• The mineralogy of the effusive silicate rocks from the Mosonik volcano, Northern Tanzania.

      Sedova, AM; Zaitsev, AN; Spratt, J (Vernadsky Institute of Geochemistry and Anlytical Chemistry of Russian Academy of Sciences (GEOKHI RAS), 2018-10-01)
      International Conference on Magmatism of the Earth and Related Strategic Metal Deposits 3-7 September, 2018 Vernadsky Institute of Geochemistry and Analytical Chemistry of Russian Academу of Sciences, Moscow, Russia. The mineralogy of the effusive silicate rocks from the Mosonik volcano, Northern Tanzania Sedova А.М.1, Zaitsev A.N.1,2, J. Spratt2 1 Department of Mineralogy, St. Petersburg State University, Saint-Petersburg, Russia, e-mail: a.sedova@spbu.ru 2Department of Core Research Laboratories, Natural History Museum, London, UK The Mosonik volcano belongs to the Neogene-Resent volcanics of the Natron-Engaruka region of the East African Rift system. It is one of several stratovolcanoes located on the northeastern tip of the Gregory Rift Valley. Mosonik is attributed as having the earliest phase of eruptions in this province (Dawson, 2008) and is dated in the range 3.18-1.28 Ma (Isaac & Curtis, 1974; Dawson, 2008). In 1961, it was mapped by the Tanganyika Geological Survey (Guest et al., 1961), with published data (Paslick et al., 1996) on the composition of minerals from basanites, nephelinites and phonolites. According to the results of this study the compositions of melilite and nephelinite, Zaitsev et al. (2015) have indicated that the Mosonik volcano could be a potential source for the Upper Laetolil Footprint Tuff 7. According to our data the main effusive rocks of Mosonic are various nephelinites and phonolites, quite often they contain xenoliths of plutonic rocks: melteigites, foyaites, ijolites, and rocks of the enclosing stratum (andesites). Carbonatites mostly occur as boulders of various sizes within creek deposits. Among nephelinites there are nephelinites s.s., phonolitic nephelinites, calcite-phonolite nephelinites and melilite nephelinites. Microphenocrysts are represented by nepheline (45-60%), pyroxenes of diopside-hedenbergite solid solution, in some cases with aegirine edging (15-30%), apatite (3-10%) and titanite (3-10%). Calcite content reaches 10% within the calcite varieties of nephelinites; sanidine up to 10% in phonolitic nephelinites, which are strongly altered. Melilite nephelinites are also characterized by the following coposition: melilite (20%), perovskite (5%), sherlomite (3%). In rare cases within the nephelinites there are microphenocrysts of nepheline. Phonolites are represented by the following species: phonolites, sodalite phonolites and calcite phonolites. Phenocrysts are represented by nepheline (40-65%), pyroxenes of the diopside-hedenbergite series, rarely with aegirine edging (10-50%), sanidine (15-40%), Mg-Fe mica (0-5%), titanite (1-10%), and apatite (0-8%). In these rocks a large number of macrophenic crystals of nepheline, pyroxene, and often sanidine are observed. The work is supported by Russian Foundation of Basic Research (grant 18-05-00835) and St. Petersburg State University (Geomodel Resource Center) References Dawson J. B. The Gregory Rift Valley and Neogene-Recent Volcanoes of Northern Tanzania. London. 2008. 112 pp. Guest N. J., James, T. C Pickering R., and Dawson J. B. Angata salei. Geol. Surv. Tanganyika. Quarter degree sheet 39. 1961 Isaac, G. L. & Curtis, G. H. Age of the Acheulian industries from the Peninj Group, Tanzania // Nature. 1974. p.249. Paslick, C., Halliday, A. N., Lange, R. A., James, D. & Dawson, J. B. Indirect crustal contamination: evidence from isotopic and chemical disequilibria in minerals from alkali basalts and nephelinites from northern Tanzania // Contributions to Mineralogy and Petrology. Vol. 125. 1996. 277–292. Zaitsev A.N., Spratt J., Sharygin V.V., Wenzel T., Zaitseva O.A., Markl G. Mineralogy of the Laetolil Footprint Tuff: A comparison with possible volcanic sources from the Crater Highlands and Gregory Rift // Journal of African Earth Sciences. Vol. 111. 2015. pp. 214–221.