Structure and metamorphism beneath the obducting Oman ophiolite: Evidence from the Bani Hamid granulites, northern Oman mountains

Abstract

The Cretaceous Semail ophiolite (northern Oman and the United ArabEmirates) includes an intact thrust slice of Tethyan oceanic crust and upper mantle formed above a northeast-dipping subduction zone that was the site of initiation of obduction. The normal metamorphic sole of the Semail ophiolite comprises a highly condensed sequence of hornblende + plagioclase ± garnet amphibolites with small enclaves of garnet + clinopyroxene granulites immediately beneath the mantle sequence peridotites, tectonically underlain by a series of epidote amphibolite and greenschist facies lithologies in a highly deformed ductile shear zone. Peak metamorphic conditions of 770-900 °C and 11-15 kbar indicate metamorphism at depths far greater than can be accounted for by the preserved thickness of the ophiolite (~15 km). In the mountains of northern Oman, the 1.2-km-thick Bani Hamid thrust sheet is composed of intensely folded granulite and amphibolite facies rocks within mantle sequence peridotites, exhumed by late-stage out-of-sequence thrusting along the Bani Hamid thrust. The Bani Hamid thrust slice includes two-pyroxene quartzites (± hornblende, cordierite, sapphirine), diopside + andradite garnet + wollastonite + scapolite marbles and calc-silicates and amphibolites (hornblende + plagioclase ± clinopyroxene ± biotite) with localized partial melting, intruded by hornblende pegmatites. The Bani Hamid granulites represent metamorphosed cherts and calcareous turbidites probably derived from the distal Haybi Complex and Oman Exotic limestones, which have an alkali basaltic substrate. Metamorphic modeling using the program THERMOCALC in the system NCKFMASHTO (Na 2 O-CaO-K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O-TiO 2 -O)gives peak pressure-temperature conditions of 850 ± 60 °C and 6.3 ± 0.5 kbar, a pressure that is much lower than that of the metamorphic sole, suggesting a different origin. The 206 Pb/ 238 U zircon dates indicate that the gabbroic crust of the ophiolite formed by ridge magmatism from before 96.1 to 95.5 Ma. The 206 Pb/ 238 U zircon dates from the metamorphic sole range from 95.7 to 94.5 Ma, and suggest that metamorphism and melting was either synchronous with or slightly postdated ridge magmatism. The Bani Hamid granulites are younger; zircon and titanite U-Pb dates span ca. 94.5-89.8 Ma. Peraluminous granitic dikes intruding the mantle sequence peridotites are as young as 91.4 Ma and likely reflect localized partial melting of crustal material during the late stage of the obduction process. A minimum of 130 km shortening is recorded by restoration of the major folds within the Bani Hamid thrust sheet, and more than 30 km offset has occurred along the west-directed breaching out-of-sequence Bani Hamid thrust. These rocks may be representative of deep-level duplexes imaged on recent seismic sections across the mountains of northern Oman-United Arab Emirates.

Synchronous Seafloor Spreading and Subduction at the Paleo‐Convergent Margin of Semail and Arabia

“…In the Emirates, U‐Pb dating on zircon fractions is available (Figure a), providing 95–96 Ma in Masafi window, and 91.5 to 93.5 Ma in various lithologies from Bani Hamid (Styles et al, ). Single‐grain zircon dating in Bani Hamid brackets these data at 94.5 for amphibolite and 92.5 on felsic pod (Searle et al, ).…”

“…The three lithologies are roughly distributed from west to east, and any steep inverted metamorphic gradient is absent (Gnos & Kurz, ). The peak metamorphism points to 800–850 °C at 9 to 6 kbar based on the stability of sapphirine (Gnos & Kurz, ; Searle et al, ), and retrograde temperatures are recorded down to ~300 °C.…”

“…(a) Recent dating on metamorphic soles and windows (black label), on alkaline intrusions (red label), and on UHP metamorphics (blue label) Styles et al (2006). are U/Pb dating on zircon population;Rioux et al (2016) andSearle et al (2015) are youngest precise single-grain date U/Pb on zircon, and εΝd data Gray, Hand, et al (2004). are Sm/Nd isochrone and single-grain date U/Pb on zircon.…”

Emplacement of Semail–Emirates ophiolite at ridge–trench collision

“…In the Bani Hamid window (Figure section 1), parageneses equilibrated in the amphibolite to granulite facies (~800°C and 9 kbar) (Gnos & Kurz, ) and include interlayered metachert, metacarbonate and metavolcanics, without any clear metamorphic gradient; all components suggest a continental slope origin (Cox, Searle, & Pedersen, ). Dating the metamorphism of Bani Hamid provided U–Pb ages spanning from 94.5 to 92.5 Ma on zircon, and up to 88.5 Ma on titanite from quartzite (Searle, Waters, Garber, Rioux, & Ambrose, ).…”

Petrogenesis of kyanite‐ and corundum‐bearing mafic granulite in a meta‐ophiolite, SE Turkey

“…In contrast, this study has documented an anticlockwise path in which a steep increase in pressure was followed by near-isobaric cooling. Anticlockwise P-T paths involving granulite facies metamorphism have also been inferred for the Semail meta-ophiolite (Oman), in which conditions reached >700-900°C,~0.9-1.5 GPa, followed by cooling and decompression (Figure 16; Hacker, 1991;Hacker & Gnos, 1997;Searle et al, 2015). The Semail meta-ophiolite also resembles the Berit in that it locally consists of a highly deformed sequence of amphibolite (AEgarnet) hosting small lenses of garnet-clinopyroxene granulite (e.g.…”