U/Pb and Pb/Pb Zircon Ages from Granitoid Rocks of Wallagga Area: Constraints on Magmatic and Tectonic Evolution of Precambrian Rocks of Western Ethiopia
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Mineralogy and Petrology (2001) 71: 251±271 U/Pb and Pb/Pb zircon ages from granitoid rocks of Wallagga area: constraints on magmatic and tectonic evolution of Precambrian rocks of western Ethiopia T. Kebede1;Ã, U. S. Kloetzli2, and C. Koeberl1 1 Institute of Geochemistry, University of Vienna, Austria 2 Laboratory for Geochronology, Institute of Geology, University of Vienna, Austria With 7 Figures Received June 7, 2000; accepted October 29, 2000 Summary The Precambrian rocks in western Ethiopia consist of high- and low-grade terranes intruded by granitoids with a wide compositional spectrum. The formation ages of these granitoid rocks are, so far, poorly understood. Single-grain zircon Pb/Pb evaporation and conventional U/Pb dating conducted on four granitoids places time constraints on their emplacement and tectonothermal events. Three granitoid magmatic events were identi®ed at 815 Ma, 700±730 Ma, and 620±625 Ma, which were marked by emplacement of the calc-alkaline Ujjukka granite and granodiorite, the anatectic Suqii-Wagga two-mica granite and the Guttin K-feldspar megacrystic granite, and the anorogenic Ganjii monzogranite, respectively. We interpret the 815 Ma age to mark a major magmatic episode in this part of Africa. A tectonothermal event at 630 Ma preceded the emplacement of the within-plate granitoids at 620±625 Ma. The decrease of ages from the calc-alkaline to anorogenic granitoids suggests a shift of magmatic styles and tectonic setting of the granitoids over a period of 200 million years. The Suqii- Wagga and Guttin granites, representing the granitoid population in the migmatitic terrane, formed as part of the successive evolution of the granitoid magmatism in the region. The presence of xenocrystic zircons of Mesoproterozoic ages in both granitoid populations emplaced into the low-grade volcanosedimentary sequence and the high- grade, often migmatitic, gneisses suggest contribution of pre-Pan-African crust to the origin and evolution of the granitoids. Conventional U/Pb studies of zircons from the à Present address: Laboratory for Geochronology, Institute of Geology, University of Vienna, Austria 252 T. Kebede et al. Guttin K-feldspar megacrystic granite and the Ganjii monzogranite yielded upper intercept ages of 3 Ga and 2 Ga, respectively, possibly indicating the presence of reworked Archean-Proterozoic crustal material. Zusammenfassung U/Pb und Pb/Pb Zirkonalter granitoider Gesteine aus dem Gebiet von Wallagga: Hinweise zur magmatischen und tektonischen Entwicklung praÈkambrischer Gesteine in AÈ thiopien Das PraÈkambrium im westlichen AÈ thiopien besteht aus hoch- und niedrigmetamorphen Basement Serien, die von Granitoiden unterschiedlichster Zusammensetzung intrudiert werden. Die Bildungsalter dieser Magmatite sind bisher nur ungenuÈgend bekannt gewesen. Neue Pb/Pb-Evaporations- und konventionelle U/Pb-Datierungen an Einzel- zirkonen von vier verschiedenen Plutoniten erlauben nun RuÈckschluÈsse auf deren Intrusionsalter und die damit verbundene tektonische Entwicklung der Region. Drei zeitlich getrennte magmatische Ereignisse lassen sich unterscheiden: Intrusion der kalk- alkalischen Ujjukka Granite um 815 Ma; Bildung der anatektischen Zweiglimmer Granite der Suqii-Wagga Suite um 700±730 Ma; Intrusion der grob porphyrischen K- Feldspat Granite von Guttin und der anorogenen Ganjii Monzogranite um 620±625 Ma. Das 815 Ma Ereignis wird als wichtige magmatische Phase in diesem Teil von Afrika interpretiert. Ein thermisches Ereignis um 630 Ma geht der Platzname von ``within- plate'' Granitoiden um 620±625 Ma voraus. Die beobachtete Altersabnahme von den kalk-alkalischen zu den anorogenen Granitoiden spricht fuÈr eine praÈgnante AÈ nderung des tektonischen Regimes uÈber einen Zeitraum von ca. 200 Ma. Die Suquii-Wagga und Guttin Granite sind in das hochgradige, migmatische Basement intrudiert. Dies mag fuÈr eine sukzessive tektonische Entwicklung dieser Abfolgen sprechen. Ererbte, mesopro- terozoische Zirkone deuten auf die Aufarbeitung praÈ-panafrikanischer Gesteine hin. Obere Einstichpunkte von den U/Pb Analysen im Altersbereich von ca. 3 Ga in den Guttin Graniten und von ca. 2 Ga in den Ganjii Monzograniten sprechen ebenfalls fuÈr die Inkorporation von proterozoischen bis archaischen Krustenkomponenten. Introduction Granitoids constitute a signi®cant proportion of the western Ethiopian Precambrian rocks (Fig. 1). Field relationships and petrography, geochemistry, and petrogenesis of the granitoids intruded into the high-grade gneisses and low-grade metasedi- mentary and metavolcanic rocks were recently studied by Kebede et al. (1999, 2000). However, the age relationships among the granites emplaced into the same terrane or between granite populations intruding contrasting terranes are still not well understood. In particular, systematic geochronological studies are lacking for the western Ethiopian Precambrian areas, except the study by Ayalew et al. (1990), who reported U/Pb and Rb/Sr dating on plutonic rocks south of the area described in the present work and attempted to put age limits on the magmatic and metamorphic evolution there. The present study, therefore, was aimed at constraining the magmatic history and establishing the sequence of events in the research area using U/Pb and Pb/Pb single-grain zircon chronometers. The results of the study, integrated with other regional data, provide a better picture of the geological evolution of the western Ethiopian Precambrian rocks. To this end, we dated samples of the Ujjukka granite and granodiorite, the Suqii- Wagga garnet-bearing two-mica leucocratic granite, the Ganjii, often porphyritic, U/Pb and Pb/Pb zircon ages from granitoid rocks of Wallagga area 253 Fig. 1. Generalized geological map of the study area (modi®ed after Kebede et al., 1999 and references therein). Tb Tertiary basalt, WPG within-plate granite, VAG volcanic arc granite, VST volcano-sedimentary terrane, GT gneissic terrane, SZ suture zone, GM Ganjii monzogranite, GKM Guttin K-feldspar megacrystic granite, SW Suqii-Wagga two-mica granite, UK Ujjukka granite and granodiorite, GG Gore-Gambela geotraverse area. Ages given in the ellipses are derived from single-grain zircon 207Pb/206Pb evaporation and U/Pb dating 254 T. Kebede et al. monzogranite, and the Guttin K-feldspar megacrystic granite (Fig. 1). These granitoids were selected based on their ®eld relationships to represent intrusion into both high- and low-grade terranes so that comparison of magmatic and tectono- thermal events are possible. Accordingly, the Ujjukka and the Ganjii granitoids represent plutons emplaced into the low-grade rocks, whereas the Suqii-Wagga and the Guttin granites represent plutons in the high-grade rocks. In this study we show that granite magmatism changed from subduction-related to anatectic to anorogenic in the time span of 815 to 620 Ma. Sample preparation and description Large rock samples, ranging from about 15 to 30 kg, were collected from the Ganjii, Guttin, Suqii-Wagga, and Ujjukka granitoids (see Fig. 1 for sample locations). The samples were crushed and panned to separate the heavy mineral fractions, which include most of the zircons present. Thereafter, the samples were sieved and the size fraction between 45 mm and 200 mm was used for further zircon mineral separation. Magnetic and heavy liquid separations were subsequently used to obtain zircon concentrates. The almost pure zircon fractions were hand-picked and classi®ed into different populations according to typology, colour, and translucency using a bino- cular microscope. The zircon typologies from different granitoid bodies are summarized in Table 1 and some representative crystals are shown in Figs. 2 and 3. In general, idiomorphic and translucent, seemingly magmatic, zircons were used for analysis. Two populations of such grains are present in samples TK117 and TK141 from the Ganjii and the Guttin granitoids, respectively. Dating of samples containing complex zircon populations is often dif®cult, as different populations may have different histories. However, Pupin (1980) was able to relate the variation in a population to different stages of magmatic crystallization, in which the successive stages of typological evolution of zircon were trapped in other minerals as their growth proceed. He also suggested that such variations within zircon population are caused by changes in physico-chemical conditions of the crystallization medium, which would still permit a closely related age for different populations of zircon in a particular sample. However, samples TK117 and TK141 contain, besides the magmatic zircon population, inherited zircons, which may have resulted in typologic variations. Such population variations might complicate the dating by yielding ages that are dif®cult to interpret, or mixing ages without geological signi®cance. Samples containing homogeneous zircons, for example, TK099 of the Ujjukka granite and granodiorite, yielded consistent ages (Table 2). Analytical methods Single-grain zircon Pb/Pb evaporation and conventional U/Pb zircon dating techniques were used. Analyses were conducted on a Finnigan MAT 262 mass spectrometer, equipped with a secondary electron multiplier-ion counter system, at the Geochronology Laboratory, Institute of Geology, University of Vienna. The ®nal 207Pb/206Pb and U/Pb ages were calculated at 2 standard deviation using the Isoplot/Ex program version 2.10 of Ludwig (1999). Table 1. Typologic classi®cation of zircons and summary of petrographic and chemical characteristics of selected