Formation And Exhumation Of High Pressure Metamorphic Rocks From Oman: New Constraints From U-Pb Dating Of Zircon And Rutile

Basement and continental shelf units structurally underlying the Semail ophiolite in Saih Hatat, NE Oman were metamorphosed under high pressure, low temperature (high P/T) conditions, and occur in two plates. The lower plate consists of eclogite facies rocks that grade westward into garnet blueschists and crossite epidote schists. The upper plate consists of thrust sheets of lower grade metasedimentary and metavolcanic rocks formed under pumpellyite - actinolite to lawsonite albite facies conditions. The eclogite facies rocks had a clockwise P-T path, with peak conditions of 12 < P < 15 kbar, and T of 550 to 580° C, a stage of near isothermal decompression from >12 kbar to ∼ 4 kbar, and a stage of near isobaric cooling followed by cooling and decompression. High P/T rocks from the upper plate are also characterized by clockwise P-T paths with a segment of near isothermal decompression, and reached peak conditions that ranged from ∼ 310° C, P < 6 kbar to 380 to 430° C, 6.5 to 9 kbar, depending on location in the thrust pile. A distinct pressure gap of > 3 kbar results from the juxtaposition of the lower and upper plates. U-Pb dating of zircon and rutile grains from two lower plate blueschists yield ages ranging from Proterozoic to Late Cretaceous: data cluster near a Late Creatceous age on the younger end of the array and spread to Mesoproterozoic values at the older end. A weighted mean of the ²³⁸U/²⁰⁶Pb ages from the youngest and most concordant fractions (one zircon and 2 rutile crystals) yields 80.1 +/- 5 Ma. These results are consistent with Rb/Sr isochron ages of 78 +/- 2 Ma obtained using phengite, clinopyroxene, and epidote from seven eclogite facies samples, and suggest that high P/T metamorphism in Saih Hatat was almost coeval with the emplacement of the Semail ophiolite. We conclude that the lower plate blueschists and eclogites formed by intracontinental thrusting and subduction of the thinned leading edge of the Oman continental margin in an east dipping subduction zone. The upper plate rocks which were part of the hanging wall of this subduction zone, were metamorphosed by tectonic loading by the ophiolite. Entrance of thicker portions of the positively buoyant continental crust into this subduction zone choked it, and caused the ductile thinning of mantle lithosphere in its hanging wall. This initiated the rapid exhumation of the blueschists and eclogites from > 40 km to 14 km ∼ 78 Ma, and led to their juxtaposition by the lower grade upper plate rocks. Erosion and crustal extension led to the final exhumation of all high P/T rocks in Saih Hatat.