Zircon ‘microvein’ in peralkaline granitic gneiss, western Ethiopia: Origin, SHRIMP U–Pb geochronology and trace element investigations Tesfaye Kebede ⁎ , Kenji Horie, Hiroshi Hidaka, Kentaro Terada Department of Earth and Planetary Systems Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan Received 27 June 2005; received in revised form 20 February 2007; accepted 5 March 2007 Editor: S.L. Goldstein Abstract A zircon ‘microvein’ composed of several hundred crystals occurs in peralkaline granitic gneiss of western Ethiopian Precambrians. U– Pb ages and trace element (U, Th, Hf, Y, REE, P, Ca, Al, Fe, and Mn) abundances of the ‘microvein’ and host granitic gneiss zircon were determined using a sensitive high mass resolution ion microprobe (SHRIMP) and electron probe microanalyzer (EPMA). Back-scattered electron (BSE) imaging of the ‘microvein’ zircon and host granite zircon, hereafter referred to as Type-I and Type-II zircon, respectively, reveal prevalent low and high mean atomic number contrast domains within individual crystals. Ubiquitous fluorite microinclusions in bright BSE domains of Type-I and less commonly, Type-II zircon suggest an early formation of fluorite that buffers F activity, causing zircon supersaturation and precipitation from a late-magmatic melt/fluid-enriched in high field strength elements (HFSEs) including Zr. The textural make up of the host peralkaline granitic gneiss and internal structural features of Type-I and Type-II zircon indicate that dark- grey BSE domains were formed by dissolution–reprecipitation owing to fluid infiltration and interaction with the primary zircon crystals. The bright and dark-grey BSE domains in Type-I zircon yield U–Pb ages of 779 ± 69 Ma and 780 ± 35 Ma, and similar domains in Type-II zircon dated at 778±49 Ma and 780±31 Ma, respectively. The primary and recrystallized domains in both zircon types have indistinguishable ages, suggesting initial crystallization shortly followed by fluid-driven alteration. The ages are identical, within analytical uncertainties, to the 776 ± 12 Ma zircon U–Pb emplacement age of a protolith of a leucocratic granitic gneiss determined from a different sample. Hence, zircon crystals forming ‘microvein’ and aggregate structures, the relatively high Th/U ratios (reaching up to 1.5) in the primary domains, high LREE/HREE, and the formation of Type-I and Type-II zircon during emplacement support a late-magmatic– hydrothermal origin. Extensive alteration of the host rock, recrystallization of young and non-metamict zircon corroborate the infiltration of orthomagmatic or hydrothermal fluids containing fluorides as a major constituent, which expelled a considerable amount of trace elements, namely, Hf, U, Th, Y, and the REEs, from the recrystallized domains of Type-I and Type-II zircon. The trace element depleted recrystallized domains characteristically contain microfractures apparently caused by differential volume expansion of the U and Th enriched primary domains or volume change during cation exchange reactions, and anomalously high Th/U ratios (∼ 0.5 to 1.0). Furthermore, the ca. 780– 776 Ma emplacement age of the protolith of the peralkaline granitic gneiss and late-stage orthomagmatic or hydrothermal activity shed light on the occurrence of older anorogenic granitoid magmatism and associated structures in western Ethiopian Precambrian terranes. © 2007 Elsevier B.V. All rights reserved. Keywords: zircon ‘microvein’;U–Pb zircon age; Trace element; Hydrothermal alteration; Peralkaline granite; Western Ethiopian Precambrian Chemical Geology 242 (2007) 76 – 102 www.elsevier.com/locate/chemgeo ⁎ Corresponding author. Water Works Design and Supervision Enterprise, P.O. Box 13979, Addis Ababa, Ethiopia. Fax: +25111 6615371. E-mail address: tesfaye_k@yahoo.com (T. Kebede). 0009-2541/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2007.03.014