Geochemistry of NW-SE Trending Palaeoproterozoic Mafic Dyke Intrusions in the Bundelkhand Craton, India and Subcontinental Lithospheric Mantle Processes

NW-SE trending mafic dykes in the Bundelkhand Craton of the Indian shield manifest Palaeoproterozoic igneous activity. These dykes are Fe-rich tholeiitic basalts with compositions varying from near primary melts to more evolved magmas (Mg#: 0.64-0.35) and show enrichment of large-ion lithophile and light rare earth elements relative to primordial mantle values. Despite very subtle variations of elemental abundances and similar structural trend, at least two groups are identified, mainly based on incompatible element ratios and rare earth element patterns. Group 1 samples are characterized by relatively low Ti/Y (av. 222 +/- 22) and Ti/Nb (av. 609 +/- 108), high Zr/TiO2 (av. 117 +/- 15), more light to heavy rare earth element fractionation (La/Vb)(CN): 2.7-5.1) and mild negative Eu anomaly (Eu* = 0.77 +/- 0.10). Group 2 samples possess high Ti/Y (av. 334 +/- 46) and Ti/Nb (av. 1349 +/- 198), low Zr/TiO2 (av. 70 +/- 7) and minor light to heavy rare earth element fractionation ((La/Vb)(CN): 1.5-2.7) without any Eu anomaly (Eu* = 0.97 +/- 0.04). Both groups show relative depletions in Nb, Sr and P while the Group 1 also shows Ti depletion. A third group is apparent but is less certain. Group 2 dykes constitute the 1.98 Ga Jhansi swarm, whilst the Group 1 and unclassified samples are likely to be of older age (similar to 2.18-2.20 Ga and similar to 2.37 Ga) based on available U-Pb ages and palaeomagnetic considerations. Despite the compositions indicate different batches of magma, the dykes have near similar petrogenetic evolutionary pattern as if the entire population constitutes a single clan. Samples of both groups (a) do not indicate significant crustal contamination (b) derived from two different batches of magmas formed by similar to 10-12% fractional melting of mantle near the spinel-garnet transition zone and (c) evolved through initial fractionation of olivine and thereafter clinopyroxene and plagioclase became important fractionation phases. The more primitive compositions of both groups indicate mantle potential temperatures of similar to 1550-1600 degrees C suggesting thermal anomaly. The dyke magmas inherited variably enriched compositions as a result of interaction of upwelling mantle melts with metasomatised subcontinental lithospheric mantle. Low H2O-CO2-rich silicate melts/fluids are likely the dominant metasomatic agents to develop the Indian subcontinental lithosphere in the Archaean (c. 3.0 Ga).