LATE PALEOZOIC COLLISION, DELAMINATION, SHORT-LIVED MAGMATISM, AND RAPID DENUDATION IN THE MEGUMA TERRANE (NOVA-SCOTIA, CANADA) - CONSTRAINTS FROM 40AR/39AR ISOTOPIC DATA

The Meguma Terrane consists of 8-15 km of early Paleozoic stratified rocks, deposited on a continental basement, that were deformed and metamorphosed during the late Paleozoic as a result of lithospheric plate collision. The oldest cleavage (previously published, whole-rock, 40Ar/39Ar plateau ages of 415-395 Ma) dates the onset of crustal thickening, which was followed by voluminous, but short-lived, Late Devonian granitic and minor mafic magmatism (380-370 Ma). This magmatism may have been the product of delamination of the lower lithosphere and upwelling of asthenosphere, which effected melting above the new Moho and resulted in intrusion at depths of 5-12 km. 40Ar/39Ar plateau ages of hornblende, muscovite, and biotite indicate that, at the present erosion level, most of the Meguma Terrane cooled through similar to 300 degrees C by 368-360 Ma, slightly earlier than for the southwestern Meguma Terrane (ca. 345 Ma). The present erosion level was exhumed by the latest Devonian - Early Carboniferous (Visean): the age of the oldest unconformably overlying rocks. Subsequent burial beneath similar to 6 km of Carboniferous sediments would not have been sufficient to completely rejuvenate older intracrystalline mica systems and result in the observed 40Ar/39Ar mica plateau ages between ca. 350 and 260 Ma. Such rejuvenation may have resulted from migration of hot fluids along shear zones derived from the lower crust and mantle, and from granitoid magma intruded at ca. 316 Ma. The recurrence of deformation, magmatism and denudation in the middle Carboniferous suggests that further delamination may have occurred. The temperature of these fluids decreased from 400-500 degrees C during the Carboniferous to 300-400 degrees C during the Early Permian.