Petrography and Mineral Chemistry of Oligocene Shoshonitic Dacites From the Central Bosnia

Abstract

Postorogenic volcanic rocks of different Tertiary ages are very common in the Sava-Vardar Zone of the Dinarides and in the southeastern part of adjoing Pannonian Basin. South of the Sava-Vardar Zone, in central Bosnia, Tertiary volcanic rocks occur within ophiolite sequences and genetically related sedimentary formations of the Dinaride Ophiolite Zone. Central Bosnia volcanic rocks are mostly dacites, and highly subordinately andesites as the members of the high-K calc-alkaline series.It appears from the mineralogical and petrographic characteristics obtained some insight into the processes that occurred during the genesis of the rocks. The presence of primary igneous minerals: clinopyroxene, orthopyroxene, hornblende and biotite from ferromagnesian minerals, and plagioclase, sanidine and quartz, indicates that the fractional crystallization played a significant role in the genesis of the rocks. Reaction edge on many rounded quartz phenocrysts indicates the possibility of magma mixing with the formation of Tertiary volcanic rocks of the central Bosnia. On magma mixing different temperature and chemical composition also indicates the existence of zoned plagioclase and amphibole phenocrysts.Complex compositional and zoning patterns of biotite and plagioclase phenocrysts and disequilibrium microstructures of plagioclase and quartz phenocrysts suggest interaction of fractionating, mantle derived melts with continental crust during a shalow level pre-eruptive stage and mixing with small amount of devolatilized phlogopite-phyric mafic magma before eruption.

Geochemistry and Petrogenesis of Oligocene Dacites From the Central Bosnia and Herzegovina With Insight in the Postcollisional Tectonic Evolution of Central Dinaridic Ophiolite Belt

“…Pseudomorph after biotite consisting of fine-grained oxides and silicates [17]. Similar reaction products of biotite in dacites were interpreted by [43] as resulting from dehydration above its stability limit attending magma mixing.…”

“…Large quartz phenocrysts are corroded and embayed (resorption textures) indicate disequilibrium with coexisting magma [17]. Such features suggest they are xenocrystals entrained into the magma from crustal rocks From the textural viewpoint we explain quartz as xenocrystal entrained in basaltic magma (disequilibrium texture) during its ascend, before mixing with felsic magma.…”

“…Plagioclase disequilibrium textures in the CBTVR are recorded by zoning combined with sievetextures of large phenocrysts [17]. These fenomena are interpreted as results of heating above the existing plagioclase liquidus by mixing with a hotter magma [41,42,43] which causes solution and change to more calcic composition of sieved zone compared to the previous zone of exposed plagioclase [44] or to at least similar composition to the matrix plagioclases [39].…”

“…Another disequilibrium feature is the development of a reaction rim on biotites. This phase can be replaced by Fe-Ti oxides with increasing reaction, and pseudomorphs after and biotite are common [17]. Reaction rims on mafic phases indicate that the water content in the magma became too low for these hydrous minerals to be stable.…”

“…Central Bosnia volcanic rocks are mostly dacites, and highly subordinately andesites as the members of the high-K calc-alkaline series. The CBTVR are plagioclase, biotite and quartz phyric with a microto cryptocrystalline matrix consists abundantly sanidine and quartz along with accessory amount of apatite, hematite, pseudobrookite, zircon, rutile, monazite [17].…”