J. Earth Planet. Sci. Nagoya Univ., Vol. 44, 21 to 27, 1997

Chemical Th-U-total Pb isochron age of zircon fromthe Mereb Granite in northern Ethiopia

Tarekegn TADESSE *, Kazuhiro S UZUKI ** and Mitsuo H OSHINO **School of Informatics and Sciences, Nagoya University, Nagoya 464-01, Japan**Department of Earth and Planetary Sciences, Graduate School of Science,Nagoya University, Nagoya 464-01, Japan(Received August 8, 1997 / Accepted September 17, 1997)

ABSTRACTThe chemical Th-U-total Pb isochron age was detemined for zircon grainsfrom the post-tectonic Mereb Granite in northern Ethiopia. Twenty-seven analyses from five zircon grains yield a well defined isochron of 545±24 Ma. Oneof the analyzed grains is chronologically zoned with a core having age range of732-828 Ma. The core is interpreted as a xenocryst. The 545±24 CHIME zircon
age, slightly younger than the previously reported Rb-Sr whole-rock isochron
age of 633±62 Ma, dates emplacement of the Mereb Granite, and agrees wellwith the 541+10/-16 Ma U-Pb zircon age for the post-tectonic Mao granite inwestern Ethiopia.

INTRODUCTION
The Precambrian geology of northern Ethiopia os characterized by thewidespread occurence of low-grade, volcanic and volcano-sedimentary rockassemblage (Kazmin, 1972; Tadesse, 1997). This assemblage belongs to theupper Proterozoic Red Sea Fold Belt of Ethiopia (Kazmin et al., 1978) or more
broadly to the neo-Proterozioc Pan-African Arabian-Nubian Shield (De Witand Chaweka, 1981), and is intruded by syn-tectonic composite granitoids andpost tectonic pink granites.
Although gross lithological similarities are pointed out between the lowgrade rocks of northern Ethiopia and those of the Arabian-Nubian shield inArabia, Egypt and the Sudan, the lack of chronological data has prevented thecorrelation of the sequence of geological events and the geodynamic setup ofthe region. Recently, Alemu (1997) obtained an Ar-Ar age of 792±2 Ma for thes y n - t e c t o n i c C h i n a i n t r u s i o n a n d a n A r- A r a g e o f 7 4 5 ± 4 M a f o r t h e s y n tectonic Rama intrusion (Fig. 1). These ages are nearly equal to the 780-760
Ma ages for the low-grade Pan-African metamorphism in western Ethiopia..
(Ayalew et al., 1990) and the Tokar area in eastern Sudan (Kroner et al., 1991).
For the post-tectonic Mereb granite, Alemu (1997) reported a Rb-Sr wholerock isochronageof633±62 Ma with an initial ratio of 0.7034±9. The Rb- Sr
isotopic data which yield this age, however, are highly scattered. Further, ourde t a i l e d p e t r o g r a ph i c ob s e r v a t i o n r e v e a l e d t h a t t h e M e r e b g r a ni t e c o n t a i n s

21

22Fig. 1.

T. Ta d e s s e e t a l .

Geological map of the area around the Mereb Granite in northern Ethiopia (modified after Tadesse, 1997).

core-mantle zircon grains. This, coupled with the high age error and apparently
high MSDW (not reported), leads us to a suspicion that the 633±62 Ma isochron
is a pseudoisochron. Therefore, the upper limit of plutonism in northern
Ethiopia still remains unclear. In order to shed more light on the tectonic evolution in northern Ethiopia, we dated the post- tectonic mereb Granite (Fig. 1)
using chemical Th-U-total Pb isochron method (CHIME; Suzuki and Adachi,1991a,b, 1994) on zircons This paper reports on the preliminary finding.
GEOLOGICAL SETTING AND SAMPLE DESCRIPTION
The Mereb Granite is located in the northernmost tip of Ethiopia (Fig. 1).
It is a batholithic in size, but the major part of the intrusion occurs in Eritrea
across the Mereb River. The granite has intruded into low grade metasedimentary unit in the south and appears to have intruded into biotite-rich tonalitic to granodioritic rock which is exposed in the northern side around Adi
Kawala (Fig. 1). The northern end of the pluton is overlain with a Phanerozoic cover.
The metasedimentary unit comprises phyllitic and and carbonaceous schist,Chemical Th-U-total Pb isochron age of zircon

23

graywacke, quartzite and minor marble layers. This unit was previously
included within the Tembien Group (Beyth, 1971; Kazmin, 1972), but recently
redefined as a distinct tectono-stratigraphic unit, Adi Abun Group (Tadesse,1997). The Mereb Granite, like other post-tectonic granitoids in the ArabianNubian Shield, deflects and/or cuts the metamorphic layering of the countryrock, and yields a narrow (less than 200 m wide) but distinct contact aureole.
The biotite-rich tonalitic to granodioritic rock in the northern side of theMereb granite is clearly deformed and are very similar to those of the syntectonic Rama and Chila Granitoids situated south of the Mereb River (Fig. 1;Tadesse, 1997; Alemu, 1997).
The typical Mereb Granite is pink and has porphyritic appearance owing tohte presence of randomly oriented K-feldspar phenocrysts. A fresh sample(Sample No. TT428) for the present CHIME dating was collected from an outcrop along the Mereb River in southern part of the intrusion (Fig. 1). It consists mainly of 42% K-feldspar, 19% plagioclase (albite-oligoclase), 23% quartz,
11% biotite and 5% hornblende. Accessory minerals include sphene, zircon,trace of epidote, allanite, apatite and iron oxide. K-feldspar is mainly
microperthitic microcline. most of plagioclase grains show zoning. The coreof zoned plagioclase grains are severely affected by secondary alteration andhence have cloudy appearance. Biotite is generally brownish and rarely altered into chlorite along the margins and cleavages. Zircon grains form shortprism of 0.06 to 0.29 mm in length. Euhedral crystals with concentric growthzoning dominate in the sample. Some zircon grains show a core-mantle structure in which unzoned cores are overgrown by zoned rim. The core is generally anhedral and corroded.
ANALYTICAL METHOD
We analyzed zircon grains in a polished thin section prepared for the conventional electron microprobe analyses. Four unzoned and one zoned zircon
grains were analyzed on JEOL JXA-733 electron microprobe equipped withthree wavelength dispersive-type spectrometers at Department of Earth andPlanetary Sciences, Nagoya University. The analysis was carried out under
the following instrument operating conditions; a 15 kV accelerating voltage,
0.02-0.15 µA current and 5 µm probe diameter. X-ray intensities were integrated 300s for the line and 150s for backgrounds at two optimum positions
on both sides of the lines The measurement was repeated twice or threetimes, and the arithmetic average was taken as true intensities. Under thecondition, the detection limits of ThO 2 , UO 2 and PbO at 2σ confidence level
were 0.012, 0.007 and 0.004 wt.%, respectively. The possible maximum errors
in the determination of ThO 2 , UO 2 and PbO are about 14% for 0.1 wt.% of theconcentrations. The details of analytical procedure and CHIME age calculation were described by Suzuki and Adachi (1991a,b, 1994).

24T. Ta d e s s e e t a l .

RESULTSA total of 30 spots on 5 zircon grains were analyzed. The ThO 2 , UO 2 andPbO analytical data (in wt.%) together with apparent ages (Ma) and UO 2 *values (sum of the measured UO 2 and UO 2 equivalent of the measured ThO 2 )are listed in Table 1. The ThO 2 concentration of zircon ranges from 0.019 to0.178 wt.%, the UO 2 concentration from 0.056 to 0.395 wt.%, and the PbO concentration from 0.0047 to 0.0307 wt.%.
Table 1.

Analytical data of the five zircon (Z1-Z5) grains from sample No. TT428 (MerebGranite). (c) core of the zoned zircon grain UO 2 * : sum of the measured UO 2and UO 2 equivalent to the measured ThO 2 of zircon.

Grain No.
Z1 (zoned)
Z1-1
Z1-2
Z1-3
Z1-4
Z1-5
Z1-6
Z1-7
Z1-8 (c)Z1-9 (c)Z1-10 (c)Z2-Z5(unzoned)
Z2-1
Z2-2
Z2-3
Z2-4
Z2-5
Z3-1
Z3-2
Z3-3
Z3-4
Z3-5
Z3-6
Z3-7
Z3-8
Z4-1
Z4-2
Z4-3
Z4-4
Z4-5
Z4-6
Z5-1ThO2(wt.%)

UO2(wt.%)PbO(wt.%)apparent age(ma.)

UO2*

0.030
0.030
0.079
0.075
0.047
0.056
0.062
0.053
0.056
0.063

0.090
0.087
0.395
0.146
0.102
0.201
0.294
0.101
0.104
0.122

0.00750.00670.03070.01280.00840.01620.02460.01220.01360.0168

543
501
525
541
518
531
561
732
785
828

0.09860.095
0.41900.16910.11610.21830.31300.11670.12070.1406

0.048
0.049
0.051
0.062
0.030
0.019
0.021
0.045
0.024
0.031
0.020
0.053
0.056
0.073
0.106
0.1460.124
0.093
0.109
0.178

0.118
0.155
0.116
0.116
0.099
0.065
0.056
0.102
0.072
0.080
0.067
0.071
0.120
0.126
0.174
0.135
0.136
0.108
0.125
0.317

0.01050.01290.00980.01060.00700.00560.00490.00750.00610.00670.00470.00670.01080.01160.01620.01360.01290.01140.01140.0275

567
543
534
561
466
563
562
466
551
537
462
550
564
560
561
542
532
595
518
531

0.13200.16970.13130.13480.10810.07100.06220.11600.07910.08920.07330.08700.13650.14780.20590.17950.17360.13620.15760.3703

25

Chemical Th-U-total Pb isochron age of zircon

0.04

545±24 Ma

PbO (wt.%)

0.03

27 analyses on 5 grains

MSWD = 0.1

intercept = -0.0002±0.0005
0.02

0.01
Maximum analyticaluncertainty
0.00
0.0

0.1

0.2

0.3

0.4

0.5

UO2* (wt.%)
Fig. 2.

Plot of PbO vs. UO 2 * for zircon grains from Sample TT428 of the Mereb Granite
in northern Ethiopia. Solid circle represents data points for chronologically unzoned grains and the rim of zoned Z1 grain, and open circle represents data
points for the core of zoned Z1 grain.

The analytical data are plotted on the PbO-UO 2 * diagram (Fig. 2). Excluding three data point for the core of Z1 grain (Table 1, Z1-8,9 and 10), all data
point including those for the rim of Z1 grain are arrayed linearly on the diagram. They are regressed with an isochron of 545±24 Ma (MSWD=0.10) withan intercept value of -0.0002±0.0005 (errors quoted in this paper are of 2σ).
The core of Z01 grain gives apparent ages of 732-828 Ma (Table 1).
DISCUSSION
The well defined isochron of 545±24 Ma passes through the origin of thePbO-UO 2 * diagram (Fig. 2). This rules out any suspicion of partial Pb loss orheterogeneity in the distribution of initial Pb. Therefore, we consider that the545±24 Ma CHIME zircon age is highly reliable.
The Mereb Granite has been dated by Alemu (1997) using the Rb-Sr wholerock isochron method. The Rb-Sr age, 633±62 Ma, is inconsistent with theCHIME zircon age. This inconsistency may not be ascribed to the simple error inboth the microprobe and isotopic analyses. Rather, we believe that the inconsistency is responsible to the old Rb-Sr age possibly resulted from isotopic
disequilibrium in the Rb-Sr system. Inspection of the Rb-Sr isochron

26

T. Tadesse et al.

diagram shows that 4 data points are not strictly aligned; the scatter is beyond analytical uncertainty and resulted in large error (Alemu, 1997 PersonalComm.). This suggests that the Rb-Sr system is not in equilibrium on thewhole-rock scale and therefore, Rb-Sr whole isochron age of 633±62 Ma might
be fortuitous. In this connection, it is important to note that the Mereb granite contains zoned zircon grains with older cores yielding apparent age upto
828 Ma (Table 1). We interpret this core as a xenocryst of older zircon surrounded by new grown in the Mereb Granite. The oldest apparent age (828Ma) is similar to the U-Pb zircon age of Syn-tectonic granitoid in western
Ethiopia (Ayalew et al., 1990). We therefore, prefer the CHIME zircon ages of545±24 Ma as dating the emplacement of the Mereb Granite. This CHIME zircon age just overlaps with the 541+10/-14 Ma U-Pb zircon age of the posttectonic Mao granite in western Ethiopia (Ayalew et al., 1990).
Further dating works is in progress to get detailed geochronological information of the syn-tectonic and post-tectonic granitoids in northern Ethiopia
using the CHIME method and Sm-Nd isotopic method.
ACKNOWLEDGEMENTS
We thank Mr. T. Nagaoka for his excellent technical assistance. Thanks arealso extended to Dr. M. Asimi and anonymous reviewer for their thoughtfulcomments. During the preparation of this work the senior author is a postdoctoral fellow sponsored by JSPS for which he is grateful. The work is
partly supported by Grant-in-Aid for JSPS fellows (No. 90096301) from theMinistry of Education, Science, Sports and Culture, Japan.
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