Provenance of Proterozoic Basal Aravalli mafic volcanic rocks from Rajasthan, Northwestern India: Nd isotopes evidence for enriched mantle reservoirs

摘要

Komatiites, komatiitic basalts and tholeiites from Nathdwara, Rajasthan, have been investigated for their Nd isotopic characteristics. Nd-143/Nd-144 vary broadly in the komatiites (0.51131-0.51252) and show a restricted range in komatiitic basalts and tholeiites (0.51120-0.51179 and 0.51141-0.51202, respectively). The corresponding epsilon Nd (epsilon(Nd)) values indicate an enriched source for these rocks. The model ages of komatiitic basalts and tholeiites cluster between 2.2 and 3.0 Ga. The komatiites show older model ages between 3.5 and 3.8 Ga. If the correlations found between Nd and Sm isotopic ratios can be interpreted as isochrones, then the komatiites point to the age of similar to 2300Ma and the komatiitic basalts and tholeiites constrain the age of similar to 1800-2000 Ma. The age of 2300 Ma for the komatiite samples is similar to the range estimated for the komatiitic basalt and tholeiite samples (1800-2000 Ma), considering the errors involved. Collectively the data suggest an age of 2300-1800 Ma for the Basal Aravalli volcanism. The variation of Nd isotopic ratios between the komatiites and the associated basalts is too large to reflect analytical bias or open system behavior for Sm and Nd. It may be due to significant mantle heterogeneity in the source region. Isotopically distinct sources for the komatiites and komatiitic basalts are possible, but simple mixing between a depleted mantle component and a crustal component in the manner of magmatic assimilation of wall-rock material is inconsistent with the observed isotopic ratios. The inference of an enriched mantle component appears inevitable, as the alternative model of mixing would have generated more siliceous high magnesian (boninitic) rocks with much lower Fe and Ti than are observed. This enriched material may have been part of the sub-continental lithospheric mantle (SCLM) under the A-ravalli craton, which could have generated the Basal Aravalli magmatic rocks. We suggest that during early middle Proterozoic the SCLM under cratonic regions consisted of relatively fertile mantle, capable of generating komatiites, tholeiites and alkalic magmas.