RECONNAISSANCE OXYGEN AND SULFUR ISOTOPIC MAPPING OF PAN-AFRICAN ALKALI GRANITES AND SYENITES IN THE SOUTHERN INDIAN SHIELD

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 deep-crust or uppermantle. A reconnaissance oxygen isotope study involving ten representative plutons yielded a dramatic increase in delta-O-18 values from 3.9 parts per thousand in the north, near the amphibolite-granulite transition zone, to 10.9 parts per thousand at the southern margin of the granulite terrane. From north to south, these intrusives define a low oxygen isotope zone (delta-O-18 = 3.9 to 4.6 parts per thousand; 3 plutons), an intermediate oxygen isotope zone (6.9 to 8.6 parts per thousand; 5 plutons) and a high oxygen isotope zone (9.2 to 10.9 parts per thousand; 2 plutons). Whole rock sulfur isotopes display positive values, with a wide spread in delta-S-34 from 2.5 parts per thousand to 14.5 parts per thousand. 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.