The solubility of potassium-feldspar in calcic plagioclase : experimental study and application to natural occurrences

Unusual feldspar intergrowths were found in two metamorphosed basalt dykes from Departure Rocks, Antarctica. They are composed of host calcic plagioclase (Kf 0 _3 An7 5_9 1 Ab9_25) and potassium-rich alkali feldspar (Kf90 _96 Ano_5 Ab0_8) lamellae. Their bulk composition is estimated to be Kf An 21 65 Ab14. The complex and yet regular lamella textures of these intergrowths suggest that they are exsolution products. This requires the existence of a formerly homogeneous plagioclase, compositionally equivalent to the estimated bulk composition of these intergrowths. Experimental studies in the binary system anorthite K-feldspar and in the albite-poor ternery feldspar system have proven the existence of such homogeneous plagioclase, which has been synthesized at 1150°C and 10 [(bar under dry conditions. The unusual feldspar intergrowths were homogenized under these same conditions, further supporting the hypothesized exsolution origin of these intergrowths. In an experiment of plagioclase crystallization in a synthetic rock system, a K-rich plagioclase (Kf 19.6 An70 Ab10 . 4) was obtained from a K-rich melt at 1140°C and 10 kbar, indicating that the proposed parental plagioclase for the unusual feldspar intergrowths can be formed in a complex (rock) system. Such K-rich melt could be generated through interaction of the dyke magma and its surrounding charnockitic magma. It is concluded that limits of solid solutions in feldspar systems are increased at high pressures due to the increase of feldspar melting temperatures. Pressure may have an effect on the extend of feldspar solid solutions, but it is considered to be very small.