Mineralogy and paragenesis of Ca-dachiardite in Cretaceous zeolitized volcanic rocks, Esquel, Chubut, Argentinian Patagonia.

Abstract

 Ca-dachiardite is present in Cretaceous volcanic breccias affected by very low grade metamorphism. The cement of the breccia shows quartz, Ca-dachiardite, mordenite, cristobalite, calcite and siderite, while in the clasts there are pectolite, prehnite, yugawaralite, Ca-dachiardite, albite, adularia, interestratified smectite/chlorite (S/C), quartz and cristobalite. Most Ca-dachiardites are found in the cement of the breccia. It occurs principally as fibrous tubes of parallel or lightly divergent bladed and tabular crystals, arranged in concentrical layers with length along 'b' and the fibers; the crystals are flattened on {001} and rarely on {100}. It shows cyclic twins in cross section normal to the length and multiple twins parallel to {001}, but single crystals are also present. The optical character shows: a=b; g:c= 43°; 2Va=55°/60°; strong dispersion r>v. The structural formulae is: Ca_1.54-1.72 K_0.82-0.98 Na₀ (Al_3.65-3.98 Si_19.95-20.25 O₄₈) 13H₂O ; T_Si= 0.83-0.85; R²⁺/(R⁺ + R²⁺)=0.61/0.68; DEC=Ca and K. The strong diffraction lines are: 3.460 Å; 3.190 Å and 1.870 Å; a₀ =18.679Å; b₀=7.488 Å; c₀ 10.267 Å; b=107°86'. The secondary mineralogical succession deposited according with the decrease in temperature and X_co2, could be: calcite (step I); pectolite, prehnite, quartz, yugawaralite, Ca-dachiardite, S/C, adularia, albite (step II, zeolite facies); mordenite and cristobalite (step III) and then, siderite and calcite (step IV). The metamorphic assemblages suggests that the crystallization conditions of Ca-dachiardite would be 200-220°C at pressures smaller than 0.5 kb, from high a_Si and a_Ca fluids.