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Journal of African Earth Sciences 47 (2007) 203–226
A-type volcanics in Central Eastern Sinai, Egypt
M.D. Samuel, H.E. Moussa, M.K. Azer *
Geology Department, National Research Centre, Dokki, Cairo 12622, Egypt
Received 26 January 2006; received in revised form 12 September 2006; accepted 6 February 2007Available online 23 February 2007
Abstract
Alkaline rhyolitic and minor trachytic volcanics were erupted �580–530 Ma ago. They occur with their A-type intrusive equivalents inSinai, southern Negev and southwestern Jordan. At Taba-Nuweiba district, these volcanics outcrop in three areas, namely, Wadi El-Mahash, Wadi Khileifiya and Gebel El-Homra. Mineralogically, they comprise alkali feldspars, iron-rich biotite and arfvedsonitetogether with rare ferro-eckermannite. Geochemically, the older rhyolitic volcanics are highly evolved, enriched in HFSE includingREE and depleted in Ca, Mg, Sr and Eu. The rhyolitic rocks of Wadi El-Mahash and Gebel El-Homra are enriched in K2O content(5.3–10.1 wt.%) and depleted in Na2O content (0.08–2.97 wt.%), while the rhyolites of Wadi Khileifiya have normal contents of alkalis.Their REE patterns are uniform, parallel to subparallel, fractionated [(La/Yb)n = 5.4] and show prominent negative Eu-anomalies. Theyare classified as alkali rhyolites with minor comendites. The younger volcanics are classified as trachyandesite and quartz trachyte (56.6–62.9 wt.% SiO2). Both older and younger volcanics represent two separate magmatic suites. The overall mineralogical and chemical char-acteristics of these volcanics are consistent with within plate tectonic setting. It is suggested that partial melting of crustal rocks yieldedthe source magma. Lithospheric extension and crustal rupture occurred prior to the eruption of these volcanics. The rather thin conti-nental crust (�35 km) as well as the continental upheaval and extensive erosion that preceded their emplacement favoured pressurerelease and increasing mantle contribution. The volatiles of the upper mantle were important agents for heat transfer, and sufficientfor the anatexis of the crustal rocks. A petrogenetic hypothesis is proposed for the genesis of the recorded potassic and ultrapotassicrhyolitic rocks through the action of dissolved volatiles and their accumulation in the uppermost part of the magma chamber.� 2007 Elsevier Ltd. All rights reserved.
Keywords: Alkaline volcanics; Taba-Nuweiba area; Mineral chemistry; Geochemistry; K-enrichment
1. Introduction
Two episodes of essentially calc-alkaline volcanic activ-ity are recognized in the late Proterozoic or Pan-Africanbelt in the Eastern Desert and Sinai, Egypt. The older epi-sode, 800–700 Ma old, (Harris et al., 1984; Kroner, 1985;Stern et al., 1991) produced the island arc Younger Meta-volcanics (Stern, 1981), metamorphosed within the greens-chist facies. These metavolcanics are associated withvolcaniclastics. The younger volcanic episode, 620–580 Ma old (Stern, 1981; Bielski, 1982, in Bentor, 1985;Ressetar and Monrad, 1983; Abdel-Rahman and Doig,1987) occurred after the accretion of the island arc onto
1464-343X/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jafrearsci.2007.02.006
* Corresponding author. Tel.: +20 2 7742962; fax: +20 2 3370931.E-mail address: [email protected] (M.K. Azer).
the East Saharan Craton. It produced the Dokhan Volca-nics that are frequently associated and intercalated withmolasse-type Hammamat clastic sediments (Gass, 1982;El-Gaby et al., 1984, 1988, 1989, 1990, 1991; Moussa,2003). After the closure of the Pan-African orogeny, a thirdvolcanic episode produced alkaline to peralkaline volcanicrocks known as Katherina Volcanics (Agron and Bentor,1981). These alkaline magmas form minor occurrencesbut are widely scattered over the entire Arabian–NubianShield (ANS), although later erosion rendered the alkalineflows relatively rare (Harris, 1982). These alkaline volcanicsand related alkali granites mark the transition to intraplatealkaline magmas, which prevailed during the Phanerozoic(Bentor, 1985; Stern et al., 1988; Black and Liegeois,1993). Reported ages for the Late Precambrian alkalinerocks from Sinai, the Eastern Desert, southern Negev
204 M.D. Samuel et al. / Journal of African Earth Sciences 47 (2007) 203–226
and SW Jordan, using the Rb-Sr whole rock isochronmethod range from ca 580 to 530 Ma (Bielski, 1982, inBentor and Eyal, 1987; Hassan, 1998; Mushkin et al.,1999; Jarrar et al., 1992). However, Mushkin et al. (1999)extend the time span for the intrusion and extrusion ofalkaline magmas in southern Negev to 600–530 Ma.
The alkaline volcanics were largely neglected in Egypt.In the Eastern Desert, Essawy (1972) recognized alkali rhy-olites at Samadai–Tunduba area (CED). Abu El-Ela (2001)revealed that the felsites of Wadi Atalla (CED) pertainmost probably to the Katherina Volcanics of South Sinai.In Sinai, there is only limited but conflicting data aboutthe geology of these volcanics at Gebel Katherina typelocality (Eyal and Hezkiyahu, 1980; El-Masry et al.,1992; Abdel Khalek et al., 1994; Eyal et al., 1994, 1995).Moreover the volcanics at Iqna Shar’a, central Sinai, wereconsidered as a good example of Katherina Volcanics(Bentor, 1985; Bentor and Eyal, 1987; Mushkin et al.,1999). However, Samuel et al. (2001a,b) concluded that
Fig. 1. (a) Location map of the studied areas; (b) photogeologic map ofphotogeologic map of Gebel El-Homra.
these volcanics, using petrological and geochemical data,represent the upper sequence of the Dokhan Volcanics(or Ferani Volcanics of Sinai).
The present work deals with the study of the alkalinevolcanics exposed at Taba-Nuweiba environs near thenorthwestern end of the Gulf of Aqaba. Three areas wereselected for investigations namely, Wadi El-Mahash(W. M.), Wadi Khileifiya (W. Kh.) and Gebel El-Homra(G. H.). The aim of this work is to elucidate the natureof these volcanics, their tectonic environment, their magmasource and modifying geological processes.
2. Geologic setting and field relations
The three studied volcanic occurrences (W. M., W. Kh.and G. H.) occur in Taba-Nuweiba district (Fig. 1a). Thesevolcanics were mapped on the regional geologic maps ofSinai (edited by Eyal et al., 1980) and Egypt (edited by Kli-tzsch et al., 1987) as Katherina Volcanics. On the recent
Wadi El-Mahash; (c) photogeologic map of Wadi Khileifiya and (d)
Fig. 1 (continued)
M.D. Samuel et al. / Journal of African Earth Sciences 47 (2007) 203–226 205
geologic map of Sinai, sheet No. 2 (edited by El-Hinnawi,1994) W. M. is occupied by alkaline granites and both W.Kh. & G. H. are occupied by metamorphosed acid volca-nics. Moreover, Essawy et al. (1997) and Abu El-Enenet al. (1999) considered the volcanics at W. M. & W. Kh.as Dokhan Volcanics.
Wadi El-Mahash (W. M.) volcanics occupy an elongatearea, 9 km long and 0.7–3 km wide, trending roughly N–Sin the middle part of the mapped area (Fig. 1b). They formhighly elevated outcrops (925 m a.s.l.) with steep slopes.The southern and northern subvolcanic granophyresintrude with sharp contacts the alkali feldspar graniteand gneisses, respectively. The volcanics are unconform-ably overlain on its western side by early Paleozoic sand-stones (A. Akarish, National Research Centre, Egypt,personal comm.) constituting the basal part of GebelGhazalani sedimentary succession.
The volcanics constitute a stratified sequence (�220 mthick) of pyroclastics (volcanic breccia, tuffs and ignimb-rites), rhyolite lava flows and subvolcanic granophyres.The tuffs and ignimbrites prevail in the central part of thevolcanic occurrence, while the lava flows are exposedmainly in the northern part. The pyroclastics started withvolcanic breccia in which the angular rock fragments attain8 cm across and are set in a tuffaceous matrix. The lithicand crystal tuffs are well bedded, and the strata are com-monly 0.3–3 m thick. The ignimbrites are highly weldedand are formed of abundant, rather oriented lenticularglass clasts showing the filamentous nature of fiamme.
The lava flows are represented by rhyolites, which arepresent as thin sheets intercalating the pyroclastics or form-ing small hills. The granophyres are more abundant thanthe rhyolites and appear to be older than the pyroclasticsand rhyolites but later than the alkali-feldspar granites.
206 M.D. Samuel et al. / Journal of African Earth Sciences 47 (2007) 203–226
The granophyres are highly porphyritic and have a coarsermatrix than the rhyolites.
Wadi Khileifiya (W. Kh.) volcanics occupy an irregularcrescent-shaped outcrop, 5.5 km long and 0.5–2.5 km wide,elongated roughly in an E–W direction in the middle partof Wadi Khileifiya area (Fig. 1c). The volcanic sequenceforms highly elevated hills with steep slopes attaining734 m (a.s.l.). Rhyolite dykes and minor intrusions cut withsharp contacts the two-feldspar granite present in thesouthern part of the area and even taking xenoliths fromit. Doleritic dikes cut across the granite but abut againstthe rhyolites. The volcanics are unconformably overlainfrom the north by Cambrian Araba Formation (El-Arabyand Abdel-Motelib, 1999), with basal polymictic conglom-erates, deposited on a flat horizontal erosional surface.
Wadi Khileifiya volcanics occur as steeply inclined tovertical successive sheets of rhyolites intruded in the middlepart of the volcanic occurrence by non-porphyritic grano-phyre sheets. The rhyolite sheets are brecciated along theircontacts with the granophyres.
The rhyolites and granophyres were followed by theeruption of trachyandesite forming a thin belt of lava flowsand pyroclastics concentrated along the southern and wes-tern contacts with the older two-feldspar granite. Thetrachyandesites are fine-grained, black in colour and con-tain xenoliths from the older acid rocks.
Gebel El-Homra (G. H.) volcanics was named Biq’atHayareah (Agron and Bentor, 1981), Gebel El-Hamra(EGPC/CONOCO geologic map, 1987) and Neshef Massif(Mushkin et al., 1999). The volcanics of Gebel El-Homraarea (Fig. 1d) occur as ridges and conical hills with steepslopes drained by almost dead wadis filled by alluvial accu-mulations capped in places by a thin layer of loess. Thenorthern ridges are mainly aligned in an E–W directionparallel to the Themed fault (Tethyan trend) separatingthese volcanics from the Late Cretaceous Sudr Chalk.These ridges attain a height of 928 m (a.s.l.), about 160 mabove the surrounding plain.
The G. H. volcanics are represented by rhyolites, sub-volcanic rhyolite porphyries, pyroclastics and ignimbrites.The rhyolites are intruded by quartz trachytes and amyg-daloidal quartz trachytes in the eastern and western partsof the map area.
Extrusive rhyolites are the most widespread rocks inthe area. They are fine-grained with rather few pheno-crysts and have orange, brown or purple colours. Thepyroclastics are less abundant and occur as minor layers(4–6 m thick) intercalating the rhyolites. They compriselapilli tuffs, banded tuffs and ash tuffs together withignimbrites. The minor subvolcanic rhyolite intrusionsform separate hills in the western part of the area; theyare distinctly highly porphyritic and rather coarsergrained than the rhyolites.
The quartz-trachytes (both massive and amygdaloidal)are younger than the surrounding rhyolites as they tendto have chilled margins along their contacts with the rhyo-lites. Agron and Bentor (1981) assumed that the quartz
trachytes may be related to the Early Cretaceous volcanicsof Ramon Province.
Field observations revealed that the volcanics in thethree studied areas are genetically related. The chronolog-ical sequence of the different rock units in the mapped areasstarts with the emplacement of two-feldspar granites (G2-granites) during the late phase of the Pan-African orogeny.This was followed by the intrusion of A-type alkali-feld-spar granites forming small discordant intrusions havingalmost circular outlines, sharp contacts and devoid ofxenoliths. The A-type granites were preceded by variouskinds of dykes, which traverse the G2-granites. Afterwards,alkaline silicic volcanics erupted like Katherina Volcanicsat Gebel Katherina (central Sinai), Gebel Amram (south-ern Negev Desert) and Wadi Araba (southwest Jordan).The Katherina Volcanics started in the studied areas withexplosive eruption and deposition of pyroclastics andignimbrites associated somewhat later with the extrusionof rhyolites. Volcanic breccia constitutes the basal part ofW. M. volcanic sequence, which represents the beginningof this cycle. Bielski (1982, in Bentor, 1985) and Jarraret al. (1992) reported Rb/Sr whole-rock isochron ages of548 ± 5 and 553 ± 11 Ma for the rhyolites of Gebel El-Homra and Wadi Araba volcanics (SW Jordan), respec-tively. The extrusion of the studied rhyolitic lavas andpyroclastics was either preceded (at Wadi El-Mahash) orsucceeded (at Wadi Khileifiya) by the emplacement of sub-volcanic granophyres having sharp contacts against therhyolites. At G. H., the small subvolcanic rhyolite-porphyry intrusions form isolated hills without mutualcontacts with their volcanic counterparts. The youngervolcanic cycle is represented by the extrusion of trachy-andesites and quartz trachytes. These trachytic extrusivescan be correlated with their monzonite and quartz syeniteplutonic equivalents in Elat area, which yielded a Rb/Srwhole-rock isochron age of 526 ± 22 Ma (Mushkin et al.,1999). This is inferred from their geographical proximityand the occurrence of similar rhyolites in both areas.
The Katherina Volcanics are unconformably overlain atWadi El-Mahash by undifferentiated early Paleozoic sand-stones or by Cambrian sandstones of the Araba Formationat Wadi Khileifiya. Subaerial erosion of the basementrocks and sedimentation of these sandstones mark theonset of the stable platform conditions in Taba-Nuweibadistrict.
3. Petrography
The older cycle of the Katherina Volcanics is repre-sented by rhyolitic volcanic and subvolcanic rocks, pyroc-lastics and ignimbrites. The younger cycle is formed oftrachyandesite lava flows and pyroclastics as well as quartztrachytes of massive and amygdaloidal nature. The petro-graphical description of the studied Katherina Volcanicsis summarized in Table 1.
From the present petrographic investigation, it is clearthat the volcanics are exclusively subaerial and are unmeta-
Table 1Petrographic description of the Katherina Volcanics in the studied areas
Area Rock type Description
Wadi El-Mahash
1. Pyroclastics Volcanic breccia is poorly sorted, predominantly angular to subangular felsic volcanic rock fragments, 50–70 mmacross, embedded in a tuffaceous matrix. The groundmass is formed of fine (ash) grains (less than 1/16 mm) andcontains abundant vesicles commonly stretched parallel to the bedding and filled with secondary quartzLithic tuffs are mainly composed of granitic and rhyolitic rock fragments with minor orthoclase and quartz crystalsand glass fragments, <2 mm across, embedded in a fine tuffaceous groundmass. The vitric fragments are rarelydevitrifiedCrystal tuffs are composed of predominant crystal fragments of quartz, orthoclase and rare plagioclase togetherwith minor rock fragments embedded in a vitric or fine tuffaceous groundmass. Rare angular rhyolite and glassfragments of variable sizes are recorded; the glass fragments show slight devitrificationBanded tuffs are very fine grained and composed of alternating laminae (<2.0 mm thick) and thin beds (<10 mmthick) of variable grain size and colours. The fine laminae are formed of very fine-grained, vitreous-ashy material,while the coarse laminae contain abundant coarse crystals and crystal fragments of quartzIgnimbrite is rhyolitic in composition and shows regular orientation of flattened lenticular glass particles (fiamme)embedded in a groundmass formed of acid volcanic glass, pumice and glass shards of irregular shape (Fig. 2a). Thegroundmass in most of the examined ignimbrites is spherulitic and devitrified into equigranular crystal aggregatesshowing mosaic texture and rarely a fluidal structure
2. Granophyre It is typically porphyritic and the phenocrysts (20–45 vol.%) may reach 5.0 mm in length. They are represented byK-feldspar, albite, quartz and sometimes biotite but rarely amphibole.These phenocrysts are set in amicrocrystalline felsic groundmass exhibiting granophyric and micrographic textures
3. Rhyolite The phenocrysts (�10 vol.%)are represented by K-feldspar and quartz embedded in microcrystalline groundmass.Micrographic intergrowth between K-feldspar and quartz surrounding porphyritic quartz crystalls is observed. Itcontains rare subhedral phenocrysts (�1 vol.%) and fine flakes of biotite showing variable degrees of alteration tochlorite and iron-oxides
WadiKhileifiya
1. Rhyolite It contains variable amounts of phenocrysts (4–20 vol.%) which represented by K-feldspar, quartz and albite. Thephenocrysts occur either as discrete crystals or as glomerophyric or cumulophyric aggregates (Fig. 2b). Thegroundmass is microcrystalline and show microgranular and/or spherulitic textures
2. Granophyre It is fine-grained with sparse phenocrysts (<3 vol.%) represented by K-feldspars intergrown with quartz in amicrographic fashion. Fine relics of biotite are present
3. Trachyandesite It occurs as thin suuccesion of alternating lava flows and pyroclastic layers. The former are porphyritic andcomposed of sodic plagioclase, alkali feldspars, biotite and amphibole set in a trachytic groundmass formed offeldspar laths and mafic microlites disposed in a subparallel arrangment marking the flow lines of the lava. Theycontain granitic and rhyolitic xenoliths (Fig. 2c). The pyroclastics are represented by crystall tuffs. They arecomposed of iron-stained feldspar, biotite and hornblende crystals and crystal fragments, 2–1/16 mm across,embedded in a dense groundmass. The groundmass is occasionally devitrified and dissected by subparallel quartzveins
Gebel-El-Homra
1. Rhyolite Rhyolites are intercalated with the pyroclastics. They contain variable amounts of phenocrysts (5–30 vol.%) whichthey occur either as discrete crystals or as glomerophyric or cumulophyric aggregates. The phenocrysts representmainly by quartz and K-feldspars with rare alkali amphibole (Fig. 2d), embedded in microcrystalline groundmassshowing micrographic and spherulitic textures. Some crystals display graphically intergrown rectangularphenocrysts of K-feldspars and quartz showing some sort of hourglass twinning (Fig. 2e). Some K-feldsparphenocrysts contain exsolved batches or stringers of albite (Fig. 2f). Few phenocrysts of anorthoclase are detectedshowing their characteristic very fine cross-hatched twinning. Irregular vesicles are recorded in the studied rhyolite
2. Pyroclastics They comprise lapilli tuffs, crystals tuffs, banded tuffs and ignimbrites. Lapilli tuffs are poorly sorted and composedof angular to subangular clasts, 5–40 mm across, comprising lithic and crystal fragments set in a fine (ash)tuffaceous groundmass (Fig. 2g). The lithic fragments are rhyolitic in composition and the crystal fragments arerepresented by quartz and K-feldspars. The crystal tuffs, banded tuffs and ignbimbrites are similar to those of W.El-Mahash area
3. Rhyoliteporphyry
It has proper rhyolite composition, but commonly contains larger amounts of phenocrysts that may constitute 50–80 vol.% of the rock. The K-feldspar phenocrysts are more than quartz phenocrysts, while the rhyolites carry morequartz phenocrysts. It also possesses coarser grained groundmass, reaching 0.4 mm in grain size, and showsgranophyric texture. No vesicles are recorded
4. Quartz-trachyte
They are differentiated into quartz trachyte and amygdaloidal quartz-trachyte. The former is porphyritic andcomposed of phenocrysts and microphenocrysts of alkali feldspars (albite and orthoclase) embedded in a fine-grained groundmass showing the characteristic trachytic texture (Fig. 2h) The latter is composed of abundantrounded and oval amygdales set in a fine-grained trachytic groundmass. The amygdales constitute about half ofthe rock volume and vary in size from 2 to 10 mm in diameter. They are filled by calcite, quartz and chlorite;quartz usually forms the outer lining
M.D. Samuel et al. / Journal of African Earth Sciences 47 (2007) 203–226 207
morphosed. The rhyolites are either alkaline or peralkalinecarrying arfvedsonite. The subvolcanics comprise grano-phyres which are very similar to G3-granites (A-type gran-
ites) together with subvolcanic rhyolite-porphyry havinghigher amounts of phenocrysts, coarser grained ground-mass and almost lack vesicles as compared to the extrusive
208 M.D. Samuel et al. / Journal of African Earth Sciences 47 (2007) 203–226
rhyolites. The fallout pyroclastics cover the entire spectrumfrom coarse volcanic breccia to fine (ash) tuffs. The ignimb-rites are rhyolitic in composition. The trachyandesites andquartz trachytes of the younger cycle are the extrusiveequivalents of the monzonite and quartz syenite plutonsexposed in southern Negev Desert (Mushkin et al., 1999).The total opaque minerals in the felsic Katherina Volcanicsrange from 0.1 to 2.0 vol.%, whereas the intermediate vol-canics contain up to 4.0 vol.%. They are represented mainlyby Ti-poor magnetite, titanomagnetite and ilmenite.Titanomagnetite occurs as homogeneous grains or as
Fig. 2. (a) Vesiculated ignimbrite at Wadi El-Mahash (C.N., X = 60); (b) FKhileifiya (C.N., X = 40); (c) Rhyolite xenolith in the trachyandesite of WadiGebel El-Homra (C.N., X = 50); (e) Micrographic intergrowth of K-feldspar athe thin K-feldspar veneer around the phenocrysts (C.N., X = 40); (f)Albite(C.N., X = 30); (g) Ill-sorted lapilli lithic tuff from Gebel El-Homra formed ofdisposed in a trachytic texture in the quartz trachyte of Gebel El-Homra (C.N
exsolved crystals predominant in the granophyres. Theexsolved magnetite exhibits a variety of fabrics with ilmen-ite, namely fine trellis-, coarse trellis-, banded-intergrowthsand composite grains. The ilmenite is present as homoge-neous discrete grains and few grains are hook-like.
4. Mineral chemistry
Chemical composition of feldspars, biotite and alkaliamphiboles from the studied volcanic-subvolcanic rockswere determined using CAMECA SX 100 electron micro-
eldspar phenocrysts showing cumulophyric texture in rhyolite at WadiKhileifiya (C.N., X = 40); (d) Arfvedsonite phenocryst in the rhyolite of
nd quartz in rhyolite of Gebel El-Homra. Note the hourglass twinning andbatches within K-feldspar phenocryst in the rhyolite of Gebel El-Homraangular to subangular rock fragments (P.L., X = 30) and (h) Albite laths., X = 30).
Table 2Average chemical compositions of alkali feldspars from the Katherina Volcanics
Volcanic cycle Older Younger
Rock type Rhyolite Granophyre Rh. Porp. Trachyandesite Qz-trachyte
Locality W. El-Mahash W. Khileifiya G. El-Homra W. El-Mahash W. Khileifiya G. El-Homra W. Khileifiya G. El-Homra
Sample no. 206 107 342 200 & 216 127 305 125 304
Minerals Or Or Ab Or Aba Or Ab Anor Or Or Or Ab Anor Or Ab AnorNo. of spots 18 2 17 11 2 19 13 3 17 19 12 6 7 10 16 4SiO2 63.73 64.53 67.67 63.53 67.59 64.2 65.52 62.95 64.46 63.85 64.66 68.21 67.39 62.23 64.89 61.43TiO2 0.01 0.01 0 0.01 0.03 0.02 0.01 0.01 0.01 0.01 0.16 0.01 0.01 0.08 0.03 0.09Al2O3 19.59 19.4 21.1 19.42 21.93 19.99 22.18 23.1 19.26 18.89 19.1 20.83 20.77 18.93 22.06 23.63FeO 0.27 0.47 0.34 0.26 0.08 0.11 0.59 0.86 0.3 0.35 0.44 0.26 0.38 0.9 0.63 1.17MnO 0.01 0.01 0.04 0.01 0 0.04 0.02 0.07 0.01 0.01 0.02 0.02 0.03 0.01 0.02 0.05MgO 0.02 0.08 0.02 0.01 0.01 0.03 0.21 0.44 0.01 0.01 0.04 0.04 0.16 0.01 0.14 0.5CaO 0.01 0 0.25 0 0.01 0.07 0.79 0.85 0.07 0.03 0.22 0.19 0.27 0.06 0.72 0.7Na2O 0.2 0.69 11.57 0.48 9.99 1.19 10.64 8.47 0.77 0.29 1.24 11.04 9.87 0.53 10.14 8.68K2O 15.82 15.11 0.23 15.13 0.04 14.29 0.57 2.68 15.21 15.73 14.01 1.08 2.32 14.63 0.7 2.31Total 99.64 100.3 101.2 98.87 99.7 99.92 100.5 99.43 100.18 99.2 99.89 101.7 101.1 97.33 99.31 98.56
Structural formula based on 32 oxygensSi 11.254 11.85 11.14 11.83 11.77 11.79 11.48 11.25 11.869 11.892 11.88 11.79 11.76 11.8 11.49 11.09Al 4.275 4.197 4.307 4.26 4.496 4.322 4.576 4.859 3.999 4.143 4.131 4.239 4.267 4.227 4.602 5.026Ti 0.001 0.002 0 0.001 0.004 0.002 0.001 0.001 0.001 0.001 0.023 0.001 0.001 0.011 0.005 0.013Fe 0.041 0.073 0.05 0.041 0.011 0.017 0.086 0.128 0.046 0.055 0.067 0.038 0.056 0.144 0.134 0.176Mn 0.002 0.001 0.006 0.002 0 0.006 0.002 0.011 0.002 0.002 0.004 0.003 0.005 0.001 0.003 0.008Mg 0.006 0.023 0.004 0.003 0.003 0.009 0.56 0.117 0.004 0.003 0.01 0.009 0.041 0.004 0.036 0.134Ca 0.002 0 0.047 0 0.001 0.013 0.149 0.164 0.014 0.006 0.043 0.035 0.051 0.013 0.136 0.136Na 0.07 0.247 3.889 0.174 3.378 0.418 3.612 2.931 0.275 0.105 0.436 3.7 3.34 0.193 3.486 3.039K 3.739 3.541 0.051 3.596 0.009 3.351 0.128 0.612 3.572 3.739 3.289 0.239 0.517 3.538 0.158 0.533Ab 1.84 6.55 97.54 4.6 99.7 11.02 98.82 79 7.11 2.73 11.62 93.12 85.46 5.1 92.08 81.98An 0.05 0 1.18 0 0.05 0.34 3.85 4.45 0.36 0.16 1.15 0.88 1.31 0.33 3.84 3.68Or 98.11 93.45 1.28 95.4 0.25 88.64 3.33 16.55 92.53 97.11 87.24 6 13.23 94.57 4.08 14.4
a Albite intergrowths in the perthite phenocrysts.
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AlbiteOligoclase Andesine Labradorite Bytownite Anorthite
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Oligoclase Andesine Labradorite Bytownite Anorthite
Ab An
Ab An
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210 M.D. Samuel et al. / Journal of African Earth Sciences 47 (2007) 203–226
probe under operating conditions of 15 kV and 20 nA.Suitable synthetic and natural mineral standards wereapplied for calibration. The analyses were carried out onabout 190 spots in different representative minerals in nineselected samples at the Institute of Mineralogy, ClausthalTechnical University, Germany. Most of the analyses wereperformed on the phenocrysts and microphenocrysts whichare chemically related to the host magma than the ground-mass. Groundmass crystals are usually too small to ana-lyze. The raw data were processed through MinpetSoftware after Richard (1995) for the calculation of thegiven structural formulae and plotted on available classifi-cation diagrams. The chemical composition of the analyzedminerals is used here to depict, to some extent, the tectono-magmatic affiliation of the studied rocks (discussed in thenext section).
Data on mineral chemistry show that the feldspars ofthe felsic Katherina Volcanics (Table 2, Fig. 3a and b)are represented only by alkali feldspars. Albite and K-feld-spar occur in the rhyolites of W. Kh. and the granophyresof W. M. Other felsic rhyolites and subvolcanic intrusionscontain K-feldspar only. Rare perthite phenocrysts andanorthoclase are detected in G. H. rhyolites and W. M.granophyres, respectively. It is to be noted that the feldsparchemistry together with the general absence of perthitessuggest low temperatures of formation (high water pres-sure). Therefore, the A-type magmas are often, but notalways, anhydrous high-temperature melts. The K-feld-spars have an average composition of Or97; some crystalsin the granophyres of W. M. possess lower Or content(Or75–Or56). The albite has an average composition ofAb98. Anorthoclase contains usually low An content (3–6%). It is remarkable that zoning in alkali-feldspars isabsent indicating that they were close to equilibriumconditions.
The trachyandesites and quartz trachytes contain onlyalkali feldspars, represented by K-feldspar, albite andanorthoclase (Table 2, Fig. 3c). The K-feldspar crystalsof the trachyandesites show variation in composition(Or99 to Or58) while those of quartz trachytes are of limitedcompositional range (Or98–Or90). The albite of both rocktypes is rather similar in composition with low An contents(av. An3 Ab92 Or5). The anorthoclase present is sodium-rich and poor in anorthite content (av. An2 Ab84Or14).
Biotite is the principal ferromagnesian mineral in therhyolites and granophyres of W. M. It occurs as subhedralphenocrysts, 0.4–1.0 mm long, as well as microphenocrysts(<0.2 mm long). Its composition is given in Table 3. Biotite
Fig. 3. (a) Or–Ab–An ternary diagram of the analyzed feldspars from therhyolites and subvolcanic rhyolite-porphyry (after Deer et al., 1992); (b) Or–Ab–An ternary diagram of the analyzed feldspars from the granophyres (afterDeer et al., 1992) and (c) Or–Ab–An ternary diagram of the analyzed feldsparsfrom the younger cycle of the Katherina Volcanics (after Deer et al., 1992).Unless otherwise indicated, the symbols used in all given figures are:X = rhyolite and rhyolite porphyry, O = tuffs (crystal, lithic and lapalli tuffs),d = ignimbrite, . = granophyre of W. Khileifiya, $ = granophyre of W. El-Mahash, j = Quatrz-trachyte, h = trachyandesite.
c
in the similar rocks in the other studied areas is very fine orhighly altered to be analyzed by the electron microprobe.The given analyses have totals around 96%; structural
Anorthoclaseo
Albite
Ab An
Oligoclase Andesine Labradorite Bytownite Anorthite
Table 3
Composition of biotites and sodic amphiboles from the older KatherinaVolcanics
Mineral Biotite
Locality Wadi El-Mahash
Roch type Rhyolite Granophyre
Sample 206 200
1 2 3 1 2 3
SiO2 40.001 39.17 39.497 36.891 35.961 36.998TiO2 1.054 1.132 0.919 1.544 2.103 1.991Al2O3 13.027 12.625 11.98 12.978 13.522 13.501FeO 29.163 28.616 29.979 31.425 30.908 30.951MnO 0.092 0.37 0.17 0.127 0.397 0.463MgO 4.699 5.913 5.513 4.372 4.201 3.932CaO 0.216 0.105 0.316 0.122 0.061 0.081Na2O 0.101 0.112 0.314 0.175 0.217 0.301K2O 6.932 7.93 7.732 7.826 8.232 7.864Total 95.285 95.973 96.42 95.46 95.602 96.082
Structural formula based on 22 oxygensSi 5.964 5.846 5.901 5.644 5.513 5.614AlIV 2.036 2.154 2.099 2.338 2.441 2.386AlVI 0.251 0.065 0.009 0 0 0.027Ti 0.118 0.127 0.103 0.178 0.243 0.227Fe2+ 3.636 3.572 3.745 4.021 3.963 3.928Mn 0.012 0.047 0.022 0.016 0.052 0.06Mg 1.044 1.316 1.228 0.997 0.96 0.889Ca 0.035 0.017 0.051 0.02 0.01 0.013Na 0.029 0.032 0.091 0.052 0.065 0.089K 1.319 1.51 1.474 1.528 1.61 1.522Fe/Fe+Mg 0.78 0.73 0.75 0.8 0.8 0.82
Mineral Sodic amphibole
Locality G. El-Homra
Rock type Peralkaline rhyolite
Sample 342
1 2 3 4 5 6 7
SiO2 48.138 47.194 47.425 46.811 47.104 47.717 46.92TiO2 0.961 0.41 1.245 0.992 1.08 0.985 1.473Al2O3 0.261 0.533 0.239 0.312 0.554 0.239 0.468FeO 34.272 33.86 33.577 33.164 32.948 33.563 33.338MnO 0.872 0.785 0.882 0.828 0.772 0.96 0.908MgO 0.092 0.204 0.086 0.201 0.337 0.07 0.17CaO 1.72 0.744 1.625 1.384 1.316 1.874 1.748Na2O 8.015 10.103 8.148 10.227 9.668 8.918 9.63K2O 1.435 0.639 1.425 0.827 1.34 1.398 1.162Total 95.766 94.472 94.652 94.746 95.119 95.724 95.817
Structural formula based on 23 oxygensSi 7.951 7.925 7.945 7.938 7.924 7.981 7.872AlIV 0.049 0.075 0.047 0.062 0.076 0.019 0.092Fe3+ 0 0 0.008 0 0 0 0Ti 0 0 0 0 0 0 0.186T-site 8 8 8 8 8 8 8.151AlVI 0.002 0.03 0 0 0.034 0.028 0Fe3+ 0.33 0.247 0.206 0 0 0 0Ti 0.119 0.052 0.157 0.127 0.137 0.124 0Mg 0.023 0.051 0.021 0.051 0.085 0.017 0.043Fe2+ 4.404 4.508 4.49 4.703 4.635 4.695 4.678Mn 0.122 0.112 0.125 0.119 0.11 0.136 0.129Ca 0 0 0 0 0 0 0.151C-site 5 5 5 5 5 5 5Ca 0.304 0.134 0.292 0.251 0.237 0.336 0.164Na 1.696 1.866 1.708 1.749 1.763 1.664 1.836B-site 2 2 2 2 2 2 2Na 0.871 1.423 0.938 1.614 1.391 1.228 1.297K 0.302 0.137 0.305 0.179 0.288 0.298 0.246A-site 1.174 1.56 1.243 1.793 1.678 1.526 1.545
ArfvedsoniteVI 3+(Al <Fe )
Ferro-Echermannite
VI 3+(Al >Fe )
Ferric-FerronyboiteVI 3+(Al <Fe )
FerronyboiteVI 3+(Al >Fe )
Ferric-nyboiteVI 3+(Al <Fe )
NyboiteVI 3+(Al >Fe )
Magnesio-Arfvedsonite
VI 3+(Al <Fe )
EchermanniteVI 3+(Al >Fe )
0.0
0.5
1.0
2+Fe
Mg/
(Mg+
)
Si in formula
8.0 7.5 7.0 6.5
Fig. 4. Classification of sodic-amphiboles from the studied rhyolites atGebel El-Homra (after Leake, 1997). The diagram parameters are:NaBP1.5; (Mg + Fe2+ + Mn2+) > 2.5; (AlVI or Fe3+) > Mn3+; Li < 0.5;(Mg or Fe2+) > Mn2+; (Na + K)A P 0.50.
M.D. Samuel et al. / Journal of African Earth Sciences 47 (2007) 203–226 211
water and halogens as well as Fe3+ are not taken in consid-eration. The analyzed crystals have restricted composi-tional range in both rhyolites and granophyres and plotin the biotite field on the Fe(t)/(Fe(t) + Mg) vs. AlIV dia-gram (Deer et al., 1966) (not shown here). The biotites ofthe rhyolites have rather higher Si and Mg and lower Ti,Fe2+ and Na contents compared with the biotites of thegranophyres. All biotites are mostly siliceous and Fe-rich(near annite) with an average FeO/MgO ratio of 6.48.
Sodic amphiboles are found as phenocrysts in the rhyo-lites of Gebel El-Homra where they attain 1.0 mm inlength. Groundmass sodic amphiboles, which are opticallysimilar to the phenocrysts, are usually too small to be ana-lyzed by the electron microprobe. The chemical composi-tions of seven representative fresh crystals of sodicamphiboles together with their structural formulae calcu-lated on the basis of 23 oxygens are given in Table 3. Theyhave limited compositional range and plot in the ferro-eck-ermannite-arfvedsonite field on the Si (in formula) vs. Mg/(Mg+Fe2+) diagram (Fig. 4) given by Leake (1997). Mostof the analyzed sodic amphiboles are arfvedsonite(AlVI
6 Fe3+) and only two crystals are ferro-eckermannite(AlVI P Fe3+).
5. Geochemistry
Only few questionable geochemical data are available onthe volcanics at Gebel El-Homra while the geochemistry ofthe volcanics at Wadi El-Mahash and Wadi Khileifiya wasnot studied before.
5.1. Analytical techniques
Both major and selected trace elements were determinedfor 70 samples by X-ray fluorescence spectrometry (Philips
Table 4
Major and trace element composition and calculated normative minerals of the Katherina Volcanics
Oxides Older Katherina cycle
W. El-Mahash
Rhyolite Ignimbrite Pyroclastics Granophyre
V. breccia L.tuff Crystal tuff Banded tuff
206 212 202 203a 209 210 211 213 215 207 205 208 214 0 201 204 200 216 217 218
SiO2 73 75.8 73.3 72.5 74.6 75.85 76.31 74.15 74.92 75.55 74.42 73.3 71.05 74.56 74 72.5 68.6 67.5 66.74 66.78
TiO2 0.36 0.15 0.2 0.32 0.16 0.16 0.13 0.22 0.14 0.1 0.16 0.15 0.26 0.06 0.18 0.3 0.6 0.53 0.68 0.55
Al2O3 11.72 12.47 11.69 11.18 11.94 11.72 11.43 12.15 12.29 10.98 12.47 12.37 14.65 11.7 12.38 12.76 13.72 15.1 14.36 15.16
Fe2O3 3.26 1.7 2.54 3.46 2.27 2.56 2.32 2.94 2.31 1.92 1.92 2.24 3 1.04 2.49 2.9 3.91 3.66 3.96 3.5
MnO 0.03 0.03 0.02 0.04 0.04 0.03 0.03 0.05 0.03 0.02 0.02 0.02 0.04 0.06 0.02 0.02 0.05 0.1 0.09 0.08
MgO 0.18 0.38 0.2 0.19 0.27 0.11 0.11 0.17 0.19 0.35 0.53 0.62 1.01 1.09 0.71 0.93 1.37 0.85 1.19 1
CaO 0.36 0.26 0.25 0.5 0.33 0.16 0.17 0.2 0.26 0.27 0.26 0.33 0.39 0.82 0.26 0.33 1.05 0.6 0.88 0.84
Na2O 1.15 2.7 0.22 1.14 2.72 4.59 2.84 2.97 3.78 1.35 2.29 1.82 3.4 3 0.23 2.59 4.49 3.65 3.72 3.81
K2O 8.46 5.26 10.05 8.6 5.69 3.4 5.45 6.05 4.67 6.23 6.05 5.8 4.66 5.2 6.96 5.45 4.95 6.25 5.4 5.64
P2O5 0.03 0.04 0.04 0.04 0.04 0.05 0.03 0.05 0.03 0.03 0.05 0.05 0.06 0.04 0.05 0.07 0.13 0.14 0.18 0.13
L.O.I 1.32 1.31 1 1.37 1.5 0.56 0.85 0.69 0.86 1.91 1.47 2.53 1.77 2.58 2.6 2.08 1.23 1.65 2.05 1.79
Total 99.87 100.1 99.51 99.34 99.56 99.19 99.67 99.64 99.48 98.71 99.64 99.23 100.29 100.15 99.88 99.93 100.1 100.03 99.25 99.28
Trace elements (in ppm)
Ba 962 798 1183 713 168 58 82 122 68 221 435 252 430 160 468 341 927 1212 988 1048
Rb 157 148 197 172 161 115 150 128 135 188 197 186 178 172 229 178 158 158 143 149
Sr 57 83 27 55 37 53 39 30 31 53 64 63 155 63 45 101 171 207 216 236
Nb 47 24 36 52 36 35 36 34 33 26 26 25 24 28 27 24 25 22 24 22
Zr 730 249 544 776 551 552 531 504 525 250 255 255 250 250 272 404 456 450 488 399
Y 55 61 60 65 59 53 61 54 49 50 52 51 68 51 57 42 35 31 36 33
Cr 9 12 7 12 5 6 7 8 6 6 16 8 28 10 23 32 8 5 8 7
Ni 7 10 9 6 8 7 5 7 5 8 8 5 17 9 22 16 7 9 9 10
Co 2 3 4 2 3 1 2 2 4 1 4 3 6 3 5 5 7 8 9 7
V 8 6 8 5 6 8 7 6 9 10 7 6 23 9 11 20 27 25 36 27
Cu 50 49 33 25 52 95 58 38 31 35 31 27 39 66 25 30 27 51 46 33
Pb 16 37 39 213 20 417 21 176 29 24 197 19 24 149 36 15 19 129 34 73
Zn 31 32 23 21 39 58 31 94 78 30 33 29 71 29 90 62 83 71 89 71
Zr/Nb 15.53 10.375 15.11 14.92 15.31 15.77 14.75 14.82 15.91 9.62 9.8 10.2 10.42 8.93 10.07 16.83 18.24 20.46 20.33 18.14
Ba/Nb 20.47 33.25 32.86 13.71 4.67 1.66 2.28 3.59 2.06 8.5 16.73 10.08 17.92 5.71 17.33 14.21 37.08 55.09 41.17 47.64
Rb/Nb 3.34 6.17 5.47 3.31 4.47 3.29 4.17 3.76 4.09 7.23 7.58 7.44 7.42 6.14 8.48 7.42 6.32 7.18 5.96 6.77
K/Nb 1494 1819 2317 1373 1312 806 1257 1477 1175 1989 1932 19.26 1612 1542 2140 1885 1644 2358 1868 2128
K2O/Na2O 7.36 1.95 45.68 7.54 2.09 0.74 1.92 2.04 1.24 4.61 2.64 3.19 1.37 1.73 30.26 2.1 1.1 1.71 1.45 1.48
Y/Nb 1.17 2.54 1.67 1.25 1.64 1.51 1.69 1.59 1.48 1.92 2 2.04 2.83 1.82 2.11 1.75 1.4 1.41 1.5 1.5
Q 38.82 39.21 33.04 37.71 36.4 35.91 38.64 33.2 34.48 43.94 37.17 40 31.28 34.74 45.38 35.08 19.9 20.11 21.47 20.22
C 1.68 1.96 0 0.79 0.8 0.31 0.62 0.47 0.62 1.63 1.8 2.68 3.45 0 4.19 2.18 0 1.49 1.16 1.51
Z 0.15 0.05 0.11 0.16 0.11 0.11 0.11 0.1 0.11 0.05 0.05 0.05 0.05 0.05 0.06 0.08 0.09 0.09 0.1 0.08
Or 50.84 31.5 60.37 52 34.33 20.4 32.63 36.19 28.02 38.08 36.46 35.49 28 31.51 42.34 32.97 29.65 37.61 32.9 34.25
Ab 1.29 23.14 1.89 1.21 23.5 39.42 24.34 25.43 32.47 11.81 19.76 15.94 29.24 26.02 2 22.43 38.5 31.45 32.45 33.12
An 1.9 1.12 1.32 2.49 1.46 0.51 0.69 0.72 1.14 1.26 1.12 1.45 1.74 3.19 1.13 1.33 2.74 2.49 3.63 3.78
Ac 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Di 0 0 0 0 0 0 0 0 0 0 0 0 0 0.61 0 0 1.58 0 0 0
Wo 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Hy 1.51 1.41 0.9 1.69 1.34 1.05 0.94 1.15 1.13 1.57 1.83 2.37 3.48 2.88 2.7 3.12 3.29 2.78 3.75 3.19
Mt 3.26 1.29 2.09 3.43 1.75 1.94 1.79 2.3 1.78 1.45 1.46 1.67 2.2 0.83 1.81 2.15 2.97 2.83 2.96 2.65
Cm 0 0 0 0 0 0 0 0 0 0 0 0 0.01 0 0.01 0.01 0 0 0 0
Il 0.7 0.29 0.39 0.62 0.31 0.31 0.25 0.42 0.27 0.2 0.31 0.29 0.5 0.12 0.35 0.58 1.15 1.02 1.33 1.07
Ap 0.08 0.1 0.11 0.1 0.1 0.12 0.07 0.12 0.07 0.08 0.13 0.13 0.15 0.1 0.13 0.18 0.32 0.37 0.46 0.33
D. I. 90.95 93.85 95.3 90.92 94.23 95.73 95.61 94.82 94.97 93.83 93.39 91.43 88.52 92.27 89.72 90.48 88.05 89.17 86.82 87.59
212M
.D.
Sa
mu
elet
al.
/J
ou
rna
lo
fA
frican
Ea
rthS
ciences
47
(2
00
7)
20
3–
22
6
Oxi
des
Old
erK
ath
erin
acy
cle
W.
Kh
ilei
fiya
G.
El-
Ho
mra
Rh
yoli
teG
ran
op
hyr
eR
hyo
lite
102
103
105
107
108
109
112
114
116
123
124
126
127
306
308
312
314a
317
319
323
324
328
329
330
331
SiO
271
.92
74.1
570
.970
.25
73.8
473
.54
72.3
574
.32
73.0
874
.46
74.7
277
.35
77.9
275
.35
75.4
274
.88
75.2
775
.33
75.3
374
.88
76.2
274
.75
74.8
876
.57
73.5
5
TiO
20.
250.
180.
260.
260.
160.
180.
240.
170.
220.
140.
180.
090.
090.
170.
210.
20.
230.
250.
140.
160.
240.
280.
230.
210.
22
Al 2
O3
12.9
312
.52
13.8
314
12.8
612
.85
13.6
13.2
113
.19
1212
.62
11.7
211
.66
10.4
910
.29.
8311
.08
10.7
810
.62
10.5
510
.77
10.6
310
.96
9.64
10.4
Fe 2
O3
2.83
3.17
3.13
2.46
2.6
2.9
3.24
2.73
2.55
2.14
1.42
1.53
1.42
3.35
3.27
3.15
33.
152.
621.
82.
943.
33.
12.
733
Mn
O0.
060.
030.
030.
040.
030.
020.
040.
040.
040.
030.
020.
020.
010.
050.
040.
020.
050.
080.
030.
020.
050.
060.
030.
10.
16
MgO
0.45
0.07
0.07
0.52
0.12
0.1
0.25
0.09
0.1
0.27
0.19
0.16
0.22
0.1
0.05
0.13
0.05
0.06
0.05
0.05
0.15
0.19
0.05
0.1
0.08
CaO
0.86
0.45
0.45
1.08
0.13
0.19
0.11
0.15
0.51
0.56
1.2
0.25
0.5
0.13
0.11
0.27
0.14
0.14
0.38
10.
220.
260.
290.
390.
39
Na 2
O3.
655
5.43
5.27
4.59
4.33
4.56
4.43
5.14
4.27
3.48
3.96
4.35
0.18
0.34
0.24
2.33
0.14
0.2
0.22
0.36
0.16
0.15
0.16
0.15
K2O
5.55
3.79
3.88
4.4
4.9
4.92
4.74
4.72
3.93
4.11
5.36
4.42
3.87
8.62
7.56
8.47
7.04
8.1
8.95
8.91
7.83
8.45
8.99
7.66
8.47
P2O
50.
050.
030.
040.
040.
020.
030.
050.
030.
030.
030.
030.
010.
020.
010.
020.
050.
020.
020.
030.
030.
050.
030.
020.
050.
04
L.O
.I1.
320.
531.
331.
090.
470.
570.
40.
40.
61.
060.
930.
630.
551.
041.
351
0.72
1.06
0.91
1.54
1.43
1.38
0.87
1.44
1.59
To
tal
99.8
799
.92
99.3
599
.41
99.7
299
.63
99.5
810
0.29
99.3
999
.07
100.
210
0.1
100.
6199
.49
98.5
798
.24
99.9
399
.11
99.2
699
.16
100.
399
.49
99.5
799
.05
98.0
5
Tra
ceel
emen
ts(i
np
pm
)
Ba
867
749
1019
1109
969
846
1049
884
1025
921
758
332
226
182
172
228
101
305
162
122
389
613
154
284
615
Rb
101
104
8391
9710
789
9410
311
896
123
137
229
203
292
169
167
204
179
189
221
216
177
186
Sr
5336
8710
134
3434
3543
5471
2832
3227
3828
2729
3228
3325
2744
Nb
2323
2021
2124
2323
2321
1726
3181
6965
7368
6469
6059
5861
61
Zr
386
401
371
378
399
383
404
400
383
370
355
516
549
1073
831
888
854
790
838
914
733
607
665
729
660
Y42
4034
3741
3942
4238
3636
4448
8193
8196
8382
8770
5963
7772
Cr
1110
109
811
810
1311
1311
1317
1516
812
1415
910
65
9
Ni
1413
1213
1015
1012
1913
1311
1313
1515
108
1212
510
48
9
Co
42
32
12
43
14
32
12
13
34
24
12
36
8
V8
1010
89
810
98
107
89
65
2110
86
65
98
65
Cu
685
1511
410
525
518
4937
4212
319
529
3722
355
1213
412
419
4
Pb
2623
316
248
483
1913
0343
1150
699
8451
440
3550
391
7320
7133
322
1640
9827
0
Zn
1044
1517
6742
8852
8397
2829
2310
288
3233
605
1128
3025
6855
Zr/
Nb
16.7
817
.43
18.5
518
1915
.96
17.5
717
.39
16.6
517
.62
20.8
819
.84
17.7
113
.24
12.0
413
.66
11.7
11.6
213
.09
13.4
512
.22
10.2
911
.47
11.9
510
.82
Ba/
Nb
37.6
932
.57
50.9
552
.81
46.1
435
.25
45.6
138
.43
44.5
743
.86
44.5
912
.77
7.29
2.25
2.49
3.51
1.38
4.49
2.53
1.77
6.48
10.3
92.
664.
6610
.08
Rb
/Nb
4.39
4.52
4.15
4.33
4.62
4.46
3.78
4.09
4.48
5.62
5.65
4.73
4.42
2.83
2.94
4.49
2.32
2.46
3.19
2.59
3.15
3.75
3.72
2.9
3.05
K/N
b20
0313
6816
1017
3919
3717
0217
1117
0414
1816
2526
1714
1110
3688
391
010
8280
198
911
6110
7210
8311
8912
8710
4211
53
K2O
/Na 2
O1.
520.
760.
710.
831.
071.
141.
041.
070.
760.
961.
541.
120.
8947
.89
22.2
435
.29
3.02
57.8
644
.75
40.5
21.7
552
.81
59.9
347
.88
56.4
7
Y/N
b1.
831.
741.
71.
761.
951.
631.
831.
831.
651.
712.
121.
691.
551
1.35
1.25
1.32
1.22
1.28
1.26
1.17
11.
091.
261.
18
Q27
.44
29.6
723
.25
20.6
227
.99
28.9
227
29.7
727
.23
32.9
731
.85
36.6
336
.57
41.0
945
.05
38.1
734
.78
43.6
139
.46
38.8
43.7
541
.09
39.1
746
.17
38.9
6
C0
00
00
0.05
0.82
0.55
00
00
00.
641.
330
01.
580
01.
410.
790.
50.
480.
25
Z0.
080.
080.
080.
080.
080.
080.
080.
080.
080.
080.
070.
10.
110.
220.
170.
180.
170.
160.
170.
190.
150.
120.
140.
150.
14
Or
33.3
322
.57
23.4
326
.48
29.2
229
.39
28.2
927
.96
23.5
424
.81
31.9
526
.27
22.8
751
.83
46.0
450
.72
4248
.953
.85
53.9
946
.89
50.9
853
.92
46.4
451
.45
Ab
31.3
842
.63
46.9
445
.439
.18
37.0
338
.96
37.5
744
.08
36.9
29.7
33.7
36.8
1.55
2.96
2.06
17.9
61.
211.
721.
913.
091.
381.
291.
391.
3
An
2.59
0.55
2.01
1.61
0.02
1.02
0.49
0.79
1.37
1.51
3.07
1.18
0.87
0.65
0.48
0.75
00.
671.
691.
520.
891.
291.
381.
742.
18
Ac
00
00
00
00
00
00
00
00
1.7
00
00
00
00
Di
1.37
1.5
0.28
3.16
0.69
00
01.
11.
161.
490.
081.
270
00.
280.
530
0.08
0.95
00
00
0
Wo
00
00
00
00
00
00
00
00
00
00.
960
00
00
Hy
1.17
0.28
1.6
0.31
0.58
0.97
1.41
0.9
0.3
0.74
00.
760.
291.
191.
155.
030.
941.
050.
730
1.13
1.3
0.78
1.14
3.15
Mt
2.19
2.44
1.99
1.93
2.06
2.27
2.52
2.14
1.98
1.64
1.1
1.18
1.09
2.64
2.49
2.45
1.55
2.43
2.1
1.44
2.26
2.57
2.46
2.1
2.32
Cm
00
00
00
00
00
00
00
00
00
00
00
00
0
Il0.
480.
340.
50.
50.
310.
350.
460.
320.
420.
270.
340.
170.
170.
330.
410.
380.
440.
480.
270.
310.
460.
540.
440.
410.
43
Ap
0.13
0.08
0.11
0.1
0.06
0.08
0.15
0.08
0.08
0.08
0.08
0.03
0.05
0.03
0.05
0.12
0.05
0.05
0.07
0.07
0.13
0.08
0.05
0.13
0.11
D.
I.92
.15
94.8
793
.62
92.5
96.3
995
.34
94.2
595
.394
.85
94.6
893
.596
.696
.24
94.4
794
.05
90.9
594
.74
93.7
295
.03
94.7
93.7
393
.45
94.3
894
91.7
1
(co
nti
nu
edo
nn
ext
pa
ge)
M.D. Samuel et al. / Journal of African Earth Sciences 47 (2007) 203–226 213
Tab
le4
(conti
nued
)
Oxi
des
Old
erK
ath
erin
aV
olc
anic
sY
ou
nge
rK
ath
erin
aV
olc
anic
s
G.
El-
Ho
mra
W.
Kh
ilei
fiya
G.
El-
Ho
mra
Rh
yoli
teIg
nim
bri
teP
yro
clas
tics
Rh
.P
orp
.T
rach
yan
des
ite
Qu
atrz
trac
hyt
eA
myg
dal
oid
al
trac
hyt
eL
apil
litu
ffC
ryst
altu
ffB
and
edtu
ffL
ava
Tu
ff
333
335
337
342a
315a
320a
321
325
307
318
338
309
310
326
305
332
117
125
115
120
302
304
340
311a
311b
SiO
274
.22
73.4
674
.973
.55
76.8
7580
.15
75.1
380
79.3
72.1
175
.33
7474
74.6
75.7
162
.05
56.6
257
.459
.65
58.6
57.1
162
.95
50.8
849
.43
TiO
20.
170.
180.
170.
260.
240.
150.
160.
180.
20.
140.
220.
190.
190.
250.
220.
141.
081.
621.
261.
131.
361.
60.
71.
351.
36
Al 2
O3
11.3
212
.47
11.0
211
.62
9.39
9.41
7.8
10.5
38.
598.
611
.95
10.3
11.6
210
.96
10.1
10.4
916
.16
15.0
316
.44
14.5
714
.23
13.4
214
.413
.13
12.9
5
Fe 2
O3
2.83
2.04
2.82
3.13
3.3
3.05
1.9
2.9
2.66
2.23
3.42
10.3
33.
292.
861.
796.
148.
17.
77.
310
.512
.23
7.65
10.5
19.
89
Mn
O0.
030.
070.
030.
050.
040.
230.
030.
10.
050.
040.
0410
.30.
030.
040.
080.
020.
140.
120.
170.
110.
160.
140.
180.
460.
63
MgO
0.13
0.07
0.15
0.13
0.08
0.09
0.04
0.03
0.08
0.06
0.12
10.3
0.06
0.07
0.07
0.07
1.99
3.29
2.98
2.36
1.43
1.96
0.85
0.43
1.33
CaO
0.29
0.28
0.11
0.24
0.43
0.45
0.17
0.25
0.24
0.39
0.42
10.3
0.42
0.46
0.91
0.48
3.63
3.43
4.95
3.15
1.88
2.15
2.1
5.29
7.65
Na 2
00.
220.
360.
243.
470.
120.
140.
180.
080.
080.
160.
2210
.30.
240.
180.
20.
144.
773.
924.
524.
543.
864.
323.
520.
510.
46
K2O
9.42
9.85
8.59
5.86
8.56
8.66
6.86
8.72
7.8
7.38
7.53
10.3
8.59
8.83
8.77
8.83
2.93
3.58
2.69
2.66
5.05
4.11
5.34
9.92
9.55
P2O
50.
040.
040.
050.
020.
030.
050.
040.
030.
020.
020.
0310
.30.
040.
040.
040.
030.
330.
540.
430.
520.
520.
660.
20.
620.
56
L.O
.I1.
611.
151.
271.
050.
911.
940.
991.
980.
650.
982.
7310
.31.
221.
561.
551.
10.
974.
451.
694.
552.
22.
852.
266.
816.
53
To
tal
100.
399
.97
99.3
599
.38
99.9
99.1
798
.32
99.9
310
0.37
99.3
98.7
910
.399
.41
99.6
899
.498
.810
0.19
100.
710
0.23
100.
5499
.79
100.
5510
0.15
99.9
110
0.34
Tra
ceel
emen
ts(i
np
pm
)
Ba
303
346
179
113
129
365
114
253
8213
124
936
418
514
022
538
680
690
770
481
616
6516
5723
0710
9910
32
Rb
197
208
161
163
166
178
194
219
204
198
218
170
266
189
249
210
9385
8370
170
119
131
285
271
Sr
4232
2550
3878
3518
2533
7336
2229
3628
430
324
476
312
220
272
385
8071
Nb
4246
6760
7068
5836
6260
6971
6858
4747
1920
2018
2929
3123
21
Zr
349
317
862
743
839
934
668
737
741
819
931
906
819
623
528
563
247
231
231
248
331
379
315
335
293
Y46
3591
7497
9473
8584
8296
9290
6561
5827
2927
2444
4747
4346
Cr
76
58
1121
2017
1716
915
108
148
328
268
109
1010
12
Ni
68
610
714
1615
1411
711
96
157
3211
2714
810
139
16
Co
24
53
23
34
12
23
22
23
1818
2216
1215
610
6
V10
87
57
85
78
99
97
77
1011
215
114
811
610
2510
1316
Cu
108
5912
05
1043
2034
2819
175
4615
1638
117
3818
1829
1011
534
28
Pb
4785
1330
114
732
896
2439
446
1516
350
4715
195
599
151
379
250
158
3334
9952
Zn
3420
1916
054
1037
1591
1154
5731
4177
6487
9910
570
159
184
172
157
143
Zr/
Nb
8.31
6.89
12.8
712
.38
11.9
913
.74
11.5
120
.47
11.9
513
.65
13.4
912
.761
12.0
410
.74
11.2
11.9
813
11.5
511
.55
13.7
811
.41
13.0
710
.16
14.5
713
.95
Ba/
Nb
7.21
7.52
2.67
1.88
1.84
5.37
1.97
7.03
1.32
2.18
3.61
5.13
2.72
2.41
4.79
8.21
42.4
245
.35
35.2
45.3
357
.41
57.1
474
.42
47.7
849
.14
Rb
/Nb
4.69
4.52
2.4
2.72
2.37
2.62
3.34
6.08
3.29
3.3
3.16
2.39
3.91
3.26
5.3
4.47
4.89
4.25
4.15
3.89
5.86
4.1
4.23
12.3
912
.9
K/N
b18
6217
7810
6481
110
1510
5798
220
1110
4410
2190
610
3210
4912
6415
4915
6012
8014
8611
1712
2714
4611
7614
3035
8037
75
K2O
/Na 2
O42
.82
27.3
635
.79
1.69
71.3
361
.86
38.1
110
997
.546
.13
34.2
336
.79
35.7
949
.06
43.9
63.0
70.
610.
910.
60.
591.
310.
951.
5219
.45
20.7
6
Y/N
b1.
10.
761.
361.
231.
391.
381.
262.
361.
351.
371.
391.
31.
321.
121.
31.
231.
421.
451.
351.
331.
521.
621.
521.
872.
19
Q36
.38
33.2
640
.73
30.9
742
.95
39.6
653
.93
41.4
649
.57
49.1
42.1
139
.44
39.1
938
.55
39.2
40.8
512
.87
8.62
6.71
13.0
310
.66
8.95
16.1
2–
–
C0.
310.
781.
260
00
00.
660
02.
840
1.27
0.36
00
00
00
0.1
00
––
Z0.
070.
060.
180.
150.
170.
190.
140.
150.
150.
170.
20.
180.
170.
130.
110.
120.
050.
050.
050.
050.
070.
080.
06–
–
Or
56.5
58.9
751
.83
35.2
851
.19
52.1
841
.74
52.7
946
.62
43.8
746
.42
52.9
51.7
853
.27
5353
.46
17.5
122
.09
16.2
116
.63
30.7
625
.04
32.3
7–
–
Ab
1.89
3.09
2.07
27.6
50.
650.
191.
570.
170.
681.
361.
942.
062.
071.
551.
731.
2140
.82
34.6
338
.99
40.6
233
.66
37.6
830
.55
––
An
1.29
1.25
0.28
00
00.
221.
140.
040.
962.
070.
981.
922.
110.
811.
9814
.26
13.4
817
.01
12.2
56.
625.
318.
05–
–
Ac
00
01.
990.
340.
90
00
00
00
00
00
00
00
00
––
Di
00
00.
971.
743.
310.
360
0.91
0.79
00.
570
01.
790.
321.
730.
784.
460.
840
1.45
1.73
––
Wo
00
00
01.
40
00
00
00
00.
650
00
00
00
0–
–
Hy
0.94
0.56
1.12
1.08
0.14
00.
31.
160.
290.
971.
551.
030.
930.
960
0.39
6.09
10.9
28.
610
.49
7.26
8.98
4.17
––
Mt
2.28
1.67
2.22
1.49
2.43
1.94
1.45
2.21
1.42
2.6
2.54
2.57
2.35
2.59
2.27
1.44
45.
074.
674.
717.
198.
035.
43–
–
Cm
00
00
00
00
00
00
00
00
0.01
00.
010
00
0–
–
Il0.
330.
350.
330.
50.
460.
290.
310.
350.
380.
270.
440.
370.
370.
480.
430.
272.
073.
212.
440.
262.
663.
131.
36–
–
Ap
0.1
0.1
0.13
0.05
0.07
0.12
0.1
0.08
0.05
0.05
0.08
0.08
0.1
0.1
0.1
0.08
0.8
1.35
1.05
1.32
1.32
1.66
0.51
––
D.
I.94
.77
95.3
294
.63
93.9
94.7
992
.03
97.2
494
.42
96.8
794
.33
90.4
794
.493
.04
93.3
793
.95
95.5
271
.265
.34
61.9
170
.28
75.0
871
.67
79.0
4–
–
aP
eral
kal
ine.
214 M.D. Samuel et al. / Journal of African Earth Sciences 47 (2007) 203–226
35 45 55 65 750
5
10
15
SiO2
ON
+K
2Oa 2
Rhyolite
Trachydacite(q> 20%)
Trachyte(q< 20%)
DaciteAndesite
Trachy-andesite
Fig. 5. Total alkalis-silica (TAS) diagram for the studied volcanic rocks(after Le Maitre et al., 1989).
M.D. Samuel et al. / Journal of African Earth Sciences 47 (2007) 203–226 215
PW 1480). Concentrations of the major elements wereobtained on fused lithium-metaborate discs while the traceelements were determined on pressed pellets. The analyticalprecision for the major oxides was found to be better than1%; however, the error may reach 10% for MnO and P2O5
due to their low concentration. Precision for trace elementsconcentrations is estimated as better than 5%. Loss on igni-tion (L.O.I.) was determined by heating powdered samplesfor 50 min at 1000 �C.
The concentration of the REE in seven samples repre-senting the rhyolites and the granophyres of the olderKatherina cycle were determined by ICP-mass spectrome-try (Perkin–Elmer Sciex Elan 6000). The ICP-MS was opti-mized to CeO/Ce-ratio of 2.5% at 400000 CPS for 10 ppbCe. All masses were measured for 1500 ms. Detection limitsfor REE are about 0.004 ppb. The analytical procedurewas as follows: (1) 100 mg of the sample was dissolved with3 ml of HF and 3 ml of HClO4 at 180 �C for 8 h in a Teflonbeaker. (2) The sample was then evaporated to near dry-ness and the residue was dissolved in 3 ml HNO3, and thisprocedure is repeated three times and then taken into solu-tion with 2 ml of HNO3 and finally diluted to 100 ml. (3)10 ppb of Rh and Re were added as internal standards;trace elements with masses above 138 were normalized toRe, whereas those with lower masses were normalized toRh.
All the analyses were carried out at the Institute of Min-eralogy and Mineral Resources, Clausthal Technical Uni-versity, Germany. Most samples were also analyzed formajor and trace elements concentrations by X-ray fluores-cence spectrometry at Saudi Geological Survey, Jeddah,Saudi Arabia. Discrepancy between the two analyses forthe major elements was in the order of 1–2% and neverexceeded 5% for trace elements.
5.2. Major elements
Major and trace elements contents of selected rockstogether with their Barth–Niggli molecular norms arepresented in Table 4. The older cycle of the KatherinaVolcanics is composed of acidic volcanic-subvolcanics(66.7–77.9 wt.% SiO2) together with rhyolitic falloutpyroclastics and ignimbrites (71.1–80.2 wt.% SiO2). It isto be noted that few tuffs and ignimbrites contain morethan 76 wt.% SiO2 which can be attributed to the pres-ence of secondary silica as proved microscopically. Theyounger cycle includes a narrow range of alkaline inter-mediate rocks represented by trachyandesite and quartztrachyte (56.6–62.9 wt.% SiO2). The chemical data ofthe amygdaloidal quartz trachytes of Gebel El-Homrareflect abnormally high CaO (5.3–7.7%) and L.O.I.(6.5–6.8%) and low SiO2 (49.4–50.9 wt.%) contents. Thisis attributed, as proved microscopically, to the presenceof a high percentage of amygdales filled by secondarycalcite, quartz and chlorite. These chemical data willnot be discussed further and are excluded from the fol-lowing graphs and calculations.
The obtained data demonstrate that the granophyres ofthe older Katherina cycle as well as the alkaline intermedi-ate volcanics of the younger cycle have normal Na2O andK2O contents. On the other hand, most of the analyzedsamples representing the felsic volcanics and their pyroclas-tics in both W. M. and G. H. are enriched in K2O content(5.3–10.1 wt.%) and depleted in Na2O content (0.08–2.97 wt.%); the felsic volcanics of W. Kh. have normal con-tents of alkalis. The wide variation in the contents of alkalisin some of the analyzed rocks will be discussed later indetails.
On the TAS diagram, recommended by the IUGS (LeMaitre et al., 1989) for the classification of volcanic rocks,all the analyzed older lava flows and their subvolcanicequivalents together with the pyroclastics plot in the rhyo-lite field; the granophyres of W. M. plot near the boundaryseparating between rhyolites and trachydacite (q > 20%).The younger alkaline intermediate volcanics fall in thefields of trachyte (q < 20%) and trachyandesite (Fig. 5).
On the A/CNK vs. A/NK diagram (Fig. 6) of Maniarand Piccoli (1989), the alkaline rocks of the older Katherinacycle are peraluminous to metaluminous, while the youngercycle is metaluminous in character. This is substantiated bythe normative corundum content (0.05–4.19%) in the pera-luminous rocks. The rhyolites and rhyolitic ignimbrites ofperalkaline affinity having molecular ratio (Na2O + K2O)/Al2O3 greater than 1 (samples 203, 314, 315, 320 & 342),are further subdivided into comendite and pantellerite usingthe diagram of Al2O3 vs. total iron as FeO (after Macdon-ald, 1974) as recommended by the IUGS (Le Maitre et al.,1989). On this diagram (Fig. 7) all the analyzed peralkalinerocks are described as comendites. Most of these rocks con-tain acmite in their norms.
The major elements variation diagrams (Harker dia-grams, Fig. 8) demonstrate that the analyzed rocks of theolder Katherina cycle show relatively smooth variationwith gradual decrease in Al2O3, Fe2O3, TiO2, MgO and
0.5 1.0 1.5 2.00.4
0.8
1.2
1.6
2.0
2.4
2.8
Peralkaline
MetaluminousPeraluminous
AN
K
ACNK
Fig. 6. Al2O3/(Na2O + K2O) vs. Al2O3/(CaO + Na2O + K2O) diagramfor the studied Katherina Volcanics (after Maniar and Piccoli, 1989).
216 M.D. Samuel et al. / Journal of African Earth Sciences 47 (2007) 203–226
P2O5 with increasing silica, while Fe2O3, TiO2, MgO andP2O5 contents decrease gradually in the younger Katherinacycle with increasing silica. Scattering in the variation dia-grams is attributed to the porphyritic nature of some ofthese rocks (Cox et al., 1979; Wilson, 1994). They alsodemonstrate that the examined Katherina Volcanics com-prise two separate suites.
5.3. Trace elements
The contents of the estimated trace elements and theirvariation trends in the studied rocks show the followingremarks:
– The analyzed rhyolites in the three studied areas havehighly comparable contents of transition metals (V,Cr, Co, Ni, Cu & Zn). W. M. and W. Kh. rhyolites have
0 2 4 6 8 10 12
5
9
13
17
21
FeO(t)
AlO
32
Coe d ic rh
lit
mn it
yoe
Pe
ri icy
it
ant llet
rhol e
Fig. 7. Separation of peralkaline volcanics in the studied KatherinaVolcanics into comendite and pantellerite types using Al2O3 vs. FeO(t)
diagram of Macdonald (1974).
rather comparable contents of HFSE (Y, Zr & Nb) andLILE (Ba, Rb & Sr). On the other hand, G. H. rhyoliteshave remarkably high HFSE and Rb contents togetherwith pronounced depletion in Ba and Sr. Generally,the obtained values of all the analyzed rhyolites matchwell with the averages and ranges of 148 samples ofA-type granites given by Whalen et al. (1987).
– The studied ignimbrites show a narrow variation rangeof most trace elements. Few elements show wide varia-tion, where, for example, Ba varies from 58 to1183 ppm, Cu from 10 to 95 ppm and Zn from 10 to58 ppm. The HFSE Nb, Zr and Y are relatively moreenriched in the ignimbrites of G. H. (with average con-tents of 69, 887 and 96 ppm, respectively) relative to W.M. ignimbrites (with average contents of 39, 591 and60 ppm, respectively).
– The fallout pyroclastics are characterized by a widerange of some trace elements. Barium varies from 68to 468 ppm, Sr from 18 to 155 ppm, Cu from 15 to175 ppm, and Zn from 11 to 94 ppm. The transitionmetals Cr, Ni, Co & V as well as Rb do not show muchvariation. The HFSE Nb, Zr & Y are usually muchmore enriched in the pyroclastics of G. H. (with aver-ages of 60, 781 & 83 ppm, respectively) as comparedto those of W. M. (with averages of only 27, 329 &53 ppm, respectively).
– The granophyres of W. M. (av. 67.4 wt.% SiO2) havehigher contents of Ba, Sr, Co, V and Zn and lower con-tents of Zr and Y compared to the granophyres of W.Kh. (av. 77.6 wt.% SiO2). The less differentiated charac-ter of W. M. granophyres is also indicated in the differ-entiation index values (Thornton and Tuttle, 1960)which average 87.9, while it averages 96.4 in W. Kh.granophyres.
– The younger cycle of the studied Katherina Volcanicsrepresented by alkaline intermediate rocks (56.62–62.95 wt.% SiO2) are generally enriched in Ba, Sr, Cr,Ni, Co, V and Zn and depleted in Rb, Nb, Y and Cuas compared with the older felsic suite. They have highercontents of Sr and V and lower contents of Ba, Rb, Nb,Zr, Y and Zn at W. Kh. relative to the similar rocks ofG. H. Besides, W. Kh. samples are relatively enriched inAl2O3, MgO, CaO, Na2O and depleted in Fe2O3 & K2Orelative to the samples of G. H. having the same silicacontent.
The distribution of some trace elements vs. silica (Har-ker diagrams) in the studied volcanic rocks is shown inFig. 9. The LILE (Ba, Rb & Sr) show different behaviourin the older Katherina felsic suite. Barium and Sr contentsgenerally decrease with silica increase, while Rb showserratic distribution. The abundance of these elements inthe older felsic suite is generally controlled by the crystalli-zation of feldspars and biotite (Green, 1980; Wilson, 1994).The low concentration of Sr in rhyolites (av. 43 ppm)reflects a low Sr content for the original melt as rhyolitesrepresent a liquid or near liquid composition. The high
0
1
2
3
4
5
0
10
20
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0
1
2
3
4
5
6
0
1
2
0
1
2
3
4
0
10
20
AlO 2
3
0
10
20
FeO 2
3
TiO
2
gOM
aOC K
O 2
aO
N2
50 60 70 80
SiO2
90
PO 2
5
50 60 70 80
SiO2
90
Fig. 8. The relation between SiO2 and major oxides in the studied Katherina Volcanics.
M.D. Samuel et al. / Journal of African Earth Sciences 47 (2007) 203–226 217
concentration of Ba in W. M. & W. Kh. rhyolites (av.920 ppm) relative to G. H. rhyolites (av. 267) is most prob-ably related to the K-feldspars content as evidenced by theresults of REE given in Section 5.4. Biotite, proved to bepresent in the rhyolites of the first two localities, may haveplayed an additional role in their Ba enrichment.
The subvolcanic granophyres and the rhyolite-porphyryshow also marked variation in the LILE. The highly differ-entiated W. Kh. granophyres and G. H. rhyolite-porphyry(74.6–77.9 wt.% SiO2) have lower contents of Ba & Sr
(225–386 ppm Ba and 28–36 ppm Sr) while the less differ-entiated W. M. granophyres (66.7–68.6 wt.% SiO2) contain927–1212 ppm Ba and 171–236 ppm Sr. The latter grano-phyres contain biotite as an essential mineral constituentand their CaO contents are higher than the former subvol-canic rocks. It is to be noted that all analyzed subvolcanicshave rather comparable contents of Rb (123–249 ppm).
The analyzed rhyolitic lavas and rhyolitic ignimbriteswith narrow range of silica content (70.3–76.8 wt.%SiO2) show marked variation in Nb (17–81 ppm), Zr
10
20 A- Within Plate Basalts B- Island Arc Basalts C- Mid Ocean Ridge Basalts
A
Zr/
Y
B
C
110 100 1000
Zr
0
1000
2000
3000
0
100
200
300
10
20
30
40
50
60
70
80
90
50 60 70 80 900
1000
2000
50 60 70 80 9020
30
40
50
60
70
80
90
100
0
100
200
300
400
500
Nb
SrB
a Rb
Y
Zr
SiO2 SiO2
Fig. 9. The relation between SiO2 and some trace elements in the studied Katherina Volcanics.
218 M.D. Samuel et al. / Journal of African Earth Sciences 47 (2007) 203–226
(249–1073 ppm) and Y (34–97 ppm) contents. The rhyo-lites and ignimbrites with normal K contents have lowerNb, Zr and Y contents, whereas those rich in K haveabnormally higher contents. The bivariate plots of Nb,Zr and Y against K2O (not shown here) show two popula-tions with an apparent gap. The first population has rathercomparable contents of these elements not changing withthe slight increase in K2O content. The K-rich populationhas almost a vertical trend with a peak around 8.5% K2O.
5.4. Rare earth elements
Concentrations of fourteen REE (La to Lu, except Pm)were determined for seven samples from the older Kathe-rina felsic suite, and given in Table 5. The chondrite-nor-
malized REE patterns are subparallel (using thechondrite values of Evensen et al., 1978) (Fig. 10). Thesamples show moderate light REE enrichment and ratherflat heavy REE patterns. The negative Eu anomaly [(Eu/Eu*)n = 0.12–0.60] is striking in these rocks and it mightindicate early fractionation of plagioclase in the magmachamber below before the eruption of the rhyolites or pla-gioclase was retained at depth as a refractory restite. Theinterrelation between the Eu anomaly and the Ba and Srconcentrations in both the analyzed subvolcanics (samples200, 216, 127 & 305) or rhyolites (samples 206, 107 & 342)reveals that the strong negative Eu anomalies are mostlyassociated with low Ba and Sr concentrations indicatingextreme K-feldspar fractionation. The effect of alkali feld-spar fractionation on the evolution of the studied suite
Table 5Contents of REE and some trace elements in the rhyolites and granophyres of the older Katherina cycle
Locality W. El-Mahash W. Khileifiya G. El-Homra
Rock type Rhyolite Granophyre Rholite Granophyre Rholite Rh. Porp.
Sample no. 206 200 216 107 127 342a 305
La 43.9 47 39.7 14.3 29.6 46.8 33.6Ce 76.2 98.5 86.1 36.7 64 100.2 82.8Pr 13.7 13 11 4.53 7.87 13.5 9.71Nd 56.6 50.3 43 17.7 30.8 54.6 38.9Sm 11.04 9.23 8.4 3.82 6.51 12.1 8.84Eu 0.65 1.03 1.33 0.73 0.24 0.7 0.57Gd 10.18 8.1 7.18 3.58 6.13 11.68 9.6Tb 1.46 1.08 0.98 0.55 0.96 1.79 1.58Dy 8.83 6.18 5.87 3.63 6.47 10.93 10.1Ho 1.78 1.23 1.16 0.79 1.36 2.19 2.01Er 5.28 3.56 3.42 2.48 4.26 6.51 5.55Tm 0.82 0.53 0.51 0.41 0.68 1 0.83Yb 5.54 3.55 3.39 2.84 4.66 6.53 5.41Lu 0.84 0.54 0.51 0.43 0.68 0.95 0.8Sum(REE) 236.82 243.83 212.55 92.49 164.22 269.48 210.03(Ce/Yb)n 3.45 6.96 6.37. 3.26 3.45 3.86 3.85(Tb/Yb)n 1.14 1.32 1.25 0.84 0.89 1.19 1.26(Eu/Eua)n 0.19 0.36 0.53 0.6 0.12 0.18 0.19Hf 16.6 10.97 10.48 9.26 16.8 17.4 13.4Ta 2.27 1.1 1.16 0.92 1.39 2.89 1.96Th 6.39 10.43 11.34 3.67 7.3 6.57 8.55Pb 28.2 10.7 16.7 11.2 53.4 30.1 70U 3.61 3.84 3.83 2.84 4.87 2.63 1.56
a Peralkaline rhyolite.
M.D. Samuel et al. / Journal of African Earth Sciences 47 (2007) 203–226 219
can be investigated by plotting Eu/Eu* vs. Ba (after Eby,1990) (Fig. 11), which shows that the studied suite depictstrends parallel or subparallel to the alkali feldspar fraction-ation trend.
6. Tectonic setting
The tectonic environment of the Katherina volcanic-sub-volcanic rocks is interpreted by an integrated approachusing all the gained information. The studied Katherina vol-
2
10
100
250
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Sam
ple/
C1
Cho
ndri
te
Rhyolite of W. Khileifiya (107)Rhyolite of W. El-Mahash (206)Rhyolite of G. El-Homra (342)Rhyolite-porphyry of G. El-Homra (305)Granophyre of W. Khileifiya (127)Granophyre of W. El-Mahash (200 & 216)
Fig. 10. Chondrite-normalized REE patterns for the rhyolites andgranophyres of the older Katherina cycle.
canic activity produced mainly rhyolites; ignimbrites andpyroclastic rocks of rhyolitic composition are prominent.At a later stage, quartz trachytes and trachyandesites wereextruded, which are less abundant than the rhyolites. Kathe-rina Volcanics erupted during an anorogenic period, shortlyafter the end of the Pan-African orogeny. This stage is char-acterized by the radical change from the subduction-related
0.01 0.1 1040
100
1000
10000
Eu/Eu*
Ba
P
AF
1
Fig. 11. Ba vs. Eu/Eu* for the rhyolites and granophyres of the olderKatherina cycle. Alkali feldspar. (AF) and plagioclase (P) fractionationtrends for high silica magma are after Eby (1990).
50 100 1000 50001
10
100
200
(+
a 2O
)/CaO
KO
N2
Zr+Nb+Ce+Y
FG
OGT
A-type
ig. 13. Zr+Nb+Ce+Y vs. (K2O + Na2O)/CaO binary diagram for somehyolites and granophyres of the older Katherina cycle (after Whalent al., 1987).
220 M.D. Samuel et al. / Journal of African Earth Sciences 47 (2007) 203–226
calc-alkaline magmatism to the post-tectonic alkaline (per-alkaline) magmatism. This alkaline magmatism was gener-ated during a period transitional from orogenic activity tointra-plate stable conditions and was characterized byextensional tectonics. The stable platform condition ismarked (in Taba-Nuweiba) by peneplanation and sedimen-tation of undifferentiated early Paleozoic sandstones orCambrian sandstones of the Araba Formation (El-Arabyand Abdel-Motelib, 1999) equivalent to Cambrian AmudeiShelomo Formation in southern Negev (Parnes, 1971).These sandstones lie unconformably over the KatherinaVolcanics at Wadi Khileifiya and Wadi El-Mahash. Basalconglomerates occur at the base of the sandstone successionmarking the surface of unconformity between the volcanicsof Wadi Khileifiya and the overlying sandstones. The con-glomerate contains abundant volcanic cobbles and pebblesderived from the underlying Katherina Volcanics and othercrystalline basement rocks.
The chemistry of the biotites indicates the tectonic envi-ronment of the studied volcanics. They have an averageFeO*/MgO ratio of 6.48 which is nearly similar to the ratioof 7.04 given for biotites in alkaline (mostly anorogenicextensional) suites including A-type granites (Abdel-Rah-man, 1994).
The overall chemical characteristics of the studied volca-nics are consistent with a within-plate tectonic setting. Thealkaline (and sometimes peralkaline) felsic volcanics showa narrow range of high SiO2 content, remarkable depletionin CaO, MgO, Sr and transition metals and high contentsof alkalis together with marked enrichment in HFSEincluding the REE. The REE patterns depict pronouncednegative Eu anomalies. These geochemical features are typ-ically characteristic of within-plate magmatism (Whalenet al., 1987; Eby, 1990, 1992). Moreover, the analyzed sam-ples fall also in the within-plate field on the Nb–Y diagram(Fig. 12) (Pearce et al., 1984). The seven analyzed Kathe-rina rhyolite and granophyre samples plot in the A-type
100 1000
ORG
1 101
10
100
1000
VAG+Syn-COLG
WPG
Nb
Y100 1000
ORG
1 101
10
100
1000
VAG+Syn-COLG
WPG
Nb
Y
Fig. 12. Y–Nb diagram for the volcanic rocks of the older Katherina cycle(after Pearce et al., 1984).
Fre
field on the Zr + Nb + Ce + Y vs. (K2O + Na2O)/CaO(Fig. 13) (Whalen et al., 1987). Similarly, the analyzed alka-line intermediate rocks of the younger cycle plot in thewithin-plate field (Fig. 14) on the Zr/Y vs. Zr diagram(Pearce and Norry, 1979) applied for andesites.
It is now widely accepted that during the closing stage ofthe Pan-African orogeny, the subduction-related calc-alka-line magmatism was replaced by post-tectonic alkalinemagmatism (Garson and Krs, 1976; Harris, 1982; El-Ramly and Hussein, 1985; Bentor, 1985). Bowden (1985)distinguished in many alkaline provinces in Africa an olderassociation related to the terminal phases of major oroge-nies (i.e. not strictly anorogenic) and a younger associationrelated to progressive uplift, long-term doming and rifting(i.e. true anorogenic). Bonin (1990) subdivided the (per-)alkaline felsic rock association, currently named anoro-genic or A-type granites, into post-orogenic and early anor-ogenic groups. The first group postdates the last orogenicmagmatic episode by less than 10 Ma and bears Mg-richmafic silicate mineralogy. The second group, which may
10
20 A- Within Plate Basalts B- Island Arc Basalts C- Mid Ocean Ridge Basalts
A
Zr/
Y
B
C
110 100 1000
Zr
Fig. 14. Zr–Zr/Y diagram for the volcanic rocks of the younger Katherinacycle (after Pearce and Norry, 1979).
0.2
1
10
100
Rb Ba Th K Ta Nb Ce Hf ZrSm Yb Y
Sam
ple/
Oce
an R
idge
Gra
nite
Rhyolite of W. Khileifiya (107)Rhyolite of W. El-Mahash (206)Rhyolite of G. El-Homra (342)Rhyolite-porphyry of G. El-Homra (305)Granophyre of W. Khileifiya (127)
Granophyre of W. El-Mahash (200 & 216)
Fig. 15. Normalized spider diagram for the rhyolites and granophyres of the older Katherina cycle.
60 65 70 75 800.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
SiO2
RRG
CEUG
iT
O2
Fig. 16. TiO2 vs. SiO2 diagram for the volcanic-subvolcanic rocks of theolder Katherina cycle (after Maniar and Piccoli, 1989). RRG: Rift-relatedgranitoids, CEUG: Continental epeirogenic uplift granitoids.
M.D. Samuel et al. / Journal of African Earth Sciences 47 (2007) 203–226 221
be synchronous with the first group, contains Fe-rich maficsilicate mineralogy. Trace elements contents in both thepost-orogenic and early anorogenic alkaline suites of meta-luminous, peraluminous and peralkaline affinities are notclearly different; however, the post-orogenic suites are gen-erally low in Nb and Y content but accompanied by amarked Ba enrichment. Bonin (1990) added that Ba isstrongly depleted in early anorogenic granites and showspronounced negative anomalies in the ORG-normalizedspidergrams. Adopting the above mentioned criteria forthe studied felsic Katherina Volcanics, it is clear that thevolcanics of Wadi El-Mahash and Wadi Khileifiya pertainto a post-orogenic suite as they contain lower values of Nb(av. 27 ppm) and Y (av. 47 ppm). On the other hand, GebelEl-Homra volcanics contain higher contents of Nb and Y,averaging 61 and 77 ppm, respectively. Moreover, the sodicamphiboles (arfvedsonite) in G. H. volcanics have chemicalcomposition similar to those of early anorogenic granites,while the biotite of W. M. is compositionally similar tothose of post-orogenic granites (Bonin, 1990). The ORG-normalized spidergrams (Fig. 15) show distinct negativeBa anomalies in the case of G. H. volcanics (av.246 ppm), which is absent in the volcanics of the two otherlocalities (av. 672 ppm).
Controversial views were previously proposed for therole of rifting during the emplacement of the KatherinaVolcanics in the type locality and neighbouring areas ofNegev and SW Jordan (Bentor, 1985; Jarrar, 1992; AbdelKhalek et al., 1994; Garfunkel, 1999; Mushkin et al.,1999). The present study showed that the studied Kathe-rina Volcanics cross-cut late Pan-African, subduction-related calc-alkaline batholiths. The eruption of thesevolcanics was preceded by the intrusion of dyke swarmsof varying composition that cut across the calc-alkalinerocks but abuts against the volcanics. The dyke swarmswere formed as a result of extension-related tectonics andmark the switch from calc-alkaline to alkaline magmatism(Friz-Topfer, 1991). The latter produced at Taba-Nuweiba
environs A-type granites together with rhyolitic and minorintermediate rocks having oversaturated and alkalinenature but lacking basalts and undersaturated rocks. Thestudied rhyolitic volcanic-subvolcanic rocks are chemicallycomparable to continental epeirogenic uplift granitoids(CEUG) of Maniar and Piccoli (1989) (Fig. 16). Therefore,the Katherina Volcanics may have been emplaced in anextensional tectonic environment accompanied by fractur-ing and crustal uplift, after the end of the Pan-Africanorogeny.
7. Petrogenesis
7.1. Source rocks
A variety of petrogenetic models have been proposed forthe A-type alkaline magmatism. Many workers accept a
222 M.D. Samuel et al. / Journal of African Earth Sciences 47 (2007) 203–226
mantle origin (e.g. Bonin and Giret, 1985; Turner et al.,1992). Others considered the A-type suites as anatecticmelts of various crustal sources (e.g. Chappell et al.,1987; Creaser et al., 1991). Stoeser and Elliott (1980) pro-posed a petrogenetic model for the A-type suites involvingtheir generation through fractionation of I-type melts,while Whalen et al. (1987) and Sylvester (1989) believe thatA-type melts were derived from restites from which I-typemelt had been extracted earlier.
The predominance of evolved acid A-type volcanics,including rhyolitic ignimbrites accompanied with minorintermediate rocks but lacking completely basalts favourthat the parent magma was generated mainly by partialmelting of crustal rocks.
10 100 3000.6
1
10
20
Sr
Rb/
Sr
Fig. 17. Rb/Sr vs. Sr diagram for the volcanic rocks of the olderKatherina cycle.
Sam
ple/
Prim
itive
man
tle
0.1
10
100
500
1
Rb Ba Th K Ta Nb La Ce Pr Sr Nd P H
Fig. 18. Primitive mantle-normalized multi-element diagram for the rhyolitestaken from Mcdonough et al. (1991).
The ratios of the highly incompatible trace elements arecharacteristic of the source region from which the magmawas extracted. Zr/Nb and Th/La ratios average 13.9 and0.221, respectively. These averages are similar to thosegiven for the continental crust by Weaver (1991) (10.86and 0.204) and Wedepohl (1994) (16.2 and 0.34).
The possible thermal source required for melting crustalrocks to generate A-type suites is a matter of debate (Clem-ens et al., 1986; Sylvester, 1989). However, it is suggestedhere that lithospheric extension and crustal rupture thatoccurred prior to the emplacement of Katherina Volcanicscaused pressure release at depth and influx of volatiles intothe crust from deeper sources. The rather thin crust inEgypt of about 35 km (Mechie and Prodehl, 1988) as wellas the continental upheaval and extensive erosion that pre-ceded the emplacement of the Katherina Volcanics (Ben-tor, 1985; Garfunkel, 1999) favoured the pressure releaseand increasing mantle contribution of volatiles (Harris,1982) which were important agents of heat transfer suffi-cient for the anatexis of the crustal rocks.
The overall trends of major and trace elements (Figs. 8and 9), the elemental ratios of Ba-Eu/Eu* (Fig. 11), Rb/Sr-Sr (Fig. 17), and the fractionated nature of the REE[(La/Yb)n = 5.4 on average) as well as the primitive mantlenormalized multi-element diagram for the rhyolitic rocks(Fig. 18) suggest that their parent magma has undergonesome degree of magmatic differentiation. Alkali feldsparswere the main fractionating phases; fractionation of bio-tite, apatite and Fe–Ti oxides played a minor role.
7.2. Mechanisms and origin of potassium enrichment
The present study revealed that the older Katherina rhy-olitic rocks of Wadi Khileifiya differ considerably fromtheir counterparts at Wadi El-Mahash and Gebel El-
f Zr Sm Eu Gd Ti Tb Dy Ho Er Tm Yb Lu Y
Rhyolite of W. Khileifiya (107)Rhyolite of W. El-Mahash (206)Rhyolite of G. El-Homra (342)Rhyolite-porphyry of G. El-Homra (305)Granophyre of W. Khileifiya (127)
Granophyre of W. El-Mahash (200 & 216)
and granophyres of the older Katherina cycle. Normalization values are
M.D. Samuel et al. / Journal of African Earth Sciences 47 (2007) 203–226 223
Homra. They occur as steeply inclined to vertically dippingsuccessive lava sheets unaccompanied by pyroclastics. Theycontain albite and K-feldspar phenocrysts. They possessnormal alkali contents with K2O/Na2O ratios less than1.54, and are enriched in Ba but depleted in HFSE. Onthe other hand, the rhyolites of the other two areas areassociated with fallout pyroclastics and ignimbrites. Theycontain K-feldspar phenocrysts; rare phenocrysts of perth-ite and anorthoclase are additionally detected in the rhyo-lites of Gebel El-Homra. The rocks of both areas arecharacterized by K2O enrichment (5.20–10.05 wt.%) andNa2O depletion (0.08–3.47 wt.%) and are mostly depletedin Ba but enriched in HFSE. In spite of the abnormalK2O/Na2O ratios, they possess almost constant total alkalicontents, which suggests exchange of Na2O by K2O. It is tobe noted here that the granophyres of the older Katherinacycle as well as the intermediate volcanics of the youngercycle have normal Na2O and K2O contents. Coeval ultra-potassic volcanics in the neighbouring areas of Elat (Agronand Bentor, 1981; Mushkin et al., 1999) and southwest Jor-dan (Wachendorf et al., 1985; Jarrar, 1992) are alsorecorded.
Selective enrichment of K2O up to 10.05 wt.% withrespect to Na2O cannot be achieved by primary magmaticprocesses, such as fractional crystallization or by partialmelting of a K-rich protolith (Steiner et al., 1975 and F.Holtz, Hanover Univ., Germany, written comm.). How-ever, this selective enrichment possibly can be achievedthrough gas transfer in the top part of the magma chamber(Sahama, 1974).
The derivation of the high-potash rocks of Wadi El-Mahash and Gebel El-Homra from a separate high-Kmagma is excluded since they are closely related to thoseof Wadi Khileifiya having normal alkali contents. More-over, the petrographically studied K-rich and ultrapotassicrhyolitic rocks do not represent crystal cumulates with highconcentration of K-feldspar phenocrysts.
Low-temperature exchange of soda by potash throughgroundwater during alteration is not feasible since the stud-ied rhyolitic rocks are always fresh and no adularia hasever been detected. Also, these rocks possess normal valuesof L.O.I. averaging 1.37% and are unaccompanied by sec-ondary minerals (cf. Ennis et al., 2000; Rougvie and Soren-sen, 2002).
High-temperature K-metasomatism assumes that pot-ash-rich alkali rhyolites were derived from a normal alkaligranitic magma through alteration during or after emplace-ment (Taylor et al., 1984). This mechanism was previouslyproposed to explain the occurrence of the potash-enrichedrhyolitic volcanics recorded in southern Negev and SWJordan. Agron and Bentor (1981) assumed that about halfof the alkali rhyolites at Biq’at Hayareah area have under-gone post-emplacement metasomatic alteration whereexsolved albite in perthite phenocrysts and primary quartzhas been replaced by K-feldspar. The present studyrevealed that all the analyzed rhyolitic volcanics and theirsubvolcanic equivalents at Gebel El-Homra (the western
continuation of Biq’at Hayareah) are K-rich rocks. Theserocks contain microscopically both K-feldspar and rareperthite phenocrysts with exsolved batches or stringers ofalbite, and verified by EPMA (see Fig. 2f and Table 2).Moreover, stained thin sections revealed the presence ofstained K-feldspar phenocrysts together with unaffectedquartz and albite intergrowths within the perthite pheno-crysts; discrete albite and other plagioclase crystals arenot detected. Also, the present petrographic investigationrevealed the presence of quartz and K-feldspar graphicintergrowths (simulating hourglass twinning) which is theresult of rather rapid simultaneous crystallization and notformed by metasomatic replacement of quartz phenocrystsby K-feldspar (cf. Agron & Bentor, op.cit.). Besides, thegraphically intergrown phenocrysts are mantled by a thinK-feldspar veneer (see Fig. 2e). This is often interpretedto indicate a drop in water pressure during extrusion(thankfully added by one of the reviewers). The drop inwater pressure moves the quartz-alkali feldspar boundaryand reducing the quartz field in the aplogranite system.So that quartz crystallization stopped and the graphicallyintergrown phenocrysts were overgrown by a K-feldsparmantle.
High-temperature K-metasomatism is characterized byan increase in K2O, Rb, Zn and a decrease in Na2O accom-panied by depletion in all REE (Kinnaird et al., 1985). Inthe studied areas, K-rich rhyolitic volcanics and normalrhyolites have comparable contents of Zn and REE. Like-wise, Rb contents in the ultrapotassic rhyolites range from115 to 292 ppm, which is not abnormally higher than thatof normal rhyolites (83–118 ppm); very high Rb content(P1000 ppm) was reported in K-metasomatized rhyoliticrocks (Kinnaird et al. op. cit.). Thus, trace element compo-sitions do not comply with high-temperature K-metasomatism.
The salient features of the ultrapotassic acidic volcanicsare: (1) They are associated with abundant pyroclasticswhich are absent in the area with normal potash content.This feature ascribes an important role for the accumula-tion of gases, vapours or volatiles in general; (2) They areextremely impoverished in Na2O but accompanied by anincrease in normative corundum content; (3) The amountof K2O reaches up to 10.05 and (4) The K-feldspars arepoor in albite content indicating that they crystallized froma magma already deficient in soda. In a lengthy discussionwith S. El-Gaby, Assiut Univ., after summarizing the sali-ent features of the ultrapotassic acidic volcanics, wereached the following consensus.
The feasible mechanism suggested below for the gene-sis of the studied ultrapotassic rhyolitic rocks depends onthe fact that Na2O dissolves more favourably than K2Oin the vapour phase (or volatiles in general) that accumu-lating in the upper part of the magma chamber and caus-ing at the end the explosive eruption and the formationof pyroclastics. Given ample time, volatiles dissolved inthe magma would separate and accumulate atop themagma chamber causing the dissolution and removal of
224 M.D. Samuel et al. / Journal of African Earth Sciences 47 (2007) 203–226
the greater part of Na2O (and a smaller part of K2O),and the explosive eruption of the volcano. However, alu-mina remaining in the magma which cannot combine toform albite (due to loss of soda) or muscovite (due todeficiency in H2O) would import more potash still avail-able in the deeper levels of the magma chamber to formmore K-feldspars; the increased normative corundumcontent might represent the amount of alumina remain-ing after the loss of potash (escaping together with thelost soda to the gaseous phase). This explains adequatelythe increased potash content (seemingly at the expense oflost soda). Normal K2O/Na2O relation occurs whereaccumulation of volatiles did not occur, i.e. lackingpyroclastics.
8. Conclusions
1. The Katherina volcanic phase at Taba-Nuweiba districtis differentiated into two cycles. The older cycle is repre-sented by explosive eruption and deposition of pyroclas-tics and ignimbrites associated with rhyolite extrusives.These rhyolitic volcanics were either preceded or suc-ceeded by the emplacement of subvolcanic granophyressimilar to G3-granites (A-type granites). The youngercycle is represented by the extrusion of trachyandesiteand quartz trachyte. The Katherina Volcanics areunconformably overlain by Cambrian sandstones,which mark the stable platform conditions in Taba-Nuweiba district.
2. The rhyolitic extrusives (lavas, fallout pyroclastics andignimbrites) of Wadi El-Mahash and Gebel El-Homraare enriched in potash and depleted in soda; the rhyolitelavas of Wadi Khileifiya unaccompanied with pyroclas-tics have normal contents of alkalis.
3. The older felsic and younger intermediate rocks repre-sent two different magmatic suites.
4. The overall mineralogical and chemical characteristicsof the Katherina Volcanics are consistent with within-plate tectonic setting but are not rift-related. The acidvolcanics of Wadi El-Mahash and Wadi Khileifiyapertain to the post-orogenic granites, while those ofGebel El-Homra are similar to early anorogenic gran-ites. The parent magma of the acid volcanic-subvolca-nics is assumed to have been generated mainly bypartial melting of crustal rocks. The volatiles fromthe upper mantle were important agents for heattransfer, and sufficient for the anatexis of the crustalrocks.
5. The potash-enriched and ultrapotassic rhyolitic volca-nics lack petrographic and geochemical evidences infavour of K-metasomatism. A substantial effect of thevolatiles accumulated in the upper part of the magmachamber together with the fact that Na2O dissolvesmore favourably than K2O in the vapour phase explainsthe soda loss while normal K2O/Na2O relation remainswhere pronounced accumulation of volatiles did notoccur, i.e. lacking pyroclastics.
Acknowledgement
The authors are greatly indebted to Prof. El-Gaby forhis fruitful discussions and valuable comments that im-proved this contribution.
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