Magma emplacement and mafic – felsic magma hybridization: structural evidence from the Pan-African Negash pluton, Northern Ethiopia Asfawossen Asrat a,b, * , Ge ´rard Gleizes c , Pierre Barbey a , Dereje Ayalew b a CRPG-CNRS, 15, Rue Notre-Dame des Pauvres, B. P. 20, 54501 Vandoeuvre-le `s-Nancy Cedex, France b Department of Geology and Geophysics, Addis Ababa University, P.O.Box 1176, Addis Ababa, Ethiopia c CNRS-UMR 5563 LMTG, Universite ´ Paul Sabatier, 38, rue des Trente-six-Ponts, 31400 Toulouse, France Received 12 January 2002; received in revised form 20 August 2002; accepted 7 September 2002 Abstract The Negash pluton (50 km 2 ) consists of late Pan-African, high-K, calc-alkaline granitoids intruded into low-grade metavolcanics – metasediments. This almost circular massif consists of monzogranites, granodiorites, diorites – gabbrodiorites, and hybrid diorites. The anisotropy of magnetic susceptibility (AMS) method was used to determine internal structures of the pluton. The foliation trajectories are concentric and inward dipping. The lineation pattern displays an external zone characterised by horizontal concentrically oriented lineations and an internal zone with NW – SE oriented lineations. These petro-structural data clearly locate the feeder zone at the north-western tip of the pluton and indicate the subsequent expansion of the magmas towards the SE. The pluton is a result of in-situ assembly of four magma batches, which were forcefully injected into pre-existing foliated country rocks in relation to transpressional tectonic regime. Two types of mafic – felsic magma interactions are recognised: homogeneous and heterogeneous hybrid diorites at the north-western part, and mingled interfaces at the diorite – granodiorite contact zones mainly visible in the eastern and south-eastern parts. The in-situ mingling along diorite – granodiorite contacts was achieved at the level of emplacement during the injection of dioritic magma into the felsic magmas while the hybrid diorites are assumed to result from two-way conduit mixing and mingling during simultaneous rising of mafic and felsic magmas. q 2003 Published by Elsevier Science Ltd. Keywords: Ethiopia; Pan-African; Granite; Anisotropy of magnetic susceptibility (AMS); Hybridization; Pluton emplacement 1. Introduction A systematic structural study of plutons using the anisotropy of magnetic susceptibility (AMS) coupled with petrographic interpretations has been proved to be a revealing technique in the understanding of internal structures of granitoids and in determining the tectonic regime during magma emplacement. This AMS method- ology has been successfully applied with efficient results to late Neo proterozoic plutons in Madagascar (Ne ´de ´lec et al., 1994), Western and Northern Africa (e.g. Ferre ´ et al., 1995, 1997, 2002; De ´le ´ris et al., 1996; Djouadi et al., 1997) and Brazil (Archanjo, 1993; Archanjo et al., 1994). These studies indicate that most plutons are emplaced in transpressive tectonic regimes. Although there are many late Pan-African plutons in Ethiopia, such structural studies are non-existent. The nature of mafic – felsic magma interactions (magma mixing and mingling) is an important element in the understanding of the dynamics of magma chambers and pluton emplacement. Most previous works generally agreed on the replenishment of a felsic magma chamber by intruding mafic magmas as the major process responsible for the hybridization (mixing and mingling) of magmatic rocks (e.g. Reid et al., 1983; Whalen and Currie, 1984; Barbarin, 1988; Zorpi et al., 1989; Michael, 1991; Didier and Barbarin, 1991 and references therein; Bateman, 1995; Wiebe, 1996; Wiebe and Adams, 1997; Snyder and Tait, 1998; Wiebe and Collins, 1998; Poli and Tommasini, 1999), while some works indicate that the intrusion of felsic magmas into mafic chambers may be more common than so far realised (e.g. Wiebe and Wild, 1983; Wiebe, 1987; Weinberg, 1997; Weinberg and Leitch, 1998). Others consider dynamic, two-way conduit mixing and hybridiz- ation along rising and during emplacement of magmas as equally important processes (e.g. Oldenburg et al., 1989; Castro et al., 1995; Seaman et al., 1995). 0191-8141/03/$ - see front matter q 2003 Published by Elsevier Science Ltd. PII: S0191-8141(02)00182-7 Journal of Structural Geology 25 (2003) 1451–1469 www.elsevier.com/locate/jsg * Corresponding author. Tel. þ251-1-55-32-14, fax. þ 251-1-55-23-50. E-mail address: asrata@ged.aau.edu.et (A. Asrat).