PRELIMINARY CHARACTERIZATION OF THE HIGHLAND-VIJAYAN BOUNDARY ZONE, SRI LANKA

Sri Lanka, a small-scale aggregate of contrasting crustal domains, has been the focus of lower crustal research for the past two decades. The timing of high-grade granulite metamorphism in the central Highland Complex (HC) of Sri Lanka is well-constrained at 570-550 Ma (Kroner et al., 1994; Prec Res; Sajeev et al., 2010, Geology). Amphibolite facies metamorphism of the adjacent Vijayan Complex (VC) to the east is less well constrained but apparently somewhat younger (ca. 510 Ma; Holzl et al., 1994; Prec Res). Additionally, the timing and nature of juxtaposition (via thrusting) of the HC above the VC is uncertain. The contact between the HC and VC, characterized by strong deformation, exotic tectonic slivers, migmatites, local serpentinite bodies, magnetite deposits, and gold mineralization is a major suture separating the Paleoproterozoic HC from the Grenville-age VC. Our goal is to ascertain the conditions and timing of deformation along this important suture. We have collected 21 oriented samples within and near (<6 km) the HC-VC boundary zone. West of the boundary (granulite terrain), coarse garnet-sillimanite-graphite gneiss with 5 mm euhedral inclusion-rich garnets are locally sheared with garnet elongate in the foliation plane. Within the boundary zone, garnet-(amphibole)-biotite gneisses exhibit a strong mesoscopic fabric consisting of ribbon quartz and compositional layering. Microscopically, static annealing is prevalent. East of the HC-VC deformation zone, coarse plagioclase-biotite orthogneisses exhibit compositional banding, augen, and incipient migmatization.

EMPA in situ spot-dating of monazite grains from texturally characterized samples collected in this study is likely to yield important information on the timing of thrusting and the timing of amphibolite metamorphism of the VC. Initial SEM imaging reveals two populations of accessory monazite from a strongly deformed boundary sample: large (300-500 micron) euhedral grains and small (30-70 micron) subrounded grains. We have also imaged monazite grains in the sheared granulite gneiss samples described above. Deformed migmatite lenses spatially and temporally inferred to be associated with thrusting (Kleinschrodt, 1994, Prec Res) are also being investigated in this study.