Volcanic Islands of the Red Sea

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.

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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.

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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 abundant. 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 mainland 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

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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.

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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). Where flows descend the old sea cliff they sometimes break, the distal parts be- coming detached from their source area. Fissures. The surface of Jebel at Tair is cut by numerous open fissures having a radial distribution and possibly showing a very slight concentration in a NNW direction parallel to the Red Sea trend. These fissures appear to have been formed by tensional stresses created by dilation and subsidence of the volcanic superstructure before and after eruption. Some of the lavas were erupted from these fissures, a very clear case being immediately NE of the landing place where a fissure, from which basalt has issued, cuts the flank of a cinder cone. (ii) Petrography. The Jebel at Tair rocks are all tholeiitic basalts. However, as most of the pyroclastic debris from the spatter cones is highly oxidized, lavas and specimens from the larger blocks in the cones were selected for more detailed study. The lavas are mesocratic, fine grained rocks with variable vesicularity and abundance of plagioclase phenocrysts, ranging from aphyric varieties to those containing 3o per cent phenocrystic plagioclase. Microphenocrysts of colourless clinopyroxene, olivine and rare magnetite are usually set in an interstitial to intergranular ground mass of calcic plagioclase, clinopyroxene, opaque ore and glass. Plagioclase, the most abundant phenocryst mineral, is present as subhedral to euhedral crystals up to Io mm long that are commonly carlsbad and albite twinned, show oscillatory and gradational zoning, display high temperature

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optics and have cores of An85_70 with rims as sodic as Ansi. Olivine (Fa14_,0) forms occasional, small subhedral, corroded microphenocrysts, most commonly partially altered to hematite or iddingsite. Colourless, anhedral microphenocrysts of clinopyroxene, rarely forming more than 3 per cent of the mode and occurring as crystals less than 2 mm across, are present in a few specimens. Discrete microphenocrysts of magnetite are rare, small grains do however occur in some plagioclase phenocrysts. Of the 2 1 specimens examined under the microscope, four contain glass and have an interstitial groundmass whereas, in the others, the groundmass is intergranular with granules of clinopyroxene (X.R.D. methods gave Ca49Mg37Fe14 on one specimen) and rarer magnetite occurring between subhedral laths of plagioclase (AnTic0). No groundmass olivine has been identified.

B) THE ZUBAIR GROUP (i) Field data. The ten islands of this group rise from a shallow water platform elongated NNW sub-parallel to the trend of the Red Sea median trough. To the north and south of the platform the depth of water increases rapidly. To the east and west the platform is separated from the flanks of the median trough by narrow north-south trenches in which the water attains depths of up to 9oo m (Figs. i and 3). It is evident from the shape of the platform and the existence of deeper water between the more widely spaced islands, that the platform has been con- constructed by the coalescence of the volcanic products from numerous eruptive centres, the present islands being the latest manifestation of this activity. Al- though there are no signs of current activity, nor is there any record of historic eruptions, many of the volcanic cones are perfect, devoid of erosional features and emit a strong su!phurous smell. Figs. 3 and 4 show that the dominant structural trend of the Zubair group is 34 o°. Not only is the platform, on which the volcanic islands stand, elongated in this direction but the vents themselves lie about this azimuth and the median fissure on Zubair is orientated at 35 o°. The trend is so obviously near parallel to the boundary fractures of the Red Sea median trough that all these features must be related to the same regional mechanism. Once the volcanic products of the various islands had been studied, it became evident that there is a common eruptive history pattern for the group as a whole. Individual islands usually represent part of this pattern (Table i). Subaerial activity invariably commenced with an explosive phase (phase i) which produced basaltic agglomerate, now yellow-buff coloured. This first phase also included subsequent effusive episodes of basaltic, often plagioclase-phyric, lavas, as for instance on Centre Peak and Jebel Zubair. A period of erosion followed, during which most of the islands were modified to a greater or lesser extent by marine erosion. In some cases (Quoin, Rugged, Saddle, Low and Connected islands) no further activity occurred. In other islands, notably the larger ones (Haycock, Saba, Zubair and Centre Peak) a second phase of activity followed the period of quiescence. This second phase commenced with the eruption of basaltic ash, scoria and spatter followed by the effusion of aphyric, feldspar and olivine-phyric basalts.

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In the case of Zubair, much of this second phase issued from a central, north- south fissure (Fig. 4). Phase z. The basaltic agglomerate and ash that forms the early part of this phase consists primarily of angular blocks of basalt ranging in diameter from 4 to 5 ° cm set in a matrix of angular to subangular basaltic ash and lapilli. Probably this first period ofsubaerial activity was phreatic, a contention supported by the presence of blocks of coral in the volcanic agglomerate, notably on Quoin Island and Jebel Zubair. Only after a tuff or agglomerate ring had been developed to protect the eruptive centre from the sea, was the effusion of lava possible. The latter took place on several islands, notably Centre Peak where an extensive flow of plagioclase-phyric basalt (Fig. 4) issued from the same centre of activity and also forms a lava lake within the agglomerate cone. The lava was undoubtedly fluid, as typical pahoehoe and broken pahoehoe surfaces are abundant. The period of quiescence that separates the two phases of activity is reflected by the presence of erosional cliffs modifying the form of the agglomerate cones of phase I, even where these are mantled by the products of phase 2. No quantitative idea, however, can be given concerning the length of this hiatus in the volcanic activity. It is evident that it varies from island to island for the cliff surrounding the agglomerate cone at the south end of Centre Peak is only a few metres high, probably representing a period of a few decades. In contrast, Saba island stands on a shallow water eroded platform of phase I debris. On this platform there is a perfect phase I cone at the eastern end with a phase 2 cone, equally perfect, immediately to its west (Fig. 4). It is evident that here, the extensive period of erosion that succeeded phase I was so effective that further phreatic activity was necessary before the sub-aerial products of phase 2 could be emitted. Phase 2. In contrast with phase x, virtually all the products of phase 2 are grey to black in colour, and show little sign of oxidation. The early activity in this phase produced basaltic ash, scoria and spatter, followed shortly after by extensive effusive activity. Scoria and spatter were emitted through localized vents on Centre Peak and Saba islands. On Zubair, as well as isolated cones, an elongated, north-south zone of scoria accumulation forms the spine of the island and overlies a prominent fissure. The products of the centres on the fissure coalesce and overlap and the vents themselves are elongated parallel to the fissure. Much of the central part of Zubair is mantled by a layer of grey ash a few centimetres thick which is locally very rich in plagioclase and olivine crystals. Lava flows have issued from both isolated vents and from cones on the central fissure of Zubair. Individual flows rarely exceed 3 m in thickness and have varied pahoehoe surfaces. On some flows the surface is of broken pahoehoe whereas others are scoriaceous. Older flows commonly carry local pockets of gravelly erosion products. The fissures in the Zubair group, especially those on Zubair itself, cannot be related entirely to pre-eruptive dilation and post eruptive subsidence of the volcanic superstructure; they may have been produced partly by a regional east-west tension related to the formation of the Red Sea median trough. (ii) Petrography. Of the 97 specimens collected from the Zubair group, 49 are from lava flows, 43 from lava blocks in agglomerate cones, and the remaining 5

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t 42"04 E

uoin Zubair Group ~ ~ 1oo

÷ f

15"05N

ZUBAIR

Connected (

/°° "~'~X

Centre Peak,/~ :o;

m_ Volcanic fissure

• • Volcanic vent

:0~ Shoal over vent

--so-- Bathymetric contours m fathoms ®

0 I0 Km I A i I 1 I

Centre Peak Zuba~r

200 ~ 400 t letres

F z o. 3. The Zubair Group.

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are tufts. From this collection, 55 specimens were petrographically studied and 16 of these chemically analysed. The Zubair group rocks are all basalts, variably vesicular, often with plagioclase and olivine phenocrysts but always with an aphanitic groundmass. The nature and abundance of the phenocryst minerals enables four petrographic types and abundances to be identified: picrite basalts

Saba Zubair

Centre Peak

(~ Volcanic cone with I Alluvium; beach deposits Fissure crater rim indicated

/ Dip on pyroclastics • Landing ~ Effusive "~ PHASE Tf / Direction of flow t~ Lighthouse ~ Explosive !

~ Effusive ~? PHASE I 0 I 2 Km ~ Explosive J I ~ J I [ I FIO. 4. Geological sketch maps of islands in the Zubair group.

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(x 5 %) ; olivine basalts (30 %) ; feldspar-phyric basalts (45 %) and trachybasalts (i o %). Picrite basalts contain between x5 and 2o % olivine. Olivine basalts have less than i o % phenocrystic olivine but phenocrysts of other minerals are very subordinate. Feldspar-phyric basalts contain large phenocrysts of calcic plagioclase with subordinate amounts of either olivine or olivine and clinopyroxene. Trachybasalts contain phenocrysts of calcic plagioclase with very subordinate olivine and clinopyroxene but differ from the feldspar-phyric basalts in that their groundmass plagioclase is probably andesine and they commonly have a well defined trachytic texture; they could properly be termed hawaiites. Plagioclase and olivine are the most abundant phenocrysts and occur as anhedral to subhedral crystals up to 4 mm long. The plagioclase phenocrysts are commonly carlsbad and albite twinned and show both oscillatory and progressive zoning to a more sodic rim. They display high temperature optics and most fall in the compositional range Anso_~o. Smaller phenocrysts are rare but when present are often in the labradorite range. The olivine phenocrysts are typ- ically fresh being Fax0_15 in the picrite basalts and Fa~0_25 in the olivine basalts; the most fayalitic specimen identified, Fas0 , occurs in the groundmass crystals of a trachybasalt. Clinopyroxene is rare as a phenocryst phase and when present is optically consistent with augite. Small octahedra of opaque spinel are common in olivines of the picrite basalt and phenocrysts of apatite occur in one trachybasalt. The groundmass of the Zubair specimens is nearly all fine grained and commonly contains glass. The alkaline affinities of the suite are shown by the ubiqui- tous olivine (Fas0). Fine grained plagioclase was identified as andesine in a trachybasalt and labradorite in an olivine basalt. Clinopyroxene granules are usually abundant and the ore minerals, apart from the spinel mentioned above, are confined to the groundmass in nearly all rocks. Apatite is often conspicuous as small needles in the groundmass of coarse grained specimens. If the rocks represent a fractionation sequence, then the phenocrysts show that olivine and spinel crystallized first, plagioclase second while spinel stops crystal- lizing, clinopyroxene appears third, and the ore mineral and apatite are late phases. One specimen has subophitic texture confirming the relative appearance of plagioclase and clinopyroxene whereas another of picrite basalt contains many leucocratic patches, coarser in grain than the enclosing rock. These are rich in sanidine with subordinate oligoclase or andesine and relatively rich in opaque ore. A little clinopyroxene, traces of biotite and conspicuous needles of quench apatite are present. This assemblage is distinctly syenitic in composition.

(c) HANISH-Z K R ORO P (i) Field data. Whereas Jebel at Tair and all the islands of the Zubair group were visited during this study, it was only possible to land and collect along the north coast ofJebel Zukur and at three localities on the sF. coast of Great Hanish. So, although the major structures and volcanic features depicted in Fig. 5 are confidently interpreted from aerial photographs and field observations, it is not possible to identify the composition of the rocks of all the volcanic centres.

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With the exception of fringing beach deposits and isolated inland pockets of blown sands, the islands are composed entirely of Recent volcanic rocks; the products of explosive and effusive activity being of about equal abundance. Rock types present belong to an alkalic association and range from alkali olivine basalts and basalts, through trachybasalts and trachyandesites to trachytes. Unlike either Jebel at Tair or the Zubair islands, this group is entirely surrounded by shallow waters; it lies well to the south of the Red Sea median trough and no associated submarine feature is indicated on bathymetric charts. The dominant structural lineament in the group is 4o°; this is particularly evident on Great Hanish, where not only is the island itself elongated in this direction but the jagged median ridge line is formed by overlapping spatter cones whose perfect unmodified form testifies to their extreme youth. Structures on Little and Suyul Hanish are also in a northeasterly direction and the spatter cones on Jebel Zukur lie along fissures with the same orientation. It is evident that the same regional stress pattern has affected all islands of the group, an area of some 1200 km 2. Although the Hanish-Zukur group differs from both the Zubair group and Jebel at Tair in structure and composition it is, nevertheless, strictly comparable in its volcanic history. The earliest sub-aerial rocks are yellow basaltic agglomerates similar to those of the phreatic phase I in the Zubair group and the oldest pyroclastics of Jebel at Tair. These phreatic agglomerates form the small islands of the Hanish-Zukur group such as Haycock and Shark island, but on the major islands are only to be found along the coasts (Fig. 5)- The greater part of the larger islands is formed of black or reddish black basaltic ash and spatter cones from which fluid lavas, with pahoehoe surfaces have issued. Zukur, in particular, is a composite cone constructed by repeated eruptions from various centres both effusive and explosive. Trachytic tholoids, from which thick stubby lava flows of the same composition have issued, were emplaced late in the history of Zukur. (ii) Petrography. In the time available, as wide a variety of petrographic types as possible were collected; 32 specimens on the north coast of Zukur, x6 on Abu Ail and 42 on the sF. side of Great Hanish. A wide range of basic rocks, including olivine- and feldspar-phyric as well as aphyric basalts and trachybasalts were found on Zukur and Great Hanish; on Abu All only olivine basalts were collected. On Zukur, lighter coloured trachyandesites and trachytes are present and, although trachytes seem to be absent on the other islands, two recent flows on the northwest side of Great Hanish are light grey, and have the blocky surface of the trachyandesite on Zukur. The relative abundance of the four main rock types, basalts, trachybasalts, trachyandesites and trachytes is difficult to estimate but trachytes seem to form less than x %, trachyandesites between 3 and 5 %, and basalt and trachybasalt over 95 % of the rocks present. Olivine-phyric, feldspar-phyric, aphyric basalts and trachybasalts are grouped together as alkali basalts whilst the trachyandesites (benmoreites) and trachytes are discussed separately.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/3/275/4884789/gsjgs.129.3.0275.pdf by guest on 01 October 2021 ~42"40E High Zukur- Hanish ,.~'~ Abu Ail Group

14"00 N ZUKUR

Spatter cinder and ash cones

~ OId lavas Tongue ~'/

~ Old yellow agglomerate cones (- Phase E Zubair) ~l~ L°w

...... Non-volcanic lineament

------Volcanic fissure LITTLE HANISH

• • Volcanic vent

{'o;° Shoal over vent Haycock

T Trachyte ~ '"

J Direction of flow "o.'~ '~!i!:~'

Height above sea-level "! -~ ~s4 in metres /.

.:.*'~i:.~"i;~ii i " ~Ouoin

. ~..=-..~i!iiii:::.;i~-~ilii-~: N Round

: ~;;:t ::i % Round i 0 0 ~,, Rocky "~"~:' ,~~ "' ,.,;)

o 5 ~OKm ,-, ~r'~ Suyul Hanish I ~ ~ , , I I *o~"-

F TO. 5. Geological sketch map of the Zukur-Hanish group.

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The alkali basalts are divided on their phenocryst content and the trachybasalts on the abundance of normative orthoclase. Most of the olivine basalts came from Abu Ail island and have iddingsite after subhedral, i mm, Fa3_5 olivine. The abnormally high forsterite content is probably due to oxidation processes as suggested by Sigurdsson and Brown (i97o) for the highly mag- nesian olivines of Kolbeinsey island, Iceland. Small brown chrome spinels occur with the olivine. The groundmass consists of seriate laths of An~ plagioclase with interstitial magnetite, titaniferous augite and olivine. Apatite needles are abundant. Plagioclase-phyric and aphyric basalts occur on both Zukur and Great Hanish. In both, the groundmass consists of plagioclase laths, 45-7 ° % (Anes_49), with interstitial granular titanaugite (one sample studied by X.R.D. methods gave Ca37Mg30F%~) and either granular magnetite or skeletal ilmenite. Fall_i3 olivine, or iddingsite after olivine, is generally present both as phenocrysts and in the groundmass; apatite is common. In the feldspar-phyric varieties there is usually 5-IO % plagioclase but it can form up to 30 %. Two generations of plagioclase phenocrysts are usually present; the older is sodic, and is either partly resorbed, or zoned to a more calcic rim (An84_70). The younger plagioclase phenocrysts are subhedral, carlsbad-albite twinned with strong oscillatory and normal zoning from An65 to An48; some parts, not necessarily the core, are highly calcic (An80_83). The trachybasalts have mildly zoned (Anss_~0) plagioclase phenocrysts up to 2"0 mm long with rare cores of An~0_80, small subhedral olivines (Fa25_51) form up to 2 % of the mode but clinopyroxene is generally only present as small brown subhedra of titanaugite always forming less than 1% of the mode. The groundmass is usually trachytic consisting of 65-80 % An49_~ plagioclase laths with intergranular titanaugite (Ca,4Mg~oFele; Ca86Mg38Fe,~) together with magnetite and, or, ilmenite and a little olivine. Accessory apatite is common. Sometimes volcanic glass, or its alteration products, occupies the spaces between the plagioclase laths. The trachyandesite description is based on three specimens from Zukur which have io% in total of sparse, small orientated phenocrysts of carlsbad-albite twinned, An39_40plagioclase, clinopyroxene and olivine in a trachytic groundmass. Magnetite occurs as small subhedra as does olivine (Fa4~ and Fa25). Euhedra of pale green clinopyroxene are rare. The groundmass consists mainly (80 %) of orientated plagioclase laths (An34_23), intergranular magnetite, greenish clinopyroxene (Ca46Mg40Fe14) and brown hornblende. Trachytic masses crop out locally on Zukur (Fig. 5); the specimens described are from a prominent trachytic tholoid I km inland from the northwest coast. Phenocrysts of plagioclase, An39_~0, dusted with inclusions, some of which are clinopyroxene, anorthoclase, sodic clinopyroxene and magnetite form between 5 and I O % of the mode. Anorthoclase occurs as small euhedra which display fine cross-hatch twinning and which, on their high refractive indices (RI > 1-54), are probably calcic. Small pale green subhedra of clinopyroxene are present and may be sodic ferrohedenbergite. Magnetite is small and anhedral. Plagioclase, An20, forms 8o% of the groundmass as elongate orientated laths separating

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(o o~ 18 ~ 0 ~ A1203 A 0 o A • A ,A~O o /% • • ~ o~• 16 O A A oA° q~o A % 14

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I I I I I 1 I 14 12 10 8 6 4 2 MgO FIe. 6. Variation diagram showing oxides plotted against MgO. Open circles--Jebel at Tair: triangles--Zubair group: .filled circles--Hanish-Zukur group.

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TABLE 2: Analyses and norms of lavas from Jebel at Tair

Specimen No. JT 4 JT, JTx2 JT, 4 JTxo JTt6 Rock Types B B B B B B

SiO2 49"38 49"54 50"48 50"3 ° 5 °.08 50"48 TiO s ,'98 2"25 x.28 2.4 ° 1.7 x "95 AI,Oj 15.o2 x4-64 x8.32 I5'4 o ,8"4° '7"56 Fe~O3 4"98 2"39 5"9 o ' "46 ' "77 3"34 FeO 6.81 9"92 2"43 xo-78 7"9° 7"o4 MnO o-2o o-2x o.,5 o.22 o'x7 o.,8 MgO 6"57 5"84 5"3 x 5"o9 4 .'6 4"5 CaO ,1.45 xo.82 x2-9 x 9"57 ,2.o8 , ,-x 4 Na~O 2"45 3"I9 2"51 3"54 2"98 3"29 K20 o.38 o.41 o-26 o'46 o'27 o'35 H20+ 0.46 o.5x 0.30 0-36 0"34 0"33 H~O-- 0.32 0.28 o',5 0"42 o',4 o"I9 Total IOO.OO xoo-oo Ioo.oo Ioo-oo Ioo.oo ,oo-oo

C.I.P.W. norms

q 2-88 -- 4"94 m -- ' "63 or 2"25 2.42 1.54 2"72 1.6o 2"07 ab 2o-73 26"99 21.24 29"95 25"2I 27"84 an 28"87 24"42 37"96 24" 78 36"o4 32" i 2 ne ...... di 22"29 24" t I 2o.3t x8"89 t9"78 18"94

WO ...... hy I I.I 3 7.88 3.8i xo-27 x t-o 4 8"35 ol k 5"64 -- 5"94 o'o4 il 3"76 4"27 2"43 4"56 3"24 3"7o mt 7.22 3.46 4.6t 2.t2 2"57 4-84 hm ~ k 2"72 ~ ~ H~O o'78 o'79 o'45 o'78 o'48 o'52

Trace elements (p.p.m.) Ba 2oo t6o x9 o Zn xoo 74 8o Ni 3 o 34 3 ° Cu 134 x22 x4 ° Zr ,65 ,o3 ,43 Sr 243 228 248 Rb x, 5 6 y 36 22 25 Nb 2o 7 7

Analyses by XRF, major oxides correct to 4-2 per cent; FeO, H~O+, H20-- by classical methods; totals recalculated to ,oo. Analysts: Dr D. I. J. Mallick; Mrs M. H. Kerr. Key to analysed rocks given in appendix. Rock Types: B = Basalt.

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O0 0 coc~ 0 c~ e~ .~ u') .~,.. ,.. ,.. r-..-, ~f".O 00~ ,f)'.O 0~00

~j ~J ~OOe~O000 ~o

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a'Aa~.~. 4: Analyses and G.LP.W. norms of lavas from the Hanish-Zukur group

Specimen H24" Hx8 AAx H26" HI 7 H3 H27 Ht H32 G2o H28" H 4 Rock Type B B B B B B B B B B B B

SiOa 47"53 46"83 47"31 45"65 45"74 45"83 45"51 45"32 47"36 48"37 46"I6 45"35 TiO9 o'93 2"x9 2"56 2.o2 3"93 3"92 3"52 3"9o 3"3 ° 2.82 I'54 2"79 AlaO s 15-56 I4.94 I6-22 16.i 5 i5.o8 x5.47 i5.24 i4-7I I6.32 I6.oI I6.93 I8.87 Fe~O a 2"89 5"66 3"89 3"or 4"o6 9"26 3"64 6"9o 4"64 5"51 4"4° 4"37 FeO 7"26 6-64 6.o8 11 "34 8"36 3-o2 8"84 5"86 6-89 5"o4 9 "61 5"37 MnO o"I4 o'x7 o.I8 0.26 0.20 0"20 0.22 o.21 0"20 0.22 0-23 o.15 MgO 9"94 9"28 7" 17 5"59 5"58 5" [ 4 5"o7 4"96 4"82 4"67 4"64 4"49 CaO 9"63 Io'33 Io'73 9"29 Io'72 [o'[I 8"9[ 9"67 9"3o 9"o4 9"39 I2"46 Na90 2.86 3"53 3"42 4" [ 3 3" 17 3" 8 t 4"95 5"o4 4"63 5"97 4" 13 3" 86 KaO o.66 o.62 x-o 5 1.32 I-23 1-46 1.[9 1.27 1.49 1.28 1.52 o'84 H~O+ 1.43 x-I 7 I-oi o'o5 ['I5 o'98 o'97 o'53 o'89 o'39 o'25 o'76 H~O-- 0.67 o.oo 0.38 0.26 o.oo o.oo o-oo o-oo o.oo 0-68 0.22 o.oo P~05 0"47 0"34 m I-oo 0.68 0"74 0"85 0-66 0"83 -- 0"99 0"48 Total 99"97 1o1.7o Ioo.oo Ioo.o 7 99"90 99"94 98"91 99"03 Ioo'67 Ioo-oo Ioo.ol 99"79

q ...... or 3"90 3"66 6.21 7"80 7"27 8"63 7"63 7"5 x 8.81 7"57 8"98 4"97 ab 24"20 24"78 22-6o 21.27 24"18 27-22 25"23 24"78 28"27 26"98 23"28 I8.4 ° an 27"68 23"09 25"8I 21.64 23"29 20.80 I5.86 13"77 19"34 I3.Ix 23"I7 31"69 ne -- 2"75 3"43 7"4t ] "43 2"72 9"o2 9"68 5"9 x 12"75 6-23 7"72 di I3.7o 2o.55 21.81 14.88 2o.52 I8.i8 i8-57 23"27 I7"o9 24"8I 14"o3 2I"37 WO ...... hy 4-61 ...... ol 16"74 12"53 8"25 I6"25 7-14 3"o7 8"3 ° t.1o 5"43 o-37 t2.16 2.14 il 1"77 4" [ 6 4"87 3"84 7"46 6.8o 6.68 7.41 6"27 5"36 2-92 5-3 ° mt 4" 19 8-21 5"64 4"36 5"89 -- 5"28 8"27 6"73 7"99 6"38 6"34 hm ..... 9.26 m 1-2o .... ap x.t I o.8o 2.36 x.6o 2"32 2-oi 1.56 1.96 2"34 x.x3

Ba 15 ° 165 3oo 3o5 335 4o5 26o Zn 65 60 70 85 9 ° 5 ° 65 Zr 2o 5 38o 3o5 335 33 ° 33 ° 22o Sr 400 640 650 515 535 665 720 Rb I5 35 3o 25 3 ° 35 25 Y 25 35 4o 35 3 ° 35 25 La 6o 6o 6o 8o 35 75 26 Nb 20 60 55 5 ° 45 50 30 Cr 290 380 55 85 x20 85 15 ° Co 1o 5 7 ° 2[5 I25 i 15 35 75 Cu

Analyses by XRF, major oxides correct to 4-2 %; FeO, H20+, H20-- by classical methods. Analyses with AA & G prefix recalculated to xoo. Trace elements in p.p.m. * Complete analysis by classical methods. Key to analysed rocks given in appendix. Analysts: Dr G. Hornung, Mrs M. H. Kerr and Dr D. I. J. Mallick. Rock Types: B ----- Basalt, TB = Trachybasalt, TA = Trachyandesite, T --= Trachyte.

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T A B L E 4 : (c°ntinued)

H33 HIO G 9 GI8 GI 7 GI x GI G23 G2I G27 G3IA G3tB Specimen B B TB TB TB TB TB TA TA TA T T Rock Type

45"3 ° 49"00 49"23 48.88 49"82 50"09 52"45 56.I7 56"87 61.22 6t.ox 61.9I SiOg. 2.86 2"73 3"3 x 3"o4 2"98 3"o8 2"29 1.6t x.4o i-oo o.68 o-71 TiO9 19.o 5 16.33 16-I 4 I6-i6 16-9o I6"83 I6"o9 I7"x3 I6"96 I7"Io 17"I4 I7"41 A12Oa x.88 2.2o 3.o6 2"74 I '79 I'7I 5"51 3"23 4"I3 2"o3 5"o9 5 .o6 Fe20 7"97 8.12 8.40 8.50 9.I4 9.i4 4-60 5"32 4 "I8 4 .06 I'2I 1"O5 FeO o.I 5 0.22 0-24 0.24 0"23 0.24 0-26 0.22 0.24 0.20 0.22 0"23 MaO 4.32 4.31 3.88 3.72 3.5 o 3-31 2.67 i "73 i "57 o'92 o'76 o-6o MgO I~"o7 7"77 8"9o 9"26 8"84 8"84 7"49 5"2o 4"83 3"28 3"33 2-65 CaO 4"92 4"98 4"52 5"I2 4"96 4"89 6-x I 6"33 6"47 6.6x 7-o7 7-I5 Na.~O 0.94 1.66 x.23 1.33 x.35 1.28 1.64 2.22 2"43 3"05 2"87 2"98 K~O 0.55 1.98 o.61 o.62 o'23 o.26 o.38 o'52 o'46 o.2I o'24 o'I3 H~O+ o.oo o.oo 0-48 0"39 0.26 0-33 o.5I 0-32 0"46 o"3a 0"38 o-I2 H~O--

0"49 0"89 ...... P205 Ioo.5o Ioo.19 1oo.oo xoo.oo Ioo-oo Ioo.oo xoo.oo 1oo'oo Ioo-oo Ioo-oo 1oo-oo Ioo'oo Total

...... 2"44 2"78 • 3"75 q 5"56 9"81 7"27 7"86 7"98 7"57 9"67 13"I2 44"36 I8"o3 16"96 17.6I or t 2.4 ° 33"29 31.84 26"95 29"70 3 x .44 42"63 5 t.o I 53"37 55"93 59 .82 60"49 ab 27.I3 I7.31 2o.12 I7.i 9 I9"87 20-20 x 1.64 x 1.78 xo.o6 7"99 6"56 6.62 an 15"83 4"79 3"47 8"87 6"65 5"38 4"91 x-38 0"74 ------ne 24"28 12"69 19"75 23"75 19"99 I9"75 I5"47 I 1.65 1 I'32 7"oi 4.08 3-22 di ...... 2.44 ------1.97 I-oo wo

...... 3-24 -- -- hy 5"47 9"89 5"73 4"63 7"07 6"75 -- 2"49 0"58 ------ol 5"43 5 "I8 6.28 5"77 5 .66 5"85 4"35 3 .o6 2.66 1-9° 1.29 1.35 il 2"73 3"19 4"44 3"97 2.6o 4-48 5.64 4"68 5"99 2"94 2"65 2.o8 mt

...... 3"26 3-62 hm 1.16 2"I0 ap

260 320 675 805 845 Io85 1425 Ba 55 80 xto II6 x2o xx2 xo 5 Zn 245 32o 3 ox 494 5 °6 6oo 525 Zr 66o 65o 347 517 455 327 27 ° Sr 2o 25 28 6I 57 83 74 Rb 25 35 45 5 ° 47 55 45 Y 70 70 La 35 45 67 95 Io7 II5 97 Nb 85 I2o Cr 200 i2o Co 39 15 20 I8 45 Cu

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t'q t,D

0 0 tO

cO I:I q)

to ,.Q "0(-.

°,.~o °~ .C: t~ i-- j S~ \ .-', I.-'" 0 \ \ \ \ o m ° o t~ \ i \o t- \ O I 0 0 \ i-- \ i I I ~ °~\i +'-6 0.-~ m Q \ °-- o\ \ \ o~ t..: ! I i i ~ O~ I~ T,O

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granules of greenish clinopyroxene and opaque oxides and giving a well developed trachytic texture. 2. Petrology Six specimens from Jebel at Tair, I6 from the Zubair group and 24 from the Hanish-Zukur group were chemically analysed and the results are in Tables 2 to 4 where analyses are arranged in descending order of MgO content. Brief petrographic details of analysed specimens are given in the Appendix. Variation diagrams in which oxides are plotted against MgO are given in Fig. 6. An alkali-silica diagram and an FMA diagram are given in Figs. 7 and 8. Differences in major elements between the three suites are small except for the balance of alkalis and silica. The Jebel at Tair rocks are high in silica and low in alkalis relative to those of Hanish-Zukur and are a little alkali-poor relative to Zubair.

A) COMPARATIVE PETROLOGY AND GEOCHEMISTRY OF THE BASALTIC ROCKS (i) Silica saturation. The C.I.P.W. norms given in Tables 2 to 4 show that the Jebel at Tair rocks are tholeiites in the sense of Yoder and Tilley (I962). The Zubair rocks are approximately critically saturated with respect to silica and may therefore be termed a transitional series. Such series are not uncommon (Coombs 1963) and are locally represented by the Miocene-Pliocene volcanic rocks of Aden and Little Aden (Cox et al. I969, 197o ) and Jebel Khariz (Gass & Mallick i968 ). The Hanish-Zukur rocks are distinctly undersaturated, though some of the trachytic fractionates are slightly quartz normative. The analyses of the basalts are shown in Figs. 9 to I I projected into the pseudo-quaternary system R~O3--- XOmYO--ZO,, details of which are given by O'Hara (I968a) and Jamieson (197o). In both the olivine and diopside projections the positions of the fields relative to the olivine-diopside-plagioclase join ('olivine gabbro thermal divide') should be noted. (ii) Incompatible elements. A few analyses of basalts from the floor of the Red Sea are available (Chase 1969; Schilling I969) and the Jebel at Tair basalts with low concentrations of K are somewhat similar to them. It is therefore of interest to examine in all the basalts the variation in K and other elements such as Ti, Ba, Sr, Rb, Zr, Nb and the lighter rare earths that are often termed 'incompatible' because their ionic charge and, or, radius does not allow them to enter readily into the crystal lattices of common basaltic minerals. Table 5 shows that the Jebel at Tair basalts are slightly enriched in K and notably enriched in light rare earths relative to the ocean floor basalts. Table 6 shows incompatible element concentrations for Zubair and Hanish-Zukur compared to Jebel at Tair, rather than to the ocean floor rocks, since full trace element data are not available for the latter group. It is anticipated that a similar behaviour would also be shown by P but our data for this element are incomplete. In this study we have included all the rocks with MgO contents in the range 3.5-6'o % since this gives the maximum number of rocks from each group which are closely comparable in terms of major elements.

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Table 6 shows that all the incompatible elements, with the exception of Ba, are enriched in the Zubair rocks relative to Jebel at Tair by factors ranging from I'37 to 3"45. Similarly, all the incompatibles are enriched in the Hanish-Zukur group relative to the Zubair group. The change in silica saturation between the tholeiites of the sea floor and Jebel at Tair on the one hand and the alkali basalts of Hanish-Zukur on the other is therefore accompanied by a general increase of the incompatible elements. In terms of both incompatible elements and silica saturation the Zubair group occupies an intermediate position. (iii) Phase relations of the basalts. In this discussion the rocks considered are those with more than 5.o % MgO. Compositions are shown in projection in Figs. 9 to Ii. One of the analysed basalts (JTI2) is rich in plagioclase phenocrysts and the projections (particularly from diopside) suggest that it has a bulk composition which lies inside the plagioclase stability field. Conversely, others such as Z4,

A M FIG. 8. Weight per cent AFM diagram. A = Na~O + K~O, F = FeO + Fe~O3, M = MgO. Open circles--Jebel at Tair: triangles--Zubair group: filled circles-- Hanish-Zukur group.

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Z37 and Z48 from the Zubair group and HI8 and H24 from Hanish-Zukur are rich in olivine phenocrysts and have bulk compositions which lie within the olivine field. The remaining rocks are aphyric or only sparsely porphyritic and appear to have bulk compositions which are near the cotectic curve for liquids which are in equilibrium with olivine, clinopyroxene and plagioclase. However the displacement of these points towards the R,O3 (alumina)-rich side of the cotectic curve in the olivine projection suggests that they may lie slightly displaced from the 4-phase cotectic, on the surface representing the loci of liquids equilibrating with olivine and plagioclase only. Petrographic evidence supports this view for the phenocryst assemblage olivine -4-plagioclase is common and the assemblage olivine -4- clinopyroxene has not been noted. To summarise, it is probable that all the islands are characterized by aphyric or sparsely porphyritic rocks having bulk compositions which lie on or close to the olivine -4-plagioclase -4-liquid surface in the pseudoquaternary system and are slightly displaced from the olivine + clinopyroxene + plagioclase + liquid cotectic. Accompanying these rocks are abundant plagioclase enriched types and subordinate olivine-rich types. The formation of the former appears to be due to the effective addition of plagioclase crystals to magmas like those represented by the aphyric rocks. A discussion of the origin of the olivine-rich rocks is deferred until later.

(B) THE DIFFERENTIATED ROCKS From the field and petrographic evidence it appears that only in the Hanish- Zukur group is an extensive series of differentiated rocks developed. Petrographic studies indicate that most of the basaltic rocks were erupted as liquids carrying phenocrysts of olivine and plagioclase, and the phase relations suggest that several of the analysed aphyric basalts have bulk compositions which are appropriate to equilibration with these two phases at low pressures. Phenocrysts of clinopyroxene are rare in both basalts and more differentiated types and it follows that if the production of the latter is due to high-level crystal fractionation then it must have been dominated by the fractionation of the plagioclase and olivine with, at certain stages, an oxide mineral. The oxide variation curves show a discontinuity which is broadly consistent with a change from olivine control to olivine + plagioclase + ore control at about the 5 % MgO level. There are insufficient data at present to justify a detailed study of the fractionation process in the more differentiated rocks, particularly to confirm the absence of clinopyroxene as a major fractionating phase at low pressures. However certain points are worthy of comment. Several of the analysed rocks are highly aluminous (JTIo, JTI2, JTI6, H4, H33 and Z2I). These have aberrant positions on the A1203, FeO and CaO against MgO plots which are consistent with their formation by plagioclase accumulation in liquids similar to those of the main group of the analysed basalts and trachybasalts. As they are all rich in plagioclase phenocrysts this origin seems reasonably certain. An alternative origin as liquids rich in potential plagio° clase which have subsequently undergone relatively advanced quasi-equilibrium crystallization cannot be substantiated in the apparent absence of aphyric

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TABLE 5 : Enrichment factors for incompatible elements in Jebel at Taft" relative to sea floor basalts

Rare Earth Elements Other Elements (from data of Schil- ling, I969) (i) (ii) La 5"36 Ti t-o2 x.45 Ce 4-85 K x-85 x.57 Pr 3" x6 Nd 2 "45 Sm x .83 Eu l "39

Column (i) is comparison of Jebel at Tair rocks with two sea floor basalts from Red Sea (Chase x969). Column (ii) compares with 33 mid-ocean ridge basalts (Chase x969).

alumina-rich types. It is likely that plagioclase enrichment has resulted from selec- tive fractionation of olivine rather than from addition of plagioclase to more normal liquids. Minor elements in the Hanish-Zukur differentiated rocks show a variation which is broadly similar to that in most differentiated alkali basalt suites (Table 4)- Ba, Zr, Rb, Y and Nb show progressive enrichment with falling MgO while Sr follows Ca.

TABLE 6 : Enrichment factors for incompatible elements in Zubair and Zukur- Hanish groups relative to Jebel at Tair

Ti K Ba Sr Zr Rb Nb Zubair x.37 2"78 0"74 1.55 1.96 3"00 3"45 Zukur-Hanish x"67 3"5° x.61 2.63 2.25 4 .00 4" 18 Analyses used are those with 3"5 to 6 ~ MgO.

C) THE PARENTAL AND PRIMARY MAGMAS

In discussing the genesis of the parental and primary magmas we are continu- ally conscious that we have seen only the products of the last phases of volcanic activity and the following comments are made on their evidence alone. If changes in composition have occurred during the formation of the volcanic cones, we have no data to substantiate this. One fact is however abundantly clear--the basahs of the three island groups vary in both their major and trace element chemistry; Jebel at Tair is tholeiitic, the Zubair Group transitional and the Hanish-Zukur group alkalic. It is this variation we now attempt to explain. First, it is evident that the aphyric rocks probably represent samples of the magmas which were immediately parental to the majority of the trachybasaltic

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Fio. 9. ,\--. ; Projection of analytical data for Jebel at "Fair basalts (specimens with MgO> ~'JT.~ .jT i'4--.,., oo+ ~, 5 ~0) into the system RgOs--XO--YO --ZOs, the parameters being repre- ¢ sented by A, 12, M & S respectively c"~2,/OI+Cpx+L I°l+gpx ,as (simulating AlgOs, CaO, MgO & SiOe). _ ,, v 7 ~, ' v Iv +, \'--.z (a) projection from olivine into 40 50 60 plane CS---MS--A. (b) projection from cllnopyroxene (ol into CA--S---M. Sfl/ A TV " A H--Z is field of Hanish-Zukur analyses, // Z is field of Zubair analyses. P is olivine- "/ Cpx÷Opx o~,~ plagioclase piercing point in Fig. (a). / Cpx÷PI+L / JT.12 /I_ _ /_ j.-l~'JT.41 / Cpx+Ol÷L JT141 • "

"7/ ,, "c.-'"" v v 10 20 (b)

~,,%H-Z "~, _~o --Z.48 ~ ., ~."~'-ez 7, ,~ .,~OI+PI+I. \ ~.'%_7.!,,c.. 'X ~J Z.?.-,;.-"-.~ O,m" \ c~ v v /v 'v Iv+L' \ 40 50 6O (a)

Sift/ A A A / Cpx+Opx

+PI+L C-- Fxo. 10.

Pll , ~." .~.,-,_ .' / OI-Cpx- PI OI Projection of data for Zubair 7" ' JLzi'ltll " > rocks. JT is field of Jebel at JZ.9 .#.lOee Z.4 Tair rocks. Other symbols as in "~ 9-" z.;,,; ,=48 ,z.3;, Fig. 9. CA" / \ H-Z , " ~// v 'v'"" v v v ~ I0 20

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•H, H.26 oH 3 -~o \ H 18" ...... ~: AA.1" -'-. _X ""-"-4 \

CS ~z/ Ol+Cpx+L ~01.+O~x.~MS v ~ "~' v v Iv+l- ~" 40 50 60 (ol

A A A •// Cpx+Opx

PL~ '''''~ "'~. ] / .... Ol-Cpx-Pl Fio. lx. / H.17 ~ /...^Z " " x • x Projection of data for Hanish- C~/'/' •H.26 Zukur rocks. Dotted line rep- ell.27 ~-~ resents maximum extent of V V V V V Opx + L field (O'Hara x968a ) 10 20 at elevated pressure. Other Ib) symbols as in Fig. 9.

and more evolved rocks. The equilibration of these liquids with olivine and plagioclase is likely to have been achieved by fractional crystallization at high levels within the volcanic superstructure and none of them is likely to be a primary magma. Second, in considering the genesis and evolution of the primary magmas, we have two major lines of reasoning. One relates the composition of volcanic products to the thermal environment, maintaining that variation in the thermal gradient will determine at what depth partial fusion of the Earth's peridotite mantle has occurred. In this hypothesis, a steep thermal gradient results in partial melting at a low pressure, and oversaturated basaltic magmas are considered to be the products of such a process. In regions of lower thermal gradient, partial melting will occur at greater depths, under higher pressures, and the products may be undersaturated and alkalic (Kushiro & Kuno I963; Kushiro I965; McBirney & Gass 1967). This model, relating the composition of primary magmas to the depth at which they were generated is attractive in that it presents a simple framework within which regional variations in basalt chemistry can be readily explained although the validity of some of the experimental evidence supporting the theory has been questioned (O'Hara, I968b ).

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The other major source of evidence is that of the experimental petrologists whose studies are presenting, with increasing exactitude, the pressure and temperature stability fields of the major basalt mineral phases (Yoder & Tilley i962 ; O'Hara 1965, 1968a, x968b; Green & Ringwood 1967). Their findings suggest that many basalts are in equilibrium with their major phases at low pressures and if generated at depth may have suffered extensive fractionation on the way to the surface to maintain equilibrium. The contention, based on this evidence, is that not only is the depth of magma generation important but also the rate at which the liquid moves towards the surface and the pressure regimes in which it fractionates. In considering the Red Sea basalts, we will discuss the origin of the Jebel at Tair tholeiites and the Hanish-Zubair alkalic basalts assuming that as the islands of the Zubair group are spatially and compositionally intermediate, they will also represent an intermediate stage in the magma genetic processes. The Jebel at Tair tholeiites could be produced by partial melting of the Earth's mantle at depths of less than 15 km (5 kb) ; a thermal gradient of at least 8o°C km -1 would be required for this to occur. From the exceptionally high heat flow reported by Girdler (i 97o) for the central trough of the Red Sea, such a gradient is entirely feasible. The widely accepted model of Cann (I97o) suggests that beneath the oceanic spreading axes, of which the Red Sea median trough is one (Falcon et al. I97o), the Earth's mantle is partially molten at depths of less than I o km. In this model, the Jebel at Tair primary magmas would be produced high in the mantle in a very low pressure environment. However, it can be argued that as the high heat flow is restricted to a narrow linear zone, this implies that it is caused by magmatic activity and is not the cause of it. If this is the case it is likely that the Jebel at Tair tholeiites originated at greater depths than 15 km and, as O'Hara (op. cit.) has argued for the ocean floor tholeiites and Clarke (i97o) for the Baffin Island tholeiites, they could well have been derived by olivine fractionation from partial melts formed at a depth of about I OO km. From what is known of likely phase relations at high pressures (e.g. Green & Ringwood i967; O'Hara i968a ) the following model can be proposed. Primary tholeiitic picrite magma was generated at c. I oo km and ascended rapidly to the surface, fractionating only olivine, to give rise to Jebel at Tair type eruptive products after some small amount of plagioclase crystallization at a high level. On the evidence available we are unable to decide unequivocally which of these models is the more likely. Linear magmatic activity, although it may well be responsible for the high surface heat flow, must be in response to thermal instability at greater depth and thereby implies that there is in that region an elevated thermal gradient within the upper part of the Earth's mantle. Conversely, there seems to be little doubt that the top of the low velocity layer in aseismic areas occurs at depths of c. i oo km and any upward deflection of this surface will modify the nature of the basalt liquids within this layer. The basalts of the Hanish-Zubair group are alkalic and undersaturated. On the thermal gradient model it could be suggested that their primary magmas were generated at higher pressure than those of Jebel at Tair and thereby at greater

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depth within the mantle (cf. Kushiro i965). To support this model some easy line of egress is needed to allow the magmas to pass into a near surface environment without fractionation. This is seemingly provided by the marked northeasterly lineation within the group (Fig. 5) which, it is suggested, may be the surface manifestation of a transform fault. However, it is equally plausible to propose that the Hanish-Zukur magmas originated at depths of c. I oo km, rose to the surface more slowly than those of Jebel at Tair, and, as a result, extensive fractionation of pyroxene bearing assemblages took place in the 6o-25 km depth range. Of the possibilities which exist, clinopyroxenite and wehrlite appear to be favoured over spinel wehrlite by published phase relations (O'Hara 1968a) because liquids equilibrating with the latter are too aluminous to correspond with the magmas concerned. Of the two remaining possibilities wehrlite seems more likely than clinopyroxenite because the existence of the necessary olivine + liquid reaction under the required pressure conditions is open to question (O'Hara op. cir.). High pressure fractionation was then followed by extensive fractionation in a near- surface environment which led to the production of trachyandesites and trachytes. For the basalts of the Zubair group we propose an intermediate mechanism; either generation of the magma at an intermediate depth on the thermal model or an intermediate rate of ascent somewhere between the fast ascent at Jebel at Tair and the slow rise of the Hanish-Zukur liquids. In the Zubair locality, ascent should be such that at times the magma fractionated sufficient olivine to reach equilibrium with olivine and orthopyroxene (harzburgite) or olivine, orthopyroxene and clinopyroxene (lherzolite). While in either condition, in the depth range 25-6o km, the residual liquid could pass through the olivine-gabbro thermal divide and become undersaturated with respect to silica. Subsequent fractionation of olivine alone could lead to the formation of magmas akin to slightly undersaturated rocks or saturated rocks such as Z4, Z I o, JZ 9 and JZ24. However lherzolite and harzburgite fractionation followed by olivine fractionation do not by themselves appear to be capable of producing magmas as undersaturated as the more extreme Zubair types nor any of the Hanish-Zukur types other than H24. The limitation is shown in Fig. I I a where the probable maximum extent of the orthopyroxene primary phase field into undersaturated compositions is indicated. 3" General discussion Basalt is the dominant rock type on all islands, but Jebel at Tair is markedly tholeiitic, the Zukur-Hanish group strongly alkalic, and the Zubair group occupies an intermediate position, compositionally as well as spatially, having basalts neither strongly alkalic nor tholeiitic. All appear to be equally recent. It is appropriate here to compare the volcanic history of Jebel at Tair with that of the Zubair and Zukur-Hanish groups. Despite the differences in the gross chemistry of the rocks of the various islands, sub-aerial activity started, in all cases, with the formation of ash and agglomerate cones. Evidence has been pre- sented which strongly suggests that this activity was phreatic, the reaction between erupting lava and sea water continuing until the vent was above sea level. From

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then, activity was sub-aerial and the volcanic products were mainly effusive but associated with unoxidized and unpalagonitized ash, spatter and scoria. Commonly, there was a variable time gap between the end of the phreatic and the beginning of the sub-aerial phases. However, although the pattern is common, individual cones rose above sea level at various times and an age correlation of the volcanic history from group to group or island to island is not valid. Structurally the three island groups have little in common. Jebel at Tair has a radial fissure pattern related, seemingly, to the inflation and subsidence of the volcanic superstructure before and after eruption through a central vent. The orientation of fissures and the alignment of cones in the Zubair group is parallel to the major Red Sea structure and the two are obviously structurally related. In the Zukur-Hanish Group fissures and cones are aligned in a northeasterly direction, a trend that seems unrelated either to Red Sea structures or to lineaments in Ethiopia or Arabia. As little as 7 ° million years ago a level eroded peneplane stretched from present day Africa to present day Arabia, and the Red Sea did not exist as such. Africa and Arabia were part of a single lithospheric plate extending from the Mid- Atlantic Ridge, and the ridges of the southern oceans, north and eastwards to the destructive margin marked by the Alpine-Himalayan chain. Some 4 ° million years ago some upper mantle mechanism, probably within the base of the lithospheric plate, caused extensive partial fusion within the mantle. This, in turn, resulted in the eruption of the alkali basalt lavas of the Ethiopian and Yemen Trap Series. Subsequently, the region covered by the recently erupted lavas was uplifted into a structure known as the Afro-Arabian dome. A smaller, but still regional domal structure, lies athwart the Red Sea centred on the intersection of the Red Sea with latitude 22°N (Gass I97o ). The fracturing of the brittle lithosphere by the updoming created profound fractures in the upper part of the lithospheric plate; these were the 'proto' Red Sea, Gulf of Aden and Ethiopian rifts. With these fractures giving easy egress to the magmas, repeated injections took place along these lines of weakness until, in the Gulf of Aden and in the Red Sea, the once contiguous continental plate was separated by newly generated basaltic 'oceanic' crust. In the Red Sea there was little in the way of sub-aerial igneous activity to herald the continental fracture, other than the Ethiopian and Yemen Traps already described, although minor alkali basalt lavas (25 ~ 5 my) occur within Miocene sediments of the Sudan coastal plain (Gass I969). Volcanism seems to be almost entirely confined to the submarine part of the Red Sea depression. The question is just how much of the Red Sea is floored by 'oceanic' basaltic crust? Opinions still differ despite a large amount of geophysical data available. One school maintains that only the central median trough is floored by oceanic crust (Girdler I958; Vine I966; Gass & Gibson 1969) whereas others (Wegener I929; Davies & Tramontini I97o and McKenzie et al. I97o ) support the idea that the entire marine area is so underlain. Since this bears on subsequent arguments it is as well to consider which of these hypotheses is the best founded. Bathymetric surveys show that the Red Sea floor, north of the Zukur-Hanish group, consists of shallow, marginal seas on each side of a deep median trough

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(Fig. I). Drilling indicates that the shallow seas are underlain by up to 5 km of Miocene evaporites (Frazier 197o ). The escarpments on either side of the Red Sea are primarily erosional features and no major Red Sea lineaments occur along the length of the African or Arabian coastal plains (Gass & Gibson i969). These are direct observations and not open to serious doubt. The controversy lies in the interpretation of geophysical data. The absence of any free air gravity anomaly over the area is, Girdler (op cit.) suggested, due to the fact that only the central trough is underlain by basaltic rocks. This contention was supported by the existence of large amplitude magnetic anomalies over the median trough and their virtual absence elsewhere. Vine (I966), on this evidence, suggested that separation of Arabia from Africa, along the Red Sea suture, took place some 5 million years ago. Recently, Davies and Tramontini (I 97o) showed that the 'basement' underlying the evaporites of the shallow marginal seas had a P-wave velocity of 6-6 km s-1. This they suggested was more compatible with an oceanic crust than with a continental basement. Further, they argued, their model could explain the gravity observations and the absence of magnetic anomalies. Putting this work into a plate tectonic context, McKenzie et al. (op. cit.) support the idea that the whole of the Red Sea is underlain by oceanic crust and suggest that the continental separation took place 15 million years ago and that the spreading rate would be about 5 cm yr -1 per ocean flank. We tentatively suggest however that only the central area is underlain by oceanic crust. This argument may find some support from field evidence in South Arabia where monoclinal flexuring towards the oceanic area of the Gulf of Aden is evident (Gass et al. 1965) and appears to support the contention of Cox (I 97 o) and Girdler et al. (I 969) that extensive lithospheric attenuation or 'necking' precedes rupture. Moreover the geophysical data strongly suggest that in the northern Red Sea continental separation has not taken place although a median rift is present (Vine, pers. comm.) and crustal attenuation is evident. Despite these arguments, there is little doubt that the volcanic islands herein described, arise from an oceanic floor or possibly, in the case of the Zukur-Hanish Group, from its margins. The problems posed by their structure and petrochem- istry relate directly to the processes of sea floor spreading (Vine x971). It is therefore suggested that with Jebel at Tair, the radial disposition of fissures is in response to the filling and evacuation of a magma chamber high in a volcanic pile which has a total elevation of over i km above the floor of the median trough. The structure of the Zubair group, rising as it does from shallower water, seems to have been more controlled by the orientation of the spreading axis, and this is evident in the alignment of the volcanic vents and the median fissure on Jebel Zubair which, it is suggested, overlie the spreading axis. The dominant lineation in the Hanish-Zukur group, in contrast, lies at 40°N, and this is strongly developed on all the larger islands of the group. The structure is entirely confined to the Red Sea and does not continue into the adjacent continents. It is therefore proposed that these islands lie over a transform fracture in the oceanic crust of the Red Sea which may mark the southerly termination of the oceanic crust of the median trough, or merely a deep fracture within it if the whole of the Red Sea

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floor is considered to be oceanic. It is also noted that the Hanish-Zukur 'line' is parallel to transform faults in the Gulf of Aden recorded by Laughton et al. (I97O). As this fracture afforded easy egress for alkalic magmas, it must penetrate deep into the mantle and could be an example of a 'leaky' transform fault (Menard I967). Finally, it should be noted that the three island groups discussed, together with the basalts of the sea floor, may constitute an evolutionary series with the impli- cation that magmatic activity in the Hanish-Zukur group started with the eruption of ocean-floor type basalts and then developed through Jebel at Tair type and Zubair type stages. A temporal change from tholeiitic to alkalic activity is of course well documented for the large oceanic islands of Hawaii (e.g. Green & Ringwood i967 pp. i67-9) and Reunion (Upton & Wadsworth I966 ). The implied decrease in the rate of magmatic ascent with time, which at the moment we can only invoke as a possibility, might to some extent be correlated with an increasing height of the magma column as the volcanic edifices grew, although the difference in height above sea level between Jebel at Tair (224 m) on the one hand and Zukur (624 m) and Great Hanish (422 m) on the other is not very great. The actual volume of erupted products in the three island groups is difficult to determine except for Jebel at Tair. Reference to the bathymetric map (Fig. I), however, suggests the possibility that the Zubair and Hanish-Zukur islands may rise from comparatively shallow water only because their eruptive products have partly or completely filled the central Red Sea trough. If it is assumed that the trough originally shallowed regularly from about i4oo m in the present vicinity of Jebel at Tair to 2oo m at Perim, while narrowing from 5 ° km to Io kin, the following approximate figures for the volumes of erupted products are ob- tained: Jebel at Tair IOO km3; Zubair group 300 km3; Hanish-Zukur group I25O km 3. This evidence is in harmony with the evolutionary model postulated above, the most primitive island being the smallest while the most evolved island group is very much larger. If the volcanic edifices have been built at comparable rates, and as in all groups the latest volcanic activity is recent and approximately contemporaneous, then eruptive activity at Hanish-Zukur must have started before that at Zubair, which in turn started before that at Jebel at Tair. The possibility that the initiation of eruptive centres has moved progressively northwards from the southern end of the Red Sea in the comparatively recent geological past is of some interest in view of the probable anticlockwise rotation of Arabia relative to Africa (e.g. Tarling I97o ) in the recent geological past.

ACKNO~CLEDGEMENTS. This paper is based on an expedition to the Red Sea in March-April 1964. We gratefully acknowledge financial support from the Royal Society and from the Universities of Leeds and Edinburgh. We are particularly indebted to the Commanding Officer, Officers and men of H.M.S. Owen without whose wholehearted help this work would not have been possible. Analytical services provided by Dr G. Hornung, Mrs M. H. Kerr and Mr C. R. Neary and the loan of a norm computer programme by Drs M. H. Hey, R. W. LeMaitre and B. C. M. Butler are gratefully acknowledged. Thanks are due to Dr Joan M. Rooke and the staff of the Electronic Computing Laboratory at the University of Leeds for translating the programmes and processing some of the data and to Dr J. G. Holland for processing the remainder. We are also indebted to Professor W. Q. Kennedy, for his help and encouragement.

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4. References BARBERI, F., Bo~I, S., F~RRARA, G., MARINELLI,G. • VARET,J. 197o. Relations between tectonics and magmatology in the northern Danakil depression (Ethiopia). Phil. Tram. R. Soc. A267, 293-3I I. CAr~, J. R. x97o. New model for the structure of the ocean crust. Nature, Lond. 226, 928-3o. CHAS~, R. L. I969 . Basalt from the axial trough of the Red Sea. In Degens, E. T. & Ross, D. A. (eds.) Hot brines and recent heavy metal deposits in the Red Sea. New York, I22-8. CLARKE, D. B. x97o. Tertiary basalts of Baffin Bay: possible primary magma from the mande. Contr. Mineral. and Petrol. 25, 203-24 . COOMBS, D. S. 1963. Trends and affinitites of basaltic magmas and pyroxenes as illustrated on the diopside-olivine-silica diagram. Miner. Soc. Am. Spec. Pap. No. z, 227-50. Cox, K. G. 197 o. Tectonics and vulcanism of the Karroo period and their bearing on the postulated fragmentation of Gondwanaland. In Clifford, T. N. & Gass, I. G. (eds.) African mag- matisra and tectonics. Edinburgh, 21 I-36. , GASS, I. G. & lk'IALLICK,D. 1. J. i969. The evolution of the volcanoes of Aden and Litde Aden, South Arabia. Q. dl geol. Soc. Lond. xa4, 283-308. & ~ x97 o. The peralkaline volcanic suite of Aden and Little Aden, South Arabia. J. Petrol. xt, 433-6x. DAVIES, D. & TRAMONTIm, C. x97o. The deep structure of the Red Sea. Phil. Tram. R. Soc. A267, 18 I--9. FALCON, N. L., GASS, I. G., GmDLER, R. W. & LAUOHTON, A. S. (organisers) x97o. A discussion on the structure and evolution of the Red Sea, Gulf of Aden and Ethiopia rift junction. Phil. Tram. R. Soc. &267, t-417. FRAZmR, S. B. x97o. Adjacent structures of Ethiopia: that portion of the Red Sea coast including Dahlak Kebir Island and the Gulf of Zula. Phil. Tram. R. Soc. Aa67, x3I-4I. GAss, I. G. 1969 Geol. Mag. xo6, 89-9o. Correspondence re Whiteman, A. J. 1968. Formation of the Red Sea depression. Geol. Mag. xo5, 23r-46. I97o. The evolution of vulcanism in the junction area of the Red Sea, Gulf of Aden and Ethiopian rifts. Phil Tram. R. Soc. Aa67, 369-8t. & GmSON, I. L. 1969. The structural evolution of the rift zones in the Middle East. Nature, Lond. 22x, 926-3o. & MALI~CK, D. I. J. 1968. Jebel Khariz: an Upper Miocene strato-volcano of comenditic affinity on the South Arabian coast. Bull. Volcan. 32, 33-88. & ~ & Cox, K. G. I965. The Royal Society volcanological expedition to the South Arabian Federation. Nature, Lond. aoS, 952-5- GmDImR, R. W. I958. The relationship of the Red Sea to the East African rift system. Q. Jl Geol. Soc. Lond. xx4, 79-Io 5. I97o. A review of Red Sea heat flow. Phil. Tram. R. Soc. #.a67, I9I-2o3. GmmN, D. H. & RINGWOOD, A. E. t967. The genesis of basaltic magmas. Contr. Mineral. and Petrol. xS, I o3-9o. JAMmSON, B. G. I966. Evidence on the evolution of basaltic magma at elevated pressures. Nature, Lond. ,,T~, 243_6. 197o. Phase relations in some tholeiitic lavas illustrated by the system R203--XO--YOuZO~. Mineralog. Mag. 37, 537-54. KusmRo, I. 1968. Compositions of magmas formed by partial zone melting of the earth's upper mande. J. geophys. Res. 73, 6i 9-34- ---- & KUNO, H. i963. Origin of primary basalt magmas and classification of basaltic rocks. J. Petrol. 4, 75-89 • LAMAKE, P. 193o. Les manifestations volcaniques post-Cr~tac~e de la Mer Rouge et des pays limitrophes. In Etudes gdologiques en Ethiopie, Somalie et Arabie meridionale. M~m. Soc. gdol. Frame. 6, 2 x-48. LAUGHTON, A. S., WHrrMAmH, R. B. & JoNEs, M. T. t97o. The evolution of the Gulf of Aden. Phil. Tram. R. Soc. A267, 227-66.

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 307

McBml~Y, A. R. & G~ss, I. G. x967. Relations of oceanic volcanic rocks to mid-ocean rises and heat flow. Earth and Planet. Sci. Lett. 2, 265-76. MAcF~mx,EN, W. A. x932. On the volcanic Zebayir Islands, Red Sea. Geol. Mag. 69, 3xo-5 . McI~Nzxe, D. P., DAvr~s, D. & MOLNAR, P. I97o. Plate tectonics of the Red Sea and East Africa. Nature Lond. 226~ 243-9. MF.N~tD, H. W. x967. Extension of Northeastern-Pacific fracture zones. Science x55, 72-4. MooN, F. W. I923. Preliminary geological report on Saint John's Island (Red Sea). Report Geological Survey Egypt, Cairo. O'~, M. J. 1965. Primary magmas and the origin of basalts. Scott. J. Geol. x, i9-4 o. O'~, M. J. 1968a. The bearing of phase equilibria studies in synthetic and natural systems on the origin and evolution of basic and ultrabasic rocks. Earth Science Reviews 4, 69-x 33- .... I968b. Are ocean floor basalts primary magma ? Nature, Lond. 220, 683-6. SCHILtaNO, J. G. I969. Red Sea floor origin: rare earth evidence. Science x65, I357-6o. SXGURVSSON H. & BROWN, G. M. I97o. An unusual enstatite-forsterite basalt from Kolbeinsey Island, north of Iceland. J. Petrol. xx, 2o5-2o. T~LING, D. H. 197o. Palaeomagnetism and the origin of the Red Sea and Gulf of Aden. Phil. Trans. R. Soc. A267, 219-26. VINF., F. J. x966. Spreading of the ocean floor: new evidence. Science x54, i4o5-i 5. x971. Sea-floor spreading In Gass, I. G., Smith, P. J. & Wilson, R. C. L. (eds.) Under- standing the Earth, Sussex, 233-249. UPTON, B. G. J. & WADSWORTH, W. J. i966. The basalts of Reunion Island, Indian Ocean. Bull. Volcan. 29, 7-24. WEOnr~R, A. x929. The origin of continents and oceans. 4th edition, Methuen, London. WVNTWORTH, C. K. & MACDONALD, G. A. 1953. Structures and forms of basaltic rocks in Hawaii. U.S. geol. Surv. Bull. 994, 1-98. YODER, H. S. & TXLLEY, C. E. I962. Origin of basalt magmas: an experimental study of natural and synthetic systems. J. Petrol. 3, 342-532.

Appendix

Brief descriptions of analysed rocks in Tables 2-4 Abbreviations P--phenocrysts, ol---olivine, cpx---clinopyroxene, pl--plagioclase, rot-- opaque minerals (mainly magnetite), trmpresent in very small amounts, occ---occasional, GM~groundmass. Percentages quoted are volume percents. Analyses are listed in decreasing order ofMgO. Plagioclase compositions are optical determinations and give approximate range of zoning. Pyroxene compositions were determined by X.R.D. methods.

Jebel at Tair (Table 2) JT 4 Basalt, lava flow, summit of island: P---ol tr, cpx microphenocrysts, pl occ: GM--p1 Aneo. JTI Basalt, lava flow, o'5 km sE of landing: almost aphyric, P--pl occ: GM---cpx Ca49Mg3TFel~. JTx2 Basalt, block in spatter cone, landing: P--ol Fa14 occ, pl Ansi-so 2 5 %, mt tr. JTI 4 Basalt, lava flow, 0. 5 km s of landing: almost aphyric P--occ pl AnT0 resorbed, cpx tr. JTio Basalt, lava flow, 4oom NW of summit: P---ol microphenocrysts 5%, pl Aneo 20 %.

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JTI6 Basalt, lava flow, I km s of landing: P---ol Fa2s occ, pl Ans0-6, IO}/o, mt tr. Zubair Group ( Table 3) Z37 Picrite basalt, lava of Phase 2, w coast Saba I: P---ol Fa10-15 I9%, cpx x % & pl 2 % present as xenocrysts. The relatively high CaO content is due to contamination by gabbroic fragments. Z48 Picrite basalt, block in agglomerate, Phase I, E side Saba I: P---ol Fal0_15 I7%. Z I o Feldsparphyric basalt, block in agglomerate of Phase x, ~ coast Haycock I: P--ol abundant small, pl abundant An~o. Z 4 Basalt, block in agglomerate of Phase x, N coast Haycock I: P---ol Fa,o 9 %, cpx tr, pl tr. Z 7 Basalt, block in agglomerate of Phase x, N COast Haycock I: P---ol Fa~0 2 %: GMmpl Anes. JZ 9 Basalt, lava flow, cone cluster ~. of median ridge, J. Zubair Phase 2: P---ol Fa~0 3"5 %, cpx tr, pl AnT~_80 8"5 %. JZ24 Basalt, lava flow, s side of large cone at N end of island, J. Zubair, Phase 2. P--abundant ol, pl, cpx. JZI2 Feldsparphyric basalt, lava flow, near sE coast J. Zubair, Phase 2: Pmcpx tr, pl Ans0_~5 I O ~/O" Z69 Feldsparphyric basalt, lava flow, landing place E coast Centre Peak I, Phase 2: P--ol i. 5 %, pl An,s 3"5 %. JZI Trachybasalt, block in agglomerate, s tip of J. Zubair, Phase I. P---ol I "5 ~0, pl Ans0_,s I "8 %. GM--pl AnTo. JZ26 Aphyric trachybasalt, lava flow, coast 2 km sw of the N tip of J. Zubair, Phase 2. JZI 7 Feldsparphyric trachybasalt, 0. 5 km from coast, w of central part of median ridge, J. Zubair, Phase 2. Z26 Feldsparphyric trachybasalt, block in agglomerate, E coast Rugged I, Phase i: P--ol i % , cpx o"3% , pl II %. Z36 Trachybasalt, block in agglomerate, s coast Saddle I, Phase I : aphyric: GM--ol Faso. Z2I Feldsparphyric trachybasalt, block in agglomerate, ~. coast Rugged I, Phase I : P--pl AnTs abundant. Z I 7 Trachybasalt, block in agglomerate, E coast Haycock I, Phase i: P--pl Anso 4 %: GM---ol Faso.

Hanish-Zukur Group (Table 4) H2 4 Basalt, block in old agglomerate cone, Gt. Hanish I, ~. coast 6 kin s of N tip of island: P--ol Fa12 9%, pl Anes-e0 5%" H 18 Basalt, block in old agglomerate cone, Gt. Hanish I, N tip: P---ol Fal5 I5%. AAI Basalt, block in agglomerate, Abu Ail I., near lighthouse: P---ol Fa2_ 50CC.

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H26 Basalt, lava flow, locality as H24: aphyric. H 17 Basalt, young lava flow, locality as H z8: glassy with occ microlites ofpl. H 3 Basalt, older lava flow, locality as H z8: aphyric. H2 7 Basalt, block in old agglomerate cone, locality as H24: aphyric. H z Basalt, older lava flow, locality as Ha8: Microporphyritic, P---ol tr, pl AnT0 2 %: GM--pl An55. H32 Basalt, block in agglomerate, Gt. Hanish I., ~ coast due w of N Round I: P--ol 2 %, pl AnTo-e5 7 %- G2o Basalt, older lava flow, Zukur, 3oom sw of landing place 2 km sE of N tip of island: P--ol Faas, pl. H28 Basalt, block in old agglomerate cone, locality as H24: aphyric: GM--pl Anss. H 4 Basalt, older lava flow, locality as H24: P--pl An~e0 z8%; GM--pl Ans0. H33 Basalt, block in agglomerate, locality as H32 : P--ol Fa~0 tr, pl 23 %. H io Basalt, block from agglomerate, locality as HI8: P---ol microphenocrysts ca. 2 %, pl AnT0-s5 5 %: GM--pl Ane0. G 9 Trachybasalt, lava flow, coast z km N of landing place, Zukur (see G2o) : P--ol Fa31, pl Ane0, mt: GM----cpx Ca3eMg3sFe,e, pl Ans0. Gz8 Trachybasalt, lava flow, landing place, Zukur (see G2o): P--~I Fa30, pl An,0-e0, mt: GM--pl Ane0. G17 Trachybasalt, lava flow, locality as GI8: P---ol Falb occ, pl An83_es moderately abundant, mt: GM--pl An55. GxI Trachybasalt, lava flow, coast z km N of landing, Zukur: P---ol Fa,, occ, pl Ans0_v0 moderately abundant, mt. GI Trachybasalt, older lava flow, 1 km N of landing, Zukur: P--ol Fasa, pl Anss abundant, mt: GM--cpx Ca44Mg,0Fele, pl An,7. G2 3 Trachyandesite, older lava flow, 2 km sw of landing, Zukur: aphyric: GM--pl An49. G2z Trachyandesite, older lava flow, z km sw of landing, Zukur: P--ol Fa4e, hornblende, cpx, pl An40, rot: GM--pl An31. G2 7 Trachyandesite, lava flow, landing place, Zukur: P--ol Fa,5, cpx, pl Ansg, mt: GM--cpx Ca4eMg,0Fe14, pl Ans4_,s. G3IA Trachyte, prominent tholoid, Nw coast, Zukur: P--cpx, pl An4o_~o, mt: GM--pl An2o, anorthoclase. G3zB Trachyte, coarse grained xenolith in G31A. Contains ol Fa4,, cpx, pl An,e_v, hornblende, apatite, mt.

Received z July x97x; revised manuscript received 5 January I972; read zo May x972. Ian Graham Gass, Department of Earth Sciences, The Open University, Walton Hall, Bletchley, Bucks. Donald Ivor John Mallick, Geological Survey, c/o British Residency, Vila, Elate, New Hebrides. Keith Gordon Cox, Department of Geology and Mineralogy, University, Parks Road, Oxford.

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DISCUSSION DR R. B. WI-IITMARSIt said that a hole recently drilled by Glomar Challenger during Leg 23 of the Deep-Sea Drilling Project is relevant to the authors' discussion of the possible southward extent of the axial valley of the Red Sea. This hole was drilled about 15 miles south of Zubayir Island in a water depth of 852 metres. A total section of 212 metres of Holocene to Late Pleistocene greenish calcareous ooze was penetrated before the hole had to be abandoned. The oldest recovered sediments were about 5oo,ooo years old and indicate therefore very rapid sedimentation of primarily biogenous material. A minimum of about 55 ° metres of sediment was present at this site so it seems unlikely that the deepest sediments are older than the Pliocene. These facts, together with an above average heat flow measurement at this site, suggest that the axial valley does in fact extend south of Zubayir Island but that it is filled with a thick sequence of biogenous Quaternary sediments.

Professor Gass thanked Dr Whitmarsh for providing this information.

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