Volcanic-Stratigraphy and Petrography of Floo...

Volcanic-Stratigraphy and Petrography of Flood Volcanics along Jita River in North-Central

Author Name : Endayen Belay Belachew, T. Suryanarayana, Dereje Ayalew, Gezahegn Yirgu

ABSTRACT

The study area is located in the North Wollo zone within the NW Ethiopian volcanic plateau. The area provides a well-preserved volcano-stratigraphic sequence from lower basalt initiation to upper lava flow termination. The main objective of the study is to establish the petrovolcanic stratigraphy of the area and to put the petrographic observations within the context of magmatic evolution or depositional history of volcanic products. To accomplish the research objectives, actual field observation, stratigraphic sampling and petrographic analysis methods were applied. The major volcanic units from bottom to top are aphyric-intergranular basalt, aphyric-trachy flow basalt, augite cumulophyric basalt, olivine-augite phyric basalt, Kfs vitrophyric rhyolite, augite phyric basalt, basaltic tuff, columnar-aphyric basalt, non-columnar-aphyric basalt, moderately welded rhyolitic tuff, Kfsphyric rhyolitic-ignimbrite, Kfs phyric rhyolite, columnar-aphyric basalt, thin layer basaltic agglomerate and slightly vesicular aphyric basalt. Petrographyically, these volcanic rocks have different mineral compositions and textures; this heterogeneity reveals that there is a variation in depth of mineral fractionation and magma flux in the lithosphere. The flows have broad changes in mineralogy and eruptive cyclicity and this suggests that they were pulsed with a fluctuating magmatic influx along with complex plumbing systems and overtime fed by shallower magmatic plumbing reservoirs. Overall, the petrovolcanic- stratigraphy findings in this study provide a new insight into the magmatic evolution of the Ethiopian CFB provinces.

Keywords: Cpx-cumulophyric, Megacrystic olivine, Magma plumbing system, Petrography,                Volcanic Stratigraphy.