Genesis and evolution of mafic and felsic magmas at Quaternary volcanoes within the Main Ethiopian Rift: Insights from Gedemsa and Fanta 'Ale complexes
Introduction
Continental rift zones are the sites of intense volcanism typically fed by bi-modal mafic–felsic magmas. Felsic magmas are usually peralkaline, variable in composition from mostly pantellerite and comenditic trachyte, and subordinately comendite (e.g., Furman, 2007, and references therein). The results of petrological studies, corroborated by geological, volcanological, geochronological and geophysical data, suggest that the tectono-magmatic evolution of continental rift zones can result generally from the activity of upwelling mantle (e.g., Bailey, 1983, Griffiths and Campbell, 1990, McKenzie and Bickle, 1988, Morgan, 1983, Nicolas et al., 1994). Very good worldwide examples of such a relationship include the Rhine Valley in Germany (e.g., Ritter et al., 2001), the Sicily Channel Rift Zone in Southern Italy (e.g., Civetta et al., 1998), and the Main Ethiopian Rift (MER) (e.g., Ebinger and Sleep, 1998, Furman et al., 2004, Furman et al., 2006a, Furman et al., 2006b, George et al., 1998, Hofmann et al., 1997, Pik et al., 1999, Shinjo et al., 2011, Stewart and Rogers, 1996).
The East African Rift, extending from Afar to Mozambique, provides a unique opportunity to study composition and evolution of a mantle plume, and its relationships with the dynamics of the overlying lithosphere. In this respect, of particular relevance is the MER that records all the different stages in the evolution of a rift, from initiation, through break-up, to incipient ocean spreading. These stages have been accompanied by intense magmatism and volcanism, variable through time from widespread to more concentrated in the rift axis flood basalt emplacement, to growth of several central edifices (e.g., Corti, 2009, and references therein). Volcanism began in southern Ethiopia at about 45 Ma (George et al., 1998), and continued with a voluminous flood basalt activity that occurred in both Ethiopia and Yemen at around 30 Ma (Pik et al., 1999) which formed a plateau of about 350,000 km3 constituted by both mafic and felsic volcanic rocks (Kieffer et al., 2004, Mohr, 1983, Mohr and Zanettin, 1988). As discussed by Wolfenden et al. (2004), the continental break-up started ca. 11 Ma ago in the northern MER, about 20 Ma after the beginning of the flood basalt activity, and propagated southwards until present. Several phases, if not all, of this magmatism were related to mantle plume activity. According to petrological inferences, the Afar plume is a long-lived feature of the mantle persisting since at least 30 Ma until present, providing a significant contribution through time to magma feeding volcanism in Ethiopia (e.g., Beccaluva et al., 2009, Furman et al., 2006a, Furman et al., 2006b).
The main extension direction of the MER changed from dominantly E–W to NW–SE (N°52 W) ca. 1.8 Ma ago with a clockwise rotation (Boccaletti et al., 1998, Boccaletti et al., 1999). The fault system associated with this rotation is known as the N°20 Wonji Fault Belt (WFB; Mohr, 1987), characterized by an extension rate of 0.5 cm per year, according to structural studies (Williams et al., 2004), or between 0.1 and 0.45 cm per year, according to geodetic measurements (Bilham et al., 1999, Pan et al., 2002). The WFB activity has determined the formation of many magmatic segments along the Rift, arranged “en echelon” (Fig. 1), and bordered by the middle Miocene Rift fault system (Ebinger and Casey, 2001, Wolfenden et al., 2004). The magmatic activity associated with the WFB represented the younger (~ 1.6 Ma) stages of the MER (e.g., Mohr, 1971). Along the Rift magmatic segments, a series of Quaternary volcanoes formed and were mostly characterized by a summit caldera (e.g., Acocella et al., 2002, Acocella et al., 2003) and extrusion of highly differentiated, peralkaline magmas with subordinate mildly alkaline basalts (e.g., Ayalew et al., 1999, Boccaletti et al., 1999, Brotzu et al., 1974, Brotzu et al., 1981, Brotzu et al., 1986, Di Paola, 1972, Merla et al., 1979, Mohr and Zanettin, 1988, Peccerillo et al., 2003, Piccirillo et al., 1979, Ronga et al., 2010, Trua et al., 1999, Yemane et al., 1999).
This article reports the results of a study focused on the Gedemsa and Fanta 'Ale volcanoes located in the central-northern part of the MER. In particular, petrological investigations have been carried out with the aim of shedding light on the genesis and evolution of plume-derived magmas, the role of asthenosphere and lithosphere in the magmagenesis, genetic relationships between mafic and peralkaline felsic rocks, and the role of crustal contamination in the course of magmas' evolution. The two volcanoes are Quaternary in age, and located in the central Rift area (Fig. 1), within individual grabens, the likely sites of maximum extension (Mohr, 1960, Mohr et al., 1978). Fanta 'Ale is an active volcano located just south of the Afar triple junction (Orsi et al., 2007), whereas Gedemsa has produced its last eruption ca. 200 ka BP (Peccerillo et al., 2003). The geology of these volcanoes has been previously described (Gibson, 1969, Gibson, 1970, Gibson, 1974, Kazmin et al., 1980a, Kazmin et al., 1980b, Merla et al., 1979, Theilhard De Chardin and Lamare, 1930, Thrall, 1973) and is summarized here together with new information collected during this work.
Section snippets
Gedemsa
The Gedemsa volcano is located in the central part of the MER (Fig. 1), about 100 km SE of Addis Ababa. It reaches a maximum elevation of about 300 m above the rift floor, and is dominated by a wide caldera with a diameter of about 8 km. The margin of the caldera is well preserved with 100–200 m high vertical inner walls, and only a small sector of its north-western portion has been affected by erosion. The pre-caldera collapse remaining rock bodies are mostly rhyolitic lavas, even though
Sampling and analytical techniques
Petrological analyses have been carried out on samples of lavas as well as pyroclastic-fallout and -current deposits, collected at Gedemsa and Fanta 'Ale. The samples are representative of the main volcano-stratigraphic units. They have been collected at variable heights along the stratigraphic sequence of each unit in order to investigate any compositional variation of the magma tapped during the course of an eruption.
Major oxide and trace element contents of Gedemsa whole-rocks were
Petrography
Basaltic lava samples have moderately porphyritic texture (porphyritic index, P.I. = 15–30 vol.%), while hawaiitic lava samples are moderately vesicular and have nearly aphyric to poorly porphyritic texture (P.I. < 5 vol.%); phenocrysts of plagioclase are dominant, with minor olivine, sometimes partially substituted by iddingsite, diopsidic clinopyroxene and sporadic microphenocrysts of Fe–Ti oxides, set in a groundmass consisting of the same phases and minor glass.
Benmoreitic and trachytic
Discussion
The major oxide and trace element variations (Fig. 3, Fig. 4, Fig. 5) exhibited by the mafic through felsic Gedemsa and Fanta 'Ale rocks are roughly compatible with fractionation of the mineral phases observed in the rocks, i.e. olivine, clinopyroxene, plagioclase, Fe–Ti oxides, apatite, quartz, anorthoclase and aenigmatite, in rough order of appearance. This suggests that fractional crystallization has played a key role as a magma evolution process at both volcanoes. Moreover, as the
Conclusions
The results of this petrological investigation on volcanic rocks from Gedemsa and Fanta 'Ale volcanoes have highlighted several aspects of the genesis and evolution of mafic and felsic magmas in the Main Ethiopian Rift. Geochemical data and modeling suggest that magmas evolved mainly through similar fractional crystallization processes. The latter accounts for the entire mafic through felsic mineralogical and geochemical (major oxide and trace element contents) variations. However, slightly
Acknowledgments
This work is dedicated to the memory of Enzo M. Piccirillo, the great scientist and good friend. The article benefited from comments and suggestions by Dr. Furman, an anonymous Reviewer, and the Editor A. Marzoli.
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