engleski

Mineralogical and geochemical proxies of the Lower Miocene sediments of Mtn. Papuk – Poljanska and Mala sections (Northern Croatia)

The Poljanska and Mala sections are located on the south-western slopes of Mtn. Papuk in the northern part of Croatia, close to the active Poljanska quarry. The Poljanska succession starts with light grey tuffitic sediments. Above the tuffitic sediment, a thin grey siltstone layer and a brown medium-grained limonitized silty sand layer with seepage cracks are exposed. The silty sand continues into dark grey layered siltstone rich in plant remains and mollusc shells. Above the fossiliferous siltstone, a lens of light grey medium-grained silty sand appears, followed by grey horizontally layered fossiliferous silt intercalated with a few cm thick coal lens. The subsequent sedimentation was represented by the exchange of clayey sandy silt and silty sand. The uppermost part of the section is dominated by horizontally laminated grey siltstones and thin bentonite and clayey, silty sand interlayers. Mala section lithotype is determined as gravelly silty sand. The modal composition of the tuffitic sediment and silty sand samples was performed to determine the provenance of the material. All samples contain quartz and feldspars as a light mineral fraction (LMF). Only the tuffitic sediment and the silty sand layer above it additionally contain volcanic glass particles in the LMF. Gravelly silty sand of the Mala section and silty sand from the middle and upper part of the Poljanska section contains muscovite and lithic fragments. Among the transparent heavy minerals (THM), garnet and pyroxene (mostly orthopyroxene) grains are predominant in the tuffitic sediment. In contrast, gravelly silty sand and silty sand above tuffitic sediment predominantly contain garnet. In the upper part of the Poljanska section, silty sands contain pyroxene as prevalent. The tuffitic sediment contains 45% of fresh volcanic glass shards in the LMF. The XRPD analyses also proved volcanic glass by the broadening diffraction maximum at 20-30 °2θ, originating from an amorphous compound. The multi-purpose SEM images of glass shards show their angular shape and abundant pipe vesicles and bubble walls. The mineralogical composition of gravelly silty sand, tuffitic sediment, siltstone, and silty sand samples were also analysed by XRPD. Among clay minerals, the main component in all samples is the dioctahedral smectite, which corresponds to beidellite. Only the uppermost part of the section contains zeolite minerals from the clinoptilolite/heulandite group. The bulk rock chemistry of tuffitic sediment and silty sand samples was conducted. In the studied samples, SiO2/Al2O3 ratio for tuffitic sediment and the following silty sand layer sample is slightly higher, 4.84 and 4.25, respectively, than in other samples (3.07–2.61). This is consistent with the analysis of the modal composition of the same sediments because these two samples contain volcanic glass shards. The K2O/Al2O3 ratio is used as an indicator of the source composition of sediments. The studied samples, excluding one sample from the upper part of the section, have the same K2O/Al2O3 ratio less or equal to 0.5, indicating feldspar-rich source rocks. The degree of chemical weathering of the source rocks was constrained by calculating the chemical index of alteration (CIA= [Al2O3/(Al2O3+CaO*+Na2O+K2O)]x100. The CIA index, between 65 and 76, indicates weak to intermediate intensity of chemical weathering in the source area. The samples of the Poljanska and Mala sections are plotted near the (CaO*+Na2O)–Al2O3) line and follow the trend of the average tonalite and gabbro weathering (Nesbit & Young, 1984). Tuffitic sediment and one silty sand sample from the upper part of the section show a more intermediate igneous provenance, while other samples show mafic igneous provenance source rocks. The 40Ar/39Ar dating method was applied on separated volcanic glass particles from the tuffitic sediment. The inverse isochron age of the volcanic glass shards of 22.2 ± 1.9 Ma is considered to be the best estimate 40Ar/39Ar age. Mineralogical analyses revealed that the deposits are of a hybrid origin, both pyroclastic and terrigenous. The pyroclastic material is mainly slightly altered into smectite and clinoptilolite heulandite type of zeolites (the uppermost part of the Poljanska section). The occurrence of clinoptilolite/heulandite-type zeolites was probably caused by a gradual increase in aridity and salinity. Bulk rock chemical analysis suggests an intermediate to prevailing mafic chemical composition source rock affinity. The new age data presented within this study does not correlate to the rhyolite volcanic activity in the North Croatian Basin (NCB) as we expected but very likely implies the existence of an even older volcanic activity in the NCB, which may have provided material for the formation of the studied rocks. Acknowledgement: The authors are very grateful to Marijan Kovačić, Monika Milošević, Mirjana Miknić, Radovan Filjak, Mirko Belak and Ivo Suša, for their help during the fieldwork. Special thanks to Janet Gabites for Ar/Ar chronology. This study was financed by the Department of Geology of the Croatian Geological Survey. References: Nesbitt, H.W. & Young, G.M. 1984. Prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations. Geochimica et Cosmochimica Acta, 48, 1523–1534.

mineralogical and geochemical proxies, Lower Miocene, Northern Croatia

nije evidentirano