Durbachite

Photomicrographs

Sericitized phenocryst of potassium feldspar – Kfs in groundmass composed of biotite – Bt, actinolite – Ac, and quartz – Qz. Biotite often encloses accessory apatite – Ap (upper photomicrographs). A large phenocryst of potassium feldspar – Kfs twinned according to the Carlsbad law alongside biotite – Bt (lower photomicrographs). Width of the photomicrographs is 2.2 mm.

Normative composition

Durbachite displays the normative composition anomalous for alkaline rocks, because it contains neither modal nor normative foids and/or alkalic pyroxenes/amphiboles. Despite this, durbachites are ultrapotassic rocks what is expressed by an increased content of the normative orthoclase – or. A high Mg content is projected in the normative hypersthene – hy and a supersaturation in Si in the normative quartz – q.

Comment A lack of Fe²⁺ in the chemical analysis caused the allocation of whole molar proportion of Ti in titanite – tn and not in ilmenite il. A small molar proportion of Fe²⁺ is a result of the addition of molar proportion of Mn, which is used in the normative ilmenite – il. The whole molar proportion of Fe³⁺ occurs in the normative hematite – hm. Small molar proportion of Ca remaining after the creation of normative anorthite and titanite remained for the creation of normative diopside – di. Also the content of normative apatite – ap is relatively high.

Chemical composition

Durbachite is very specific, even anomaly plutonic magmatic rock. It is a metaluminous, alkalic, ultrapotassic rock with the K₂O content > 3 wt. %, the K₂O/Na₂O ratio > 2 and the MgO content > 3 wt. % (Foley et al., 1987). According to Mitchell and Bergman (1991), ultrapotassic rocks exhibit the molar K/Na ratio > 3. Typical melanocratic durbachites from the Sedlec – Prčice locality have K/Na ratios ranging between 3.76 and 5.26, and the medium-dark varieties between 3.92 and 4.82. Ultrapotassic rocks of the World are prevailingly volcanic or hypoabbysal, whereas their plutonic equivalents are very rare. Durbachite is significantly enriched in MgO compared to common syenite, and it differs from the syenite also in terms of increased Al₂O₃ and CaO contents. Durbachites are also significantly enriched in LREE, showing a negative slope of the normalized distribution of the rare elements and the Eu/Eu* ratio = 0.8-0.9. Taktiež sú významne obohatené o LREE s negatívnym sklonom normalizovanej krivky vzácnych zemín a vyznačujú sa Eu anomáliou Eu/Eu* = 0,8-0,9. Durbachite originate from a mantle-derived magma modified by mingling with a crust-derived leucogranite magma in various ratios, and/or by a fractionation. It is supposed that the lithospheric mantle must have been moderately depleted prior to the melting by separation of the basaltic component and subsequently metasomatically enriched (Holub 1997).