Formation and exhumation of blueschists and eclogites from NE Oman:: new perspectives from Rb-Sr and 40Ar/39Ar dating

Seven eclogite facies samples from lithologically different units which structurally underlie the Semail ophiolite were dated by the 40Ar/39Ar and Rb-Sr methods. Despite extensive efforts, phengite dated by the (40)A/39Ar method yielded saddle, hump or irregularly shaped spectra with uninterpretable isochrons. The total gas ages for the phengite ranged from 136 to 85 Ma. Clinopyroxene-phengite, epidote-phengite and whole-rock-phengite Rb-Sr isochrons for the same samples yielded ages of 78 +/- 2 Ma. We therefore conclude that the eclogite facies rocks cooled through 500 degreesC at c. 78+/-2. Ma, and that the 40Ar/39Ar dates can only constrain maximum ages due to the occurrence of excess Ar inhomogeneously distributed in different sites. Our new results lead us to conclude that high-pressure metamorphism of the Oman margin took place in the Late Cretaceous, contemporaneous with ophiolite emplacement. Previously published structural and petrological data lead us to suggest that this metamorphism resulted from intracontinental subduction and crustal thickening along a NE-dipping zone. Choking of this subduction zone followed by ductile thinning of a crustal mass wedged between deeply subducted continental material and overthrust shelf and slope units facilitated the exhumation of the eclogite facies rocks from depths of c. 50 km to 10-15 km within c. 10 Ma, and led to their juxtaposition against overlying lower grade rocks. Final exhumation of all high- pressure rocks was driven primarily by erosion and assisted by normal faulting in the upper plate.