Introduction Yemen basement Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol105.Issue2-4.2001:

Precambrian Research 105 2001 357 – 369 Precambrian basement character of Yemen and correlations with Saudi Arabia and Somalia Martin J. Whitehouse a, , Brian F. Windley b , Douglas B. Stoeser c , Salah Al-Khirbash d , Mahfood A.O. Ba-Bttat e , Abdullah Haider f a Swedish Museum of Natural History, Box 50007 , SE- 104 05 Stockholm, Sweden b Department of Geology, Uni6ersity of Leicester, Leicester LE 1 7 RH, UK c United States Geological Sur6ey, MS 905 , Den6er Federal Center, Den6er, CO 80225 , USA d Uni6ersity of Sana ’ a, P.O. Box 13499 , Sana ’ a, Yemen e Department of Earth and En6ironemental Sciences, Kuwait Uni6ersity, P.O. Box 5960 , Safat, 13060 , Kuwait f Geological Museum, Øster6oldgade 5 - 7 , 1350 Copenhagen K., Denmark Received 23 February 1999; accepted 27 May 1999 Abstract The Precambrian basement of Yemen occupies a key location in the Pan-African orogen of Gondwana. This paper reviews geological, isotopic and geochronological data and presents new Pb- and Nd-isotope data which help define distinct gneiss terranes within this basement, constraining correlations of these terranes with neighbouring regions of Saudi Arabia and Somalia. Existing whole-rock Pb- and Nd-isotopic data are also summarised. These data should facilitate a more objective assessment of the contribution of the Yemen Precambrian to Cenozoic magmatism associated with the opening of the Red Sea and the Gulf of Aden. © 2001 Elsevier Science B.V. All rights reserved. Keywords : Pb- and Nd-isotopes; Precambrian; Yemen www.elsevier.comlocateprecamres

1. Introduction

The Precambrian basement of Yemen is located between the collage of low-grade, mainly island arc terranes of the Arabian – Nubian Shield ANS to the west and the high-grade polycyclic, mainly gneissic terranes of the Mozambique Belt to the south, which extends via Somalia and Ethiopia to eastern Africa. Thus, the Precambrian basement of Yemen provides a key link in our understand- ing of the Pan-African orogen of Gondwana Kro¨ner, 1985; Stern, 1994. The Precambrian rocks of Yemen underlie much of the Cenozoic volcanic cover and therefore the trends, linea- ments, varied age and heterogeneous character of the Precambrian rocks may also have had a sig- nificant structural and chemical influence on the evolution of the volcanic-dominated continental margins of Yemen. Corresponding author. Fax: + 46-8-5195-4031. E-mail address : martin.whitehousenrm.se M.J. White- house. 0301-926801 - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 9 2 6 8 0 0 0 0 1 2 0 - 0 A number of recent studies have been published on the Precambrian geology of eastern Saudi Ara- bia Stoeser and Stacey, 1988; Quick, 1991; Agar et al., 1992 and the Precambrian rocks of north- ern Somalia Sacchi and Zanferrari, 1987; Lenoir et al., 1994; Kro¨ner and Sassi, 1996. Added to this, there is an expanding literature on the Pre- cambrian basement of Yemen Stoeser et al., 1991; Whitehouse et al., 1993, 1994, 1998; Wind- ley et al., 1996, which is likely to have consider- able influence upon the development of Cenozoic volcanism and structure in the region. In their geochronological study of Yemen basement rocks, Whitehouse et al. 1998 proposed tentative corre- lations with Precambrian basement rocks of Saudi Arabia and Somalia. The aim of the present paper is to review the main tectonic units and isotopic characteristics existing geochronological and Nd- isotope data; new Pb- and Nd-isotope data of the Yemen basement and further outline correlations with Saudi Arabia and Somalia.

2. Yemen basement

2 . 1 . Geological obser6ations The lithological associations of the terranes in Yemen are summarised in Table 1 modified after Whitehouse et al., 1998. In this paper, we use the term terrane to imply a tectonic unit characterised by distinctive geological, geochemical, isotopic andor geochronological features within the boundaries of major structural discontinuities. From a combination of the geological relation- ships and our geochronological data, we conclude that the Precambrian basement of Yemen is com- prised of an alternation of early Precambrian gneissic terranes and late Proterozoic island arcs, which were accreted together to form an arc- gneiss collage during, the Pan-African orogeny. This accretion gave rise to NNENE-trending terranes with prominent parallel lineaments which are principally the suture zones and associated mylonite zones along and close to the boundaries of the alternating terranes. For example, a major suture possibly correlative with the Nabitah su- ture of Saudi Arabia Fig. 1 is manifested near Hajjah by a \ 30 m wide, vertical, highly de- formed ophiolite consisting of alternating tectonic slices of serpentinites and gabbros. This is the southern end of a 1200 km long suture zone Quick, 1991, which is a major zone of weakness and fluid transport in the continental crust. Fur- ther southeast the suture zones between the Abas, Al-Bayda, Al-Mahfid and Al-Mukalla terranes are less prominent than the Nabitah suture but nevertheless are represented today by zones of ductile and brittle deformation several kilometres wide. This is seen along the suture between the Abas and Al-Bayda terranes where there is also a 1 km wide mylonite zone. The suture zones be- tween the gneiss and arc terranes in Yemen are major crustal scale tectonic boundaries, which might be expected to have influenced the develop- ment of later geological structures such as sedi- mentary basins and lava fields. 2 . 2 . Isotopic and geochronological features 2 . 2 . 1 . Nd-isotopic data Windley et al. 1996 have presented neodymium isotope data for representative lithologies from the Yemen Precambrian terranes. Additionally, in this study, we present new data Table 2 from a reconnaissance set of samples collected during a 1975 traverse of the Abas and Al Bayda terranes Grolier et al., 1977; Appendix A. These data display considerable variation within each terrane, reflecting the basement het- erogeneity andor mixture of age components. Fig. 2 presents histograms of the depleted mantle model ages for granitic gneisses, granites and gabbros from the Abas, Al-Bayda and Al-Mahfid terranes of Yemen this study and data from Windley et al., 1996, together with data from basement rocks i.e. pre-750 Ma Pan-African in- trusives, including paragneisses, of the Afif ter- rane of Saudi Arabia data from Agar et al., 1992. The Al-Mahfid terrane is clearly distinctive, yielding the only late Archean model ages, and two of these samples have been dated at 2.55 Ga Whitehouse et al., 1998, confirming the presence of late-Archean crust in Yemen it also should be noted that a c 3 Ga model age is obtained from a metasediment of the Abas terrane, Table 1. Pan- M .J . Whitehouse et al . Precambrian Research 105 2001 357 – 369 359 Table 1 Comparative geology and isotopic data from terranes in Yemen, Saudi Arabia and northern Somalia YemenSaudi Arabia Northern Somalia Isotopic data Description Description Terrane Terrane Isotopic data complex Asir Saudi Amphibolite-facies gneisses Yemen: none. Saudi Arabia: alternating with 840–750 Ma diorites and Arabia greenschist-grade supracrustal ?Yemen tonalites; evolved granitoids \ 570 Ma 1 belts Amphibolite-facies Afif Saudi Yemen: none. Saudi Arabia: Arabia basement — 1800 Ma orthogneisses alternating with UPb; t DM 2400–1650 Ma; ?Yemen meta-volcanic belts. Many 750; Siham arc — 750–700 granitic plutons Andean-type continental margin? Ma 2 Abas Yemen Orthogneisses — t DM Amphibolite-facies 2300–1300 Ma 3,4 ; 950–750 orthogneisses containing belts Ma UPb 5 of supracrustal rocks rhyolites, schists, amphibolites, tuffs, marbles Al Bayda Greenschist-grade island Granitoids — t DM Basalts: 700–640 Ma 7 Greenschist-grade basalts and Abdulkadir Yemen arc-type rhyolites, andesites, tuffs, phyllites, quartzites, 2500–2000 Ma 4 ; Dykes — marbles 700–600 Ma ArAr 6 basalts and tuffs. Granitic plutons. Ophiolites. Andesite-rhyolite dykes Al-Mahfid Amphibolite-facies Old gneisses — t DM Mora Qabri Amphibolite facies gneisses Granitic gneisses: 840 –720 Yemen Bahar Ma UPb; xenocrysts: 3000–2700 Ma 4 ; 2900–2550 containing amphibolites, orthogneisses containing gabbro bodies and two Ma UPb 5 ; granitoids — marbles, quartzites, and 1820–1400 Ma 8 generations of supracrustal gabbro bodies. Granulite t DM 2200–1300 Ma 4 ; 750 Ma relics. UPb 5 rocks amphibolites, quartzites, marbles, rhyolites None Maydh Mait Greenschist-grade basalts, Greenschist-grade island Basalts: 700-640 Ma 7 Al-Mukalla tuffs, pelitic-clastic sediments arc-type tuffis, rhyolites, Yemen basalts, lava breccias. Granitic plutons, mafic dykes Granite — 562 Ma 9 Ghabar group Inda Ad group Very low-grade clastic Granites: 550 Ma 7 Very low-grade clastic sediments and limestones. sediments and limestones. Yemen Granites. Late Granites. Late Proterozoic–Cambrian Proterozoic-Cambrian 1 Quick 1991 summarising data of Calvez et al. 1983 and Stoeser 1985. 2 Agar et al. 1992. 3 Stoeser et al. 1991. 4 Windley et al. 1996. 5 Whitehouse et al. 1998. 6 Ba-Bttat 1991. 7 Sassi et al. 1993. 8 Kro¨ner and Sassi 1996. 9 Greenwood and Bleackley 1967. M .J . Whitehouse et al . Precambrian Research 105 2001 357 – 369 Table 2 Pb- and Nd-isotopic data from the Precambrian of Yemen a 207 Pb 204 Pb Locality 206 Pb 204 Pb 208 Pb 204 Pb Lithology a Sm ppm Nd ppm 147 Sm 144 Nd 143 Nd 144 Nd t DM Ga Sample Abas terrane 38.503 granite Y92-51 15.543 17.465 44°59E 14°22N 38.687 mig-gn 45°1.7E 14°21.0N 18.198 15.661 Y92-71 Y92-73 mig-gn ‘‘ 18.137 15.649 38.724 38.814 18.445 15.678 Y92-74 mig-gn ‘‘ 38.637 mig-gn ‘‘ 18.029 15.660 Y92-75 Y92-76 mig-gn ‘‘ 18.337 15.670 39.206 38.871 mig-gn Y92-77 15.665 18.313 ‘‘ 38.825 mig-gn ‘‘ 18.026 15.673 Y92-78 40.151 granite 45°1.7E 14°20N 18.027 15.676 Y92-91 41.046 gr-gn Y92-12A1 15.632 18.188 45°1.7E 14°16N 40.836 gr-gn ‘‘ 18.227 15.668 Y92-12A3 Y92-12A6 gr-gn ‘‘ 18.068 15.772 40.530 39.711 gr-gn Y92-12B1 15.734 18.051 ‘‘ 40.197 gr-gn ‘‘ 18.093 15.775 Y92-12B2 40.016 gr-gn ‘‘ 17.979 15.612 Y92-12B3 39.167 18.001 15.745 Y92-12B4 gr-gn ‘‘ 39.879 gr-gn ‘‘ 18.031 15.700 Y92-12B5 40.483 gr-gn ‘‘ 18.447 15.741 Y92-12B6 39.095 Y92-151 15.690 17.930 45°20.7E 14°17.7N gr-gn Y92-152 gr-gn ‘‘ 17.761 15.702 39.149 38.709 17.802 16.202 Y92-155 aug-gn ‘‘ 38 . 331 gr-gn 6.014 31.68 0.1147 0.511793 20 2.01 45°05.1E 14°21.1N 17 . 621 15 . 614 MJG76-19B 4.087 21.77 0.1135 0.512108 8 1.51 MJG76-24 gr-dyke 45°10.9E 14°19.5N 5.661 34.26 0.0999 0.511725 11 1.84 38 . 058 gr-gn MJG76-25 15 . 606 . 414 45°13.1E 14°18.1N 37 . 842 granite 15.99 73.08 0.1323 0.51263 5 1.38 45°21.8E 14°15.2N 17 . 485 15 . 561 MJG76-29A 7.582 37.70 0.1216 0.512348 7 1.25 MJG76-32A granite 45°23.7E 14°10.5N 17 . 596 15 . 552 37 . 756 3.710 19.97 0.1182 0.512107 10 1.59 MJG76-10 metased 44°54.9E 14°24.6N metased 2.227 10.67 0.1261 0.511224 17 3.29 45°10.9E 14°19.5N MJG76-23 2.155 10.35 0.1258 0.511271 9 3.19 MJG76-23 rpt metased ‘‘ 2.831 17.68 0.0968 0.512305 6 10.3 metased MJG76-31 45°22.5E 14°12.6N 38.262 gneiss 0.511944 10 44°40.0E 15°11.4N 17.549 15.599 F25 15.547 37.933 0.512073 10 gneiss F27 44°41.3E 15°12.0N 17.322 Al-Bayda island arc 45°27’E 14°07’N 17.251 15.498 gabbro Y92-191 37.153 15.637 40.678 45°43’E 14°03’N Y92-341 18.702 granite 15.642 38.851 45°’44’E 14°04’N Y92-351 18.017 granite 40.065 Y92-371 15.706 18.297 45°’42’E 13°57’N granite Y92-391 granite 45°48’E 13°54’N 19.184 15.665 41.893 41.518 19.186 15.674 BY-63 dyke-host 45°47.2’E 13°53.7’N 37.870 gabbro 45°46.0’E 13°52.0’N 17.320 15.427 BY-111 37.599 diorite 45°46.4E 13°52.0N 17.914 15.605 BY-113 39.102 BY-115 qtz-diorite 15.561 17.635 45°47.1E 13°52.0N M .J . Whitehouse et al . Precambrian Research 105 2001 357 – 369 361 Table 2 Continued 207 Pb 204 Pb Locality 206 Pb 204 Pb 208 Pb 204 Pb Lithology a Sm ppm Nd ppm 147 Sm 144 Nd 143 Nd 144 Nd t DM Ga Sample BY-117 qtz-diorite 45°47.4E 13°52.0N 17.944 15.589 38.860 1.196 6.521 0.1109 0.512414 9 1.02 37 . 219 gr-gn MJG76-34 15 . 487 17 . 350 45°’26.8E 14°06.5N 37 . 109 gr-dyke 4.533 29.86 0.0917 0.511622 7 1.85 45°39.7E 13°59.5N 16 . 871 . 451 MJG76-36A 3.768 15.75 0.1446 0.511971 8 2.51 MJG76-42A diorite 45°45.8E 14°03.7N 4.042 19.61 0.1246 0.511433 17 2.87 37 . 357 . 115 . 498 MJG76-43 diorite 45°48.2E 14°04.8N 37 . 239 aug-gn 0.843 4.237 0.1202 0.512078 8 1.66 45°34.0E 14°01.2N 16 . 928 . 453 MJG76-52C gr-gn 5.036 23.53 0.1293 0.512081 17 1.84 45°32.3E 14°02.9N MJG76-53A 2.531 13.30 0.1150 0.511783 5 2.04 37 . 695 MJG76-53C 15 . 457 17 . 102 ‘‘ gr-gn 6.487 30.89 0.1270 0.511961 6 2.00 MJG76-49B 45°32.3E 13°57.7N metased 4.762 16.66 0.1728 0.512789 9 1.12 metased 45°27.5E 14°05.5N MJG76-57A Al-Mahfid terrane 15.848 42.961 46°55.1E 14°3.0N Y92-471 21.472 gr-gn 15.730 41.341 46°54.4E 14°2.7N Y92-481 19.449 gr-gn 42.371 Y92-482 17.246 27.793 ‘‘ gr-gn Y92-483 gr-gn ‘‘ 21.497 15.859 42.659 42.094 24.706 16.975 Y92-491 gr-gn 46°46E 14°02N 40.392 gr-gn 46°51.6E 14°3.0N 18.842 15.921 Y92-501 42.015 gr-gn 46°51.0E 14°3.0N 21.496 16.395 Y92-511 45.242 Y92-521 18.404 37.599 46°45E 14°02N leucogn 43.239 gr-gn 46°0.6E 13°53.7N 18.432 15.655 BY-15E 41.501 19.317 15.705 BY-21 gr-gn 45°57.8E 13°50.3N 44.368 gr-gn 46°2.7E 13°51.9N 23.096 16.645 BY-30A BY-30B gr-gn 46°2.5E 13°51.5N 28.362 17.362 39.707 38.640 gr-gn BY-36B 16.059 19.893 45°59.7E 13°50.1N 38.711 gr-qn 45°47.4E 13°42N 17.654 15.569 BY-92 40.829 granite 45°59.4E 13°53.1N 19.400 15.743 BY-18A 41.149 19.459 15.748 BY-18B granite 45°59.4E 13°53.1N 40.452 granite 45°54.5E 13°49.8N 19.458 15.703 BY-19A 40.632 granite 46°2.8E 13°51.8N 19.026 15.745 BY-30C 43.433 BY-36C 15.753 18.934 45°59.7E 13°50.1N granite 39.853 granite 45°45.8E 13°42.1N 17.642 15-602 BY-91 39.402 granite BY-93 15.579 17.689 45°48E 13°42.6N 41.365 granite 46°0.2E 13°53.6N 18.229 15.636 BY-106 BY-16K2 amphibolite 45°54.1E 13°46.5N 18.198 15.617 37.968 37.918 17.859 BY-24A amphibolite 45°55.7E 13°48.4N 15.624 West Yemen F4 amphibolite 0.512860 10 43°25.0E 15°1.0N 18.921 15-559 37.558 0.513180 10 37.572 19.127 15-586 F6 granite ‘‘ 38.378 amphibolite 0.512755 10 43°23.3E 14°58.0N 19.172 15.603 F22 16.658 39.011 0.512721 10 ‘‘ F23 19.927 gneiss a Abbreviations in lithology column: mig-gn, migmatitic gneiss; gr-gn, granitic gneiss; gr-dyke, granitic dyke; leucogn, leucogneiss; aug-gn, augen gneiss; metased, metasediment. Standard ion exchange separation techniques for Pb were followed. Samples were analysed on a VG 54E mass spectrometer in Oxford Y92 series or a Finnigan MAT262 mass spectrometer at USGS, Menlo Park MJG76 series. Ratios in italics indicate measurements on separated feldspars. Pb-isotopic ratios are corrected for mass fractionation of −0.15 per atomic mass unit derived from replicate measurements of NBS 981 standard. Overall analytical error is c 9 0.1 2s. SmNd data MJG76 series were obtained at USGS, Menlo Park using analytical techniques described by Whitehouse et al. 1992 Nd-isotope ratio reproducibility9 0.00001, 2s. Depleted mantle model ages t DM assume the model of DePaolo et al. 1991. F-series data kindly made available by J.E. Baker, University of London Pb normalisation and errors as Oxford data; Nd-isotope ratio reproducibility 9 0.00001, 2s; only isotope ratio is available for these analyses. African age c 760 Ma intrusives andor gneisses in the Al-Mahfid and Abas terranes show a range of model ages, which indicate the involvement of older crust in their genesis and, for the Abas terrane, the observation of model ages as old as c 2.3 Ga, together with the documentation of a 2.6 Ga core in a 760-Ma zircon Whitehouse et al., 1998 suggests the possibility that this crust might be Archean. Samples from the Al Bayda arc terrane exhibit a wide range of SmNd model ages, not only in granitoids, which might be ex- pected to sample underlying gneissic basement to the arc, but also in more mafic samples e.g. diorite MJW76-43 which has a t DM of 2.87 Ga. The Afif terrane data are quite different from any of the Yemen terranes for which isotopic data exist, both in crystallisation age since no mid- Proterozoic ages have yet been recorded from Yemen and in depleted mantle model age charac- teristics. It should be stressed, however, that sam- pling in both regions has not, to date, been extensive. 2 . 2 . 2 . Pb-isotopic data In Table 2 and Fig. 3, we present whole-rock Pb-isotopic data from a variety of lithologies within the Abas and Al-Mahfid gneiss terranes and the Al-Bayda island arc terrane, as well as feldspar analyses for the Grolier et al. 1977 samples from the Abas and Al Bayda terranes. Additionally, we present data for four samples from western Yemen together with two Abas Fig. 1. Summary geologic and tectonic map of Yemen modified after Windley et al. 1996 see this reference for data sources. Ranges of t DM model ages presented by Windley et al. 1996 are annotated for the Abas, Al-Bayda and Al-Mahfid terranes in Yemen. Asterix symbol main map, above inset B shows the position of the F-series samples from west Yemen. Inset A shows a simplified map of terranes and boundaries in Yemen in relation to those in eastern Saudi Arabia. Inset B shows relations with comparable terranes in northern Somalia; Abdulkadir A, Maydh M, Qabri BaharMora QBM and Inda Ad IA. See Table 1 for geological descriptions and geochronology. Fig. 2. Histogram of depleted mantle model ages t DM ; De- Paolo et al., 1991 for granitoids, granitic gneisses and mafic rocks M from the Abas, Al-Bayda and Al-Mahfid terranes of Yemen. Data from this study and Windley et al. 1996. Data from pre-750 Ma basement rocks from the Afif terrane of Saudi Arabia are shown for comparison Agar et al., 1992; P-paragneiss. Numbers for some samples refer to UPb zir- con crystallisation ages in Ga; Whitehouse et al., 1998. subsequently, it remains possible to make mean- ingful first order comparisons for the uranogenic Pb system 235 U and 238 U decaying, respectively, to 207 Pb and 206 Pb because initial compositions are constrained to lie along lines whose slopes are controlled only by age i.e. isochrons. For the Abas terrane, Whitehouse et al. 1998 have re- ported c 760 Ma UPb zircon ages from two typical granitic gneisses. Projecting the Abas com- positions back along 760 Ma isochrons indicates a slightly larger range of 207 Pb 204 Pb ratios for com- parable 206 Pb 204 Pb ratios to the Afif terrane of Saudi Arabia : 800 – 600 Ma Stacey and Kramers 1975 model 206 Pb 204 Pb; Group III and filled symbols in Fig. 3a, most likely due to a greater range of pre-760 Ma UPb ratios andor different protolith ages. In addition, there are no 760 Ma projected initial compositions or feldspar analyses from the Abas samples of the Grolier et al. 1977 collection that are less radiogenic than Afif Group III, indicating the dominant continen- tal character of these gneisses compared with the juvenile arc terranes of the Saudi Arabian shield Groups I and II, Fig. 3a. A similar inference may be made about the Al-Mahfid granitic gneisses which have been dated at c 760 Ma Whitehouse et al., 1998; Fig. 2a, although it is clear from the inset diagram that the highly radio- genic whole-rock compositions of many of the Al-Mahfid samples project to much higher 207 Pb 204 Pb ratios for 600 – 800 Ma Stacey and Kramers 1975 model 206 Pb 204 Pb ratios than either the Abas or Afif samples. A late-Archean igneous crystallisation age with a later episode of Pan- African Pb-loss has been described in the zircons from two of the Al-Mahfid granitic gneisses Whitehouse et al., 1998 and the highly radio- genic compositions require high UPb ratios both before and after Pan-African age events. The Al-Bayda granitoid lithologies show similar be- haviour to the Abas granitic gneisses, although the Al-Bayda gabbros plot close to the Group I and II arc rocks of Saudi Arabia. An unusual feature of the feldspar Pb-isotope data from the Al Bayda arc terrane is that these display highly unradiogenic compositions, plotting below the Stacey and Kramers 1975 terrestrial Pb isotope growth curve and to the left of the fields defined analyses, these have been kindly made available to us by J.E. Baker. These are shown in relation to the fields for feldspar Pb-isotopic compositions of Pan-African rocks in Saudi Arabia — groups I and II represent arc rocks, respectively, west and east of the Nabitah suture zone Stoeser and Stacey, 1988 and Group III represents 750 Ma and younger intrusives in the Afif terrane Agar et al., 1992; pre-750 Ma pre-Siham arc granitoids and metasediments Agar et al., 1992 are shown as individual data points. The feldspar Pb-isotopic data from Saudi Arabia are considered by Agar et al. 1992 to reflect compositions in the interval c 850 – 650 Ma, representing shield-wide tectonic andor metamorphic events. Although it is not ideal to compare feldspar Pb-isotopic data, which essentially represent initial isotopic compositions at the time of last igneous or metamorphic crys- tallisation, with whole-rock data, which include a component of radiogenic Pb developed in situ by feldspars from arc rocks of Saudi Arabia Groups I and II, Fig. 3a. Furthermore, as dis- cussed above, these rocks have unusually high SmNd model ages for juvenile Pan-African arc rocks. These features of the data may be ex- plained if juvenile Al Bayda arc rocks ? c 750 Ma, Whitehouse et al., 1998 assimilated substan- tial amounts of ancient crustal material or if the arc itself contains accreted fragments of older continental material which should be tested by further geochronological studies. The four sam- ples from western Yemen project back into the Group II Pb field of Saudi Arabia, suggesting closer affinity with the arc terranes of Saudi Ara- bia than with either the continental Afif terrane, or the Abas and Al-Mahfid terranes of Yemen. Interpretation of whole-rock data for the com- bined uranogenic – thorogenic Pb-isotope system- Fig. 3. Pb-isotopic data from Precambrian terranes in Saudi Arabia Stoeser and Stacey, 1988; Agar et al., 1992 and Yemen this study. Inset diagrams maintain the same aspect ratio as the main diagram i.e. the slope of reference isochrons will be the same as in the main diagrams, the area of which is shaded. Reference terrestrial Pb-isotope growth curves after Stacey and Kramers 1975. Fig. 4. Correlation plots of uranogenic and thorogenic Pb-iso- tope ratios 206 Pb 204 Pb and 208 Pb 204 Pb, respectively with Nd-isotope ratios 143 Nd 144 Nd; whole-rock data only. terranes may be expected to influence the compo- sition of any recent volcanics erupted through the crust. In Fig. 4, we show whole-rock 208 Pb 204 Pb and 206 Pb 204 Pb ratios Table 2 plotted against whole-rock 143 Nd 144 Nd Table 2 and Windley et al., 1996. A number of observations may be made using these isotope correlation diagrams: 1 only the Al-Mahfid granitic gneisses extend to 143 Nd 144 Nd ratios B c 0.5115 o Nd 0 B − 22; 2 while there is clear overlap in Nd-isotopic compositions \ 0.5115 between Abas granitoids and gneisses, Al-Mahfid granitic gneisses and Al- Bayda plutonic rocks, the Abas compositions are in general characterised by low and restricted Pb-isotopic signatures; 3 basement rocks of western Yemen have PbNd signatures that are highly distinctive from those of the Abas, Al- Bayda and Al-Mahfid terranes.

3. Correlations with neighbouring regions