Volcanic islands of the Red Sea IAN G. GASS, DONALD I. J. MALLICK & KEITH G. COX CONTENTS i Description of the islands and petrography of the rocks . 277 (A) Jebel at Tair 277 (B) The Zubair Group 280 (e) The Hanish-Zukur Group . 285 2 Petrology 295 (A) Comparative petrology and geochemistry of the basaltic rocks °95 (B) The differentiated rocks 297 (c) The parental and primary magmas • 098 3 General discussion • 3o2 4 References 3o6 SUMMARY The Recent volcanic islands of the Red Sea mediate stages. Two alternative petrogenetic are (i) Jebel at Tair, a single small volcano of models are discussed to account for this grada- tholeiitic basalt lava; (2) the Zubair Islands tional behaviour. One derives the parental with pyroclastic cones and flows intermediate magma from successively greater depths, the between tholeiite and alkali basalt and with other considers derivation by successively picrite basalt and trachybasalt blocks in the greater fractionation on route to the surface. agglomerates; (3) the Hanish-Zukur Islands The relationship of the volcanoes to the open- with alkali basalts accompanied by trachy- ing of the Red Sea is discussed. Possibly, erup- basalts, trachyandesites and trachytes to- tive activity was initiated at the southern end gether with pyroclastic rocks. The chemistry and is migrating northwards in response to of 46 lava specimens indicates that a grada- the anticlockwise rotation of Arabia relative to tional series exists between the sea-floor Africa. The Red Sea axial trough may die out basalts (K-poor tholeiites) and the alkali southwards owing to vocanic fill from the basalts of Hanish-Zukur, with the rocks of Hanish-Zukur volcanoes. Jebel at Tair and Zubair representing inter- EXCEPT for those lying close to the Ethiopian coast, islands formed by Recent volcanism occur only in the southern central part of the Red Sea where there are three separate islands or island groups. From north to south these are Jebel at Tair, the Zubair group and the Hanish-Zukur group (Fig. I). Jebel at Tair is a single, near circular island occupying I o km s and formed of a thin carapace of tholeiitic lava flows that have issued from a central vent and overlie basaltic agglomerates. Some 3 ° km to the SSE of Jebel at Tair is the Zubair group, consisting of I o islands together with numerous rocks and shoals occupying 26 km 2. The smaller islands of this group consist mainly of yellow basaltic volcanic agglomerate whereas the three larger islands have extensive flows of olivine- phyric, plagioclase-phyric and aphyric basalts intermediate in composition Jl geol. So¢. Lond. vol. z29, x973, pp. 275-3IO, t I figs. Printed in Northern Ireland. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/3/275/4884789/gsjgs.129.3.0275.pdf by guest on 01 October 2021 276 L G. Gass, D. L J. Mallick & K. G. Cox 390E / (~ ~) lool 42°E/ 18° N~'~ Massawao,( ~ ~ ~~°Farisan Is ) ~ Jebel at ~.a~a ~an \ zub, ~ ARABIA t]..Gre.atc~.~ittle ( 'S°N ] Mamsnor. Hanish) / 45°E Bathymetric contours shown ~illi at 100 fathom intervals :~-!i~: 0 Perim ~E 12°N ~ /45 °E A B --- - 20°1 c D 4001 6001 800 J Fathoms E ~ F F zo. I. General map of the southern part of the Red Sea. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/3/275/4884789/gsjgs.129.3.0275.pdf by guest on 01 October 2021 Volcanic islands of the Red Sea 277 between tholeiite and alkali basalt. The Hanish-Zukur islands lie 85 km to the ssE of Zubair, and are considerably larger; Jebel Zukur itself is I2o km t, Great Hanish 72 km 2 and Little Hanish 8 km 2. Numerous volcanic vents occur on the major island where pyroclastic debris and lavas seem to be about equally abun- dant. The rocks present in this group are mainly alkali basalts although types ranging through trachybasalt and trachyandesite to trachyte have been identified on Jebel Zukur. The salient structural feature of the Red Sea is its median trough which, although well developed in the central section, narrows and becomes shallower towards the south. Jebel at Tair lies within the well developed central section, the Zubair group within the southern restricted extremity where the trough becomes shallower, and the Hanish-Zukur group is to the south of the trough, entirely surrounded by shallow water (Fig. I). The relationship of the islands to the median trough is emphasized by the alignment of the volcanic vents and structures, which, on Jebel at Tair bracket the trend of the median trough at this latitude, while volcanic vents of the Zubair group lie on lines parallel to the trough margins. However, the vents of the Hanish-Zukur group lie along northeasterly lines, a direction apparently unrelated either to the median trough or to structures on either side of the Red Sea in Ethiopia and Saudi Arabia. Several other islands in the Red Sea are formed of volcanic rocks. Perim (Fig. I), at the southern entrance to the Red Sea, is an erosional remnant of the western flank of a Mio-Pliocene volcano which itself is the westernmost of six large central vent volcanoes that lie along the coast of Arabia between Aden and the Red Sea (Gass et al. 1965). The volcanic islands near to the Ethiopian main- land are most closely related in space and composition to the Recent basic vol- canics of the north--central Afar depression (Fig. I) (Barberi et al. 197o ). Further north in the Red Sea, St. John's island and The Brothers, although partly of volcanic or sub-volcanic rocks, are fragments of crystalline basement (Moon 1923) detached from either the African or the Arabian mainland by faulting during the formation of the Red Sea depression. Published data on the Red Sea volcanic islands described herein seem to be restricted to descriptive notes by MacFadyen (I932) and Lamare (193o). x. Description of the islands and petrography of the rocks (A) JEBEL AT TAIR (i) Field observations. Jebel at Tair (Fig. 2), is the uppermost part of a roughly conical volcanic edifice rising from the centre of the median trough of the Red Sea about x2oo m below sea level (Fig. I). It is the only volcano in the Red Sea that appears to be active, though the present activity is only fumarolic and there is no unequivocal historic record of eruption. The highest point (244 m) is the central vent of the volcano. There is clear evidence of two periods of eruptive activity separated by a dormant phase during which sea cliffs were cut by wave action; as a result, the island has a distinctive profile (Fig. 2). An almost flat apron of very recent lava forms a coastal plain around most of the island. The Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/3/275/4884789/gsjgs.129.3.0275.pdf by guest on 01 October 2021 278 L G. Gass, D. L J. Mallick & K. G. Cox form of the old sea cliffs, which terminate the coastal plain inland, has been modified by a veneer of younger lavas flowing from a central vent. For the most 41"49E Jebel at Tair t ~N~ I "15"33'N fumarolic cinder cone ~ Latest, brown Newer lavas, Open fissure cinder cones post-date cliffs ~ Old, brown ~ Older lavas, pre-date Geological boundary cinder cones last cliff cutting '~.r~\~ Old sea cliff ~ Oldest, yellow cinder cones • Landing / Dip fi Lighthouse =(" Direction of flow 0 ! 2 Km .......... Road I, i ~ l I I Fxo. ~. Geological sketch map of Jebel at Tair based on field studies in the NW quadrant and aerial photograph interpretation. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/3/275/4884789/gsjgs.129.3.0275.pdf by guest on 01 October 2021 Volcanic islands of the Red Sea 279 part, the cliff is completely mantled by the younger flows and this is particularly so in the northeast quadrant. In the southeast, however, where a large cinder cone formed a topographic feature too high to be covered by the younger lavas, the erosional sea clifl~ are exposed. Above the cliff the ground rises gently with an average slope of about 7° to form a low dome with a slightly convex profile. This dome is crowned by two prominent cinder cones the larger of which is the location of the present fumarolic activity. Pyroclastic cones. It is possible to date the pyroclastic cones on the basis of their colour for the oldest material weathers to a pale yellow shade, cones of intermediate age are greyish to brown, whereas the younger cones are composed of brown scoria often with a red tinge. The oldest pyroclastic material, which appears to pre-date the cliff cutting episode, is the large cone exposed in the cliffs on the sE coast. Similar yellow pyroclastics form remnants of cones at localities 0- 9 km s and 0"7 km s~ of the landing place, at a locality 0. 5 km sw of the landing place, and near the south coast, ssw of the summit. The latest phase of pyroclastic activity is represented by the two prominent and uneroded cones that form the summit and are the site of fumarolic activity. Here, steam emerges from small sulphur- and gypsum-encrusted vents in the scoria. Lavas. The majority of the island is covered by basaltic lava flows many of which appear to have originated in the summit area. Macroscopic variation is slight and no attempt has been made to identify flows of varying composition, texture or age. The individual flows are less than 2-3 m thick, and the surface structures developed are mainly of the pahoehoe type though there has been much fragmentation of ropy chilled surfaces to give a rough and blocky surface most appropriately termed broken pahoehoe (Wentworth & MacDonald I935).