EMPA dating of monazite from high grade metamorphic rocks along 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; Sajeev et al., 2010). Amphibolite facies metamorphism of the adjacent Vijayan Complex (VC) to the east is less well constrained but apparently somewhat younger (Holzl et al., 1994). 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.;EMPA in-situ spot-dating of monazite grains record timing of peak and retrograde metamorphism of HC rocks. Thirty spot ages from eight monazite grains from the HC reveals a dominance of Pan African ages (533-613 Ma), and minor Paleoproterozoic and Mesoproterozoic ages (1069-1872 Ma). The qualitative monazite textural descriptions suggest that the age data are a function of chemical domains formed during and/or re-absorption of monazite. The average age for higher Y concentrations (>4000 ppm) is 558 Ma and 572 Ma age for lower Y concentrations (<4000 ppm). These ages consistent with a regionally widespread peak metamorphic age of 570 Ma for HC followed by retrograde metamorphism during exhumation at about 555-545 Ma (Sajeev et al., 2010).;Forty spot ages from seven monazite grains from the Boundary Zone preserved a dominance of Pan African spot ages between 595 and 635 Ma. Also, Paleoproterozoic spot ages (1858-1868 Ma) and Mesoproterozoic spot ages (1255-1350 Ma) are preserved in Boundary Zone rocks. Pre Pan African Neoproterozoic ages (700-1000 Ma) recorded from the Boundary Zone are absent in HC rocks analyzed here.;The Boundary Zone age data suggest shearing of HC rocks at moderate P-T conditions at 595-635 Ma and may therefore date the initial juxtaposition of the HC and VC terranes. The crustal thickening during and after the juxtaposition may have ultimately led to the peak metamorphism of Highland Complex at 570 Ma. Peak metamorphism was followed closely by rapid exhumation of the HC rocks beginning at 558 Ma in the east and 551 Ma in the western HC terrane.;21 oriented samples were collected 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.