Paragenesis of unusual Fe-cordierite (sekaninaite)-bearing paralava and clinker from the Kuznetsk coal basin,Siberia, Russia

Sekaninaite (X_Fe > 0.5)-bearing paralava and clinker are the products of ancient combustion metamorphism in the western part of the Kuznetsk coal basin, Siberia. The combustion metamorphic rocks typically occur as clinker beds and breccias consisting of vitrified sandstone–siltstone clinker fragments cemented by paralava, resulting from hanging-wall collapse above burning coal seams and quenching. Sekaninaite–Fe-cordierite (X_Fe = 95–45) is associated with tridymite, fayalite, magnetite, ± clinoferrosilite and ±mullite in paralava and with tridymite and mullite in clinker. Unmelted grains of detrital quartz occur in both rocks (<3 vol% in paralavas and up to 30 vol% in some clinkers). Compositionally variable siliceous, K-rich peraluminous glass is <30% in paralavas and up to 85% in clinkers. The paralavas resulted from extensive fusion of sandstone–siltstone (clinker), and sideritic/Fe-hydroxide material contained within them, with the proportion of clastic sediments ≫ ferruginous component. Calculated dry liquidus temperatures of the paralavas are 1,120–1,050°C and 920–1,050°C for clinkers, with calculated viscosities at liquidus temperatures of 10^1.6–7.0 and 10^7.0–9.8 Pa s, respectively. Dry liquidus temperatures of glass compositions range between 920 and 1,120°C (paralava) and 920–960°C (clinker), and viscosities at these temperatures are 10^9.7–5.5 and 10^8.8–9.7 Pa s, respectively. Compared with worldwide occurrences of cordierite–sekaninaite in pyrometamorphic rocks, sekaninaite occurs in rocks with X_Fe (mol% FeO/(FeO + MgO)) > 0.8; sekaninaite and Fe-cordierite occur in rocks with X_Fe 0.6–0.8, and cordierite (X_Fe < 0.5) is restricted to rocks with X_Fe < 0.6. The crystal-chemical formula of an anhydrous sekaninaite based on the refined structure is | textK_0.02 |(textFe_1.54^{2 +} textMg_0.40 textMn_0.06 )_{Upsigma 2.00}^M [(textAl_1.98 textFe_0.02^{2 +} textSi_1.00 )_{Upsigma 3.00}^T1 (textSi_3.94 textAl_2.04 textFe_0.02^{2 +} )_{Upsigma 6.00}^T2 textO₁₈ ]. left| {{text{K}}_{0.02} } right|({text{Fe}}_{1.54}^{2 + } {text{Mg}}_{0.40} {text{Mn}}_{0.06} )_{Upsigma 2.00}^{M} [({text{Al}}_{1.98} {text{Fe}}_{0.02}^{2 + } {text{Si}}_{1.00} )_{Upsigma 3.00}^{T1} ({text{Si}}_{3.94} {text{Al}}_{2.04} {text{Fe}}_{0.02}^{2 + } )_{Upsigma 6.00}^{T2} {text{O}}_{18} ].