Contrib Mineral Petrol (2007) 154:115–118 DOI 10.1007/s00410-007-0182-z
REPLY
Reply to Comment by F. Boudier and A. Nicolas on ‘‘Dating the geologic history of Oman’s Semail Ophiolite: insights from U–Pb geochronology’’ by C.J. Warren, R.R. Parrish, M.P. Searle and D.J. Waters C. J. Warren Æ M. P. Searle Æ R. R. Parrish Æ D. J. Waters
Received: 9 September 2006 / Accepted: 8 January 2007 / Published online: 10 February 2007 Springer-Verlag 2007
We thank Boudier and Nicolas for renewing the longstanding discussion on the origin and emplacement of the Semail Ophiolite in Oman. The new U–Pb age data published by Warren et al. (2003, 2005), along with a wealth of previously published lithological, geochemical, structural and metamorphic data, allow the assessment of whether the Supra-Subduction Zone model or the Mid-Ocean Ridge model for ophiolite genesis and emplacement best fits the available reliable constraints. The Mid-Ocean Ridge (MOR) model (e.g. Boudier and Coleman 1981; Boudier et al. 1988; Nicolas 1989) suggests that: 1. 2. 3.
The ophiolite originated at a Mid-Ocean Ridge. The detachment originated at or close to the spreading centre. The detachment was shallow-dipping, shown as only 10–15 km deep in its present position.
The Supra-Subduction (SSZ) model (e.g. Pearce et al. 1981; Searle and Malpas 1980, 1982; Searle and
Communicated by I. Parsons. C. J. Warren (&) Department of Earth Sciences, Dalhousie University, Halifax, NS B3H 4J1, Canada e-mail:
[email protected] M. P. Searle D. J. Waters Department of Earth Sciences, Oxford University, Parks Rd., Oxford OX1 3PR, UK R. R. Parrish NERC Isotope Geoscience Laboratory, Keyworth, Nottingham NG12 5GG, UK
Cox 1999, 2002; Warren et al. 2003, 2005) alternatively suggests that: 1. 2.
3.
4.
The ophiolite originated at a spreading centre above a NE-dipping subduction zone. The detachment initiated at the subduction zone where old, cold oceanic crust was underthrust beneath young, hot oceanic lithosphere (the eventual ophiolite). The detachment was initially steeply dipping to account for the high pressures recorded in the metamorphic sole (11–14 kbar; Gnos 1998; Searle and Cox 2002). The subduction zone was still active 16 My after ophiolite formation and during the latest stages of ophiolite emplacement, when the Arabian continental margin was subducted and metamorphosed under eclogite facies conditions (ca. 20 kbar; Searle et al. 1994, 2004; Warren and Waters 2006).
The geochemistry of the ophiolite lavas has been analysed in much detail, in part to resolve the discussion around Point 1, whether the ophiolite was formed in a MOR or SSZ setting. The lower ophiolite lavas (Geotimes or V1) are similar to MORB, whereas the higher Lasail V2 lavas have low-Ti, arc-like geochemical signatures and the youngest, highest, Salahi V3 lavas have geochemical signatures more similar to within-plate basalts (Pearce et al. 1981; Godard et al. 2003, 2006). As pointed out by Godard et al. (2006), the incompatible element compositions of the Geotimes and Lasail lavas overlap both typical MORB and typical back-arc lavas, and therefore cannot be used conclusively in Oman to distinguish between these two tectonic settings. The geochemical data are, however, compatible with the SSZ model, as they indicate that
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the initial ophiolitic lavas originate from a relatively uncontaminated mantle source (at which time the subduction zone was nascent), whilst the later lavas originated from a more contaminated mantle source. Point 2 concerns whether the site of the detachment was along the ridge axis (MOR model) or along a subduction zone (SSZ model). If the detachment initiated along the Mid-Ocean Ridge axis, then the protolith of the detachment footwall (present-day metamorphic sole) would be the same as the hangingwall (Semail Ophiolite). There are both lithological and geochemical arguments to suggest that this is not the case. The Semail ophiolite is formed of mainly tholeiitic gabbros and basalts, with only minor intercalated sediments (cherts). In Oman, the base of the ophiolite (the metamorphic sole) is marked by a narrow zone of mafic amphibolite facies rocks, underlain by greenschist facies quartzites and marbles of obvious sedimentary origin (e.g. Allemann and Peters 1972; Searle and Malpas 1980, 1982; Hacker 1994; Hacker et al. 1996; Gnos 1998). In the Bani Hamid thrust slice in the United Arab Emirates, the sole mainly contains granulite and amphibolite facies quartzites and marbles with only minor garnet amphibolites (Gnos and Kurz 1994; Searle and Cox 1999, 2002). The thickness of the sedimentary rocks in the sole and the relative proportion of mafic amphibolites compared with sedimentary greenschists is dissimilar to that found in the ophiolite, which is dominated by mafic rocks with only very few intercalated sediments. If the emplacement thrust was generated at the mid ocean ridge, the metamorphic sole protolith would have been the nascent oceanic crust, which would not have been overlain with thick sedimentary sequences at this time. It therefore seems more plausible that the young ophiolite was thrust over old, cold, oceanic crust, with a thick sedimentary cover, and it is this which became the metamorphic sole. In the UAE the amphibolites of the metamorphic sole also contain intrusions, layers and pods of high Tiperidotites and gabbros which are chemically dissimilar to anything found in the ophiolite sequence (Searle and Malpas 1982; Searle 1984). They are, however, similar to certain ankaramites, nephelinites and alkaline basalts found within the Haybi Complex, the thrust sheet immediately beneath the metamorphic sole. Whilst the lithological and geochemical origin of the metamorphic sole is far from perfectly understood (although new high quality geochemical data is slowly appearing, e.g. Ishikawa et al. 2005), the metamorphic sole appears to be lithologically and geochemically more similar to the underlying Haybi Complex than the overlying Semail ophiolite (Searle and Malpas
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1982). The possibility still exists that the sole amphibolites formed from basalts not preserved or exposed elsewhere in the Oman Mountains. Either way, the data suggest that the metamorphic sole did not form from a Semail ophiolite protolith, as the MOR model would suggest. The SSZ model better fits the available data. Point 3 concerns the steepness of the detachment, with the MOR model favouring a shallow initial detachment (10–15 km deep) and the SSZ model favouring detachment along a steeper, deeper subduction zone. Thermal 1D modelling by Hacker (1991), which assumed that the peak pressures recorded by the metamorphic sole were equivalent to the presently preserved thickness of the ophiolite, could not replicate peak metamorphic temperatures recorded in the sole. Subsequent investigation of the metamorphic sole amphibolites, however, yielded peak pressures of 11–14 kbar (Gnos 1998; Searle and Cox 2002), increasing the depth of formation to >40 km. As this is deeper than the thickness of the presently preserved ophiolite section, is implies that the amphibolites must have formed in a subduction zone setting. Mantle temperatures of ca. 1,200C are geologically plausible for 40 km depth in the mantle, and could heat the subducted footwall to the recorded peak metamorphic temperatures of 840–870C. Heating in such a manner could replicate the inverted thermal gradient recorded in the metamorphic sole (Gnos 1998; Searle and Malpas 1980, 1982). The structure of the metamorphic sole suggests that subsequent shearing condensed the thermal gradient during the obduction and exhumation process. An important point to add here is that even with the most conservative pressure estimate of 8 kbar for the amphibolites of the metamorphic sole, the implied depth of formation, at ~25 km, is still deeper than the present-day preserved thickness of the ophiolite (~15 km). The amphibolites must therefore have formed in a subduction zone setting. However, as highlighted by Boudier and Nicholas in their comment, there are no structures in the ophiolite peridotites indicating major tectonic thinning. The amphibolites, having formed in a subduction zone, must have been exhumed, via subduction channel return-flow, buoyancy or other mechanism to a depth of 10–15 km before being accreted onto the base of the ophiolite. The age of the amphibolites, at 94.48 ± 0.23 Ma (Warren et al. 2005), indicates the timing of their metamorphism in the subduction zone, and not the timing of their juxtaposition against the base of the ophiolite section. This timing currently remains unresolved. White micas from phyllitic
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metacherts and calc-silicates from the lower, greenschist facies sedimentary sequence of the metamorphic sole yield 40Ar/39Ar cooling ages of 90–93 Ma (Hacker et al. 1996). The metasedimentary part of the metamorphic sole, which underlies the mafic amphibolites, must have been accreted and have cooled through the mica closure temperature by this time. Any model of Semail ophiolite genesis and evolution must also explain the formation and evolution of the continental margin-derived eclogites at As Sifah (Point 4), as long as it could be shown that they formed during the same period of tectonic evolution. The new U–Pb ages of the As Sifah eclogite peak metamorphism (Warren et al. 2003, 2005) combined with metamorphic thermobarometry (Searle et al. 1994; Warren and Waters 2006), suggest that these rocks reached peak metamorphic pressures of ca. 20 kbar at 78.95 ± 0.3 Ma. They must therefore have formed in a subduction zone, and the simplest and most plausible scenario is that they formed in the same subduction zone as the metamorphic sole amphibolites, yet 15 My later. The MOR model does not and cannot account for the formation of the eclogites nor for the subduction zone metamorphism of the sole amphibolites. We therefore conclude that the new U–Pb age data presented by Warren et al. (2003, 2005), in conjunction with a wealth of previously published structural, lithological, geochemical and thermobarometric data, support the SSZ model of formation of the ophiolite and the concomitant subduction zone metamorphism of the sole. Detachment of the ophiolite most probably initiated along the subduction zone, as a pre-existing weak zone. Old, cold oceanic crustal sediments and basalts were subducted beneath the young, hot ophiolite, and were metamorphosed under garnet amphibolite conditions within 1 My of the crystallisation of the ophiolite trondhjemites. They were rapidly exhumed and accreted onto the base of the ophiolite before 90– 93 Ma, when micas in the metasedimentary units cooled through the closure temperature for argon diffusion (Hacker et al. 1996). Fifteen million years later the continental margin of Oman was subducted down the same subduction zone, metamorphosed to eclogite facies conditions and subsequently rapidly exhumed (Searle et al. 2004).
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117 Boudier F, Coleman RG (1981) Cross section through the peridotite in the Semail ophiolite. J Geophys Res 86:2573– 2592 Boudier F, Ceuleneer G, Nicolas A (1988) Shear zones, thrusts and related magmatism in the Oman ophiolite: initiation of thrusting at an ocean ridge. Tectonophysics 151:275–296 Gnos E (1998) Peak metamorphic conditions in garnet amphibolites beneath the Semail ophiolite: implications for an inverted pressure gradient. Int Geol Rev 40:281–304 Gnos E, Kurz D (1994) Sapphirine-quartz and sapphirinecorundum assemblages in metamorphic rocks associated with the Semail Ophiolite (United Arab Emirates). Contrib Mineral Petrol 116:398–410 Godard M, Dautria J-M, Perrin M (2003) Geochemical variability of the Oman ophiolite lavas: relationship with spatial distribution and paleomagnetic directions. Geochem Geophys Geosyst 4(6):66–80. doi:10.1029/2002GC00452 Godard M, Bosch D, Einaudi F (2006) A MORB source for low-Ti magmatism in the Semail ophiolite. Chem Geol 234:58–78 Hacker BR (1991) The role of deformation in the formation of metamorphic gradients: Ridge subduction beneath the Oman Ophiolite. Tectonics 10:455–473 Hacker BR (1994) Rapid emplacement of young oceanic lithosphere: Argon geochronology of the Oman ophiolite. Science 265:1563–1565 Hacker BR, Mosenfelder JL, Gnos E (1996) Rapid emplacement of the Oman Ophiolite: thermal and geochronological constraints. Tectonics 15:1230–1247 Ishikawa T, Fujisawa S, Nagaishi K, Masuda T (2005) Trace element characteristics of the fluid liberated from amphibolite-facies slab: inference from the metamorphic sole beneath the Semail ophiolite and implication for boninite genesis. Earth Planet Sci Lett 240:355–377 Nicolas A (1989) Structures of ophiolites and dynamics of oceanic lithosphere. Kluwer, Amsterdam Pearce JA, Alabaster T, Shelton AW, Searle MP (1981) The Oman Ophiolite as a Cretaceous arc-basin complex: evidence and implications. Philos Trans R Soc Lond Ser A 300:299–317 Searle MP (1984) Alkaline peridotite, pyroxenite and gabbroic intrusions in the Oman Mountains, Arabia. Can J Earth Sci 21:396–406 Searle MP, Cox JS (1999) Tectonic setting, origin and obduction of the Oman Ophiolite. Geol Soc Am Bull 111:104–122 Searle MP, Cox JS (2002) Subduction zone metamorphism during formation and emplacement of the Semail Ophiolite in the Oman Mountains. Geol Mag 139:241–255 Searle MP, Malpas J (1980) The structure and metamorphism of rocks beneath the Semail ophiolite of Oman and their significance in ophiolite obduction. Trans R Soc Edinburgh 71:247–262 Searle MP, Malpas J (1982) Petrochemistry and origin of subophiolite metamorphic and related rocks in the Oman Mountains. J Geol Soc Lond 139:235–248 Searle MP, Waters DJ, Martin HN, Rex DC (1994) Structure and metamorphism of blueschist–eclogite facies rocks from the northeastern Oman Mountains. J Geol Soc Lond 151:555–576 Searle MP, Warren CJ, Waters DJ, Parrish RR (2004) Structural evolution, metamorphism and restoration of the Arabian continental margin, Saih Hatat region, Oman Mountains. J Struct Geol 26:451–473 Warren CJ, Waters DJ (2006) Oxidised eclogites and blueschists from Oman: PT path modelling in the NCFMASHO system.
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118 J Metamorph Petrol 24:783–802. doi:10.1111/j.15251314.2006.00668.x Warren CJ, Parrish RR, Searle MP, Waters DJ (2003) Dating the subduction of the Arabian continental margin beneath the Semail ophiolite. Oman. Geology 31:889–892
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Contrib Mineral Petrol (2007) 154:115–118 Warren CJ, Parrish RR, Waters DJ, Searle MP (2005) Dating the geologic history of Oman’s Semail Ophiolite: insights from U–Pb geochronology. Contrib Mineral Petrol 150:403–422
