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

Abstract

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 εNd(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. © Oxford University Press 2004; all rights reserved.