Origin of clinopyroxene megacrysts from the 1.1 Ga Chigicherla-4 kimberlite (CC4), Dharwar craton, southern India: Implications for multi-stage metasomatism of the sub-continental lithospheric mantle

Megacrysts are large monomineralic crystals with an unknown relationship with the host rock and hence of debatable origin. We present major and trace element chemistry of clinopyroxene (cpx) megacrysts from the ca. 1.1 Ga diamondiferous CC4 kimberlite pipe, Dharwar craton, southern India in order to understand their parentage and ambient lithospheric conditions. The megacrysts were classified into three groups based on their chemistry. Two Cr-poor (Group-I and III) and one Cr-rich (Group-II) populations were identified. The Group-I diopsides have low Al2O3, Na2O, and REE, and positive Zr and Hf anomalies similar to those from deep-seated sheared peridotites. The Group-II and III diopsides, have higher Al2O3 and Na2O than the Group-I. The Group-II diopsides have elevated HREE and lower LREE than the Group-III. Thermobarometry indicates a pressure range of 56-67 Kbar for the Group-I samples, making them the deepest. The Group-II and the Group-III megacrysts appear to be shallower compared to the Group-I. Equilibrium melt composition calculated for all the three groups show a silicate-rich end member for the Group-I, whereas the Group-II samples show a carbonatitic equilibrium melt composition. Equilibrium melt composition of the Group-III megacrysts indicate elevated Nb, Zr, Hf, LREE, and HREE than that of natural basalt, carbonatite, and fresh kimberlite. Inter-element relations indicate that the Group-I and II are of metasomatic origin, whereas the Group-III samples suggest a fractionated history. Other trace element ratios indicate a deeper silicate and shallower carbonate melt related metasomatic overprint.

Automated summary

We conducted a study on the major and trace element chemistry of clinopyroxene megacrysts from the Dharwar craton in southern India to understand their origin and the lithospheric conditions. The megacrysts were classified into three groups based on their chemistry, indicating a possible relationship with metasomatism and fractionation processes.