Host: The Japanese Association of Mineralogists, Petrologists and Economic Geologists
Almost all the geologic basement of Sri Lanka is underlain by Precambrian high-grade metamorphic rocks, which have been subdivided into four lithological units as follows: the Wanni Complex, the Highland Complex, the Kadugannawa Complex and the Vijayan Complex (e.g. Hiroi et al., 1990; Kroner et al., 1991) based on the difference of Nd-model ages. At Gintota near Galle, calc-silicate rock occurs as layers into charnockite belonging to the Highland Complex. The calc-silicate rocks composed mainly of clinopyroxene (Cpx) Scapolite (Scp) -bearing layers, and minor of Scp and Alkali-feldspar (Afs) layers, and wollastonite (Wo) layers. Judging from its mode of occurrence in the field, the calc-silicate rock probably formed during high-grade regional metamorphism of the calcic sediments originally. Simultaneously, the layers of the calc-silicate rocks cut by the veins up to 50 cm composed mainly of platy Wo. Cpx and Scp of coarse grain size are locally formed near the Wo vein intruding discordantly to the calc-silicate rock. Associated minerals are plagioclase (Pl), apatite, titanite (Spn), calcite (Cal), quartz (Qtz), graphite and pyrrhotite, except for Cpx, Scp, Wo and Afs. In the present study, we deal its mode of occurrence and mineralogical properties, and discuss the geneses of the calc-silicate minerals at this locality. Description of main minerals: Cpx occurs as aggregates of euhedral to anhedral crystals up to 15 cm. The mineral is dark-green in hand specimen. XMg value of the mineral is 0.53-0.60. The cell parameters are a=9.767(8), b=8.957, c=5.258(4) Å, β=105.58(4). Wo occurs as platy crystals characteristically in the vein. The mineral is white to pale yellow in color. It is monoclinic with the following cell parameters; a=15.409(4), b=7.311, c=7.066(2) Å, β=95.36(2). Scp found as a constituent mineral of the layer. The mineral occurs as aggregates of subhedral to anhedral crystals. It is gray in color and colorless in thin section. The chemical composition is uniform. The meionite content varies from 72 to 87 mole %. The cell parameters calculated from the powder data are a=12.165(3), c=7.567(3) Å. Afs found as melt pots and a constituent mineral of the layer. The mineral occurs as anhedral crystals. The chemical composition of the mineral is Or85-100. Genesis: THERMOCALC v.3.2 (Powell and Holland, 1988) and the thermodynamics data set (Holland and Powell, 2002) were used to construct activity-corrected partial petrogenetic grids. In the CASV model system, the formation of minerals such as Wo and Scp, and the metamorphic history were considered. Since the Scp is in equilibrium with Cpx, Afs, and Spn, the peak metamorphic temperature is estimated to be above 800°C. It is possible that aCO2 was to be greater 0.2 and less than about 0.5, judging from the absence of grossular. The breakdown reactions of Scp and Woe are respectively indicators for cooling. The increase in aCO2 is presumed from the presence of symplectite textures such as Cal+Qtz, Pl+Qtz, and Scp+Qtz that may be of retrograde origin.
