Rapid emplacement of the Oman ophiolite: Thermal and geochronologic constraints

Abstract

In understanding of ophiolite emplacement requires knowledge of the elapsed time between igneous crystallization and intraoceanic thrusting, and the rate and duration of that thrusting. Hornblende 4øAr/39Ar ages demonstrate that the igneous oceanic crust in Oman crystallized and cooled to-825 K in 1-2 m.y. Hornblende ages from metamorphic rocks and from cross-cutting dikes require that the basal metamorphic thrust fault beneath the ophiolite also cooled below-825 K in 1-2 m.y. Motion along the sole thrust accounted for 200 km of displacement at a rate of 100-200 mm/yr. On the basis of age relationships and thermal considerations, we favor a twostage model for the initial stages of Samall ophiolite emplacement: intraoceanic thrusting over < 2-m.y.-old lithosphere at 150 km/m.y. parallel to a spreading ridge for 1-2 m.y., followed by equally rapid and brief thrusting over cold and old lithosphere. Preservation of the Samail ophiolite is a direct result of its young age and positive buoyancy at the time of ocean closure, and we propose that all ophiolites that originated near spreading centers and were emplaced onto continents were young at the time of intraoceanic thrusting. Locations of Ar/ Ar samples from this study are indicated by sample number. Ages (in Ma) of U/Pb zircon samples [Tilton et al., 1981] are preceded with a "z". Orientations of sheeted dikes after Nicolas et al. [1988]. spreading and subsequent melt extraction during intraoceanic thrusting [Ernewein et al., 1988; Pfiumio, 1991]. Plutonic rocks comprise an early gabbroic series (60-80 vol% of the crust) and a late wehrlitic series (20-40%) [Browning and Smewing, 1981; Juteau et al., 1988; Lippard et al., 1986]. The early series is a gabbro-dioriteplagiogranite suite that was the source of the sheeted dike complex and the lower extrusive unit [Lippard et al., 1986; Nicolas and Boudier, 1991]. The wehrlitic series is dominantly wehrlite, with subordinate dunite, gabbro, plagiogranite, and granite, which intruded all younger crustal rocks before complete crystallization of the gabbroic sequence. It is genetically related to the middle extrusive sequence, V2, and is inferred to have been emplaced during

Mantle xenolith‐bearing Maastrichtian to Tertiary alkaline magmatism in Oman

“…Although Jebel Ja'alan is situated in a forebulge position with respect to the Eastern Ophiolite Belt-Batain nappe thrust system, it is unlikely that the loading from this relatively thin nappe stack (<10 km) alone would be sufficient unless thrusting along reactivated faults occurred. The early thrusting (obduction) of the Semail ophiolite was similarly associated with alkaline volcanism [Allemann and Peters, 1972;Robertson et al, 1990;Hacker et al, 1996] which suggests that loading of a continental edge with an ophiolite causes deep-reaching deformation in the continental crust manifested by localized alkaline magmatism due to extension.…”

Rapid conversion of an oceanic spreading center to a subduction zone inferred from high-precision geochronology

“…The process of subduction initiation may last on the order of only 2-3 My (assuming a convergence rate of ∼5 cm/y) (1), a small fraction of the lifetime of a subduction zone, and is therefore unlikely to be captured by present day observations. Ophiolites that have similar crystallization ages to the metamorphic age of their sole, like the Palawan ophiolite, are not uncommon (19,25,26). These other examples, however, lack dates for the protolith of the sole.…”

“…Metamorphic soles commonly have metamorphic cooling ages that are similar to the igneous crystallization ages of their overlying ophiolites (19,25,26), implying that the overlying ophiolite was still very young or formed during or shortly after subduction initiated. However, because these cooling ages reflect the time of metamorphic cooling and not necessarily the time when the original igneous crust formed, the similarity in ages between the metamorphic sole and overlying ophiolite may be interpreted in several ways: (i) subduction initiation between older lithospheric plates followed by rapid slab rollback and seafloor spreading that generates the ophiolite eventually preserved with the sole (10); (ii) subduction initiation of variably older lithosphere beneath an already active spreading center (27,28); (iii) subduction initiation along weak detachment faults at some distance from a spreading center (12), or (iv) underthrusting of young lithosphere at a spreading center (13) (Fig.…”

Thermochronologic constraints on deformation and cooling history of high‐ and ultrahigh‐pressure rocks in the Qinling‐Dabie orogen, eastern China

“…White mica, ubiquitous in the host paragneisses and common in mylonitized eclogites, was assumed to be either muscovitic or phengitic based on paragenesis. The 4øArfl9Ar analyses were conducted at the Stanford University geochronology laboratory (see Hacker et al [1996a] for analytical details). Typically, all samples gave highly radiogenic gas fractions requiring atmospheric corrections of 10% or less, and less than 5% on average for white mica, in particular.…”