PETROGENESIS OF THE MAGLAJ VOLCANICS, CENTRAL BOSNIA

Abstract

InBosnia and Herzegovina, Tertiary volcanic rocks occur within two geotectonically different areas: (1) in northeasternBosnia, the wider area of Srebrenica and, (2) to a lesser extent in centralBosnia, the wider areas of Maglaj, Teslić and Nemila (Kolići). The major and trace element variations in the rocks surrounding Maglaj indicate their high-K calc-alkaline character, and are consistent with fractionation of the observed phenocryst assemblages (plagioclase, sanidine, biotite and hornblende). The scatter in some graphs cannot be explained by simple crystal fractionation from a common parent magma. More likely, such variations are affected by variable contribution and assimilation of continental crust within the magmas generated in the mantle. The presence of reverse zoned plagioclase phenocrysts, as well as a resorbed rim enclosing the coexisting quartz phenocrysts, can be interpreted by mixing of magmas with different temperatures and compositions.

All analyzed rocks show enrichment of the LILEs over the HFSEs and have significant negative Ta-Nb, P and Ti anomalies, and positive U and Pb anomalies, which are characteristic of subduction-related volcanic rocks generated in (post) collisional zones. Chondrite-normalized REE patterns exibit enrichment on the LREEs over the HREEs with (La/Yb)_cn ranging from 21.4 to 21.9. All analyzed rocks have small negative Eu anomalies (Eu/Eu^*=0.86-0.89) which suggests that plagioclase fractionation played a minor role in the genesis of the dacites. Ratios of some diagnostic elements versus SiO₂ diagrams are best explained as a result of assimilation of varying degree in the continental crust during magma ascent and continuing fractional crystallization. High values of LILE/HFSE ratios in K/Ti (6.6 to 11.9), K/Zr (124-169), K/Nb (1598-2692) and Ba/Nb (44-65) and negative anomalies of Ti and Nb in the volcanic rocks surrounding Maglaj can be explained as the result of complex processes in the magmatic system originally derived from a mantle wedge.