The northeast Arabian margin that is exposed in windows below the Semail Ophiolite in Oman and United Arab Emirates is largely unmetamorphosed, but nevertheless limited domains of significantly metamorphosed rocks do exist. These can be grouped into three distinct metamorphic style types that are entirely distinct from each other and together nearly encompass the entire spectrum of metamorphism possible in crustal lithosphere.

The Banni Hamid Granulites are an enigmatic sliver of meta-quartzites, mafic granulite, calcsilicates and carbonates with oceanic provinence that has experienced metamorphic conditions that are entirely incompatible with typical subduction, obduction or crustal over thrusting tectonic settings. By applying equilibrium thermodynamics (THERMOCALC v3.1), conventional thermobarometry and interpretation of developed matrix assemblages within published petrogenetic grids and pseudosections in FMAS, KFMASH and NCKFMASH. All of these P-T estimates are all restricted to between 900-970 ºC and 6.0-8.0 kb and are centred on conditions of 935±39 ºC and 6.4±1.0 kb, defining an average thermal gradient of 42.7±5.7 ºC/km. These results are critical to constraining plausible tectonic scenarios responsible for the generation of these metamorphic rocks. There are currently two competing tectonic models to explain metamorphism in the NE Arabian Margin, below the Semail Ophiolite sheet. These are a simple one-stage overthrusting of the Semail Ophiolite sheet generating metamorphism during the intra-oceanic thrusting stage and a two-stage evolution involving an earlier subduction zone component followed by overthrusting of the ophiolite sheet. Neither of these subduction/obduction scenarios could have generated the high-T/low-P metamorphic conditions experienced. We propose that metamorphism of the Banni Hamid Granulites occurred early in the evolution of the margin, possibly in a lithospheric extension setting or by being subducted below an active spreading ridge. High-grade metamorphism occurred prior to obduction at the NE Arabian margin and has been dated by a single peak metamorphic garnet with Sm-Nd age of 129±15 Ma and must be older than a mixed age of 103.5±5.1 Ma from a sample containg two garnet generations. The margins of the Banni Hamid Granulite sliver were reworked by amphibolite to greenschist facies mylonites during emplacement onto the NE Arabian margin. Garnet from a mylonitized granite sill in the reworked margin have been dated at 88.8±4.0 Ma and all U-Pb SHRIMP ages from zircon and monazite grains in calcsilicate samples pool between 90-92 Ma. These younger age determinations are consistent with the accepted age of obduction of the Semail Ophiolite sheet onto the SE Arabian Margin.

Three distinct metamorphic terrane types are recognised within the northeast Arabian margin, below the Semail Ophiolite in Oman and United Arab Emirates.
(1) Moderate-P/moderate-T (18-23 ºC/km) greenschist to upper-amphibolite facies metamorphic soles immediately below the obducted Semail Ophiolite sheet.
(2) High-P/low-T (7-15 ºC/km) eclogite, blueschist and prehnite-pumpellyite facies metamorphism in the Saih Hatat Window.
(3) Low-P/high-T (>35-44 ºC/km) granulite facies metamorphism in the Banni Hamid Granulite Inlier.
These metamorphic styles are entirely distinct from each other and together nearly encompass the entire spectrum of metamorphism possible in crustal lithosphere. Previous attempts to explain metamorphism of the NE Arabian Margin have sought to explain these disparit domains within a single tectonic scenario. There are three competing tectonic models to explain metamorphism in the NE Arabian Margin, below the Semail Ophiolite sheet: (1) Simple one-stage overthrusting of the Semail Ophiolite sheet generating metamorphism during the intra-oceanic thrusting stage. (2) Two-stage evolution involving an earlier subduction zone component followed by overthrusting of the ophiolite sheet. (3) Our prefered three-stage evolution involved early (approximately 104-129 Ma) granulite metamorphism in an environment unrelated to typical obduction or subduction, generating the Banni Hamid Granulites. This was followed by a two-stage subduction/obduction evolution that first generated high-P metamorphism in the Saih Hatat Window at 110 Ma in a continental margin subduction zone. Metamorphic sole metamorphism occurred at 85-95 Ma within an outboard intra-oceanic subduction zone and these rocks were obducted onto the continental margin along with the ophiolite sheet.

Investigation of six metamorphic sole localities, document remarkably similar peak metamorphic conditions and evolutions from throughout the 500km length of the Oman Mountains. Maximum temperatures recorded in sole rocks, immediately adjacent to the mantle lithosphere harzburgite, average 743±13 ºC at pressures averaging 10.7±0.4 kb (i.e. 32-46 km). The pooled mean of the average thermal gradient in the metamorphic soles is 20.0±2.2 ºC/km. Field relationships that support formation by contact metamorphism of subducting oceanic plate against hot mantle lithosphere are; the soles are thin inverted aureoles, have steep DT/Dx field gradients and metamorphic fabrics and sole contact are concordant. The high pressures recorded dismiss a single-stage process of contact metamorphism occurring during obduction of the ophiolite sheet (only 15 km thick), and imply a two-stage process involving an intra-oceanic subduction zone scenario. However, the moderate average thermal gradients of 18-23 ºC/km are inconsistent with a typical subduction zone against a continental margin (i.e. 6-12 ºC/km), implying high heat flow from the over-riding oceanic mantle lithosphere. Obduction of the Semail Ophiolite sheet transported parts of the metamorphic sole rocks, both upwards and laterally, onto the SE Arabian Margin at 95-85 Ma. This obduction stage was not responsible for a prograde metamorphic cycle, though was responsible for low-grade and spatially restricted retrograde metamorphism in the NE Arabian Margin rocks and the metamorphic sole rocks in particular.