Paper 202 Disc
An early Miocene age for a high-temperature event in gneisses from Zabargad Island (Red Sea, Egypt): mantle diapirism? D. Bosch* and O. Bruguier Laboratoire de GeÂochimie Isotopique, CNRS-UMR 5567, cc 066,Universite de Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex 05, France
ABSTRACT U±Pb zircon data from a felsic gneiss located at the contact zone recrystallization under metamorphic conditions. The 22.4 Myr with the central peridotite body of Zabargad Island (Red Sea, Miocene age is thus interpreted as dating a high-temperature Egypt) provide an age of 23.2 + 5.9 Myr consistent with the metamorphic event. The proximity between the studied sample 238U±206Pb age of the youngest concordant grain (22.4 + 1.3 and the peridotite supports previous conclusions which regard Myr). Concordant grains indicate new zircon growth and/or parts of the peridotites from Zabargad Island as an asthenopheric resetting whereas slightly discordant analyses suggest mantle diapir which intruded the thinned Pan-African continental participation of an older zircon component whose age cannot crust during the early stages of the Red Sea opening. be defined precisely. SEM back-scattered imaging further reveals the occurrence of zoned domains almost completely erased by complex internal structures attributed to extensive Terra Nova, 10, 274±279, 1998
in harzburgite (Trieloff et al., 1997). date (Brueckner et al., 1995) from a Introduction Both ages are identical, within errors, felsic gneiss located a few hundred In the last decade, much attention has of a 18.4 + 1.0 Myr zircon age obtained metres from the contact zone, between been focused on the tectonothermal by Oberli et al. (1987). Although there the central peridotite body and the and geochemical evolution of rocks is a good agreement between the three gneisses. These ages, however, provide outcropping on Zabargad Island (Red methods, Oberli's zircon value has been no constraints on the timing of juxta- Sea, Egypt). This island, though of presented only in an abstract, thus position of gneisses and peridotites. In limited size (& 4km2), has an almost making it difficult to properly evaluate. order to solve this fundamental pro- unique geodynamic setting, 50 km west K±Ar and Ar±Ar ages have also been blem, and because of the known resis- of the Red Sea axis, and reveals a close questioned for decoupling of parent tance to alteration of the mineral zir- association of mafic/ultramafic rocks and daughter nuclides (Villa, 1990), or con, we focused on the U±Pb dating of Ahed with a metamorphic gneiss complex. for their meaning in terms of a geolo- single zircon grains extracted from a Bhed Understanding the geodynamic evolu- gical event (uplift, hydrothermalism or felsic gneiss located at the contact zone Ched tion of the island was expected to elu- emplacement age) due to their known with the central peridotite body. Dhed cidate the processes operating in a susceptibility to low-grade events. An Ref marker young rift-setting environment and alternative interpretation considers the Geological setting Fig marker crust/mantle interface tectonics. Since peridotites and the gneisses to be Pan- Table marker the first work of Bonatti et al. (1981), African (Brueckner et al., 1988, 1995) Located about 90 km south-east of the Ref end however, genetic models have been lim- and that juxtaposition of both rock Raà s BaÃnas peninsula and about 50 km Ref start ited by the the scant data on the timing types occurred shortly after differentia- west of the Red Sea axis, Zabargad of juxtaposition between the perido- tion from a common depleted mantle island (Fig. 1) presents a petrographic tites and the gneisses, as well as the source & 700 Ma. This is apparently association dominated by three bodies nature of the mafic/ultramafic rocks. supported by the so-called SLAP error- of fresh ultramafic rocks. The southern One interpretation considers the peri- chron resulting from alignment of body is constituted by plagioclase peri- dotites as an asthenospheric mantle whole rock peridotite samples and dotites with a great abundance of dia- diapir intruding a Pan-African conti- CPX separates in the Sm±Nd isochron base dikes at the contact with the over- nental crust during the early stages of diagram (Brueckner et al., 1988). This lying metasedimentary formation. The the Red Sea opening (e.g. Nicolas et al., model, however, has been challenged in central and northern massifs are spinel 1987). The few attempts to date the time two recent papers on the grounds of lherzolites. Three generations of am- of intrusion of the peridotites are con- fluid inclusion (Boullier et al., 1997) phiboles are present in all three perido- sistent with this view. Nicolas et al. and noble gases and argon chronologi- tite bodies and have been classified into (1985), for example, inferred a K±Ar cal studies (Trieloff et al., 1997). In both three generations related to distinct age of 23 + 7 Myr for amphiboles ex- models, gneisses are seen to represent metasomatic events (Agrinier et al., tracted from an amphibolite. This value remnants of the Pan-African continen- 1993). The northern and central massifs concurs with the more precise Ar±Ar tal deep crust left behind during open- are in contact with a complex polygenic age (18.7 + 1.3 Myr) given by hornble- ing of the Red Sea rift. Pan-African assemblage constituted by Pan-African nde extracted from a pegmatoidal ages for the metamorphic gneiss com- granulite-facies gneisses (Bosch, 1990; pocket of a coarse grained hornblendite plex are well established by Sm±Nd and Lancelot and Bosch, 1991; Brueckner Rb±Sr mineral isochrons (Lancelot and et al., 1995) equilibrated at a depth of *Correspondence: Bosch, 1991; Brueckner et al., 1995) as over 30 km (Boudier et al., 1988) and by E-mail: [email protected] well as by a zircon Pb±Pb evaporation igneous pyroxenites and gabbros un-
274 *C 1998 Blackwell Science Ltd Paper 202 Disc
Terra Nova, Vol 10, No. 5, 274±279 Mantle diapirism, Zagarbad Island? . D. Bosch and O. Bruguier ......
SAUDI dard techniques (e.g. Bosch et al., 1996; ZABARGAD ISLAND ARABIA Pidgeon et al., 1996) where only the best