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Comment on ''Dating the Geologic History of Oman's Semail Ophiolite Contrib Mineral Petrol DOI 10.1007/s00410-007-0189-5 COMMENT Comment on ‘‘dating the geologic history of Oman’s Semail ophiolite: insights from U–Pb geochronology’’ by C. J. Warren, R. R. Parrish, D. J. Waters and M. P. Searle Franc¸oise Boudier Æ Adolphe Nicolas Received: 3 March 2006 / Accepted: 9 January 2007 Ó Springer-Verlag 2007 Warren et al. 2005 present a new set of precise radio- features such as low Ti, light-rare-earth-element genic dates constraining the history of genesis and depleted patterns, and low Zr/Hf ratios relative to emplacement of the Semail ophiolite of Oman. The N-MORB. These features are carried only by the sec- new U–Pb data concern the eclogites preserved at the ondary volcanism overlying the normal MORB-type margin of the Arabian shield, and the metamorphic crust. Moreover, the significance of the ‘primitive’ sole of the ophiolite, as well as plagiogranites intruding geochemical signature has been questioned (e.g., the ophiolite. Although the paper is oriented toward Hofmann 2004), especially in the case of Oman the continental subduction recorded by the eclogite, it (Godard et al. 2003, 2006; Ishikawa et al. 2002, 2005). contains (p. 414 and 419) a pointed allusion to the The new U/Pb ages on zircon published in Warren Supra-Subduction Zone (SSZ) origin of the ophiolite et al.’s article show, with an error bar as low as ‘‘... the metamorphic sole formed during or very rap- 200,000 year, that the genesis of the trondhjemites (the idly after ophiolite crystallization. The synchroneity of last magmatic products in the ophiolite pile), at initial metamorphic sole formation with final stages of 95.3 ± 0.2 Ma, slightly predates the metamorphism of ophiolite crystallization is much more consistent with a the sole at 94.48 ± 0.23 Ma. The other interesting thing SSZ zone origin for the metamorphic sole than a about these new data is that the U/Pb ages on zircon [Mid-Ocean Ridge] (MOR) setting’’, a conclusion that mark the peak of metamorphism (850°C) and not the is not discussed in the paper and a so far unanswered closure of hornblende (500°C) as recorded by previous controversial question. Ar–Ar dating on amphibole (Hacker 1994). A differ- We wish, when considering these excellent new data ence in age smaller than 1 Ma between the last episode which constrain the previous chronology of ophiolite of formation of the ophiolite and its first thrusting over genesis versus emplacement thrusting, to recall the oceanic crust confirms the emplacement model pub- terms of the controversy, and emphasize that these lished by our group. We therefore question model 1A, results reinforce a previous tectonic model published proposed by Warren et al., reproduced here in Fig. 2, by our group (Boudier and Coleman 1981; Boudier and ask: how can such a SSZ model for the genesis of et al. 1988; Nicolas 1989), summarized in Fig. 1.In the ophiolite be reconciled with their new age data. addition, we wish to recall that the SSZ origin of the Model 1A (Fig. 2), favored by Warren et al. 2005,of Semail ophiolite, as proposed in the initial Pearce et al. an ophiolite generated in a SSZ environment and (1981) contribution, is based mainly on geochemical detached along the subduction plane was developed by the Open University group (Pearce et al. 1981; Alabaster et al. 1982; Searle and Cox 1999, 2002). It Communicated by I. Parsons. faces several problems that have been raised previ- F. Boudier (&) Á A. Nicolas ously (Nicolas 1989): Tectonophysique, Universite´ Montpellier 2, • In this SSZ model, the lithosphere subducted Place Eugene Bataillon, 34095 Montpellier, France e-mail: [email protected] beneath the ophiolite is millions of years older 123 Contrib Mineral Petrol Fig. 1 a Genesis and ‘detachment’ model of the Semail ophiolite Hacker and Mosenfelder 1996) constrains the detachment close in Oman (after Boudier et al. 1988). The ophiolite represents a to the ridge axis. In addition, the subparallelism of the basal large segment of oceanic lithosphere generated at an oceanic contact of the Oman ophiolite with the overlying stratigraphic ridge axis. The lithospheric plate detached comprises V1 units at the scale of the map (Nicolas et al. 2000) constrains a low ‘geotimes’ volcanics of MORB affinity and the continuous angle detachment plane. The secondary volcanism (V2 or Lasail- stratigraphy of the ophiolite components assigns its genesis to Alley) of ‘arc affinity’ is attributed to intra-oceanic thrusting or fast spreading conditions. The very high inverse thermal gradient associated shallow subduction as also proposed by Ishikawa et al. of the dynamo-metamorphism recorded at the sole of the (2002; 2005). b Thermal structure at fast spreading ridge axis ophiolite fits the overthrusting of a hot, thin, young oceanic (5 cm/year half spreading rate) after Kusznir (1980); Morton and lithosphere (Dewey 1976). The fact that the ages of the accreted Sleep (1985). The 1,000°C isotherm is taken as the limit of the lithosphere (biostratigraphic dating, Tipitt et al. 1981), are very mechanical lithosphere, used as the detachment plane at the close to those of the metamorphic sole (Ghent and Stout 1981; ridge Model 1: NE-directed Subduction Model 1A Oman margin Semail ophiolite SW NE HT Metamorphic Sole Saih Hatat HP rocks Fig. 2 Model 1A after Searle et al. (1994; 2003) in Warren et al. neous problems of heating the basal contact and a huge thinning (2005). According to this model, the ophiolite was exhumed with of the mantle section of the ophiolite initial shear along the subduction fault plane, raising simulta- than the accreting ophiolite. Thermal modeling, the ridge requires a large thinning, as also pointed including shear heating (Hacker, 1991), indicates out by Hacker (1991). This is not supported by our that this lithosphere, whatever the depth, cannot be structural mapping in the mantle section of the heated to replicate the peak metamorphic temper- Semail ophiolite (Nicolas et al. 2000). We have atures (~850°C) recorded in the metamorphic sole. mapped kilometer-sized shear zones with mylonitic • In order to generate the large and fast extension peridotites, but these are very steep, and they relate recorded in the Semail ophiolite in a SSZ environ- to ridge segmentation (Nicolas et al. 2000). Within ment (i.e as in Fig. 2 of Searle and Cox 1999), the mantle section no major low-angle thrusts, classical models invoke a retreating (or roll over) which are necessary to produce this large internal subduction and sinking of the slab at depth, below thinning (Casey and Dewey 1984), have been the new spreading center. Most recent compilations observed. The only flat-lying shear zone is located (Lallemand et al. 2005) state that in present-day at the base of the mantle unit, just above the convergent systems, SSZ spreading is observed for metamorphic sole. Its deformation, at 1,000–900°C, subducted plates with dips larger than 51°. Bringing is in continuity with that of the underlying granu- back a steep subduction plane to some 20 km below lites and amphibolites of the metamorphic sole. 123 Contrib Mineral Petrol Upsection in the main mantle, this lower temper- Ghent ED, Stout MZ (1981) Metamorphism at the base of the ature horizon grades, over 1–2 km, into a section Semail ophiolite, Southeastern Oman mountains. J Geophys Res 86:2557–2571 recording plastic flow at temperatures of about Godard M, Dautria JM, Perrin M (2003) Geochemical variability 1,200°C, related to ridge activity. of the Oman ophiolite lavas: relationship with spatial distribution and paleomagnetic directions. Geochem Geo- These considerations make the SSZ model 1A phys Geosyst 4(6):1–15 geometrically and thermally unrealistic. Moreover, the Godard M, Bosch D, Einaudi F (2006) A MORB source for low new age constraints do not support it. Conversely, the Ti magmatism in the Semail ophiolite. Chem Geol (in press) model of detachment of the lithospheric plate close to Hacker BR (1991) The role of deformation in the formation of metamorphic gradient: ridge subduction meneath the Oman the ridge axis of a fast spreading center (Fig. 1) ophiolite. Tectonics 10(2):455–473 accounts for the field and thermal constraints, and the Hacker BR (1994) Rapid emplacement of young oceanic tight chronology deduced from Warren et al.’s new lithosphere: argon geochronology of the Oman ophiolite. data. The detachment occurred at the base of the Science 265:1563–1565 Hacker BR, Mosenfelder JL (1996) Metamorphism and defor- newly formed lithosphere. The metamorphic sole, mation along the emplacement thrust of the Samail ophi- located at the base of the overriding plate, records olite, Oman. Earth Planet Sci Lett 144:435–451 temperatures as high as ~850°C (Ghent and Stout 1981; Hofmann AW (2004) Sampling mantle heterogeneity through Hacker 1994) with an apparent inverted thermal gra- oceanic basalts: isotopes and trace elements. In: Carlson RW (ed) Treatise on geochemistry, vol 2. Elsevier, Oxford dient as high as 4,000°C/km (Hacker and Mosenfelder San Diego, pp 61–101 1996). This metamorphic sole could have developed Ishikawa T, Nagaishi K, Umino S (2002) Boninitic volcanism in only below rocks at a T ~ 1,000°C. Thermal models of the Oman ophiolite: Implications for thermal condition fast spreading ridges (Kusznir 1980; Morton and Sleep during transition from spreading ridge to arc. Geology 30(10):899–902 1985) indicate that at 1 Ma (Fig. 1b), the lithosphere is Ishikawa T, Fujisawa S, Nagaishi K, Masuda T (2005) Trace only 10 km thick after 1 Ma of spreading at a rate of element characteristics of the fluid liberated from amphib- 5 km/Ma, the thickness that is recorded on average in olite-facies slab: Inference from the metamorphic sole Oman ophiolite sections.
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