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Title: The geology and petrology of the Engaruka-Oldowyo L'Engai-Lake Natron area of northern Tanganyika territory
Author: Guest, Norman James
ISNI:       0000 0004 0123 5151
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 1953
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This region· lies .in the northern part of the Tanganyika Gregory Rift Valley. Amongst certain aspects which are con-' sidered in less detail are the questions of tectonics and age, the rocks of the basement complex, and the geology of Lake Natron. The main work consists of the systematic petrography of a group of Older Extrusives, andesitic. and basaltic rocks, and of a series of Younger Extrusives, soda-rich alkaline rocks, including· the carbonat1tes of the volcano, Kerimasi. The former occurred after a period of rift faUlting;- they are associated with larger volcanoes, and cover greater areas than the probable Upper Pleistocene Younger Extrusives. Utiliz1ng 12 ne~l chemical analyses of rocks, 2 partial analyses, and incorporating all additional known rock analyses, a regional magmatic trend is 'outlined, with a parent magma of ... basic-andes1t1c composition, be11eved to have been derived from / an o11vine-basalt magma. This parent magma eXh1b1ts different1ation by fr~ctiona1 crystall1sat10n together with alteration through contamination w1th the basement rocks. To the east, .on the. volcanoes of Ketumbaine and Gelai, the magmatic trend is essentially, basic-andesite, andes1te, Mawenzi-, trachyandesite, phonolite, phonolitic-trachyte, trachyte. NepheI1ne-Irlelabasalt, nepheline-andesite, i\\ugear1 te, and nephe1initic-phono1ites were also noted. Similar rocks were mapped' on the rift wall to the west. After further rift-faulting, the nephelinitic-phonolite magma was further contaminated with the basement rocks, as shown -by the fenites and tveitlsites noted on the active volcano of Oldonyp L'Engai. Moreover, the nephelinitic-phono1ite magma was desil~cated by the assimilation of basement dolomitic limestone, and carbonatite was formed. Subsequently the source of the 1imesto~e having becon~ exhausted, the parent magma returned to near its pre-deailication composition. A detailed geological map has been constructed, and photomicrographs have been made or slides or most or the rook types.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available