engleski

Mineralogy of altered tephra layers in the Upper Jurassic Lemeš deposits near Maovice (Dalmatia, Croatia)

Two bentonite layers near Maovice village in the Dalmatian hinterland intercalate within typical Lemeš deposits (dolomitic limestones and dolomite with chert intercalations) (BRAUN, 1991). The XRD analyses of samples revealed smectite mineral as the principal mineral in both samples accompanied by calcite, cristobalite, plagioclase, quartz, biotite and augite in the Bunarić sample and calcite, quartz, zeolite and cristobalite in Gornji Ždanj sample. Dioctahedral smectite mineral was determined on oriented samples on the basis of maxima with d(001)≈ 14.83 &Aring ; ; , d(001)≈ 15.02 &Aring ; ; (air dried), d(001)≈ 16.68 &Aring ; ; , d(001)≈ 16.62 &Aring ; ; (etilenglycole saturated), d(001) ≈ 9.79 &Aring ; ; , d(001) ≈ 9.58 &Aring ; ; (heated) for Bunarić and Gornji Ždanj samples, respectively. To determine the heulandite group zeolite procedure after ALIETTI, 1972 was used. X-ray powder patterns were recorded on the original Gornji Ždanj sample (d(020)≈ 8.98 &Aring ; ; ), and on samples heated at 400°C for 24 hours and at 550°C for additional 24 hours. The observed contraction (d(020)≈ 8.35 &Aring ; ; ) and destruction of the lattice indicate heulandite type 1. Samples, coated with carbon, were investigated on SEM equipped with EDS. Microprobe chemical composition of volcanic crystaloclasts in Bunarić sample (biotite (Mg# = 55.8-61.3 ; TiO2 = 4.2-4.7 wt.%, BaO = 0.18-0.58 wt.%), sanidine (Or65.9-74.9Ab24.4-33.5An0.65-1.34), subcalcic augite (Wo36.5En41.1-41.5Fs22.0-22.3), hyperstene (Wo1.6En59.2Fs39.2) and potassium tschermakite) and whole rock chemical composition suggest a high-K calc-alkaline (riodacitic-rhyolitic) volcanic arc as the source of tephra. Alteration of tephra to the assemblage consisting of montmorillonite and heulandite suggests diagenesis at around pH~8, typical for the relatively deep marine setting within the carbonate platform. Different fossils, mostly radiolarian, are abundant in both samples.

bentonite; altered tephra layers; Upper Jurassic; Lemeš deposit; Maovice; Dalmatia; Croatia

nije evidentirano