J__Petrology-2004-ASRAT-1147-79

Petrology and Isotope Geochemistry of thePan-African Negash Pluton, NorthernEthiopia: Mafic---Felsic Magma InteractionsDuring the Construction of Shallow-levelCalc-alkaline Plutons

A. ASRAT1,2*, P. BARBEY1, J. N. LUDDEN1, L. REISBERG1,G. GLEIZES3 AND D. AYALEW2

1CRPG---CNRS, 15, RUE NOTRE-DAME DES PAUVRES, BP 20, 54501 VANDOEUVRE-LE`S-NANCY CEDEX, FRANCE2DEPARTMENT OF GEOLOGY AND GEOPHYSICS, ADDIS ABABA UNIVERSITY, PO BOX 1176, ADDIS ABABA, ETHIOPIA

3CNRS---UMR 5563 LMTG, UNIVERSIT!EE PAUL SABATIER, 38, RUE DES TRENTE-SIX PONTS, 31400 TOULOUSE, FRANCE

RECEIVED JULY 15, 2002; ACCEPTED NOVEMBER 27, 2003

The Negash pluton consists of monzogranites, granodiorites, hybridquartz monzodiorites, quartz monzodiorites and pyroxene monzo-diorites, emplaced at 608 ! 7Ma (zircon U---Pb) in low-gradevolcaniclastic sediments. Field relationships between mafic and felsicrocks result from mingling and hybridization at the lower interface ofa mafic sheet injected into partially crystallized, phenocryst-laden,granodiorite magma (back-veining), and hybridization duringsimultaneous ascent of mafic and felsic magmas in the feeder zonelocated to the NW of the pluton. The rock suite displays low 87Sr/86Sr(608) (0"70260---0"70350) and positive eNd(608) values(3"9 to5"9), along with fractionated rare earth element patterns[(La/Yb)N 9"9---17"7], enrichment in large ion lithophile ele-ments (Ba, U, K, Pb and Sr) and depletion in Nb and Th comparedwith the primitive mantle. Monzogranites, granodiorites and hybridquartz monzodiorites define a calc-alkaline differentiation trend,whereas the quartz monzodiorites have higher Fe/Mg ratios. Thepyroxene monzodiorites show anomalously high Ti/Zr, Ti/Y andTi/V ratios, suggesting that they are cumulates. Chemical modellingsuggests that pyroxene and quartz monzodiorites could derive from acommon gabbrodioritic parent by fractional crystallization. Struc-tural and chemical data suggest that (1) the pluton results from theassembly of several, low-viscosity, melt-rich batches (sheeting/dyk-ing), differentiated in intermediate chambers prior to their emplace-ment; (2) in situ differentiation is limited to the coarse-grainedpyroxene monzodiorites; (3) mafic---felsic magma interactions at theemplacement level were essentially limited to mingling.

KEY WORDS: mafic---felsic intrusion; magma mingling; Ethiopia; Pan-African

INTRODUCTIONMafic---felsic magma interactions have been recognizedas important processes during the construction of graniticplutons (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; Janouseket al., 2000). Co-mingling is considered as the dominantprocess accounting for the structures observed in plutons,whereas thorough mixing is thought to occur in chambersat depth, prior to magma emplacement. Most studiesagree that mingling is related to the replenishment of afelsic magma chamber by mafic magma intrusion anddepends strongly on the relative viscosities of the mag-mas, which control the rheology (Fernandez & Barbarin,1991; Fernandez & Gasquet, 1994; Hallot et al., 1996).Wiebe & Collins (1998) provided a general model forthe formation of sheet-like bodies, which were describedby Wiebe (1993) as mafic and silicic layered intrusions(MASLI). Other studies have considered dynamic, two-way conduit mingling and hybridization during emplace-ment of magmas as an equally important process

JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 PAGES 11471179 2004 DOI: 10.1093/petrology/egh009

*Corresponding author. Telephone: 251 1 55 32 14. Fax: 251 1 5523 50. E-mail: [email protected]

Journal of Petrology 45(6) # Oxford University Press 2004; all rightsreserved

at Universidad Catlica del Norte on March 13, 2014

http://petrology.oxfordjournals.org/Downloaded from

Camila Beln Riera SotoEl Fe indica diferenciacin, y el Mg indica que proviene del manto.

Camila Beln Riera Soto

(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 ourstudy is to investigate the relationships between magmainteractions and the dynamics of pluton growth. In a

Fig. 1. (a) Geological sketch map of the Northern metamorphic terrain of Ethiopia (modified after Tadesse et al., 1999; Asrat et al., 2001).(b) Geological map of the Negash pluton along with sampling sites, the foliation trajectories in the surrounding country rocks, and the main septa ofthe country rocks within the pluton (modified after Asrat et al., 2003). (c) A synthetic cross-section along the line A---A0 marked in (b).

JOURNAL OF PETROLOGY VOLUME 45 NUMBER 6 JUNE 2004

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previous study (Asrat et al., 2003), we presented a structuralinvestigation of the pluton using the anisotropy of mag-netic susceptibility. We showed that the Negash plutondisplays two major types of mafic---felsic magma inter-actions: (1) injection of monzodioritic magma into felsicmagmas, which favoured in situ mingling of monzodior-ites and granodiorites along their contacts; (2) simulta-neous ascent of monzodioritic and granodioritic magmasthrough the same conduit, which led to thoroughhybridization and formation of homogeneous hybridmonzodiorites.In the present paper we use petrological, chemical and

isotopic data to (1) describe the systematic mineralogicaland geochemical variations of the whole suite, (2) con-strain 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 ofemplacement of shallow-level calc-alkaline plutons.

GEOLOGICAL SETTING, ROCK

TYPES AND FIELD RELATIONSHIPSThe Northern metamorphic terrain of Ethiopia consistsof a series of thick, inhomogeneous volcano-sedimentaryassemblages that belong to the Arabian---Nubian Shield(ANS) of the Pan-African orogen (900---500Ma). TheANS is a juvenile, subduction-related, accreted terraneformed by lateral crustal growth through arc---arc accre-tion (Krooner 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 volcanicassemblages are calc-alkaline and lack evidence of anypre-Pan-African continental crust. A review of the avail-able geochronological data (Asrat et al., 2001) suggests theexistence of three periods of granitic magmatism in boththe ANS and the Mozambique Belt (800---885, 700---780and 540---660Ma), 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 themiddle of a low-grade metamorphic inlier in the Mekele---Adigrat area (Fig. 1a), and is one of several calc-alkalineplutons, which occur to the north especially in the Axumarea. They are syn- to post-tectonic granites, monzogra-nites, granodiorites, diorites and subordinate gabbros,which have mantle-like Sr and Nd isotopic ratios andbelong to three magmatic events at 800, 750 and550Ma (Rb/Sr Sm/Nd and U---Pb zircon ages). Furtherdetails about these granites have been reported by Alemu(1998) and Tadesse et al. (2000).

Rock typesThe 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 atrend from the monzodiorite to the monzogranite fields.We distinguish: (1) coarse-grained pyroxene monzo-diorites and microgranular biotite---hornblende---quartzmonzodiorites, both containing variable proportions ofpyroxene (referred to as mafic rocks); (2) microgranular,biotite---hornblende---quartz monzodiorites, devoid ofpyroxene and with higher proportion of quartz andK-feldspar (referred to as hybrid rocks); (3) hornblende-bearing biotite tonalites, granodiorites and monzogra-nites (referred to as felsic rocks); (4) biotite---(muscovite)pegmatites, aplites and microgranites. An overviewof the modal compositions and textures is given inTable 1.

Field relationshipsThe pluton displays a crescent-shaped lithological zoningthat varies inwards from monzogranite through grano-diorite, quartz monzodiorite and pyroxene monzodiorite,to hybrid quartz monzodiorite and granodiorite in thenorthwestern part (Fig. 1b). Locally, granodiorites showigneous layering with modal grading. Dykes of aplite,pegmatite and microgranite (a few centimetres to 10mwide) are common near contact zones in the eastern andwestern parts of the pluton. Metamorphic septa(kilometre-long and hectometre-wide) are also commonthroughout the pluton and outline the crescent-shaped

1

2

3

4 5

6P

Q

A

Quartz monzodiorites

Pyroxene monzodiorites

Aplites and microgranitesHybrid quartz monzodiorites

Granodiorites, tonalites

Monzogranites

4. quartz monzodiorite quartz monzogabbro5. quartz diorite quartz gabbro6. monzodiorite monzogabbro

1. monzogranite

2. granodiorite

3. tonalite

50

50

Fig. 2. Quartz---Alkali feldspar---Plagioclase (Q---A---P) classificationdiagram (Streckeisen, 1976). Arrow indicates medium-K calc-alkalinedifferentiation trend (Lameyre & Bowden, 1982).

ASRAT et al. MAFIC---FELSIC MAGMA INTERACTION

1149



structure. Based on structural data, Asrat et al. (2003)concluded that:(1) the pluton was assembled by successive injection of

four magma pulses (monzogranite, granodiorite, pyrox-ene and quartz monzodiorites, quartz monzodiorite andgranodiorite) into already foliated country rocks;

(2) magmatic foliations and lineations convergetowards the NW, suggesting that the feeder zone islocated at the northwestern tip of the pluton;(3) the obliquity in the orientations of the magmatic

foliations and of the metamorphic septa at the northernand southern borders of the pluton is symmetrical

Table 1: Modal compositions and major petrographic characteristics of rocks from

the Negash pluton (modal compositions in vol. %)

Rock type Petrographic description

Monzogranite

Qtz (25---30), Kfs (15---25), Pl (30---40),

Bt (10---15), Hbl (5---10), Acc (%1)

& porphyritic to equigranular, coarse- to medium-grained; grain size of plagioclase and K-feldspar mayreach 20mm in length, whereas biotite and amphibole are 1---3mm long;

& allotriomorphic, normally and oscillatory zoned oligoclase (An11---30), which may locally contain smallmicas and quartz grains (sieve textures); microcline as phenocrysts or interstitial grains;Granodiorite

Qtz (20---25), Kfs (10---15), Pl (40---45),

Bt (10---15), Hbl (7---12), Acc (%1)& subhedral to euhedral biotites; large, unzoned calcic amphiboles (magnesiohornblendes);& interstitial quartz; occasional myrmekites and biotite---quartz symplectites;

Tonalite

Qtz (15---20), Kfs (5---10), Pl (50---55),

Bt (10---15), Hbl (10---15), Acc (%1)

& no significant alteration, with the exception of some transformation of plagioclase to epidote, sericite,muscovite and locally calcite in some samples (contact zones);

& abundant titanite with ilmenite and titanomagnetite inclusions; apatite and zircon are common accessories.

Quartz monzodiorite

Qtz (5---15), Kfs (%5), Pl (50---60),Bt (8---15), Hbl (10---20), Opx (%5),Cpx (%2), Acc (2---5)

& porphyritic to equigranular; minerals are medium- to fine-grained (0.5---10mm for plagioclase, 0.5---2mmfor ferromagnesian minerals);

& normally zoned oligoclase and andesine (An18---46); complex, patchy zoned plagioclase grains with resorbedrims and overgrowths; locally centimetre-long unzoned laths; K-feldspar xenocrysts;

& subhedral to euhedral biotites (some phlogopites); weakly and inversely zoned (Fe-rich core) amphibole(magnesiohornblende---ferrotschermakite); amphibole is commonly rimmed with actinolite;

Pyroxene monzodiorite

Qtz (0---5), Kfs (%3), Pl (50---65),Bt (10---20), Hbl (10---20), Opx (3---10),

Cpx (%5), Acc (%10)& prismatic orthopyroxene, locally abundant; restricted clinopyroxene;& abundant hexagonal, acicular (42mm long) apatite; ilmenite and rhombic to rounded magnetite andtitanomagnetite; titanite and zircon are common.

Hybrid quartz monzodiorite

Qtz (10---15), Kfs (%7), Pl (45---50),Bt (10---20), Hbl (10---20), Acc (%1)

& same texture and grain size as in the quartz monzodiorites;& normally zoned oligoclase---andesine (An16---33); occasional complex, patchy zoned plagioclases withresorbed rims and overgrowths; locally centimetre-long unzoned laths; K-feldspar xenocrysts;

& two types of biotite; weakly and normally zoned magnesiohornblende;& abundant titanite rimming ilmenite and titanomagnetite; abundant hexagonal, acicular (42mm long)apatite, and zircon;

& biotite, plagioclase and hornblende are partially replaced by epidote, sericite, chlorite, muscovite andoccasionally calcite close to contacts with the felsic rocks.

Pegmatite, aplite

Qtz (35---45), Kfs (30---40), Pl (15---25),

Bt (3---7), Acc (%1)

& quartz and K-feldspar locally as micrographic intergrowths;& normally zoned sodic plagioclase (An3---8);& subhedral to anhedral biotite;& accessories are pyrite and zircon.

Microgranite

Qtz (20---35), Kfs (15---25),

Pl (20---30), Bt (5), Ms

(20), Acc (%5)

& porphyritic with coarse plagioclase laths and quartz phenocrysts;& thin muscovite flakes; rare anhedral biotite;& accessories are pyrite and zircon.

Mineral abbreviations according to Kretz (1983). Acc, accessories.


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with respect to the NW---SE pluton axis, suggestingthat magma transfer was from the NW towardsthe SE.The relationships between monzogranites---granodiorites

and monzodiorites in the Negash pluton can besubdivided into three main types: (1) a large body ofquartz monzodiorites and pyroxene monzodioriteswithin the granodiorites displaying a complex contactzone (eastern and southeastern part of the pluton);(2) hybrid quartz monzodiorites intimately associatedwith granodiorites (northwestern part of the pluton);(3) widespread centimetre- to metre-sized quartzmonzodiorite enclaves in the monzogranites and grano-diorites. We report here only on the main features oftheir mutual relationships. Further information and

illustration of these relationships have been given byAsrat et al. (2003).

The southern granodiorite---monzodioritecontactThe cross-section A---A0 (Fig. 1c) shows that the largemonzodioritic body in the eastern and southeastern partof the pluton forms a shallowly dipping unit. Its lowercontact with the granodiorites is marked by (1) lobateinterfaces with interfingering of granodiorites into quartzmonzodiorites at a decametre scale (see Asrat et al., 2003,fig. 4a); (2) abundant granitic pipes several metres inlength and a few centimetres to c. 30 cm in diameter(Fig. 3a); (3) vertical, metre-wide dykes consisting of a

hbl

btbt

bt

bt

bt

(a)

(b)

(c) (e)

(d)

Fig. 3. Field photographs of the relationships between granodiorites and quartz monzodiorites: (a) granitic pipes and (b) K-feldspar-phenocrystladen microgranular mafic enclaves at the base of the mafic sheet (southeastern part); (c) mingling structures between highly porphyriticgranodiorites and quartz monzodiorites; (d) mingling between mafic and felsic rocks in the northwestern part (enclaves are parallel to a well-developedmagmatic foliation in the granodiorites); (e) partly resorbed biotite flakes within a euhedral hornblende from a hybrid quartz monzodiorite(scale bar represents 0"5mm).


1151



breccia of angular monzodioritic blocks within a more orless hybridized granitic matrix; (4) veins that cut throughthe monzodiorites and are extremely enriched in mega-crysts of K-feldspar set in a quartz and biotite ground-mass (see Asrat et al., 2003, fig. 4d); (5) evidence of localintense mingling between the felsic and mafic rocks(Fig. 3b and c).As shown by Asrat et al. (2003), the lineations in the

granodiorites are sub-horizontal and circumferential,whereas those in the monzodiorites (including thosenear the contact with the granodiorites) are orientedNW---SE and plunge gently towards the NW. Also, theanisotropy and fabric are very different: low and linear inthe monzodiorites, but higher and planar in the grano-diorites. These differences, suggesting local deformationregimes related to the successive emplacement of thecorresponding two magmas, are considered to reflectthe forceful emplacement, across the floor of the cham-ber, of a monzodiorite sheet leading to flattening of theadjacent partially crystallized granodiorites.

The northwestern part of the plutonThe northwestern part of the pluton consists dominantlyof quartz monzodiorites and granodiorites that showevidence for pervasive mingling and hybridization. Themafic and felsic lithologies are interleaved vertically or athigh angle ('45(), on a metre scale (Fig. 3d; see also Asratet al., 2003, fig. 5b). In some cases, the mafic rocks formdistinct lobes and boudins enclosed by the felsic rocks.The mafic rocks are variously hybridized, as suggested bytheir more leucocratic character and by the presence ofabundant rounded K-feldspar megacrysts along withocellar quartz grains. Subsequent discussion of hybridrocks or hybrid quartz monzodiorites refers to theserocks.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---felsiclayering along with sub-vertical foliation and sub-horizontal lineation patterns. This zone, which possiblyacted as a pathway for the successive uprise of magmasduring a short span of time, as evidenced by the mag-matic microstructures and contacts, is considered as theinferred feeder zone.

Microgranular monzodioritic enclavesMicrogranular monzodioritic enclaves are ubiquitous inisolation or as swarms in the monzogranites and grano-diorites. They are circular to elliptical, and centimetre- tometre-sized (see, e.g. Asrat et al., 2003, fig. 5d). Theycommonly contain rounded K-feldspar megacrysts,locally in high proportion (Fig. 3b). The contact withthe host monzogranite or granodiorite is sharp, orlobate with interfingering between the felsic and mafic

lithologies. They locally have quenched margins with thehost rocks.

MINERAL TEXTURE AND

CHEMISTRYQuartz and feldsparsQuartz occurs as both subhedral, 5mm in diameter,rounded crystals and as interstitial grains. Alkali feldsparin the felsic rocks occurs both as anhedral perthitic grains(Or94---97) in the groundmass and as centimetre-sizedphenocrysts. In the hybrid quartz monzodiorites, itoccurs 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 snow-flake textures in some quartz monzodiorites. It generallyexhibits discontinuous normal zoning with sodic rims.However, spongy calcic cores and reverse zoning arecommon. Plagioclase composition ranges from An11 toAn48 (Electronic Appendix: http://www.petrology.oupjournals.org), with 0"5---4% Or component. Largerplagioclase crystals (46mm) have more calcic coresthan smaller ones (52mm) within the same sample. Inthe felsic rocks, compositions are clustered (An11---30) witha median at An20 (Fig. 4a). Cores show compositionalvariations from sample to sample (An13---30) but littlevariation within the same sample. In the hybrid rocks,plagioclase displays (1) compositions intermediatebetween the felsic and mafic rocks (Fig. 4b) with coresdisplaying higher An contents (An16---33) than rims(An17---26), and (2) reverse zoning with calcic rims(An27---33) overgrowing normally zoned cores. In themafic rocks, compositions are scattered (Fig. 4c) with abimodal distribution (medians at An26 and An43). A largevariation in anorthite content of plagioclase cores occursfrom sample to sample (An18---45 in quartz monzodioritesand An28---48 in pyroxene monzodiorites) and in somecases within the same sample (e.g. An25---45 and An18---43).

Ferromagnesian mineralsPyroxenes do not exceed 5 vol. % of the mode ofthe quartz monzodiorites, but may exceed 10% in pyr-oxene monzodiorites. Orthopyroxene (66---79 mol %En,18---30%Fs and 2---5%Wo) occurs as large, yellowishgreen to colourless, prismatic grains (42mm) and assmall euhedral crystals (0"5---1mm). They show veryweak normal zoning from Mg-rich cores to Fe-richrims. Pigeonite (66---72 mol %En, 22---28%Fs and5---6%Wo) occurs as discrete grains in association withorthopyroxene.Green to brownish green amphiboles occur both in

mafic and felsic rocks. The hybrid quartz monzodioritescontain euhedral hornblendes with inclusions of resorbed


1152



biotite in their cores (Fig. 3e). Amphiboles are generallycalcic and Al2O3 rich (46"0 wt %; Electronic Appendix:http://www.petrology.oupjournals.org). They are mag-nesiohornblendeor tschermakite in themonzogranitesandgranodiorites (XMg 0"52---0"63), in the hybrid quartzmonzodiorites (XMg 0"55---0"73) and in the pyroxenemonzodiorites (XMg 0"47---0"79, Fig. 5a). They aremagnesiohornblende, tschermakite or ferrotscherma-kite in the quartz monzodiorites (XMg 0"43---0"58).

Amphiboles in the felsic rocks show mainly the eden-ite---hornblende substitution trend, whereas in the maficand hybrid rocks they show pargasite---tschermakite---hornblende substitution (Fig. 5b). Fe3 content recast bystoichiometry is negligible, in most cases close to zero.Actinolitic retrogression rims are observed in amphibolesfrom the pyroxene monzodiorites.Biotite forms large subhedral to euhedral crystals

(5---10mm in length) with numerous inclusions of Fe---Tioxides and apatite. Dendritic biotite was observed locallyin some hybrid rocks. The biotites are unzoned and mosthave XMg 5 0"66 (Electronic Appendix: http://www.petrology.oupjournals.org) and correspond to biotitesensu stricto (Fig. 6). Biotites in the felsic rocks cluster atnearly the same XMg (0"53---0"58), whereas those in thequartz monzodiorites show higher variation in XMg(0"46---0"62). Biotites from pyroxene monzodiorites arephlogopites (XMg 0"69---0"73). In the hybrid quartzmonzodiorites, they fall in two groups: (1) those fromsamples collected at the contact zones with the maficand felsic rocks have XMg (0"54---0"58) similar to thequartz monzodiorites; (2) those collected away from con-tact zones are more magnesian (XMg 0"61---0"68).

Accessory mineralsZircon is an accessory phase in all rocks of the suite. Itcommonly occurs as euhedral, prismatic to bipyramidalcrystals, up to 400 mm in length in granodiorites andmonzogranites. Backscattered scanning electron (BSE)images of zircon grains show that they consist of euhedralmagmatic 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 2mmlong and as needle-like inclusions in plagioclase, ferro-magnesian and oxide minerals, suggesting early crystal-lization. It is present in all rock types, but is especiallyabundant in the hybrid and mafic rocks (up to 2"5% ofthe 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. Itsmajor-element composition is homogeneous in all rocktypes, except for very slight enrichment in FeO(0"5---1"8 wt %) in the rims.In the felsic and hybrid rocks, ilmenite and titanomag-

netite (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 theserocks. In the mafic rocks, euhedral or rounded magnetitecrystals are found in addition to ilmenite and titanomag-netite grains; the magnetite occurs both in isolation andwithin titanite. Fe---Ti oxides do not exceed 5 vol. % inthe quartz monzodiorites, but may reach 10% of themode in the pyroxene monzodiorites.

An%

0

8

16

N

10 20 30 5040

(b)

0

8

16

N

10 20 30 5040

(a)

0

8

16

NMafic rocks

(N = 72)

(c)

10 20 30 5040

Felsic rocks(N = 100)

median = 20.4mean = 20.3 + 3.8_

(N= 24)median= 42.6

mean = 41.3 + 4.2_

(N= 48)median= 26.0

mean = 26.3 + 3.4 _

Hybrid rocks(N = 47)

median = 21.8mean = 22.0 + 3.6_

Fig. 4. Histograms of the plagioclase anorthite content in (a) monzo-granites and granodiorites, (b) hybrid quartz monzodiorites and(c) quartz monzodiorites and pyroxene monzodiorites.


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CONDITIONS OF EMPLACEMENTDetermining temperature and pressure conditions ofmagma emplacement and consolidation, aswell asmagmawater content, is a prerequisite for estimating magmaviscosity, which, in turn, directly controls emplacementand interaction mechanisms. However, it should benoted that these data, derived from mineral assemblages,are strongly dependent on calibration and, therefore, canprovide only provisional geological information. A sum-mary of thermobarometric and oxybarometric data dis-cussed below is given in Table 2.

Apatite and zircon thermometryApatite 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 sus-pected to be cumulates (see Discussion). P2O5 decreaseswith increasing SiO2 (Fig. 7a), implying crystallization ofapatite and, hence, saturation of the parent melts inP2O5. Zr decreases with increasing SiO2 only in thehybrid and felsic rocks (Fig. 7b), suggesting crystallizationof zircon and, therefore, melt saturation in Zr only forthese rocks. The crystallization of apatite and zirconis also consistent with the decrease in Nb and Y fromthe quartz monzodiorites to the monzogranites (Fig. 7cand d).The apatite thermometer gives temperatures in the

following ranges: 836---886(C (felsic rocks), 790---912(C(hybrid quartz monzodiorites) and 824---950(C (quartzmonzodiorites). The zircon thermometer applied only

0.0

0.5

1.0

0.9

XM

g

Si

Magnesiohornblende Tschermakite

FerrotschermakiteFerrohornblende

Ferro-actinolite

Actinolite

Tremolite

8.0

8.0

7.5 7.0 6.5 6.0 5.5

5.5

PargasiteEdenite

TschermakiteTremolite

0.0

0.5

1.0

(Na +

K) A




Hybridmonzodiorites

(a)

(b)

Monzogranitesand granodiorites

Hybridmonzodiorites

Hybrid quartz monzodiorites

Pyroxene monzodioritesQuartz monzodiorites

Granodiorites, tonalitesMonzogranites

Hornblende Pyroxene monzodiorites


Fig. 5. Composition of amphiboles in the classification diagrams of (a) Leake et al. (1997) and (b) Deer et al. (1992). Na, K and Si in atoms performula unit.


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Camila Beln Riera SotoIndicardores de P y T

2.0

2.2

2.4

2.6

2.8

3.0

Al V

I

1.00.2 0.4 0.6 0.8

XMg

PH

LO

GO

PIT

E

BIO

TIT

E

Eastonite

Phlogopite

Quartz monzodiorites Pyroxene monzodiorites


Hybridmonzodiorites




Granodiorites, tonalites

Monzogranites

Fig. 6. Composition of biotites in the classification diagram of Deer et al. (1992). AlVI in atoms per formula unit.

Table 2: Summary of averages and ranges of temperature ( (C), log fO2 (bar) and pressure (kbar)estimated for the Negash granitoids

Rock type, method Tmin Tav Tmax fO2min fO2av fO2max Pmin Pav Pmax

Monzogranite

Ap 846 863 880

Zrn 741 754 767

Hbl---Pl 684 719 762

Fe---Ti 536 605 682 )22.9 )19.9 )16.9Al---Hbl 2.2 3.4 4.6

Granodiorite, tonalite

Ap 836 864 886

Zrn 737 756 768

Hbl---Pl 697 734 786

Fe---Ti 598 675 753 ---20.1 ---17.5 ---14.9

Al---Hbl 2.3 3.1 4.5

Hybrid quartz monzodiorite

Ap 790 840 912

Zrn 712 739 756

Hbl---Pl 682 732 788

Fe---Ti 521 589 622 )23.5 )20.5 )19.2Al---Hbl 2.4 3.0 3.5

Quartz monzodiorite

Ap 729 879 950

Hbl---Pl 724 796 914

Fe---Ti 594 680 770 )20.3 )17.6 )14.3

Thermobarometric methods: Ap, apatite saturation thermometry (Harrison &Watson, 1984); Zrn, zircon saturation thermo-metry (Watson & Harrison, 1983); Hbl---Pl, hornblende---plagioclase thermometry (Blundy & Holland, 1990); Fe---Ti, ilmenite---titanomagnetite thermometry and oxybarometry (Spencer& Lindsley, 1981); Al---Hbl, Al-in-hornblende barometry (Anderson& Smith, 1995).


1155



to the hybrid and felsic rocks gives lower temperatureestimates of 737---768(C and 712---756(C, respectively.Preservation of high temperatures in the quartz monzo-diorites is consistent with the fine-grained textures sug-gesting quenching. Discrepancy between apatite andzircon thermometry has been attributed to several possi-ble causes: (1) excess apatite, which may not fractionateefficiently from the melt; (2) the saturation model may notbe appropriate for the rocks considered (e.g. Hoskin et al.,2000); (3) apparent saturation may be due to local dis-equilibrium (Bacon, 1989); and/or (4) lower temperatureestimates of zircon thermometer representing tempera-tures closer to the solidus (e.g. Wyllie, 1984; Anderson,1996). Our data fall into the low-temperature granitecategory defined by Miller et al. (2003), although they lackinheritance. Accounting for the fact that apatite is anearly crystallized phase in the Negash pluton, we suggestthat the lower temperatures given by zircon thermometry,compared with those obtained from apatite, reflect melt

Zr undersaturation at the source. In this case, tempera-tures obtained from apatite thermometry should be closerto liquidus temperatures, whereas those obtained fromzircon thermometry should be considered as minimumestimates.

Hornblende---plagioclase thermometryHornblende---plagioclase thermometry (Blundy &Holland, 1990) can be applied to rocks that crystallized inthe interval 550---1100(C. The prerequisite for the appli-cation of this method is that plagioclase should be lessanorthitic thanAn92 and the amphiboles should have Si57"8 a.p.f.u. The pressure range used in the temperatureestimation (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 othermethods: the felsic and hybrid rocks give temperatureestimates of 684---786(C and 682---788(C, respectively,

0.0

0.5

1.0

1.5

706560555045SiO2

706560555045SiO2

50

100

150

200

250

300




Granodiorites

Monzogranites

(b) (d)

(a) (c)

Zr

P2O5

0

5

10

15

20

5

10

15

20

25

Y

Nb

Fig. 7. SiO2 vs (a) P2O5, (b) Zr, (c) Nb, and (d) Y. !, quartz monzodiorite samples N9-2 and N9-47; shaded inverted triangles, hybrid quartzmonzodiorite samples N9-13, N9-19 and N9-41.


1156



whereas the mafic rocks give temperatures of795---856(C.

Al-in-hornblende barometryThe Al-in-hornblende barometer of Anderson & Smith(1995), which takes into account the temperature depen-dence, was applied to hornblendes in rocks that containthe recommended seven-phase assemblage (hornblende,biotite, plagioclase, K-feldspar, quartz, titanite andFe---Ti oxides). The Al contents of amphibole cores andrims show no significant difference. The temperaturerange (710---950(C) used in pressure estimation is thatdetermined from apatite and zircon thermometry. Thepressures obtained for the felsic and hybrid rocks rangefrom 2"2 to 4"6 kbar, with averages ranging from 3"0 to3"4 kbar (Table 2). These values are reasonable estimatesconsidering the presence of andalusite in the contactaureole, which implies pressure lower than 4"5 kbar,according to the position of the Al-silicate triple pointgiven by Pattison (1992), which appears to be the mostreliable (Cesare et al., 2003).

Fe---Ti oxide thermobarometryThe Fe---Ti oxide thermometer and oxybarometer(Spencer & Lindsley, 1981) was applied to coexistingilmenite and titanomagnetite that satisfy the test ofBacon & Hirschmann (1988). The estimated ranges intemperatures are 536---753(C (felsic rocks), 521---622(C(hybrid rocks), 594---770(C (quartz monzodiorites) and615---742(C (pyroxene monzodiorites). All these valuesare 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 rela-tively low fO2 close to the fayalite---magnetite---quartz(FMQ) buffer. However, some mafic samples with highertemperature of equilibration plot close to the nickel---nickel oxide (NNO) buffer (Fig. 8). The fO2 estimatesare also likely to represent values re-equilibrated duringcooling, as suggested by the regular decrease in log fO2with falling temperature. The temperatures and fO2indicate that titanite was stable in the presence of quartzand magnetite, consistent with petrographic data.

Magma water contentMagma water content is another important parameter, inaddition to P---T---fO2 conditions, that influences meltcompositions, crystallization conditions and viscosity ofgranitic 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 asmelt composition, make determination of this parameterdifficult. Water content can be estimated empirically bycomparison with available experimental data. Scailletet al. (1998) indicated that most silicic volcanic rocks andtheir plutonic equivalents have a dissolved H2O contentof 4---6 wt % for a wide temperature range (700---900(C).Scaillet & Evans (1999) proposed an experimental cali-bration of water content in magma of dacitic com-position, at 2"2 kbar and log fO2 NNO 2"7. TheP---T---fO2 conditions of emplacement of the Negash plu-ton, which are not far from these experimental data,allow the water fugacity to be roughly estimated for theparent melt of the felsic rocks. Accordingly, the phaserelationships in these rocks (stability of hornblende,absence of orthopyroxene, and plagioclase less anorthiticthan An50) suggest water contents '6 wt %. On the basisof the experimental data of Scaillet & Evans (1999) andusing representative Altot values of hornblende (1"3---1"5a.p.f.u.), the water content in the 750---850(C tempera-ture range is estimated to be of the order of 5"5---6"5 wt %for the felsic rocks. In contrast, the water content ofthe mafic rocks was probably significantly lower, asdeduced from the presence of pigeonitic pyroxene inthe less evolved compositions (pyroxene monzodiorites).In summary: (1) apatite thermometer yields tempera-

tures close to the liquidus (from 836---886(C for thefelsic rocks to 824---950(C for the quartz monzo-diorites), whereas the lowest Fe---Ti oxide temperatures(550---750(C) suggest near-solidus or subsolidus re-equilibration; (2) fO2 values, although within the rangeof 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

log

fO2 (

bars

)

IW

MH

FMQ

-10

-15

-20

-25

tit + m

ag +

qtz

hed+

ilm

tit+

fay

500 600 700 800


Hybrid quartz monzodioritesQuartz monzodiorites

GranodioritesMonzogranites

T(C)

hed +

ilm

NNO

Fig. 8. Plot of log fO2 vs temperature. All buffers are from Frost(1991); titanite magnetite quartz ilmenite hedenbergite O2 and titanite fayalite ilmenite hedenbergite O2 equilibriafrom Wones (1989).


1157



to NNO); (3) phase relationships suggest that H2O con-tents were high in the felsic rocks but low in the pyroxenemonzodiorites; (4) the presence of andalusite in thecontact aureole, as well as pressure estimates, suggest ashallow level of emplacement, as generally observed insimilar types of plutons.

MAJOR AND TRACE ELEMENT DATAElement abundancesWhole-rock compositions (Tables 3 and 4) display a bulkmedium- 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, withthe exception of the most differentiated ones, and theAl2O3/(CaO Na2OK2O)molar and Al2O3/(Na2OK2O)molar ratios are negatively correlated, forming arough linear trend from the mafic to the felsic rocks(Fig. 9b). The pegmatite, aplite and microgranite dykesare slightly peraluminous and form two distinct groups:sodic with K2O 52 wt % and potassic with K2O44 wt %. MgO and FeO contents range from 1"73 to8"97 and from 3"16 to 14"17 wt %, respectively, goingfrom the felsic to the mafic rocks. The whole suite can,therefore, be considered as magnesian according to theclassification of Frost et al. (2001). The rocks define a calc-alkaline trend in the AFM triangular plot (Fig. 9d), withthe exception of the quartz monzodiorites, which displayhigher Fe/Mg ratios (tholeiitic affinity).Pyroxene monzodiorites, granodiorites and monzo-

granites have similar mg number [MgO/(MgO FeOtot)] values of 0"36---0"39. They differ from the quartzmonzodiorites, most of which are significantly less mag-nesian (mg number 0"28---0"30); samples N9-2 and N9-47 are exceptions that show mg number (0"36) similar tothose of the former rock types. Mg number of hybridrocks are similar to those of the pyroxene monzodioritesand felsic rocks (0"36---0"37), with the exception of twosamples (N9-16 and N9-44) that have higher mg numbervalues (*0"48). The pyroxene monzodiorites and thequartz monzodiorites with the lowest silica contents areremarkably 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 rangingfrom 9"9 to 17"7. LREE fractionation decreases fromfelsic [(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 pyrox-ene monzodiorites have similar normalized La and Ybvalues (LaN 40---70, YbN 3---5), whereas the quartzmonzodiorites have overlapping to higher LaN (50---100)

and higher YbN (5---7); the hybrid quartz monzodioriteshave the highest LaN contents (LaN 70---120). Althoughthe Negash pluton can be described as calc-alkaline, theREE patterns are unlike typical Andean calc-alkalineplutons where middle and heavy REE (MREE andHREE) flatten out at about 10 times chondrite (Atherton& Sanderson, 1985). Rather, they closely resemble thoseof the Mesozoic plutonic rocks from Patagonia (Rapela &Pankhurst, 1996).

Inter-element relationshipsPrimitive mantle-normalized trace element patterns(Fig. 11) indicate similar geochemical characteristics forall of the rock types, although some differences appear inthe relative sizes of the peaks and troughs. Most samplesshow spikes in Ba, U, K, Pb and Sr and troughs in Rb, Thand Nb, but the trough in Nb is more pronounced in thehybrid and felsic rocks than in the mafic rocks. Th/Uratios (1"8---3"9) fall in the range of values (2---4) publishedfor medium-K suites (Gill, 1981). The REE and othertrace-element patterns of the quartz monzodiorites aresimilar 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 pyroxenemonzodiorites are similar to that of island-arc basaltsfrom Vanuatu (Peate et al., 1997), with the exception ofhigher Nb, strong positive anomalies in Ti and P, andlower HREE. The felsic and hybrid rocks show similarconcave-upward REE patterns (Fig. 10) and troughs in Pand Ti and spikes in Zr.Plots of selected major and trace elements vs SiO2 are

presented in Fig. 12a---f for elements linked mainly tofeldspars and in Fig. 12g---l for elements linked to ferro-magnesian and oxide minerals. Element concentrationsdisplay systematic variations, which can be summarizedas follows.(1) Al2O3, K2O, Na2O, Sr and Ba increase in both

the mafic and hybrid rocks, whereas Al2O3, Na2O, Srand to some extent Ba decrease in the felsic rocks, withincreasing silica content. CaO is anti-correlated withAl2O3 and decreases from pyroxene monzodiorites tomonzogranites. Rb behaves as an incompatible elementin the whole suite. On the whole, MgO, FeOtot, TiO2,V, Cr and Ni decrease with increasing silica in all rocktypes.(2) Pyroxene monzodiorites have high FeOtot, MgO,

CaO, TiO2, V, Cr and Ni but low Al2O3 contents. Threesamples have high TiO2 contents ('4"5 wt %) and onesample (N9-25) high P2O5 concentrations (1"13 wt %).Even though high TiO2 contents can be found in tholeii-tic and alkali basalts, the pyroxene monzodiorites showhigher Ti/Zr and Ti/Y ratios (200---274 and 1587---2444,respectively) compared with the primitive mantle (116,


1158



Table3:Whole-rockmajor(wt%)andtrace(ppm

)elementcompositionofmonzodioriticrocksfrom

theNegashpluton

Rock

type:

Pyroxenemonzodiorites

Quartz

monzodiorite

Hybrid

quartz

monzodiorite

Sam

ple:

N9-25

N9-21

N9-29

N9-42

N9-3

N9-47

N9-4

N9-43

N9-2

N9-24

N9-19

N9-13

N9-41

N9-16

N9-44

SiO

246. 66

48. 11

48. 30

49. 67

49. 76

51. 48

53. 24

53. 74

53. 87

55. 74

50. 99

53. 12

53. 74

55. 18

58. 31

Al 2O3

13. 00

10. 01

14. 61

16. 13

15. 48

13. 15

16. 17

16. 32

13. 49

15. 64

17. 35

18. 09

18. 40

14. 59

16. 07

Fe 2O3tot

12. 61

14. 17

11. 74

11. 47

11. 53

11. 46

10. 37

9.40

10. 72

9.06

9.48

8.21

7.57

7.57

6.04

MnO

0.14

0.16

0.13

0.13

0.13

0.14

0.12

0.10

0.13

0.10

0.10

0.09

0.09

0.10

0.09

MgO

7.03

8.97

6.76

4.41

4.83

6.35

3.58

3.71

5.97

3.84

5.50

4.52

4.19

6.98

5.37

CaO

8.91

9.15

7.91

7.85

7.94

7.26

6.49

6.32

5.68

5.73

5.77

6.01

5.65

5.69

5.33

Na 2O

2.86

2.23

3.08

3.84

3.64

3.11

4.10

4.24

2.92

4.07

4.56

4.75

4.85

3.32

4.48

K2O

0.91

0.93

1.31

1.37

1.05

1.30

1.95

1.98

2.08

1.99

2.87

2.09

2.63

2.90

2.44

TiO

24.86

4.76

4.48

3.09

3.32

4.06

2.68

2.49

2.76

2.27

2.08

1.48

1.58

1.10

1.02

P2O5

1.13

0.70

0.38

1.26

1.22

0.52

1.01

0.87

0.68

0.72

0.43

0.54

0.46

0.28

0.48

LOI

0.62

0.76

1.66

1.21

1.48

0.68

0.46

0.82

1.72

0.83

0.87

1.14

0.87

1.87

0.78

Total

98. 73

99. 95

100.36

100.43

100.38

99. 51

100.17

99. 99

100.02

99. 99

100.00

100.04

100.03

99. 58

100.41

Ba

440

373

535

535

636

557

738

884

775

859

726

963

833

779

883

Cs

0.3

0.5

0.3

0.5

0.3

0.5

0.7

0.6

0.9

1.0

1.1

0.7

0.7

1.2

0.6

Rb

1714

2022

1621

3538

3739

6240

5060

49

Sr

961

656

906

1318

1443

874

1077

1171

791

1085

1067

1391

1326

871

1581

Y18

1811

2223

1623

2219

2022

2221

1415

Zr

146

127

9899

120

163

171

219

202

197

275

251

191

164

201

Ta

0.9

0.8

0.8

1.0

0.9

0.8

1.0

0.9

0.8

0.8

1.8

0.5

1.1

0.6

0.6

Hf

3.2

3.0

2.3

2.5

2.8

3.6

3.8

5.0

4.5

4.2

6.2

5.1

4.0

3.9

4.7

Nb

1313

1115

1311

1713

1212

209

147

7

Pb

3.1

3.4

3.4

5.3

4.7

4.9

6.4

8.0

5.9

7.0

7.8

7.5

7.3

5.4

7.1

Th

1.0

1.1

0.8

1.2

1.0

1.2

2.2

2.4

1.2

1.4

4.2

2.7

2.3

2.9

1.8

U0.4

0.59

0.3

0.6

0.5

0.5

0.7

0.9

0.6

0.7

1.7

0.7

0.9

1.1

0.8

V224

246

198

198

189

199

142

138

146

136

178

165

150

148

115

Ni

124

187

167

3249

131

1832

103

4785

6674

196

146

Cr

278

561

448

5363

358

1945

303

88140

87129

389

238

Cu

6072

5040

3650

2930

3635

7849

3813

12

Zn

141

161

124

143

128

138

149

126

146

120

121

9494

100

88


1159



Rock

type:

Pyroxenemonzodiorites

Quartz

monzodiorite

Hybrid

quartz

monzodiorite

Sam

ple:

N9-25

N9-21

N9-29

N9-42

N9-3

N9-47

N9-4

N9-43

N9-2

N9-24

N9-19

N9-13

N9-41

N9-16

N9-44

La

23. 3

17. 4

13. 3

26. 9

26. 4

18. 7

37. 6

34. 6

23. 2

26. 9

36. 5

48. 3

28. 5

24. 9

34. 0

Ce

54. 7

44. 3

30. 9

66. 3

64. 8

42. 4

81. 8

77. 8

56. 2

62. 4

83. 0

102.0

68. 2

54. 1

73. 7

Pr

7.7

6.2

4.4

9.5

9.1

5.9

10. 6

10. 4

7.3

8.2

10. 3

11. 8

8.6

6.4

9.1

Nd

34. 2

28. 6

1940. 4

41. 5

24. 8

45. 5

42. 9

32. 6

34. 1

42. 6

43. 6

34. 5

23. 9

36. 8

Sm

7.3

6.6

4.0

9.1

8.8

5.5

9.2

8.6

6.9

7.1

7.9

7.5

6.8

4.3

6.2

Eu

2.6

2.1

1.7

3.1

3.2

1.9

3.3

2.8

2.3

2.6

2.3

2.4

1.9

1.4

1.7

Gd

6.3

5.7

3.6

7.4

7.8

5.4

7.5

6.8

6.0

6.0

6.3

5.7

5.1

3.3

4.3

Tb

0.75

0.7

0.50

0.91

0.96

0.59

0.99

0.90

0.78

0.78

0.87

0.80

0.70

0.50

0.58

Dy

3.90

3.64

2.51

4.53

5.18

3.30

4.90

4.67

3.81

3.93

4.62

4.14

3.94

2.80

3.24

Ho

0.60

0.60

0.43

0.81

0.80

0.59

0.77

0.81

0.71

0.74

0.80

0.80

0.74

0.48

0.54

Er

1.45

1.54

1.06

1.93

1.94

1.51

1.96

2.06

1.71

1.73

2.06

2.03

1.93

1.27

1.47

Tm

0.20

0.19

0.14

0.22

0.25

0.20

0.25

0.28

0.23

0.25

0.30

0.31

0.27

0.18

0.19

Yb

1.08

1.12

0.82

1.53

1.46

1.26

1.42

1.7

1.49

1.53

1.68

1.84

1.77

1.19

1.26

Lu

0.16

0.17

0.11

0.20

0.22

0.16

0.21

0.21

0.24

0.22

0.29

0.29

0.28

0.19

0.20

Mgno.

0.38

0.41

0.39

0.30

0.32

0.38

0.28

0.30

0.38

0.32

0.39

0.38

0.38

0.51

0.50

A/C

NK

0.59

0.47

0.70

0.73

0.72

0.67

0.78

0.79

0.77

0.81

0.82

0.86

0.87

0.77

0.81

SREE

144

119

82173

172

112

206

195

143

156

200

232

163

125

173

Eu/Eu+

1.1

1.0

1.4

1.1

1.2

1.1

1.2

1.1

1.1

1.2

1.0

1.1

0.9

1.1

0.9

(La/Yb) N

14. 4

10. 4

10. 8

11. 8

12. 1

9.9

17. 7

13. 6

10. 4

11. 8

14. 5

17. 6

10. 8

14. 0

18. 0

(Gd/Y

b) N

4.7

4.1

3.5

3.9

4.2

3.4

4.2

3.2

3.2

3.1

3.0

2.4

2.3

2.2

2.7

Fe 2O3tot,totalironas

Fe3;LOI,loss

onignition.MgnumberMgO/(MgOFeO

tot);A/C

NKAl 2O3/(CaO

Na 2OK2O)molar.

Table3:continued


1160



Table 4: Whole-rock major (wt%) and trace (ppm) element composition of granitic rocks from the Negash

pluton

Rock type: Granodiorite Monzogranite

Sample: N9-39 N9-17 N9-36 N9-45 N9-23 N9-37 N9-15 N9-30 N9-14

SiO2 59.6 61.89 62.05 64.58 65.64 66.14 68.28 64.66 66.46

Al2O3 18.21 16.75 17.25 16.83 17.10 16.09 15.28 16.73 15.7

Fe2O3tot 5.40 5.17 4.64 3.76 3.39 3.45 3.16 3.74 3.7

MnO 0.05 0.06 0.05 0.04 0.03 0.03 0.04 0.04 0.05

MgO 3.33 3.04 2.75 2.12 1.74 2.03 1.73 2.25 2.02

CaO 4.00 4.05 3.30 3.37 3.03 3.04 2.55 3.06 2.85

Na2O 5.16 4.75 4.82 4.98 5.20 4.77 4.33 4.77 4.52

K2O 2.67 2.60 3.43 2.85 2.68 3.15 3.52 3.23 3.31

TiO2 0.82 0.91 0.66 0.61 0.55 0.54 0.51 0.52 0.57

P2O5 0.23 0.25 0.17 0.18 0.15 0.16 0.14 0.14 0.16

LOI 0.54 0.55 0.87 0.68 0.49 0.54 0.41 0.85 0.63

Total 100.01 100.02 99.99 100.00 100.00 99.94 99.95 99.99 99.97

Ba 947 1049 953 1120 1053 779 774 952 809

Cs 0.6 0.9 1.0 0.4 1.3 0.7 1.4 1.2 1.5

Rb 56 48 65 39 55 54 70 63 66

Sr 1381 1261 1119 1125 1075 978 908 1120 978

Y 8 16 8 9 10 8 11 8 10

Zr 133 192 133 162 158 125 154 119 162

Ta 0.3 0.6 0.3 0.3 0.4 0.3 0.4 0.3 0.5

Hf 2.9 4.5 3.1 3.9 3.6 3.2 4.1 2.9 4.3

Nb 3 7 4 4 4 4 5 3 5

Pb 7.0 9.3 8.2 8.9 9.9 8.3 13.8 11.7 15.3

Th 1.0 2.1 1.7 0.9 1.7 1.6 5.1 2.7 4.4

U 0.4 0.9 0.7 0.5 0.8 0.7 1.4 1.0 1.4

V 100 99 81 66 66 61 53 65 64

Ni 62 52 51 34 26 36 27 40 34

Cr 104 93 80 56 42 63 50 66 59

Cu 26 33 ------- ------- 86 6 7 30 33

Zn 79 76 68 54 51 50 47 53 57

La 13.8 22.6 15.5 14.9 12.8 15.2 23.4 16.6 22.7

Ce 29.7 54.6 29.4 33.0 28.2 30.6 44.0 32.0 45.6

Pr 3.6 6.9 3.4 4.0 3.6 3.5 4.9 3.8 5.3

Nd 14.7 26.9 13.2 15.4 14.0 13.6 18.7 14.2 19.1

Sm 2.7 5.0 2.1 2.5 2.8 2.4 3.3 2.5 3.5

Eu 0.9 1.5 0.8 0.9 0.9 0.85 0.97 0.74 0.93

Gd 2.0 3.8 1.8 2.3 2.2 1.92 2.32 2.04 2.40

Tb 0.28 0.56 0.25 0.32 0.33 0.27 0.36 0.29 0.34

Dy 1.81 3.06 1.44 1.65 1.77 1.53 1.82 1.60 1.74

Ho 0.32 0.54 0.25 0.34 0.35 0.30 0.37 0.28 0.34

Er 0.85 1.46 0.77 1.01 0.78 0.83 0.97 0.76 0.92

Tm 0.13 0.21 0.11 0.14 0.13 0.11 0.14 0.14 0.14

Yb 0.84 1.34 0.75 0.86 0.85 0.73 0.97 0.76 0.86

Lu 0.13 0.20 0.13 0.14 0.13 0.12 0.14 0.12 0.15


1161



286), normal mid-ocean ridge basalt (N-MORB; 82, 273)and ocean island basalt (OIB; 61, 593) values of Sun &McDonough (1989),orcomparedwithIABvalues (73,261)of Peate et al (1997). They also have high P/Zr ratios (17---34) compared with subalkaline basalts [%15 according toWinchester & Floyd (1976)]. Ti/V ratios (116---136) arealso anomalously high for basaltic melts, which are com-monly%50 (Woodhead et al., 1993) and reach 100 only inOIB and alkali basalts (Shervais, 1982). The positive Tiand P anomalies are especially clear in the multi-elementplot of Fig. 11d.(3) 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-rockand mineral compositions and lower Cr and Ni contents.Only two samples, N9-2 and N9-47, have mg numbersimilar to those of the other lithologies, but they havelower Al2O3 and Sr and higher Ni and Cr contents(Table 3 and Fig. 12). The three pyroxene monzodioriteswith the lowest silica contents (Table 3) have high TiO2contents (3"09---4"04 wt %).(4) Three samples of hybrid quartzmonzodiorites (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 miner-alogically distinguishable by their high modal abundanceof K-feldspar xenocrysts. The remaining two samples(N9-16, N9-44) have higher MgO, Cr and Ni contentsand significantly higher mg number.

ISOTOPE GEOCHEMISTRYZircon U---Pb isotopic dataZircons were extracted from a monzogranite samplecollected at the northeastern border of the Negash plu-ton. The most transparent, inclusion-free, and fully euhe-dral zircon grains with brilliant surfaces and sharp

edges were hand-picked from a 50---200 mm fraction.Four groups of zircons were identified on the basis of size,colour, and morphology:

group (i), 5100 mm, colourless, transparent, euhedral,prismatic to bipyramidal;

group (ii), 100---150 mm, transparent, colourless, euhe-dral, elongated to acicular and prismatic;

group (iii), 100---150 mm, rose to pink, euhedral,bipyramidal;

group (iv), 150---200 mm, rose to pink, euhedral, prismatic.

The internal structures of selected zircon grains wereobserved by BSE microscopy, before and after isotopicmeasurements. None of the grains are metamict andall display well-preserved euhedral, systematic growthzones around nearly euhedral cores (Fig. 13), suggestinga magmatic origin without subsequent resorption andrecrystallization.Sixteen zircon grains representing all four groups were

analysed using a CAMECA IMS-1270 ion microprobe atCNRS---CRPG, Nancy. Details of analytical and workingconditions have been given by Deloule et al. (2002). Theage calculations are based on the isotopic ratios correctedfor background noise and common lead (using 204Pb).The U and Pb abundances are calculated on the basisof the Zr2O vs UO2 correlation for the standard zircon91500 with an age of 1062"4! 0"4Ma (Wiedenbeck et al.,1995). The relative sensitivity factor for Pb and U usedfor samples was defined from an empirical linear relation-ship between UO/U and Pb/U (Compston et al.,1984), using all the measurements performed on thestandards. The 207Pb/206Pb ratios are directly deter-mined from each spot analysis.Results are given in Table 5 and concordia diagrams

are shown in Fig. 13. Weighted mean ages and discordialines were determined using the Isoplot program (Ludwig,1991). The four zircon groups give the following results.

Rock type: Granodiorite Monzogranite

Sample: N9-39 N9-17 N9-36 N9-45 N9-23 N9-37 N9-15 N9-30 N9-14

Mg no. 0.41 0.40 0.40 0.39 0.36 0.40 0.38 0.40 0.38

A/CNK 0.98 0.93 0.98 0.97 1.01 0.96 0.98 0.99 0.97

SREE 72 129 70 77 69 72 102 76 104

Eu/Eu+ 1.2 1.0 1.3 1.1 1.1 1.2 1.0 1.0 0.9

(La/Yb)N 11.0 11.3 13.8 11.5 10.1 14.0 16.2 14.7 17.6

(Gd/Yb)N 1.9 2.3 1.9 2.1 2.0 2.1 1.9 2.2 2.2

Fe2O3tot, total iron as Fe3; LOI, loss on ignition; -------, below detection limit; Mg number MgO/(MgO FeOtot); A/CNK

Al2O3/(CaO Na2O K2O) molar.

Table 4: continued


1162



Group (i) yields discordant 206Pb/238U ages rangingfrom 514 ! 14 to 588 ! 13Ma, with a weighted mean206Pb/238U age of 563 ! 35Ma [mean square weighteddeviation(MSWD)7"6].Thediscordialineyieldsapoorlydefined upper intercept at 687 ! 77Ma (MSWD 1"4).

Group (ii) yields discordant 206Pb/238U ages rangingfrom 518 ! 14 to 658 ! 15Ma, with the exception ofgrain 1b at 809 ! 26Ma, which was discarded from theage calculation. Their weighted mean 206Pb/238U age is591 ! 81Ma (MSWD 20) and they define a discordia

6545 55 75

0

1

2

3

4

5

6

7

K2O

SiO2

SHOS

HONI

TIC SE

RIES

Gabbro Gabbrodiorite

Granite

CALC-ALKALINE SERIES

Diorite

HIGH

-K C

ALC-

ALKA

LINE

SERI

ES

ARC THOLEIITE SERIES

Granodiorite (a)




Granodiorites

Monzogranites

Aplites and microgranites

CaONa2O

K2O

(c) (d)

1.00.5

1.0

1.6

2.2

2.8Metaluminous

Peralkaline

Peralu-minous

(b)

Al2O3 / (CaO + Na2O + K2O)mol

Al 2

O3 /

(N

a2O

+ K

2O

) mol

MgO

FeO

1

2

3

Na2O + K2O

Fig. 9. Classification diagrams: (a) K2O---SiO2 diagram (after Peccerillo & Taylor, 1976); (b) alumina saturation index diagram;(c) Na2O---K2O---CaO diagram [arrow indicates differentiation trend from Atherton et al. (1979)]; (d) AFM diagram showing the more iron-richchemistry of the quartz monzodiorites: 1, boundary between calc-alkaline and tholeiitic fields from Irvine & Baragar (1971); 2, trend of the Cascadescalc-alkaline lavas from Carmichael (1964); 3, western Cascades Oligocene tholeiitic series from McBirney (1984).


1163



line with an upper intercept at 611! 23Ma (MSWD 3"6)(Fig. 13a).Group (iii) consists of two concordant grains at 597 !

16 and 622 ! 14Ma, and of discordant grains, with206Pb/238U ages ranging from 528 ! 14 to 738 !24Ma (Fig. 13b). The weighted mean 206Pb/238U age is611 ! 68Ma (MSWD 19). These analyses define adiscordia line with an upper intercept at 608 ! 6Ma(MSWD 2"3).Group (iv) yields 206Pb/238U ages ranging from 562 !

15 to 712 ! 19Ma with a concordant grain at 605 !16Ma (Fig. 13c). The weighted mean 206Pb/238U age is616 ! 48Ma (MSWD 7"7). All the analyses define adiscordia line with an upper intercept at 608 ! 7Ma(MSWD 1"3).

The three groups corresponding to the larger zircongrains (100---200 mm) give consistent upper intercept agesat 611! 23, 608! 6 and 608! 7Ma, which are identicalwithin errors to both the concordant single grain ages(597! 16, 605! 16 and 622! 14Ma) and the respectiveweighted mean 206Pb/238U ages. The smaller discordantzircon grains give younger 206Pb/238U ages, possibly sug-gesting partial resetting. Therefore, we consider theupper intercept age at 608 ! 7Ma as representative ofthe emplacement age of the Negash pluton.

Whole-rock Sr and Nd isotopic dataWhole-rock Sr---Nd isotopic data (Table 6) show limitedvariation of measured 87Sr/86Sr (0"70332---0"70475) and143Nd/144Nd (0"51249---0"51262) ratios. Initial isotopic

Monzogranites and granodiorites

Rock

/ C

hondrite

s

1

10

100

Rock

/ C

hondrite

s

1

10

100


(a) (b)

(c) (d)

Quartz monzodiorites Pyroxene monzodiorites

LuYbTmErHoDyTbGd EuSmNdPrCeLa LuYbTmErHoDyTbGd EuSmNdPrCeLa

LuYbTmErHoDyTbGd EuSmNdPrCeLa LuYbTmErHoDyTbGd EuSmNdPrCeLa

Fig. 10. Chondrite-normalized REE patterns for the four main rock types of the Negash pluton. The shaded area corresponds to the compositionalfield of pyroxene monzodiorites. Normalizing values are from Nakamura (1974).


1164



ratios, recalculated at 608Ma, fall within a restrictedrange (Fig. 14a), with low 87Sr/86Sr ratios (0"70260---0"70296), and positive eNd(608) (3"9 to 5"9). Oneexception is a hybrid quartz monzodiorite, which has ahigher 87Sr/86Sr ratio (0"70350), despite having aneNd(608) value (4"8) typical of the sample suite. All ofthe samples plot close to the mantle array and within thefield of the Arabian---Nubian magmatic rocks (Fig. 14a).They have Sr---Nd isotopic ratios comparable with thoseof Neoproterozoic granites from NE Sudan (Fig. 14b).However, they differ from some Neoproterozoic granitesof northern Ethiopia, from the Neoproterozoic crust ofsouthern Ethiopia, and from Palaeoproterozoic andArchaean basement rocks of eastern Ethiopia, most of

which are characterized by higher 87Sr/86Sr initial ratiosand, for some of them, by lower eNd(608) values (Fig. 14b).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 hybridquartz monzodiorites (0"70274---0"70285, one value at0"70350), to the quartz monzodiorites and pyroxenemonzodiorites (0"70282---0"70296). The eNd(608) valuesare lower on average in the monzogranites and pyroxenemonzodiorites than in the granodiorites and hybrid quartzmonzodiorites (Fig. 14a). On the whole, most sampleshave an isotopically moderately depleted signature.However, the low Sr initial ratios, which appear to be verylow compared with other calc-alkaline arc granitoids,

Rock

/ P

rim

itive

Mantle

LuYb

YDy

TiEu

SmZr

NdP

SrPr

PbCe

LaK

NbU

ThBa

RbCs

LuYb

YDy

TiEu

SmZr

NdP

SrPr

PbCe

LaK

NbU

ThBa

RbCs

1

10

100

Monzogranites,granodiorites

(a) (b)Hybrid quartz monzodiorites

Rock

/ P

rim

itive

Mantle

LuYb

YDy

TiEu

SmZr

NdP

SrPr

PbCe

LaK

NbU

ThBa

RbCs

1

10

100

(c)Quartz monzodiorites


LuYb

YDy

TiEu

SmZr

NdP

SrPr

PbCe

LaK

NbU

ThBa

RbCs

(d)

N-MORB

UL

OIB

IAB

Fig. 11. Primitive-mantle-normalized trace element patterns for the four main rock types of the Negash pluton. The shaded area corresponds to thecompositional field of pyroxene monzodiorites. Normalizing values and compositions of N-MORB and OIB are from Sun & McDonough (1989),IAB from Peate et al. (1997) and ultramafic lamprophyre (UL) from Riley et al. (2003).


1165



seem to be typical of most Neoproterozoic granitoid rocksfrom the Arabian---Nubian Shield (Fig. 14).

DISCUSSIONSource of the mafic and felsic rocksInitial Sr and Nd isotopic plots (Fig. 14) show that theNegash mafic and felsic rocks, like the granites from

NE Sudan, have more mantle-like Sr isotopic signaturesthan other Neoproterozoic rocks from NorthernEthiopia. The low and homogeneous initial Sr and highNd isotopic ratios along with Pan-African depleted man-tle Nd model ages (0"70---0"92Ga) imply a source domi-nated by a mantle-derived component. This suggests ajuvenile source for the rock suite, although the Nd ratiosare lower than those of MORB at 608Ma (Fig. 14a).

300

500

700

900

1100

SiO2

706560555045

SiO2

706560555045706560555045

5

25

45

65

Rb

10

12

14

16

18Al2O3

7065605550451

2

3

4

5

6

Na2O

Ba

45 50 55 60 65 702

4

6

8

10

CaO

45 50 55 60 65 70500

1000

1500

Sr

(a) (b)

(c) (d)

(e) (f)

Fig. 12. Harker variation diagrams for selected major (wt %) and trace (ppm) elements. Grey fields highlight the hybrid quartz monzodiorites.!, quartz monzodiorite samples N9-2 and N9-47; shaded inverted triangles, hybrid quartz monzodiorite samples N9-13, N9-19 and N9-41.


1166



These isotopic signatures are typical of both OIB andIAB, but the trace element patterns, which display a clearsubduction signature, argue in favour of arcs. The multi-element patterns (Fig. 11) with spikes in Cs, Ba, Sr andPb, 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 fluidsfollowing dehydration of a subducted slab, as observed inmodern island-arc environments (e.g. Woodhead et al.,1993; Gamble et al., 1996).

3

6

9

12

15

50

100

150

200

250

SiO2 SiO2

V

FeOtot

45 50 55 60 65 700

2

4

6

8

10

45 50 55 60 65 700

100

200

300

400

500

600

45 50 55 60 65 700

100

200

45 50 55 60 65 70

45 50 55 60 65 70

45 50 55 60 65 70

0

1

2

3

4

5

TiO2

MgO

Cr

Ni

150

50


Pyroxene monzodioritesQuartz monzodiorites

Granodiorites

Monzogranites

(g) (h)

(i) (j)

(k) (l)

Fig. 12. Continued


1167



Petrogenesis of the rocks from theNegash plutonPossible petrogenetic processes for the origin of theNegash mafic and felsic rocks can be variable differentia-tion of mantle-derived magmas by fractional crystallization,

partial melting of underplated igneous rocks, or partialmelting of Pan-African juvenile island-arc crust orimmature sediments. Mixing and mingling have also tobe considered as possible processes for the genesis ofquartz monzodiorites. Significant contamination by

740

700

660

620580

540

0.075

0.085

0.095

0.105

0.115

0.125

0.75 0.85 0.95 1.05

207Pb/235U

150

100

86

100

15

17 18

16

614 643

712

681

605

562

(c)

20

6 Pb/2

38U

680640

600560

520

480

0.07

0.08

0.09

0.10

0.11

0.65 0.75 0.85 0.95

(a)

Upper Intercept at611 + 23 MaMSWD = 3.6

_

1 3

613

64 639

658

530518 120120

100

100

780

740

700

660620

580

500

0.07

0.09

0.11

0.13

0.7 0.9 1.1

120

86

100 100

5

9 61

6

738

738

667

622

552

528

597

(b)

Upper Intercept at608 + 6 Ma

MSWD = 2.3

_

Upper Intercept at608 + 7 Ma

MSWD = 1.3

_

20

6 Pb/2

38U

20

6 Pb/2

38U

(iii)

(iv)

(ii)

Fig. 13. BSE images and concordia diagrams for zircons from a monzogranite of the Negash pluton (sample N9-14). The analytical spots and thecorresponding 206Pb/238U ages for each zircon grain are marked. (a) 100---150mm, colourless, transparent, euhedral, elongated zircons (with theexception of grain 1b not shown); (b) 100---150mm, pink, euhedral, bipyramidal zircons; (c) 150---200mm, pink, euhedral, prismatic zircons. Labels(ii), (iii) and (iv) refer to zircon groups of Table 5.


1168





older continental crust can be ruled out because of thelow initial 87Sr/86Sr ratios and high eNd(t) values.

Pyroxene monzodioritesPyroxene monzodiorites have coarse-grained textureswith abundant euhedral grains of apatite and Fe---Tioxides enclosed within subhedral to euhedral pyroxeneand hornblende poikiloblasts. Such textures that havebeen reported in monzodiorites from the Patagonian

batholith are attributed to orthocumulates (Rapela &Pankhurst, 1996). Although the pyroxene monzodioritesshow mantle-like Sr and Nd isotopic signatures and havethe most magnesian amphiboles and biotites of the wholesuite, their Cr (5600 ppm) and Ni (5200 ppm) contentsare too low to represent primary basaltic melts. More-over, they have very high Ti/Zr, Ti/Y, Ti/V, P/Zr andP/Y ratios compared with the primitive mantle and withcommon basaltic melts (Fig. 11d), especially island-arcand back-arc basalts (Woodhead et al., 1993). We

Table 5: Ion-probe U---Pb isotopic data for zircons from a monzogranite of the Negash pluton

Anal. spot 206Pb/204Pb Concn (ppm) Corrected ratios (atomic ratios) Ages (Ma)

Pb U Th 207Pb/235U !s+ 206Pb/238U !s 206Pb/238U !s 207Pb/235U !s

Group (i): 5100 mm, colourless, transparent, euhedral, bipyramidal zircons

11 4632 15.3 199 65.2 0.747 0.025 0.0897 0.0030 554 18 566 15

12a 5920 11.5 161 47.3 0.683 0.020 0.0831 0.0023 514 14 528 12

12b 9171 8.2 112 35.3 0.717 0.021 0.0855 0.0024 529 14 549 12

39a 14910 36.4 468 158.8 0.752 0.018 0.0905 0.0021 559 12 570 10

39b 1100 23.0 261 69.3 0.951 0.025 0.1028 0.0024 631 14 679 13

47a 11046 23.3 284 79.1 0.804 0.019 0.0955 0.0022 588 13 599 11

47b 2516 56.1 720 317.4 0.833 0.021 0.0906 0.0021 559 12 615 12

Group (ii): 100---150 mm, colourless, transparent, euhedral, elongated zircons

1a 6293 8.2 114 25.5 0.694 0.020 0.0836 0.0023 518 14 535 12

1b 2652 6.6 57 13.6 1.104 0.044 0.1337 0.0046 809 26 755 21

3a 3087 6.5 88 23.4 0.710 0.022 0.0857 0.0025 530 15 545 13

3b 10101 6.8 80 24.4 0.814 0.023 0.0997 0.0028 613 16 605 13

64a 18442 49.3 551 378.3 0.891 0.025 0.1043 0.0025 639 14 647 13

64b 30271 104.2 1128 548.6 0.895 0.021 0.1075 0.0025 658 15 649 11

Group (iii): 100---150 mm, pink, euhedral, bipyramidal zircons

5a 10388 12.6 121 32.6 0.998 0.034 0.1213 0.0041 738 24 703 17

5b 2555 14.2 170 39.8 0.798 0.025 0.0970 0.0027 597 16 596 14

6a 251 11.6 158 44.0 0.669 0.025 0.0854 0.0024 528 14 520 15

6b 2642 8.8 86 28.3 0.996 0.031 0.1179 0.0035 718 20 702 16

9a 1163 12.0 128 34.0 0.901 0.031 0.1090 0.0032 667 18 652 16

9b 1017 12.7 165 37.0 0.744 0.022 0.0894 0.0025 552 15 565 13

61 19854 81.2 933 354.8 0.840 0.020 0.1013 0.0025 622 14 619 11

Group (iv): 150---200 mm, pink, euhedral, prismatic zircons

15 6485 18.6 217 56.2 0.811 0.024 0.0999 0.0028 614 16 603 13

16 11911 6.5 72 17.8 0.872 0.027 0.1049 0.0032 643 18 637 14

17a 2281 18.7 221 70.2 0.809 0.025 0.0984 0.0028 605 16 602 14

17b 4000 20.3 259 115.0 0.758 0.022 0.0911 0.0026 562 15 573 12

18a 3486 23.8 249 85.9 0.919 0.030 0.1114 0.0036 681 21 662 16

18b 2062 28.4 283 158.9 0.960 0.027 0.1168 0.0033 712 19 683 14

31 14002 23.4 289 69.2 0.784 0.019 0.0942 0.0022 580 13 588 11

*The error (s) represents the individual common lead statistical error, the error associated with the common lead correctionand the systematic error associated with the U/Pb calibration procedure (Deloule et al., 2002).


1169




interpret these chemical characteristics as a result ofaccumulation of Fe---Ti oxide and apatite, and we there-fore consider these rocks as cumulates formed by frac-tional crystallization from a basaltic parent melt. The linkbetween pyroxene monzodiorites and quartz monzodior-ites, suggested by their association in the field and bysimilar REE and trace-element patterns, will be discussedin the next section.

Quartz monzodiorites and hybrid rocksThe quartz monzodiorites have distinctly lower whole-rock mg number values and mineral XMg ratios com-pared with the hybrid quartz monzodiorites, for thesame range of silica content. This precludes any geneticlink between these two types of quartz monzodioritesand, therefore, suggests that they originate from twodistinct magmas. These magmas probably evolvedunder distinct oxygen fugacity conditions, as suggestedby their distinct mg number, which imply fractionation ofan Fe-rich phase (probably Fe---Ti oxide) for the hybridquartz monzodiorites (Fig. 9d). Major element modelling

(Table 7) further suggests that the pyroxene and quartzmonzodiorites can be derived from a common parentmelt 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 aparent melt with the composition of the quartz monzo-diorite sample N9-4. The composition of the modelledsolid residue is similar to that of the pyroxene monzodior-ites. This is corroborated by trace element modellinginvolving compatible and incompatible trace elementssuch as V and Rb (Fig. 15c), and in agreement with theREE contents, which are lower in the pyroxene monzo-diorites than in the quartz monzodiorites.The hybrid quartz monzodiorites consist of two distinct

groups. The one with high Al2O3, K2O, Na2O, Rb andBa concentrations (Figs 9 and 12a, c, d) corresponds tosamples containing a significant modal abundance ofK-feldspar xenocrysts and, therefore, can be consideredto result from mingling with phenocryst-laden felsic mag-mas (Fig. 3b). The microtextures and compositionalzoning of plagioclase (spongy cores, reverse zoning, calcicspikes) further suggest hybridization. Such evidence raises

Table 6: Sr and Nd isotopic data for whole rocks from the Negash pluton

Sample Rb Sr 87Rb/86Sr 87Sr/86Sr (2s) 87Sr/86Sr(608) Sm Nd 147Sm/144Nd 143Nd/144Nd (2s) 143Nd/144Nd(608) eNd(608) TDM (Ga)

Monzogranites

N9-14 60.1 818 0.213 0.704447 (36) 0.70260 2.98 16.9 0.106 0.512486 (14) 0.51206 4.1 0.83N9-30 48.8 864 0.163 0.704086 (16) 0.70267 1.79 10.1 0.107 0.512518 (26) 0.51209 4.6 0.79Granodiorites, tonalites

N9-15 61.4 772 0.230 0.704749 (11) 0.70275 2.93 17.0 0.104 0.512509 (19) 0.51209 4.7 0.78N9-23 42.1 805 0.151 0.704052 (25) 0.70274 2.21 10.7 0.125 0.512578 (21) 0.51208 4.4 0.84N9-36 59.3 1063 0.161 0.704227 (26) 0.70283 2.45 13.8 0.107 0.512528 (20) 0.51210 4.8 0.78N9-17+ ------- ------- 0.111 0.703816 (37) 0.70285 4.53 24.6 0.111 0.512556 (11) 0.51211 5.1 0.77N9-45 38.0 1090 0.101 0.703689 (33) 0.70281 2.48 13.9 0.107 0.512565 (20) 0.51214 5.6 0.73Hybrid quartz monzodiorites

N9-13 35.7 1247 0.083 0.703517 (26) 0.70280 6.64 39.9 0.100 0.512509 (23) 0.51211 5.0 0.76N9-41 40.0 1371 0.084 0.704230 (25) 0.70350 6.22 29.0 0.129 0.512614 (20) 0.51210 4.8 0.82N9-44 41.0 1335 0.089 0.703536 (20) 0.70276 5.49 33.5 0.099 0.512550 (12) 0.51216 5.9 0.70Quartz monzodiorites and pyroxene monzodiorites

N9-4 32.7 985 0.096 0.703710 (32) 0.70288 9.47 46.8 0.122 0.512578 (25) 0.51209 4.6 0.82N9-21 8.4 417 0.058 0.703320 (22) 0.70282 4.44 20.0 0.134 0.512594 (27) 0.51206 4.0 0.90N9-25 12.2 836 0.042 0.703324 (26) 0.70296 6.92 31.3 0.133 0.512623 (15) 0.51209 4.7 0.84N9-29 18.9 874 0.063 0.703496 (31) 0.70295 4.35 19.1 0.137 0.512602 (23) 0.51206 3.9 0.92

Analytical errors on isotopic ratios (error in the last two digits of the Sr and Nd isotopic ratios) are expressed as 2s ( 2standard errors of the mean). CHUR composition used to calculate eNd values is 143Nd/144Nd 0.512638 and 147Sm/144Nd0.1967; the decay constant is 6.54, 10)12. Nd model ages are calculated according to the depleted mantle model of Michardet al. (1985).*The initial Sr ratio for this sample is calculated on the basis of 87Rb/86Sr ratio (0.111) determined by ICP-MS (Table 6),because the Rb/Sr ratio of this sample determined by isotope dilution is about 70% higher than that obtained by ICP-MS,unlike the other samples whose isotope dilution and ICP-MS results agree within c. 10%.


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Camila Beln Riera Sotox2,9




the question of whether in situ binary mixing can explainthe genesis of the hybrid quartz monzodiorites. Amphi-bole, biotite and whole-rock compositions show that thehybrid rocks cannot result from only two-componentmixing 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. Thesignificant enrichment of the hybrid rocks in both LREE(LaN 70---120 in the hybrid quartz monzodiorites

compared with 50---100 in the quartz monzodiorites and40---70 in the pyroxene monzodiorites and felsic rocks)and HREE (YbN 6---8 in the hybrid monzodioritescompared with 5---7 in the quartz monzodiorites and3---5 in the pyroxene monzodiorites and felsic rocks)also provides evidence that the hybrid rocks cannotbe explained by simple in situ mixing between themafic and felsic end-members. This, therefore, calls foranother, unseen, more primitive mafic magma (highermg number and Ca, lower Ti and P) and implies that

Fig. 14. (a) Plot of eNd(t) vs (87Sr/86Sr)i at 608Ma for the four main rock types of the Negash pluton (error bars are 2s) compared with the mantlearray (ONions et al., 1979) and with the field of the Arabian---Nubian Shield magmatic rocks (Duyvermann et al., 1982; Stern & Krooner, 1993).(b) Plot of eNd(t) vs (87Sr/86Sr)i at 608Ma showing the isotopic signature of the rocks of the Negash pluton compared with other granites of equivalentage in NE Africa (Stern & Abdelsalam, 1998; Tadesse et al., 2000), with the Neoproterozoic upper crust of Southern Ethiopia, and with thePalaeoproterozoic and Archaean basement of Eastern Ethiopia (Teklay et al., 1998).


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Table 7: Result of petrogenetic modelling compared with the composition of the most differentiated quartz

monzodiorites from the Negash pluton

Solid residue Px monzodiorite sample N9-25

Opx Amph Bt Pl-45 Ilm Mnt Ap Residue

wt %

SiO2 54.5 44.0 39.5 56.5 0.0 0.0 0.0 45.6 47.6

Al2O3 1.5 10.5 14.5 27.5 0.0 0.0 0.0 16.0 13.3

FeO 17.0 13.0 13.5 0.0 47.0 85.0 0.0 13.1 12.9

MgO 24.5 14.0 17.5 0.0 0.0 0.0 0.0 7.5 7.2

CaO 2.0 11.5 0.0 10.0 0.0 0.0 100.0 9.3 9.1

Na2O 0.0 2.5 0.0 5.8 0.0 0.0 0.0 3.1 2.9

K2O 0.0 1.0 10.5 0.2 0.0 0.0 0.0 1.1 0.9

TiO2 0.5 3.5 4.5 0.0 53.0 15.0 0.0 4.4 5.0

Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

Mass fraction 0.19 0.09 0.09 0.49 0.05 0.06 0.03 1.00

Partition coefficients Bulk D

ppm

Rb 0.03 0.29 4.5 0.06 0 0 0 0.47

V 0.9 1.5 1.5 0.1 12 10 0 1.69

Parent melt Modelled residual melt Qtz monzodiorite sample N9-24

wt %

SiO2 54.0 54.5 54.9 55.5 56.1 56.8 56.7

Al2O3 16.4 16.4 16.4 16.5 16.5 16.5 15.9

FeO 10.5 10.4 10.2 10.1 9.9 9.7 9.2

MgO 3.6 3.4 3.2 3.0 2.7 2.3 3.9

CaO 6.6 6.4 6.3 6.1 5.9 5.7 5.8

Na2O 4.2 4.2 4.3 4.4 4.4 4.5 4.1

K2O 2.0 2.0 2.1 2.1 2.2 2.3 2.0

TiO2 2.7 2.6 2.5 2.4 2.3 2.2 2.3

Total 100 100.0 100.0 100.0 100.0 100.0 100.00

Mass fraction 1.0 0.95 0.9 0.85 0.8 0.75

Parent melt Modelled residual melt Qtz monzodiorite sample N9-24

ppm

Rb 35 36 37 38 39 41 39

V 142 137 132 127 122 116 136

Parent melt Modelled residual solid Px monzodiorite sample N9-25

Rb 35 17 17 17 17 18 17

V 142 236 232 227 223 219 224

Data source for calculations: minerals from the pyroxene monzodiorites; parent melt is qtz monzodiorite sample N9-4. Datasource for partition coefficients: Ringwood (1970); Mahood & Hildreth (1983); Ewart & Griffin (1994); Horn et al. (1994);Sisson (1994).


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differentiation (involving hybridization) must haveoccurred before emplacement.

Granodiorites and monzogranitesThe low Sr and high Nd isotopic ratios of the granitoidssuggest they may have been produced by partial meltingof mantle-derived material with short crustal residencetime, or fractional crystallization of basaltic melt. How-ever, it is not clear from the available data which processexplains the origin of the felsic rocks.Partial melting of underplated mafic material is a pos-

sible process that may account for the origin of felsic rocks

in general, as shown from geochemical (e.g. Williamsonet al., 1992; Tepper et al., 1993; Petford & Atherton, 1996)and experimental studies (e.g. Helz, 1976; Spulber &Rutherford, 1983; Beard & Lofgren, 1989, 1991; Thyet al., 1990; Rushmer, 1991; Wolf & Wyllie, 1994; Rapp& Watson, 1995). Applying this model to the Negashgranitoids is consistent with: (1) the high Sr contents,implying that plagioclase was incorporated into themelt; (2) the concave-upward REE patterns and lack ofEu anomalies, suggesting the predominance of residualamphibole in the source (see Tepper et al., 1993); (3) thestrongly fractionated HREE-depleted patterns (YbNaround 4---5 times chondrite), implying the presence of

3

6

9

12

15FeOtot

45 50 55 60 65 70

(a)

SiO2SiO2

45 50 55 60 65 700

1

2

3

4

5 (b)

(c)

10 20 30 40 50

240

120

180

0.05

0.95

0.85

0.75

0.150.25

0.50

0.90

0.70solid

residue

residualmelt

Rb

V

1

2

21

r

pp

m

r




Granodiorites

Monzogranites

TiO2

m

Fig. 15. Major- and trace-element modelling showing that the pyroxene monzodiorites may have been derived by fractional crystallization from amelt with the composition of a quartz monzodiorite. The residual solid has a mineralogical composition similar to that of the pyroxenemonzodiorites and consists of opx:hbl:bt:pl:ilm:mgn:ap (19:9:9:49:5:6:3, wt %). (a) and (b) FeOtot and TiO2 vs SiO2 plots: r (solid residue) andm (residual melt) correspond to a residual melt fraction of 0"8; p (parent melt) has the composition of sample N9-4 (Table 3); shown for comparisonare the differentiation trends of (1) the tholeiitic series from the Galapagos and (2) calc-alkaline rocks from the Cascades (McBirney, 1984). (c) V---Rbplot showing the fractionation trend of the same parent melt (numbered dots correspond to the fractions of melt or solid). Partition coefficients anddetails of calculation are given in Table 7.


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garnet in the residue; (4) the temperatures estimated fromthe apatite thermometer (836---886(C for the felsic rocks),which are consistent with experimental data indicatingmelting temperatures of 850---950(C for the generation offelsic melts (Beard & Lofgren, 1991).The granodiorites and monzogranites are unlikely to

have been derived by fractional crystallization from thequartz monzodiorites as they belong to two distinct series(Figs 9d and 15a, b). The apparent linear array shown bythe felsic rocks, the hybrid quartz monzodiorites and thepyroxene monzodiorites in Fig. 9d cannot be interpretedin terms of fractional crystallization

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