Evidence of Neoarchean adakitic alkaline magmatism in the Eastern Dharwar Craton

Archean alkaline magmatism is exceptionally rare in the geological record, offering valuable insights into Archean tectonic processes, mantle evolution, and crustal growth. This study presents in situ titanite U-Pb geochronology, trace element geochemical data, and bulk-rock geochemical results for an adakitic syenite pluton from the Danduvaripalle area of the Eastern Dharwar Craton, southern India. The investigated pegmatoidal leucocratic syenite forms an undeformed, oval-shaped plug with an NNE-SSW orientation, emplaced in a regional extensional to transtensional crustal setting. The syenite is predominantly composed of pure end-members of alkali feldspar, aegirine-augite, diopside, titanite, actinolite, apatite, and magnetite.

Bulk-rock geochemical analyses indicate a metaluminous nature, exhibiting highly fractionated REE patterns with marked middle REE depletion, resulting in an overall spoon-shaped REE profile typical of melts affected by high-pressure amphibole and garnet fractionation. The rocks exhibit elevated La/Yb (>45) and Sr/Y (>100) ratios, with pronounced heavy REE depletion (Yb < 0.7 ppm, Y < 8 ppm), which confers a distinct adakitic geochemical signature. Extreme Lu/Gd ratios (<1) in titanite further confirm strong heavy REE depletion in the parental melt. Primitive mantle-normalized multi-element patterns display ‘crust-like’ signatures, notably with negative Nb-Ta-Ti anomalies. Titanite grains are exceptionally enriched in incompatible trace elements, reflecting the evolved nature of the melt from which they crystallized. Chondrite-normalized REE patterns for titanite show extreme LREE enrichment with minimal HREE, resulting in steeply fractionated trends. Additionally, low La/Ce (<0.4) and high Ce/Nd (>1) ratios in titanite indicate an oxidizing condition of the melt. Overall, the geochemistry (adakitic traits, high Th/U and low Nb/U) supports derivation from a mafic lower crust preconditioned by subduction- and accretion-related processes, which, upon partial melting, produced K- and SiO₂-rich melts with adakitic characteristics and crust-like multi-element patterns.

LA-ICP-MS in situ U-Pb dating of unzoned titanite from two representative samples yields crystallization ages of 2526 ± 5 Ma (n = 19, MSWD = 2) and 2514 ± 7 Ma (n = 24, MSWD = 3.5). These ages likely correspond to the final stage of collision vis-à-vis slab breakoff, which facilitated extensional magmatism. This resulted in the emplacement of syenite in a post-collisional extensional regime, rather than an active subduction-related setting. Between ~2600 and 2500 Ma, the Eastern Dharwar Craton underwent widespread felsic plutonism, including tonalites, granodiorites, granites, and syenites, which were associated with the final stage of cratonization. The studied syenite is therefore interpreted to constitute a component of this terminal magmatic event during craton stabilization.