EMPA MONAZITE GEOCHRONOLOGY FROM THE EASTERN HIGHLAND COMPLEX AND HIGHLAND-VIJAYAN BOUNDARY ZONE, SRI LANKA

UHT granulite metamorphism at 570 Ma in the central Highland Complex (HC), Sri Lanka, was followed by retrogressive metamorphism associated with isothermal decompression at 550 Ma (Kroner et al., 1994; Sajeev et al., 2010). However, the timing of juxtaposition (via thrusting) of the HC above the Vijayan Complex (VC) to the east remains uncertain. The HC-VC boundary zone (BZ), characterized by strong deformation, exotic tectonic slivers, and mineralization is a major suture separating the Paleoproterozoic HC from the Grenville-age VC. EMPA in-situ spot-dating of metamorphic monazite from the eastern part of HC near the boundary zone reveal a dominance of Pan African ages (533-613 Ma) and minor Proterozoic ages ranging from 1069-1872 Ma. Qualitative monazite textural analysis suggests the ages are a function of chemical domains formed during growth and/or re-absorption of monazite. Chemical analyses from the Pan African age population separate into two clusters: low Y spots yield an average age of 572 Ma whereas higher Y spots yield an average age of 558 Ma, interpreted as the time of peak and retrograde metamorphism respectively. These results compare remarkably well with the SHRIMP U-Pb zircon and monazite ages reported by Sajeev et al. (2010) from samples located ca. 50 km to the west.

Monazite from within the BZ preserve a dominance of Pan African EMPA spot ages between 595 and 635 Ma, and minor relict Proterozoic ages ranging from 717-1868 Ma. Pre Pan African Neoproterozoic ages (700-1000 Ma) recorded from the Boundary Zone are absent in HC rocks analyzed here. The BZ age data suggest shearing of VC rocks at moderate P-T conditions at 595-635 Ma and may therefore date the initial juxtaposition of the HC and VC terranes. If so, terrane collision and continued crustal thickening during and after the juxtaposition may have ultimately led to peak metamorphism of the HC at 570 Ma. Rapid exhumation followed peak metamorphism of the HC rocks beginning at 558 Ma in the eastern HC and 550 Ma in the central HC.