Correlations with neighbouring regions

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

Whitehouse et al. 1998 discuss possible ter- rane correlations between the Precambrian of Ye- men and neighbouring regions of Saudi Arabia and Somalia. For the purposes of this review, we reproduce and embellish this discussion here, to- gether with new insights provided by the Pb-iso- tope data. Table 1 modified after Whitehouse et al., 1998 summarises available geological, iso- topic and geochronological data. 3 . 1 . Saudi Arabia Windley et al. 1996 correlated terranes in northwest Yemen and Saudi Arabia on the basis of descriptions of the observed geology together with reasonable geometric constraints since there are, to date, no geochronologic andor isotopic data from northwest Yemen. The two Precam- brian basement terranes we recognise in northwest Yemen may be correlated with the Asir andor Afif terranes of Saudi Arabia Stoeser and Camp, 1985; Johnson, 1998 by direct extrapolation of the boundary between them represented by the 680 – 640-Ma Nabitah orogenic belt Quick, 1991; atics shown in Fig. 3b is more complicated be- cause initial compositions will lie along a pro- jected slope determined by age and ThU ratio. Thus, although it is reasonable to assume a c 760 Ma age for projection, little may assumed about whole rock ThU ratios for these rocks and in fact whole-rock ThU ratios derived from mea- sured Th and U concentrations would likely be in error because of the mobility of U in near-surface weathering environments. Qualitatively, however, it is clear that some of the Al-Mahfid samples exhibit much lower post-750 Ma ThU than the Abas samples since they show a similar range of 208 Pb 204 Pb ratios for much greater corresponding values of 206 Pb 204 Pb Fig. 3b, inset. 2 . 2 . 3 . Whole-rock PbNd isotope correlations In addition to facilitating terrane correlations, the isotopic compositions of the Yemen basement Fig. 1. This correlation assumes that there are no hidden sinistral strike-slip faults belonging to the Najd system between c 18°N where the Nabitah suture is obscured by the Wadi Tarib batholith and the Yemen – Saudi Arabia border at c 17.5°N see also discussion by Johnson and Stewart, 1996 and Whitehouse et al., 1996. North-northeast of Sada, the inferred continuation of the Nabitah belt is marked by a 40 km long series of ophiolite complexes together with island-arc type andesites, rhyolites, pillow basalts, and silicic to intermedi- ate tuffs Michel et al., 1989. The belt extends south-southwest to Hajjah where an extensive ophiolite Windley et al., 1996, comprising de- formed serpentinites, gabbros and deformed sheeted basic dykes, is cut by undeformed basic dykes. The suture zone is stitched by post-tectonic granites. To the west of the Nabitah orogenic belt, the Asir terrane consists of alternating belts of green- schist-grade volcanic and sedimentary rocks and high-grade gneisses. In southern Saudi Arabia, the eastern part of the Asir terrane occurring within the Nabitah orogenic belt has undergone exten- sive plutonism and high-grade metamorphism. The terrane to the east of the Nabitah orogenic belt in Yemen, which we tentatively correlate with the Afif terrane of Saudi Arabia cf. Johnson, 1998, who correlates it with the Asir, comprises monotonous orthogneisses of unknown age inter- calated with arc-type pillow basalts, andesites and rhyolites, the gneisses and lavas being intruded by undated post-tectonic, granitic to gabbroic plu- tons Michel et al., 1989. The arc has a continen- tal basement and is tentatively interpreted as a Pan-African Andean-type continental margin. At present, there are no isotopic data available from the Asir and Afif terranes as we interpret them in Yemen. If our correlation is reasonable then the Pb-isotopic compositions for the western Asir terrane should overlap with Group I of Saudi Arabia, with more radiogenic compositions Groups II and III for the eastern ?Afif terrane. The occurrence of rocks with Group II Pb-iso- topic signature c 70 km south of Hajjah asterix symbol in Fig. 1 suggests that the hidden south- ward extension of the Nabitah suture passes to the west of this point. 3 . 2 . Somalia In Table 1, we summarise available geological and isotopic data from terranes in Yemen and Somalia, although the type of isotopic data is not readily comparable. There are no Nd model ages t DM from terranes in Somalia to compare with the Yemen data presented by Windley et al. 1996 and conversely, the large amount of UPb geochronology available for northern Somalia Kro¨ner and Sassi, 1996 can be compared with only a handful of UPb zircon dates currently available from the Al-Mahfid terrane of Yemen Whitehouse et al., 1998. As a result, most of the correlations are based upon geological and geo- metric Gulf of Aden fracture zone patterns con- straints. Where geochronological data can be compared, this is done to emphasise the probable validity of the correlation. Four terranes in Yemen correlate well geologi- cally with four similar terranes in northern Soma- lia Sassi et al., 1993; Lenoir et al. 1994; Kro¨ner and Sassi, 1996 as follows Table 1: The Al- Bayda and Al-Mukalla arcs are equivalent, re- spectively, to the juvenile volcanic-dominated Abdulkadir and Maydh Mait terranes in Soma- lia. The terranes in Somalia, however, are much narrower than those in Yemen suggesting south- ward discontinuation or destruction of the arcs between the bordering accreted gneiss terranes. Thus, further south in Somalia only gneissic ter- ranes are present, and we suggest that this is the manner in which the arc-gneiss terranes of Yemen pass along strike into the gneissic Mozambique belt of Ethiopia and East Africa. The intervening Al-Mahfid gneissic terrane is similar in lithologies, long crustal history and predominant east-west strike to the Qabri Bahar – Mora terrane com- plex in Somalia. Within the Al-Mukalla arc, folded and meta- morphosed volcanic rocks are unconformably overlain by the sedimentary Ghabar Group which is unmetamorphosed or virtually unmetamor- phosed, although folded and cleaved Beydoun, 1966. The basal sedimentary rocks are tuffs and conglomeratic sandstones, which contain pebbles of granite and volcanic rocks, derived by erosion of the arc, and these are overlain by limestones, dolomites, calcareous sandstones, quartzites, silt- stones and shales. Beydoun 1966 suggested the Ghabar Group is equivalent to the Inda Ad Group of northern Somalia. The Inda Ad Group consists of low to very low grade clastic sedimen- tary rocks and marbles Sassi et al., 1993; Kro¨ner and Sassi, 1996. Abbate et al. 1981 suggested that the elastic rocks were derived by erosion of a volcanic arc. Both the Ghabar and Inda Ad Groups are intruded by post-tectonic granites, and both have been assigned to the uppermost Proterozoic to Lower Cambrian. About 250 km east of Al-Mukalla town, there are the remains of another possible island arc which, considering the distance, is probably dis- tinct from the Al-Mukalla arc, although the inter- vening ground is covered by younger sedimentary rocks. This can be termed the Tha’lab arc after the Tha’lab Group of Beydoun 1966. The late-Archean crust forming, part of the Al-Mahfid terrane of Yemen t DM model ages of c 3.0 – 2.7 Ga, Windley et al., 1996, confirmed by UPb SHRIMP analyses, Whitehouse et al., 1998 has not to date been recorded in the Qabri Bahar- Mora complexes terrane in northern Somalia. The 1820 – 1400-Ma zircon xenocrystic ages from granitoids within these terranes in Somalia remain undetected in the Al-Mahfid terrane although, like the absence of late-Archean crust in Somalia, this might represent an artefact of the presently limited sampling in both terranes. Evidence of the earliest Pan-African event of Somalia 840 Ma, Kro¨ner and Sassi, 1996 has not been found in the Al-Mahfid terrane of Ye- men. In the latter, the earliest Pan-African age so far recorded is 760 Ma in intrusive granitoids Whitehouse et al., 1998, although involvement of older crust is indicated by c 1.3 – 2.2 Ga t DM model ages Windley et al., 1996. A 814 – 778 Ma gabbro-syenite belt is promi- nent in the Qabri Bahar – Mora complexes ter- rane. Comparable gabbro bodies up to 6 km across occur in the Al-Mahfid terrane and some are discordant to the host gneisses Isakin and Degtyariov, 1990, like those in Somalia, but they have not yet been isotopically dated. We agree with Kro¨ner and Sassi 1996 that these gabbros could be related to subduction – accretion pro- cesses further north. This event may relate to an early stage of the collision of the Abas and Al- Mahfid terranes which involved accretion of the intervening 760 Ma Al-Bayda oceanic arc. South- eastward dipping subduction responsible for the accretion of this arc might have emplaced sub- duction-derived? gabbro-syenite complexes into the deep crust of the gneissic terrane to the east and heat input from the underlying subduction zone could have caused partial melting of that gneissic terrane with resultant formation of crustal melt granites. Zircons from a granodioritic gneiss in the Qabri Bahar complex have a 207 Pb 206 Pb age of c 760 Ma Kro¨ner and Sassi, 1996, which these authors associated with migmatisa- tion and anatexis caused by heating of the crust by the gabbro-syenite bodies. This event could be related to the emplacement of the crustal-melt dominated granitoid sheets at 760 Ma in the Al-Mahfid terrane.

4. Summary