Abstract:
Mineral chemistry, bulk-rock geochemistry and radiogenic isotope (Sr and Nd) data of surface and sub-surface samples of early-Cretaceous Kerguelen mantle plume linked ultrapotassic intrusive rocks from previously unstudied localities of Raniganj and Jharia Gondwana sedimentary basins, Damodar Valley, eastern India, are presented. Despite considerable textural diversity and variable mineralogy these rocks display broadly similar geochemistry highlighting their co-genetic nature. Their bulk-geochemical and petrographic characteristics are similar to those of ultramafic lamprophyres and liquidus mineral composition are closer to that of lamproites and are strikingly comparable to ultrapotassic rocks reported from the Denizli region (Western Anatolia, Turkey), Karinya Syncline and Mt. Bundey (Australia) and the Polayapalle, eastern Dharwar craton (southern India). Incompatible trace element concentrations (e.g., Sr, Zr, Nb, Ta etc.) and their ratios (Ce/Pb, Nb/U, Nb/Yb, Th/Yb) reveal limited influence of crustal contamination and involvement of a predominantly within-plate (plume) and minor subduction-derived components in their magmas. Initial Sr–Nd isotopic ratios of the Damodar Valley ultrapotassic intrusives suggest their derivation from source regions with long term incompatible element enrichment relative to that of Bulk Earth which are very different from those of (i) kimberlites and orangeites from India and southern Africa and (ii) primitive Kerguelen plume component but indistinguishable from those of the pristine Kerguelen mantle plume derived basalts. The depleted mantle (TDM) model ages (0.95–1.4 Ga) of the Damodar Valley ultrapotassic rocks are strikingly similar to (i) those of the Deccan-age orangeites from the Bastar craton, central India, and (ii) the emplacement ages (1.1–1.4 Ga) of kimberlites and lamproites from the eastern Dharwar craton, southern India. The Gondwana ultrapotassic rocks represent small degree-partial melts derived from a depleted harzburgitic source subsequently metasomatised by carbonate- and rutile-rich fluids/melts, presumably derived from Kerguelen plume, within the garnet stability field at lithospheric depths. A temporal difference of ~ 500 Ma in the source enrichment of Jharia and Raniganj ultrapotassic intrusives, coupled with their distinct incompatible element signatures, bring out involvement of a heterogeneous mantle in their genesis.
