U-PB GEOCHRONOLOGY OF THE SRI-LANKAN BASEMENT

U-Pb ages of zircons and monazites from ortho- and paragneisses representing the major crystalline basement units of Sri Lanka establish a new geochronological framework, which is consistent with previously published work on Sm-Nd model ages, Rb-Sr whole-rock ages, and U-Pb mineral ages. In the Highland Complex, three newly dated orthogneisses, one previously dated by Baur, et at. (1991), and one dated by Kroner and Williams ( 1992), all intruded 1.85 to 1.9 Ga ago. This widespread intrusion event occurred soon after the end of sediment deposition, which is estimated at 1.9 to 2.0 Ga using upper-intercept ages of zircons from metapelites in the Highland Complex, Within the boundaries of this complex, established by Sm-Nd model age mapping (Milisenda et al., 1994), only one orthogneiss has been found which is substantially younger than 1.8 Ga. This is the metatonalite from Polonnaruwa, which intruded 670 Ma ago (Baur et al., 1991). Three orthogneisses from the Vijayan Complex, which lies to the east of the Highland Complex, yield similarly consistent crystallization ages of 1023 +/- 8, 1016 +/- 28 and 1033 +/- 121 Ma. In the Wanni Complex, which lies to the west of the Highland Complex, a sillimanite-bearing gneiss, which is apparently of metasedimentary origin but contains zircons of igneous appearance, yields a primary crystallization age of 793 +/- 4 Ma, consistent with a previously dated orthogneiss from Kurunegala of 770 Ma (Baur et al., 1991 ). In addition, single-zircon data by Kroner et al. ( 1994) indicate primary crystallization ages of approximately 1.0 to 1.1 Ga for orthogneisses from the Vavuniya Charnockite Province which is considered to be part of the Wanni Complex. These results confirm the age contrast between the Highland Complex on the one hand, and the Vijayan Complex and Wanni Complex on the other, as previously inferred from Sm-Nd model ages (Milisenda et al., 1988, 1994). Post-tectonic granitoids and pegmatites intruded throughout all three complexes approximately 550 Ma ago. The age of regional high-grade metamorphism (mostly granulite facies in the Highland Complex, mostly amphibolite facies in the Vijayan Complex, and both granulite and amphibolite facies in the Wanni Complex) is constrained to be less than 670 Ma, the primary crystallization age of the Polonnaruwa metatonalite. It is more closely defined by lower-intercept zircon ages, which include one concordant age of 608 +/- 4 Ma and one discordant but well-defined age of 611 +/- 23 Ma, and by a nearly concordant monazite age of 592 +/- 2 Ma. This indicates that peak metamorphic conditions occurred approximately 610 Ma ago. Numerous other lower-intercept ages between 590 and 550 Ma and post-tectonic intrusion ages of 550 Ma set a lower age limit for high-grade conditions and deformation. The temperature history between 610 and 550 Ma is not yet well resolved, and it is not clear whether there was a separate thermal event 5 50 Ma ago. Previously published whole-rock Rb-Sr and Sm-Nd isochrons for gneisses from the Highland Complex, yielding ages of approximately 2 Ga and greater, and previously interpreted to date the time of granulite-grade metamorphism, must be reinterpreted in the light of the new data presented here as well as other recently published results (e.g. Burton and O'Nions, 1990b; Baur et al., 1991 ), and in view of the accumulated evidence for very limited element mobility during high-grade metamorPhism. These whole-rock ages are in fact identical or close to the ages of primary granitoid emplacement rather than the age of granulite-facies metamorphism.