Analysis of genetic relations between Eocene volcanic lavas in Shirinsu area (South of Lowshan) based on petrographic and geochemical evidences


Eocene volcanic rocks in Shirinsu area, consist mainly of alkali olivine basalt lavas, intermediate andesitic- trachyandesitic lavas and dacitic- rhyodacitic ignimbrites. A number of textural and mineralogical relations observed in thin sections and some geochemical evidences indicate that basaltic and andesitic magma in this area have a common origin and are affected by crystal fractionation, magma mixing (mingling and blending) and crustal contamination. These include the existence of glomeroporphyric and glomerocryst assemblages of mafic minerals, inequilibrium textures such as sieve texture in plagioclase, compositionally bimodal phenocrysts of plagioclase and clinopyroxene, hetrogenous composition of rock and the presence of basic enclaves and micropillows with opacitized groundmass in acidic volcanic rocks with light-coulered glassy matrix, existence of xenoliths and xenocrysts with reaction rims, etc. and dispersions and various trends observed in geochemical diagrams. On the other hand, intermediate andesitic rocks are the result of mixing between basaltic magma and it,s fractionated parts which were contaminated by crustal materials during the ascend and emplacement in upper crust. Based on existing evidences, it seems that voluminous dacitic and rhyodaciyic ignimbrites have a different origin in comparision with basalts and andesites. Acidic volcanics are most probably the products of crust partial melting which during emplacement of basic magma in crust, mixing (mainly mingling) is occurred between these magmas.