2020 年 115 巻 2 号 p. 202-215
Petrology and geochemistry (including Sr and Nd isotopes) of two lamprophyre dykes, intruding the Archaean granitic gneisses at Sivarampeta in the diamondiferous Wajrakarur kimberlite field (WKF), eastern Dharwar craton, southern India, are presented. The Sivarampeta lamprophyres display porphyritic–panidiomorphic texture comprising macrocrysts/phenocrysts of olivine, clinopyroxene (augite), and mica set in a groundmass dominated by feldspar and comprising minor amounts of ilmenite, chlorite, carbonates, epidote, and sulphides. Amphibole (actinolite–tremolite) is essentially secondary in nature and derived from the alteration of clinopyroxene. Mica is compositionally biotite and occurs as a scattered phase throughout. Mineralogy suggests that these lamprophyres belong to calc–alkaline variety whereas their bulk–rock geochemistry portrays mixed signals of both alkaline as well as calc–alkaline (shoshonitic) variety of lamprophyres and suggest their derivation from the recently identified Domain II (orogenic–anorogenic transitional type mantle source) from eastern Dharwar craton. Trace element ratios imply melt–derivation from an essentially the garnet bearing–enriched lithospheric mantle source region; this is further supported by their 87Sr/86Srinitial (0.708213 and 0.708507) and ‘enriched’ εNdinitial (−19.1 and −24.2) values. The calculated TDM ages (2.7–2.9 Ga) implies that such enrichment occurred prior to or during Neoarchean, contrary to that of the co–spatial and co–eval kimberlites which originated from an isotopically depleted mantle source which was metasomatized during Mesoproterozoic. The close association of calc–alkaline shoshonitic lamprophyres, sampling distinct mantle sources, viz., Domain I (e.g., Udiripikonda) and Domain II (Sivarampeta), and kimberlites in the WKF provide further evidence for highly heterogeneous nature of the sub–continental lithospheric mantle beneath the eastern Dharwar craton.
