Journal of the Geological Society, London, Vol. 155, 1998, pp. 281–295. Printed in Great Britain.
Crustal evolution and terrane correlation in the eastern Arabian Shield, Yemen: geochronological constraints MARTIN J. WHITEHOUSE 1 , BRIAN F. WINDLEY 2 , MAHFOOD A. O. BA-BTTAT 2,3 , C. MARK FANNING 4 & DAVID C. REX 5 1 Department of Earth Sciences, University of Oxford, Oxford OX1 3PR, UK Present address: Swedish Museum of Natural History, Box 50007, S-104 05 Stockholm, Sweden (e-mail:
[email protected]) 2 Department of Geology, University of Leicester, Leicester LE1 7RH, UK 3 Arab Petroleum Services Co. Ltd, PO Box 48, Aden, Yemen 4 Research School of Earth Sciences, Australian National University, Canberra ACT2601, Australia 5 Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK Abstract: Ion-microprobe U–Pb zircon and 40Ar–39Ar geochronology is presented for two gneiss terranes and an island arc terrane in the Arabian Shield of Yemen. These data confirm the presence of late Archaean (c. 2.95–2.55 Ga) crust in the Al-Mahfid (eastern) terrane, previously inferred from Nd model age determinations; a minor late Archaean component in the Abas (western) terrane is revealed by a single c. 2.6 Ga zircon core. No ages have been obtained corresponding to documented c. 1.8–1.4 Ga events in neighbouring Somalia and Saudi Arabia, but both gneiss terranes are strongly affected by c. 760 Ma Pan-African reworking which is recorded by Pb loss in zircons of the Al-Mahfid terrane and zircon crystallization ages of granitic gneisses and sheets in both the Al-Mahfid and Abas gneiss terranes. 40 Ar–39Ar ages obtained from intrusive rocks in the intervening Al-Bayda arc terrane provide a lower age limit of 615 Ma upon terrane assembly. Correlation of pre-Pan-African events within the Arabian–Nubian Shield remains problematic because of the still limited sampling and available geochronology. Keywords: Arabian Shield, Yemen, Pan-African Orogeny, absolute age, terranes.
Assembly of the Arabian–Nubian Shield during the PanAfrican (900–550 Ma; Kröner 1985) involved juvenile island arcs and micro-continental fragments of poorly known affinity. Elucidation of the chronology of events within these terranes is important for correlations both within the Arabian–Nubian Shield itself, and for continental distribution prior to Arabian– Nubian Shield accretion. For the southern Afif continental terrane exposed in the eastern part of the shield in Saudi Arabia, Agar et al. (1992) have presented conventional and ion-microprobe U–Pb zircon geochronology, feldspar common Pb, and whole-rock Sm–Nd isotopic data. A protolith age of c. 1.7 Ga was determined for a charnockite from Jabal Khida (Fig. 1) using conventional zircon U–Pb with Sm–Nd data indicating an older, possibly late Archaean, precursor. Paragneisses from the southern Afif terrane yielded a range of Sm–Nd depleted mantle model ages (tDM) and detrital zircon populations with ages of c. 1.6–1.8 Ga and 2.4–2.6 Ga. In northern Somalia, Kröner & Sassi (1996) have reported midProterozoic ages in the range 1.4–1.8 Ga from xenocrystic zircons in Pan-African (840–720 Ma) granitic gneisses. Further evidence for the existence of ancient continental terranes in the Arabian–Nubian Shield has been presented by Windley et al. (1996) who identified several gneiss terranes separated by island arcs in the Precambrian of Yemen. Two of these terranes, the Abas (western) and Al-Mahfid (eastern), yielded Sm–Nd model ages (tDM) in the range 1.7–2.3 Ga and 1.3–3.0 Ga respectively, with the oldest recognizable gneisses in the Al-Mahfid terrane having tDMs between 2.7 and 3.0 Ga. The Proterozoic model ages obtained by Windley et al. (1996) together with other Sm–Nd data from the Abas terrane
(Stoeser et al. 1991) cannot be used to constrain directly the ages of the gneisses which probably represent mixtures of juvenile (mantle-derived) Pan-African material with older components. The potential complexity of older components, such that a Pan-African age gneiss might, by analogy with the southern Afif terrane, contain late Archaean, mid- and late Proterozoic zircons, can be unravelled only by detailed geochronology. In this paper we present zircon U–Pb geochronology for samples from the Abas and Al-Mahfid terranes obtained with the Sensitive High-Resolution Ion Micro-Probe (SHRIMP) instruments located at the Australian National University. Additionally, we present Ar–Ar data from the Al-Bayda island arc terrane which separates the Abas and Al-Mahfid gneiss terranes. These data have important implications for the evolution of continental terranes in Yemen, the timing of terrane accretion in the eastern part of the Arabian–Nubian Shield, as well as for correlation of gneiss terranes within the Arabian–Nubian Shield and wider ranging Gondwanaland reconstructions (Stern 1994).
Geological setting and sample location Windley et al. (1996) have presented geological descriptions and Nd isotopic data from two gneiss terranes in central southern Yemen (Fig. 1), the Abas and Al-Mahfid, and only brief details are reproduced here. Gneisses and metamorphosed supracrustal rocks form the two main components of the Abas gneiss terrane. Predominantly homogeneous, hornblende–biotite orthogneisses, some 281
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Fig. 1. Map of the Precambrian outcrop in Yemen (adapted from Windley et al. 1996) showing delineation of gneissic and island arc terranes. Approximate locality of samples from this study are indicated by numbered circles as follows: (1) Y92-7/6; (2) Y92-15/1; (3) BY104, BY109, BY116 and BY119; (4) Y92-47/1, Y92-48/1, Y92-50/1 and Y92-51/1. Inset diagrams show suggested terrane correlations with adjoining areas: (A) Saudi Arabia (locality of Jabal Khida charnockite mentioned in text is marked by asterix) and (B) northern Somalia (A, Abdulkadir; QBM, Qabri Bahar–Mora; M, Mait; IA, Inda Ad).
exhibiting extensive migmatization, contain many amphibolite dykes (
