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Ž . Earth and Planetary Science Letters 152 1997 217–231 U–Pb dating on single detrital zircon grains from the Triassic Songpan–Ganze flysch ž Central China /: provenance and tectonic correlations O. Bruguier a,) , J.R. Lancelot a , J. Malavieille b a Laboratoire de Geochimie ´ Isotopique, CNRS-UMR 5567, UniÕersite´ de Montpellier II, case 066, Place Eugene ` Bataillon, Montpellier, Cedex 5 34095 France b Laboratoire de Geophysique ´ et Tectonique, CNRS-UMR 5573, UniÕersite´ de Montpellier II, case 060, Place Eugene ` Bataillon, Montpellier, Cedex 5 34095 France Received 1 May 1997; revised 4 August 1997; accepted 4 August 1997 Abstract Ž . 2 The Songpan–Ganze flysch belt Central China covers a huge triangular area of more than 200,000 km and is bounded by the continental blocks of South China, North China and the Tibetan plateau. Detrital zircons extracted from three flysch samples collected in the central part of the belt were analyzed grain by grain using the U–Pb method. Two samples of Middle Triassic sandstones, collected at different locations in the belt, provide identical results, which suggests similar source regions. The detrital zircons yield a wide range of ages and indicate their principal derivation from Mid-Proterozoic Ž 1.8–2.0 Ga. source rocks with minor contribution from late Archean Ž ca. 2.5–2.6 Ga. material. The discordance and Pb loss patterns from low-U zircons indicate disturbances during a subsequent event which may be of Caledonian age, as suggested by concordant zircon grains at ca. 420 and 450 Ma. One sample collected within the Palang Shan Pass zone provides concordant zircon grains at around 230 Ma Ž 231"1 Ma and 233"1 Ma .. These Triassic ages are synchronous to flysch deposition and suggest intense geological activity Ž calc-alkaline volcanism . at that time in the area close to the basin. The data support an origin of the clastic material mainly from a northeastern landmass, corresponding to the southern margin of the Sino–Korean craton. To a lesser degree, inputs from the Yangtze craton and possibly from the northern margin of the basin Ž Kunlun arc. are also detected. The age spectrum from the Upper Triassic sandstone is significantly different and shows predominance of Sinian Ž ca. 760 Ma. grains, probably derived from the Yangtze craton. This change in the source region is interpreted as reflecting the tectonic evolution of this area and in particular as being linked to the late Triassic collision between South China and North China. In the Middle Triassic, while subduction of the Songpan sea northward beneath the North China plate was still taking place, continental subduction of South China in the Dabie region was responsible for uplift of the overriding plate Ž i.e. the Sino–Korean craton. which supplied large volumes of sediments. During the Late Triassic, clockwise rotation of the South China block uplifted the Indo-Sinian part of the Qinling belt and closed the basin. As the accretionary wedge was thickening along the southern margin of North China, detritus derived from this source region were unable to reach the flysch basin. The age spectrum presented by detrital zircons indicates ) Corresponding author. E-mail: bruguier@dstu.univ-montp2.fr 0012-821Xr97r$17.00 q 1997 Elsevier Science B.V. All rights reserved. Ž . PII S0012-821X 97 00138-6
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 Ge´ochimie 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 with the central peridotite body of Zabargad Island (Red Sea, Egypt) provide an age of 23.2 + 5.9 Myr consistent with the 238 U± 206 Pb age of the youngest concordant grain (22.4 + 1.3 Myr). Concordant grains indicate new zircon growth and/or resetting whereas slightly discordant analyses suggest participation of an older zircon component whose age cannot be defined precisely. SEM back-scattered imaging further reveals the occurrence of zoned domains almost completely erased by complex internal structures attributed to extensive recrystallization under metamorphic conditions. The 22.4 Myr Miocene age is thus interpreted as dating a high-temperature metamorphic event. The proximity between the studied sample and the peridotite supports previous conclusions which regard parts of the peridotites from Zabargad Island as an asthenopheric mantle diapir which intruded the thinned Pan-African continental crust during the early stages of the Red Sea opening. Terra Nova, 10, 274±279, 1998 Ahed Bhed Ched Dhed Ref marker Fig marker Table marker Ref end Ref start Introduction In the last decade, much attention has been focused on the tectonothermal and geochemical evolution of rocks outcropping on Zabargad Island (Red Sea, Egypt). This island, though of limited size (& 4km 2 ), has an almost unique geodynamic setting, 50 km west of the Red Sea axis, and reveals a close association of mafic/ultramafic rocks with a metamorphic gneiss complex. Understanding the geodynamic evolution of the island was expected to elucidate the processes operating in a young rift-setting environment and crust/mantle interface tectonics. Since the first work of Bonatti et al. (1981), however, genetic models have been limited by the the scant data on the timing of juxtaposition between the peridotites and the gneisses, as well as the nature of the mafic/ultramafic rocks. One interpretation considers the peridotites as an asthenospheric mantle diapir intruding a Pan-African continental crust during the early stages of the Red Sea opening (e.g. Nicolas et al., 1987). The few attempts to date the time of intrusion of the peridotites are consistent with this view. Nicolas et al. (1985), for example, inferred a K–Ar age of 23 + 7 Myr for amphiboles extracted from an amphibolite. This value concurs with the more precise Ar–Ar age (18.7 + 1.3 Myr) given by hornblende extracted from a pegmatoidal pocket of a coarse grained hornblendite *Correspondence: E-mail: bosch@dstu.univ-montp2.fr in harzburgite (Trieloff et al., 1997). Both ages are identical, within errors, of a 18.4 + 1.0 Myr zircon age obtained by Oberli et al. (1987). Although there is a good agreement between the three methods, Oberli’s zircon value has been presented only in an abstract, thus making it difficult to properly evaluate. K–Ar and Ar–Ar ages have also been questioned for decoupling of parent and daughter nuclides (Villa, 1990), or for their meaning in terms of a geological event (uplift, hydrothermalism or emplacement age) due to their known susceptibility to low-grade events. An alternative interpretation considers the peridotites and the gneisses to be Pan- African (Brueckner et al., 1988, 1995) and that juxtaposition of both rock types occurred shortly after differentiation from a common depleted mantle source & 700 Ma. This is apparently supported by the so-called SLAP errorchron resulting from alignment of whole rock peridotite samples and CPX separates in the Sm–Nd isochron diagram (Brueckner et al., 1988). This model, however, has been challenged in two recent papers on the grounds of fluid inclusion (Boullier et al., 1997) and noble gases and argon chronological studies (Trieloff et al., 1997). In both models, gneisses are seen to represent remnants of the Pan-African continental deep crust left behind during opening of the Red Sea rift. Pan-African ages for the metamorphic gneiss complex are well established by Sm–Nd and Rb–Sr mineral isochrons (Lancelot and Bosch, 1991; Brueckner et al., 1995) as well as by a zircon Pb–Pb evaporation date (Brueckner et al., 1995) from a felsic gneiss located a few hundred metres from the contact zone, between the central peridotite body and the gneisses. These ages, however, provide no constraints on the timing of juxtaposition of gneisses and peridotites. In order to solve this fundamental problem, and because of the known resistance to alteration of the mineral zircon, we focused on the U–Pb dating of single zircon grains extracted from a felsic gneiss located at the contact zone with the central peridotite body. Geological setting Located about 90 km south-east of the Râs Baˆ nas peninsula and about 50 km west of the Red Sea axis, Zabargad island (Fig. 1) presents a petrographic association dominated by three bodies of fresh ultramafic rocks. The southern body is constituted by plagioclase peridotites with a great abundance of diabase dikes at the contact with the overlying metasedimentary formation. The central and northern massifs are spinel lherzolites. Three generations of amphiboles are present in all three peridotite bodies and have been classified into three generations related to distinct metasomatic events (Agrinier et al., 1993). The northern and central massifs are in contact with a complex polygenic assemblage constituted by Pan-African granulite-facies gneisses (Bosch, 1990; Lancelot and Bosch, 1991; Brueckner et al., 1995) equilibrated at a depth of over 30 km (Boudier et al., 1988) and by igneous pyroxenites and gabbros un- 274 *C 1998 Blackwell Science Ltd
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