Fluid inclusion studies in datolite of low grade metamorphic origin from a Jurassic pillow basalt series in northeastern Hungary

Abstract

Abstract:The Jurassic pillow basalt blocks in the Szarvaskő Unit in the SW-Bükk Mountains are incorporated into an accretionary mélange of Cretaceous age which had been displaced from the Dinarides to NE-Hungary during the Alpine-Carpathian collision. The pillow basalt series is a part of an incomplete remnant of an ophiolite-like sequence from which ultramafic units and sheeted dikes are almost completely absent. At the studied Egerbakta quarry, closely packed pillow, pillow fragmented hyaloclastite breccia and peperitic facies of submarine basaltic volcanism were recognized. Peperitic facies was formed by local admixture of unconsolidated siliciclastic sediment into the basaltic lava. Basaltic pillows contains short and thin prehnite-chlorite-carbonate-quartz veins formed during the interaction of seawater with the cooling lava, as well as cross-cutting datolite-prehnite-quartzchlorite-albite bearing veins. Calcite in the short cooling cracks crystallised from upheated seawater at around 160 • C temperature and 0.5-0.6 kbar pressure (water depth of about 5-6 km). Datolite precipitations in veins can be classified into at least three different textural-morphological types but all of them contain methane bearing primary fluid inclusion assemblages, with variable methane/water ratios, suggesting an inhomogeneous entrapment from a heterogeneous aqueous-carbonic fluid. End member primary fluid inclusions contain aqueous liquid with methane rich vapour phase and apparently methane only inclusions characterized by liquid phase on room temperature. A combination of results from fluid inclusion microthermometry, Raman spectroscopy (used not only for phase identification but also for determination of salinities) and temperatures calculated from chlorite compositions, show that the precipitation of datolite had taken place from low salinity (0.2-2 NaCl equiv. mass%) aqueous-methane fluids under increasing temperature and pressure (from 160 to 210 • C, and from 0.6 to 1.1 kbar) conditions. The elevated pressures and temperatures in comparison to the submarine hydrothermal processes suggest that the datolite bearing mineral parageneses had been formed during the progression of Alpine very low grade metamorphic processes.

Raman study of datolite CaBSiO4(OH) at simultaneously high pressure and high temperature

“…The borosilicate mineral datolite CaBSiO 4 (OH), which is a common secondary mineral in mafic igneous rocks (e.g. associated with zeolites and carbonates in the vesicles of seafloor basalts), can also participate in this process because of its high thermal and baric stability. In spite of comparably small abundance (i.e.…”

European Current Research on Fluid Inclusions — ECROFI XXI Leoben (Austria)

“…Gabriela Kiss et al [10] investigated epigenetic processes in Jurassic pillow basalts. Fluid inclusions in datolite-rich cross-cutting veins provides information about very-low grade metamorphic processes.…”