Rare-earth elements in some Egyptian metavolcanics

Chemical Geology, 43 (1984) 233--239

233

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

R A R E - E A R T H ELEMENTS IN SOME EGYPTIAN METAVOLCANICS

M. HAMED HATHOUT

Geological Department, Faculty of Education, Menoufia University, Shebin El Kom (Egypt) (Received January, 25, 1983; revised and accepted September 28, 1983)

ABSTRACT Hathout, M.H., 1984. Rare-earth elements in some Egyptian metavolcanics. Chem. Geol., 43: 233--239. Ten samples from Egyptian metavolcanic formations ( Q e n a - Q u s e i r area, central Eastern Desert) have been analyzed for rare-earth elements (REE). The studied rock types include metabasalt, metaandesite and metadacite. On the basis of REE data, it is suggested that these volcanic rocks represent at least two magma types, a tholeiitic and a calc-alkaline magma. INTRODUCTION

The metavolcanic rocks of the Precambrian basement complex in the eastern Desert of Egypt, are the most extensive particularly in the central and southern parts. The present work deals with rare-earth elements (REE) in selected samples from the metavolcanics of the Q e n a - Q u s e i r area in the central Eastern Desert with the two-fold aim of providing new data on these volcanics; and discussing their genetic significance. RESULTS

The analytical results are summarized in Table I. The data were obtained by instrumental neutron activation technique described in Jacobs et al. (1977). Accuracy and precision of analysis, on the basis of a standard synthetic basalt and on replicate analyses, are considered to be better than 10%. Table I indicates the deviation from the mean values of two samples. The samples analyzed here are from the Q e n a - Q u s e i r area metavolcanics. The sampled localities include G. Hadarba, Wadi E1 Kereim--E1 Abiad, Wadi Zeidun and Wadi E1 Gidami. The geology has been given by E1 Ramly and Akkad (1960), Assaf (1973) and E1 Ghawaby (1973). The bulk-rock chemistry and the petrographical description of all samples have been given by H a t h o u t (1975).

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© 1984 Elsevier Science Publishers B.V.

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235 DISCUSSION AND CONCLUSIONS

All the studied volcanics have been subjected to low-grade metamorphism (they most commonly occur as greenschist facies). Numerous studies have used REE analyses to interpret the magmatic parentage of altered or metamorphosed volcanic rocks (e.g., Philpotts et al., 1971; Herrmann et al., 1974). The effect of metamorphism on the REE patterns has not been studied in great detail. However, it is generally believed that for basaltic rocks this effect is of minor importance (e.g., Herrmann and Wedepohl, 1970; Tanaka and Sugisaki, 1973; Montigny and All6gre, 1974). Frey et al. (1968) and Kay and Senechal (1976) stated that REE are only slightly affected by alteration and metamorphism in crystalline rocks. Frey et al. (1974) and Herrmann et al. (1974) concluded that low-grade metamorphism (~400°C greenschist) did not significantly change the REE pattern. Herrmann et al. (1974) added that minor fluctuations may exist but in no case could light REE (LREE) enriched lava be changed to LREE-depleted lava. Owing to the arguments given above, as well as the close similarity of the REE data of the analyzed rock types to those of their fresh equivalents as will be shown later, it would appear that these metamorphosed volcanic rocks should give REE patterns and abundances indicative of the original rocks. The REE data of the investigated rock types including metabasalt, metaandesite and metadacite will be considered in the following discussions. Metabasalt

Fig. 1 shows the chondrite-normalized REE patterns of the metabasalts. As shown in this figure, the metabasalts are characterized by the presence of nearly chondritic (or LREE-depleted) and LREE-enriched patterns. The total REE abundances of samples 2, MA2-B and 34-K basaltic rocks fall well within the range for abyssal tholeiites (Frey et al., 1974; Schilling, 1975) with absolute REE concentrations which are ~ 5 - - 1 0 times those of chondrites. These samples have nearly chondritic REE patterns with slight convex-upwards REE distributions. The REE patterns of these rocks differ from those of mid-ocean ridge basalts (MORB's) by the absence of a strong LREE depletion. An exception is sample 34-K which has a distinct LREEdepleted pattern. Field relations at the Qena-Quseir area indicate a close association of the basic metavolcanic rocks with serpentinized ultramafic bodies (mostly of refractory dunitic and rarely of peridotitic composition) which would suggest that they are genetically related. In a recent work (Hathout, 1983), the author found that some Qena-Quseir peridotites have major- and traceelement contents which are consistent with estimates of mantle composition. The REE pattern of these peridotites is nearly parallel to that of the chondrite composition but the overall concentration levels are ~ 4 times those of chondrites. Calculations [Shaw's (1970) equations] , reveal that 20--30%

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Fig. 1. Chondrite-normalized REE abundance patterns of metabasalts from the QenaQuseir area: (1) a peridotite rock from the same area (Hathout, 1983); (2)olivine tholeiite D: (East Pacific Rise; Frey et al., 1968); and (3) oceanic island tholeiite (Kohala Mt., Hawaii; Frey et al., 1968) are shown for comparison. partial melting of a mantle source material similar to the Q e n a - Q u s e i r peridotites is required to generate a basaltic melt with a REE absolute abundance and distribution nearly alike to those of LREE-depleted basalts. Thus with respect to REE, such a peridotite is a suitable source material for basaltic rocks like 2, MA:-B and 34-K. On the o th er hand, the REE data of samples 10 and NZ-10A indicate that c o mp ar ed to chondrites the L R E E are enriched in relation to heavy REE (HREE). LREE-enriched patterns are known for basaltic rocks from the MidAtlantic Ridge at 45°N (Frey et al., 1968) and at DSDP Leg 2, Site 10 (Frey et al., 1974). It is unlikely that these basalts were derived by fractional crystallization from LREE-depleted tholeiites; instead, such basalts require a different mantle source (a more LREE-enriched source and/ or more clinopyroxene-rich source).

Metaandesite The four analyzed samples were collected from G. Hadarba where metaandesites constitute the most predominating rock t ype relative to the ot her metavolcanics. The REE distributions (relative to chondrites) of these samples are characterized by steadily increasing L RE E enrichments from Lu to La (Fig. 2). The chondrite-normalized REE patterns of these andesites are

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rocks from the Qena-Quseir area. similar to those of calc-alkaline and alkaline rocks of island arcs (Jake~ and Gill, 1970). It seems unlikely to produce an andesitic liquid similar in composition to G. Hadarba metaandesites by partial melting of a basalt parent-rock because the magmas formed by melting of an oceanic tholeiite keep the LREE depletion typical of the source rock (Dostal et al., 1977; Lopez-Escobar et al., 1977). All the analyzed andesitic rocks have a distinctly fractionated HREE pattern with a normalized Tb/normalized Yb ratio significantly higher than unity (Tbn/Yb n = 1.5--2.1). Garnet is the only c o m m o n rock mineral which has H R E E solid--liquid partition coefficients greater than unity. As judged from garnet which is strongly discriminated among H R E E (having DY/bL significantly higher than D S/L Tb ; Arth, 1976), it could be suggested that these andesitic rocks were probably produced by partial melting of a garnetbearing mantle source. Me tadacite

Field evidences reveal that there are more felsic metavolcanics well developed in Wadi Kereim E1 Abiad. The REE pattern of the analyzed rock sample NZ-13-A) is characterized by a fractionated LREE, least fractionated HREE ( T b n / Y b n ~ 1) and an Eu depletion (Fig. 2). These evidences preclude derivation of such dacite by fractional crystallization of G. Hadarba andesites. On the other hand, the metadactie rock NZ-13A and the metabasalt rock NZ-10A, which are from the same occurrence, may be genetical-

238

ly related. Nevertheless, production of such a dacite by fractional crystallization of a basaltic magma similar in composition to that of NZ-10A is not permissible because the total content of the REE of the dacite (~ 80 ppm) is not much greater than that of the basalt ( ~ 5 6 ppm). This could eliminate direct fractional crystallization as a mechanism for the genesis of such dacite from basaltic liquids. ACKNOWLEDGMENTS

The author expresses his deep gratitude to Prof. Dr. F. Frey for help and facilities in neutron activation techniques.

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239 Lopez-Escobar, L., Frey, F.A. and Vergara, M., 1977. Andesites and high alumina basalts from the central-south Chile, high Andes: geochemical evidence bearing on their petrogenesis. Contrib. Mineral. Petrol., 63: 199--228. Montigny, R. and All4gre, C.J., 1974. A la recherche des oc6ans perdus: les ~clogites de Vendee, t6moin m6tamorphis6s d'une ancienne crofite oc4anique. C.R. Acad. Sci. Paris, 279: 543--545. Philpotts, J.A., Schnetzler, C.C. and Hart, S.R., 1971. Geochemical aspects of some Japanese lavas. Earth Planet. Sci. Lett., 12" 89--96. Schilling, J.G., 1975. Rare earth variations across normal segments of the Reykjanes Ridge, 60°--53°N, Mid-Atlantic Ridge, 2°--29°S and evidence on the composition of the underlying low velocity layer. J. Geophys. Res., 80 (11). Shaws, D.M., 1970. Trace element fractionation during anatexis. Geochim. Cosmochim. Acta, 34: 237--243. Tanaka, T. and Sugisaki, R., 1973. Successive eruption of alkaline and tholeiite magmas in a Japanese Palaeozoic geosynclinal body with special reference to REE features. J. Petrol., 14: 489--507.