U-Pb Geochronology and Zr Saturation Temperatures for ... · U-Pb Geochronology and Zr Saturation...

----------------------------------------------JJoouurrnnaall ooff SSeebbhhaa UUnniivveerrssiittyy--((PPuurree aanndd AApppplliieedd SScciieenncceess))--VVooll..66 NNoo..22 ((22000077)) ---------------------------------------- 85

U-Pb Geochronology and Zr Saturation Temperatures for Granitoid

Rocks from Abu Murrat Area, North Easthern Desert, Egypt Aboazom, A. S.* , Abdel Ghani, M.S.**

Abstract Abu Murrat granitic rocks represent one of the Pre-Cambrian acidic intrusions

of the northeasthern desert Egypt (~100km2). Zircon morphology, U-Pb geochronology and zircon saturation temperatures of these rocks have been studied in order to understand the crystallization history. These granitic rocks have been classified as three different intrusions: granodiorite, monzogranite and more evolved alkali granite. The crystal morphology of zircon grains exhibits the dominance of subrounded shape possibly due to the partial melting during the igneous activity before the emplacement of the intrusions. Moreover, these zircon grains are unzoned and core-free crystals.

These granitic rocks represent homogeneous U-Pb ages together with undisturbed isotopic compositions, which suggest and preclude any major role played by inherited materials or post-magmatic events. This may suggest that the igneous processes involved in derivation of these rocks were dominated by magmatic ones. However, slight U-Pb discordance of monzo-and alkaligranite analyses are possibly due to amount of structural damages within grains.

It is proposed that all three intrusions were dominated by basic source materials, with two main episodes of crystallization, one for granodiorite (658.6Ma) with temperatures in the range of 808-830oC and another for more differentiated monzogranite (621.86Ma).

The crystallization of monzogranite probably occurred after new pulses of basic magma with high Zr contents were introduced into the magma chamber. Such magma composition would raise the crystallization temperature to 880-896oC and increase zircon population in this intrusion (monzogranite). Thereafter, the rest of magma differentiated with time and crystallized another phase of monzogranite at 613.6-612.9Ma., until the crystallization of alkali granite occurred at 598.9Ma with temperatures consisten with those of granitic rocks (769-779oC).

Key words: Zircon; Saturation temperatures; Egypt, Abu Murrat; U-Pb geochronology

*Earth Science Department- Faculty of Science- University of Sebha. **Geology Department- Faculty of Science- Minia University- Egypt..

Aboazom, A. S. and Abdel Ghani, M.S. ------------------------------------------------------------------------------------------------

86 ----------------------------------------------JJoouurrnnaall ooff SSeebbhhaa UUnniivveerrssiittyy--((PPuurree aanndd AApppplliieedd SScciieenncceess))--VVooll..66 NNoo..22 ((22000077)) ----------------------------------------

1. Introduction Several studies concerning the U-

Pb content of accessory minerals in geochronology from igneous, metamorphic and sedimentary rocks have been undertaken in the last forty years to unravel the role of radiogenic Pb in most geological processes, (e.g. Halliday, et al., 1984;Aftalion and Max, 1987; O'Connor, et al., 1989; and Ekwueme and Kroner, 2006).

Many of these studies are concerned with elucidation of crystallization conditions, metamorphic events and sedimentary processes of some accessory minerals and the processes affecting their U-Pb isotopic compositions.

The melt-precipitated zircon, in many igneous rocks, is dominantly a closed U-Pb isotopic system. So zircon is a particularly valuable mineral for U-Pb geochronology.

This is because zircon incorporates uranium but little lead during crystallization and therefore a high proportion of radiogenic lead in zircon has resulted from in situ decay of uranium.

Zircon is also extremely resistant to both chemical and physical weathering (Pidgeon, 1992).

In the meantime, in recent years several studies dealing with the behavior of accessory minerals in granitic rocks, have been undertaken in order to investigate the Zr and P2O5 saturation temperatures (e. g.

Watson and Harrison, 1983; Harrison and Watson, 1984; and Aboazom, 1995, 2004).

These saturation temperatures were considered as the temperatures of the peak of partial melting (Harrison, et al., 1987).

However, Aboazom (2004) used these saturation temperatures in conjunction with temperatures derived from feldspar thermometry for granitic intrusions to confirm their reliability.

The granitic rocks of Gabal Abu Murrat area which crop out on the NE desert, Egypt have been classified into three phases and mineralogical characteristics. These are granodiorite, monzogranite, and alkali granite, (Sabet et al., 1976; Abdel Ghani, et al., 1995). These acidic rocks and other related intrusions have crystallized during Neoproterozoic igneous activity at 550-650Ma (Fullagar 1978, Ghuma and Rogers 1978 and Bentor 1985).

In this contribution we present U-Pb data of separated zircon grains alongside with the Zr saturation temperatures for the different granitic types from Gabal Abu Murrat, to elucidate the effect of radiogenic Pb during their crystallizations.

The determined Zr-saturation temperatures are used to detect the order of crystallization of these

----------------------------------------------------------- U-Pb Geochronology and Zr Saturation Temperatures for Granitoid Rocks


intrusions and subsequently their crystallization history.

2. Geological setting The investigated acidic rocks are

part of Pan-African Arabian-Nubian shield (ANS), lying in the northeastern desert (NED) of Egypt, and cover an area of about 100 km2 (Fig.1).

These rocks have been classified by Abdel Ghani et al. (1995) based

on their geochemical character as older granites and younger granites. The older granites contains granodiorite, whereas, the younger granites includes both monzogranite and alkali granite.

N2650

o

-

33 50o -

Granodiorite

Monzogranite

Alkali granite

Dykes

Phanerozoic

Faults

0 1 2 km

W.Abu Ghali

W.Abu Murrat

G. Abu Murr at

Red Se a

Eastern D

eser t

West ern D

esert

Sin

ai

Study area

W.Barud

Fig. 1: Geological map of Gabal AbuMurrat showing sample localities

(Modified after, Abdel Ghani et al., 1995) The granodiorites form relatively

subdued and are intruded in some places by alkali granite as dykes and pegmatitic veins.

However they grade imperceptibly into monzogranite.

The monzogranites are represented by relatively higher

relief terranis compared to the associated granodiorites, and alkali granites.

They possess mixed and gradational contacts with granodiorite (Fig.1).

However, Sabet et al., (1976) and Abdel Ghani et al. (1995) confirmed


88 --------------------------------------------JJoouurrnnaall ooff SSeebbhhaa UUnniivveerrssiittyy--((PPuurree aanndd AApppplliieedd SScciieenncceess))--VVooll..66 NNoo..22 ((22000077)) ----------------------------------------

their gradational contacts with the later alkali granites, and hence suggested that this intrusion might be considered as a transitional phase between granodiorite and more differentiated alkali granites.

Alkali granite comprises the dominant rock type in Gabal Abu Murrat acidic intrusions and forms the higher mountain elongated in NW-SE trend. This intrusion shows both sharp and gradational contacts with the earlier intrusions (granodiorite and monzogranite).

The emplacement of Gabal Abu Murrat acidic rocks and other related rocks in the area lie mostly in NW-SE direction and might have been structurally controlled by pre-

existing fractures as has been suggested by Sabet et al., (1972). Thereafter, this conclusion is consistent with the proposal introduced by Bentor (1985) and Avigad et al. (2003), which presumes that the emplacement of acidic intrusions related to the Gabal Abu Murrat acidic rocks comes because of igneous activity of post-tectonic processes during the Neoproterozoic time.

The presence of xenoliths of amphibolitic composition within the granodiorite and in monzogranite clearly suggests that these rocks intruded through amphibolitic crustal materials for their final emplacement.

3. Petrography and Zircon Morphology This paper is concerned mainly

with zircon grains, hence the petrography of host rocks is only desribed below: Granodiorites are coarse to medium grained, occasionally containing porphyritic texture and composed essentially of andesine, quartz, alkali feldspar, biotite and hornblende, along-side with accessory minerals such as allanite, zircon, sphene, epidote and Fe-Ti oxides. Secondary chlorite is also present.

Monzogranite, mainly medium grained, with porphyritic texture, comprises of alkali feldspar, oligoclase and quartz. Biotite is the main mafic mineral, some muscovite

flakes, zircon, apatite and Fe-Ti oxide occur as accessory minerals. Alkali granite is medium to coarse grained and consist of orthoclase, microcline as phenocrysts along side with quartz, albite, biotite and rare hornblende and muscovite.

Zircon, apatite, epidote and Fe-Ti oxide are the accessory minerals that frequently cluster with the major mafic phases.

Zircon texture has been widely used for granitic rocks as an indicator for the presence of inherited crustal materials for example (Gupta, 1973, Aftalion and Max, 1987, Paterson et al. 1989,



O'Connor et al. 1989, Paterson et al., 1992 and Aboazom, 2004).

However, to the best of knowledge of the present authors, there are no such investigations that have been reported for zircon from these granitic rocks.

Therefore, zircon crystals from three different intrusions were studied here from Abu Murrat granitoid rocks.

They show subhedral to anhedral, squat crystals and are mostly clear and colourless, often fragmented and few have some cracks.

However, the clear overgrowths were not found and zoning is very rare.

These zircons are dominated by colorless grains and these are mostly included in feldspar and quartz grains, and rarely associated with patches of secondary chlorite in granodiorite samples.

This is consistent with the findings of Patchett and Jocelyn (1979), which showed that zircon included in quartz and feldspar should have an amount of U≤ 300ppm (Table 1).

Inclusions from apatite needles have been observed within zircon grains.

4. U-Pb Isotope Results

The U-Pb results for zircons from Abu Murrat granitic rocks are shown in Table1 and Fig.2.

The data represent one sample from granodiorite rocks (analysis 6), four samples from monzo-granite (analyses 2,3,4 and 5) and one sample from alkali granite (analysis1).

The granodiorite analysis plots on Concordia at 658.6Ma and probably represents age crystallization of the zircon.

However, the other sequence of analyses defines a discordia (or semi-discordia) with a higher intercept of ~612.9Ma (analysis 2).

Accepting that there are two different magmas were emplaced at

mentioned times (658.6Ma and about 621.86Ma), (Patchett and Jocelyn 1979, Aftalion and Max, 1987).

The monzogranite and alkali granite analyses define an apparent Pb-loss line (Fig. 2), with analysis 4 mostly disturbed.

According to Williams, (1992) who stated that the structural damage within the zircon crystals from any one rock are difference and resulting in difference isotopic signature from one grain to another.

This could fairly explain the apparent Pb difference withinn analyses of monzogranite and within the study intrusions as well.



The analyses 1and 4 might be most structurally damaged and then more isotopically disturbed by later

events ( see Sommerauer 1976, Medenbach 1976 and O'Connor et al. 1989).

Table 1 Isotopic results of zircon for granitoid rocks from Murrat Area. Fraction UPb Atom percent radiogenic Pb Atomic ratios Apparent ages (Ma) S.No. (µm) ppm ppm 206Pb/204Pb 206Pb 207Pb 208Pb 207Pb/206Pb 207Pb/235U 206Pb/238U 207Pb/206Pb 1 61-85NM 230 21 1417 84.99 6.02 8.99 0.059771 0.695650 0.084081 598.90±1.5 2 45-61NM 151 18 1371 83.24 5.97 10.79 0.061218 0.806550 0.097031 612.90±2.9 3 60-84NM 280 28.5 1633 85.76 6.01 8.23 0.061318 0.736930 0.088658 613.60±1.8 4 84-101NM 157 36.5 814 82.19 6.47 11.34 0.062014 0.654390 0.078508 620.97±2.7 5 61-85NM 159 36.6 795 80.31 6.42 13.27 0.062104 0.717150 0.086041 621.86±2.1 6 45-61NM 169.5 20 1595 83.85 6.23 9.92 0.065831 0.841610 0.099125 658.60±2.5 1: Alkali granite; 2,3,4,5: Syeno-monzo granites and 6: Granodiorite. The isotopic concentrations are corrected for common lead with composition 206Pb/204Pb=16.25, 207Pb/204Pb=15.51, 208Pb/204Pb=35.73. The apparent ages are calculated using the decay constants given by Steiger and Jager (1977): 238Uλ=1.55125× 10-10, 235Uλ=9.8485× 10-10. NM=non-magnetic.

As mentioned above, apart from zircons from the granodiorite which plot on Concordia at 658.6Ma, all other zircons made a chord which cuts the Concordia at about 612.9Ma, so the possible interpretation is that there are two

periods of zircon crystallization at 658.6Ma and 621.86Ma and that the original zircons represented a relatively homogeneous population (Halliday et al., 1984).

Fig. 2 Concordia diagram presenting the zircon data from Abu Murrat granitoid rocks,

name of rocks as presented in table 1.


--------------------------------------------JJoouurrnnaall ooff SSeebbhhaa UUnniivveerrssiittyy--((PPuurree aanndd AApppplliieedd SScciieenncceess))--VVooll..66 NNoo..22 ((22000077)) ---------------------------------------- 91

It is of geological importance to note that for most of the analyses situated on or close to the Concordia, which rule out the possibility of inherited radiogenic lead, the conclusion is consistent with the textural investigations reported in this study, and similar works of O'Connor et al. (1989).

Assuming only a recent lead loss, a chord between the origin of Concordia (analysis 2) and data, point (analysis 4) yields an upper intercept of 612.9Ma.

This result suggests that the zircons from monzogranite were crystallized in two different phases, first one at 621.86Ma and another at 612.9Ma, followed by crystallization of alkali granite at 598.9Ma.

Their U-Pb system was completely reset at the time of intrusions crystallization (Aboazom 1995, 2004).

However, due to the damage in some of these zircon crystals, their isotopic composition were slightly disturbed and some Pb-loss occur.

5. Zirconium Saturation Temperatures

The behaviour of some accessory minerals (Zircon and apatite) during crustal anatexis has been well documented in recent years (e.g. Watson and Harrison,1983, Harrison and Watson, 1984 and Aboazom, 2004).

Indeed the saturation behavior of zircon in crustal melts has been investigated by Watson (1979) and Watson and Harrison (1983), who have shown that Zr solubility during periods of crustal melting increases with increasing temperature and cationic ratio.

They provide a saturation model in the form of equation: ln DZ r

zircon m elt/= -3.80-[0.85x(M-

1)]+(12900/T);

Where: D Z rz i r c o n m e l t/ is the

concentration ratio of Zr in a stoichiometric zircon to that in the

melt; T is the absolute temperature (i.e. in K) and M is the cation ratio (Na + K + 2Ca)/(Si x Al). Moreover, in this model the melt should contain at least 2wt.% H2O.

Using geochemical data for whole rock samples of studied intrusions from Abdel Ghani et al. (1995) in conjunction with Zr concentration of a stoichometric zircon (517000 ppm) from Deer et al. (1993), in order to calculate Zr saturation temperatures for granodiorite, monzo-granite and alkali granite.

The data presented in Table 2, show that the granodiorite has a Zr saturation temperatures of 808-830±30oC, the monzo-granite has a Zr saturation temperatures of 880-896 ± 30oC, and alkali granite has a Zr saturation temperatures of 769-779 ± 30oC.



Zr saturation temperatures may correspond to the temperatures at the peak of anatexis within their

uncertainty as recommended by Harrison et al. (1987).

Table 2 Zircon solubility results Sample SiO2(Wt%) Zr (ppm) M Zr saturation (ToC) 416 75.39 82 0.80 769 443 72.77 93 0.81 779 415 67.71 359 0.96 896 468 68.80 304 0.94 880 410 60.63 252 1.31 830 401 58.47 202 1.33 808 416 and 443 are alkali granite; 415 and 468 are monzogranite; 410 and 401 are granodiorite. Chemical data from Abdel Ghani et al., 1995.

Watson et al. (1989) have shown

that the Zr saturation temperatures can be changed with some physical processes.

In the meantime, Aboazom (1995, 2004) has confirmed the reliability of this model in granitic rocks using temperatures calculated by temperatures acquired by coexisting plagioclase and alkali feldspar model for the same rocks, within their expected errors.

Therefore, the calculated temperatures indicate that the granodiorite and alkali granite have

crystallized at temperatures expected for granitoid magmas (Clarke, 1992). In contrast, temperatures of monzogranite appear to have crystallized at slightly higher temperatures and suggesting higher temperatures of their parental magma (Watson and Harrison, 1983).

This was possibly due to the influx of more basic magma pulses with high Zr concentration (Table 2) into this granitic magma before the emplacement of monzo-granite and alkali granite.

6. Discussion

The data presented here show low U contents of Abu Murarat zircons, and slight discordant ages.

Such features might reflect the slight Pb loss probably related to the variation in structural damage of crystals or to the igneous activity during the ascent of these intrusions.

On textural study basis Clemens et al. (1986) have shown that zircon was one of the first mineral to crystallize and was entirely melt-precipitated. In the meantime, the zircon grains studied here had well rounded shape.



Such crystal forms may be attributed to the partial melting of zircon grains during the igneous activity before the emplacement of these rocks (Halliday et al., 1984 and Ekwueme and Kroner, 2006).

The low U-contents of studied zircon grains along-side their homogeneous nature (unzoned), could fairly reflect their crystallization at one and later magmatic stage for each intrusion (Pidgeon, 1992).

Indeed such a process is always accompanied by a reduction in U and Pb concentrations.

The possible mechanisms advanced, to explain the development of unzoned, low U and Pb zircon grains include magmatic and post magmatic processes.

The unzoned zircon is generally concordant or slightly discordant, since analyses 1, 3, 4 and 5 fall out of the Concordia, possibly reflecting the slight Pb-loss.

This Pb loss is consistent with slight damage of zircon crystal structure, since zircon susceptibility to disturbance increase with increasing damage to its crystal structure.

In the meantime this varies with U content, as with increasing structure damage, U content increases, such conclusions has been reported here (Table1) Pidgeon and Aftalion (1978); Williams et al. (1988); and Williams (1992).

Infact, Williams (1992) argued, on the zircon morphology grounds that crystals similar to type D of Pupin (1980) rarely contain inheritance.

Such conclusion might be supported by recorded uniform age and undisturbed isotopic composition.

All 207Pb/206Pb ages determined on zircon grains fall within the range 598.9-658.6Ma (Table1), and Concordia plot Fig.2. Infact these data points intersect the Concordia at 612.9 and 658.6Ma.

Therefore the age of zircon from granodiorite (658.6Ma) and monzogranite (612.9Ma), probably are very close to the crystallization ages of these intrusions.

However, the rest of monzogranite analyses (3, 4 and 5) and alkali granite data (1) situated close to the Concordia and have a 207Pb/206Pb age ranging from 621.86 to 613.6Ma. for monzogranite, and 598.9Ma. for alkali granite indicating little or no inherited radiogenic lead.

This might represent or be assumed to represent recent lead loss, with a chord of data yielding an upper intercept of 612.9Ma.

Infact, this result suggests that the zircon from monzogranite and alkali granite were crystallized and their U-Pb system were completely reset at the period in the range from 598.9 to 621.86Ma.



Williams (1992) pointedout that the zircons younger than the mid-Proterozoic must have an accurate measured age, obtained by 206Pb/238U method.

Avigad et al., (2003) suggested that the granitic rocks in northeasthern desert of Egypt were crystallized in Neoproterozoic. This is in agreement with result obtained in this study for granitic rocks of Abu Murrat area.

Zirconium saturation temperature model has been adopted here in order to calculate precise timing of initial zircon precipitation.

However, the zircon saturation temperatures for the magma were reported to be about 940oC, (Williams 1992.

This is consistent with the clouser temperature for U-Pb isotopic system of zircon (950oC) (Rollinson 1993).

Watson and Harrison (1983) have confirmed that, even while zircon grain separated gravitationally from differentiated magma, this saturation model can identify instances of zircon saturation even when the crystal remain suspended in the magma.

In this context, zircons from Gabal Abu Murrat acidic intrusions have given zircon saturation temperatures in the range of 769-896oC. These temperatures are consistent with those of granitic magma (Clarke, 1992;Pitcher, 1993).

However, the slight higher temperature for monzogranite (880-896oC), clearly goes in favor of introduction of new pulses of basic melts into the magma chamber after the crystallization of granodiorite has been completed at 658.6Ma.

Fullagar (1978) and Ghuma and Rogers (1978) pointed out that the granitic rocks of northeastern Africa were derived 500-600Ma ago from the upper mantle or lower crust source.

This conclusion was based on mineral ages by Rb-Sr method, indeed, the mineral ages always considered as a minimum ages as diffusion loss of radiogenic daughter products generally significant above temperatures of 300oC (Damon, 1968).Thus with respect to the clouser temperature of zircon about 950oC Rollison (1993), the temperatures calculated here might represent the crystallization temperatures at the period of 598.9-658.6Ma from mantle materials without significant crustal involvement. Such conclusion is support by the absence of inheritance zircon in these intrusions.

The derivation of granodiorite and monzogranite (analysis 2) from partial melting of mantle materials, will require greater temperatures (800-900oC) than those granite derived from melting of crustal material (700oC) (Watson and Harrison, 1983 and Liew, 1984).



Such mantle melts produce somewhat more mafic melt (i.e. larger M value).

Thereafter high M value for granodiorite (M=1.31-1.33, Table 2) would reflect a magma at 808-830±30oC, such a magma would not become saturated until a concentration of Zr reach 340ppm. Infact, this could explain the very low abundance of zircon grains in this intrusion. In the meantime, the monzogranite magma, which have M value between 0.94-0.96 and alkali granite magma have M value between 0.80-0.81 will dissolve zircon grain in even low Zr concentration.

Thus, the recorded saturation temperatures for monzo- and alkali granites (> 700oC), clearly provide basis for interpreting the observed general absence of inherited zircon from Abu Murrat acidic intrusions.

Fig.2 shows that the granodiorite and analysis 2 from monzogranite

crystallized at 658.6Ma. and 612.9Ma. respectively.

That means, after the crystallization of granodiorite at 658.6Ma. and during the next 37Ma., new pulses of basic composition with high Zr content (see Table, 2) were incorporated into the magma chamber and raised their temperature, and subsequently many zircon grains crystallized. Thereafter, the fractional crystallization processes would modify and differentiate the remaining magma until alkali granite composition was reached, however this could happen without any new magma coming in. The alkali granite and some of the monzogranite samples have suffered slight Pb-loss as has be shown by their position on Concordia diagram, (Fig. 2), which might be due to crystal structure damage of zircon grains.

7. Conclusion Zircons from Gabal Abu Murrat

acidic intrusions showed unzoned subhedral to well rounded crystal forms. These crystal shapes are mainly due to the partial melting of zircon grains during the emplacement of these granitic rocks. The 207Pb/206Pb ages reported in this study are suggestive of two period of crystallization, one for granodiorite at 658.6Ma, and the other at

621.86Ma for monzogranite, followed by a span of 37Ma at which the magma was differentiated and then crystallized another phase of monzogranite (613.6-612.9Ma.), and finally alkali granite at 598.9Ma with little U-Pb discordance possibly due to the difference in structural damage between zircon crystals.

The calculated zircon saturation temperatures are consistent with



such periods of crystallization. These temperatures indicated that the crystallization of granodiorite occurred at about 800 to 830oC, then some new pulses of basic magma were introduced into the magma chamber in a span of 37Ma. However, these basic magma pulses have higher zirconium concentration

(Table 2) and thereafter, the crystallization of monzogranites with high crystallization temperatures (880-896 oC) occurred at a span of ca 9Ma producing two different phases, then the rest of magma differentiated and crystallized more evolved acidic intrusion (i. e. alkali granite) at lower temperatures (769-779 oC).

Acknowledgements The assistance of isotope analyses

by Prof. Yori Barisovich Marin from Institute of Nuclear Physics, Moscow, Russian Federal Republic, is highly appreciated. Prof. Asran M.

Hasan from geology department Sohag University, Egypt, is also thanked for his valuable discussion and enormous help during the preparation of this manuscript.

Appendix Analytical methods

The rock samples were crushed in a jaw crusher, grounded by a disc mill and heavy minerals were separated by using a wilfley table, heavy liquids and a Frantz magnetic separator. The recovered zircons were sieved into different size fractions, which were further spilt up according to their magnetic susceptibility.

After hand picking to about 98.9% purity and acid washing, some of the non-magnetic size fractions were used to remove metamict grains by air abrasion technique of Krogh (1982).

These zircon fractions were then used for analysis of their U-Pb

contents and isotopic compositions respectively.

The analytical technique to extract Pb and U from zircon follow closely the method of Krogh (1973) and Pidgeon and Aftalion (1978).

The isotopic ratios of Pb and U were obtained using a solid sourceVacuum Generators, semi-automatic M30 mass spectrometer.

The isotopic ratios obtained for NBS radiogenic lead standard SRM 983 are 207Pb/206Pb = 0.071167 ± 29, 208Pb/206Pb = 0.013631±26, and 206Pb/204Pb = 2753±10. All errors are quoted at 2 sigma.

The isotopic concentrations are corrected for common lead using the composition 206Pb/204Pb = 16.25,



207Pb/204Pb = 15.51 and 208Pb/204Pb = 35.73.

This is the composition of the Pb contained in the nitric acid used, assumed to be the main source of

contamination. (All above analytical techniques were carried out in the Institute of Nuclear Physics, Moscow, Russian Federal Republic).

حتديد أعمار و درجات حرارة التجانس للصخور اجلرانيتية من منطقة أبو مرات مشال شرق الصحراء الشرقية مصر

محمد صابر عبد الغنى ،على صالح ابوعزوم

الخالصةر رق مص مال ش ى ش رى ف ل الكمب ا قب خور م د ص ومرات أح ل أب ة لجب خور الجرانيتي ل الص تمث

(100km2) . طة ون بواس ار الزرآ د أعم ون ، تحدي ارجى للزرآ كل الخ ة الش ت دراس ، ) U-Pb(تمى الظروف المصاحبة ك من أجل التعرف عل باالضافةإلى حساب درجات حرارة تجانس الزرآون وذل

وى . لعملية التبلر ى جرانوديورايت ومونزوجرانيت وجرانيت قل ة إل ذه الصخور الجرانيتي د قسمت ه . وقى إنصهاره جزئي ك إل زى ذل د يع دائرى وق كله ال ون بش ورات الزرآ ارجى لبل ر الخ از المظه الل يمت ًا خ

ة تموضع الجرانيت ال عملي ل إآتم ك قب ة وذل ات الناري ورات الزرآون ال . العملي ى ان بل ذا باالضافة إل ه .تحتوى على اى نوع من النطاقية

ى ا يشير إل دوره ربم ذى ب تمثل هذه الصخور الجرانيتية أعمارًا متجانسة مع ترآيب نظائرى منتظم والذى . أى عمليات ما بعد التبلراستبعاد أى دور للمواد الموروثة أو هذا فى نفس الوقت ربما يعزز المقترح ال

واد ر للم دور األآب ا ال ا يغلب عليه ذه الصخور ربم يشير إلى ان العمليات النارية المسئولة عن تموضع ها ة عن غيره ى . القاعدي دان البسيط ف ا أن الفق ت Pb آم ات المونزوجرانيت والجراني ى عين واضح ف

.الذى ربما يعزى إلى التشوهات البنيوية للبلوراتالقلوى ود ة ق إن آل الصخور الثالت ه ف اءًا علي دة بن رحلتين واح ى م ك عل ة وذل واد قاعدي رت من م ون تبل تك

ت نة 658.6(للجرانوديوراي ون س دى ) ملي ى م رارة ف ة ح رى 830-808بدرج ة واألخ ة مئوي درج . مليون سنة 621.86) مونزوجرانيت(للصخور األآثر تمايزًا

ى نسبة وى عل دة من الصهير القاعدى المحت د إضافة دفعات جدي ا حدث بع تبلر المونزوجرانيت ربمة الصهير وم لغرف ر . عالية من الزرآوني ع درجة حرارة تبل وع من الصهير هو المسئول عن رف ذا الن ه

ورات الزرآ 896-880المونزوجرانيت ما بين دل بل ذا الصخر درجة مئوية وزاد من مع ومن . ون فى هد ر عن ورًا أخ ت وأعطت ط رور الوق ع م ة الصهير م ايزت بقي نة 612.9-613.6(ثمتم ون س ن ) ملي مد وى عن ت القل ر الجراني ى ان تبل ى وصلت إل ت حت رارة 598.9 المونزوجراني ة ح نة بدرج ون س ملي

.درجة مئوية 779-769مناسبة للصخور النارية تتراوح بين



8. References Aboazom, A. S., 1995.

Petrogenesis of Palaeocene granites, Isle of Skye N.W. Scotland. Unpublished PhD. Thesis, Glasgow University.

Aboazom, A. S., 2004. Texture and radiogenic isotope study of accessory minerals from Skye granites. N. united kingdom. J. Sebha University, pure and applied science, 3(1), 116-135.

Abdel Ghani, M. S., Mahallawi, M. M. and Ahmed, A. F. 1995. Geochemistry and petrogenesis of the granitoid rocks of Gebal Abu Murrat area northeastern desert Egypt. Delta j. Sci. 19 (2) 156-186.

Aftalion, M. and Max, M. D. 1987. U-Pb zircon geochronology from the Precambrian Annagh Division gneisses and the Termon Granite, N.W. County Mayo, Ireland. J. Geol. Soc. Lond. 144: 401-406.

Avigad, D., Kolodner, K., McWilliams, M. Persing, H. and Weissbord, T. 2003. Origin of northern Gondwana Cambrian sandstone revealed by detrital zircon SHRIMP dating. Geology, 31 (3) 227-230.

Bentor, Y. K., 1985. The crustal evolution of the Arabo-Nubian massif with special reference to the Sinai Peninsula: Precambrian research, 28: 1-74.

Clarke, D. B., 1992. Granitoid rocks, Chapman and Hall, London.

283p. Clemens, H. J. D., Holloway, J.R. and White, A.J.R., 1986. Origin of an A-type granite: experimental constraints. Am.Mineral.71:317-324.

Damon, P. E., 1968. Potassium-Aragon dating of igneous and metamorphic rocks with applications to the basin ranges of Arizona and Sonora. In: radiometric dating Geologists (ed E. I. Hamilton and R. M. Farquhar). Interscience, New York, 1-72.

Deer, W.A., Howie, R.A. and Zussman, J., 1993. An introduction to the rock forming minerals. 2nd-ed. Longman, London and New York. 696p.

Ekwueme, B. N. and Kroner, A., 2006. Single zircon ages of migmatic gneisses and granulites in the Obudu Plateau: Timing of granulites-facies metamorphism in southeastern Nigeria. J.African Earth. Sci., 44, issues 4-5, 459-469.

Fullagar, P. D., 1978. Pan-African age granite of northeastern Africa: New or reworked sialic materials. Symposium on the Geology of Libya, V3, 1051-1058.

Ghuma, M. A. and Rogers, J. W., 1978. Pan-African evolution in Jamahiriya, and North Africa. Symposium on the geology of Libya, V3, 1059-1064.

Gupta, L. N., 1973. Unusually large zircons from the Leinster pluton. Mineral. Mag. 39: 253-255.



Halliday, A. N., Aftalion, M., Upton, B. G. J., Aspen, P. and Jocelyn, J., 1984. U-Pb isotopic ages from granulite facies xenoliths from Partan Craig in the Midland Valley of Scotland. Trans. R. Soc. Edinb. Earth Sci. 75: 71-74.

Harrison, T. M. and Watson, E. B., 1984. The behaviour of apatite during crustal anatexis: equilibrium and kinetic considerations. Geochim. Cosmochim. Acta, 48, 1467-1477.

Harrison, T., M., Aleinikoff, J. N., and Compston, W., 1987. Observation and controls on the occurrence of inherited zircon in Concord-type granitoids. New Hampshire. Geochim. Cosmochim. Acta, 51, 2549-2558.

Krogh, T. E., 1973. A low contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determination. Geochim. Cosmochim. Acta, 37:485-494.

Krog. T. E. (1982) Improved accuracy of U-Pb zircon ages by the creation of more concordant systems using an air abrasion technique. Geochim. Cosmochim. Acta, 46, 637-649.

Liew, T. C., 1984. A Nd, Sr, and U-Pb zircon isotopic study of granitoid batholiths of Penninsular Malaysia. PhD. thesis, Australian National University.Canberra.

Medenbach, O., 1976. Geochemie der Elemente in zircon und ihre

raumliche Verteilung-eine Untersuchung mit der Elektronenstrachlmikrosonde. Dr. diss., Ruprecht-karl- Universitat, Heidelberg.

O'Connor, P. J., Aftalion, M., Kennan, P. S., 1989. Isotope U-Pb ages of zircon and monzonite from Leinster granite, south Ireland. Geol. Mag. 126 (6) 725-728.

Patchett, P. J. and Jocelyn, J., 1979. U-Pb zircon ages for late Precambrian igneous rocks in South Wales. J. Geol. Soc. 136 (1) 13-19.

Paterson, B. A., Stephens, W. E. and Herd, D. A., 1989. Zoning in granitoid accessory minerals as revealed by backscattered electron imagery. Mineral. Mag. 53: 55-61.

Paterson, B. A., Stephens, W. E. and Rogers, G., Williams, I. S. Hinton, R. W., Herd, D. A.,1992. The nature of zircon inheritance in two granite plutons. Trans. R. Soc. Edinb. Earth Sci. 83: 459-471.

Pidgeon, R. T., 1992. Recrystallization of oscillatory-zoned zircon: some geochronological and petrological implications. Contrib. Mineral. Petrol. 110: 463-472.

Pidgeon, R. T. and Aftalion, M., 1978. Cogenetic and inherited zircon U-Pb systems in granites:palaeozoic granites of Scotland and England. In Bowes, D. R. and Leake, B. E. (eds) Crustal evolution in northeastern Britain and adjacent regions, 183-248.Geol. J. Spec. Issue 10.



Pitcher, W. S., 1993. The nature and origin of granite. Chapman and Hall, London. 321p.

Pupin, J. P., 1980. Zircon and granite petrology. Contrib. Mineral. Petrol.73: 207-220.

Rollinson, H., 1993. Using geochemical data: evaluation, presentation, interpretation. Longman, Essex, England. 352p.

Sabet, A. H., EL Gaby, S. and Zalat, A. A. (1972) Geology of the basement rocks in the northern parts of El-Shayib and safaga sheets, Eastern Desert. Ann. Geol. Surv. Egypt. 2: 111-128.

Sabet, A.H., Bessoneko, V.V. and Bykov, B.A., 1976.The intrusive complexes of central Eastern Desert of Egypt. Ann. Geol. Surv. Egypt. 8., 53-73.

Sommerauer, J., 1976. Die chemisch-physikalische Stabiliatat naturlicher zircon and U-(Th) Pb system. Dr diss., Eidg. Techn. Hochshule, Zurich.

Steiger, R. H., and Jager, E.(1977) Subcommissionon geochronology: Convention on the use of decay constants in geo- and cosmmochronology. Earth Planet.Sci. Letters, 36, 359-362.

Watson, E. B., 1979. Zircon saturation in felsic liquids: experimental results and applications to trace element geochemistry. Contrib. Mineral. Petrol. 70: 407-419.

Watson, E. B. and Harrison, T. M., 1983. Zircon saturation revisited: temperature and composition effects in a variety of crustal magma types. Earth plant. Sci. lett., 64, 295-304.

Watson, E. B., Vicenzi, E. P. and Rapp, R. P. (1989) Inclusion/host relations involving accessory minerals in high grade metamoephic and anatectic rocks. Contrib. Mineral. Petrol., 101:220-231.

Williams, I. S., 1992. Some observation on the use of zircon U-Pb geochronology in the study of granitic rocks. Trans. R. Soc. Edinb.: Earth Sci. 83, 447-458.

Williams, I.S., Chen, Y.D. Chappell, B.W. and Compston, W., 1988. Dating the sources of Bega Batholith granites by ion microprobe. Geol. Soc. Aust. Abstr. 21. 424

U-Pb Geochronology and Zr Saturation Temperatures for ... · U-Pb Geochronology and Zr Saturation...

Documents

Transcript of U-Pb Geochronology and Zr Saturation Temperatures for ... · U-Pb Geochronology and Zr Saturation...