2025 July LIP of the Month

A newly identified ca. 1.94 Ga mafic dyke swarm in the Bastar craton and its relation to 1.88 and 1.85 Ga swarms

Ankur Ashutosh¹, Ulf Söderlund², Amiya K. Samal^1,2*, Gulab C. Gautam¹, Rajesh K. Srivastava¹, Richard E. Ernst³, Hafida El Bilali³

¹Department of Geology, Banaras Hindu University, Varanasi-221005, India

²Department of Geology, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden

³Department of Earth Sciences, Carleton University, Ottawa, Canada K1S 5B6

*Corresponding author’s e-mail: amiyasamal007@gmail.com

Extracted and modified from:

1. Introduction

The Indian Shield has experienced multiple LIP-scale magmatic events, recorded as mafic dykes and sill complexes across its different cratons and basins. The Bastar craton preserves mafic dyke swarms from ca. 2.70 Ga to 1.42 Ga with diverse orientations and geochemistry (Fig. 1a) (Srivastava and Singh, 2003; French et al., 2008; Srivastava and Gautam, 2009; Ratre et al., 2010; Das et al., 2011; Pisarevsky et al., 2013; Shellnutt et al., 2018, 2019; Samal et al., 2019, 2021; Liao et al., 2019; Pandey et al., 2020; Srivastava et al., 2021; Panda et al., 2023). Based on U–Pb dating, six well-defined swarms have been recognized: (i) NW-trending 2.37–2.36 Ga Bhanupratappur, (ii) ENE-trending ca. 2.25 Ga Chhura, (iii) NNW-trending 1.89–1.88 Ga Bastanar, (iv) NNW-trending ca. 1.85 Ga Sonakhan, (v) NW-trending ca. 1.78 Ga Geedam, and (vi) N–NNE-trending 1.44–1.46 Ga Lakhna. The NW–NNW dykes are most widespread, representing magmatic events at 2.37–2.36 Ga, 1.89–1.88 Ga, ca. 1.85 Ga, and ca. 1.78 Ga. From the central-western part of the craton, Ashutosh et al. (2024) identified two geochemical groups among the NNW-trending 1.89–1.88 Ga dykes; Group 1 belongs to the Bastanar swarm, while undated Group 2 likely also does, reflecting shallower mantle melting. This study provides the first U–Pb ID-TIMS baddeleyite date and detailed geochemistry for Group 2, allowing reassessment of their petrogenesis and regional dyke classification.

2. Recognition of a new ca. 1.94 Ga Pakhanjore swarm

U–Pb ID-TIMS baddeleyite dating of sample PJ22/5 from the Pakhanjore area of the central western Bastar craton yields a concordia age of 1944 ± 6 Ma (upper intercept; Fig. 2), defining a previously unrecognized magmatic event, termed as the Pakhanjore swarm (Fig. 1b). Comparison among dated NNW-trending ca. 1.89–1.88 Ga dykes shows clear geochemical differences: central-western Bastar dykes exhibit inclined REE patterns {(La/Yb)_N = 4.2–9.2}, southern Bastar dykes display LREE-enriched, nearly flat HREE patterns {(La/Yb)N = 2.4–3.4}, while the ca. 1.94 Ga Pakhanjore dyke has moderate LREE enrichment with a flat HREE pattern {(La/Yb)N = 1.6–3.2}. Examination of undated Group 2 dykes (Ashutosh et al., 2024) in the central-western Bastar craton reveals similar LREE-enriched and flat HREE patterns, overlapping with the ca. 1.94 Ga dyke. This geochemical similarity indicates that these undated Group 2 dykes likely belong to the newly identified Pakhanjore swarm.

3. Geochemical Characteristics and Petrogenesis

The geochemical trends of the 1.94 Ga dyke samples (both the new samples of this study and Group 2 dykes of the ca. 1.89–1.88 Ga swarm) suggest early olivine and clinopyroxene fractionation, with Fe-Ti oxides crystallizing only at a late stage when the melt volume was low. The ME-spidergram (Fig. 3a) shows pronounced negative Nb–Ta anomalies, weak Ti anomalies, and enrichment in LILEs and LREEs. Although arc-like in appearance, these features are inconsistent with the regional tectonic setting and are better explained by (i) crustal contamination of mantle melts during ascent or magma chamber residence, or (ii) partial melting of a metasomatized SCLM source (e.g., Sandeman and Ryan, 2008; Pearce et al., 2021). The REE profiles (Fig. 3b) display a fanning pattern, from flat to steep LREE-enrichment, attributable to variable crustal input or melting of a heterogeneous mantle. Covariance of contamination proxies (Th/Nb, Th/La, Th/Yb) with differentiation indicator (decreasing SiO₂) highlights strong Th/La–SiO₂ and moderate Th/Nb–SiO₂ positive correlations (Fig. 4a, b). Enrichment of Th relative to Nb, La, and Yb (Sun and McDonough, 1989; Rudnick and Fountain, 1995) and increasing Th/La–Th/Yb trends confirm in situ crustal assimilation. On the Th/Yb–Nb/Yb plot (Fig. 4c; Pearce, 2008), rising Th/Yb with limited variation of Nb/Yb places samples outside the MORB–OIB array, reinforcing crustal overprinting. Collectively, the data indicate that in situ crustal assimilation during differentiation within the magma chamber was more important than inheritance from a metasomatized source.

To quantify these effects, assimilation–fractional crystallization (AFC) modeling was applied (cf. DePaolo, 1981). Most samples deviate from the pure FC path but broadly approach the AFC trajectory with r ≈ 0.3, implying fractional crystallization as the dominant process with moderate assimilation (Fig. 5a–d). Primitive mantle-normalized ME patterns also align with modeled curves (Fig. 5e). The dyke samples fall within the AFC curve at F ≈ 0.7, consistent with ~30% fractional crystallization from a relatively undifferentiated magma.

Trace element ratios (Nb/La, La/Ba, La/Nb) indicate a mantle source influenced by both lithospheric and asthenospheric components. Nb/La (0.60–0.86) lies between lithospheric (<0.5) and asthenospheric (>1) values (Smith et al., 1999), while La/Ba (0.01–0.2) and La/Nb (1.1–1.7) further support this transitional character, though minor crustal input cannot be excluded. Dy/Yb (1.56–1.70), with (Tb/Yb)_N < 1.8 and (Gd/Yb)_N < 2, suggests melting near the garnet–spinel transition but closer to the spinel field (Davidson et al., 2013; Wang et al., 2002). Moderately fractionated (La/Yb)_N (1.3–1.9) reflects variable partial melting. Non-modal batch melting modeling of spinel peridotite indicates 5–15% partial melting of a spinel-rich mantle source for the Pakhanjore swarm.

4. Relationship between the ca. 1.94, 1.89-1.88 and ca. 1.85 Ga dyke swarms

The Bastar craton records three mafic magmatic episodes at ca. 1.94, 1.89–1.88, and ca. 1.85 Ga, all showing NNW trend, implying emplacement under a common stress regime. The 1.89–1.88 Ga Bastanar swarm is the most extensive, extending into the Dharwar craton and correlating with intrusions in the Himalaya and Yilgarn craton, consistent with a mantle plume centered on the eastern Indian Shield (French et al., 2008; Rao et al., 2023). The ca. 1.85 Ga Sonakhan swarm likely represents a younger phase of this plume system, with geochemical subgroups linking it to both the Bastanar and the older ca. 1.94 Ga Pakhanjore swarm (Table 1). Possible correlation with the Yalgoo swarm of Western Australia suggests India–Australia connections during plume-driven breakup (Shellnutt et al., 2019).

The ca. 1.94 Ga Pakhanjore swarm may extend more widely and could be linked with the ca. 1.96 Ga Mundargi swarm (Yadav et al., 2020) of the Dharwar craton, hinting at a broader Pakhanjore–Mundargi LIP. While the plume center of this older event remains uncertain, it may also lie in the eastern Indian Shield (Fig. 6). Despite distinct chemistries, the three swarms likely represent successive pulses of plume-related magmatism, consistent with LIPs elsewhere that display multiple magma types (Ernst, 2014; Pearce et al., 2021).

We propose two possible models (A & B) for the genetic link between these events: (i) Model A- the 1.89–1.88 Ga Bastanar and ca. 1.85 Ga Sonakhan swarms are considered parts of a single LIP with a plume center on the eastern Indian Shield, whereas the ca. 1.94 Ga Pakhanjore (–Mundargi?) swarm represents an earlier, separate plume event, possibly also sourced from the east. (ii) Model B: all the three swarms are linked to a single plume; however, this would require the Indian lithosphere to have remained stationary for about 110 Myr, a scenario that is unlikely because such long-lived plume activity is atypical for terrestrial LIPs.

5. 1.96–1.88 Ga magmatism vs. Pranhita–Godavari Basin evolution

The NNW-trending Pranhita–Godavari (PG) basin lies between the Bastar and Dharwar cratons and preserves Gondwana and Proterozoic successions over an Archean basement (Radhakrishna and Naqvi, 1986; Chaudhuri et al., 2015). The ~450 km long basin evolved through multiple rifting phases (Paleoproterozoic–Cretaceous), with sedimentation beginning during the Paleoproterozoic, as indicated by Ar–Ar glauconite ages (1686–1620 Ma) and U–Pb zircon constraints (~1615 Ma; Conrad et al., 2011; Amarasinghe et al., 2015). Basin development is broadly placed between ca. 1.9–1.6 Ga, consistent with ca. 1.6 Ga Mul granites and granulite metamorphism in the Bhopalpatnam region (Dora et al., 2021; Meshram et al., 2021).

Spatially, the PG basin aligns with the NNW-trending ca. 1.94 Ga Pakhanjore, 1.89–1.88 Ga Bastanar, and 1.85 Ga Sonakhan dyke swarms (Fig. 1a), suggesting a possible tectono-magmatic link. Unlike other Purana basins (Cuddapah, Chhattisgarh, Gwalior, Bijawar), the PG basin lacks reported Proterozoic magmatism, which can be explained by two possibilities: (i) magmatism exists in the subsurface but is buried beneath thick Gondwana cover (Kale and Pillai, 2022), or (ii) the PG basin represents a failed rift where plume-induced lithospheric stretching occurred without significant magmatic input (Dubey et al., 2025).

6. Conclusions

• A U–Pb ID-TIMS age of 1944 ± 6 Ma defines the Pakhanjore swarm, possibly linked with the ca. 1.96 Ga Mundargi swarm, forming a broader 1.96–1.94 Ga LIP.

• Geochemistry indicates derivation from a spinel-rich mantle, with magma evolution dominated by fractional crystallization and moderate crustal assimilation; partial melting was 5–15%.

• Despite similar trends, the 1.89–1.88 Ga Bastanar and ca. 1.85 Ga Sonakhan swarms likely represent a separate plume event; the ca. 1.94 Ga Pakhanjore swarm is independent.

• Spatial and temporal alignment with the Pranhita–Godavari basin suggests a potential tectono-magmatic link.

References