engleski

Does Tuff Geochemistry Control Its Diagenetic Rate? – Case Study of the Ugljevik Basin Tuffs from the Miocene Pannonian Basin System (NE Bosnia and Herzegovina)

The Ugljevik or Semberija Basin is located in the Sava Zone of the Dinarides fold-and-thrust belt at the southern margin of the Pannonian Basin System (UNEN et al. 2019, MANDIC et al., 2012). Numerous tuffs from early and middle Miocene are recovered in post-orogenic lacustrine deposits of the Dinarides Lake System (DLS) making a record of an extensive volcanic activity (DE LEEUW et al., 2012, KRSTIĆ et al., 2001). The Ugljevik Basin, in contrast to the most of Dinarides basins, has been affected by the Badenian marine transgression (MANDIC et al., 2019, PAVELIĆ & KOVAČIĆ, 2018). This created relatively stable marine environment, which made diagenetic patterns of analyzed tuffs more robust compared to those weathered under altering depositional conditions (e.g. ZHAO et al., 2017). Six middle Miocene tuffaceous horizons (13.86 to 12.6 Ma, MANDIC et al., 2019) intercalated between the marls and limestones were sampled for the purpose of this research. Tuffs show high levels of alteration giving rise to the formation of ubiquitous clay matrix in which only the remnants of volcanic glass are preserved. The loss of ignition values (16.5 to 36.5 wt. %) are in line with the high content of authigenic clay minerals, which are largely consisted of illite and different intermediates of illite-smectite. Calcite is another major mineral, while quartz, muscovite, biotite, plagioclase, K-feldspars and amphiboles represent minor phases. Presence of calcite and aragonite is likely linked to the abundance of fossiliferous content. Chemical composition of tuffs is featured by low amounts of K (K2O = 0.53-3.18 wt. %) and Na (NaO = 0.05-1.28 wt. %), and strong variations in the content of Ti (TiO2 = 0.07-0.23 wt. %) and Si (SiO2 = 20.29-54.90 wt. %). The Nb/Y vs Zr/Ti (PEARCE et al., 1996) discrimination diagram largely defines these tuffs as intermediate rocks, with only one sample classified as rhyolite. Concentrations of HFS elements seem to decrease with time (Hf = 4.63-1.22 ppm, Ta = 2.6-0.28 ppm), while the Mg# and Cr concentrations exhibit the opposite trend (Mg# = 33.63-89.5 ; Cr = 9.3-118 ppm). Generally, the original magmatism must have been evolved (~6- 40 times chondrite concentrations) with a modest enrichment of LREE over HREE [(La/Lu)cn = 6.00-11.34]. A clear trend has been documented between the crystallinity of the main alteration product of tuffs – illite-smectite – and the age of tuffs, which in turns reflects a decrease in magmatic evolution of tuffaceous geochemistry (Figure 1.). Thus, well- crystallized but disordered illite-smectite (FWHM = ~0.8°) dominates in younger and less evolved tuffs, while in the older ones, more evolved in nature, a poorly crystallized illite-smectite (FWHM = ~1.42°) renders a major alteration product. We hypothesize that the reactiveness of less evolved magmatic material, prone to deuteric alterations, may explain the correlation patterns presented herein. Future research which will include larger dataset and additional techniques will lend further insights into the problematic of diagenetic evolution of Miocene tuffs from DLS.

tuffs, diagenesis, Dinarides, southern Pannonian Basin, clays, mineralogy

nije evidentirano