Metamorphism of the SE Arabian Margin, Oman: Current Problems
Doctor Ben Goscombe
Professor David Gray (The University of Melbourne)
Robert Gregory (Southern Methodist University)
The Southeast Arabian margin exposed in windows below the Semail Ophiolite in Oman and United Arab Emirates.
This margin is largely unmetamorphosed (less than or equivalent to zeolite facies), however, limited domains of significantly metamorphosed rocks do exist. These group into three distinct metamorphic styles:
Moderate-P/moderate-T (18-23 ºC/km) greenschist to upper-amphibolite facies metamorphic soles immediately below the obducted Semail Ophiolite sheet. High-P/low-T (9-16 ºC/km) eclogite, blueschist and prehnite-pumpellyite facies metamorphism in the Saih Hatat Dome. Low-P/high-T (>27-35 ºC/km) granulite facies metamorphism in the Banni Hamid granulite Window.
These metamorphic styles are entirely distinct from each other and together nearly encompass the entire spectrum of metamorphism possible in crustal lithosphere.
Models to explain the metamorphism in the SE Arabian Margin, below the Semail Ophiolite sheet:
- Simple one-stage overthrusting of the Semail Ophiolite sheet generating metamorphism during the intra-oceanic thrusting stage (e.g. Searle & Malpas, 1980; Ghent & Stout, 1981; Montigny et al ., 1988; Hacker, 1991; Hacker et al ., 1996; Hacker & Gnos, 1997; Gnos, 1998).
- Two-stage evolution involving an earlier subduction zone component followed by overthrusting of the ophiolite sheet (e.g. Gray & Gregory, 2000, etc).
- Three-stage evolution involving early granulite metamorphism in an environment unrelated to obduction or subduction followed by the two-stage subduction/obduction evolution.
The preliminary results imply an evolution involving three metamorphic events in a 4-stage tectonic process, to generate the metamorphic terranes recognised in the post-Pan-African SE Arabian Margin.
High-T/low-P metamorphism of the Banni Hamid granulites occurred at an unknown age prior to all other metamorphic events, and in an unknown tectonic environment (possibly involving lithospheric extension). Peak conditions of approximately 850-900 ºC and 4.0-6.0 kb were attained, defining an average thermal gradient of 43.0-61 ºC/km. Subduction of the continental margin at 110 Ma to depths of at least 53 km in a subduction zone environment to produce eclogites and blueschists of the Saih Hatat Dome with average thermal gradients of approximately 6-12 ºC/km.
Possibly temporally closely associated with stage-2, was subduction of oceanic crust below oceanic crust in a second, outboard subduction zone. Subduction to depths of typically 39 km and a maximum of 53 km are recorded in metamorphic sole rocks. Contact metamorphism of a thin aureole in the subducting plate, immediately below the mantle lithosphere harzburgite, generated temperatures that average 787±26 ºC immediately adjacent to the contact. The pooled mean of the average thermal gradient in the contact metamorphic soles is 20.8±2.8 ºC/km. The high pressures recorded (11.0±0.7 kb), dismiss a single-stage process of contact metamorphism occurring during obduction. Field relationships (thin inverted aureole, concordant contact and metamorphic fabric) are nevertheless consistent with contact metamorphism against mantle lithosphere. The high pressures imply a two-stage process involving a subduction zone scenario. However, moderate to high average thermal gradients of 18-23 ºC/km are inconsistent with a typical subduction zone (6-12 ºC/km). The high heat flow required was most plausibly supplied by 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 SE Arabian Margin rocks and the metamorphic sole rocks in particular.