in trace and rare earth elements. Geochemical studies point out that aphanitic and macrocrystic varieties of Pipe 2 kimberlite to be products of the same mantle source but have experienced different evolutionary histories. The inferred primary magma of Pipe 2 kimberlite (for both aphanitic and macrocrystic) is suggested to have a composition of ∼33 wt% SiO2, Mg# ∼75.6 and ∼460 ppm Ni. Pipe 2 primary magma can be derived by using forward melting model assuming ∼1% partial melting of a source enriched in light REE by a factor of ∼11 x chondrite and almost chondritic heavy REE with 2% residual garnet. Differences between aphanitic and macrocrystic varieties in their chondrite normalized REE abundance patterns can be explained by about 5% crystal fractionation of primary magma and not by variations in the degree of partial melting. The primitive mantle normalized trace element patterns exhibit significant negative K, Sr, P, Ti and Hf anomalies that are interpreted to be characteristic of the primary magma. Combined petrology and whole rock geochemistry including compatible and incompatible element abundances and their ratios confirm that Pipe 2 kimberlite intrusions are archetypal Group I kimberlites similar to other kimberlites of EDC and not lamproites.