Carbonatitic and Alkaline Melt‐Induced Mantle Heterogeneity for Origin of Lamproites: Evidence From Petrology, Mineralogy and Geochemistry of Lamproites From Bokaro Coalfields of Gondwana Basin, Eastern India

ABSTRACT The spectacular mineralogy and geochemistry of lamproites are inherited from their mantle source, which suggests heterogeneity of sub‐continental lithospheric mantle (SCLM). Mantle heterogeneity may be a result of various metasomatic melt activities. The Gondwana basin lamprophyres and lamproites have some unique geochemical characters like higher abundances of TiO 2 , Nb, Hf, Zr and total REE which make them different from similar rock suites of other parts of the globe. These unique characters along with negative ε Nd (t) values (−2.88 to −3.52, this study) suggest that their mantle source might have been modified by multiple phases of metasomatism. The present research focuses on two major categories of lamproites—phlogopite lamproites from east Bokaro and olivine lamproites from west Bokaro coalfields. The phlogopite lamproites are relatively enriched in SiO 2 , K 2 O, Nd, Sm, Y, Zr and Hf compared to olivine lamproites, which are SiO 2 poor, but enriched in MgO, Fe 2 O 3T , CaO and Cr. The higher abundance of Nb and total REE along with high Zr/Hf and initial 87 Sr/ 86 Sr ratios in Bokaro lamproites suggest carbonatite metasomatism which was pervasive throughout the mantle peridotite. Higher abundance of SiO 2 , Al 2 O 3 , K 2 O, TiO 2 and Rb in east Bokaro lamproites indicate metasomatism of the mantle source by alkaline‐Ti‐rich silicate melt, which was not all pervasive but occurred as metasomatic phlogopite‐apatite‐rutile veins. Besides these, evidence of relict ancient subduction‐zone metasomatism is indicated by Ta‐trough and Pb‐crest in OIB‐normalised diagram. It can be concluded that the mantle source for east Bokaro lamproites had greater inputs from phlogopite‐apatite‐rutile bearing metasomatic veins with lesser contributions of carbonatite‐metasomatised peridotite wall‐rock; whereas for west Bokaro lamproites the source inputs were dominantly derived from carbonatite‐metasomatised peridotite wall‐rock.