Reconnaissance oxygen and sulfur isotopic mapping of Pan-African alkali granites and syenites in the southern Indian Shield

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The granulite facies deep crustal segment in the southern Indian Shield evidenced a prominent alkaline silicic magmatic event of within-plate affinity at ca. 550–750 Ma, represented by a suite of alkali granite and syenite plutons, emplaced on or close to regional fault systems. Their mineral associations and geochemical patterns are typical of alkaline plutons, invoking magma derivation from the deepcrust or uppermantle. A reconnaissance oxygen isotope study involving ten representative plutons yielded a dramatic increase in δ¹⁸O values from 3.9‰ in the north, near the amphibolite-granulite transition zone, to 10.9‰ at the southern margin of the granulite terrane. From north to south, these intrusives define a low oxygen isotope zone (δ¹⁸O = 3.99 to 4.6‰; 3 plutons), an intermediate oxygen isotope zone (6.9 to 8.6‰; 5 plutons) and a high oxygen isotope zone (9.2 to 10.9‰; 2 plutons). Whole rock sulfur isotopes display positive values, with a wide spread in δ³⁴S from 2.5‰ to 14.5‰. Although the isotopic patterns of some of these plutons are consistent with varying degrees of fluid-rock exchange or supracrustal interaction, our data are in broad agreement with source heterogeneities, and suggest the possibility of isotopic gradients in the subcontinental mantle beneath the southern Indian Shield.