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Geochronology and Geochemistry of the Essimingor Volcano: Melting of Metasomatized Lithospheric Mantle Beneath the North Tanzanian Divergence Zone (East African Rift)

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Geochronology and Geochemistry of the Essimingor Volcano: Melting of Metasomatized Lithospheric Mantle Beneath the North Tanzanian Divergence Zone (East African Rift) LITHOS-02833; No of Pages 16 Lithos xxx (2012) xxx–xxx Contents lists available at SciVerse ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos Geochronology and geochemistry of the Essimingor volcano: Melting of metasomatized lithospheric mantle beneath the North Tanzanian Divergence zone (East African Rift) S. Mana a,⁎, T. Furman b, M.J. Carr a, G.F. Mollel c, R.A. Mortlock a, M.D. Feigenson a, B.D. Turrin a, C.C. Swisher III a a Department of Earth and Planetary Sciences, Rutgers University, NJ 08854, USA b Department of Geosciences, Pennsylvania State University, University Park, PA 16802, USA c Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Canada T6G 2N8 article info abstract Article history: The North Tanzanian Divergence zone (NTD), at the southern end of the eastern branch of the East African Received 9 May 2012 Rift, is part of one of Earth's few currently active intra-continental rift systems. The NTD preserves a complex Accepted 23 September 2012 tectono-magmatic evolution of a rift in its early stage of activity. The oldest magmatism reported in the NTD Available online xxxx is associated with the centrally located Essimingor volcano. Although major element oxides show narrow compositional variations suggesting fractional crystallization, trace element abundances and Sr–Nd–Pb iso- Keywords: North Tanzanian Divergence topic data have complex distributions that require open-system processes. The more primitive samples fl 40Ar/39Ar dating (MgO>9 wt.%) likely re ect partial melting of a metasomatized lithospheric mantle characterized by residual Lead isotope garnet, phlogopite and minor amphibole. The range of radiogenic Pb isotopic compositions indicates the Neodymium isotope presence of mixing between this source and the lithosphere of the western branch of the East African Rift Strontium isotope (Toro Ankole and Virunga). Laser-incremental heating of selected samples gives 40Ar/39Ar ages that range from 5.76±0.02 Ma to 5.91±0.01 Ma, suggesting an age roughly 2 myr younger than previously reported. © 2012 Elsevier B.V. All rights reserved. 1. Introduction (NTD). The earliest occurrence of volcanism in the NTD appears to be Late Miocene or Early Pliocene, thus much younger than in the northern The East African Rift (EAR) is an intracontinental rift system char- part of the EAR. The NTD is characterized by an abrupt widening of the acterized by a succession of extensional basins linked and segmented rift at around 3°S where it extends from the Plio-Quaternary Kilimanjaro by accommodation zones (Chrorowicz, 2005). Volcanism in the EAR volcano in the east to the large Late Pleistocene caldera of Ngorongoro in is considered to be a result of one or more mantle plumes from the west (Fig. 1). This 200 km wide zone contrasts markedly with the undetermined depths of origin rising to the base of the lithosphere 50–60 km width of the majority of the East African Rift, and is compara- (e.g. Ebinger and Sleep, 1998; George et al., 1998; Lin et al., 2005; ble to the width observed in Turkana, northern Kenya. From north Nyblade et al., 2000; Owens et al., 2000; Smith, 1994). Along the to south, the NTD volcanoes (Fig. 1) include the poorly studied Gelai length of the EAR, numerous volcanoes with highly variable volumes near the Kenya–Tanzania border, and the nearby well-known Oldoinyo and compositions have erupted both prior to and in conjunction Lengai, Earth's only active carbonatite volcano; southward, the Late with the onset of documented extension at ~30 Ma (Baker et al., Miocene Essimingor volcano is considered the oldest pre-rift volcano of 1996; Hofmann et al., 1997). The age, duration and geochemical evo- the NTD based on a K–Ar age of around 8 Ma from Bagdasaryan et al. lution of these volcanoes serve as primary bases for unraveling the (1973). The present day north–south rift valley in the NTD is thought tectonomagmatic development and history of the EAR. to have been established about 1.2 Ma (MacIntyre et al., 1974)and In northern Tanzania, magmatic activity and rifting are superimposed the older volcanoes to have erupted in a pre-rift tectonic depression upon a pre-existing zone of crustal weakness: the tectonic contact (Dawson, 1992). between the Archean Tanzanian Craton to the west and the Proterozoic The detailed geochemical, spatial and temporal evolution of the Mozambique Mobile Belt to the east (Smith, 1994; Fig. 1). Baker et al. NTD remains unclear, due to a lack of sampling, geochemical and geo- (1972) referred to this region as the North Tanzanian Divergence zone chronological data (see summaries in Dawson, 2008 and Le Gall et al., 2008). The age data are highly variable in quality and many are isolated K–Ar dates. To address these uncertainties and to better unravel the ⁎ Corresponding author at: Earth and Planetary Sciences, Rutgers University, 610 Taylor Road, Piscataway, NJ 08854, USA. Tel.: +1 732 445 2044; fax: +1 732 445 3374. tectonomagmatic history of the NTD, we have initiated a systematic pro- E-mail address: [email protected] (S. Mana). gram focused on providing detailed geochemical and geochronological 0024-4937/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.lithos.2012.09.009 Please cite this article as: Mana, S., et al., Geochronology and geochemistry of the Essimingor volcano: Melting of metasomatized lithospheric mantle beneath the North Tanzanian Divergence zone (East African Rift), Lithos (2012), http://dx.doi.org/10.1016/j.lithos.2012.09.009 2 S. Mana et al. / Lithos xxx (2012) xxx–xxx a 4°N 0 Km 5 c Shombole 2°S 0° 10’ Kenya S 3° 22’ Lake Eastern Branch Natron Western Branch Tanzanian NTD Craton 4°S Mozambique 2°S Belt 30’ 8°S Gelai 28°E 32°E 36°E 40°E Oldoinyo Longido Lengai 2°S Kerimasi S 3° 30’ E 36° 00’ Essimingor 50’ Engelosin Ketumbeine Embagai Olmoti Shira Kibo Loolmalasin Mawenzi Lemagrut Kilimanjaro Tarosero 3°S Sadiman Ngorongoro 10’ Meru Monduli Ngurtoto crater Oldeani Burko Lake Eyasi 3°S ESSIMINGOR 30’ 0 Km 20 Lake Manyara Tanzania Masai Plateau b 3°S 50’ 35°E20’ 35°E40’ 36°E 36°E20’ 36°E40’ 37°E 37°E20’ 37°E40’ Fig. 1. (a) Map of the southern sector of the East African Rift showing major regional faults, including the boundary between the Archean Tanzanian Craton and the Proterozoic Mozambique Belt and location of the North Tanzanian Divergence (Chrorowicz, 2005; Nyblade et al., 2000). The black star represents the location of the crustal contaminant used in the discussion (TA-47, Manya et al., 2007). (b) Map of the North Tanzanian Divergence sector of the East African Rift. The shaded area represents the Neogene to Recent volcanism as in Dawson (1992). (c) Essimingor volcano and the location of the samples selected in this study (inclusive of Paslick et al., 1995 location: BD214 (3°23′6″S, 36°2′ 15″E), BD216 (3°23′6″S, 36°2′36″E) and BD247 (3°24′30″S, 36°3′40.7″E) from Dawson personal communication). Map modified from GeoMapApp and CNES/SPOT image 2010. data for key northern Tanzanian volcanoes (Mollel et al., 2008, 2009, slopes of Essimingor. The ages are 4.89±0.09 and 4.68±0.09 Ma for 2011). Here, we report our findings on the centrally located Essimingor one sample, and 3.23±0.07 and 3.20±0.06 Ma for the other. Based volcano, which likely record the earliest volcanic activity in the NTD. on these data Evans et al. (1971) concluded that Essimingor was active during the Pliocene, probably before 4.8 Ma. Bagdasaryan 2. This study et al. (1973) reported two additional K–Ar analyses from Essimingor based on a single whole rock sample of melaphonolite lava. The We collected 40 well-located lava samples from the S–SW slopes of precise location on Essimingor from where this sample was collected Essimingor volcano (Fig. 1, Table 1). Samples are not evenly distributed is not known. Duplicate K–Ar analyses of this sample yielded ages of across the volcano because of access difficulties; nevertheless an 8.1±1 Ma and 7.35±0.65 Ma. These K–Ar dates are commonly attempt was made to collect a variety of lithologies among the visible used to establish Essimingor as the oldest volcano of the NTD (e.g. range of lava types in the area. Unfortunately a geologic map of the Dawson, 2008; Le Gall et al., 2008). volcano is not available and due to the extensive vegetation we were Whole rock, matrix, and when possible, nepheline separates were unable to produce a stratigraphic section of the samples collected. We analyzed using 40Ar/39Ar laser incremental-heating techniques for report here the results of 40Ar/39Ar dating, major and trace element twelve selected samples (Table 2). Samples were crushed, sieved analyses, and Pb, Sr, and Nd isotope analyses for the Essimingor sam- between 300 and 600 μm (50 and 30 mesh sizes), and washed with dis- ples. In the laboratory, thin sections were made to determine pheno- tilled water in an ultrasonic bath, loaded into wells of Al irradiation disks, cryst assemblages and degree of weathering (summary available in wrappedinAlfoilandsealedinaquartz–glass tube. The samples, along Appendix A). Twenty two samples were further prepared for geochem- with multiple splits of the monitor mineral, Fish Canyon Sanidine (FC2= ical analyses, and 12 of these were dated by 40Ar/39Ar geochronology. 28.02±0.16 Ma; Renne et al., 1998), were irradiated in the central thim- ble of the USGS TRIGA reactor in Denver for 20 min.
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