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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
* Corresponding author. Telephone: +251 1 55 32 14. Fax: +251 1 55 23 50. E-mail: asrata{at}geol.aau.edu.et
The 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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