Paper No. 56-3
Presentation Time: 2:00 PM
PETROGENESIS OF INTERMEDIATE LAVAS AT HASANDAG STRATOVOLCANO, CENTRAL ANATOLIA
A suite of 34 samples from Hasandag stratovolcano in the Central Anatolian Volcanic Province provide evidence of the competing roles of magma mixing and low-pressure fractional crystallization in generating andesite lavas. Samples were analyzed for major and trace elements in conjunction with detailed petrographic analysis. The samples range in composition from basalt to dacite, ~49-67 wt.% SiO2. Bulk geochemical trends suggest fractionation as a dominant process for the full suite of analyzed lavas. We classify ~20 samples with silica content in the andesite-dacite range as “intermediate”. Plots of key elements (e.g., TiO2, P2O5, V) against MgO show that the majority of data define linear trends between primitive and evolved endmember compositions, while a smaller subset show variations characteristic of progressive fractionation of major and minor phenocryst phases. There is abundant evidence for mixing processes in the intermediate samples, e.g., very fine grained inclusions (enclaves; 1-3 cm across) in hand sample. In thin section, mineral disequilibria textures and assemblages are common. Embayed quartz phenocrysts with pyroxene reaction rims are present within centimeters of olivine crystals in some samples. Plagioclase phenocrysts commonly display disequilibrium features, including resorption and sieve textures. When assessing these features, two distinct populations of plagioclase phenocrysts can be identified. One population comprises large, tabular crystals that often contain internal dissolution structures surrounded by clear crystal rims, while the other population is zoned, typically smaller than 0.5 mm and lath-like in shape. We interpret these separate groups of plagioclase feldspars to be derived from constituent endmember magmas that mixed to create the final erupted composition: the smaller plagioclase phenocrysts formed in a basaltic melt and the larger crystals coming from a felsic melt. There is direct evidence for the latter group: large homogeneous plagioclase phenocrysts are present in a wide range of compositions, including dacites with ~67 wt.% SiO2. These disequilibrium features, alongside linear major element oxide trends, strongly indicate magma mixing as the dominant process responsible for creating erupted intermediate lavas at Hasandag stratovolcano.