Composition and total-Pb model ages of monazite from high

Mineralogy and Petrology (1998) 62:269-289 Mineralogy a n ( 1

Petrology © Springer-Verlag 1998 Printed in Austria

Composition and total-Pb model ages of monazite from high-grade paragneisses in the Abu Swayel area, southern Eastern Desert, Egypt

E Finger 1 and H. Mo Helmy 2

1 Institut ftir Mineralogie, Universit~t Salzburg, Austria 2 Geological Department, Faculty of Science, E1 Minia University, E1 Minia, Egypt

With 5 Figures

Received January 18, 1997; revised version accepted January 20, 1998

Summary

Monazites from high-grade metapelitic paragneisses from the southern Eastern Desert of Egypt (Abu Swayel area) were analysed with the electron microprobe mainly in an attempt to broadly constrain the metamorphic ages of the rocks by means of chemical Th(U)-Pb dating. Two samples were investigated, one showed weak signs of a greenschist facies overprint, the other one did not. For each sample, weighted average ages were calculated from long-time analyses of 18 (16) individual grains with a 5 gm beam placed in the grain centres. The average ages were almost the same (636~= 10 Ma, 633±10Ma). The monazites appeared chemically fairly uniform und homogeneous in both samples with ThO2 contents of ca. 3.3-4.5wt.%, UO2 0.4-1.2wt.%, La203 12-13 wt.%, Nd203 11-13wt.%, Y203 1.8-2.6wt.%. Some larger grains displayed a weak concentric zoning in the BSE image with increasing brightness near the rims. A microprobe traverse was laid across a zoned monazite from the slightly retrogressed sample. It was found that the U and Y contents were somewhat higher in the outer growth shell. The high Y contents at the rims argue for crystal growth under prograde temperature conditions and against a retrograde overgrowth. There appeared to be a tendency that the model ages become slightly younger towards the crystals rim (645i15 Ma in the core section versus 633+16Ma in the rim section of the profile). However, the observed differences are interpreted as equivocal due to the limited resolution of EMP monazite dating.

Clearly, the results do not support previous hypotheses, according to which Abu Swayel gneisses should belong to pre-Panafrican, mid-Proterozoic metamorphic sequences. Instead, the data accord with other 600-650 Ma metamorphic ages recently

270 F. Finger and H. M. Helmy

recognized near the contact of the East Sahara Craton and the Arabian Nubian Shield. The best interpretation is that high-grade metamorphism at that time occurred in connection with collisional crustal thickening, when a Panafrican terrane assembly was attached to the east Sahara Craton from the (present day) east. This event appears to be distinct from an earlier phase of high-grade regional metamorphism between ca. 700 and 750 Ma, which has been documented in other parts of the Arabian Nubian Shield.

Zusammenfassung

Zusammensetzung und Gesamtblei-ModellaIter von Monazit aus hochgradig metamor- phen Paragneisen der Abu Swayel Region, siidliche Ostwiiste, Agypten

Monazite aus amphibolitfaziellen metapelitischen Paragneisen der Abu Swayel Region in Siid~igypten (SW Eastern Desert) wurden mit der Mikrosonde analysiert, unter anderem um die Metamorphosealter der Gesteine auf dem Weg einer chemischen Th(U)-Pb Datierung annfiherungsweise zu bestimmen.

Zwei Proben wurden untersucht, eine davon wies eine leichte retrograde 0berpr~igung unter grtinschieferfaziellen Bedingungen auf. In jeder Probe wurden 18 bzw. 16 Monazite, jeweils in den Kornzentren, mit langer Zahlzeit und mit auf 5 gm defokussiertem Strahl analysiert. Die aus den Analysen errechneten mittleren Th(U)-Pb Alter waren in beiden Proben praktisch gleich (636+10Ma, 633~10Ma). Auch die Chemismen der Monazite erwiesen sich in beiden Proben als sehr ~ihnlich und ziemlich homogen (ThO2 3.3-4.5wt.%, UO2 0.4-1.2wt.%, La203 12-13wt.%, Nd203 11- 13 wt.%, Y203 1.8-2.6 wt.%). Nur einzelne gr6gere K6rner liegen im BSE-Bild einen ganz schwachen konzentrischen Zonarbau mit gr6gerer Helligkeit am Rand erkennen. Durch ein solches Korn aus der leicht retrograden Probe wurde ein chemisches Profil gelegt um den Zonarbau zu quantifizieren bzw. die Altershomogenit~it zu testen. Die Randzone wies dabei leicht erh6hte Gehalte an U and Y auf. Die hohen Y Werte sprechen far eine Kristallisation unter hohen Temperaturen und gegen ein retrogrades Weiterwachsen. Die chemischen Modellalter waren im Kornzentrum im Schnitt etwas h6her als am Kornrand (645± 15,633+ 16 Ma). Allf~illige Zeitdifferenzen im Wachstum oder Bleiverlusteffekte waren allerdings innerhalb des methodischen Fehlers nicht sicher aufl6sbar.

Die untersuchten Paragneise wurden bisher meist als Reste eines mittelprotero- zoischen, pr~i-panafrikanischen Kristallins angesehen. Die chemischen Modellalter fiir die Monazite best~itigen diese Ansicht aber nicht, sondern passen vielmehr gut zu anderen Metamorphosealtern von etwa 600 bis 650Ma, welche in der letzten Zeit mehrfach entlang der Grenzzone zwischen dem Ostsahara-Kraton und dem Arabisch- Nubischen Schild dokumentiert wurden. Die wahrscheinlichste Erklgrung ist, dab diese Regionalmetamorphose durch das Andocken panafrikanischer Terrane an den Kraton verursacht wurde.

1. In t roduc t ion

The chemical Th-U-Pb dating of monazite with the electron microprobe (EMP) has been regarded in several recent studies as a powerful new tool in geological research (Suzuki et al., 1991; Montel et al., 1996; Rhede et al., 1996). Although the method is certainly by far not as precise and reliable as the conventional mass spectrometric dating of monazite, it appears highly attractive for obtaining quickly and simply first rough age estimates for rocks, which are geochronologically new

Composition and total-Pb model ages of monazite from high-grade paragneisses 271

ground. The experience available attests the method of EMP monazite dating a sufficient reliability at moderate precision of about +20-30 Ma for Palaeozoic and older rocks (cf. Montel et al., 1996; Rhede et al., 1996). However, as with all new techniques, there are questions and uncertainties involved, that can only be addressed through testing on specific case studies. Applying EMP monazite dating to various geological problems is therefore a useful task not only as an attempt to gather age information, but also to better assess the potential of the method and to become aware of possible major error risks. Furthermore, data obtained in such studies are useful for broadening our general knowledge of monazite as a mineral, particularly with reference to its crystallochemical variability in rocks and its behavior during distinct geological processes.

In this study, EMP monazite dating was applied to a Precambrian paragneiss formation in Egypt (Abu Swayel area, Eastern Desert), for which highly contradictory age estimates of metamorphism have been previously published, ranging from ca. 1200Ma to post-800Ma. It was expected that EMP monazite dating would give new independent age information for these rocks.

2. Geological background

The Precambrian basement of the Arabian-Nubian shield in eastern Egypt (Eastern Desert) is viewed mostly as an orogenic amalgamation of island arcs, immature basin sediments and oceanic crust. These different formations or terranes developed during the Panafrican cycle s.l., i.e. between ca. 900 and 600Ma, in an oceanic environment (Kr6ner et al., 1992), in part probably in a back-arc basin (Shackleton, 1994), offshore of the East Sahara Craton. The latter constitutes the basement west of the fiver Nile (Fig. 1). It is not yet clear whether the terranes accreted with each other and the cratonic mass to the west during one single event in the late Precambrian (the Panafrican orogeny s.s.) or stepwise during several orogenic episodes. Further to the south, in the Sudan, it appears that formations comparable to the Eastern Desert rocks were attached to the East Sahara Craton during two collisional phases; between 720 and 700Ma and between 650 and 600Ma (Harms et al., 1994; Stern et al., 1994).

Large masses of late- to post-tectonic volcanics and granitoids flooded this composite crust finally at the end of the Precambrian, between ca. 620 and 550 Ma. The particularly intense magmatism may have been caused by large-scale delamination of the lower crust (Stem et al., 1994). A phase of crustal uplift and extension is documented by the exhumation of metamorphic core complexes, and molasse basin formation, at about 600-580Ma (Fritz et al., 1996). This was followed again by a phase of transpressional thrusting and folding (Greiling et al., 1994). The present-day geological structures of the basement outcrops in the Eastern Desert appear to be mainly the result of these late, Neoproterozoic geological processes, which considerably blured the earlier pre-600 Ma history.

There is considerable discussion over whether pre-Panafrican cratonic crust is also present in the Egyptian basement east of the river Nile. This appears to be a common belief especially among Egyptian geologists (see e.g. Hassan and Hashad, 1990; El Gaby, 1990), but has been considered unlikely for example by Harris et al. (1984), Stern and Hedge (1985), KrOner et al. (1990, 1994). The


2 9 ~' _

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M E D I T E R R A N E A N SEA

CAIRO

S A H A R A

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Tab~

0 |,,

f'n

N 2 a o

. . , . . ._ .1 km

2 7 ~ -

2 5 ° -

2 3 ° -

, s.,. Safaga

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I i z 29° a~o 3'3o a'so sTo

Fig. 1. Exposed Precamblian basement in Egypt and Saudi Arabia, with tentative eastern boundary between the pre-Panafrican East Sahara Craton and the Panafrican Arabian- Nubian Shield [map slightly modified from Greiling et al. (1994)]. Shown are the positions of the Abu Swayel study area (AS), the Hafafit (H) and Meatique (M) domes as prominent outcrops of amphibolite facies paragneisses

possible candidates for such old crustal fragments are foliated quartzofeldspathic, amphibolite facies metamorphic rocks, as these are exposed in various gneiss- domes throughout .the Eastern Desert. Most of the rocks are paragneisses with compositions ranging from meta- to peraluminous (El Rarely and Akaad, 1960). Large amounts of such paragneisses occur in the Abu Swayel area in the southern


Eastern Desert. Here, the amphibolite facies metamorphism has been attributed to a ca. 1200 Ma old tectonothermal event (El Shazly et al., 1973; Hassan and Hashad, 1990). However, these high age estimates are mainly based on Rb-Sr whole rock isochrons with unclear significance. Monazites from two peraluminous paragneiss samples from the Abu Swayel gneiss complex have now been used for electron microprobe dating in an effort to better constrain the metamorphic age of the formation.

3. The local geological situation

The Abu Swayel crystalline complex is situated in the southwestern part of the Eastern Desert, close to the inferred eastern margin of the East Sahara Craton (Fig. 1). It is separated tectonically in the northeast from low metamorphosed volcanics, mudstones, graywackes and conglomerates by a shear zone (Fig. 2). Three major lithologies have been mapped in the Abu Swayel crystalline complex: metasedimentary rocks, metamorphosed ultramafic/mafic units and unmetamor- phosed granites. The general direction of dip of the metamorphic series is moderately steep eastward to northeastwards.

The high grade metasediments, considered as a part of the Pre-Panafrican basement by some authors (e.g. El Shazly et al., 1973; Hassan and Hashad, 1990), mainly comprise hornblende gneisses plus considerable amounts of more aluminous biotite-gneisses (often garnet and sillimanite bearing) and some marbles. We dated monazite from two samples of sillimanite-garnet-paragneisses. In other paragneiss lithologies, monazite appears to be absent or very rare.

The ultramafic/mafic rocks of the Abu Swayel massif have been interpreted as fragments of a Panafrican ophiolite thrust, and as the northwestern continuation of the Allaqi-Heiani ophiolite belt (KrOner et al., 1987). A small body of an amphibole-rich rock is of considerable economic importance and is the site of the Abu Swayel Cu-Ni prospect. However, the largest volumes of the ultramafic/mafic rocks occur ca. 15 km southeast of the Abu Swayel mine, in a relatively high structural position (Fig. 2). There, a serpentinite body is intruded by a large metagabbro mass, the formation age of which was constrained at 735±15Ma through zircon dating (KrOner et al., 1992). Both rocks are metamorphosed in the greenschist facies. Another ca. 13kin long and 500m wide serpentinite body occurs to the south-east of the mine in a lower structural position. It is closely associated with marbles and may have had a separate origin, unrelated to the ophiolite thrust in the east.

As in many other places in the Eastern Desert, the granites may be subdivided into an older suite of grey granites, and several younger granite types including the characteristic "red granites". A Rb-Sr whole rock age of 605 Ma is available for the older granites of the Abu Swayel area (El Shazly et al., 1973).

The metamorphic and tectonothermal evolution of the Abu Swayel crystalline complex is still poorly understood. High-grade metamorphism appears to be of the medium-pressure type (ca. 600°C, 6kbar - Helmy, 1996). The available thermobarometric data give no equivocal information whether this phase of metamorphism was collision-related or not. However, a collision model appears quite likely from a number of geological grounds (see discussion section).

274

22 °

F. Finger and H. M. Helmy

2 2 ° ,

22°: 33 ° 35" 33 ° 40"

Mudstone, greywacke, conglomerate

Metavolcanics

Various granites

33 ° 45"

Mylonitic rocks

"- Shear zone

ABU SWAYEL CRYSTALLINE COMPLEX

M e t a s e d i m e n t s Ultramafie/mafic rocks

Marble

Hornblende schist and gneiss

Biotite schist and gneiss

Quartzo-feldspathic schist

l l Amphibole rocks

~ ] Serpentinite and related rocks

~ Gabbro, diorite, quartz diorite

Fig. 2. Geological sketch map of the Abu Swayel area, compiled by Helmy (1996), mainly according to mapping results of E1 Shazly et al. (1977). Box marks the Abu Swayel mine area where the samples for this study have been taken


4. Petrography of the samples investigated

Both samples are intensly foliated, fine-grained gneisses, that consist of mainly quartz (ca. 50vo1.%), red-brown biotite (30-40%), 5-10% sillimanite and 5-10% oligoclase. Further constituents are a few garnet porphyroblasts between 0.5 and 3 mm in size, some muscovite, some K-feldspar, some ilmenite, accessory apatite, zircon (both mostly rounded), xenotime and monazite. The latter is surrounded by particularly strong pleochroic haloes in biotite.

The foliation is mainly defined by oriented biotite and sillimanite. The garnets are synkinematic and have pressure shadows. The quartz fabric is well annealed and displays mosaic structure. A few larger postkinematic biotite and muscovite sheets overgrew the foliation in an oblique angle.

Sample HS 17 shows no signs of secondary alteration. However, sample HS 11 displays a slight overprint in greenschist facies, expressed through occasional chloritization of biotite combined with strong sagenite exsolution. A few thin low- T shear bands filled with fine-grained muscovite and chlorite cut the main foliation in a steep angle.

However, even in this sample, more than 90 percent of the red-brown amphibolite facies biotite remained stable, suggesting that the later greenschist facies overprint did not lead to a significant redistribution of elements among the rock forming minerals. Therefore, it was to expect that the amphibolite facies stage of rock formation could be constrained by means of chemical monazite dating without any complications inherent to retrogression.

Helmy (1996) has carried out some mineral analyses and thermobarometric investigations on the monazite bearing metapelitic paragneisses of the Abu Swayel area.

The muscovites contain 6.17-6.25Si pfu ( 0 = 2 2 ) , indicating minimum pressure conditions of ca. 4kb (Massone and Scheyer, 1987). The Mg/Mg+Fe of the muscovites is 0.4-0.5, the paragonite component is between 12 and 19 mol%.

The garnets have a composition of typically Alm67Prp16Grs3Sps13Adrl in their cores and Alm72Prp14Grs3Sps~ 1Adro at their rims. GASP barometry (Koziol, 1989) gives pressures of ca. 4-5 kb for rim compositions of garnet-plagioclase pairs and ca. 6 kb for core compositions.

Biotites have an uniform X~g of 0.49-0.51, the average Si and Ti contents are 5.34 and 0.2 cations pfu (O = 22), respectively. The average A1TM content is 2.66 pfu.

Garnet-biotite pairs, analysed at their rims, yielded temperatures of around 565°C (calculated after Ferry and Spear, 1990 at 5kb, with the garnet activation model of Bermann, 1990). The higher XMg in the garnet cores suggests that these rim temperatures represent retrograde diffusion equilibria and that peak temperatures were probably higher. Precise estimates of the P, T evolution of the rocks would require a detailed study of zoning and inclusions in garnet, which is presently not available and also beyond the scope of this paper.


5. Results of monazite dating

5.1 Theoretical basis of the method and analytical procedure

It has been shown in a large number of conventional isotope studies that U-Th-Pb dating of monazites typically yields concordant crystallization ages. The reason for this is that the U-Th-Pb system of monazite is particularly resistent to postcrystallization disturbance and mostly free from inheritance (see e.g Parrish, 1990).

Furthermore, monazites have the advantage of incorporating only relatively low amounts of common lead, which means that lead in monazite is almost entirely radiogenic and a mixture of Pb 2°s (from the Th 232 decay), Pb 206 (from U 238) and Pb 2°7 (from U235). This and the high probability of concordant ages permits a chemical dating of monazite. As the mineral normally contains high amounts of Th and U, radiogenic lead can accumulate within less than 100 Ma to concentrations that fall within the detection range of the electron microprobe. Particularly suited for the method are monazites of Palaeozoic age and older. Assuming that Th in monazite is only Th 232, and that the U involves the isotopes U 235 and U 238 in a known constant ratio of 99.28 : 0.72 (Steiger and Jiiger, 1977), a chemical model age (T) may be calculated for every microprobe point analysis, by solving the equation:

Pb = Th/232' (e ;~232.T - 1)'208 + U/238"0.9928" (e ~238~T - 1)'206

+ U/238"0.0072" (e x235*T - 1)'207

For Th, U, Pb, the measured elemental concentrations, as obtained from the microprobe, are inserted into the equation. The A symbols are the decay constants of 232Th, 235U and 238U (Steiger and Jiiger, 1977).

On the basis of the analytical errors resulting from the counting statistics of the microprobe, a 2-sigma error may be attributed to every calculated model age by propagating the individual analytical errors for Th, U and Pb through the above equation (MonteI et al., 1996).

A major problem inherent to EMP monazite dating is that single point analyses typically suffer from large 2-sigma errors in the range of ca. 30-90 Ma, and that reasonable age constraints for a rock can only be obtained through a statistical treatment of a larger set of single point analyses. Such a weighted average age and its statistical error are geologically meaningful when all analysed monazite domains crystallized at the same time and experienced no lead loss later. However, the relatively large errors of microprobe analyses at low concentration levels complicate the identification of zones of inheritance or lead loss within monazites, which, although not common, may occasionally be present (De Wolf et al., 1993; Kingsbury et al., 1993). This means that average ages calculated for monazite populations may sometimes be shifted slightly to the one or the other side of the actual geological event due to inconspicious lead loss or inheritance.

For the present study the measurements were carried out on a Jeol JX 8600 equipped with three spectrometers. All elements were measured in the wave-length dispersive mode. Apart from Th, U, Pb, the elements R La, Ce, Pr, Nd, Y, Si and Ca


were routinely determined to warrent a reasonable ZAF correction. Mc~ 1 lines were chosen for Th, U and Pb, Lozl for La, Y, Ce, LB1 for Pr and Nd, and Kod for R Si and Ca. Commercially available synthetic calibration standards were used. In order of a particularly precise analysis of the Pb in the 0.1-0.2wt.% range, the probe current was set to 250 nA at 15 kV. For Pb, counting times of 80-100 s (peak) and 2x40-50 s (background) were preset for each analysis point. Counting times for the Th and U were 30 s (2x 15 s) and 50 s (2x25 s), respectively. All other elements were determined with 10 s (2 x5 s) counting times. The small Y interference on the Pb Mo~ line was eliminated by measuring a Pb-free Yttrium standard and routinely corrected for the monazite analyses by linear extrapolation (Montel et al., 1996). A small Th interference on U Mo~ was also empirically corrected.

The 2-sigma errors as resulting from the counting statistics of the microprobe under the chosen analysis conditions were typically ±0.04-0.05wt.% for Th, +0.025-0.030wt.% for U and ±0.018-0.020wt.% for Pb.

To control the quality of the analyses, monazites with precisely known concordant U-Pb ages have been measured together with the specimen. Weighted average model ages of 3434-7 Ma and 3404-7 Ma were obtained for these standard monazites during two different sessions (see below). These data are in perfect agreement with a concordant U-Pb age of 3414-2 Ma, determined by conventional U-Pb isotope dating (Friedl, 1997).

5.2 Textural observations

Monazites were detected in two normal polished thin sections, by means of BSE imaging. In sample HS 11, several dozen monazite crystals were found. However, most of these were very small. Roughly 20 grains in the section were somewhat larger between 10 and 30gm in diameter, 19 of these grains were analysed. A detailed chemical profile was laid across one grain (grain 19). Sample HS 17 contained less monazite. Sixteen grains with a diameter of 10-30 gm were analysed. The shapes of the monazites are basically the same in both samples and range from isometric to slightly elongated. The contours of the grains are typically round and not euhedral. The monazite interiors were mostly free of inclusions and looked mostly homogeneous in the BSE image. Only in a few cases, and in both samples, some monazite rims showed a slightly elevated brightness.

Some of the analysed grains were included in garnet, some in biotite or quartz and some were situated along the grain boundaries of the major minerals. From the experiences available from other geochronological studies, it may be reasonably assumed that monazites of metapelitic paragneisses are not detrital, but formed during prograde metamorphism (see e.g. Parrish, 1990; Smith and Barreiro, 1990; Suzuki et al., 1994; Finger et al., 1996). Lanzirotti and Hanson (1996) suggested that retrograde chloritization of biotite may eventually cause a release of LREEs and monazite growth under greenschist facies conditions. We have searched for such textures in sample HS 11, but found no indication for renewed monazite growth in connection with chloritization of biotite.


5.3 Single grain analyses

Electron microprobe data were acquired during two different sessions. During the first, 34 grains from the two samples were analysed with a 5 ~tm beam, which was always placed into the grain centres to reduce any potential influence from the host minerals on the X-ray spectrum to as low a level as possible. Also, the risk of secondary lead loss is probably lowest in the grain centres.

All 34 analyses yielded near ideal monazite stoichiometry, with fairly low crystallochemical variability (see Table 1 and Table 2). There appeared to be no

Table l. Examples of monazite analyses from samples HS l l and HS 17 (grain centres). Mineral formulas were calculated on the basis of 4 oxygens. The analyses have a ca 5% deficit in the totals and slightly low cation sums in the A [9] position, because Sin, Gd and the HREEs were not determined. Br and Hu are the theoretical percentages of brabantite and huttonite solid solution in monazite as recast from the Ca and Si formula units

\

S a m p l e 11 S a m p l e 17

Grain No. 5 8 9 3 5 16

SiO2 0,16 0,06 0,14 0,17 0,09 0,10 P205 30,00 29 ,10 29,08 29,18 29,21 29,61 CaO 0,80 0,94 0,84 0,86 0,96 0,96 Y203 1,93 2,34 1,88 1,88 1,94 2,32 La203 12,79 12,64 12,98 13,07 13 ,27 12,55 Ce203 28,94 28 ,68 29,20 28,45 28 ,72 27,26 Pr203 3,14 3,39 3,15 3,59 3,46 3,33 Nd203 12,07 12 ,02 12,11 12,19 12 ,07 12,23 ThO2 3,82 3,84 3,94 4,06 4,50 4,35 UO2 0,49 0,90 0,46 0,54 0,68 0,85 PbO 0,15 0,19 0,14 0,16 0,17 0,19 Total 94,29 94,10 93,91 94,13 95,08 93,75

Si 0,006 0 ,002 0,006 0,007 0 ,004 0,004 P 1,022 1 ,007 1,008 1,008 1 ,005 1,019 Ca 0,034 0,041 0,037 0,037 0 ,042 0,042 Y 0,041 0 ,051 0,041 0,041 0 ,042 0,050 La 0,190 0,191 0,196 0,197 0 ,199 0,188 Ce 0,427 0 ,429 0,438 0,425 0 ,427 0,406 Pr 0,046 0,051 0,047 0,053 0,051 0,049 Nd 0,174 0 ,176 0,177 0,178 0 ,175 0,177 Th 0,035 0 ,036 0,037 0,038 0 ,042 0,040 U 0,004 0,008 0,004 0,005 0 ,006 0,008 Pb 0,002 0,002 0,002 0,002 0 ,002 0,002

Mo (ss) 92,25 91,36 92,05 91,80 90 ,93 90,68 Br 7,07 8,39 7,36 7,51 8,69 8,90 Hu 0,68 0,25 0,59 0,69 0,39 0,42

tetr. 1,029 1 ,010 1,014 1,015 1 ,009 1,023 A(9) 0,953 0 ,984 0,978 0,976 0 ,986 0,962


Table 2. Th, U, Pb contents (wt. % elements) and model ages of monazites from samples HS 11 and HS 17, and laboratory standard F5, calculated after the method of Montel et al. (1996). F5 monazites are from a fraction dated by U-Pb mass spectrometry at a concordant age of 341~z2 Ma (Ffiedl, 1997)

Grain Th U Pb Age (Ma)

SAMPLE HS Ii

1 3.454 0.403 2 3.073 0.539 3 3.346 0.668 4 3.280 0.688 5 3.354 0.436 6 3.373 0.777 7 3.573 0.613 8 3.378 0.791 9 3.462 0.402

i0 3.376 0.777 ii 3.088 0.517 12 3.318 0,688 13 3.206 0.557 14 3.348 0.752 15 3.392 0.786 16 3.524 0.595 17 3.260 0.675 18 3.592 0.622

SAMPLE HS 17

1 3.449 0.480 2 3.926 0.564 3 3.567 0.474 4 3.638 0.722 5 3.953 0.580 6 2.978 0.456 7 3.646 0.633 8 3.506 0.396 9 3.468 0.516

i0 3.644 0.469 Ii 3.938 0.577 12 2.948 0.489 13 3.667 0.626 14 3.525 0.393 15 3.670 0.631 16 3.820 0.749

0.137 0.139 0 153 0 153 0 135 0 175 0 168 0 179 0 133 0 164 0 138 0 153 0 149 0 173 0 174 0 152 0 157 0 161

STANDARD F-5

636 ± 93 636 ± 92 611 ± 80 614 ± 80 626 ± 93 655 ± 75 666 ± 79 662 ± 70 617 ± 88 614 ± 75 641 ± 88 611 ± 76 654 ± 84 658 ± 72 646 ± 70 615 ± 77 634 ± 77 634 ± 75 636 + i0

0 148 654 ± 88 0 166 638 ± 73 0 146 634 ± 87 0 168 620 ± 74 0 162 613 ± 76 0 131 650 ± 99 0 166 641 ± 77 0 143 659 ± 92 0 150 645 ± 82 0 149 638 ± 81 0 175 666 ± 72 0.132 642 ± 92 0.161 623 ± 74 0.135 621 ± 88 0.157 605 ± 73 0.174 614 ± 67

633 ± I0

(first session)

I-i 7.157 1.230 0.170 2-1 9.326 0.788 0.176 2-2 9.458 0.798 0.191 3-1 6.511 1.145 0.158 3-2 6.438 1.116 0.153 4-1 7.491 1.173 0.174 4-2 7.380 1.135 0.177 5-1 12.790 0.818 0.232 5-2 12.114 0.807 0.225

STANDARD F-5 (second

l-1 9.222 1.390 0.210 1-2 9.159 1.386 0.205 1-3 9.154 1.385 0.212 2-1 8.923 1.412 0.195 2-2 8.973 1.373 0.203 2-3 8.861 1.358 0.202 2-4 8.919 1.381 0.200 3-1 7.124 1.828 0.207 4-1 5.884 1.396 0.168 5-i 7.825 1.169 0.185 5-2 6.541 1.325 0.167 6-1 5.821 1.465 0.152 6-2 5.710 1.378 0.151 6-3 5.694 1.337 0.147 7-1 6.430 1.366 0.156 7-2 6.307 1.306 0.170 7-3 6.417 1.344 0.160

342 ± 39 331 ± 36 355 ± 35 347 ± 42 337 ± 42 346 ± 38 359 ± 37 336 ± 28 341 + 29 343 + 7

session)

343 ± 33 337 ± 34 349 +_ 33 323 ± 35 339 + 34 341 ± 34 335 _+ 33 362 ± 43 362 ± 43 356 ± 39 346 ± 41 322 + 42 332 + 44 328 ± 45 321 ± 41 362 + 42 333 ± 42 340 .+ __7.7


compositional difference between the monazites of sample HS 11 and HS 17. The Th contents always lay between ca. 3 and 4.5 wt.% oxides. Uranium substitution (ca. 0.4 to 0.9wt.% UO2) was generally relatively high; this is often observed in monazites from paragneisses. The UO2 contents were positively correlated with the Y203 contents (1.8-2.5 wt.%). The latter are also moderately high, and in the typical range for monazites that grew under middle amphibolite facies conditions (Franz et al., 1996). The LREE systematics of the measured monazites was fairly constant, La/Nd ratios displayed only a slight variation (Table 1). The charge balance for the Th substitution is mainly established through a brabantite CaTh(PO4)2 component.

Model ages for 34 analyses are given in Table 2. For each sample a weighted average age was calculated, following the suggestions of Montel et al. (1996) and using the geochronological programm of Ludwig (1980). The results were almost identical in both samples (633±10Ma and 636±10Ma respectively). All single monazite analyses overlap in error with these mean ages and the whole data set fulfills the criteria of a Gaussian distribution. There is no evidence for any systematic age differences between the monazites included in garnet, those in biotite of quartz, or those situated along the grain boundaries of the major minerals.

All this suggests that the analysed monazites formed at about the same time and that their Th-U-Pb system did not suffer significant post-crystalline alteration, at least as far as the grain centres are concerned. Since complete resetting during the greenschist facies event in unrealistic, it may be assumed that the calculated average ages approximately date monazite crystallization, and thus, amphibolite facies metamorphism of the rocks.

5.4 A chemical profile across a zoned monazite

During a second analytical session, a chemical profile was recorded across a ca. 30 gm sized monazite (grain 19) from the slightly retrogressed sample HS 11. The grain was one of those which showed a slight concentric zoning in the BSE image. The aim was to quantitatively investigate the zoning patterns and the element covariations, and to test whether a systematic age difference exists between central and marginal points. Analyses were carried out with a focused beam in ca. 2.5 gm steps (Table 3, Fig. 3).

A relatively homogeneous, ca. 15 gm wide central zone (analysis points 6-12) may be distinguished from a ca. 5-10 gm wide rim zone with enhanced UO2 and Y203 contents. It is apparent that the weak zoning features recognized in the BSE image result from the higher concentrations of these heavier atoms.

The element distribution patterns recorded in the profile are complex and not well correlated. Only UOa and YaO3 display a quite simple, more or less sympathetic positive correlation throughout the whole traverse with low values in the centre and a steady increase of concentrations in the rim zone. Although clearly increasing from the core to the rim zone, ThO2 is negatively correlated with the UO2 in the rim section and decreases again outwards.

It can be seen from Table 3 that the incorporation of the 4 + cations U and Th is mainly balanced by the substitution of Ca 2+ for R E E 3+ (brabantite component) and


Table 3. Microprobe data for grain 19 (see Fig. 3). Mineral formulas were calculated on the basis of 4 oxygens. The analyses have a ca. 5% deficit in the totals and slightly low cation sums in the A [9] position, because Sm, Gd and the HREEs were not determined. Br and Hu are the theoretical percentages of brabantite and huttonite solid solution in monazite as recast from the Ca and Si formula units. Model ages given are calculated after the method of Montel et al. (1996)

Point 2 3 4 5 6 7 8 9 10 11 12 13 14

SiO2 0,11 0,11 0 ,13 0,11 0,11 0 ,15 0 ,13 0 ,19 0 ,14 0 ,12 0,11 0,11 0,12 P205 29,60 30,63 29,41 29,52 29,31 29,37 29,31 29,15 29,27 29,01 29,41 29,85 29,40 CaO 0,96 0 ,97 0 ,99 0 ,99 0 ,77 0 ,76 0,81 0 ,79 0,75 0 ,76 0 ,84 0 ,96 1,02 Y203 2,59 2 ,59 2 ,38 2 ,12 1 ,82 1,93 2 ,16 1 ,94 1,86 1 ,92 1,96 2 ,36 2,51 La203 12,75 12,86 12,77 13,20 13,07 12,79 12,77 12,92 13,04 12,93 13,09 13,08 12,77 Ce203 27,74 28,28 27,52 28,07 28,37 27,97 27,96 28,07 28,53 28,23 28,24 27,98 27,46 Pr203 3,51 3 ,43 3 ,36 3 ,48 3 ,66 3 ,49 3 ,58 3 ,54 3 ,39 3 ,46 3 ,63 3 ,53 3,37 Nd203 12,33 12,31 11,70 11,47 12,06 12,27 12,28 12,32 12,44 12,29 12,15 11,90 11,72 ThO2 3,99 4 ,19 4 ,39 4 ,47 3 ,82 3 ,96 4 ,02 3 ,92 3 ,77 3,81 3 ,93 4,11 4,03 UO2 1,02 1,05 0 ,93 0,81 0 ,45 0 ,53 0 ,45 0 ,50 0 ,45 0 ,46 0 ,58 0 ,92 1,17 PbO 0,20 0,21 0 ,20 0 ,19 0 ,14 0 ,15 0 ,15 0 ,15 0 ,14 0 ,14 0 ,16 0 ,18 0,21 Total 94,59 96,42 93,56 94,23 93,41 93,20 93,44 93,32 93,60 92,96 93,91 94,77 93,54

Si 0,004 0,004 0,005 0,004 0,005 0,006 0,005 0,008 0,006 0,005 0,004 0,004 0,005 P 1,013 1,022 1,016 1,015 1,017 1,018 1,015 1,013 1,014 1,013 1,015 1,017 1,016 Ca 0,042 0,041 0,043 0,043 0,034 0,034 0,035 0,035 0,033 0,034 0,037 0,041 0,044 Y 0,056 0,054 0,052 0,046 0,040 0,042 0,047 0,042 0,040 0,042 0,043 0,051 0,054 La 0,190 0,187 0,192 0,198 0,197 0,193 0,193 0,196 0,197 0,197 0,197 0,194 0,192 Ce 0,411 0,408 0,411 0,417 0,425 0,419 0,419 0,422 0,427 0,426 0,421 0,412 0,410 Pr 0,052 0,049 0,050 0,052 0,055 0,052 0,053 0,053 0,051 0,052 0,054 0,052 0,050 Nd 0,178 0,173 0,171 0,166 0,176 0,179 0,179 0,181 0,182 0,181 0,177 0,171 0,171 Th 0,037 0,038 0,041 0,041 0,036 0,037 0,037 0,037 0,035 0,036 0,036 0,038 0,037 U 0,009 0,010 0,008 0,007 0,004 0,005 0,004 0,005 0,004 0,004 0,005 0,008 0,011 Pb 0,002 0,002 0,002 0,002 0,002 0,002 0,002 0,002 0,002 0,002 0,002 0,002 0,002

tetr. 1,018 1,026 1,021 1,019 1,021 1,024 1,021 1,020 1,020 1,018 1,020 1,022 1,020 A(9) 0,971 0,957 0,966 0,968 0,965 0,959 0,966 0,967 0,967 0,970 0,968 0,965 0,967

Mo(ss) 90,96 90,80 90,41 90,52 92,38 92,16 92,02 92,14 92,55 92,39 91,91 90,96 90,44 Br 8,60 8 ,75 9 ,06 9 ,03 7 ,16 7 ,22 7 ,42 7 ,04 6 ,88 7 ,10 7 ,63 8 ,59 9,07 Hu 0,44 0 ,44 0 ,53 0 ,45 0 ,47 0 ,62 0 ,56 0,81 0 ,58 0,51 0 ,46 0 ,45 0,48

Th 3,506 3,683 3,855 3,929 3,356 3,481 3,533 3,448 3,313 3,352 3,452 3,610 3,539 U 0,876 0,901 0,791 0,682 0,370 0,439 0,375 0,418 0,374 0,381 0,486 0,783 1,004 Pb 0,182 0,196 0,188 0,176 0,133 0,145 0,139 0,138 0,131 0,135 0,148 0,170 0,194 Age 632 653 644 632 646 651 646 635 640 651 648 609 628 Error 69 67 69 72 97 90 93 92 98 96 88 72 65

only to a minor degree by an exchange of Si 4+ for pS+ in the tetrahedral sites (huttonite substitution). Nevertheless, it is interesting to note that the huttonite/ brabantite ratios are clearly higher in the grain centre (Table 3). La/Nd ratios show a slight outward fractionation through the core section, but this trend is reversed in the rim section.

282

12

5 t

4 -

0

point I

Ma -, 800 -

700

6 0 0 - -

50O

4 0 O

F. Finger and H. M. Helmy

La203 ~o--~_~_ o ° / - - / 0 ~ 0 ~ ~ o _ _ e . / ~ o ~ o ~ O j t o O ~ o

Nd203~A~..._ A / J "'~L--~ A

ThO

v ~ ~v'~ ~ v ~ V ~ v _ ~ _ _ v ~ v ~ v _ _ _ / v ~ V ~ - v

Y O 2 3

I ~ m / l / n ~ i ~ m ~ n ~ n J UO 2 4~--~'~---~___~1 ' j , j j O j e '

I I I I I I I I I I I t I 2 3 4 5 6 7 8 9 10 11 12 13 14 I I I I I I I I I I I I I _ _

rim core rim

14

12

-4

- 8 0 0

- 7 0 0

- 6 0 0

- 5 0 0

2 . 4 0 0

Fig. 3. Chemical profile across grain 19 (cf. Table 3). Upper diagram features the variation of UO2, Y203, ThO2, La203 and Nd203. X-axis scale is in wt.% oxides. Lower diagram shows the calculated model ages with +2-sigma error bars for the single points

The 34 analyses of grain centres (Tables 1, 2) never reached the high uranium concentrations that were recorded at the margins of grain 19 (Fig. 4). Approximately one third of these analyses match more or less with those of the core section of grain 19. The rest are intermediate relative to core and rim composition of grain 19 and feature also a clear positive correlation between uranium and yttrium. Thus, it may be concluded that the chemical variation displayed by grain 19 is largely representative of the overall chemical systematics of monazite in the two investigated metapelitic paragneisses.


1,2

1,0

0 , 8 "

0,6 "

0,4 -

0,2

UO 2

+ +

+~- + +

J; + 4- +

+ +

+ ~ ' ~ +

0 +~ O0 ~ 0

"~-H- + +

Y203

0 , 0 ' I ' I ' I ' I ' I '

0,0 0,5 1,0 1,5 2,0 2,5 3,0

Fig. 4. U O 2 v e r s u s Y203 diagram with plots of all monazite analyses from samples HS 11 and HS 17. Crosses are analyses in grain centres, circles show the chemical zoning recorded in grain 19 (filled circles: rim section, open circles: core section; see text and Table 3)

6. Discussion

The petrological significance of the elemental covariation observed in the monazites of the two metapelitic paragneiss samples is not easy to assess. Due to the clear chemical discontinuities between core and rim zone of grain 19, it would seem that monazite growth occurred over an extended period of the metamorphic rock evolution. The two zones probably formed at two different metamorphic stages characterized by changing REE, Y, Th and U availabilities. However, this does not necessarily point at two distinct metamorphic events. For example, the major mineral reactions that are crossed on a prograde P, T path may well be responsible for consumption or release of larger amounts of distinct trace elements and may thus produce a discontinous trace element zoning in contemporaneously forming monazite, without requiring significant P, T variations. Speculations that the outer shell of the monazite grain might have grown during retrogression under greenschist facies conditions are incompatible with the relatively high Y203 contents near the grain margin. Due to the solvus relationship between monazite and xenotime (Heinrich et al., 1996), Y and HREE contents in monazites can be used to estimate temperature conditions. When xenotime is additionally present in paragneisses, as in the present case, an elevated Y content at


monazite rims suggests monazite growth under prograde rather than under retrograde temperature conditions. During regional metamorphism, monazites may start to grow in the upper greenschist facies (Franz et al., 1996). Such early stage greenschist facies monazites may then be overgrown successively by new monazite shells during the further prograde P, T evolution until peak conditions are reached.

The model ages appear to be on average slightly higher in the central parts of grain 19 (Fig. 3). However, the mean age calculated for the core zone (645:~15 Ma for analysis points 6-11) overlaps in error with the mean age calculated for the rim zone (633:~16 Ma for analysis points 2-5 and 13-14). The significance of the slight age difference is thus unclear and the method of EMP monazite dating is obviously not sensitive enough to resolve geological events within a 10-20Ma time span. Alternatively, the generally lower model ages of the marginal points 2, 13, 14 may perhaps be explained in terms of lead loss from the outer few micrometers of the crystal.

However, despite of the limited resolution of EMP monazite dating on a detailed time scale, the data presented here provide strong support for the

0,24 -

0,22 -

0,20

0,18

0,16

0,14

0,12

0,10

0,08

0,06

0,04

0,02

0,00

1000 750 6 6 0 6 2 0 360. ,320,

i / Pb /

/

O / i

° /

l t o V

/ t z~ HS11 / / j / ..s17

' ,,, Grain 19

/ / / /

/ / I / / / 2 (Y /1// / Th*

' I ~ 1 ' I ' I ' I ' I ' I = I '

0 2 4 6 8 10 12 14 16 18

Fig. 5. Total Pb vs Th* isochrone diagram after Suzuki et al. (1994) with plots of all monazite analyses obtained during this study. Th* values have been recast with the model ages given in Tables 2 and 3. Open and filled circles are data for laboratory standard F5 gathered during the two different analytical sessions. Regression lines include all sample and standard data points respectively. Time scale shown is based on the position of zero- intersect isochrones


assumption that amphibolite facies metamorphism in the host rocks is not 1200 Ma old, but occurred most likely around 620-650 Ma.

All monazite data from samples HS 11 and HS 17 are graphically presented in a Pb versus Th* isochrone diagram (Fig. 5, Suzuki et al., 1994), together with the results obtained for the laboratory standard F5. A regression line calculated for the standard data almost perfectly reproduces the recommended age value of 341 Ma and at the same time intersects the Y-axis very close to the origin. This demonstrates the accuracy of the Th, U, Pb analytics, since the F5 age standard contains only negligible amounts of common lead, as expressed in the high 2°6pb/ 2°4pb ratios determined by mass spectrometry (Friedl, 1997). A regression line for all data from samples HS 11 and HS 17 intersects the Y-axis also close to the origin, suggesting that common lead can be neglected here as well. This justifies the application of the model age calculation procedure of Montel et al. (1996), which was generally used throughout this paper.

7. Conclusions

Previously published geochronological investigations of the Precambrian basement in the Eastern Desert have concentrated on the determination of formation ages of the (meta)igneous rocks, mainly by means of zircon dating (e.g. Stern and Hedge, 1985; Krrner et al., 1992, 1994). Geochronometric methods known as particularly effective for dating high-grade metamorphic events, such as monazite dating, have to our knowledge not been applied as yet, and this may be one reason why the metamorphic evolution of the area is still very poorly constrained.

The chemical monazite model ages presented in this paper for Abu Swayel paragneisses thus provide new important geochronological information. Obviously, the amphibolite facies metamorphism in the Abu Swayel basement rocks occurred much later than previously assumed. The data are suggestive of a high-grade regional metamorphic event around 620-650 Ma.

There are several lines of evidence suggesting that this amphibolite facies metamorphism was related to a major collisional tectonic event. Firstly, many of the post-600 Ma geological features of the Eastern Desert basement may be best explained in terms of an extensional collapse of previously thickened crust (uplift of metamorphic core complexes, Molasse basin formation, large masses of granites - see Greiling et al., 1994). Secondly, high-pressure relics have been recorded in the striking continuation of the Abu-Swayel ophiolite thrust (Taylor et al., 1993). The lack of high-P assemblages in the investigated metapelitic samples does not contradict the hypothesis of a collision-related metamorphic event. It is normal that the basement in collision zones is mainly affected by intense medium- pressure regional metamorphism, following thrusting and uplift (Biittner and Kruhl, 1997; Dachs, 1990). Thirdly, a phase of deformation and metamorphism occurred at about the same time just west of the fiver Nile, at the eastern margin of the East Sahara Craton (Sultan et al., 1994).

Thus, it is likely that a Panafrican terrane assembly attached to the East Sahara Craton between ca. 620 and 650 Ma. As mentioned earlier, a metamorphic event of roughly the same age has been documented further south in the Sudan, and has been interpreted as probable collisional metamorphism (Harms et al., 1994). It


would appear that a Neoproterozoic collision event can be traced over a long section of the East Sahara Craton plate margin, implying that an extended island arc or continental terrane docked from the (present day) east.

This probable collisional metamorphism at ca. 620-650 Ma is distinct from an earlier phase of high-grade metamorphism documented in other places of the Arabian Nubian Shield: In the Hafafit dome, migmatite textures in paragneisses are cut by tonalite and trondhjemite intrusions dated at ca. 700 Ma (Greiling et al., 1994; KrOner et al., 1994). A sample of such a migmatitic paragneiss from the Hafafit dome (Neumayr et al., 1996) was recently subjected to EMP monazite dating in Salzburg, using the same analytical procedure as described in this paper, and several model ages of ca. 710 Ma were obtained (Finger and Neumayr, unpubl. data). It seems that migmatization in the Hafafit dome occurred roughly contemporaneously with the ca. 700-720Ma regional metamorphic event in northern Sudan (Harms et al., 1994).

The presence of fragments of an (overthrusted?) cratonic basement in the Eastern Desert Call not be ruled out definitely and still remains unproven. Middle Proterozoic to Archaen Pb and Nd model ages and single-grain zircon ages (Harris et al., 1984; Wust et al., 1987) from metasedimentary rocks occurring not far away from the Abu Swayel area, indicate that at least some old cratonic detritus is involved.

Basically, this study highlights the feasibility of dating high-grade metamorphism in paragneisses with moderate precision through analysing monazites by electron microprobe. Apart from the considerably higher expenditure of time and money, conventional U-Pb isotope dating may have been quite problematic in the present case, since the monazites are small and thus not easy to extract. On the other hand, there are few good alternatives for dating the amphibolite facies metamorphism in the rocks. Zircons are usually not suitable for this purpose because of their detrital origin and their particularly refractory nature, mica ages always carry the risk of being partially reset during the later greenschist facies overprint, and the garnets in the rocks commonly contain large numbers of inclusions, so that Sm-Nd dating is also problematic.

The study does, of course, also disclose the limits of the method of EMP monazite dating. For example, potential time differences between two monazite forming events (e.g. during prograde metamorphism) in the range of less than 20 Ma cannot be resolved reasonably in most cases. However, in cases where the metamorphic evolution of an area is so little constrained, monazite dating with the electron microprobe serves as a valuable tool for obtaining a first rough age estimate. Reconaissance studies with this quick and simple method may greatly aid carefully directed selection of suitable samples for subsequent, more precise conventional isotope geochronological techniques.

Acknowledgements

Our thanks go to the anonymous journal reviewers, who provided us with helpful comments. We would also like to thank E Schitter, H. P. Steyrer, and G. Riegler for drafting some of the figures and for various other technical assistances. M. Roberts kindly improved


the English. The work was supported by the Austrian Fonds zur Ftrderung der wissenschaftlichen Forschung (grant 11674 Geo).

References

Berman RG (1990) Mixing properties of Ca-Mg-Fe-Mn garnets. Am Mineral 75:328-344 Biittner St, Kruhl JH (1997) The evolution of a late-Variscan high-T/low-P region: the

south-eastern margin of the Bohemian Massif. Geol Rundschau 86:21-38 Dachs E (1990) High-pressure mineral assemblages and their breakdown-products in

metasediments south of the Grossvenediger, Tauern Window, Austria. Schweiz Mineral Petrogr Mitt 66:145-161

De Wolf CP, Belshaw N, O'Nions RK (1993) A metamorphic history from micron-scale 2°7pb/2°6pb chronometry of Archean monazite. Earth Planet Sci Lett 120:207-220

El Gaby S, List FK, Tehrani R (1990) The basement complex of the Eastern Desert and Sinai. In: Said R (ed) The geology of Egypt. Balkema, Rotterdam, pp 175-184

El Shazley EM, Hashad AH, Sayyah TA, Bassyouni FA (1973) Geochronology of the Abu Swayel area, South Eastern Desert. Egypt J Geol 17:1-18

E1 Shazley EM, Bassyoumi FA, Abdel Khalelk ML (1977) Geology of the Greater Abu Swayel Area, Eastern Desert, Egypt. Egypt J Geol 19:1-41

El Rarely ME Akaad MK (1960) The basement complex in the CED of Egypt between lat. 24°30 ~ and 25040 '. Geol Surv Egypt Pap No 8: 33p

Exley RA (1980) Microprobe studies on REE-rich accessory minerals: implications for Skye granite petrogenesis and REE mobility in hydrothermal systems. Earth Planet Sci Lett 48:97-110

Ferry JM, Spear FS (1978) Experimental calibration of the partitioning of Fe and Mg between biotite and garnet. Contrib Mineral Petrol 66:113-117

Finger F, Benisek A, Broska L Friedl G, Haunschmid B, Schermaier A, Schindlmayr A, Schitter F, Steyrer HP (1996) Altersdatieren yon Monaziten mit der Elektronen- mikrosonde - Eine wichtige neue Methode in den Geowissenschaften. Erweiterte Kurzfassungen TSK 6. Fakultas Universit~itsverlag, Wien

Franz G, Andrehs G, Rhede D (1996) Crystal chemistry of monazite and xenotime from Saxothuringian-Moldanubian metapelites, NE Bavaria, Germany. Eur J Mineral 8: 1097-1118

Friedl G (1997) U/Pb-Datierungen an Zirkonen und Monaziten aus Gesteinen vom 6sterreichischen Anteil der Bthmischen Masse. Thesis, University of Salzburg, pp 242

Fritz H, Wallbrecher E, Khudeir AA, Abu El Ela F, Dallmeyer DR (1996) Formation of Neoproterozoic metamorphic core complexes during oblique convergence (Eastern Desert, Egypt). J African Earth Sci 23:311-329

Greiling RO, Abdeen MM, Dardir AA, El Akhal H. El Ramly MF, Kamal El Din GM, Osmann AE Rashwan AA, Rice AHN, Sadek MF (1994) A structural synthesis of the Proterozoic Arabian Nubian Shield in Egypt. Geol Rundschau 83/3:484-501

Harms U, Darbyshire DPF, Denkler T, Hengst M, Schandelmeier H (1994) Evolution of the Neoproterozoic Delgo suture zone and crustal growth in Northern Sudan: geochemical and radiogenic isotope constraints. Geol Rundschau 83/3:591-603

Harris NBW, Hawkesworth CL, Ries AC (1984) Crustal evolution in northeast and east Africa from model Nd ages. Nature 309:773-776

Hassan MA, Hashad AH (1990) Precambrian of Egypt. In: Said R (ed) The geology of Egypt. Balkema, Rotterdam, pp 201-245

Heinrich W, Andrehs G, Franz G (1997) Monazite-xenotime miscibility gap thermometry. I. An empirical calibration. J Met Geol 15:3-16


Helmy HM (1996) Precious metal and base metal mineralization at Abu Swayel and Um Samiuki, Eastern Desert, Egypt. Thesis, E1 Minia University Egypt, 238p (unpublished)

Kingsbury JA, Miller CF, Wooden JL, Harrison TM (1993) Monazite paragenesis and U-Pb systematics in rocks of the eastern Mojave Desert, California, U.S.A: implications for thermochronometry. Chem Geol 110:147-167

Koziol AM (1989) Recalibration of the garnet-plagioclase-AlzSiOs-quartz barometer. Am Mineral 73:216-223

KrOner A, Greiling RO, Reischmann T, Hussein IM, Stern R J, Diirr S, Kriiger J, Zimmer M (t987) Pan-African crustal evolution in the Nubian segment of northeast Africa. In: KrOner A (ed) Proterozoic lithospheric evolution. Am Geophys Union Geodyn Ser 17: 235-257

KrOnerA, Eyal M, Eyal Y (1990) Early Pan-African evolution of the basement around Elat, Israel, and the Sinai Peninsula revealed by single-zircon evaporation dating, and implication for crustal accretion rates. Geology 18:545-548

KrOner A, Todt W, Hussein IM, Mansour M, Rashwan AAA (1992) Dating of late Proterozoic ophiolites in Egypt and the Sudan using the single zircon evaporation technique. Precamb Res 59:15-32

KrOner A, Kriiger J, Rashwan AAA (1994) Age and tectonic setting of granitoid gneisses in the Eastern Desert of Egypt and south-west Sinai. Geol Rundschau 83/3: 502-513

Lanzirotti A, Hanson GN (1996) Geochronology and geochemistry of multiple generations of monazite from the Wepawaug Schist, Connecticut, USA: implications for monazite stability in metamorphic rocks. Contrib Mineral Petrol 125:332-340

Ludwig KR (1980) Calculation of uncertainties of U-Pb isotope data. Earth Planet Sci Lett 46:212-220

Massonne H J, Schreyer W (1987) Phengite geobarometry based on the limiting assemblage with K-feldspar, phlogopite, and quartz. Contrib Mineral Petrol 96:212-224

MonteI JM, Foret S, Veschambre M, Nicollet Ch, Provost A (1996) A fast, reliable, inexpensive insitu dating technique: electron microprobe ages on monazite. Chem Geol 131:37-53

Neumayr P, Hoinkes G, PuhI J, Mogessie A (1996) Metamorphic evolution of two Panafrican metamorphic core complexes in the Eastern Desert of Egypt: tectonic implications. Mitt CIsterr Mineral Ges 141:165-167

Parrish RR (1990) U-Pb dating of monazite and its application to geological problems. Can J Earth Sci 27:1431-1450

Rhede D, Wendt I, FOrster HJ (1996) A three-dimensional method for calculating independent chemical U/Pb- and Th/Pb-ages of accessory minerals. Chem Geol 130: 247-253

Shackleton RM (1994) Review of Late Proterozoic sutures, ophiolitic mOlanges and tectonics of eastern Egypt and north-east Sudan. Geol Rundschau 83/3:537-546

Smith HA, Barreiro B (1990) Monazite U-Pb dating of staurolite grade metamorphism in pelitic schists. Contrib Mineral Petrol 105:602-615

Steiger RH, Jiiger E (1977) Subcommission on geochronology: convention on the use of decay constants in geo- and cosmochronology. Earth Planet Sci Left 36:359-362

Stern J, KrOner A, Bender R, Reischmann T Dawoud AS (1994) Precambrian basement around Wadi Halfa, Sudan: a new perspective on the evolution of the East Sahara Craton. Geol Rundschau 83/3:564-577

Stern R J, Hedge CE (1985) Geochronologic and isotopic constraints on Late Proterozoic crustal evolution in the Eastern Desert of Egypt. Am J Sci 285:97-127

Sultan M, Tucker RD, E1 Alfy Z, Atria R, Ragab AG (1994) U-Pb (zircon) ages for the gneissic terrane west of the Nile, southern Egypt. Geol Rundschau 83/3:514-522.


Suzuki K, Adachi M, Tanaka T (1991) Middle Precambrian provenance of Jurassic sandstone in the Mino Terrane, central Japan: Th-U-total Pb evidence from an electron microprobe monazite study. Sediment Geol 75:141-147

Suzuki K, Adachi M, Kajizuka I (1994) Electron microprobe observations of Pb diffusion in metamorphosed detrital monazites. Earth Planet Sci Lett 128:391-405

Taylor WEG, Hamed El, Kazzaz YA, Rashwan AA (1993) An outline of the tectonic framework for the Pan-African orogeny in the vicinity of Wadi Um Relan, SE Desert, Egypt. In: Thorweihe U, Schandlmeier H (eds) Geoscientific research in NE Africa and adjacent areas (Earth Evol Sci). Vieweg, Wiesbaden, pp 195-226

Wust HJ, Todt W, KrOner A (1987) Conventional and single grain zircon ages for metasediments and granite clasts from the Eastern Desert of Egypt: evidence for active continental margin evolution in Pan-African times. Terra Cognita 7:333-334

Authors' addresses: F. Finger, Institut fiir Mineralogie, Universit~it Salzburg, Hellbrunner- strasse 34, A-5020 Salzburg, Austria; H. Helmy, Geological Department, Faculty of Science, E1 Minia University, E1 Minia, Egypt

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Composition and total-Pb model ages of monazite from high