Subduction initiation metamorphism: evidence from the Metamorphic Sole of the Oman Ophiolite

Subduction initiation metamorphism: evidence from the Metamorphic Sole of the Oman Ophiolite
Mike Searle, Matthew Rioux, Josh Garber

The Metamorphic Sole of the Semail Ophiolite in Oman consists of an inverted sequence of metamorphic rocks accreted along the base of the ultramafic mantle sequence (harzburgites, dunites)1,2. The highest grade garnet + clinopyroxene amphibolites (0.8–1.3 GPa, ~700–900 °C)3 crop out immediately beneath the Semail thrust and are structurally underlain by grt- and cpx-free amphibolites, epidote amphibolites and a series of greenschist facies meta-sediments (marbles, meta-cherts, shales, etc). U-Pb zircon age data indicate that prograde sole metamorphism ranged between 96.7-95.2 Ma, immediately pre-dating and overlapping with formation of the overlying ophiolite gabbroic – basaltic crust (96.1–95.2 Ma), and preserving a record of the first ~1.5 Myr of subduction initiation4,5,6. A gradient in the initiation of high-grade metamorphism from the northwest (96.7 Ma) to southeast (96.0–95.7 Ma) may record propagation of the nascent subduction zone and/or variations in subduction rate along the length of the ophiolite6. The granulite-amphibolite metamorphic sole was exhumed from depths of 40-45 km along the subduction channel and accreted to base of the obducting ophiolite. The subduction zone was choked by the arrival at the trench of ~1-km-thick oceanic seamounts (Oman ‘Exotics’) comprising unsubductable Triassic reef carbonates overlying alkali basalt ocean island tholeiitic volcanic substrate. The Semail ophiolite with the metamorphic sole welded along the base, plus the oceanic rocks between it and the Arabian continental margin (proximal Hawasina complex and distal Haybi complex) were emplaced a minimum of 300 km from NE to SW during the period from 95-80 Ma when the leading margin of the continental plate was subducted to a depth of more than 100 km to form the As Sifah eclogites7. The entire ophiolite obduction process lasted ~23 Ma8.