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Journal of Petrology 45(6) © Oxford University Press 2004; all rights reserved
Petrology and Isotope Geochemistry of the Pan-African Negash Pluton, Northern Ethiopia: Mafic–Felsic Magma Interactions During the Construction of Shallow-level Calc-alkaline Plutons
1 CRPG–CNRS, 15, RUE NOTRE-DAME DES PAUVRES, BP 20, 54501 VANDOEUVRE-LÈS-NANCY CEDEX, FRANCE
2 DEPARTMENT OF GEOLOGY AND GEOPHYSICS, ADDIS ABABA UNIVERSITY, PO BOX 1176, ADDIS ABABA, ETHIOPIA
3 CNRS–UMR 5563 LMTG, UNIVERSITÉ PAUL SABATIER, 38, RUE DES TRENTE-SIX PONTS, 31400 TOULOUSE, FRANCE
RECEIVED JULY 15, 2002; ACCEPTED NOVEMBER 27, 2003
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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTIONThe Negash pluton consists of monzogranites, granodiorites, hybrid quartz monzodiorites, quartz monzodiorites and pyroxene monzodiorites, emplaced at 608 ± 7 Ma (zircon U–Pb) in low-grade volcaniclastic sediments. Field relationships between mafic and felsic rocks result from mingling and hybridization at the lower interface of a mafic sheet injected into partially crystallized, phenocryst-laden, granodiorite magma (back-veining), and hybridization during simultaneous ascent of mafic and felsic magmas in the feeder zone located to the NW of the pluton. The rock suite displays low 87Sr/86Sr(608) (0·70260–0·70350) and positive
Nd(608) values (+3·9 to +5·9), along with fractionated rare earth element patterns [(La/Yb)N = 9·9–17·7], enrichment in large ion lithophile elements (Ba, U, K, Pb and Sr) and depletion in Nb and Th compared with the primitive mantle. Monzogranites, granodiorites and hybrid quartz monzodiorites define a calc-alkaline differentiation trend, whereas the quartz monzodiorites have higher Fe/Mg ratios. The pyroxene monzodiorites show anomalously high Ti/Zr, Ti/Y and Ti/V ratios, suggesting that they are cumulates. Chemical modelling suggests that pyroxene and quartz monzodiorites could derive from a common gabbrodioritic parent by fractional crystallization. Structural and chemical data suggest that (1) the pluton results from the assembly of several, low-viscosity, melt-rich batches (sheeting/dyking), differentiated in intermediate chambers prior to their emplacement; (2) in situ differentiation is limited to the coarse-grained pyroxene monzodiorites; (3) mafic–felsic magma interactions at the emplacement level were essentially limited to mingling. KEY WORDS: mafic–felsic intrusion; magma mingling; Ethiopia; Pan-African
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INTRODUCTION |
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Mafic–felsic magma interactions have been recognized as important processes during the construction of granitic plutons (e.g. Whalen & Currie, 1984
; Wiebe, 1987, 1996; Vernon et al., 1988; Didier & Barbarin, 1991; Michael, 1991; Bateman, 1995; Castro et al., 1995; Wiebe & Collins, 1998; Wilcox, 1999; Collins et al., 2000; Janousek et al., 2000). Co-mingling is considered as the dominant process accounting for the structures observed in plutons, whereas thorough mixing is thought to occur in chambers at depth, prior to magma emplacement. Most studies agree that mingling is related to the replenishment of a felsic magma chamber by mafic magma intrusion and depends strongly on the relative viscosities of the magmas, which control the rheology (Fernandez & Barbarin, 1991; Fernandez & Gasquet, 1994; Hallot et al., 1996). Wiebe & Collins (1998) provided a general model for the formation of sheet-like bodies, which were described by Wiebe (1993) as mafic and silicic layered intrusions (MASLI). Other studies have considered dynamic, two-way conduit mingling and hybridization during emplacement of magmas as an equally important process (e.g. Carrigan, 1994; Castro et al., 1995; Seaman et al., 1995; Collins et al., 2000).
In the Negash pluton, Northern Ethiopia (Fig. 1), felsic and mafic rocks display various relationships. The mafic rocks occur as swarms of enclaves, or as dispersed, kilometre-sized, sheet-like bodies. The purpose of our study is to investigate the relationships between magma interactions and the dynamics of pluton growth. In a previous study (Asrat et al., 2003), we presented a structural investigation of the pluton using the anisotropy of magnetic susceptibility. We showed that the Negash pluton displays two major types of mafic–felsic magma interactions: (1) injection of monzodioritic magma into felsic magmas, which favoured in situ mingling of monzodiorites and granodiorites along their contacts; (2) simultaneous ascent of monzodioritic and granodioritic magmas through the same conduit, which led to thorough hybridization and formation of homogeneous hybrid monzodiorites.
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In the present paper we use petrological, chemical and isotopic data to (1) describe the systematic mineralogical and geochemical variations of the whole suite, (2) constrain the age and sources of the end-member magmas, (3) characterize the petrogenesis of the main rock types, and (4) discuss their implications for the mechanisms of emplacement of shallow-level calc-alkaline plutons.
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GEOLOGICAL SETTING, ROCK TYPES AND FIELD RELATIONSHIPS |
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The Northern metamorphic terrain of Ethiopia consists of a series of thick, inhomogeneous volcano-sedimentary assemblages that belong to the Arabian–Nubian Shield (ANS) of the Pan-African orogen (900–500 Ma). The ANS is a juvenile, subduction-related, accreted terrane formed by lateral crustal growth through arc–arc accretion (Kröner et al., 1987
; Stern, 1994), in which mafic–felsic plutonism played an important role (Tadesse et al., 1999; Asrat et al., 2001). The granitoid and the volcanic assemblages are calc-alkaline and lack evidence of any pre-Pan-African continental crust. A review of the available geochronological data (Asrat et al., 2001) suggests the existence of three periods of granitic magmatism in both the ANS and the Mozambique Belt (800–885, 700–780 and 540–660 Ma), encompassing syn-, late- and post-tectonic granites. The Negash pluton is one of the late-tectonic bodies (e.g. Beyth, 1972; Garland, 1980; Asrat, 1997; Tadesse, 1997; Alemu, 1998). It crops out in the middle of a low-grade metamorphic inlier in the Mekele–Adigrat area (Fig. 1a), and is one of several calc-alkaline plutons, which occur to the north especially in the Axum area. They are syn- to post-tectonic granites, monzogranites, granodiorites, diorites and subordinate gabbros, which have mantle-like Sr and Nd isotopic ratios and belong to three magmatic events at 800, 750 and 550 Ma (Rb/Sr Sm/Nd and U–Pb zircon ages). Further details about these granites have been reported by Alemu (1998) and Tadesse et al. (2000).
Rock types
The Negash pluton is a small body, 8 km in diameter, which consists of mafic and felsic rocks (Fig. 1b). In the Q–A–P classification diagram (Fig. 2), they define a trend from the monzodiorite to the monzogranite fields. We distinguish: (1) coarse-grained pyroxene monzodiorites and microgranular biotite–hornblende–quartz monzodiorites, both containing variable proportions of pyroxene (referred to as mafic rocks); (2) microgranular, biotite–hornblende–quartz monzodiorites, devoid of pyroxene and with higher proportion of quartz and K-feldspar (referred to as hybrid rocks); (3) hornblende-bearing biotite tonalites, granodiorites and monzogranites (referred to as felsic rocks); (4) biotite–(muscovite) pegmatites, aplites and microgranites. An overview of the modal compositions and textures is given in Table 1.
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Field relationships
The pluton displays a crescent-shaped lithological zoning that varies inwards from monzogranite through granodiorite, quartz monzodiorite and pyroxene monzodiorite, to hybrid quartz monzodiorite and granodiorite in the northwestern part (Fig. 1b). Locally, granodiorites show igneous layering with modal grading. Dykes of aplite, pegmatite and microgranite (a few centimetres to 10 m wide) are common near contact zones in the eastern and western parts of the pluton. Metamorphic septa (kilometre-long and hectometre-wide) are also common throughout the pluton and outline the crescent-shaped structure. Based on structural data, Asrat et al. (2003)
concluded that:
- the pluton was assembled by successive injection of four magma pulses (monzogranite, granodiorite, pyroxene and quartz monzodiorites, quartz monzodiorite and granodiorite) into already foliated country rocks;
- magmatic foliations and lineations converge towards the NW, suggesting that the feeder zone is located at the northwestern tip of the pluton;
- the obliquity in the orientations of the magmatic foliations and of the metamorphic septa at the northern and southern borders of the pluton is symmetrical with respect to the NW–SE pluton axis, suggesting that magma transfer was from the NW towards the SE.
The relationships between monzogranites–granodiorites and monzodiorites in the Negash pluton can be subdivided into three main types: (1) a large body of quartz monzodiorites and pyroxene monzodiorites within the granodiorites displaying a complex contact zone (eastern and southeastern part of the pluton); (2) hybrid quartz monzodiorites intimately associated with granodiorites (northwestern part of the pluton); (3) widespread centimetre- to metre-sized quartz monzodiorite enclaves in the monzogranites and granodiorites. We report here only on the main features of their mutual relationships. Further information and illustration of these relationships have been given by Asrat et al. (2003).
The southern granodiorite–monzodiorite contact
The cross-section A–A' (Fig. 1c) shows that the large monzodioritic body in the eastern and southeastern part of the pluton forms a shallowly dipping unit. Its lower contact with the granodiorites is marked by (1) lobate interfaces with interfingering of granodiorites into quartz monzodiorites at a decametre scale (see Asrat et al., 2003, fig. 4a); (2) abundant granitic pipes several metres in length and a few centimetres to c. 30 cm in diameter (Fig. 3a); (3) vertical, metre-wide dykes consisting of a breccia of angular monzodioritic blocks within a more or less hybridized granitic matrix; (4) veins that cut through the monzodiorites and are extremely enriched in megacrysts of K-feldspar set in a quartz and biotite groundmass (see Asrat et al., 2003, fig. 4d); (5) evidence of local intense mingling between the felsic and mafic rocks (Fig. 3b and c).
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As shown by Asrat et al. (2003)
, the lineations in the granodiorites are sub-horizontal and circumferential, whereas those in the monzodiorites (including those near the contact with the granodiorites) are oriented NW–SE and plunge gently towards the NW. Also, the anisotropy and fabric are very different: low and linear in the monzodiorites, but higher and planar in the granodiorites. These differences, suggesting local deformation regimes related to the successive emplacement of the corresponding two magmas, are considered to reflect the forceful emplacement, across the floor of the chamber, of a monzodiorite sheet leading to flattening of the adjacent partially crystallized granodiorites.
The northwestern part of the pluton
The northwestern part of the pluton consists dominantly of quartz monzodiorites and granodiorites that show evidence for pervasive mingling and hybridization. The mafic and felsic lithologies are interleaved vertically or at high angle (
45°), on a metre scale (Fig. 3d; see also Asrat et al., 2003, fig. 5b). In some cases, the mafic rocks form distinct lobes and boudins enclosed by the felsic rocks. The mafic rocks are variously hybridized, as suggested by their more leucocratic character and by the presence of abundant rounded K-feldspar megacrysts along with ocellar quartz grains. Subsequent discussion of ‘hybrid rocks’ or ‘hybrid quartz monzodiorites’ refers to these rocks.
As shown by Asrat et al. (2003), the northwestern part of the pluton is limited by a magmatic high-strain zone (Suluh shear zone), which shows vertical mafic–felsic layering along with sub-vertical foliation and sub-horizontal lineation patterns. This zone, which possibly acted as a pathway for the successive uprise of magmas during a short span of time, as evidenced by the magmatic microstructures and contacts, is considered as the inferred feeder zone.
Microgranular monzodioritic enclaves
Microgranular monzodioritic enclaves are ubiquitous in isolation or as swarms in the monzogranites and granodiorites. They are circular to elliptical, and centimetre- to metre-sized (see, e.g. Asrat et al., 2003, fig. 5d). They commonly contain rounded K-feldspar megacrysts, locally in high proportion (Fig. 3b). The contact with the host monzogranite or granodiorite is sharp, or lobate with interfingering between the felsic and mafic lithologies. They locally have quenched margins with the host rocks.
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MINERAL TEXTURE AND CHEMISTRY |
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Quartz and feldspars
Quartz occurs as both subhedral, 5 mm in diameter, rounded crystals and as interstitial grains. Alkali feldspar in the felsic rocks occurs both as anhedral perthitic grains (Or94–97) in the groundmass and as centimetre-sized phenocrysts. In the hybrid quartz monzodiorites, it occurs as isolated rounded megacrysts (up to 10 vol. %) or in veins.
Plagioclase forms complexly zoned phenocrysts in all rock types and centimetre-long unzoned laths with snowflake textures in some quartz monzodiorites. It generally exhibits discontinuous normal zoning with sodic rims. However, spongy calcic cores and reverse zoning are common. Plagioclase composition ranges from An11 to An48 (Electronic Appendix: http://www.petrology.oupjournals.org), with 0·5–4% Or component. Larger plagioclase crystals (>6 mm) have more calcic cores than smaller ones (<2 mm) within the same sample. In the felsic rocks, compositions are clustered (An11–30) with a median at An20 (Fig. 4a). Cores show compositional variations from sample to sample (An13–30) but little variation within the same sample. In the hybrid rocks, plagioclase displays (1) compositions intermediate between the felsic and mafic rocks (Fig. 4b) with cores displaying higher An contents (An16–33) than rims (An17–26), and (2) reverse zoning with calcic rims (An27–33) overgrowing normally zoned cores. In the mafic rocks, compositions are scattered (Fig. 4c) with a bimodal distribution (medians at An26 and An43). A large variation in anorthite content of plagioclase cores occurs from sample to sample (An18–45 in quartz monzodiorites and An28–48 in pyroxene monzodiorites) and in some cases within the same sample (e.g. An25–45 and An18–43).
Ferromagnesian minerals
Pyroxenes do not exceed 5 vol. % of the mode of the quartz monzodiorites, but may exceed 10% in pyroxene monzodiorites. Orthopyroxene (66–79 mol % En, 18–30% Fs and 2–5% Wo) occurs as large, yellowish green to colourless, prismatic grains (>2 mm) and as small euhedral crystals (0·5–1 mm). They show very weak normal zoning from Mg-rich cores to Fe-rich rims. Pigeonite (66–72 mol % En, 22–28% Fs and 5–6% Wo) occurs as discrete grains in association with orthopyroxene.
Green to brownish green amphiboles occur both in mafic and felsic rocks. The hybrid quartz monzodiorites contain euhedral hornblendes with inclusions of resorbed biotite in their cores (Fig. 3e). Amphiboles are generally calcic and Al2O3 rich (>6·0 wt %; Electronic Appendix: http://www.petrology.oupjournals.org). They are magnesiohornblende or tschermakite in the monzogranites and granodiorites (XMg = 0·52–0·63), in the hybrid quartz monzodiorites (XMg = 0·55–0·73) and in the pyroxene monzodiorites (XMg = 0·47–0·79, Fig. 5a). They are magnesiohornblende, tschermakite or ferrotschermakite in the quartz monzodiorites (XMg = 0·43–0·58). Amphiboles in the felsic rocks show mainly the edenite–hornblende substitution trend, whereas in the mafic and hybrid rocks they show pargasite–tschermakite–hornblende substitution (Fig. 5b). Fe3+ content recast by stoichiometry is negligible, in most cases close to zero. Actinolitic retrogression rims are observed in amphiboles from the pyroxene monzodiorites.
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Biotite forms large subhedral to euhedral crystals (5–10 mm in length) with numerous inclusions of Fe–Ti oxides and apatite. Dendritic biotite was observed locally in some hybrid rocks. The biotites are unzoned and most have XMg < 0·66 (Electronic Appendix: http://www.petrology.oupjournals.org) and correspond to biotite sensu stricto (Fig. 6). Biotites in the felsic rocks cluster at nearly the same XMg (0·53–0·58), whereas those in the quartz monzodiorites show higher variation in XMg (0·46–0·62). Biotites from pyroxene monzodiorites are phlogopites (XMg = 0·69–0·73). In the hybrid quartz monzodiorites, they fall in two groups: (1) those from samples collected at the contact zones with the mafic and felsic rocks have XMg (0·54–0·58) similar to the quartz monzodiorites; (2) those collected away from contact zones are more magnesian (XMg = 0·61–0·68).
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Accessory minerals
Zircon is an accessory phase in all rocks of the suite. It commonly occurs as euhedral, prismatic to bipyramidal crystals, up to 400 µm in length in granodiorites and monzogranites. Backscattered scanning electron (BSE) images of zircon grains show that they consist of euhedral magmatic growth zones surrounding euhedral cores.
Fluorapatite (39–41 wt % P2O5, 51–53 wt % CaO and 2·2–3·2 wt % F) occurs as prismatic crystals up to 2 mm long and as needle-like inclusions in plagioclase, ferromagnesian and oxide minerals, suggesting early crystallization. It is present in all rock types, but is especially abundant in the hybrid and mafic rocks (up to 2·5% of the mode in pyroxene monzodiorites).
Titanite is ubiquitous and particularly abundant in the felsic and hybrid rocks. It occurs as isolated, euhedral, brownish crystals or as aggregates of twinned crystals. Its major-element composition is homogeneous in all rock types, except for very slight enrichment in FeO (0·5–1·8 wt %) in the rims.
In the felsic and hybrid rocks, ilmenite and titanomagnetite (6–20 wt % TiO2) occur as inclusions in titanite, where they form small, subhedral to rounded grains. Isolated ilmenite or magnetite crystals are rare in these rocks. In the mafic rocks, euhedral or rounded magnetite crystals are found in addition to ilmenite and titanomagnetite grains; the magnetite occurs both in isolation and within titanite. Fe–Ti oxides do not exceed 5 vol. % in the quartz monzodiorites, but may reach 10% of the mode in the pyroxene monzodiorites.
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CONDITIONS OF EMPLACEMENT |
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Determining temperature and pressure conditions of magma emplacement and consolidation, as well as magma water content, is a prerequisite for estimating magma viscosity, which, in turn, directly controls emplacement and interaction mechanisms. However, it should be noted that these data, derived from mineral assemblages, are strongly dependent on calibration and, therefore, can provide only provisional geological information. A summary of thermobarometric and oxybarometric data discussed below is given in Table 2.
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Apatite and zircon thermometry
Apatite and zircon thermometry (Watson & Harrison, 1983
; Harrison & Watson, 1984) were applied to the rocks that show evidence of saturation in P2O5 and Zr, with the exception of the pyroxene monzodiorites suspected to be cumulates (see Discussion). P2O5 decreases with increasing SiO2 (Fig. 7a), implying crystallization of apatite and, hence, saturation of the parent melts in P2O5. Zr decreases with increasing SiO2 only in the hybrid and felsic rocks (Fig. 7b), suggesting crystallization of zircon and, therefore, melt saturation in Zr only for these rocks. The crystallization of apatite and zircon is also consistent with the decrease in Nb and Y from the quartz monzodiorites to the monzogranites (Fig. 7c and d).
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, quartz monzodiorite samples N9-2 and N9-47; shaded inverted triangles, hybrid quartz monzodiorite samples N9-13, N9-19 and N9-41.The apatite thermometer gives temperatures in the following ranges: 836–886°C (felsic rocks), 790–912°C (hybrid quartz monzodiorites) and 824–950°C (quartz monzodiorites). The zircon thermometer applied only to the hybrid and felsic rocks gives lower temperature estimates of 737–768°C and 712–756°C, respectively. Preservation of high temperatures in the quartz monzodiorites is consistent with the fine-grained textures suggesting quenching. Discrepancy between apatite and zircon thermometry has been attributed to several possible causes: (1) excess apatite, which may not fractionate efficiently from the melt; (2) the saturation model may not be appropriate for the rocks considered (e.g. Hoskin et al., 2000
); (3) apparent saturation may be due to local disequilibrium (Bacon, 1989); and/or (4) lower temperature estimates of zircon thermometer representing temperatures closer to the solidus (e.g. Wyllie, 1984; Anderson, 1996). Our data fall into the ‘low-temperature granite’ category defined by Miller et al. (2003), although they lack inheritance. Accounting for the fact that apatite is an early crystallized phase in the Negash pluton, we suggest that the lower temperatures given by zircon thermometry, compared with those obtained from apatite, reflect melt Zr undersaturation at the source. In this case, temperatures obtained from apatite thermometry should be closer to liquidus temperatures, whereas those obtained from zircon thermometry should be considered as minimum estimates.
Hornblende–plagioclase thermometry
Hornblende–plagioclase thermometry (Blundy & Holland, 1990) can be applied to rocks that crystallized in the interval 550–1100°C. The prerequisite for the application of this method is that plagioclase should be less anorthitic than An92 and the amphiboles should have Si < 7·8 a.p.f.u. The pressure range used in the temperature estimation (2·0–4·6 kbar) is that determined by the Al-in-hornblende barometer, described in the next paragraph. The results are consistent with those found by the other methods: the felsic and hybrid rocks give temperature estimates of 684–786°C and 682–788°C, respectively, whereas the mafic rocks give temperatures of 795–856°C.
Al-in-hornblende barometry
The Al-in-hornblende barometer of Anderson & Smith (1995), which takes into account the temperature dependence, was applied to hornblendes in rocks that contain the recommended seven-phase assemblage (hornblende, biotite, plagioclase, K-feldspar, quartz, titanite and Fe–Ti oxides). The Al contents of amphibole cores and rims show no significant difference. The temperature range (710–950°C) used in pressure estimation is that determined from apatite and zircon thermometry. The pressures obtained for the felsic and hybrid rocks range from 2·2 to 4·6 kbar, with averages ranging from 3·0 to 3·4 kbar (Table 2). These values are reasonable estimates considering the presence of andalusite in the contact aureole, which implies pressure lower than 4·5 kbar, according to the position of the Al-silicate triple point given by Pattison (1992), which appears to be the most reliable (Cesare et al., 2003).
Fe–Ti oxide thermobarometry
The Fe–Ti oxide thermometer and oxybarometer (Spencer & Lindsley, 1981) was applied to coexisting ilmenite and titanomagnetite that satisfy the test of Bacon & Hirschmann (1988). The estimated ranges in temperatures are 536–753°C (felsic rocks), 521–622°C (hybrid rocks), 594–770°C (quartz monzodiorites) and 615–742°C (pyroxene monzodiorites). All these values are significantly lower than those estimated by horn- blende–plagioclase, apatite and zircon thermometry, and probably suggest re-equilibration during cooling.
Oxygen fugacities (log fO2), determined from the model of Spencer & Lindsley (1981), vary between –20·5 and –16·4, and most of the samples display relatively low fO2 close to the fayalite–magnetite–quartz (FMQ) buffer. However, some mafic samples with higher temperature of equilibration plot close to the nickel–nickel oxide (NNO) buffer (Fig. 8). The fO2 estimates are also likely to represent values re-equilibrated during cooling, as suggested by the regular decrease in log fO2 with falling temperature. The temperatures and fO2 indicate that titanite was stable in the presence of quartz and magnetite, consistent with petrographic data.
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Magma water content
Magma water content is another important parameter, in addition to P–T–fO2 conditions, that influences melt compositions, crystallization conditions and viscosity of granitic magmas (e.g. Johannes & Holtz, 1996
). However, the lack of accurate models and the strong dependence of H2O content on temperature and pressure, as well as melt composition, make determination of this parameter difficult. Water content can be estimated empirically by comparison with available experimental data. Scaillet et al. (1998) indicated that most silicic volcanic rocks and their plutonic equivalents have a dissolved H2O content of 4–6 wt % for a wide temperature range (700–900°C). Scaillet & Evans (1999) proposed an experimental calibration of water content in magma of dacitic composition, at 2·2 kbar and log fO2 = NNO + 2·7. The P–T–fO2 conditions of emplacement of the Negash pluton, which are not far from these experimental data, allow the water fugacity to be roughly estimated for the parent melt of the felsic rocks. Accordingly, the phase relationships in these rocks (stability of hornblende, absence of orthopyroxene, and plagioclase less anorthitic than An50) suggest water contents 6 wt %. On the basis of the experimental data of Scaillet & Evans (1999) and using representative Altot values of hornblende (1·3–1·5 a.p.f.u.), the water content in the 750–850°C temperature range is estimated to be of the order of 5·5–6·5 wt % for the felsic rocks. In contrast, the water content of the mafic rocks was probably significantly lower, as deduced from the presence of pigeonitic pyroxene in the less evolved compositions (pyroxene monzodiorites).
In summary: (1) apatite thermometer yields temperatures close to the liquidus (from 836–886°C for the felsic rocks to 824–950°C for the quartz monzodiorites), whereas the lowest Fe–Ti oxide temperatures (550–750°C) suggest near-solidus or subsolidus re-equilibration; (2) fO2 values, although within the range of crystallization fugacity of arc-related batholiths (e.g. Czamanske et al., 1981; Speer, 1987), are probably re-equilibrated (typical arc magmas have fO2 values closer to NNO); (3) phase relationships suggest that H2O contents were high in the felsic rocks but low in the pyroxene monzodiorites; (4) the presence of andalusite in the contact aureole, as well as pressure estimates, suggest a shallow level of emplacement, as generally observed in similar types of plutons.
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MAJOR AND TRACE ELEMENT DATA |
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Element abundances
Whole-rock compositions (Tables 3 and 4) display a bulk medium- to high-K calc-alkaline trend (Fig. 9a and c), with high alkali and CaO contents (K2O + Na2O = 3·16–8·25; K2O/Na2O = 0·29–0·87; CaO = 2·55–9·15 wt %). The rocks are metaluminous, with the exception of the most differentiated ones, and the Al2O3/(CaO + Na2O + K2O)molar and Al2O3/(Na2O + K2O)molar ratios are negatively correlated, forming a rough linear trend from the mafic to the felsic rocks (Fig. 9b). The pegmatite, aplite and microgranite dykes are slightly peraluminous and form two distinct groups: sodic with K2O <2 wt % and potassic with K2O >4 wt %. MgO and FeO contents range from 1·73 to 8·97 and from 3·16 to 14·17 wt %, respectively, going from the felsic to the mafic rocks. The whole suite can, therefore, be considered as magnesian according to the classification of Frost et al. (2001)
. The rocks define a calc-alkaline trend in the AFM triangular plot (Fig. 9d), with the exception of the quartz monzodiorites, which display higher Fe/Mg ratios (tholeiitic affinity).
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Pyroxene monzodiorites, granodiorites and monzogranites have similar mg number [=MgO/(MgO + FeOtot)] values of 0·36–0·39. They differ from the quartz monzodiorites, most of which are significantly less magnesian (mg number = 0·28–0·30); samples N9-2 and N9-47 are exceptions that show mg number (0·36) similar to those of the former rock types. Mg number of hybrid rocks are similar to those of the pyroxene monzodiorites and felsic rocks (0·36–0·37), with the exception of two samples (N9-16 and N9-44) that have higher mg number values (
0·48). The pyroxene monzodiorites and the quartz monzodiorites with the lowest silica contents are remarkably rich in TiO2 (4·48–4·86 and 3·09–4·06 wt %, respectively). Also, they display high K2O (0·9–1·3 wt %) and P2O5 (up to 1·26 wt %) contents.
All of the rocks have light rare earth element (LREE)-enriched patterns (Fig. 10) with (La/Yb)N ratios ranging from 9·9 to 17·7. LREE fractionation decreases from felsic [(La/Sm)N = 2·8–4·5] through hybrid (2·6–4·0) to mafic (1·6–2·5) rocks. The felsic rocks and the pyroxene monzodiorites have similar normalized La and Yb values (LaN = 40–70, YbN = 3–5), whereas the quartz monzodiorites have overlapping to higher LaN (50–100) and higher YbN (5–7); the hybrid quartz monzodiorites have the highest LaN contents (LaN = 70–120). Although the Negash pluton can be described as calc-alkaline, the REE patterns are unlike typical Andean calc-alkaline plutons where middle and heavy REE (MREE and HREE) flatten out at about 10 times chondrite (Atherton & Sanderson, 1985). Rather, they closely resemble those of the Mesozoic plutonic rocks from Patagonia (Rapela & Pankhurst, 1996).
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Inter-element relationships
Primitive mantle-normalized trace element patterns (Fig. 11) indicate similar geochemical characteristics for all of the rock types, although some differences appear in the relative sizes of the peaks and troughs. Most samples show spikes in Ba, U, K, Pb and Sr and troughs in Rb, Th and Nb, but the trough in Nb is more pronounced in the hybrid and felsic rocks than in the mafic rocks. Th/U ratios (1·8–3·9) fall in the range of values (2–4) published for medium-K suites (Gill, 1981
). The REE and other trace-element patterns of the quartz monzodiorites are similar in shape to those of the pyroxene monzodiorites (Figs 10 and 11), but they differ in having higher trace-element concentrations. The patterns of the pyroxene monzodiorites are similar to that of island-arc basalts from Vanuatu (Peate et al., 1997), with the exception of higher Nb, strong positive anomalies in Ti and P, and lower HREE. The felsic and hybrid rocks show similar concave-upward REE patterns (Fig. 10) and troughs in P and Ti and spikes in Zr.
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Plots of selected major and trace elements vs SiO2 are presented in Fig. 12a–f for elements linked mainly to feldspars and in Fig. 12g–l for elements linked to ferromagnesian and oxide minerals. Element concentrations display systematic variations, which can be summarized as follows.
- Al2O3, K2O, Na2O, Sr and Ba increase in both the mafic and hybrid rocks, whereas Al2O3, Na2O, Sr and to some extent Ba decrease in the felsic rocks, with increasing silica content. CaO is anti-correlated with Al2O3 and decreases from pyroxene monzodiorites to monzogranites. Rb behaves as an incompatible element in the whole suite. On the whole, MgO, FeOtot, TiO2, V, Cr and Ni decrease with increasing silica in all rock types.
- Pyroxene monzodiorites have high FeOtot, MgO, CaO, TiO2, V, Cr and Ni but low Al2O3 contents. Three samples have high TiO2 contents (4·5 wt %) and one sample (N9-25) high P2O5 concentrations (1·13 wt %). Even though high TiO2 contents can be found in tholeiitic and alkali basalts, the pyroxene monzodiorites show higher Ti/Zr and Ti/Y ratios (200–274 and 1587–2444, respectively) compared with the primitive mantle (116, 286), normal mid-ocean ridge basalt (N-MORB; 82, 273) and ocean island basalt (OIB; 61, 593) values of Sun & McDonough (1989), or compared with IAB values (73, 261) of Peate et al (1997). They also have high P/Zr ratios (17–34) compared with subalkaline basalts [
15 according to Winchester & Floyd (1976)]. Ti/V ratios (116–136) are also anomalously high for basaltic melts, which are commonly 50 (Woodhead et al., 1993) and reach 100 only in OIB and alkali basalts (Shervais, 1982). The positive Ti and P anomalies are especially clear in the multi-element plot of Fig. 11d.
- Quartz monzodiorites are distinct from other rock types, especially hybrid rocks with the same range in SiO2, in that they have significantly less magnesian whole-rock and mineral compositions and lower Cr and Ni contents. Only two samples, N9-2 and N9-47, have mg number similar to those of the other lithologies, but they have lower Al2O3 and Sr and higher Ni and Cr contents (Table 3 and Fig. 12). The three pyroxene monzodiorites with the lowest silica contents (Table 3) have high TiO2 contents (3·09–4·04 wt %).
- Three samples of hybrid quartz monzodiorites (N9-13, N9-19, N9-41) fall off the bulk trends in the Harker plots (Fig. 9a and Fig. 12a, c, d), as a result of their high K2O, Al2O3, Na2O and Rb contents. These samples are mineralogically distinguishable by their high modal abundance of K-feldspar xenocrysts. The remaining two samples (N9-16, N9-44) have higher MgO, Cr and Ni contents and significantly higher mg number.
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ISOTOPE GEOCHEMISTRY |
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Zircon U–Pb isotopic data
Zircons were extracted from a monzogranite sample collected at the northeastern border of the Negash pluton. The most transparent, inclusion-free, and fully euhedral zircon grains with brilliant surfaces and sharp edges were hand-picked from a 50–200 µm fraction. Four groups of zircons were identified on the basis of size, colour, and morphology:
- group (i), <100 µm, colourless, transparent, euhedral, prismatic to bipyramidal;
- group (ii), 100–150 µm, transparent, colourless, euhedral, elongated to acicular and prismatic;
- group (iii), 100–150 µm, rose to pink, euhedral, bipyramidal;
- group (iv), 150–200 µm, rose to pink, euhedral, prismatic.
- group (ii), 100–150 µm, transparent, colourless, euhedral, elongated to acicular and prismatic;
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Sixteen zircon grains representing all four groups were analysed using a CAMECA IMS-1270 ion microprobe at CNRS–CRPG, Nancy. Details of analytical and working conditions have been given by Deloule et al. (2002)
. The age calculations are based on the isotopic ratios corrected for background noise and common lead (using 204Pb). The U and Pb abundances are calculated on the basis of the Zr2O vs UO2 correlation for the standard zircon 91500 with an age of 1062·4 ± 0·4 Ma (Wiedenbeck et al., 1995). The relative sensitivity factor for Pb and U used for samples was defined from an empirical linear relationship between UO+/U+ and Pb+/U+ (Compston et al., 1984), using all the measurements performed on the standards. The 207Pb/206Pb ratios are directly determined from each spot analysis.
Results are given in Table 5 and concordia diagrams are shown in Fig. 13. Weighted mean ages and discordia lines were determined using the Isoplot program (Ludwig, 1991). The four zircon groups give the following results.
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Group (i) yields discordant 206Pb/238U ages ranging from 514 ± 14 to 588 ± 13 Ma, with a weighted mean 206Pb/238U age of 563 ± 35 Ma [mean square weighted deviation (MSWD) = 7·6]. The discordia line yields a poorly defined upper intercept at 687 ± 77 Ma (MSWD = 1·4).
Group (ii) yields discordant 206Pb/238U ages ranging from 518 ± 14 to 658 ± 15 Ma, with the exception of grain 1b at 809 ± 26 Ma, which was discarded from the age calculation. Their weighted mean 206Pb/238U age is 591 ± 81 Ma (MSWD = 20) and they define a discordia line with an upper intercept at 611 ± 23 Ma (MSWD = 3·6) (Fig. 13a).
Group (iii) consists of two concordant grains at 597 ± 16 and 622 ± 14 Ma, and of discordant grains, with 206Pb/238U ages ranging from 528 ± 14 to 738 ± 24 Ma (Fig. 13b). The weighted mean 206Pb/238U age is 611 ± 68 Ma (MSWD = 19). These analyses define a discordia line with an upper intercept at 608 ± 6 Ma (MSWD = 2·3).
Group (iv) yields 206Pb/238U ages ranging from 562 ± 15 to 712 ± 19 Ma with a concordant grain at 605 ± 16 Ma (Fig. 13c). The weighted mean 206Pb/238U age is 616 ± 48 Ma (MSWD = 7·7). All the analyses define a discordia line with an upper intercept at 608 ± 7 Ma (MSWD = 1·3).
The three groups corresponding to the larger zircon grains (100–200 µm) give consistent upper intercept ages at 611 ± 23, 608 ± 6 and 608 ± 7 Ma, which are identical within errors to both the concordant single grain ages (597 ± 16, 605 ± 16 and 622 ± 14 Ma) and the respective weighted mean 206Pb/238U ages. The smaller discordant zircon grains give younger 206Pb/238U ages, possibly suggesting partial resetting. Therefore, we consider the upper intercept age at 608 ± 7 Ma as representative of the emplacement age of the Negash pluton.
Whole-rock Sr and Nd isotopic data
Whole-rock Sr–Nd isotopic data (Table 6) show limited variation of measured 87Sr/86Sr (0·70332–0·70475) and 143Nd/144Nd (0·51249–0·51262) ratios. Initial isotopic ratios, recalculated at 608 Ma, fall within a restricted range (Fig. 14a), with low 87Sr/86Sr ratios (0·70260–0·70296), and positive Nd(608) (+3·9 to +5·9). One exception is a hybrid quartz monzodiorite, which has a higher 87Sr/86Sr ratio (0·70350), despite having an Nd(608) value (+4·8) typical of the sample suite. All of the samples plot close to the mantle array and within the field of the Arabian–Nubian magmatic rocks (Fig. 14a). They have Sr–Nd isotopic ratios comparable with those of Neoproterozoic granites from NE Sudan (Fig. 14b). However, they differ from some Neoproterozoic granites of northern Ethiopia, from the Neoproterozoic crust of southern Ethiopia, and from Palaeoproterozoic and Archaean basement rocks of eastern Ethiopia, most of which are characterized by higher 87Sr/86Sr initial ratios and, for some of them, by lower Nd(608) values (Fig. 14b).
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Within the Negash pluton, there is a slight increase of the 87Sr/86Sr initial ratios from the monzogranites (0·70260–0·70267) through the granodiorites and hybrid quartz monzodiorites (0·70274–0·70285, one value at 0·70350), to the quartz monzodiorites and pyroxene monzodiorites (0·70282–0·70296). The
Nd(608) values are lower on average in the monzogranites and pyroxene monzodiorites than in the granodiorites and hybrid quartz monzodiorites (Fig. 14a). On the whole, most samples have an isotopically moderately depleted signature. However, the low Sr initial ratios, which appear to be very low compared with other calc-alkaline arc granitoids, seem to be typical of most Neoproterozoic granitoid rocks from the Arabian–Nubian Shield (Fig. 14). |
DISCUSSION |
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Source of the mafic and felsic rocks
Initial Sr and Nd isotopic plots (Fig. 14) show that the Negash mafic and felsic rocks, like the granites from NE Sudan, have more mantle-like Sr isotopic signatures than other Neoproterozoic rocks from Northern Ethiopia. The low and homogeneous initial Sr and high Nd isotopic ratios along with Pan-African depleted mantle Nd model ages (0·70–0·92 Ga) imply a source dominated by a mantle-derived component. This suggests a juvenile source for the rock suite, although the Nd ratios are lower than those of MORB at 608 Ma (Fig. 14a). These isotopic signatures are typical of both OIB and IAB, but the trace element patterns, which display a clear subduction signature, argue in favour of arcs. The multi-element patterns (Fig. 11) with spikes in Cs, Ba, Sr and Pb, and troughs in Nb, are consistent with magma sources involving melting of a high field strength element (HFSE)-depleted mantle that has been fluxed by fluids following dehydration of a subducted slab, as observed in modern island-arc environments (e.g. Woodhead et al., 1993
; Gamble et al., 1996).
Petrogenesis of the rocks from the Negash pluton
Possible petrogenetic processes for the origin of the Negash mafic and felsic rocks can be variable differentiation of mantle-derived magmas by fractional crystallization, partial melting of underplated igneous rocks, or partial melting of Pan-African juvenile island-arc crust or immature sediments. Mixing and mingling have also to be considered as possible processes for the genesis of quartz monzodiorites. Significant contamination by older continental crust can be ruled out because of the low initial 87Sr/86Sr ratios and high Nd(t) values.
Pyroxene monzodiorites
Pyroxene monzodiorites have coarse-grained textures with abundant euhedral grains of apatite and Fe–Ti oxides enclosed within subhedral to euhedral pyroxene and hornblende poikiloblasts. Such textures that have been reported in monzodiorites from the Patagonian batholith are attributed to orthocumulates (Rapela & Pankhurst, 1996). Although the pyroxene monzodiorites show mantle-like Sr and Nd isotopic signatures and have the most magnesian amphiboles and biotites of the whole suite, their Cr (<600 ppm) and Ni (<200 ppm) contents are too low to represent primary basaltic melts. Moreover, they have very high Ti/Zr, Ti/Y, Ti/V, P/Zr and P/Y ratios compared with the primitive mantle and with common basaltic melts (Fig. 11d), especially island-arc and back-arc basalts (Woodhead et al., 1993). We interpret these chemical characteristics as a result of accumulation of Fe–Ti oxide and apatite, and we therefore consider these rocks as cumulates formed by fractional crystallization from a basaltic parent melt. The link between pyroxene monzodiorites and quartz monzodiorites, suggested by their association in the field and by similar REE and trace-element patterns, will be discussed in the next section.
Quartz monzodiorites and hybrid rocks
The quartz monzodiorites have distinctly lower whole-rock mg number values and mineral XMg ratios compared with the hybrid quartz monzodiorites, for the same range of silica content. This precludes any genetic link between these two types of quartz monzodiorites and, therefore, suggests that they originate from two distinct magmas. These magmas probably evolved under distinct oxygen fugacity conditions, as suggested by their distinct mg number, which imply fractionation of an Fe-rich phase (probably Fe–Ti oxide) for the hybrid quartz monzodiorites (Fig. 9d). Major element modelling (Table 7) further suggests that the pyroxene and quartz monzodiorites can be derived from a common parent melt with high iron and titanium contents (Fig. 15a and b). The most silica-rich quartz monzodiorite (sample N9-24) can be derived by 80% fractional crystallization of a parent melt with the composition of the quartz monzodiorite sample N9-4. The composition of the modelled solid residue is similar to that of the pyroxene monzodiorites. This is corroborated by trace element modelling involving compatible and incompatible trace elements such as V and Rb (Fig. 15c), and in agreement with the REE contents, which are lower in the pyroxene monzodiorites than in the quartz monzodiorites.
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The hybrid quartz monzodiorites consist of two distinct groups. The one with high Al2O3, K2O, Na2O, Rb and Ba concentrations (Figs 9 and 12a, c, d) corresponds to samples containing a significant modal abundance of K-feldspar xenocrysts and, therefore, can be considered to result from mingling with phenocryst-laden felsic magmas (Fig. 3b). The microtextures and compositional zoning of plagioclase (spongy cores, reverse zoning, calcic spikes) further suggest hybridization. Such evidence raises the question of whether in situ binary mixing can explain the genesis of the hybrid quartz monzodiorites. Amphibole, biotite and whole-rock compositions show that the hybrid rocks cannot result from only two-component mixing involving quartz monzodiorites and felsic rocks. The overlap in hornblende and biotite compositions (Figs 5 and 6) implies that some cumulate component (pyroxene monzodiorites) may have been involved. The significant enrichment of the hybrid rocks in both LREE (LaN = 70–120 in the hybrid quartz monzodiorites compared with 50–100 in the quartz monzodiorites and 40–70 in the pyroxene monzodiorites and felsic rocks) and HREE (YbN = 6–8 in the hybrid monzodiorites compared with 5–7 in the quartz monzodiorites and 3–5 in the pyroxene monzodiorites and felsic rocks) also provides evidence that the hybrid rocks cannot be explained by simple in situ mixing between the mafic and felsic end-members. This, therefore, calls for another, unseen, more primitive mafic magma (higher mg number and Ca, lower Ti and P) and implies that differentiation (involving hybridization) must have occurred before emplacement.
Granodiorites and monzogranites
The low Sr and high Nd isotopic ratios of the granitoids suggest they may have been produced by partial melting of mantle-derived material with short crustal residence time, or fractional crystallization of basaltic melt. However, it is not clear from the available data which process explains the origin of the felsic rocks.
Partial melting of underplated mafic material is a possible process that may account for the origin of felsic rocks in general, as shown from geochemical (e.g. Williamson et al., 1992; Tepper et al., 1993; Petford & Atherton, 1996) and experimental studies (e.g. Helz, 1976; Spulber & Rutherford, 1983; Beard & Lofgren, 1989, 1991; Thy et al., 1990; Rushmer, 1991; Wolf & Wyllie, 1994; Rapp & Watson, 1995). Applying this model to the Negash granitoids is consistent with: (1) the high Sr contents, implying that plagioclase was incorporated into the melt; (2) the concave-upward REE patterns and lack of Eu anomalies, suggesting the predominance of residual amphibole in the source (see Tepper et al., 1993); (3) the strongly fractionated HREE-depleted patterns (YbN around 4–5 times chondrite), implying the presence of garnet in the residue; (4) the temperatures estimated from the apatite thermometer (836–886°C for the felsic rocks), which are consistent with experimental data indicating melting temperatures of 850–950°C for the generation of felsic melts (Beard & Lofgren, 1991).
The granodiorites and monzogranites are unlikely to have been derived by fractional crystallization from the quartz monzodiorites as they belong to two distinct series (Figs 9d and 15a, b). The apparent linear array shown by the felsic rocks, the hybrid quartz monzodiorites and the pyroxene monzodiorites in Fig. 9d cannot be interpreted in terms of fractional crystallization either, because the REE patterns (Fig. 11) show that the three rock types cannot represent the residual melt, the parent melt and the cumulate, respectively. We cannot exclude a genetic link between the felsic rocks and the hybrid quartz monzodiorites through fractional crystallization, considering the similarity of their mg number values and REE patterns. Nevertheless, the microgranular textures of the hybrid rocks, suggesting melt quenching and the presence of K-feldspar xenocrysts inherited from the partially crystallized felsic rocks, preclude any in situ differentiation (separate batch melts). The granodiorites and monzogranites cannot be linked by in situ fractional crystallization, because in a cooling chamber, the less differentiated rocks should have been at the base, i.e. the reverse of what is observed (granodiorites above monzogranites). However, the chemical diversity of the granodiorites (59–69% SiO2) and their coarse-grained texture with rounded quartz grains and abundant K-feldspar phenocrysts, which indicate prolonged crystallization, suggest that differentiation occurred in situ. This is consistent with the presence of microgranite dykes, which indicate that felsic material was transferred as melt. It is also in agreement with the evolution of mafic and silicic layered intrusion systems, as suggested by Wiebe & Collins (1998).
In summary, petrological and geochemical data indicate that: (1) the main rock types forming the Negash pluton were derived from already differentiated magmas; (2) at least three distinct magma types, now represented by the quartz monzodiorites, the hybrid rocks and the felsic rocks, contributed to the construction of the pluton; (3) the pyroxene monzodiorites are likely to represent in situ differentiation of melt with the composition of quartz monzodiorites; (4) the chemical diversity of the granodiorites may possibly result from in situ differentiation.
Mafic–felsic magma interactions
All the rocks of the Negash pluton can be considered as hybridized to some extent, as shown by textural evidence: widespread occurrence of feldspar xenocrysts in the mafic rocks; plagioclases with patchy cores, corroded rims and more calcic zones over normally zoned crystals; and euhedral hornblende crystals with inclusions of resorbed biotites in their cores (contamination of granodioritic magma by influx of monzodioritic magma).
Two types of interaction between felsic and mafic rocks can be distinguished. The first corresponds to emplacement of a monzodiorite sheet within the granodiorites and monzogranites (Fig. 1b). The lower interface of this sheet, visible in the southern half of the pluton, is characterized by in situ mingling structures (e.g. felsic pipes, brecciated dykes, granitic veins, microgranular mafic enclaves with high modal abundance of K-feldspar xenocrysts). These structures, which closely resemble those described for mafic and silicic layered intrusions (Wiebe & Collins, 1998), are interpreted as the result of rheological instability. The second type (northwestern part of the pluton, Fig. 1b) consists of mingled magmas with higher mafic/felsic magma ratio, higher fragmentation of the mafic material within the felsic matrix, abundant net veining and hybridized microgranular mafic enclaves. It is interpreted as the result of conduit mixing and mingling (see Carrigan, 1994). These two types of interaction, which mainly involved mingling, correspond to two successive stages of construction of the Negash pluton.
Interface mingling and magma viscosities
The mingling structures at the mafic–felsic interfaces can be indicative of the physical properties, particularly the viscosities, of the interacting magmas. The fine-grained, generally phenocryst-free, texture of the mafic rocks (pyroxene monzodiorites excepted), implying undercooling (high 
T, high nucleation and low growth rates), suggests that they were emplaced as crystal-poor melts. The viscosity of crystal-poor basaltic melts at liquidus temperature (c. 1200°C) is given as 101–102 Pa s (e.g. McBirney & Murase, 1984; Johannes & Holtz, 1996).
The viscosity of the felsic magma of the Negash pluton can be roughly estimated using empirical models and by comparison with experimental data. Calculation of the melt viscosity was made using the equation of Shaw (1972), whole-rock major element compositions from Table 4, and assuming temperatures of 900°C and 750°C for the granodiorites and monzogranites, respectively, and melt water contents of 4–6 wt %. This yields viscosity values of 102·1–104·6 Pa s, in agreement with the values given by Clemens & Petford (1999) for leucogranitic to tonalitic melts (103·2–106·3 Pa s). Experimental data of Scaillet et al. (2000) were obtained for dacitic bulk compositions in the temperature interval between the liquidus and solidus (920–680°C). Their starting material (SiO2 65 wt %, initial H2O 6·9 wt %) is compositionally close to the Negash granodiorites (SiO2 59–69 wt %, H2O 6 wt %). The melt viscosity within the temperature range determined for the Negash felsic rocks (c. 750–850°C) can be estimated at 103·5–104·5 Pa s, whereas the magma viscosity may reach 107 Pa s. The data of Scaillet et al. (2000) further suggest that, in the 850–750°C range, the fraction of melt decreases from 60 to 40 wt %, i.e. that the magma crosses the locking particle threshold (Vigneresse et al., 1996) at 750°C and can start deforming like a solid. This is in agreement with the presence of abundant feldspar xenocrysts in the mafic rocks, which indicates that mafic magmas were injected through or into partially crystallized, phenocryst-bearing, felsic material. This is also consistent with the type of deformation recorded in the peripheral granodiorites and monzodiorites (flattening), whereas the monzodiorite sheet displays flow deformation (constriction). The instability of the lower interface of the large monzodioritic sheet, leading to a folded surface accompanied by mingling structures, such as felsic pipes, dykes and veins (Asrat et al., 2003), suggests the interaction of low-viscosity magmas (e.g. Fernandez & Barbarin, 1991; Fernandez & Gasquet, 1994; Hallot et al., 1996; Scaillet et al., 2000) and, therefore, possible remelting of the felsic material at the interface, as suggested by heat balance considerations.
Interactions between mafic–felsic magmas led to complex structures, which resulted from both mafic magma intruding felsic and felsic intruding mafic. Nevertheless, these occurred at different scales and at different stages of pluton construction. At pluton scale, the mafic magma entered the felsic magma chamber, whereas locally at the interface between the two magma types, the crystal-rich felsic magma transiently back-veined the mafic material. If the viscosity of the mafic magma remains low enough after the back-veining process, the felsic veins can mechanically be destroyed and K-feldspar incorporated in the hybrid. This process, which was described by Collins et al. (2000) from the Kameruka pluton, can account for the major- and trace-element composition of some hybrid quartz monzodiorite samples.
Implications for pluton construction
The petrological and geochemical data, along with field and structural data (Asrat et al., 2003), have significant implications for the construction of the Negash pluton. Structural data indicate that the pluton was constructed by assembly of successive magma batches. The mineral compositions and whole-rock chemistry suggest that the main lithologies of the Negash pluton (pyroxene monzodiorites, quartz monzodiorites, hybrid quartz monzodiorites, granodiorites and monzogranites) cannot be derived simply by in situ fractional crystallization, nor by simple in situ mixing between the mafic and felsic end-members. This implies that some differentiation occurred before magma emplacement, either in deep-seated intermediate chambers or in magmatic conduits. Mafic–felsic magma interactions in the pluton were limited to mingling between mafic pulses and partially crystallized granitic material, both at the pluton scale in response to forceful magma injection and at a local scale in response to instabilities of the mafic–felsic interfaces caused by inverted density gradients. Shallow-level plutons appear to result from aggregation of melt that differentiated elsewhere (e.g. Pitcher, 1979; Roberts et al., 2000; Barbey et al., 2001).
The presence in the pluton of abundant septa of country rocks (Fig. 1b), which are sub-parallel to the planar fabric and display a very high aspect ratio (Asrat et al., 2003), suggests that the felsic rocks were initially emplaced as sheets within the schistosity of the country rocks. This is consistent with the low viscosities of both the mafic and felsic magmas deduced from structures and emplacement conditions, and suggests that the Negash pluton was constructed from melt-rich magmas injected as sills, according to the sheeting/dyking (Clemens & Mawer, 1992; Petford et al., 1993) or layering (Wiebe & Collins, 1998) models. The preservation of the overall geometry of the mafic sheet and septa of the country rocks, along with the lineation and foliation patterns, suggests that no major convective overturn occurred at the scale of the pluton after the emplacement of the mafic magmas. However, this does not preclude local convection and melt percolation, as suggested by the limited occurrence of igneous layering.
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SUPPLEMENTARY DATA |
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Supplementary data for this paper are available on Journal of Petrology online.
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APPENDIX: ANALYTICAL METHODS |
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Mineral compositions were analysed with a CAMECA SX-50 electron microprobe (Service Commun de Microanalyse, Université Henri Poincaré, Nancy). Operating conditions were 20 nA sample current, 15 kV accelerating potential, counting times of 20 s and a beam diameter of 1 µm. Calibration was made on a combination of silicates and oxides. Data reductions were performed using the PAP correction procedure (Pouchou & Pichoir, 1991
). Whole-rock major and trace elements were analysed by inductively coupled plasma atomic emission spectrometry (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS) (CRPG–CNRS, Nancy), respectively. Analytical uncertainties are given as 2% for major elements, and as 5% or 10% for trace element concentrations (except REE) higher or lower than 20 ppm, respectively. Precision for REE is estimated at 5% when chondrite-normalized concentrations are >10 ppm and at 10% when they are lower.
Separation of Rb–Sr and Sm–Nd was performed according to the methods of Michard et al. (1985) and Boher et al. (1992). Rb, Sr, Sm and Nd concentrations were determined by isotope dilution. Rb isotopic compositions used for concentration calculations were determined using an Elan 6000 ICP-MS system. Sr, Nd and Sm isotopic compositions were measured using a Finnigan MAT-262 mass spectrometer. Measured 87Sr/86Sr and 143Nd/144Nd ratios were normalized to 86Sr/88Sr = 0·1194 and 146Nd/144Nd = 0·7219, respectively. Repeated analyses of the NBS-987 Sr standard yielded an average value of 87Sr/86Sr = 0·710205 ± 23 (2
). Thus all Sr isotopic ratios in Table 6 have been corrected by +0·000035 to make them consistent with the accepted value of 0·71024 for this standard. Repeated analyses of our internal J-M standard yielded an average value of 143Nd/144Nd = 0·511095 ± 16 (2). The value of this standard differs by 0·000738 ± 0·000018 from that of the La Jolla standard, measured less frequently in our laboratory. Thus the measured J-M value corresponds to a La Jolla value of 0·511833. For this reason, all Nd isotopic ratios in Table 6 have been corrected by +0·000025 to make them consistent with the accepted value of 0·511858 for this standard. The blanks for Sr and Nd are negligible (<2 ng for Sr and 0·4 ng for Nd) compared with the quantities of Sr and Nd extracted from the samples.
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ACKNOWLEDGEMENTS |
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We are grateful to C. Spatz, S. Barda and A. Kohler for technical assistance. Our sincere gratitude goes to Mr and Mrs Vilain, T. Nardos, T. Yemane, D. Hailu and Yonas for their invaluable assistance during the field work. We are indebted to C. G. Barnes, W. J. Collins, V. Janousek and N. Petford for their thorough and constructive reviews, and to P. D. Kempton for her careful editorial handling. They helped us very much to improve this paper. This work was supported by a Ph.D. research grant to A.A. from the French Ministry of Foreign Affairs and by funding from INSU–CNRS Ethiopie 2000 Project. We would like to acknowledge the Department of Geology and Geophysics, Addis Ababa University, for logistical support during the field work. This paper is CRPG Contribution 1659.
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FOOTNOTES |
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* Corresponding author. Telephone: +251 1 55 32 14. Fax: +251 1 55 23 50. E-mail: asrata{at}geol.aau.edu.et
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