Zircon ‘microvein’ in peralkaline granitic gneiss, western Ethiopia: Origin, SHRIMP U–Pb geochronology and trace element investigations (original) (raw)

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 darkgrey 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-magmatichydrothermal 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.