Dating continental subduction beneath the Samail Ophiolite: garnet,
zircon, and rutile petrochronology of the As Sifah eclogites, NE Oman
Abstract
Studies of the high-pressure (HP) As Sifah eclogites in the NE part of
the Saih Hatat window, Oman, have used different radiometric dating
results (Ar/Ar, Sm-Nd vs. U-Pb, Rb-Sr) to interpret disparate tectonic
models for the timing, geometry, and cause of continental subduction –
including its association with the Samail Ophiolite. To determine the
absolute timing of continental subduction, we coupled geochronological
and geochemical analyses of major (garnet) and accessory phases (zircon,
rutile) from the highest-grade metamorphic rocks in the Saih Hatat (As
Sifah eclogites). Early Permian (283.8+/-0.7 Ma) tuffaceous zircon cores
are consistent with earlier interpretations that the As Sifah rocks were
sourced from a distal portion of the Arabian continental margin. Data
from a range of bulk compositions, metamorphic assemblages, and rock
textures consistently suggest a single metamorphic event, with garnet
growth starting by ~81 Ma and ceasing by
~77 Ma, with slight but consistent offsets in the timing
of metamorphic (re)crystallization between different lithologies. These
new data confirm previous structural, metamorphic, and geochronological
interpretations for continental HP metamorphism in a single NE-dipping
subduction zone beneath the already obducted Samail Ophiolite; there is
no robust evidence for a ~110 Ma event or a
continental-ward dipping subduction zone. Combined with literature
constraints, our data suggest that the As Sifah unit was subducted and
exhumed relatively slowly (~5 mm/yr) compared to other
continental high-pressure settings - likely associated with the dragging
to mantle depths by a mafic root, followed by long residence in the
lower to middle crust.