In 2004 Goscombe was invited by Prof. David Gray to investigate the metamorphic rocks immediately below the Semail ophiolite in Oman and UAE. This was a small component of a much larger project on ophiolite obduction being run by Prof. David Gray in collaboration with Prof. Robert Gregory. A Review Article on the metamorphism of the NE Arabian Margin has been published by Gondwana Research, March, 2020.

Metamorphic response within different subduction–obduction settings preserved on the NE Arabian margin

Ben Goscombe^1,2*, David A. Foster², David Gray³, David Kelsey^4,5, Ben Wade⁶

¹Integrated Terrane Analysis Research (ITAR), 18 Cambridge Rd, Aldgate, 5154, SA, Australia.

²Department of Geological Sciences, University of Florida, Gainesville, Florida, 32611, USA.

³School of Earth Sciences, University of Tasmania, Hobart, Tasmania, Australia.

⁴ Department of Geology, University of Adelaide, Adelaide, 5005, Australia.

⁵Geological Survey of Western Australia, 100 Plain Street, East Perth, 6004. Australia.

⁶Adelaide Microscopy, University of Adelaide, Adelaide, 5005, Australia.

*Corresponding author: ben.goscombe@gmail.com. http://www.terraneanalysis.com.au

Gondwana Research, Focus Review, published March, 2020

Abstract

Metamorphic rocks form a minor component of the NE Arabian margin in Oman and the United Arab Emirates (UAE). Conditions span almost the entire range of crustal metamorphism from very high-P/low-T eclogite and blueschist to high-P/moderate-T epidote- to upper-amphibolite and low-P/high-T granulite facies. The NE Arabian margin experienced at least six metamorphic events, each characterized by distinct peak metamorphic temperature, depth of burial, average thermal gradient and timing. Synthesis of the available metamorphic data defines five different tectonic settings that evolved during the middle Cretaceous: [1] The Saih Hatat window exposes former continental margin crust that was buried and metamorphosed in a SW-dipping subduction system. Lower-plate units in the window include relict oceanic crust with eclogite (M1–M2) parageneses that recrystallized at pressures of ~14–23 kbar under very low thermal gradients of 7–10 ºC/km. Peak metamorphism occurred at ~110 Ma. Peak assemblages were overprinted by garnet–glaucophane-blueschist foliations (M3) at about ~104–94 Ma that formed at ~10–15 kbar and 10–15 ºC/km during the first-stage of isothermal exhumation. [2] Metamorphic soles in the footwall of the Semail ophiolite experienced a two-stage history of deep burial and peak metamorphism at ~96–94 Ma, followed by retrogression during obduction onto the continental margin between ~93–84 Ma. Peak metamorphic garnet–clinopyroxene–hornblende–plagioclase assemblages (M4s), exposed at highest structural levels, formed at 743 ± 13 ºC and 10.7 ± 0.4 kbar, indicating Barrovian thermal regimes of 20.0 ± 2.2 ºC/km. Burial of seafloor sediments and oceanic crust to ~38 km depth, was attained within a short-lived, NE-dipping intra-oceanic subduction system. The relatively high average thermal gradient during the peak of metamorphism was the result of heating after subcretion onto the base of hanging-wall oceanic lithosphere. [3] The Bani Hamid terrane consists of seafloor cherts and calcareous turbidites, metamorphosed to low-P/high-T granulite condition at ~96–94 Ma. Diagnostic assemblages (M4b) such as orthopyroxene–cordierite–quartz–plagioclase and orthopyroxene–sapphirine–hercynite–quartz–plagioclase, formed at conditions averaging ~915 ± 35 ºC, ~6.1 ± 0.9 kbar and ~42.9 ± 6.5 ºC/km. The elevated average thermal gradient, combined with significant depths of burial, is anomalous for typical oceanic settings. This suggests that these sea-floor sediments were buried to ~22 km depths within the intra-oceanic subduction system, accreted onto the hanging-wall, and metamorphosed at high-T during subduction of a recently active spreading ridge. [4] A plausible plate tectonic arrangement that can accounts for the different metamorphic elements on the Arabian margin is one composed of divergent subduction systems: a relatively long-lived SW-dipping subduction zone at the continental margin, and a short-lived, NE-dipping intra-oceanic subduction system. Consumption of the intervening oceanic crust led to obduction of the Semail ophiolite and accreted metamorphic soles from the upper-plate of the floundered outboard subduction system. SW-directed obduction was initiated between 93.7–93.2 Ma and continued until ~84 Ma, producing lower-amphibolite to sub-greenschist facies retrograde fabrics in the metamorphic soles (M5) and sub-metamorphic melange in the footwall. [5] The lower-plate of the Saih Hatat window was reworked by top-to-NE extensional shear at epidote-greenschist facies grades (M6) between ~84–76 Ma. Crustal-scale structures were reactivated as extensional detachments that telescoped the continental margin, leading to isothermal decompression and development of an asymmetric core complex that segmented the Semail ophiolite and formed the Saih Hatat domal window.

Summary of all available PT determinations with uncertainties and P–T paths for the different metamorphic parageneses preserved on the NE Arabian margin. Note that conditions for the different tectono-metamorphic terranes are non-overlapping and spread over a large portion of possible crustal metamorphic conditions, indicating entirely distinct tectonic settings of formation. Thin P–T paths are as reported in the literature and thick lines are best-estimate pooled paths for the different tectono-metamorphic units.

Interpretive metamorphic map of the NE Arabian margin based on published geology and metamorphic maps (Searle and Malpas, 1980, 1982; Ghent and Stout, 1981; Lippard et al., 1986; Goffe et al., 1988; El-Shazly et al., 1990 and 2001; El-Shazly and Liou, 1991; Le Métour et al., 1993; Gnos and Kurz, 1994; El-Shazly, 1994 and 1995; Hacker and Mosenfelder, 1996; Hacker and Gnos, 1997; Jolivet et al., 1998; Gnos, 1998; Searle and Cox, 1999, 2002; Gray and Gregory, 2000 and 2003; Gnos and Searle, 2001; Miller et al., 2002). Inset in the top right is simplified metamorphic facies modified after Goscombe et al. (2009), and used as the legend for the map and colour coding of the tectono-stratigraphic rock units (bottom left). Available metamorphic and cooling age determinations from metamorphic rocks below the Semail Ophiolite are indicated. Data sources are: U–Pb (Tilton et al., 1981; Warren et al., 2003, 2005; Gray et al., 2004b; Styles et al., 2006; Goodenough et al., 2010; Rioux et al., 2012, 2013, 2016; Searle et al., 2015; Roberts et al., 2016; This paper), Ar–Ar (El-Shazly and Lanphere, 1992; Gnos and Peter, 1993, 1995; Hacker and Mosenfelder, 1996; Hacker et al., 1996; El-Shazly et al., 2001; Gray et al., 2004a,b; Warren et al., 2011), K–Ar (Allemann and Peters, 1972; Searle, 1980; Lanphere, 1981; Boudier et al., 1985; Montigny et al., 1988; El-Shazly and Lanphere, 1992; Gnos and Peters, 1993; Hacker et al., 1996; El-Shazly et al., 2001; Gray et al., 2004b), Sm–Nd garnet (Gray et al., 2004b), fission track (Saddiqi et al., 1995; Poupeau et al., 1998; Grantham et al., 2003; Tarapoanca et al., 2010; Jacobs et al., 2015), and (U–Th)/He (Jacobs et al., 2015).

A model for the time sequence evolution of tectonic settings for the formation of Bani Hamid terrane and metamorphic soles based primarily on metamorphic constraints as discussed in the text. (a) Stages of development of an intra-oceanic subduction system with subducted spreading ridge, leading to low-P/high-T metamorphism of supracrustal Bani Hamid rocks at depths of ~21 km. (b) Stages of development of an intra-oceanic subduction system leading to contact-type moderate-P/moderate-T metamorphism of supracrustal metamorphic sole rocks at depths of ~35–40 km, prior to later obduction onto the continental margin. Blue – metamorphosed rocks. Red – active thrust surfaces, asthenosphere upwelling and thermal anomalies. White circles – track the particle paths of the metamorphic rocks investigated.

Schematic sequence of events within the Saih Hatat window. See Figure (25) for wider context. Main active structures during the time period are indicated in red.

Expanded sections showing the proposed arrangement of double divergent subduction systems that can account for structural geometries, timings and conditions of metamorphism in the different metamorphic terranes obducted onto and then preserved on the NE Arabian margin. This is a composite figure and does not represent a specific time slice, and incorporates six metamorphic events labelled M1–M2, M3, M4s, M4b, M5 and M6. The illustrated tectonic settings for the different age metamorphic parageneses (ringed) are at the time of peak metamorphism and expanded further in Figure (23). (a) An inboard SW-directed continental margin subduction system produced M1–M2 eclogite and M3 blueschists parageneses in upper-plate and lower-plate settings in the Saih Hatat window (Gray and Gregory, 2003; Grey et al., 2004, 2005). Late-stage M6 exhumation of the Saih Hatat Window as an extensional core complex is discussed in text. (b) An outboard intra-oceanic NE-directed subduction system associated with low-P/high-T M4b granulite parageneses of the Bani Hamid terrane. Supra-crustal oceanic sediment protoliths were subducted to ~21 km depth and accreted into the upper-plate before subduction of a recently active spreading ridge, producing the extreme metamorphic conditions in the over-riding Bani Hamid rocks. (c) The outboard intra-oceanic NE-directed subduction system is associated with formation of younger moderate-P/moderate-T M4s parageneses in metamorphic soles. Supra-crustal oceanic sediment and basalt protoliths were subducted to ~35–40 km depth, accreted into the upper-plate and welded to the base of the hanging wall mantle lithosphere that supplied additional heat and contact-type metamorphism giving the moderate thermal regimes. (d) M5 low-P/low-T retrograde parageneses developed in metamorphic soles and footwalls during obduction of the Semail Ophiolite sheet, derived from the newly formed oceanic lithosphere in the upper-plate of the outboard subduction system. Inset map of diagramatic plate arrangement is consistent with the metamorphic constraints, and modified after Stampfli and Borel (2002) and Gray et al. (2005). Blue lines indicate approximate location of sections. 

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