Petrogenetic and geochemical constraints on ca. 1.89–1.88 Ga Bastanar mafic dyke swarm, Bastar craton, India: Insights into MORB- and OIB-type contributions and interactions with metasomatized subcontinental lithospheric mantle

Abstract

The Bastar craton of the Indian Shield hosts several generations of mafic dyke swarms of various trends, compositions, and ages, which span from ca. 2.7 Ga to 1.42 Ga. This study focuses on geochemical attributes of the ca. 1.89–1.88 Ga NNW-trending Bastanar swarm, aiming to address a perceived discrepancy between its arc-like geochemistry and the influence of a heterogeneous mantle source. To resolve the intra-swarm geochemical variations, we conducted a comprehensive geochemical characterization and petrogenetic interpretation of the ca. 1.89–1.88 Ga Bastanar swarm. The samples from this swarm are categorized into two distinct groups, primarily based on their geochemical composition. The Group 1 samples exhibit higher TiO₂ (1.06–1.86 wt%), (La/Yb)_N (7.2–8.6), (Gd/Yb)_N (2- 2.27), Nb (14.6–16.6 ppm), Th (1.23–3.03 ppm) and Zr (104–118.72 ppm) concentrations than the Group 2 samples. Furthermore, rare-earth element patterns and variations in high-field strength element contents in the Group 1 samples suggest derivation from a deeper, less depleted mantle source resembling an OIB/less MORB-type. This inference is further supported by higher TiO₂/Yb, Zr/Nb, and Nb/Y ratios. In contrast, the Group 2 samples indicate derivation from a shallower, more MORB/less OIB-type depleted mantle source, as evidenced by lower TiO₂/Yb, Zr/Nb, and Nb/Y ratios. Variations in Dy/Yb and Gd/Yb ratios confirm the involvement of variable mantle sources, implying the derivation of the Group 1 and 2 samples from garnet-rich and spinel-rich lherzolite mantle sources, respectively. The absence of consistent negative Nb-Ta-Ti anomalies in the Group 1 samples suggests an uncontaminated nature, ruling out any role of crustal contamination. On the other hand, the Group 2 samples display negative Nb-Ta-Ti anomalies with enriched LREE and LILE patterns, indicating the involvement of crustal components in their genesis. A trace-element modelling suggests that the ca. 1.89–1.88 Ga mafic dyke swarm exhibits significant intra-swarm variability, with at least two distinct source components contributing to its genesis – a depleted MORB-type and an enriched OIB-type mantle. Notably, the geochemical characteristics of the Group 2 samples suggest interaction with a metasomatized mantle source, possibly enriched by fluids from an earlier subducted slab. Geochemical evidence presented in this work supports Archean subduction-related processes for the crustal growth of the Bastar craton and highlight the enduring influence of a metasomatized sub-continental lithospheric mantle on subsequent magmatism over millions of years.