Sm-Nd and Zircon U-Pb ages from garnet-bearing eclogites, NE Oman: constraints on High-P metamorphism |
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Authors: | David R Gray Martin Hand Richard A Armstrong Robert T Gregory |
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Institution: | a School of Earth Sciences, University of Melbourne, Melbourne, Victoria 3010, Australia b CERG, Dept of Geology and Geophysics, University of Adelaide, Adelaide, South Australia 5005, Australia c PRISE, School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia d Department of Geological Sciences, Southern Methodist University, Dallas, TX 75275, USA |
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Abstract: | Poly-deformed and poly-metamorphosed glaucophane-eclogite mega-boudins beneath the Samail Ophiolite, Oman record an early subduction-related high-P metamorphism as well as subsequent overprinting deformation and metamorphism related to exhumation. Previously published Rb/Sr ages of 78 Ma and 40Ar/39Ar ages of 82-79 Ma record the major NE-directed shearing event that partially exhumed the eclogites to a shallower crustal level. New Sm/Nd garnet-garnet leachate-whole rock isochron data from garnet-bearing eclogite assemblages in the As Sifah subwindow in NE Oman are 110±9 Ma (DG02-87D); 5-point isochron) and 109±13 Ma (DG02-86E; 3-point isochron). On the basis of microfabric and field structural relationships these ages are interpreted to reflect the timing of prograde, peak high-P metamorphism in the rocks structurally beneath the Samail Ophiolite. This metamorphism clearly predates the age of formation of the obducted Samail oceanic lithosphere (97-94 Ma) as well as the subsequent obduction onto the margin (80-70 Ma). A U-Pb SHRIMP zircon age from small (<200 μm in length) zircons with herring-bone textured zoning from DG02-87D indicate that rapid zircon growth associated with high-Si phengites occurred at 82±1 Ma. Zircon growth is possibly related to liberation of Zr on garnet breakdown during decompression metamorphism under high-P conditions with exhumation. These data require that crustal stacking models attendant with ophiolite obduction are inappropriate to explain the Oman high-P metamorphism. |
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Keywords: | subduction eclogite facies Oman Sm-Nd dating U-Pb SHRIMP zircon dating |
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