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SmNd, RbSr and ThUPb zircon ages of syn- and post-tectonic granitoids from the Axum area of northern Ethiopia
Journal of African Earth Sciences, Vol. 30, No. 2, p p 3 1 3 - 3 2 7 . 2000
Pergamon
PIhS0899-5362(00)00022-1
2 0 0 0 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0 8 9 9 - 5 3 6 2 / 0 0 S- see front matter
Sm-Nd, Rb-Sr and Th-U-Pb zircon ages of syn- and post-tectonic granitoids from the Axum area of northern Ethiopia TAREKEGN TADESSE 1'2'*, MITSUO HOSHINO 2, KAZUHIRO SUZUKP and SHIGERU IIZUMI 4 tEthiopian Institute of Geological Surveys, PO Box 2302, Addis Ababa, Ethiopia 2School of Informatics and Sciences, Nagoya University, Nagoya 464-8601, Japan 3Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan 4Department of Geoscience, Shimane University, Matsue 690-8504, Japan
ABSTRACT--Sm-Nd, Rb-Sr and Th-U-Pb zircon ages for four syn-tectonic and t w o post-tectonic granitoids from the Axum area of northern Ethiopia are determined. Two of the syn-tectonic granitoids (the Azeho and Deset) are intrusions into structurally southeast facing, predominantly tholeiitic arc metavolcanics and associated metasediments situated west of the central steep zone in the area. The other t w o syn-tectonic granitoids (the Chila and Rama) are intrusions into structurally northwest facing metasediments and calc-alkaline metavolcanics at the eastern part of the steep zone. One of.the post-tectonic granites (the Sibta Granite) occurs west of the central steep zone and the second (the Shire Granite) cuts the central steep zone. Preliminary geochemical data of all the granitoids show that they are enriched in large ion lithophile elements, depleted in high field strength elements and have I-type characteristics, similar to calc-alkaline granitoids in subduction-related volcanic arc setting. The geochronological data indicate three discrete intrusive events: - 8 0 0 Ma to the east of the central steep zone, - 7 5 0 Ma to the west and a post-tectonic intrusion at around 550 Ma. The - 7 5 0 and - 8 0 0 Ma ages of the intrusives are interpreted as minimum ages of arc magmatism in the respective blocks across the central steep zone, and the - 5 5 0 Ma age of the post-tectonic granites records the final magmatic event in the region. The contrast of age across the central steep zone emphasises that this zone is a major structural element that might have played a significant role during the accretion of structurally and lithologically contrasting tectonostratigraphical blocks. © 2000 Elsevier Science Limited. All rights reserved. RI~SUMI~--Des ~ges obtenus par la Sm-Nd, Rb-Sr et Rh-U-Pb analyse de zircons ont 6te determines sur quatre granito'fdes syn-tectoniques et deux post-tectoniques de la region d'Axoum dans le nord de I'Ethiopie. Les deux granitoTdes syn-tectoniques d'Azeho et de Deset, & I'ouest de la zone centrale verticale de la region, sont intrusifs dans des metavolcanites essentiellement & composition de tholeiites d'arc et des metasediments associes structures vers le sud-est. Les deux autres granito'fdes syn-tectoniques de Chila et de Rama, & I'est de la zone centrale verticale, sont intrusifs dans des metasediments et des metavolcanites calco-alcalines structures vers le nord-ouest. Le granite post-tectonique de Sibta affleure & I'ouest de la zone centrale verticale et celui de Shire recoupe la zone centrale verticale. Les donnees chimiques preliminaires montrent que tous les granito'fdes sont enrichis en elements legers de grande taille, appauvris en elements de potentiel ionique eleve et ont des caracteres de type I, semblables aux granito'fdes calco-alcalins des arcs volcaniques de subduction. Les donnees chronologiques indiquent trois episodes distincts d'intrusion: - 8 0 0 Ma ~ I'est de la zone centrale verticale, - 7 5 0 Ma ~ I'ouest et post-tectonique vers 550 Ma. Les ~ges d'intrusion ~ - 8 0 0 et - 5 0 Ma s'interpretent comme les ~ges minimum du
*Corresponding author [email protected] Journal o f African Earth Sciences 3 1 3
T. TADESSE et al. magmatisme d'arc dans les blocs de part et d'autre de la zone centrale verticale et I'&ge des granites post-tectoniques & -550 Ma enregistre le dernier 6pisode magmatique de la r~gion. La diff6rence d'&ges de part et d'autre de la zone centrale verticale montre que cette zone est un 616ment structural majeur qui a pu jouer un rSle important Iors de I'accr~tion de blocs tectonostratigraphiques diff~rents par leur structure et leur lithologie. © 2000 Elsevier Science Limited. All rights reserved. (Received 4/11/98: revised version received 22/3/99: accepted 16/3/99)
INTRODUCTION The southern part of the Arabian Nubian Shield (ANS), exposed in northern Ethiopia and Eritrea, is geologically among the least studied areas in the East African Orogen (EAO: Stern, 1994). Most regional tectonomagmatic syntheses of the EAO across this area (e.g. Vail, 1983; Berhe, 1990) are mainly based on relatively old data obtained from reconnaissance photo interpretation augmented by limited field traverses (Kazmin, 1972; Beyth, 1972). Many researchers (e.g. Kazmin et aL, 1978; Chawaka and de Wit, 1981; Berhe, 1990) proposed a possible geological link between the low grade rocks of northern Ethiopia and those of the Saudi Arabia, Egypt and the Sudan in the north, on the basis of gross lithological similarities and structural continuity. Recently, Drury and Berhe (1993), Teklay (1997), de Souza Filho and Drury (1998) and Tadesse etaL (1998a, b) demonstrated that the area developed over a subduction-related island-arc setting and evaluated the suggested link. Unlike the northern parts of the ANS, age constraints in the southern parts are still scanty and insufficient to reveal the continuum of magmatism, sedimentation and hence the assembly of different arc terranes/tectonostratigraphical blocks. Rogers et al. (1965) reported K-Ar dates for a few granitic intrusives of northern Ethiopia. Most of the K-Ar dates obtained by these authors cluster at - 6 0 0 Ma and provide little information on the magmatic, metamorphic and tectonic events of the region, owing to the lack of detailed field data. Recently reported radiometric dates obtained from syn- and post-tectonic intrusives of northern Ethiopia (Alemu, 1997; Tadesse etal., 1997) and felsic metavolcanics and intrusive rocks of the Nakfa Terrane in Eritrea (Teklay, 1997) provide useful information on the age of magmatism and crustal growth in the region. These dates show that crustal growth in the region started before 850 Ma (single-zircon Pb-Pb evaporation age for felsic metavolcanics: Teklay, 1997) and lasted for around 300 Ma (545 Ma of post-tectonic Mereb granite: Tadesse etal., 1997). Yet, these are from geographically different parts of the vast low grade terrane of northern Ethiopia and Eritrea. This is an area that has been shown to have evolved through complex accretion involving oceanic 314 Journal of African Earth Sciences
fragments, immature to mature volcanic arc assemblages and various pre- and post-accretionary metasedimen-tary rocks (Drury and Berhe, 1993; de Souza Filho and Drury, 1998; Tadesse etal., 1998b). Therefore, examining the area in much more detail by employing dating of closely spaced geological formations across the area is of paramount importance to unravel the geodynamic evolution of the region. In this study the Rb-Sr and Sm-Nd internal and whole rock isochron and chemical Th-U-total Pb zircon isochron ages (CHIME-zircon: Suzuki and Adachi, 1991, 1994) have been determined for some syn- and post-tectonic granitoids from the Axum area of northern Ethiopia (Fig. 1 ). Also presented are the preliminary major and trace element data for representative samples from the granitoids. A manuscript describing the details of the geochemical 'characteristics of the granitoids is in preparation and will be presented elsewhere.
REGIONAL GEOLOGICAL FRAMEWORK Northern Ethiopia is underlain by low grade metavolcanosedimentary and plutonic rocks of a typical Pan-African, Arabian Nubian Shield assemblage (see Vail, 1983). With the exception of a few local stratigraphical units that rest unconformably over these volcanosedimentary assemblages, the entire layered succession in northern Ethiopia and Eritrea was mapped as the Tsaliet Group (Beyth, 1972). In the absence of radiometric age dating, the stratigraphical sequence of Ethiopian basement rocks were established on the basis of metamorphic grade and structural complexities (e.g. Kazmin, 1972). Noting that the metamorphic grade of Ethiopian basement rocks varies from high grade, locally granulite-facies in southern Ethiopia through lower amphibolite-facies in western Ethiopia, to low grade greenschist-facies in northern Ethiopia, previous researchers (e.g. Kazmin, 1972; Kazmin et al., 1978) have assumed that the Tsaliet Group is the youngest sequence of metamorphic rocks in the country. These assumptions are seriously challenged by new radiometric data (e.g. Teklay etal., 1993; Ayalew etal., 1990), which showed that the crustal growth of the region
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T. TADESSE et al. is essentially of Neoproterozoic age and hence metamorphic grade does not relate to stratigraphical relationships. This is in line with the emerging consensus on the geological evolution of east and northeast Africa (e.g. Berhe, 1990; Stern, 1994). The findings also support horizontal accretionary tectonics (Kr6ner et al., 1987), as opposed to progressive layercake stratigraphical build-up suggested by previous workers. Part of the Tsaliet Group exposed within the Axum map sheet of northern Ethiopia has been recently remapped (Tadesse, 1997). Field data supplemented by volcanic geochemistry from this area reveal more than four tectonically bounded, lithologically, structurally and geochemically distinct blocks that form an east to west accreted intra-oceanic arc assemblage (Tadesse et al., 1998a). These discrete assemblages are divided into structurally opposite facing successions by a central steep zone (CSZ, Fig. 1 ). West of the central steep zone, a sequence of imbricated metavolcanics and metasediments of the Adi Hageray and Adi Nebrid blocks occur. These blocks are bounded by shear zones. The geochemistry of volcanic rocks in these western blocks varies from mid-oceanic ridge basalt (MORB) in the Adi Hageray Block, through mixed, MORB-IAT (islandarc tholeiite ) chemistry in the Zager mafic-ultramafic belt, to immature island-arc tholeiite (IIAT) in the Adi Nebrid Block (Fig. 1). The 'telescoped' lateral sequence broadly resembles volcanic rocks developed in modern supra-subduction zone settings (cf. Pearce et aL, 1984b). At the eastern side of the CSZ, the structurally northwest facing succession includes fine-grained, commonly carbonaceous phyllite and chert with minor marble and graywacke of the Chile Block, which is thickened by a duplex of thrusts (Fig. 1 ). This distinctive metasedimentary succession rests on a northwest dipping detachment above predominantly tholeiitic to calc-alkaline gabbro and associated mafic-ultramafic rocks of the Daro Tekli area. This, in turn, is followed eastwards and structurally downwards by a more evolved calc-alkaline island-arc assemblage of the Adwa Block (Fig. 1). This complex, west to east succession of the Axum area geographically makes up the bulk of the 850630 Ma Nakfa Terrane of Eritrea (Drury and Berhe, 1993; Teklay, 1997). A n y stratigraphical and structural links between this area and areas to the north and south proposed by previous works (e.g. Kazmin, 1972; Kazmin eta/., 1978; Chawaka and de Wit, 1981 ) must be re-evaluated by identifying the accretionary assemblages similar to those described in this area.
316 Journal of African Earth Sciences
THE GRANITOIDS: FIELD RELATIONSHIPS AND PETROGRAPHY At both sides of the CSZ of the Axum area there are intrusive rocks of variable dimension and relative emplacement ages (Fig. 1 ). Two deformed, syn-tectonic granitoids have been dated from either side of the central steep zone, as have t w o post-tectonic granites. One of the post-tectonic granites is located at the western side of the CSZ and the second one is at the central part of the area, cutting the CSZ (Fig. 1 ).
The syn-tectonic granitoids The t w o dated granitoids from the western side of the steep belt are the Azeho and Deset Granitoids (Fig. 1 ). The Azeho Granitoid intrudes the chaotically mixed rocks of the Adi Hageray Block, which incorporates tectonically mixed metasediments and metavolcanics, together with lenses of mafic and ultramafic rocks. Most of this granitoid occurs to the north in Eritrea across Mereb River. South of the Mereb River, within the study area, it shows considerable variation in its mafic mineral content. The mafic-rich variety is severely altered and its primary igneous minerals are replaced by secondary minerals such as epidote, clinozoisite, sericite, calcite and chlorite. These secondary minerals occur together with relicts of coarse-grained, partly recrystallised K-feldspar and plagioclase, and quartz. In strongly deformed zones it has a mylonitic fabric defined by ribboned quartz, recrystallised plagioclase and sericite. In these mylonites, K-feldspar occurs as porphyroclasts and commonly exhibits mortaring along grain boundaries. The shear fabric is generally penetrative, steeply dipping and associated with L-S fabrics. Marginal zones of the intrusive show maximum strain, and in places the contact between the intrusive and the country rock is diffuse. Kinematic indicators, such as rotated porphyroclasts of feldspars and S-C fabrics, show dominant sinistral strike-slip movement. This deformation post-dates the early northwest verging thrust faults and associated mesoscopic folds in the area (Tadesse, 1996, 1997). The Deset Granitoid is situated in the Adi Nebrid Block (Fig. 1 ) and intrudes predominantly tholeiiticarc metavolcanics and associated metasediments. This granitoid is composite in nature comprising diorite, granodiorite and granite. The granodioritic rock is locally xenolithic, especially along marginal zones. No conclusive field evidence is observed on the relative sequence of emplacement of this varied rock facies. The sample that has been dated (KK1399, Fig. 1 ) is a granite composed of microcline microperthite, plagioclase (Ans_lo), quartz, biotite and a significant amount of Fe oxide, Zircon, apatite and sphene occur
Sm-Nd, Rb-Sr and Th-U-Pb zircon ages of syn- and post-tectonic granitoids, Axum area, Ethiopia in accessory amounts. Deformation fabrics are not well-developed in the interior of the body, but low grade alteration such as chlorite after biotite, and sericite after plagioclase, are common in thin sections. Like other syn-tectonic granitoids in the area (see below), this granitoid is only marginally deformed and characterised by a narrow zone of hybrid rocks. The regional fabric (S 2) is parallel to the fabric in the marginal part of the granite and tends to wrap around it without being cut by the intrusive. On the eastern side of the CSZ t w o syn-tectonic granitoids occur in the Chila and Rama areas (Fig. 1 ). They intrude into the predominantly metapelitic rocks of the Chila Block. These and other syntectonic granitoids in the area (Fig. 1 ) are composite in nature and range in composition from diorite to granite: granodiorite and tonalite being the most dominant, and diorite with gabbro the least significant by volume. In the intrusives, all cross-cutting relationships show that the most mafic member is the oldest in the sequence. These syn-tectonic intrusions generally have well-foliated marginal zones. The Chila Granitoid possesses a relatively wide (ca 300 m) hybridised zone along its margins. The hybridised zones are typically banded where light granitic materials are alternating with dark, maficrich country rock. In places, it typically resembles migmatites in a high grade terrane. Such features are common to the hybridised marginal zones of syntectonic granitoids. In contrast, the Rama Grani-toid lacks such hybridised zones along its margins, perhaps due to its shallow outward dipping intrusive contact. In both the Chila and Rama Granitoids, granodioritic and tonalitic rock varieties are generally xenolithic, mafic enclaves varying in size from a few tens of millimetres to 0.5 m, with commonly diffuse but occasionally sharp contacts. Such features may suggest mixing of mafic and felsic magmas as part of these granitoids' petrogenesis. The density and size of xenoliths, and intensity of deformation, decrease rapidly towards the centres of the bodies. The fabric post-dates the oblique southeast verging thrusts and associated dextral strike-slip shearing. Therefore, the intrusive post-dates the first and most intense deformation, but predates or is synchronous with the second phase of deformation (Tadesse, 1997). Granodiorite samples (TT570, KM 1500, KM 1503 and MB916; Fig. 1 ) from the Chila Granitoid chosen for dating are composed of plagioclase (Anl~_2~), quartz, biotite, hornblende and K-feldspar. Sphene, zircon, apatite and minor oxides occur in accessory amounts. Some plagioclase grains show normal compositional zoning in which the central calcic-rich zones are partly saussuritised. Biotite is partly altered
to chlorite and commonly contains some zircon inclusions with pleochroic haloes. Hornblende is marginally altered to chlorite and actinolite. Overall mineralogical and textural features suggest that the intrusive is affected by a low grade metamorphic overprint. The sample from the Rama Granitoid (TT900) is a mafic tonalite and is composed of plagioclase (Anlo_2o), quartz, biotite, hornblende and K-feldspar. Biotite and hornblende account for up to 20% of the modal composition of the lock. This sample also shows evidence of partial alteration of the constituent minerals especially of the mafic components.
Post-tectonic granitoids Unlike the syn-tectonic granitoids, the post-tectonic granitoids show no compositional facies variations, and they are dominated by pink granite. The Mereb Granite, east of the CSZ was recently dated by the CHIME-zircon method (Tadesse eta/., 1997). In this study the Sibta Granite (TT950), situated at the western side of the CSZ, and the Shire Granite (AG1117) that marginally cuts the CSZ and itself is covered by extensive Phanerozoic rocks (Fig. 1 ), have been dated. The post-tectonic granites generally have a sub-circular shape, indicating emplacement in stress-free conditions. No tectonic fabric is developed inside and along the margins of these intrusives. Narrow deformed zones are locally observed along the contact of the Shire Granite, which may be attributable to emplacement-related strain. All granites of this generation have steep intrusive contacts that cut the regional fabric, and are often associated with contact metamorphic aureoles up to 200 m wide. They are typically massive and pink with larger than 3 cm phenocrystic K-feldspars. K-feldspars (commonly microcline showing microperthitic texture), plagioclase (An8_12) and quartz, together with biotite and a subordinate amount of hornblende, are common constituents. Accessory minerals include zircon, sphene, apatite and Fe oxides. Almost all minerals are fresh, except in a few examples where plagioclase grains have cloudy cores filled with tiny, needlelike sericites.
ANALYTICAL METHODS Geochemical analysis The dated samples have been analysed, as have a few others, from the suites of each of the intrusive bodies for major and trace element chemistry. The analysis was performed by fully automated the Rigaku RX12000 XRF spectrometer at the Department of Geoscience, Shimane University. The method described by Kimura and Yamada (1996) was
Journal of African Earth Sciences 317
T. TADESSE et al.
followed to complete the analysis. FeO is determined by titration and total H20 contents are measured by conventional loss on ignition (LOI).
Rb, Sr, Sm and Nd isotope analysis Coarse-grained mineral fractions, such as K-feldspar and plagioclase, were hand-picked from crushed rock samples. The fine-grained mineral fractions, such as biotite and hornblende, were obtained by using an isodynamic magnetic separator. Extraction of Sr, Nd and Sm from powdered samples followed the method described by Kagami e t al. (1987). Isotopic analysis was performed using a Finnigan MAT 262 mass spectrometer equipped with 5 Faraday cup collectors at the Department of Geoscience, Shimane University. The 87Sr/86Sr and 143Nd/144Nd ratios were normalised to 8eSr/88Sr = O. 1194 and 14eNd/144Nd= 0.7219, respectively. The 87Sr/86Sr ratio of NBS987 and 143Nd/144Nd ratio of the La Jolla standards were each measured five times during this study. The respective means of ratios are 0.710269 _+0.000010 (2a; N = 5 ) a n d 0.511841 + 0 . 0 0 0 0 0 5 (2(~; N = 5 ) . Rubidium and Sr contents of the samples were determined by XRF (Rigaku RXI 200) from glass beads. Depending on the reproducibility of the data for standard samples (G-2), estimated uncertainties for the Rb/Sr ratios were ca 0 . 5 % (2(~). Samarium and Nd concentrations of the samples were obtained by isotope dilution techniques using a 149Sm/15°Nd mixed spike. Estimated errors for the Sm/Nd ratio are ca 0.1% (2(~). The Rb-Sr and Sm-Nd isochron ages were calculated using the equation of York (1 966). The ThO 2, UO 2 and PbO analysis of zircon Zircon grains were separated from crushed rock samples by panning and hand-picking. The grains from individual samples were mounted on glass slides using petropoxy 1 53, and polished with diamond paste until the grains were thinned to approximately half their original thickness. Thus the crosssection provides the core-mantle relationships and any overgrowths. The ThO 2, PbO and UO 2 concentrations of the zircon grains were analysed on a JEOL JXA-733 electron microprobe equipped with three wavelength dispersive-type spectrometers at the Department of Earth and Planetary Sciences, Nagoya University. Analyses were performed under the following operating conditions: a 15 kV accelerating voltage, 0.02-0.1 5 p A current and 5 pm probe diameter. X-ray intensities were integrated over a period of 400 s for the line and 200 s for background at two optimum positions on both sides of the lines. The measurements were repeated twice and the arithmetic mean was taken as the true intensity.
318 Journal of African Earth Sciences
Under the conditions, the detection limits of ThO 2, UO 2 and PbO at the 2(~ confidence level were 0.01 2, 0 . 0 0 7 and 0 . 0 0 4 w t % , respectively. The maximum possible errors in the determination are about 8% for 0.1 w t % of ThO 2 and UO 2 concentrations and about 14% for the 0.02 w t % PbO c o n c e n t r a t i o n . F u r t h e r d e t a i l s of a n a l y t i c a l p r o c e d u r e s and CHIME age c a l c u l a t i o n are described by Suzuki and Adachi (1991, 1994). It was noted that there are some problems in applying the chemical method to age determination of zircon (Suzuki and Adachi, 1991 ). One of the most serious problems is the isotopic age discordance owing to Pb loss. However, Krogh (1982) showed that clear zircon w i t h gem quality gives concordant U-Pb isotopic ages or shows least Pb loss. Following this, the zircon separates used for dating in this study include only homogeneous, clear and transparent grains of appropriate quality.
RESULTS Geochemistry The analytical data for dated granitoids and representative samples form their suites are given in Table 1 and the geochemical characteristics of the granitoids is shown in Fig. 2. Generally the post-tectonic granites are higher in total alkali contents than the syn-tectonic varieties at the same silica value (Table 1 ). Among the syntectonic granitoids, the Chila and Rama samples are more mafic and are characterised by high CaO and AI203 contents (Table 1 ). It is also noted that, for the same silica level, post-tectonic granites have relatively higher AI203 contents than the granitic samples of syn-tectonic intrusives (compare the Azeho samples with that of the Sibta, Mereb and Shire samples: Table 1 ). Titanium contents are con-sistently low for all the samples, while concentrations of Zr decrease with an increase in silica content. Apart from these broad major and trace element concentration variations, the samples from post- and syn-tectonic granitoids show some common geo-chemical features in terms of other major and trace element criteria. All the granitoids are metaluminous, or only marginally peraluminous, especially the post-tectonic ones (Fig. 2a), and have I-type (Chappell and White, 1974; Whalen e t al., 1987) character-istics (Fig. 2b, c). The primitive mantle-normalised (after Sun and McDonough, 1989) trace element distribution pattern of the granitoids (Fig. 2d) shows that all of them are characterised by depletion in Nb, P and Ti, which indicates fractionation of mineral phases containing these elements at some time during the history of the source magma, or removal of these elements during fractionation
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processes in the host magma. However, the most notable feature is progressive enrichment of elements from more compatible, high field strength elements (HFSE) to least compatible, large-ion lithophile elements (LILE), which is characteristic of a subduction-related, calc-alkaline differentiation trend (Pearce et al., 1984a). In conventional tectonic discrimination diagrams (Pearce etal., 1984a), all the
320 Journal of African Earth Sciences
granitoids plot in the volcanic arc granite field (Fig. 2e, f). Therefore, the preliminary data indicates a subduction-related volcanic arc setting for the granites, consistent with the geochemistry of the extrusive rocks in the area (Tadesse etal., 1998b). A more detailed description of the geochemistry of the Chila and Rama Granitoids is given by Alemu (1997), and a full description of petrological and
Sm-Nd, Rb-Sr and Th-U-Pb zircon ages of syn- and post-tectonic granitoids, Axum area, Ethiopia Table 2. Sm and Nd analytical data for the whole rocks and minerals Sample
Nd (ppm) Sm (ppm)
I*'Sm/'*4Nd
'*~Nd/'**Nd (2~) sNd(T)
TDM
Azeho granitoid GS1957 GS1957 GS1957 GS1957 GS1979
(whole rock) (plagioclase) (K-feldspar) (felsic portion) (whole rock)
28.50 46.27 8.07 7.60 26.20
5.22 8.00 1.81 1.72 4.21
0.111 0.105 0.139 0.137 0.096
0.512483 0.512447 0.512619 0.512607 0.512408
(08) (09) (09) (09) (18)
5.32
0.99
5.34
0.96
35.61
6.02
0.102
0 . 5 1 2 4 7 7 (02)
3.50
0.92
19.46 22.98 21.07 34.64
3.24 4.10 4.12 3.74
0.101 0.109 0.118 0.065
0.512318 0.512361 0.512392 0.512169
(13) (09) (08) (08)
3.69 3.79 3.32 4.45
1.13 1.15 1.21 1.01
19.28 15.52 19.90
4.11 2.67 4.88
0.129 1.040 0.148
0.512452 0.512316 0.512544
11) 12) 13)
2.3
1.26
62.34
8.56
0.083
0.512368
16)
4.91
0.89
13.27
1.73
0.08
0 . 5 1 2 3 6 3 (22)
5.94
0.91
Deset granitoid KK1399
Chila granitoid TT570 (whole rock) KM1500 (whole rock) KM1503 (whole rock) MB916 (whole rock)
Rama granitoid TT900 (whole rock) TT900 (plagioclase) TT900 (biotite and hornblende)
Sibta granite TT950 (whole rock)
Shire granite A G l 1 1 7 (whole rock)
Number in parentheses is the 2~ error in the last t w o digits of the Nd isotopic ratio.
geochemical characteristics of the granitoids of northern Ethiopia is given with additional whole rock and mineral chemistry elsewhere (Hoshino and Tadesse, in prep).
Geochronology Analytical data of Rb, Sr, Sm, Nd concentrations and isotope ratios are given in Tables 2 and 3. The ThO 2, UO 2, UO2* (the sum of UO 2 and UO 2 equivalent of measured ThO 2 for the zircon) and PbO concentrations, together with apparent ages obtained from individual measurement spots on zircon grains, are also given in Table 4. Isochron diagrams are shown in Figs 3 and 4. All errors quoted in this paper for age and initial isotopic ratios are of 2~ values.
Syn-tectonic granitoids The Azeho syn-tectonic granitoid One sample (GS1957) from this granitoid and mineral separates of the same sample yield an internal SmNd isochron (MSWD=0.11) age of 756+_33 Ma (initial Nd ratio=O.511929 [14]: Fig. 3a). The age calculated from the slope of two whole rock samples of the granitoid (GS1957 and GS1979) is 763 _+24 Ma (initial Nd ratio = 0.511928: Fig. 3b). Since the
Sm-Nd system is considered to be unaffected by low grade metamorphic overprint and the whole rock and internal isochron ages are identical within error, the 756 + 33 Ma age is interpreted as a close estimation for the crystallisation age of the Azeho Granitoid. On the other hand, Rb-Sr isotope data of the samples show a wide scatter on an isochron diagram (not shown), indicating that the Rb-Sr isotope system is not in equilibrium. Zircon grains were unable to be separated from the samples for dating by the CHIME method. The Sm/Nd ratios of the two samples are low, and calculated ~Nd(7~eMa~values are positive and nearly identical (+ 5.31 and + 5.35, respectively: Table 2). The TDM age of the samples are 0.99 and 0.96 Ga, respectively (Table 2). These Nd isotope data strongly suggest an origin of the granitoid from depleted source materials - possibly island-arc tholeiite of Neoproterozoic age or depleted mantle w i t h o u t any significant involvement of older continental crust (cf. Harris et al., 1990).
The Deset Granitoid Nine zircon grains were separated for sample KK1399 collected from this intrusion (Fig. 1 ). The zircon grains are short euhedral prisms ranging in grain size from 60 to 100 pro. All the grains are
Journal of African Earth Sciences 321
T. TADESSE et al. Table 3. Rb and Sr analytical data for whole rocks and minerals Sample
Rb (ppm) Sr (ppm)
~/Rb/~Sr
UZSr/~Sr (2(~)
Azeho Granitoid GS1957 (whole rock) GS1957 (plagioclase) GS1957 (K-feldspar) GS1957 (felsic portion) GS1979 (whole rock)
90 112 112 66 68
175 292 1 22 89 162
1.38 1.03 2.47 2.00 1.13
0.720929 0.718577 0.732220 0.724959 0.71 3880
(10) (14) (11) (11) (11
Deset Granite KK1399 (whole rock) KK1399 (plagioclase) KK1399 (K-feldspar) KK 1399 (biotite) KK1399 (felsic portion)
110 52 144 603 75
530 751 669 87 553
0.56 0.19 0.58 18.66 0.36
0.708576 0.704804 0.709185 0.815594 0.707051
(11) (09) (11 (13) (11)
Chila Granitoid T T 5 7 0 (whole rock) KM1 500 (whole rock) KM1 503 (whole rock) MB916 (whole rock)
55 53 63 48
505 599 808 7901
0.29 0.24 0.21 0.16
0.706573 0.705968 0.705711 0.705231
(09) (09) (09) (12)
Rama granitoid TT900 (whole rock)
76
406
0.50
0.709301 (09)
Sibta Granite TT950 (whole rock)
164
292
1.51
0 . 7 1 7 1 3 9 (10)
TT950 (biotite) TT950 (plagioclase) T T 9 5 0 (K-feldspar)
349 31 152
112 658 281
8.22 O. 13 1.46
0 . 7 6 8 3 2 4 (11 ) 0 . 7 0 4 7 3 8 (09) 0 . 7 1 6 5 0 6 (10)
75 120 71 504
864 942 1033 128
0.23 0.34 0.18 11.48
Shire Granite AG1117 AG1117 AGl117 AGl117
(whole rock) (K-feldspar) (plagioclase) (biotite)
0.705510 0.706809 0.704992 0.795408
(09) (09) (09) (13)
Number in parentheses is the 2~ error in the last t w o digits of the Sr isotope ratio
compositionally homogeneous and inclusion free. The analytical data (Table 4) are plotted on a UO2* versus PbO diagram (Fig. 4a). The data points show linear arrangement on the diagram and regressed with an isochron of 757 _+30 Ma (MSWD =0.35). Since the regression line passes through the origin, secondary Pb loss or gain in the system can largely be ruled out (see Suzuki and Adachi, 1991). The 7 5 7 + 3 0 Ma age is interpreted here as the best estimate of the age of crystallisation of the Deset syn-tectonic granitoid. Minerals were also separated from the same sample and analysed for Rb-Sr sys-tematics (Table 3). The data yield an internal error-chron age of 6 3 7 + 3 3 Ma (MSWD=6.6: not shown), much younger than the CHIME-zircon age. This age probably reflects a post-crystallisation thermal event or has no geological meaning. The Sm/Nd ratio for
322 Journal of African Earth Sciences
the sample is low, the sNd c7~7Mal value is positive ( + 3 . 5 ) with calculated TDM=O.92 Ga. This is not significantly different from the Azeho Granitoid and may suggest either a genetic link between both intrusions or similar source materials. The Chila Granitoid The Rb-Sr isotope data for four whole rock samples are well-aligned and regressed with an isochron (MSWD=O.5) of 7 2 6 + 3 0 Ma (initial Sr ratio= 0.703528 [50]: Fig. 3c). Three zircon grains were separated from sample TT570 and their CHIMEzircon age analysed. The zircon grains vary in size from 50 to 80 pro, free of inclusions and transparent with well-developed crystal faces. No internal zoning is visible under the microscope. Fourteen spots on three grains yielded a CHIME isochron age of 806 +
Sm-Nd, Rb-Sr and Th-U-Pb zircon ages of syn- and post-tectonic granitoids, Axum area, Ethiopia
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Journal of African Earth Sciences 323
T. TADESSE et al.
0.5127
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0.5126
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Azeho granitoid(GS1957) 1: Plagioclase, 4 W , hoe,,,-,, l 2 rock, : . 3: Felsicp o r t i ~
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0.5126
i
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0.5124 0.10
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Age: 756 + 33 Ma
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Age: 793 + 51 Ma / I. R: .511778 (22) .~ MSWD: .26 ~ i
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0.12
0.13
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147
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~
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0.51235 0.09
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0.80 0.78
0.7060
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0.15 87
0.76
0.72
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0.70
0.25
0.30
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144
Sin/ Nd
Sin/ Nd 0.51250
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8
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86
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Figure 3. Sm-Nd and Rb-Sr whole rock and internal isochron diagrams o f the granitoids. Uncertainties for the initial ratios o f all the isochrons are s h o w n as the numbers in brackets for the last two digits. Data from Tables 2 and 3.
21 Ma ( M S W D = 0 . 0 2 : Fig. 4b). This age is significantly older than the whole rock Rb-Sr isochron age (726 + 30 Ma), but is indistinguishable, within error, from the Ar-Ar (hornblende) plateau age of 792 + 7 Ma reported for the same granitoid by Alemu (1997). This CHIME-zircon age is interpreted as a good estimation of the crystallisation age of the Chila Granitoid. The Rb-Sr whole rock age is probably reflecting isotopic resetting during either metamorphic or hydrothermal overprint, Kr6ner etal. (1991 ) reported a widespread Rb-Sr isotopic re-equilibration around 720 Ma from northern parts of the Nubian Shield. It is also worth noting that Alemu (1997) has
324 Journal of African Earth Sciences
reported an Ar-Ar (biotite) age of 725 + 4 Ma from the sample which yielded the Ar-Ar hornblende plateau age of 792 + 7 Ma, and ascribed this discor-dance to the difference in Ar-Ar blocking temperature for biotite and hornblende. The samples from the Chila Granitoid have low Sm/ Nd ratios of 0.06-0.11, and have a restricted range of positive gNd (ao6Ma) values between + 3.92 and + 4.26 (Table 2). The TDMages for the samples range from 1.01 to 1.25. The Sm-Nd data, together with the low initial Sr ratio (0.7035), indicate that the source material of the Chila Granitoid is also primitive perhaps juvenile island-arc crust or mantle source, or
Sm-Nd, Rb-Sr and Th-U-Pb zircon ages of syn- and post-tectonic granitoids, Axum area, Ethiopia
D;se, g,:i,oi
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31 analyses on 9 grains Age: 757_+30Ma M S ~ "
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Chila granitoid 14 analyses on 3 grains Age: 8 0 6 + 2 1 Ma
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/
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Post-tectonic granites
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age has a large error owing to the limited spread in Sm/Nd isotope ratio, it is identical, within error, to the CHIME-zircon age of the nearby Chila Granitoid (806_+21 Ma). This age is interpreted as a close estimation of the crystallisation age of the Rama Granitoid. The Rb-Sr data of these samples do not align in the isochron diagram, again indicating a postcrystallisation disturbance of the Rb-Sr isotope system. Zircon grains separated from this sample are very fine-grained (< 40/Jm) in size. The grains are short prismatic, yellow and contain inclusions. The UO2* versus PbO plots of data from 18 spots on five grains are aligned linearly and yield a CHIMEzircon age of 7 4 0 + 4 2 Ma (Fig. 4c). This age is much younger than the Sm-Nd internal isochron age, but identical, within error, to the Ar-Ar biotite age of 745 + 5 Ma (Alemu, 1997) obtained for the same granitoid. The CHIME-zircon age is also identical, within error, to the whole rock Rb-Sr age of Chila Granitoid (726 + 30 Ma, see above). The linear array of data, coupled with the regression line passing through the origin on the diagram, indicates that the grains are isotopically homogeneous and mark some geological event. A possible explanation for this is that the zircon grains are of metamorphic origin or associated with hydrothermal alteration, and hence the CHIME-zircon age is the date of such a late event. The sample has a low Sm/Nd ratio, a positive sNd (793Ma)value ( + 2.92) and hasa TDM age of 1.26 Ga, (Table 2). These values are not significantly different from that of the Chila Granitoid and may suggest common source materials.
[_JAnalyticaluncertainty 0.1
0.2
0.3
0.4
UO *(~%) Figure 4. Plots of U02* versus PbO showing CHIME-zircon isochron diagrams for the respective intrusive bodies. Data from Table 4.
a combination of both, as emphasised by the occurrence of mafic (gabbroic and dioritic) bodies and mafic enclaves associated with the pluton.
The Rama Granitoid The Sm-Nd isotope data for whole rock (TTgO0) and mineral separates of the felsic (mainly plagioclase) and mafic (biotite + hornblende) components yield a three point internal isochron (MSWD = O. 26) age of 7 9 3 + 51 Ma (Fig. 3d). Although the isochron
The Sibta and Shire Granites One whole rock sample (TT950), together with Kfeldspar, plagioclase and biotite separates from the Sibta Granite, yield a Rb-Sr internal errorchron (MSWD =6.12) age of 5 5 0 + 3 5 Ma (Fig. 3e). The internal Rb-Sr errorchron (MSWD=3.6) age of the Shire Granite is 559 + 22 Ma. (Fig. 3f). Since these granites are neither deformed nor affected by metamorphism, it is assumed that the intrusives have not been subjected to any excessive post-emplacement isotopic disturbance. Therefore, the ~550 Ma age is interpreted as a close estimation for the crystallisation age of post-tectonic granites in the area. These ages are identical, within error, to the CHIME-zircon age of the Mereb post-tectonic granite ( 5 4 5 + 2 4 Ma: Tadesse eta/., 1997) and the U-Pb zircon age of 541 _+ 14 Ma of the Mao post-tectonic granite in western Ethiopia (Ayalew eta/., 1990). These post-tectonic granites have also low Sm/Nd and Rb/Sr ratios. The gNdt55oMa~values are positive ( + 4.91 and + 3.50 for the Sibta and Shire Granites,
Journal of African Earth Sciences 325
T. TADESSE et al.
respectively: Table 2). The calculated TOMage is 0.91 Ga for the Sibta Granite and 0.89 Ga for the Shire Granite. The initial Sr isotopic ratio (at 550 Ma) for the Sibta Granite is 0.7052 and 0.7036 for the Shire Granite (not shown). These and the Sm-Nd isotopic features are consistent with magma generation from an intracrustal source with a short residence history.
DISCUSSION AND CONCLUSIONS The preliminary geochemical data indicate that the granitoids in the northern Ethiopian low grade terrane have a chemistry similar to subduction-related volcanic arc granitoids. The Sm-Nd and Rb-Sr isochron and the Th-U-total Pb zircon (CHIME-zircon) isochron ages for the representative granitods of the area revealed three distinct magmatic events: at - 8 0 0 , - 750 and - 5 5 0 Ma. These ages are similar to the events recorded for arc magmatism, accretion and late-stage intra-crustal melting in other parts of the ANS (e.g. Kr6ner e t a L , 1991 ; Abdelsalam and Stern, 1996). More specifically, the oldest age reported here is identical, within error, to the single zircon Pb-Pb evaporation age of 811 Ma of a syntectonic granitoid in the Nakfa Terrane of Eritrea (Teklay, 1997) and a U-Pb zircon age of 820 Ma for a syn-tectonic granitoid from western Ethiopia (Ayalew et al., 1990). Since the granitoids are intrusions into the volcanosedimentary arc assemblage and postdate at least one phase of deformation that affects the host rock, the oldest age reported here is a minimum age of arc magma-tisim in the region. An older age of - 8 5 0 Ma (single zircon Pb-Pb evaporation method) has been reported from felsic metavolcanics of the Nakfa Terrane (Teklay, 1997). The ages of the post-tectonic granites in the Axum area ( - 5 5 0 Ma) are also identical to those reported from western Ethiopia. This uniformity in radiometric ages from western to northern Ethiopia, and further north to Eritrea, argue for virtually con-temporaneous arc magmatism and crustal growth in this part of the ANS, and therefore questions the validity of previous conclusions on Ethiopian base-ment stratigraphy (Kazmin etal., 1978). Despite the area studied being situated at the southernmost part of the ANS low grade zone, from which it passes southwards to high grade, partly reworked Mozam-bique Belt rocks, the low initial Sr isotopic ratio and primitive Nd isotopic characteristics, t o g e t h e r w i t h Neoproterozoic depleted mantle ages of the intru-sives, indicate an insignificant contribution of any older, pre-Pan-African crust in the generation of the granitic magmas (cf. Harris et al., 1990). Considering the age and isotopic differences of the granitoids in terms of the local geology, it is interesting
3 2 6 Journal o f ,4 frican Earth Sciences
to note that the ages of the syn-tectonic granitoids are geographically separate: the older (-800 Ma) are intrusions into the structurally north-west facing succession of Chila Block metasediments situated east of the central steep zone (Fig. 1). They are perhaps related to the evolved, calc-alkaline islandarc volcanic rocks of the Adwa Block (Tadesse, et al. 1998b). On the other hand, those granitoids with - 7 5 0 Ma ages are intrusions into the structurally s o u t h e a s t facing, i m m a t u r e island-arc/suprasubduction zone succession of the Adi Hageray and Adi Nebrid tectonostratigraphical blocks (Tadesse et al., 1998b; Fig. 1 ). Granitoids from both sides of the CSZ have positive sNd m values, but the former have lower 8Nd(T) values and older TDMmodel ages than the latter (Table 2). Tadesse etaL (1998b) suggested, on the basis of volcanic geo-chemistry and field relationships, that the structurally southeast facing succession west of the CSZ must have formed over a consumed back-arc basin, while the northwest facing succession east of the CSZ represents a mature island-arc assemblage. The ages and isotopic data presented above, therefore, are consistent with the field and geochemical variations and suggest that the CSZ is in effect a fundamental structure in the area that marks the boundary between a younger, immature island-arc/supra-subduction zone succession in the west and a relatively older, calc-alkaline island-arc succession in the east. Therefore, the CSZ appears to have been active during the accretion of these contrasting tectono-stratigraphical blocks. Northwards, in Eritrea, the same structure extends as an axial zone for the full north-south length of the Nakfa Terrane (Drury, 1998 pets. comm.). There too, it is associated with a high strain zone, steep structure and incorporates deformed granitic and amphibolite grade metamorphic rocks (Drury, 1998 pers. comm., Drury and de Souza Filho, 1998). Unfortunately, suitable samples have not been obtained for dating the sheet-like granitoids occurring along the steep zone that might have constrained the movement in the zone. However, the movement along this CSZ predates the 550 Ma post-tectonic granite that cuts it and post-dates the 750 Ma syn-tectonic granitoids. It is possible that isotope resetting in some of the granitoids, and some zircon ages around 730-740 Ma, might be due to arc accretion in the region.
ACKNOWLEDGEMENTS The authors thank Prof. Y. Sawada for his guidance and advice during the XRF work. They are also thankful to Ms Akasaka for helping with the Rb-Sr and Sm-Nd extraction and Mr T. Nagaoka for his technical assistance. Thoughtful review of the manuscript by Drs
Sm-Nd, Rb-Sr and Th-U-Pb zircon ages of syn- and post-tectonic granitoids, Axum area, Ethiopia
F. Henjes-Kunst and S. Drury improved the earlier version. This work is partly supported by the Grant-in-Aid for JSPS fellow (No.10096301 ) (T.T.) from the Ministry of Education, Science and Culture, Japan. REFERENCES Abdelsalam, M.G., Stern R.J., 1996. Sutures and shear zones in the Arabian-Nubian Shield. Journal African Earth Sciences 23, 2 8 9 - 3 1 0 . Alemu, T., 1997. Petrology, Geochemistry and Geochronology of Granitoids from the Axum area, northern Ethiopia. M.Sc. thesis, University of Vienna, Austria, 106p. Ayalew, T., Bell, K., Moore, M.J., Parish, R.R., 1990, UPb and Rb-Sr geochronology of western Ethiopian Shield. Geological Sciety America Bulletin 120, 1309-1316. Berhe, S.M., 1990. Ophiolites in Northeast and East Africa: implications for Proterozoic crustal growth. Journal Geological Society London 147, 41-57. Beyth, M., 1972. The geology of central and western Tigray: Ph.D. thesis, Rheinische Friedrich-Wilhelms Universitat, Bonn, West Germany. Chappell, B., White, A.J.R., 1974. Two contrasting granite types. Pacific Geology 8, 1733-1734. Chawaka, S., de Wit, M.J., 1981. Plate tectonic evolution of Ethiopia and the origin of its mineral deposits: an overview. In: Chawaka, S., de Wit, M.J. (Eds.), Plate Tectonics and Metallo-genesis: Some Guidelines to Ethiopian Mineral Deposits. Ethio-pian Geological Survey, Addis Ababa, Ethiopia, pp. 15-129. De Souza Filho, C.R.,d Drury, S.A., 1998. A Neoproterozoic supra-subduction terrane in northern Eritrea, NE Africa. Journal Geological Society London 155, 551-566. Drury, S.A., Berhe, S.M., 1993. Accretion tectonics in the northern Eritrea revealed by remotely sensed imagery. Geological Magazine 130, 177-190. Drury, S.A., de Souza Filho, C.R., 1998. Neoproterozoic terrane assemblages in Eritrea: review and prospects. Journal African Earth Sciences 27, 331-348. Harris, N.B.W., Gass, I.G., Hawkesworth, C.J., 1990. A geochemical approach to allochthonous terranes: a PanAfrican case study. Philosophical Transactions Royal Society London 331, 533-548. Kagami, H., Iwata, M., Sano, S., Honma, H., 1987. Sr and Nd isotope c o m p o s i t i o n s and Rb, Sr, Sm and Nd concentrations of standard samples. Technical Report Series B, Okayama University, Japan, pp. 1-16. Kazmin, V., 1972. The Geology of Ethiopia. Note No. 821051-12, Ethiopian Institute of Geological Surveys, Addis Ababa, Ethiopia, 208p. Kazmin, V., Shiferaw, A., Balcha, T., 1978. The Ethiopian basement: stratigraphy and possible manner of evolution. Geologische Rundschau 67, 531-548. Kimura, J., Yamada, Y., 1996. Evaluation of major and trace element XRF analyses using a flux to sample ratio of two to one glass beads. Journal Mineralogy, Petrology Economic Geology 91, 62-72. Krogh, T.E., 1982. Improved accuracy of U-Pb zircon ages by the creation of more concordant systems using air abrasion technique. Geochimica Cosmochimica Acta 46, 637-649. KrSner, A., Grilling, R.O., Reischmann, T., Hussien, I.M., Stern. R.J., Durr, St., Kruger, J., Zimmer, M., 1987. Pan-African crustal evolution in northeast Africa. In: KrSner, A. (Ed.), Proterozoic Lithospheric Evolution. American Geophysical Union, pp. 235-257. KrSner, A., Stern, R.J., Linnebacker, P., Manton, W., Reischmann, T , Hussien, I.M., 1991. Evolution of Pan-
African island arc assemblages in the south Red Sea Hills, Sudan and in SW Arabia as exemplified by geochemistry and geochronology. Precambrian Research 53, 99-118. Pearce, J.A., Harris, N.B.W., Tindle, G.A., 1984a. Trace e l e m e n t d i s c r i m i n a t i o n diagrams for the t e c t o n i c interpretation of granitic rocks. Journal Petrology 25, 956983. Pearce, J.A., Lippard, S.J., Roberts, S., 1984b. Characteristics and tectonic significance of supra-subduction zone ophiolites. Geological Society London 16, 77-94. Rogers, A.S., Miller, J . A . , Mohr, P.A., 1965. Age determination on some Ethiopian Basement Rocks. Nature 206, 1021-1023. Stern, R.J., 1994. Neoproterozoic (900-550 Ma) arc assembly and continental collision in the East African orogen: Implications for the consolidation of Godwanaland. Annual Review Earth Planetary Sciences 22, 319-351. Sun, S.-S., McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts. In: Saunders, D., Norry, M.J. (Eds.), Magmatism in the Ocean Basins. Geological Society London, Special Publication 42, 313-345. Suzuki, K., Adachi, M., 1991. Precambrian provenance and Silurian metamorphism of the Tsubonosawa paragneiss in the South Kitakami terrane, northeast Japan, revealed by Chemical Th-U-total Pb isochron ages of monazite, zircon and xenotimes. Geochemical Journal 25, 357-376. Suzuki, K., Adachi, M., 1994. Middle Precambrian detrital monazite and zircon from the Hida gneiss on Oki-Dogo Island, Japan. Their origin and implications for the correlation of basement gneiss of Southwest Japan and Korea. Tectono-physics 235, 277-292. Tadesse, T., 1996. Structure across a possible intra-oceanic suture zone in the low-grade Pan-African rocks of northern Ethiopia. Journal African Earth Sciences 23, 375-381. Tadesse, T., 1997. The Geology of Axum area (ND 37-6). Memoir No. 9, Ethiopian Institute of Geological Survey, Addis Ababa, Ethiopia, 184p. Tadesse, T., Hoshino, M., Sawada, Y., 1998a. Lithological, structural and g e o c h e m i c a l c o n s t r a i n t s from the Neoproterozoic low-grade rocks of northern Ethiopia: implications for crustal growth in the region. Journal African Earth Sciences 27 (1A), 194-195. Tadesse, T., Hoshino, M., Sawada, Y., 1998b. Geochemistry of low-grade metavolcanic rocks from the Pan-African of Axum area, northern Ethiopia. Precambrian Research 99, 101-124. Tadesse, T., Suziki, K., Hoshino, M., 1997. Chemical ThU-total Pb isochron age of zircon from the Mereb Granite in northern Ethiopia. Journal Earth Planetary Science Nagoya University 44, 21-27. Teklay, M., 1997. Petrology, Geochemistry and Geochronology of Neoproterozoic Magmatic Arc Rocks from Eritrea: Implications for Crustal Evolution in the Southern Nubian Shield, Memoir 1, Department of Mines, Eritrea, Asmara, 125p. Teklay, M., KrSner, A., Oberhansli, R., 1993. Reconnaissance Pb-Pb zircon ages from Precambrian rocks in eastern and southern Ethiopia and an attempt to define crustal province. In: Thorweihe, U., Schandelmeier, H. (Eds.), Geoscientific Research in Northeast Africa. A.A. Balkema, Rotterdam, pp. 133-138. Vail, J.R., 1983. Pan-African crustal accretion in northeast Africa. Journal African Earth Sciences 1, 285-294. Whalen, J.B., Currie, K.L., Chappell, B.W., 1987. A-type granites: geochemical chrcateristics, discrimination and petrogenesis. Contributions Mineralogy Petrology 95, 407419. York, D., 1966. Least-square fitting of straight line. Canadian Journal Physics 44, 1079-1086.
Journal of African Earth Sciences 327
Pergamon
PIhS0899-5362(00)00022-1
2 0 0 0 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0 8 9 9 - 5 3 6 2 / 0 0 S- see front matter
Sm-Nd, Rb-Sr and Th-U-Pb zircon ages of syn- and post-tectonic granitoids from the Axum area of northern Ethiopia TAREKEGN TADESSE 1'2'*, MITSUO HOSHINO 2, KAZUHIRO SUZUKP and SHIGERU IIZUMI 4 tEthiopian Institute of Geological Surveys, PO Box 2302, Addis Ababa, Ethiopia 2School of Informatics and Sciences, Nagoya University, Nagoya 464-8601, Japan 3Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan 4Department of Geoscience, Shimane University, Matsue 690-8504, Japan
ABSTRACT--Sm-Nd, Rb-Sr and Th-U-Pb zircon ages for four syn-tectonic and t w o post-tectonic granitoids from the Axum area of northern Ethiopia are determined. Two of the syn-tectonic granitoids (the Azeho and Deset) are intrusions into structurally southeast facing, predominantly tholeiitic arc metavolcanics and associated metasediments situated west of the central steep zone in the area. The other t w o syn-tectonic granitoids (the Chila and Rama) are intrusions into structurally northwest facing metasediments and calc-alkaline metavolcanics at the eastern part of the steep zone. One of.the post-tectonic granites (the Sibta Granite) occurs west of the central steep zone and the second (the Shire Granite) cuts the central steep zone. Preliminary geochemical data of all the granitoids show that they are enriched in large ion lithophile elements, depleted in high field strength elements and have I-type characteristics, similar to calc-alkaline granitoids in subduction-related volcanic arc setting. The geochronological data indicate three discrete intrusive events: - 8 0 0 Ma to the east of the central steep zone, - 7 5 0 Ma to the west and a post-tectonic intrusion at around 550 Ma. The - 7 5 0 and - 8 0 0 Ma ages of the intrusives are interpreted as minimum ages of arc magmatism in the respective blocks across the central steep zone, and the - 5 5 0 Ma age of the post-tectonic granites records the final magmatic event in the region. The contrast of age across the central steep zone emphasises that this zone is a major structural element that might have played a significant role during the accretion of structurally and lithologically contrasting tectonostratigraphical blocks. © 2000 Elsevier Science Limited. All rights reserved. RI~SUMI~--Des ~ges obtenus par la Sm-Nd, Rb-Sr et Rh-U-Pb analyse de zircons ont 6te determines sur quatre granito'fdes syn-tectoniques et deux post-tectoniques de la region d'Axoum dans le nord de I'Ethiopie. Les deux granitoTdes syn-tectoniques d'Azeho et de Deset, & I'ouest de la zone centrale verticale de la region, sont intrusifs dans des metavolcanites essentiellement & composition de tholeiites d'arc et des metasediments associes structures vers le sud-est. Les deux autres granito'fdes syn-tectoniques de Chila et de Rama, & I'est de la zone centrale verticale, sont intrusifs dans des metasediments et des metavolcanites calco-alcalines structures vers le nord-ouest. Le granite post-tectonique de Sibta affleure & I'ouest de la zone centrale verticale et celui de Shire recoupe la zone centrale verticale. Les donnees chimiques preliminaires montrent que tous les granito'fdes sont enrichis en elements legers de grande taille, appauvris en elements de potentiel ionique eleve et ont des caracteres de type I, semblables aux granito'fdes calco-alcalins des arcs volcaniques de subduction. Les donnees chronologiques indiquent trois episodes distincts d'intrusion: - 8 0 0 Ma ~ I'est de la zone centrale verticale, - 7 5 0 Ma ~ I'ouest et post-tectonique vers 550 Ma. Les ~ges d'intrusion ~ - 8 0 0 et - 5 0 Ma s'interpretent comme les ~ges minimum du
*Corresponding author [email protected] Journal o f African Earth Sciences 3 1 3
T. TADESSE et al. magmatisme d'arc dans les blocs de part et d'autre de la zone centrale verticale et I'&ge des granites post-tectoniques & -550 Ma enregistre le dernier 6pisode magmatique de la r~gion. La diff6rence d'&ges de part et d'autre de la zone centrale verticale montre que cette zone est un 616ment structural majeur qui a pu jouer un rSle important Iors de I'accr~tion de blocs tectonostratigraphiques diff~rents par leur structure et leur lithologie. © 2000 Elsevier Science Limited. All rights reserved. (Received 4/11/98: revised version received 22/3/99: accepted 16/3/99)
INTRODUCTION The southern part of the Arabian Nubian Shield (ANS), exposed in northern Ethiopia and Eritrea, is geologically among the least studied areas in the East African Orogen (EAO: Stern, 1994). Most regional tectonomagmatic syntheses of the EAO across this area (e.g. Vail, 1983; Berhe, 1990) are mainly based on relatively old data obtained from reconnaissance photo interpretation augmented by limited field traverses (Kazmin, 1972; Beyth, 1972). Many researchers (e.g. Kazmin et aL, 1978; Chawaka and de Wit, 1981; Berhe, 1990) proposed a possible geological link between the low grade rocks of northern Ethiopia and those of the Saudi Arabia, Egypt and the Sudan in the north, on the basis of gross lithological similarities and structural continuity. Recently, Drury and Berhe (1993), Teklay (1997), de Souza Filho and Drury (1998) and Tadesse etaL (1998a, b) demonstrated that the area developed over a subduction-related island-arc setting and evaluated the suggested link. Unlike the northern parts of the ANS, age constraints in the southern parts are still scanty and insufficient to reveal the continuum of magmatism, sedimentation and hence the assembly of different arc terranes/tectonostratigraphical blocks. Rogers et al. (1965) reported K-Ar dates for a few granitic intrusives of northern Ethiopia. Most of the K-Ar dates obtained by these authors cluster at - 6 0 0 Ma and provide little information on the magmatic, metamorphic and tectonic events of the region, owing to the lack of detailed field data. Recently reported radiometric dates obtained from syn- and post-tectonic intrusives of northern Ethiopia (Alemu, 1997; Tadesse etal., 1997) and felsic metavolcanics and intrusive rocks of the Nakfa Terrane in Eritrea (Teklay, 1997) provide useful information on the age of magmatism and crustal growth in the region. These dates show that crustal growth in the region started before 850 Ma (single-zircon Pb-Pb evaporation age for felsic metavolcanics: Teklay, 1997) and lasted for around 300 Ma (545 Ma of post-tectonic Mereb granite: Tadesse etal., 1997). Yet, these are from geographically different parts of the vast low grade terrane of northern Ethiopia and Eritrea. This is an area that has been shown to have evolved through complex accretion involving oceanic 314 Journal of African Earth Sciences
fragments, immature to mature volcanic arc assemblages and various pre- and post-accretionary metasedimen-tary rocks (Drury and Berhe, 1993; de Souza Filho and Drury, 1998; Tadesse etal., 1998b). Therefore, examining the area in much more detail by employing dating of closely spaced geological formations across the area is of paramount importance to unravel the geodynamic evolution of the region. In this study the Rb-Sr and Sm-Nd internal and whole rock isochron and chemical Th-U-total Pb zircon isochron ages (CHIME-zircon: Suzuki and Adachi, 1991, 1994) have been determined for some syn- and post-tectonic granitoids from the Axum area of northern Ethiopia (Fig. 1 ). Also presented are the preliminary major and trace element data for representative samples from the granitoids. A manuscript describing the details of the geochemical 'characteristics of the granitoids is in preparation and will be presented elsewhere.
REGIONAL GEOLOGICAL FRAMEWORK Northern Ethiopia is underlain by low grade metavolcanosedimentary and plutonic rocks of a typical Pan-African, Arabian Nubian Shield assemblage (see Vail, 1983). With the exception of a few local stratigraphical units that rest unconformably over these volcanosedimentary assemblages, the entire layered succession in northern Ethiopia and Eritrea was mapped as the Tsaliet Group (Beyth, 1972). In the absence of radiometric age dating, the stratigraphical sequence of Ethiopian basement rocks were established on the basis of metamorphic grade and structural complexities (e.g. Kazmin, 1972). Noting that the metamorphic grade of Ethiopian basement rocks varies from high grade, locally granulite-facies in southern Ethiopia through lower amphibolite-facies in western Ethiopia, to low grade greenschist-facies in northern Ethiopia, previous researchers (e.g. Kazmin, 1972; Kazmin et al., 1978) have assumed that the Tsaliet Group is the youngest sequence of metamorphic rocks in the country. These assumptions are seriously challenged by new radiometric data (e.g. Teklay etal., 1993; Ayalew etal., 1990), which showed that the crustal growth of the region
Sm-Nd, Rb-Sr and Th-U-Pb zircon ages of syn- and post-tectonic granitoids, Axum area, Ethiopia
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T. TADESSE et al. is essentially of Neoproterozoic age and hence metamorphic grade does not relate to stratigraphical relationships. This is in line with the emerging consensus on the geological evolution of east and northeast Africa (e.g. Berhe, 1990; Stern, 1994). The findings also support horizontal accretionary tectonics (Kr6ner et al., 1987), as opposed to progressive layercake stratigraphical build-up suggested by previous workers. Part of the Tsaliet Group exposed within the Axum map sheet of northern Ethiopia has been recently remapped (Tadesse, 1997). Field data supplemented by volcanic geochemistry from this area reveal more than four tectonically bounded, lithologically, structurally and geochemically distinct blocks that form an east to west accreted intra-oceanic arc assemblage (Tadesse et al., 1998a). These discrete assemblages are divided into structurally opposite facing successions by a central steep zone (CSZ, Fig. 1 ). West of the central steep zone, a sequence of imbricated metavolcanics and metasediments of the Adi Hageray and Adi Nebrid blocks occur. These blocks are bounded by shear zones. The geochemistry of volcanic rocks in these western blocks varies from mid-oceanic ridge basalt (MORB) in the Adi Hageray Block, through mixed, MORB-IAT (islandarc tholeiite ) chemistry in the Zager mafic-ultramafic belt, to immature island-arc tholeiite (IIAT) in the Adi Nebrid Block (Fig. 1). The 'telescoped' lateral sequence broadly resembles volcanic rocks developed in modern supra-subduction zone settings (cf. Pearce et aL, 1984b). At the eastern side of the CSZ, the structurally northwest facing succession includes fine-grained, commonly carbonaceous phyllite and chert with minor marble and graywacke of the Chile Block, which is thickened by a duplex of thrusts (Fig. 1 ). This distinctive metasedimentary succession rests on a northwest dipping detachment above predominantly tholeiitic to calc-alkaline gabbro and associated mafic-ultramafic rocks of the Daro Tekli area. This, in turn, is followed eastwards and structurally downwards by a more evolved calc-alkaline island-arc assemblage of the Adwa Block (Fig. 1). This complex, west to east succession of the Axum area geographically makes up the bulk of the 850630 Ma Nakfa Terrane of Eritrea (Drury and Berhe, 1993; Teklay, 1997). A n y stratigraphical and structural links between this area and areas to the north and south proposed by previous works (e.g. Kazmin, 1972; Kazmin eta/., 1978; Chawaka and de Wit, 1981 ) must be re-evaluated by identifying the accretionary assemblages similar to those described in this area.
316 Journal of African Earth Sciences
THE GRANITOIDS: FIELD RELATIONSHIPS AND PETROGRAPHY At both sides of the CSZ of the Axum area there are intrusive rocks of variable dimension and relative emplacement ages (Fig. 1 ). Two deformed, syn-tectonic granitoids have been dated from either side of the central steep zone, as have t w o post-tectonic granites. One of the post-tectonic granites is located at the western side of the CSZ and the second one is at the central part of the area, cutting the CSZ (Fig. 1 ).
The syn-tectonic granitoids The t w o dated granitoids from the western side of the steep belt are the Azeho and Deset Granitoids (Fig. 1 ). The Azeho Granitoid intrudes the chaotically mixed rocks of the Adi Hageray Block, which incorporates tectonically mixed metasediments and metavolcanics, together with lenses of mafic and ultramafic rocks. Most of this granitoid occurs to the north in Eritrea across Mereb River. South of the Mereb River, within the study area, it shows considerable variation in its mafic mineral content. The mafic-rich variety is severely altered and its primary igneous minerals are replaced by secondary minerals such as epidote, clinozoisite, sericite, calcite and chlorite. These secondary minerals occur together with relicts of coarse-grained, partly recrystallised K-feldspar and plagioclase, and quartz. In strongly deformed zones it has a mylonitic fabric defined by ribboned quartz, recrystallised plagioclase and sericite. In these mylonites, K-feldspar occurs as porphyroclasts and commonly exhibits mortaring along grain boundaries. The shear fabric is generally penetrative, steeply dipping and associated with L-S fabrics. Marginal zones of the intrusive show maximum strain, and in places the contact between the intrusive and the country rock is diffuse. Kinematic indicators, such as rotated porphyroclasts of feldspars and S-C fabrics, show dominant sinistral strike-slip movement. This deformation post-dates the early northwest verging thrust faults and associated mesoscopic folds in the area (Tadesse, 1996, 1997). The Deset Granitoid is situated in the Adi Nebrid Block (Fig. 1 ) and intrudes predominantly tholeiiticarc metavolcanics and associated metasediments. This granitoid is composite in nature comprising diorite, granodiorite and granite. The granodioritic rock is locally xenolithic, especially along marginal zones. No conclusive field evidence is observed on the relative sequence of emplacement of this varied rock facies. The sample that has been dated (KK1399, Fig. 1 ) is a granite composed of microcline microperthite, plagioclase (Ans_lo), quartz, biotite and a significant amount of Fe oxide, Zircon, apatite and sphene occur
Sm-Nd, Rb-Sr and Th-U-Pb zircon ages of syn- and post-tectonic granitoids, Axum area, Ethiopia in accessory amounts. Deformation fabrics are not well-developed in the interior of the body, but low grade alteration such as chlorite after biotite, and sericite after plagioclase, are common in thin sections. Like other syn-tectonic granitoids in the area (see below), this granitoid is only marginally deformed and characterised by a narrow zone of hybrid rocks. The regional fabric (S 2) is parallel to the fabric in the marginal part of the granite and tends to wrap around it without being cut by the intrusive. On the eastern side of the CSZ t w o syn-tectonic granitoids occur in the Chila and Rama areas (Fig. 1 ). They intrude into the predominantly metapelitic rocks of the Chila Block. These and other syntectonic granitoids in the area (Fig. 1 ) are composite in nature and range in composition from diorite to granite: granodiorite and tonalite being the most dominant, and diorite with gabbro the least significant by volume. In the intrusives, all cross-cutting relationships show that the most mafic member is the oldest in the sequence. These syn-tectonic intrusions generally have well-foliated marginal zones. The Chila Granitoid possesses a relatively wide (ca 300 m) hybridised zone along its margins. The hybridised zones are typically banded where light granitic materials are alternating with dark, maficrich country rock. In places, it typically resembles migmatites in a high grade terrane. Such features are common to the hybridised marginal zones of syntectonic granitoids. In contrast, the Rama Grani-toid lacks such hybridised zones along its margins, perhaps due to its shallow outward dipping intrusive contact. In both the Chila and Rama Granitoids, granodioritic and tonalitic rock varieties are generally xenolithic, mafic enclaves varying in size from a few tens of millimetres to 0.5 m, with commonly diffuse but occasionally sharp contacts. Such features may suggest mixing of mafic and felsic magmas as part of these granitoids' petrogenesis. The density and size of xenoliths, and intensity of deformation, decrease rapidly towards the centres of the bodies. The fabric post-dates the oblique southeast verging thrusts and associated dextral strike-slip shearing. Therefore, the intrusive post-dates the first and most intense deformation, but predates or is synchronous with the second phase of deformation (Tadesse, 1997). Granodiorite samples (TT570, KM 1500, KM 1503 and MB916; Fig. 1 ) from the Chila Granitoid chosen for dating are composed of plagioclase (Anl~_2~), quartz, biotite, hornblende and K-feldspar. Sphene, zircon, apatite and minor oxides occur in accessory amounts. Some plagioclase grains show normal compositional zoning in which the central calcic-rich zones are partly saussuritised. Biotite is partly altered
to chlorite and commonly contains some zircon inclusions with pleochroic haloes. Hornblende is marginally altered to chlorite and actinolite. Overall mineralogical and textural features suggest that the intrusive is affected by a low grade metamorphic overprint. The sample from the Rama Granitoid (TT900) is a mafic tonalite and is composed of plagioclase (Anlo_2o), quartz, biotite, hornblende and K-feldspar. Biotite and hornblende account for up to 20% of the modal composition of the lock. This sample also shows evidence of partial alteration of the constituent minerals especially of the mafic components.
Post-tectonic granitoids Unlike the syn-tectonic granitoids, the post-tectonic granitoids show no compositional facies variations, and they are dominated by pink granite. The Mereb Granite, east of the CSZ was recently dated by the CHIME-zircon method (Tadesse eta/., 1997). In this study the Sibta Granite (TT950), situated at the western side of the CSZ, and the Shire Granite (AG1117) that marginally cuts the CSZ and itself is covered by extensive Phanerozoic rocks (Fig. 1 ), have been dated. The post-tectonic granites generally have a sub-circular shape, indicating emplacement in stress-free conditions. No tectonic fabric is developed inside and along the margins of these intrusives. Narrow deformed zones are locally observed along the contact of the Shire Granite, which may be attributable to emplacement-related strain. All granites of this generation have steep intrusive contacts that cut the regional fabric, and are often associated with contact metamorphic aureoles up to 200 m wide. They are typically massive and pink with larger than 3 cm phenocrystic K-feldspars. K-feldspars (commonly microcline showing microperthitic texture), plagioclase (An8_12) and quartz, together with biotite and a subordinate amount of hornblende, are common constituents. Accessory minerals include zircon, sphene, apatite and Fe oxides. Almost all minerals are fresh, except in a few examples where plagioclase grains have cloudy cores filled with tiny, needlelike sericites.
ANALYTICAL METHODS Geochemical analysis The dated samples have been analysed, as have a few others, from the suites of each of the intrusive bodies for major and trace element chemistry. The analysis was performed by fully automated the Rigaku RX12000 XRF spectrometer at the Department of Geoscience, Shimane University. The method described by Kimura and Yamada (1996) was
Journal of African Earth Sciences 317
T. TADESSE et al.
followed to complete the analysis. FeO is determined by titration and total H20 contents are measured by conventional loss on ignition (LOI).
Rb, Sr, Sm and Nd isotope analysis Coarse-grained mineral fractions, such as K-feldspar and plagioclase, were hand-picked from crushed rock samples. The fine-grained mineral fractions, such as biotite and hornblende, were obtained by using an isodynamic magnetic separator. Extraction of Sr, Nd and Sm from powdered samples followed the method described by Kagami e t al. (1987). Isotopic analysis was performed using a Finnigan MAT 262 mass spectrometer equipped with 5 Faraday cup collectors at the Department of Geoscience, Shimane University. The 87Sr/86Sr and 143Nd/144Nd ratios were normalised to 8eSr/88Sr = O. 1194 and 14eNd/144Nd= 0.7219, respectively. The 87Sr/86Sr ratio of NBS987 and 143Nd/144Nd ratio of the La Jolla standards were each measured five times during this study. The respective means of ratios are 0.710269 _+0.000010 (2a; N = 5 ) a n d 0.511841 + 0 . 0 0 0 0 0 5 (2(~; N = 5 ) . Rubidium and Sr contents of the samples were determined by XRF (Rigaku RXI 200) from glass beads. Depending on the reproducibility of the data for standard samples (G-2), estimated uncertainties for the Rb/Sr ratios were ca 0 . 5 % (2(~). Samarium and Nd concentrations of the samples were obtained by isotope dilution techniques using a 149Sm/15°Nd mixed spike. Estimated errors for the Sm/Nd ratio are ca 0.1% (2(~). The Rb-Sr and Sm-Nd isochron ages were calculated using the equation of York (1 966). The ThO 2, UO 2 and PbO analysis of zircon Zircon grains were separated from crushed rock samples by panning and hand-picking. The grains from individual samples were mounted on glass slides using petropoxy 1 53, and polished with diamond paste until the grains were thinned to approximately half their original thickness. Thus the crosssection provides the core-mantle relationships and any overgrowths. The ThO 2, PbO and UO 2 concentrations of the zircon grains were analysed on a JEOL JXA-733 electron microprobe equipped with three wavelength dispersive-type spectrometers at the Department of Earth and Planetary Sciences, Nagoya University. Analyses were performed under the following operating conditions: a 15 kV accelerating voltage, 0.02-0.1 5 p A current and 5 pm probe diameter. X-ray intensities were integrated over a period of 400 s for the line and 200 s for background at two optimum positions on both sides of the lines. The measurements were repeated twice and the arithmetic mean was taken as the true intensity.
318 Journal of African Earth Sciences
Under the conditions, the detection limits of ThO 2, UO 2 and PbO at the 2(~ confidence level were 0.01 2, 0 . 0 0 7 and 0 . 0 0 4 w t % , respectively. The maximum possible errors in the determination are about 8% for 0.1 w t % of ThO 2 and UO 2 concentrations and about 14% for the 0.02 w t % PbO c o n c e n t r a t i o n . F u r t h e r d e t a i l s of a n a l y t i c a l p r o c e d u r e s and CHIME age c a l c u l a t i o n are described by Suzuki and Adachi (1991, 1994). It was noted that there are some problems in applying the chemical method to age determination of zircon (Suzuki and Adachi, 1991 ). One of the most serious problems is the isotopic age discordance owing to Pb loss. However, Krogh (1982) showed that clear zircon w i t h gem quality gives concordant U-Pb isotopic ages or shows least Pb loss. Following this, the zircon separates used for dating in this study include only homogeneous, clear and transparent grains of appropriate quality.
RESULTS Geochemistry The analytical data for dated granitoids and representative samples form their suites are given in Table 1 and the geochemical characteristics of the granitoids is shown in Fig. 2. Generally the post-tectonic granites are higher in total alkali contents than the syn-tectonic varieties at the same silica value (Table 1 ). Among the syntectonic granitoids, the Chila and Rama samples are more mafic and are characterised by high CaO and AI203 contents (Table 1 ). It is also noted that, for the same silica level, post-tectonic granites have relatively higher AI203 contents than the granitic samples of syn-tectonic intrusives (compare the Azeho samples with that of the Sibta, Mereb and Shire samples: Table 1 ). Titanium contents are con-sistently low for all the samples, while concentrations of Zr decrease with an increase in silica content. Apart from these broad major and trace element concentration variations, the samples from post- and syn-tectonic granitoids show some common geo-chemical features in terms of other major and trace element criteria. All the granitoids are metaluminous, or only marginally peraluminous, especially the post-tectonic ones (Fig. 2a), and have I-type (Chappell and White, 1974; Whalen e t al., 1987) character-istics (Fig. 2b, c). The primitive mantle-normalised (after Sun and McDonough, 1989) trace element distribution pattern of the granitoids (Fig. 2d) shows that all of them are characterised by depletion in Nb, P and Ti, which indicates fractionation of mineral phases containing these elements at some time during the history of the source magma, or removal of these elements during fractionation
Sm-Nd, Rb-Sr and Th-U-Pb zircon ages of syn- and post-tectonic granitoids, Axum area, Ethiopia
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Journal of African Earth Sciences 319
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processes in the host magma. However, the most notable feature is progressive enrichment of elements from more compatible, high field strength elements (HFSE) to least compatible, large-ion lithophile elements (LILE), which is characteristic of a subduction-related, calc-alkaline differentiation trend (Pearce et al., 1984a). In conventional tectonic discrimination diagrams (Pearce etal., 1984a), all the
320 Journal of African Earth Sciences
granitoids plot in the volcanic arc granite field (Fig. 2e, f). Therefore, the preliminary data indicates a subduction-related volcanic arc setting for the granites, consistent with the geochemistry of the extrusive rocks in the area (Tadesse etal., 1998b). A more detailed description of the geochemistry of the Chila and Rama Granitoids is given by Alemu (1997), and a full description of petrological and
Sm-Nd, Rb-Sr and Th-U-Pb zircon ages of syn- and post-tectonic granitoids, Axum area, Ethiopia Table 2. Sm and Nd analytical data for the whole rocks and minerals Sample
Nd (ppm) Sm (ppm)
I*'Sm/'*4Nd
'*~Nd/'**Nd (2~) sNd(T)
TDM
Azeho granitoid GS1957 GS1957 GS1957 GS1957 GS1979
(whole rock) (plagioclase) (K-feldspar) (felsic portion) (whole rock)
28.50 46.27 8.07 7.60 26.20
5.22 8.00 1.81 1.72 4.21
0.111 0.105 0.139 0.137 0.096
0.512483 0.512447 0.512619 0.512607 0.512408
(08) (09) (09) (09) (18)
5.32
0.99
5.34
0.96
35.61
6.02
0.102
0 . 5 1 2 4 7 7 (02)
3.50
0.92
19.46 22.98 21.07 34.64
3.24 4.10 4.12 3.74
0.101 0.109 0.118 0.065
0.512318 0.512361 0.512392 0.512169
(13) (09) (08) (08)
3.69 3.79 3.32 4.45
1.13 1.15 1.21 1.01
19.28 15.52 19.90
4.11 2.67 4.88
0.129 1.040 0.148
0.512452 0.512316 0.512544
11) 12) 13)
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8.56
0.083
0.512368
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4.91
0.89
13.27
1.73
0.08
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5.94
0.91
Deset granitoid KK1399
Chila granitoid TT570 (whole rock) KM1500 (whole rock) KM1503 (whole rock) MB916 (whole rock)
Rama granitoid TT900 (whole rock) TT900 (plagioclase) TT900 (biotite and hornblende)
Sibta granite TT950 (whole rock)
Shire granite A G l 1 1 7 (whole rock)
Number in parentheses is the 2~ error in the last t w o digits of the Nd isotopic ratio.
geochemical characteristics of the granitoids of northern Ethiopia is given with additional whole rock and mineral chemistry elsewhere (Hoshino and Tadesse, in prep).
Geochronology Analytical data of Rb, Sr, Sm, Nd concentrations and isotope ratios are given in Tables 2 and 3. The ThO 2, UO 2, UO2* (the sum of UO 2 and UO 2 equivalent of measured ThO 2 for the zircon) and PbO concentrations, together with apparent ages obtained from individual measurement spots on zircon grains, are also given in Table 4. Isochron diagrams are shown in Figs 3 and 4. All errors quoted in this paper for age and initial isotopic ratios are of 2~ values.
Syn-tectonic granitoids The Azeho syn-tectonic granitoid One sample (GS1957) from this granitoid and mineral separates of the same sample yield an internal SmNd isochron (MSWD=0.11) age of 756+_33 Ma (initial Nd ratio=O.511929 [14]: Fig. 3a). The age calculated from the slope of two whole rock samples of the granitoid (GS1957 and GS1979) is 763 _+24 Ma (initial Nd ratio = 0.511928: Fig. 3b). Since the
Sm-Nd system is considered to be unaffected by low grade metamorphic overprint and the whole rock and internal isochron ages are identical within error, the 756 + 33 Ma age is interpreted as a close estimation for the crystallisation age of the Azeho Granitoid. On the other hand, Rb-Sr isotope data of the samples show a wide scatter on an isochron diagram (not shown), indicating that the Rb-Sr isotope system is not in equilibrium. Zircon grains were unable to be separated from the samples for dating by the CHIME method. The Sm/Nd ratios of the two samples are low, and calculated ~Nd(7~eMa~values are positive and nearly identical (+ 5.31 and + 5.35, respectively: Table 2). The TDM age of the samples are 0.99 and 0.96 Ga, respectively (Table 2). These Nd isotope data strongly suggest an origin of the granitoid from depleted source materials - possibly island-arc tholeiite of Neoproterozoic age or depleted mantle w i t h o u t any significant involvement of older continental crust (cf. Harris et al., 1990).
The Deset Granitoid Nine zircon grains were separated for sample KK1399 collected from this intrusion (Fig. 1 ). The zircon grains are short euhedral prisms ranging in grain size from 60 to 100 pro. All the grains are
Journal of African Earth Sciences 321
T. TADESSE et al. Table 3. Rb and Sr analytical data for whole rocks and minerals Sample
Rb (ppm) Sr (ppm)
~/Rb/~Sr
UZSr/~Sr (2(~)
Azeho Granitoid GS1957 (whole rock) GS1957 (plagioclase) GS1957 (K-feldspar) GS1957 (felsic portion) GS1979 (whole rock)
90 112 112 66 68
175 292 1 22 89 162
1.38 1.03 2.47 2.00 1.13
0.720929 0.718577 0.732220 0.724959 0.71 3880
(10) (14) (11) (11) (11
Deset Granite KK1399 (whole rock) KK1399 (plagioclase) KK1399 (K-feldspar) KK 1399 (biotite) KK1399 (felsic portion)
110 52 144 603 75
530 751 669 87 553
0.56 0.19 0.58 18.66 0.36
0.708576 0.704804 0.709185 0.815594 0.707051
(11) (09) (11 (13) (11)
Chila Granitoid T T 5 7 0 (whole rock) KM1 500 (whole rock) KM1 503 (whole rock) MB916 (whole rock)
55 53 63 48
505 599 808 7901
0.29 0.24 0.21 0.16
0.706573 0.705968 0.705711 0.705231
(09) (09) (09) (12)
Rama granitoid TT900 (whole rock)
76
406
0.50
0.709301 (09)
Sibta Granite TT950 (whole rock)
164
292
1.51
0 . 7 1 7 1 3 9 (10)
TT950 (biotite) TT950 (plagioclase) T T 9 5 0 (K-feldspar)
349 31 152
112 658 281
8.22 O. 13 1.46
0 . 7 6 8 3 2 4 (11 ) 0 . 7 0 4 7 3 8 (09) 0 . 7 1 6 5 0 6 (10)
75 120 71 504
864 942 1033 128
0.23 0.34 0.18 11.48
Shire Granite AG1117 AG1117 AGl117 AGl117
(whole rock) (K-feldspar) (plagioclase) (biotite)
0.705510 0.706809 0.704992 0.795408
(09) (09) (09) (13)
Number in parentheses is the 2~ error in the last t w o digits of the Sr isotope ratio
compositionally homogeneous and inclusion free. The analytical data (Table 4) are plotted on a UO2* versus PbO diagram (Fig. 4a). The data points show linear arrangement on the diagram and regressed with an isochron of 757 _+30 Ma (MSWD =0.35). Since the regression line passes through the origin, secondary Pb loss or gain in the system can largely be ruled out (see Suzuki and Adachi, 1991). The 7 5 7 + 3 0 Ma age is interpreted here as the best estimate of the age of crystallisation of the Deset syn-tectonic granitoid. Minerals were also separated from the same sample and analysed for Rb-Sr sys-tematics (Table 3). The data yield an internal error-chron age of 6 3 7 + 3 3 Ma (MSWD=6.6: not shown), much younger than the CHIME-zircon age. This age probably reflects a post-crystallisation thermal event or has no geological meaning. The Sm/Nd ratio for
322 Journal of African Earth Sciences
the sample is low, the sNd c7~7Mal value is positive ( + 3 . 5 ) with calculated TDM=O.92 Ga. This is not significantly different from the Azeho Granitoid and may suggest either a genetic link between both intrusions or similar source materials. The Chila Granitoid The Rb-Sr isotope data for four whole rock samples are well-aligned and regressed with an isochron (MSWD=O.5) of 7 2 6 + 3 0 Ma (initial Sr ratio= 0.703528 [50]: Fig. 3c). Three zircon grains were separated from sample TT570 and their CHIMEzircon age analysed. The zircon grains vary in size from 50 to 80 pro, free of inclusions and transparent with well-developed crystal faces. No internal zoning is visible under the microscope. Fourteen spots on three grains yielded a CHIME isochron age of 806 +
Sm-Nd, Rb-Sr and Th-U-Pb zircon ages of syn- and post-tectonic granitoids, Axum area, Ethiopia
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Journal of African Earth Sciences 323
T. TADESSE et al.
0.5127
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Figure 3. Sm-Nd and Rb-Sr whole rock and internal isochron diagrams o f the granitoids. Uncertainties for the initial ratios o f all the isochrons are s h o w n as the numbers in brackets for the last two digits. Data from Tables 2 and 3.
21 Ma ( M S W D = 0 . 0 2 : Fig. 4b). This age is significantly older than the whole rock Rb-Sr isochron age (726 + 30 Ma), but is indistinguishable, within error, from the Ar-Ar (hornblende) plateau age of 792 + 7 Ma reported for the same granitoid by Alemu (1997). This CHIME-zircon age is interpreted as a good estimation of the crystallisation age of the Chila Granitoid. The Rb-Sr whole rock age is probably reflecting isotopic resetting during either metamorphic or hydrothermal overprint, Kr6ner etal. (1991 ) reported a widespread Rb-Sr isotopic re-equilibration around 720 Ma from northern parts of the Nubian Shield. It is also worth noting that Alemu (1997) has
324 Journal of African Earth Sciences
reported an Ar-Ar (biotite) age of 725 + 4 Ma from the sample which yielded the Ar-Ar hornblende plateau age of 792 + 7 Ma, and ascribed this discor-dance to the difference in Ar-Ar blocking temperature for biotite and hornblende. The samples from the Chila Granitoid have low Sm/ Nd ratios of 0.06-0.11, and have a restricted range of positive gNd (ao6Ma) values between + 3.92 and + 4.26 (Table 2). The TDMages for the samples range from 1.01 to 1.25. The Sm-Nd data, together with the low initial Sr ratio (0.7035), indicate that the source material of the Chila Granitoid is also primitive perhaps juvenile island-arc crust or mantle source, or
Sm-Nd, Rb-Sr and Th-U-Pb zircon ages of syn- and post-tectonic granitoids, Axum area, Ethiopia
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0.02
0.0l 0.00 0.0
age has a large error owing to the limited spread in Sm/Nd isotope ratio, it is identical, within error, to the CHIME-zircon age of the nearby Chila Granitoid (806_+21 Ma). This age is interpreted as a close estimation of the crystallisation age of the Rama Granitoid. The Rb-Sr data of these samples do not align in the isochron diagram, again indicating a postcrystallisation disturbance of the Rb-Sr isotope system. Zircon grains separated from this sample are very fine-grained (< 40/Jm) in size. The grains are short prismatic, yellow and contain inclusions. The UO2* versus PbO plots of data from 18 spots on five grains are aligned linearly and yield a CHIMEzircon age of 7 4 0 + 4 2 Ma (Fig. 4c). This age is much younger than the Sm-Nd internal isochron age, but identical, within error, to the Ar-Ar biotite age of 745 + 5 Ma (Alemu, 1997) obtained for the same granitoid. The CHIME-zircon age is also identical, within error, to the whole rock Rb-Sr age of Chila Granitoid (726 + 30 Ma, see above). The linear array of data, coupled with the regression line passing through the origin on the diagram, indicates that the grains are isotopically homogeneous and mark some geological event. A possible explanation for this is that the zircon grains are of metamorphic origin or associated with hydrothermal alteration, and hence the CHIME-zircon age is the date of such a late event. The sample has a low Sm/Nd ratio, a positive sNd (793Ma)value ( + 2.92) and hasa TDM age of 1.26 Ga, (Table 2). These values are not significantly different from that of the Chila Granitoid and may suggest common source materials.
[_JAnalyticaluncertainty 0.1
0.2
0.3
0.4
UO *(~%) Figure 4. Plots of U02* versus PbO showing CHIME-zircon isochron diagrams for the respective intrusive bodies. Data from Table 4.
a combination of both, as emphasised by the occurrence of mafic (gabbroic and dioritic) bodies and mafic enclaves associated with the pluton.
The Rama Granitoid The Sm-Nd isotope data for whole rock (TTgO0) and mineral separates of the felsic (mainly plagioclase) and mafic (biotite + hornblende) components yield a three point internal isochron (MSWD = O. 26) age of 7 9 3 + 51 Ma (Fig. 3d). Although the isochron
The Sibta and Shire Granites One whole rock sample (TT950), together with Kfeldspar, plagioclase and biotite separates from the Sibta Granite, yield a Rb-Sr internal errorchron (MSWD =6.12) age of 5 5 0 + 3 5 Ma (Fig. 3e). The internal Rb-Sr errorchron (MSWD=3.6) age of the Shire Granite is 559 + 22 Ma. (Fig. 3f). Since these granites are neither deformed nor affected by metamorphism, it is assumed that the intrusives have not been subjected to any excessive post-emplacement isotopic disturbance. Therefore, the ~550 Ma age is interpreted as a close estimation for the crystallisation age of post-tectonic granites in the area. These ages are identical, within error, to the CHIME-zircon age of the Mereb post-tectonic granite ( 5 4 5 + 2 4 Ma: Tadesse eta/., 1997) and the U-Pb zircon age of 541 _+ 14 Ma of the Mao post-tectonic granite in western Ethiopia (Ayalew eta/., 1990). These post-tectonic granites have also low Sm/Nd and Rb/Sr ratios. The gNdt55oMa~values are positive ( + 4.91 and + 3.50 for the Sibta and Shire Granites,
Journal of African Earth Sciences 325
T. TADESSE et al.
respectively: Table 2). The calculated TOMage is 0.91 Ga for the Sibta Granite and 0.89 Ga for the Shire Granite. The initial Sr isotopic ratio (at 550 Ma) for the Sibta Granite is 0.7052 and 0.7036 for the Shire Granite (not shown). These and the Sm-Nd isotopic features are consistent with magma generation from an intracrustal source with a short residence history.
DISCUSSION AND CONCLUSIONS The preliminary geochemical data indicate that the granitoids in the northern Ethiopian low grade terrane have a chemistry similar to subduction-related volcanic arc granitoids. The Sm-Nd and Rb-Sr isochron and the Th-U-total Pb zircon (CHIME-zircon) isochron ages for the representative granitods of the area revealed three distinct magmatic events: at - 8 0 0 , - 750 and - 5 5 0 Ma. These ages are similar to the events recorded for arc magmatism, accretion and late-stage intra-crustal melting in other parts of the ANS (e.g. Kr6ner e t a L , 1991 ; Abdelsalam and Stern, 1996). More specifically, the oldest age reported here is identical, within error, to the single zircon Pb-Pb evaporation age of 811 Ma of a syntectonic granitoid in the Nakfa Terrane of Eritrea (Teklay, 1997) and a U-Pb zircon age of 820 Ma for a syn-tectonic granitoid from western Ethiopia (Ayalew et al., 1990). Since the granitoids are intrusions into the volcanosedimentary arc assemblage and postdate at least one phase of deformation that affects the host rock, the oldest age reported here is a minimum age of arc magma-tisim in the region. An older age of - 8 5 0 Ma (single zircon Pb-Pb evaporation method) has been reported from felsic metavolcanics of the Nakfa Terrane (Teklay, 1997). The ages of the post-tectonic granites in the Axum area ( - 5 5 0 Ma) are also identical to those reported from western Ethiopia. This uniformity in radiometric ages from western to northern Ethiopia, and further north to Eritrea, argue for virtually con-temporaneous arc magmatism and crustal growth in this part of the ANS, and therefore questions the validity of previous conclusions on Ethiopian base-ment stratigraphy (Kazmin etal., 1978). Despite the area studied being situated at the southernmost part of the ANS low grade zone, from which it passes southwards to high grade, partly reworked Mozam-bique Belt rocks, the low initial Sr isotopic ratio and primitive Nd isotopic characteristics, t o g e t h e r w i t h Neoproterozoic depleted mantle ages of the intru-sives, indicate an insignificant contribution of any older, pre-Pan-African crust in the generation of the granitic magmas (cf. Harris et al., 1990). Considering the age and isotopic differences of the granitoids in terms of the local geology, it is interesting
3 2 6 Journal o f ,4 frican Earth Sciences
to note that the ages of the syn-tectonic granitoids are geographically separate: the older (-800 Ma) are intrusions into the structurally north-west facing succession of Chila Block metasediments situated east of the central steep zone (Fig. 1). They are perhaps related to the evolved, calc-alkaline islandarc volcanic rocks of the Adwa Block (Tadesse, et al. 1998b). On the other hand, those granitoids with - 7 5 0 Ma ages are intrusions into the structurally s o u t h e a s t facing, i m m a t u r e island-arc/suprasubduction zone succession of the Adi Hageray and Adi Nebrid tectonostratigraphical blocks (Tadesse et al., 1998b; Fig. 1 ). Granitoids from both sides of the CSZ have positive sNd m values, but the former have lower 8Nd(T) values and older TDMmodel ages than the latter (Table 2). Tadesse etaL (1998b) suggested, on the basis of volcanic geo-chemistry and field relationships, that the structurally southeast facing succession west of the CSZ must have formed over a consumed back-arc basin, while the northwest facing succession east of the CSZ represents a mature island-arc assemblage. The ages and isotopic data presented above, therefore, are consistent with the field and geochemical variations and suggest that the CSZ is in effect a fundamental structure in the area that marks the boundary between a younger, immature island-arc/supra-subduction zone succession in the west and a relatively older, calc-alkaline island-arc succession in the east. Therefore, the CSZ appears to have been active during the accretion of these contrasting tectono-stratigraphical blocks. Northwards, in Eritrea, the same structure extends as an axial zone for the full north-south length of the Nakfa Terrane (Drury, 1998 pets. comm.). There too, it is associated with a high strain zone, steep structure and incorporates deformed granitic and amphibolite grade metamorphic rocks (Drury, 1998 pers. comm., Drury and de Souza Filho, 1998). Unfortunately, suitable samples have not been obtained for dating the sheet-like granitoids occurring along the steep zone that might have constrained the movement in the zone. However, the movement along this CSZ predates the 550 Ma post-tectonic granite that cuts it and post-dates the 750 Ma syn-tectonic granitoids. It is possible that isotope resetting in some of the granitoids, and some zircon ages around 730-740 Ma, might be due to arc accretion in the region.
ACKNOWLEDGEMENTS The authors thank Prof. Y. Sawada for his guidance and advice during the XRF work. They are also thankful to Ms Akasaka for helping with the Rb-Sr and Sm-Nd extraction and Mr T. Nagaoka for his technical assistance. Thoughtful review of the manuscript by Drs
Sm-Nd, Rb-Sr and Th-U-Pb zircon ages of syn- and post-tectonic granitoids, Axum area, Ethiopia
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