BURIAL AND EXHUMATION OF THE LONG-RANGE INLIER AND ITS SURROUNDINGS, WESTERN NEWFOUNDLAND - RESULTS OF AN APATITE FISSION-TRACK STUDY

The Long Range Inlier, a steep-sided plateau underlain mainly by Grenv illian gneisses, is the most prominent topographic feature of western Newfoundland. Apatite fission-track analysis of 31 samples from the Lo ng Range Inlier and its surroundings yielded measured apparent ages of 343-152 Ma. Age versus elevation plots, track-length distributions, a nd model thermal histories indicate that the region experienced slow c ooling in the late Paleozoic, with apparent exhumation rates of 7-9 m. Ma(-1) and cooling rates of 0.08-0.28 degrees C.Ma(-1). Model thermal histories suggest that the present upper surface of the Long Range pla teau cooled below similar to 120 degrees C in Ordovician times. The th ermal histories are compatible with, but do not require, some exhumati on of the Long Range Inlier along Acadian thrust faults. Results from Early Carboniferous sedimentary rocks of the Deer Lake Basin are simil ar to Long Range Inlier data from similar elevations, implying that at some time between similar to 350 and 300 Ma, the entire region was bu ried to depths sufficient to induce total annealing (T > 120 degrees C ) in these samples. Closure ages determined from model thermal histori es indicate that regional cooling to temperatures below similar to 120 degrees C began before 300 Ma. The Carboniferous sedimentary cover wa s largely removed by Jurassic time, perhaps in response to lowering of regional base level by rifting associated with the opening of the Atl antic Ocean.