magmatic evolution of the suqii-wagga garnet-bearing two ... · magmatic evolution of the...

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Journal of African Earth Sciences, Vol. 32, No. 2, pp. 193-221, 2001 © 2001 Elsev=er Sc=ence Ltd

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Magmatic evolution of the Suqii-Wagga garnet-bearing two- mica granite, Wallagga area, western Ethiopia

T. KEBEDE 1, C. KOEBERL 1'* and F. KOLLER 2 ~lnstitute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria

2Institute of Petrology, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria

ABSTRACT--The Suqii-Wagga two-mica granite, situated in the western Ethiopian Precambrian, is emplaced in a high-grade migmatitic terrane. It is composed of feldspars + quartz + muscovite + biotite ± garnet + zircon ± allanite ± apatite + Fe-Ti oxides + Fe sulphide. Textural studies and microprobe analyses revealed two generations of almandine-spessartine-rich magmatic garnet. The first is euhedral, fine-grained (300-350 pm), commonly occurs as inclusions in plagioclase and alkali feldspars, and exhibits chemical zoning with almandine-rich cores and spessartine-rich rims. In contrast, the second variety is medium- to coarse-grained (1-7 mm) and shows reverse zoning with spessartine-rich cores and almandine-rich rims. Primary and secondary muscovites were discriminated based on the concentrations of Ti, Fe, Mn and Na. Biotite is characterised by a higher alumina saturation index than biotites of other granitoids in the area, suggesting considerable alumina concentration in the source magma. Garnet-biotite thermometry and phengite barometry were used to estimate the P-T conditions of crystallisation for the Suqii-Wagga two-mica granite pluton at - 7 kbar and - 670°C. Mineral paragenesis, the composition of aluminous minerals and the P-T conditions of crystallisation indicate that samples containing fine-grained garnet crystallised earlier than those containing medium- to coarse-grained garnet. Field and petrographic investigations, mineral chemistry, and whole rock major and trace element studies suggest that the Suqii-Wagga two-mica granite has the characteristics of anatectic granite. Highly variable normative Ab/Or ratios suggested melting under varying a,2 o conditions and/or source characteristics. The relatively high Rb/Sr, Rb/Ba and low CaO/Na20 ( < 0.3) ratios indicate the derivation of the granitic magma from a plagioclase-poor politic source. Moreover, pronounced negative Eu anomalies and large ion lithophile element modelling suggested crystal fractionation involving plagioclase. The presence of the Suqii-Wagga Granite Pluton implies a significant contribution of older mature crustal material to the magmatic evolution of the area. © 2001 Elsevier Science Limited. All rights reserved.

R¢SUME~--Le granite & deux micas de Suqii-Wagga, situ~ dans le socle prdcambrien de I'Ouest de I'Ethiopie, se met en place darts un encaissant mdtamorphique de haut degrd et migmatitique. II est composd de feldspaths + quartz + muscovite + biotite + grenat + zircon _ allanite ± apatite + oxydes Fe- Ti + sulfures de Fe. Deux g6ndrations de grenat magmatique riches en almandin-spessartine sont mis en dvidence sur la base d'arguments texturaux et d'analyses chimiques par microsonde. Les premiers, automorphes et finement grenus (300-350 pm), apparaissent g6n6ralement en inclusions dans les plagioclases et les feldspaths potassiques et montrent une zonation chimique avec des coeurs et des bordures respectivement riches en almandin et en spessartine. Au contraire, la seconde gdn6ration de grenat est de granulom6trie moyenne & grossi~re (1-7 mm) et poss~de une zonation inverse, ceeurs riches en spessartine et bordures riches en almandin. Deux types de muscovite sont discriminds sur leurs teneurs en Ti, Fe, Mn et Na. La biotite est caract6risde par un indice de saturation en alumine supdrieur & celui des biotites des autres granitdides du secteur, ce qui traduit le caract~re alumineux tr~s 61evd du magma parent. Les gE~-thermom~es (biotite-grenat) et-barom~tres (phengites) ont ~d utilis6s pour esfimer les conditions P-T de cristallisation du granite & deux micas de Suqii-Wagga ~ - 7 kbar et -670°C. La paragen6se mindralogique, la composition des phases mindrales alumineuses et les conditions P-Tde cristallisation

*Corresponding author [email protected]

Journal of African Earth Sciences 193

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indiquent que les dchantillons contenant les grenats de petite taille ont cristallis6 avant ceux qui renferment les gros grenats. Les observations pdtrographiques et de terrain, la chimie des min(}raux, et la composition en dldments majeurs et en traces des roches montrent que le granite & deux micas de Suqii-Wagga poss~le les caractdres d'un granite d'anatexie. Des variations importantes du rapport Ab/Or normatives sugg~rent des variations des conditions de all2 o pendant la fusion et/ou des caractdristiques des matdriaux sources. Des rapports Rb/Sr relativement dlevds et CaO/Na20 faibles ( < 0.3) supposent que le magma granitique est issu d'une source pdlitique pauvre en plagioclase. D'autre part, un processus de cristallisation fractionnde impliquant le plagioclase est indiqu6e par une anomalie ndgative marqude en Eu et par une moddlisation bas6e sur les dl6ments lithophiles & fort rayon ionique. L'existence du pluton granitique de Suqii-Wagga implique la forte contribution d'une croOte mature ancienne dans la magmatisme de cette rdgion. © 2001 Elsevier Science Limited. All rights reserved.

(Received 22/12/99: accepted 3/5/00)

I N T R O D U C T I O N

The Suqii-Wagga two-mica granite is located 20-35 km southeast of the town of Gimbi and underlies a characteristically convex topography. The rock unit is compact and massive (without any trace of mineral alignment), medium- to coarse-grained and hypidio- morphic inequigranular. Generally the Suqii-Wagga two-mica granite is leucocratic, containing subordin- ate mafic minerals, and pegmatitic and aplitic dykes cut it at the top. The pegmatite is mineralogically simple and devoid of any mineralogical zoning, whereas the aplite is mainly composed of feldspars and quartz with rare micas. The Suqii-Wagga two-mica granite is emplaced parallel to the meridional to sub-meridional structural trends in the high-grade migmatitic gneiss terrane, which includes biotite gneiss, hornblende- biotite gneiss, amphibolite and granitic gneiss.

Field relationships and detailed petrographic investigations, geochemical characteristics, petrogenesis, types of source magmas and emplacement age of the Suqii-Wagga two-mica granite have not yet been studied. As this granite is emplaced into the high-grade gneissic terrane, which was interpreted by Kazmin eta / . (1979) (based on structural data) as pre-Pan-African basement, the results of this study, together with other studies of western Ethiopian Precarnbrian rocks (e.g. Ayalew and Peccerillo, 1998; Kebede et al . , 1999), can help to establish the relationship between the granitoid rocks emplaced in the low- and high-grade terranes.

Thus, in this paper the authors report new data on petrography, mineral chemistry, bulk major and trace element compositions of the Suqii-Wagga two-mica granite, and the gneissic country rock samples. They use the textural relationships and chemical composition of the aluminous minerals to constrain the origin and evolution of this granite body. Tectonic setting, crystallisation sequences, pressure and temperature of crystallisation, subsolidus textural and compositional modifications, and the origin of the source magma are discussed.

G E O L O G I C A L SETT ING

The western Ethiopian Precambrian rocks constitute high-grade gneissic terrane and low-grade volcano- sedimentary sequences (Fig. 1 ). Shear zones mark the lithological boundary between these two distinct terranes (Abraham, 1989). The Homa granite, as it intrudes between the contrasting low- and high-grade terranes (Fig. 2), has recorded the e f fects of shear deformation (Kebede et aL , 2001). Plutonic rocks, ranging in composition from gabbro to granite, characteristically intruded the low-grade rock assemblage. The low-grade meta-volcano-sedimentary rocks contain isolated ultrarnafic bodies, which may be remnants of an ophiolitic sequence (e.g. Berhe, 1990). The low-grade terrane extends south into Gore-Gambella, an area where it is known as the Birbir

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194 Journal of African Earth Sciences

Magmatic evo/ution of the Suqi/-Wagga garnet-bearing two-mica granite, Wallagga area, western Ethiopia

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Domain (Moore et al., 1986; Ayalew et al., 1990; Ayalew and Peccerillo, 1998). Ultimately, it is truncated by the northwest-southeast running Surma mega-shear zone (Davidson, 1983) in the south- western verge of Ethiopia. The northern extension of this low-grade belt is also exposed in northern Ethiopia (Tadesse, 1996) and in Eritrea and Sudan (Drury and Berhe, 1993; Drury and de Souza Filho, 1998). The high-grade rocks are affected by emplacement of pre- to syn-kinematlc plutonic rocks of dioritic-granitic composition and are generally exposed ~n river cuts that are otherwise mostly covered by Tertiary volcanic rocks. The Suqii-Wagga two-m~ca granite is one of the granitic plutons in the migmatitic gneiss (Fig. 3a, b) terrane.

PETROLOGY AND GEOCHEMISTRY

Analytical methods Bulk major, minor and trace element analyses, and petrographic investigations were conducted on 11 representative samples of Suqd-Wagga two-mica granite and four samples of migmatised gneissic country rocks. Microprobe mineral analyses were done on three selected granite samples.

Whole-rock major and minor oxides and trace elements, including Rb, Sr, Y, Zr, Nb, Co, Ni, Cu, Zn, V, Cr and Ba abundances, were analysed at the University of the Witwatersrand, Johannesburg, South Africa, using X-ray fluorescence (XRF) spec- trometry. The major- and minor-elements were done on fused glass disks and the trace elements on pressed pellets of powders. The precision and accuracy of the XRF analyses are detaded in Reimold et aL (1994). The rare earth elements (REE) and other trace elements were analysed by instrumental neutron activation analysis (INAA) at the Institute of Geochemistry, University of Vienna. For INAA, several international rock standards, such as AC-E, Allende, and G-2 (cf. Govlndaraju, 1989) were used. The samples were irradiated for eight hours at a neutron flux of 2 x 1012 n cm 2 s 1at the 250 kW TRIGA Mark-II reactor of the Atomic Institute of the Austrian Universities, Vienna. Analytical methods, correction procedures (flux, geometric, interference and back- ground), as well as data on precision and accuracy of the INAA method, are described in Koeberl (1993).

Mineral analyses were conducted on a four spectrometer Cameca SXlO0 microprobe at the Institute of Petrology, University of Vienna. The accelerating voltage was 15 kV, with a beam current and beam diameter of 20 nA and 1 pm, respectively. Well-characterised natural mineral standards were used to calculate the concentrations of the unknowns. Atomic number, absorption and fluorescence

corrections, and data reduction was conducted following the PAP procedure (Pouchou and Pichoir, 1991).

Petrography and mineral chemistry The Suqii-Wagga two-mica granite is mainly composed of plagioclase (PI), K-feldspar (Kfs) and quartz (Qtz) with minor muscovite (Ms), biotite (Bt), + garnet (Grt), zircon (Zrn), +allanite (AIn), _+apatite (Ap), +topaz (Toz), _+ cassiterite (Cst), + fluorite, + Fe-Ti oxides and Fe sulphide. Quartz occurs in various forms, such as large anhedral grains and graphic and myrmekitic Jntergrowth with PI and Kfs, respectively. Other important textural features are shown in Fig. 4a-f. The mineral abbreviations are those given by Kretz (1983). Numbers of O atoms considered in mineral formula recalculations are as recommended by Deer et al. (1992).

Feldspars Representative analyses of feldspars and recalculated formulae are given in Table 1. The plagioclase composition ranges from Ab926An7Or04 to Ab85 4An13 3 Or12' Partial replacement of PI with microcline (Mc) was rarely encountered. The rock generally appears to be undeformed; however, bent albite twin lamellae, suggestwe of mechanical deformation, were observed. Alkali-feldspar compositionally ranges from Or98 9Abl 1 to Ors~ 5Abe85 and Ab947An48Oro s to Ab989Ano 3Oro 9 Alkali-feldspars occur as large grains of orthoclase (Or), crosshatched Mc, perthite and as intergranular grains. Their occurrence as interstitial grains suggests a late stage crystallisation from residual melt. The rare occurrence of graphic rater- growths further indicates simultaneous crystallisation of Kfs and Qtz.

Muscovi te Muscovite from the Suqii-Wagga two-mica granite is characterised by two texturally and chemically distinct varieties. Bivariate plots of Fe, Mn and Na versus Ti, and a Na-Ti-(Fe + Mg) ternary plot help to discriminate the high- and Iow-Ti Ms end members (Fig. 5a-d). A textural study revealed that the high-Ti Ms occurs as well-developed large tabular crystals with irregular termination often showing mica twinning. These large Ms grafns are considered to be a primary magmatic crystallisation product from peraluminous magma. Studies by Miller eta/. (1981 ) and Speer (1984) also confirmed that primary magmatic Ms contains higher Ti content than Ms of secondary origin. Zen (1988) stressed the usefulness of Ti enrichment in Ms as an indicator of magmatic origin, because it is not easily affected by subsolidus reaction, as is Mg, Fe, Na and K. Pnmary Ms (PMs), besides its higher TiO 2 ( > 0.4


Magmatic evolution of the Suqii-Wagga garnet-bearing two-mica granite, Wallagga area, western Ethiopia

Figure 3. Field appearance o f the high-grade migmatlt ic gneiss country rock. (a) Both the interlayered leucosomes and melanosomes are folded. Such structures are common m the high-grade gneiss. (b) Pytlgmatic folding was found to occur in biotite gneiss close to the Suqii-Wagga two-mica granite. The granitic leucosomes also show pinch and swel l structures.

wt%) abundance, is characterised by rather homogeneous FeO (4-6 wt%), restricted MnO (< 0.2 wt%) and variable Na20 (0.05-0.46 wt%) contents. The PMs often show interlayering with stringers of Fe oxide (Fig. 4a). These opaque inclusions generally run parallel to the cleavage and also along the margins of Ms. Early crystallising Ms can have a high Fe content, which upon cooling exsolves as Fe oxides (Deer et al., 1962). Ignoring the effect of Li and V, which were not analysed, the AI w deficiency and the types of cations that filled the octahedral site vacancies (Fig. 5e-f) also differentiate the PMs and SMs.

On the other hand, the Iow-Ti secondary Ms (SMs) normally occurs dendritically intergrown with Qtz as a replacement product along the cracks and resorbed rims of Grt crystals (Fig. 4b, c) and as parallel intergrowth with Bt. The SMs is generally fine-grained and is a product of subsolidus reactions of the primary minerals and circulating fluids. Phillips et al. (1972) suggested a retrogressive reaction involving Kfs and a H20-rich fluid [3KAISi308+H20=KAI2(AISi3) O10(OH) 2 + 6SiO 2 + K20] to form Ms-Qtz symplectite. The overall composition of the SMs seems to be in- fluenced by the compositions of the parent minerals


T. KEBEDE et al.

Figure 4. Back-scattered secondary electron images, showing textural relationships of the constituent minerals. (a) Large primary muscowte (PMs) (hght grey) containing stringers of Fe oxtdes (white). It also reveals marginal resorption and recrystalhsatlon of secondary muscowte (SMs) and quartz (dark grey) symplectite (left centre). (b) Medium- to coarse*grained (MCG) garnet (Grt) crystal (white), containing inclusions of Ms, K-feldspar (Kfs), quartz (Qtz), chlorite (Chl) and corundum. The resorbed margtns and fractures are generally lined with SMs. Note that the grain boundary with the adjacent Kfs (light grey, left top) has no reaction rim.

and interacting fluids or the associated minerals, such as Grt and Bt, which contain considerable amounts of Fe and Mn. The SMs associated with Grt and Bt generally have relatively higher Fe and Mn than those coexisting with feldspars and Qtz. A systematic variation

in concentrations of Fe and Mn occurred in SMs that coexists with Grt. Contents of these elements in SMs increase towards its grain boundary with Grt. Se- condary Ms, containing relatively higher Mg and Ti than the other SMs, was observed mantling chlorite

198 Journal o f A frlcan Earth Sciences


Figure 4. continued. Back-scattered secondary electron images, showing textural relationships o f the consti tuent minerals. (c) Close up view o f marginal breakdown o f Grt shown above in (b) into SMs (centre). The SMs contains streaks o f remnant Grt and is dendritically intergrown with Qtz. The dark grey mineral (left bottom) is pyrophyll ite. (d) Idlomorphic fine-grained (FG) Grt (centre, white) embedded in plagioclase (PI).

grains. Both the Chl and Ms appear to be alteration products after biotite. Despite these compositional variations, the SMs is always distinct from the PMs.

Biotite Biotite generally occurs in subhedral to anhedral forms and is often altered to Chl, Ms, Ep and opaques

along cleavage traces and margins. Rare inter- growths of Mc and Bt were found in sample TK049. Relatively high concentrat ions of AI203 (18- 21 wt%), FeO (23-26 wt%) and TiO 2 (2-3 wt%, with a few exceptions) characterise Bt grains of the Suqii-Wagga two-mica granite. The biotite is classified as annite with a significant proportion of


T. KEBEDE et al.

Figure 4. continued. Back-scattered secondary electron images, showing textural relationships of the constituent minerals. (e) Partially altered garnet (centre) in Kfs (light grey). The alteration products include SMs, Chl and pyrophyllite. (f) A l l the aluminous minerals, Ms (light grey), Bt (white elongated mineral from lower right to centre) and FG Grt (centre) coexist with each other. The dark grey mineral is Qtz.

siderophyll ite. Biotite coexist ing with Grt has significantly lower TiO 2 (Table 2; analysis #090). The Ti-poor analyses contain high AI w with a value close to 2 and generally deviate from other Bt analyses of the unit. This Bt is presumed to be an alteration product of Grt, which normally has low concentrations of Ti and is highly aluminous. The

Fe/(Fe + Mg) ratios are generally uniform and range from 0.80 to 0.83 (Table 2; analysis #709 with relatively higher MgO is an exception). The alumina saturation index (ASI) of the Bt in the Suqii-Wagga two-mica granite is significantly higher than that from samples of other granitoids and associated rocks in the region. Such high values of ASI in Bt



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coexisting with Ms and other peraluminous minerals reflect increased alumina act iv i ty in the magma (Zen, 1988).

Garnet Garnet crystals were analysed from three garnet- bearing samples (TK047, TK049, TK079) of the Suqii- Wagga two-mica granite. Representative microprobe analyses and formulae recalculated on the basis of 24 O are given in Table 3. Ferric Fe was calculated following the procedures outlined by Droop (1987). End member compositions were determined using the method of Deer et aL (1992). Overall the garnets from the three samples form a solid solution of almandine-spessartine, which const i tute 9 3 - 9 6 tool.% of the total. The other end members, such as pyrope, andradite, grossular and very rare uvarovite, together make up the remainder.

Detailed petrographic and microprobe studies revealed that the Grt in the garnet-bearing samples is of two types. For simplicity, these two varieties are referred to as fine-grained (FG) and medium- to coarse- grained (MCG).

The FG Grt ( 300 -350 pm) is of uniform size, generally euhedral (except where affected by resorption and subsolidus reaction), free of inclusions, associated with Ms and Bt and commonly occurs embedded in PI and Kfs (Fig. 4d, e). Plagioclase and Grt are also found together as inclusions in Kfs. Among others, the FG Grt forms a stable relationship with PI, as demonstrated by distinct grain boundaries without any reaction rims (Fig. 4d). On the other hand, microprobe studies showed that FG Grt reacted with Kfs to form aggregates of secondary minerals, such as chlorite, pyrophyllite and secondary muscovite. The FG Grt exhibits characteristic zoning, with a relatively Fe-rich and Mn-poor core and a relatively Mn-rich and Fe-poor rim (Table 3; Fig. 6a).

The MCG type (1-7 mm), on the other hand, is subhedral (as it often shows resorbed margins), contains inclusions of Ms, Kfs, Qtz, pyrophyllite, carbonate and Fe-Ti oxides along margins, and micro- fractures. Biotite is not observed to coexist with this type of garnet. The MCG variety also shows zoning but in the opposite direction of the fine-grained type, i.e. with Mn-rich cores and Fe-rich rims (Fig. 6b, c). Leake (1967) recognised garnet zoning similar to the latter in aplites within the Galway Granite, western Ireland. He explained the zoning as a result of Rayleigh fractionation process during crystal growth.

The absence of inclusions and perfect euhedral crystals in the FG variety, according to Zen (1988), suggests equilibrium with melt during crystallisation. The FG and MCG varieties contain - 3 8 - 4 3 mol.% and - 3 9 - 4 8 mol. % spessartine, respectively (Table

3). Magmatic Grt with characteristic almandine- spessartine solid solutions has been reported from highly peraluminous granites elsewhere (e.g. Clarke, 1981; Miller and Stoddard, 1981; Hogan, 1996). Similarly, both types of garnets of the Suqii-Wagga Granite with almandine-spessartine solid solutions represent magmatic phases. Miller and Stoddard (1981 ) explained the enrichment of Mn in garnets of strongly peraluminous magma by the absence of ferromagnesian minerals such as Hbl, which have a higher affinity for Mn than Bt.

Accessories Clusters of zircon crystals (8-11 grains) were found to occur typically at mineral grain boundaries and as inclusions in other minerals. Electron microprobe analyses and back-scattered electron images have shown that some zircon grains contain inclusions of Chl and Ms. These types of zircons may represent xenocrysts, which encompassed the other minerals, presumably during metamorphic crystallisation and growth. Allanite is generally replaced by a dark brown metamict phase and is also marginally replaced by carbonate. Fluorite often occurs lining the grain boundaries and micro-fractures, particularly in sample TK078.

Whole rock chemistry Major, minor and trace elements Major oxides, trace elements and normative mineral compositions of the Suqii-Wagga two-mica granite are given in Tables 4 and 5 and Fig. 7a-k. Restricted ranges of SiO 2 concentrations (74 .4-76 .2 wt%) characterise the garnet-bearing Suqii-Wagga two-mica granite. The abundance of Fe203, CaO, Sr and Ba tend to decrease with increasing SiO 2, whereas TiO 2, AI203 and MgO show less variation. Na20 and K20 have an inverse relationship and show large variations (Fig. 8a, b). Nevertheless, the Suqii-Wagga two-mica granite pluton, unlike other granitoids in the region (Kebede et aL, 1999; Kebede et aL, 2001), has a rather homogeneous major element composition. Most of the samples, with the exception of TK048, TK089 and TKO93a, are corundum-normative. In addition, all samples that contain Grt have relatively high MnO (0.03-0.04 wt%) (Fig. 7d).

Compared with the other granitoid bodies in the region (Kebede et aL, 1999; Kebede et aL, 2001 ), the Suqii-Wagga Granite is characterised by rela- t ively low K/Rb; high Rb/Sr, Rb/Zr, Y/Zr and Nb/Zr; and variable Rb/Ba, Ba/Sr, Ce/Nb, Tb/Ta and Y/Nb ratios (see also Table 5). The relatively low concentrations of high field strength elements, including Zr, suggest the presence of zircon and other minor phases in the precursor rocks (residual) that depleted the


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elements from the source magma, or the precursor rock may have originally been depleted in Zr. However, the narrow range in the abundance of Zr suggests that zircon was a crystallising phase (Hanson, 1989), as shown by the presence of magmatic zircons in the modal composition.

Rare earth elements Chondrite-normalised REE plots of the Suqii-Wagga two-mica granite and the high-grade gneiss country rock are shown in Fig. 9a-c. Most of the Suqii-Wagga two-mica granite samples (except TK070) show a moderate to strong negative Eu anomaly, suggesting fractionation of feldspars. Cullers and Graf (1984) suggested that melting of igneous source rocks with negative Eu anomalies may produce a melt with a strong negative anomaly. Thus, the strong Eu negative anomaly could be the combined effect of feldspar fractionation and source characteristics. The samples have vanable LREE enrichments, LREE/HREE ratios, and absolute REE concentrations (Fig. 9a-c). The

garnet-bearing samples have flat REE patterns with slightly enriched HREE abundances (Fig. 9a). Garnet has variable chondrite-normalised REE patterns depending on the composition of host rocks and/or grade of metamorphism; however, Grt always has higher HREE than LREE contents (e.g. Henderson, 1982; Taylor and McLennan, 1985; Bea, 1996). Therefore, garnet could be responsible for the relatively higher concentrations of the HREE and variable absolute REE concentrations.

Samples TK089 and TKO93a, with lower absolute REE content, have HREE/LREE ratios near unity. These samples also contain very low amounts of Grt. Sam- ples TK071 a, TK071 b and TKO76b, however, show LREE enrichments, a flat HREE pattern and high LREE/ HREE ratios (LaN/Yb N = 4.79 and 6.83, respectively). Replacement of Bt by Ms+opaques and Kfs by Ms + Qtz is ubiquitous in these samples, suggesting significant fluid interaction. Petrographic studies showed the presence of allanite in the modal composition. This mineral normally concentrates the LREEs



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(Bea, 1996). The LREE enrichment presumably resulted from assimilation of the gneissic country rocks, which generally are characterised by enriched LREEs and depleted HREEs, with the LaN/Yb N ratio ranging from 8 to 24 (Fig. 9c; Table 5). Sample TKO70 has low REE contents, a small positive Eu anomaly (Eu/Eu* = 1.11 ), a relatively high LaN/Yb N ratio (4.72) and a distinct chondrite-normalised REE pattern. Very low REE content, as that observed in

TK070, is common in differentiated S iO j i ch rocks of a particular suite of rocks (e.g. Cullers and Graf, 1984).

The hornblende-biotite gneiss sample (TKO86) of the country rock has a relatively elevated absolute REE content and, in particular, a Ce anomaly indicating some alteration. Marsh (1991) and Mongelli (1 993) suggested that a Ce anomaly is related to the insolubility of CeO 2 (if Ce 4÷ occurs


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instead of Ce 3+) that results in differential enrichment of Ce compared to the other REEs, which are more soluble and leached by circulating ground water. The overall REE patterns of the gneissic country rocks are similar to the patterns of quartz- intermediate greywackes (Taylor and McLennan, 1985). Such a similarity may indicate a sedimentary precursor rock for the migmatit ic gneissic rocks.

D I S C U S S I O N

T e c t o n i c d i s c r i m i n a t i o n

Trace element tectonic discriminations (e.g. Pearce et aL, 1984) for the samples of the Suqii-Wagga two- mica granite showed plots at the boundaries of WPG, syn-COLG and r A G (Fig. 10). Thus, these trace element discriminations do not sufficiently characterise the SuqiioWagga two-mica granite, either



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because the concentration of Rb was signif icantly decreased as a result of subsolidus fluid interaction, which is apparent from textural modification and formation of secondary minerals, or because the HFS elements are enriched (variably in garnet-bearing samples) and leads to scatter in the plots. However, R 1 and R 2 parameters, as defined by Batchelor and Bowden (1985), constrained the Suqii-Wagga Granite as anatectic two-mica leucogranite (Fig. 11 ). This is also supported by the occurrence of aluminous minerals (Ms, Bt and Grt), which are generally considered to be characteristic of peraluminous granites (e.g. Anderson and Rowley, 1981 ; Clemens and Wall, 1981).

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1 0 0 0

.~ 1 O0

g~ lO

! (b)

Suqli-Wagga two-mica granite I--~-- TK(~7~hl (contaminated >)

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

,c) TKO94a

Gneisslc country rocks

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er T m Yb Lu

Figure 9. Chondrite-normalised rare earth element plots. (a) and (b) The Suqii-Wagga two-mica granite and (c) the gneissic country rock samples (normalisation factors are after Taylor and McLennan, 1985).

calibrated by Ferry and Spear (1978), can be used for garnet containing Xca[Ca/(Ca + Fe + Mg + Mn)] + X M , [ M n / ( M n + F e + M g + C a ) ] up to - 0 . 2 and biotite Al~V +T i up to - 0 . 1 5 . As XM, in the Suqii- Wagga two-mica granite is on the order of 0 . 3 8 - 0.48, the Ferry and Spear (1978) thermometer was not suitable. The thermometers of Ganguly and Saxena (1984) and Williams and Grambling (1990) yielded unreasonably low temperatures, probably due to the effect of high XM, on Fe-Mg mixing in garnet. Nevertheless, the thermometric calibration of Indares and Mart ignole (1 985) recorded three


T. KEBEDE et al.

10 g

1 I0

0

1 1 10 100

Yb+Ta (ppm]

Figure 10. Discrimination of the Suqii-Wagga two-mica granite using plot o f Rb versus Yb + Ta (field boundaries are after Pearce e t a l . , 1984). ORG: Ocean ridge granite; Syn-COLG: syn-collislon granite; VAG: volcanic-arc granite; WPG: within- plate gramte.

signif icantly di f ferent temperatures at - 4 4 0 ° C , 500°C and - 670°C. The - 670°C temperature

is interpreted as a reasonable approximation of the crystallisation temperature of the Suqii-Wagga two- mica granite (see also Fig. 12). The other two temperatures ( - 450°C and - 500°C), on the other hand, are considered to represent the cooling path

of the body as indicated in Fig. 12. Relatively low temperatures of - 4 1 0 ° C were estimated from a two-feldspar thermometer (Whitney and Stormer, 1977). This result may also have been affected by subsolidus perthitic exsolution.

Phengite barometry Garnet in the Suqii-Wagga two-mica granite generally occurs in equilibrium either with PI or Ms + Bt, but never with PI + Ms + Bt. Therefore, it was not possible to determine the pressure of crystallisation using barometric calibrations (e.g. Ghent and Stout, 1981 ) that are based on the equilibrium assemblage Grt + Bt + PI + Ms. However, the muscovite in the Suqii-Wagga two-mica granite contains a considerable celadonitic component, which makes it suitable for phengite barometry. Velde (1965, 1967) calibrated the phengite thermobarometer based on the content of celadonite, which raises with increasing pressure. This barometer is best suitable for low P-Tconditions. Later Massonne and Schreyer (1987) formulated a new geobarometric calibration that shows a strong linear increase of Si per formula unit (pfu) with pressure and a moderate decrease of Si with temperature. In this study, the authors used the graphic solution of this calibration as given by Anderson (1996) to estimate the Pot crystallisation of the Suqii-Wagga two-mica granite at - 7 kbar corresponding to the highest T (670°C). Pressures of - 3 -5 kbar were recorded for lower cooling temperature ranges ( - 4 0 0 - 500°C). Pressures were calculated for extensions

2500

] 500

500

' ' ' I . . . . I ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' '

1 Mantle fractionates 2 Pre-plate collision /

3 Post-collision uplift 4 Late-orogenic 5 Anorogenic J \ 1 6 Syn-collision J \ 7 Post-orogenic J \

- - 2

, , , I , , , , I , , , , I , , a ~ I . . . . I , , ,

500 1000 1500 2000 2500 3000

R1

Figure 11. R~ versus R 2 diagram classifwng the Suqii-Wagga two-mica granite as anatectic granite (field boundaries are after 8atchelor and Bowden, 1985). R 7 =4Si- 11(Na+K)-2(Fe+ Ti); R 2 = 6 C a + 2 M g + A I .



o .Q v

400 500 600 700 800 900

Temperature [°C]

40

E

J¢: 20 ~.

Figure 12. P-T condit ions o f mel t generation, crystall isation and cooling path. Water-saturated granite solidus, muscovite and biot i te stabi l i ty curves and their approximate ranges o f dehydration are from Hyndman (1981). The stabi l i ty curve o f Grt is from Clarke (1981). The overlapping stabil i ty fields o f Ms +Bt +Gr t are drawn based on the stabi l i ty f ield o f muscovite as indicated by the horizontally hatched field. The patterned area in the overlapping fields o f the aluminous minerals represents crysta//isation conditions o f the Suqii-Wagga two-mica granite. The broken line with arrow indicates the cooling path. Two geothermal gradients, at 20°C km -r and 30°C km ~, are shown.

below and above crystall isation temperatures to monitor the variability of the barometer. Accordingly, the barometer shows a significant uncertainty of + 1 kbar for a + 74°C error in temperatures. Anderson (1996) estimated that a + 50°C variation, and an analyt ical uncer ta inty of _+0.05 atoms Si pfu, results in an overall error of _+ 2 kbar. Nevertheless, the relatively high pressure ( - 7 kbar) obtained is geological ly reasonable as this granite body is emplaced in a migmatitic high-grade rock. To exclude the effects of subsol idus reactions on the deter- minat ions, only analyses from coarse-grained Ti- rich PMs crystals were used. Ult imately, the P-T condit ions of crystall isation at - 7 kbar and 670°C fall wi th in the overlapping stabi l i ty fields of Ms + Bt + Grt (Fig. 12), indicating that they are reasonably well constrained.

Crystallisation sequence The crystal l isat ion sequence in the Suqii-Wagga two-mica granite was outlined using the AFM phase diagram (Fig. 13) of Miller and Stoddard (1981 ). Muscovi te general ly occurs together wi th other aluminous minerals, specif ically Bt and Grt. There are no cases where Bt and Grt coexisted w i thout muscovi te. The fine-grained Grt wi th Ms and Bt (Fig. 4f) suggests crystal l isat ion at the t r ibutary reaction point (Fig. 13). In contrast, the MCG variety is only found in associat ion wi th Ms, implying crystal l isat ion along the Ms-Grt phase boundary (Fig. 13). Textural relationships and P-Tof crystal- l isation suggested that the FG Grt variety crystallised at relatively higher temperatures than the MCG one, indicating temperature decrease in the order Ms + B t - ,Ms + Bt + FG Grt--~Ms + MCG Grt during


T. KEBEDE et al.

A

o " + " °

y / \ Garnet ~kk

F 50 M

Figure 13. AFM phase diagram (after Miller and Stoddarcl, 1981) relating the crystallisation order o f the aluminous minerals. Shaded areas indicate the respective mineral analyses. The arrows (1 -4 ) indicate possible crystallising phase assemblages. Ar rows on the cotectic lines indicate directions o f decreasing temperatures. A = AI-(2Ca + Na + K); F = Fe + Mg; M = Mn.

crystallisation. Crystallisation sequences involving different parageneses of the aluminous minerals (Ms + Bt, Ms + Bt + Grt, Ms + Grt) from peraluminous granitic magmas were discussed by Abbott (1981 ), Clemens and Wall (1981 ), Miller and Stoddard (1981, 1982), Speer and Becker (1992) and Hogan (1996). The occurrence of FG Grt inclusions in PI, according to Flood and Vernon (1988), suggests that crystallisation of FG Grt ceased while PI continued to crystallise. On the other hand, the spessartine contents of the late-crystallising garnets decrease towards the rim (Hogan, 1996), which is also the case in the MCG Grt of the unit. Medium- to coarse- grained Grt is never found as inclusion in PI or any other mineral. Figure 6d shows a decrease of almandine from the core of the FG Grt in TK047 to the core of the MCG Grt in TK079. Assuming crystal fractionation, in which Mn is enriched in the evolved magma (Hall, 1965; Hsu, 1968; Miller and Stoddard, 1981 ), the FG Grt ceased to grow before the beginning of crystall isation of the MCG Grt, as indicated by a decrease of the abundance of almandine and an increase in the abundance of spessartine from FG to the core of MCG garnets, but with a significant compositional gap between the two (Fig. 6d). Biotite, which crystallised along Bt-Ms cotectics or at the tr ibutary reaction point, may break down as the

melt cools along the Ms-Grt cotectic line (cf. Ehlers, 1972). Breakdown of Bt by reaction with the melt results in Grt +Ms (Cawthorn and Brown, 1976; Miller and Stoddard, 1981), and the absence of other crystallising ferromagnesian mineral phases that take up Fe, can cause reverse zoning in the MCG Grt by increasing the concentration of Fe in the melt as crystallisation proceeds. This is supported by the annite-rich end member composition and extremely rare occurrence of Bt in samples containing MCG Grt, which presumably crystallised along the Ms-Grt cotectic at relatively lower temperatures than the FG garnet.

The mineralogical compositions and textures of the Suqii-Wagga two-mica granite were also significantly affected by subsolidus interactions of the consti tuent minerals with H20-rich fluids, as suggested from the hydrous nature of the secondary minerals. Occurrence of carbonate as an alteration product is also suggestive of CO 2 in the fluid phase. Both Grt varieties are partially or wholly altered to secondary mineral phases, such as pyrophyllite, secondary Ms, carbonate, Fe-Ti oxides and quartz. The textural modification resulting from the intergrowth of quartz and secondary Ms can be ascribed to subsolidus reactions of primary minerals (e.g. feldspars and PMs) with residual H20- rich fluid.



Origin and evolution of the magma The mineralogical composition (particularly muscovite, biotite and garnet), emplacement in a migmatitic terrane and the absence of intermediate and/or basic rock associations favours the origin of the Suqii- Wagga two-mica granite by the partial melting of crustal rocks. In a normative Ab-Qtz-0r ternary diagram (Fig. 14), all samples cluster fairly close to the granite minimum melting region, further suggesting an anatectic derivation of the source magma (cf. Barbarin, 1996). The data show tendencies to migrate towards the Ab-Qtz sideline, indicating a relatively high all2 o melting condition. This is consistent with the study by Barbarin (1996) who showed that two- mica granites are largely produced by 'wet' anatexis of crustal materials in orogenic areas which experi- enced crustal thickening accompanied by shearing and thrusting. Thompson (1996) showed that the Ab and Or contents vary significantly when all2 o changes from 0 to 1 (Fig. 14). The Suqii-Wagga two-mica granite generally has high Na/K ratios (reaching up to 2.32), suggesting also melt generation at high all2 o and P~t,~ (e.g. Huang and Wyllie, 1975; Hall, 1996). According to Philpotts (1990), the portions containing high normative Ab may have crystallised from a magma

generated at the H20-saturated solidus. Further melting at H20-undersaturated conditions progressively enriched the melt with an Or-rich component (Fig. 8a, b). Therefore, similar continuously changing melting conditions (P-T-all2 o) and assimilation of the gneissic country rock could be the contributing factors for the major oxides and trace element compositional variations observed within the unit.

The characteristic pronounced negative Eu anomalies and very low concentrations of Sr in the Suqii-Wagga two-mica granite samples indicate the generation of the magma from a plagioclase-poor precursor rock (cf. Sylvester, 1998). The garnet- bearing samples of the Suqii-Wagga Granite compared to the other samples have higher Rb/Sr, Rb/Ba and Ba/Sr ratios. According to Sylvester (1998), magmas of peraluminous granites with such chemical characteristics are derived by partial melting of clay-rich and plagioclase-poor pelitic sediments. Furthermore, the relatively low K/Rb ratios in the Suqii-Wagga two-mica granite (113-1 97), compared to other granitoids in the region (e.g. Kebede etaL, 2001 ), are suggestive of biotite in the residue of the partial melt.

Qtz

Ab 50 Or

Figure 14. Normative Ab-Qtz-Or ternary plot. The Suqii-Wagga two-mica granite samples cluster at, and close to, the pseudotemary minimum granite melt. Min imum melt ing composit ions and dependence on water act iv i ty are mod i f i ed f rom Thompson (1996) and references therein. Total P increases towards the feldspar sideline, and temperature increases with decreasing Ab/Or ratio (Thompson, 1996). The sol id arrow indicates the direction o f increase o f all2 o in the source magma o f the Suqii-Wagga two- mica granite.


7". KEBEDE et al.

The Suqii-Wagga two-mica granite is characterised by low AI203/TiO2 ( < 60) and CaO/Na20 (<0.3) (Fig. 15). Low CaO/Na20 ratios (<0 .3 ) characterise peraluminous granite magmas derived from clay-rich, plagioclase-poor ( < 5 % ) pelitic rocks (Sylvester, 1998). He also suggested that high temperature (_>875°C) melting induced by basaltic magma under- plating in collisional orogens produces peraluminous granites wi th low AI203/TiO 2 ratios. In strongly peraluminous granites derived from pelitic sediments, the Rb/Ba and Rb/Sr ratios are expected to be high (Sylvester, 1998). However, these ratios in the samples of the Suqii-Wagga two-mica granite are scattered, with very low values for the ratios for some samples (Table 5). The low Rb/Ba and Rb/Sr ratios in samples (TK070, TK071 a, TK071 b, TKO76b) with relatively higher CaO/Na20 (0.26-O.31) are possibly caused by mixing (assimilation?) with the gneissic country rocks, which are characterised by higher concentrations of Sr and Ba, as well as significantly higher CaO/Na20 (0.55-1.98), than the Suqii-Wagga Granite samples. Assuming the samples with low CaO/Na20 ratios were uncontaminated, mixing of 5 - 25% of the gneissic rock samples (depending on their ratios) is required to produce the average CaO/Na20 ratios of 0.27 shown by the contaminated granite samples. In Fig. 16, it is also shown that the contaminated samples, with higher K20 than Na20 (Table 4), appear to be affected by K-feldspathisation. However, the trend of the other samples, as indicated by a pronounced negative Eu anomaly (Fig. 9a), is controlled by fractional crystallisation of plagioclase (Fig. 16).

Crystallisation of the Suqii-Wagga two-mica granite at P-Tconditions of - 7 kbar and - 6 7 0 ° C suggests emplacement at a crustal depth of - 20 -25 km. The country rocks are not extensively migmatised, ruling out an in situ crystallisation of the body. Therefore, it is not unreasonable to infer that the melt was generated in the lower crust, from where it later segregated and was emplaced.

Regional implication The emplacement of the Suqii-Wagga two-mica granite in the migmatised biotite gneiss in the eastern high- grade gneissic block (Fig. 2), its content of aluminous minerals such as Ms, Bt and Grt, and its geochemical characteristics indicate that it is an anatectic granite. Generally, anatectic granites are believed to be associated with crustal thickening as a result of con- t inent-cont inent collision tectonics (e.g. Pitcher, 1987). Hence, the occurrence of the Suqii-Wagga two- mica granite body implies that continent-continent or arc-continent collision processes may have contri- buted to the crust formation in the region. Furthermore, a significant involvement of continent-derived clay- rich p lagioc lase-poor mature sed iments (e.g. Sylvester, 1998) in the source magma is indicated. This observat ion supports the idea that micro- cont inents might have been involved in the accre- t ionary processes that are believed to have formed the Arabian Nubian Shield (ANS) (e.g. Gass, 1977). The gneissic country rocks and underlying precursor rocks are probably the northern extension of the Kenyan Mozambique Mobile Belt (Key et al., 1989;

o

1 0 . C

1 .C

0 . 1

1 0

i i i ~ = = i = i ,

1 0 0

A 1 2 0 3 / T i O 2

1 0 0 0

Figure 15. AI203/TiO 2 versus CaO/Na20 plot of the Suqii-Wagga two-mica granite. The trapezoid represent compositional range of the strongly peraluminous granites (after Sylvester, 1998).


Magmatic evolution of the Suqfi-Wagga garnet-bearing two-mica granite, Wallagga area, western Ethiopia

10000

1000

1 O0

10

Kfs

~ n 8 taminated

. . . . . . . . ~ . . . . = = = l l , = * = , , , ,

0 1 O0 1000 10000

Ba (ppm)

Figure 16. Barium and Sr modelling of the Suqii-Wagga granite. The arrows a and b indicate the effects of K-feldspathisation and fract ional crystal l isatlon o f plagioclase, respectively. The corresponding arrows to plagloclase (PI), K-feldspar (Kfs) and biotite (Bt) show which minerals control the variations (partition co- efficients are from Rollinson, 1993, and references therein).

Mosley, 1993), which is believed to form the high- grade basement rocks in Ethiopia (e.g. Kazmin et al., 1978, 1979). The arc-arc accretion (e.g. Abdelsalam and Stern, 1996) is considered a major crust forming process in the ANS. However, as the ANS narrows southward (Fig. 1), the involvement of the northern extension of the relatively older Mozambique Belt rocks becomes important. The occurrence of in- herited zircons of Mesoproterozoic age in members of the granitoid populations emplaced in the high- grade gne iss ic and the l ow-g rade vo lcano- sedimentary terranes (Kebede et aL, 2000, 2001 ) supports the involvement of older pre-Pan-African crusts in the origin and magmatic history of the Precambrian rocks of western Ethiopia. Accord- ingly, the geochemical features of the Suqii-Wagga two-mica granite may have been largely inf lu- enced by the involvement of the older crustal rocks. Kebede et aL (2000, 2001) report that the Suqii- Wagga two-mica granite was emplaced at - 700 Ma during the arc-cont inent coll ision of east and west Gondwana wi th the ANS at 7 5 0 - 6 5 0 Ma (Abdelsalam and Stern, 1996, and references therein).

CONCLUSIONS

All available data, including field and petrographic relationships, mineral chemistry, and bulk major and trace element composit ions indicate that the Suqii- Wagga two-mica granite is anatectic in origin. The

granitic melt was presumably generated by partial melting of a meta-pelitic source at lower crustal depths ( > 25 km) at - 700 Ma during arc-continent collision, which may have thickened the crust. The negative Eu anomalies, low CaO/Na20 (<0.3), as well as low concentrations of Sr in the Suqii-Wagga Granite indicate the generation of the source magma from a plagioclase-poor metapelit ic rock. The low K/Rb ratios in the unit suggest the presence of biotite in the unmelted residue. Large ion lithophile element (Sr and Ba) modelling and principal minerals (PI, Kfs, Bt) fractionation trends indicate modification of the magma by plagioclase crystal fractionation and possibly K-feldspathisation. The significant variation in Na20 concentration within the Suqii-Wagga two- mica granite is related to varying all2 o during partial melting and presumed compositional variations within the precursor rock. The aluminous minerals recorded P-T condit ions of crystall isation at - 7 kbar and

670°C, which in turn suggests that the Suqii-Wagga two-mica granite was emplaced deep in the migmatitic gneissic terrane. The textures and mineralogical compositions of the granite were modified by interaction with H20- and CO2-rich circulating fluids, as indicated by ubiquitous hydrous and carbonate secondary minerals. Overall, the occurrence of the Suqii-Wagga two-mica granite indicates crustal thickening as a result of arc-continent collision, which is an obser- vation that is consistent with the idea that micro- continents might have had a significant role in the evolution of the ANS.


T. KEBEDE et al.

ACKNOWLEDGEMENTS

T.K. is grateful to T. Alemu for logistic assistance and important discussions in the field. The authors very much appreciate T. Ntaflos (Institute of Petrology, University of Vienna) for access to, and assistance with, the electron microprobe. S. Farrell (University of the Witwatersrand, Johannesburg) is thanked for help with the XRF analyses. The authors are also grateful to Profs A. Peccerillo and A. Mogessie for their helpful reviews of the manuscript. In addition, T.K. would like to acknowledge the (3sterreichischer Akademischer Austauschdienst (0AD) for a Ph.D. scholarship. Laboratory work was supported by the Austrian FWF, grant Y58-GEO (to C.K.). Editorial handling - P. Bo wden

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magmatic evolution of the suqii-wagga garnet-bearing two ... · magmatic evolution of the...

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