Rb-Sr and Sm-Nd geochronology of the Eppawala metamorphic rocks and carbonatite, Wanni complex, Sri Lanka

Petrological studies on the surrounding metamorphic rocks of the Eppawala c arbonatite body, Wanni complex, Sri Lanka, revealed that these rocks had be en metamorphosed under amphibolite to granulite facies conditions. Garnet-s illimanite-biotite gneiss shows lower range of metamorphic temperature (730 -770 degreesC) than the migmatite gneiss (750-780 degreesC) and the pressur e varies from 6.6-7.8 kbar to 5.6-6.4 kbar respectively. The metamorphic ag e of the garnet sillimanite-biotite gneiss and migmatite gneiss dated 607+/ -23 Ma and 626+/-16 Ma, respectively for mineral - whole-rock isochron in S m-Nd system. These ages are compatible with the ages of regional high-grade metamorphism occurred 610-550 Ma in the three crustal units in Sri Lanka. Rb-Sr system for biotite, apatite and whole-rock fractions suggests 493+/-5 Ma for the Eppawala carbonatite body. This age indicates the cooling age o f the biotite. The presence of non-crystalline carbonatite matrix and large hexagonal apatite crystals suggests a slow cooling history. Further, low c losure temperature of biotite in Rb-Sr system suggests that the intrusion a ge of carbonatite body should be more than 493 Ma, but non-metamorphosed na ture provides evidence that the intrusion age of the carbonatite body shoul d be less than the period of regional metamorphism 610-550 Ma. Therefore, E ppawala carbonatite body has a strong possibility to be a late to post magm atic intrusion. The other late to post magmatic intrusions in the Wanni com plex and Highland complex are dated between 580-550 Ma. Therefore, the most probable intrusion age of the Eppawala carbonatite body is suggested to be around 550 Ma.