| Summary: | Bayat Region have been recognized as fossils of tertiary volcanoes in Southern Mountains, Central
Java; with volcanic bodies such as dikes, sills, pyroclastics, and lava flows. Former studies reveals
there were four times of magmatisms occured in Southern Mountains which aged early Oligocene to
middle Miocene. However, former studies have not proved whether the outcropped volcanic rocks are
originated from the same magmatism through geochemistry study. This research is aimed to complete
the previous study by comparing the bulk rock geochemistry of basaltic pillow lavas in Jarum Village,
approximately 3 km to the south of Pendul Hill, with the Pendul intrusion body itself. Methods applied
are bulk rock geochemistry analysis (ICP-AES) and trace elements using ICP-MS, supported by
petrographic analysis. Bivariate plots constructed to show cogenetic trends between two sets of above
mentioned samples, while spider diagrams are drawn showing magma origin in respect with the
comparison between the abundancae of LILE towards HFSE. By geochemical meaning, Pendul
gabbroic–dioritic intrusion in Jiwo Hills shows early stage of magmatism while basaltic pillow lavas
in Jarum Village, as the younger-aged volcanic body, is expected to originate from the same
magmatism. The expected result of this research is a cogenetic trend on bivariate plots and in AFM
diagram, as well as the decrease of siderophile compatible trace elements in spider diagrams between
the two sets of samples. This experiment geochemically proves the process of magma evolution and
establishes petrogenetic model of Bayat magmatism in a specific scope.
Keywords: bulk rock chemistry, petrography, cogenetic trend, magma evolution
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