Petrogenesis of Granitoid Rocks of Assosa Area, Western Ethiopia
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Date
2017-06
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Addis Ababa Universty
Abstract
The main objective of this research study to constrain the petrogenesis of granitoid rock of Assosa area. To achieve objective successfully, different research method had been done like: field investigation, petrographic analysis and description and geochemical data analysis and interpretation. Assosa granite rocks dominated by alkali feldspar, plagioclase feldspar and quartz mineral in addition to these the muscovite, biotite, sphene, and hornblend, garnet and opaque mineral occurred. Granitoid rock samples of the area characterized by medium to coarse-grained texture, euhedral to subhedral crystal shape, granular, intergranular texture of opaque mineral(Fe-Ti oxides) and alteration of plagioclase and alkali-feldspar lead to formation of sericite/muscovite and chlorite. An Assosa granitoid rock is categorized under alkali-feldspar granite and syenogranite of plutonic igneous rock classifications on QAP diagram. Scattered data follows and negative trend correlation of some major element versus to SiO2 indicate the occurrence of fractional crystallization and alteration or mobility elements. It has charactestics of both volcanic arc granitoid (VAG) using trace element tectonic discriminators like: Rb versus Y +Nb and Rb versus Yb + Ta and syn-collisional granitoid (Syn-COLG) Nb versus Y, which resembles subduction zone or island arc environment related granitoid rock formation. The Enrichments of LREE, negative Eu anomaly and slight to moderate flat HREE on Chondrite-normalized and highly deplement of Sr, Ti, P and negative anomaly of Nb and Ta multi variation element pattern diagram indicate common source region and arc-related magmas result from more mafic magmatic source that undergo fractional crystallization. High contents Uranium, Thorium and Potassium in both normalized and variation element diagram and negative anomalies of Nb and Ta indicate that granite of the area are mainly from continental crust of arc-related magmatism. Based on variation diagram of compatible (Sr) versus incompatible (Rb) on log scale representation wide variation in the concentration of Sr but the concentration Rb remain constant. This show that Assosa granite mainly resulted by fractional crystallization of mafic magma related to arc setting
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Petrogenesis of Granitoid Rocks