Mantled alkali-feldspar megacrysts from the marginal part of the Karkonosze granitoid massif (SW Poland)

Abstract

Mantled alkali feldspar megacrysts from the porphyritic granite variety of the Karkonosze Variscan pluton (SW Poland) have been studied. The feldspars contain numerous inclusions of minerals (mainly plagioclases), which form inner rims, marking successive zones of megacryst growth, and they are surrounded by a plagioclase mantle (rapakivi texture). Although, due to maturation and coarsening, the feldspars display a heterogeneous exsolutions pattern, in the core part some homogeneous sectors (without visible decomposition) are also preserved. Chemical composition of alkali feldspar megacrysts (including barium concentration) and of plagioclases (inner inclusions and mantle) has been determined and used for evaluation of thermal conditions of melt crystallization. The highest calculated temperature for single equilibrium and close-to-equilibrium pairs of un-decomposed domains in alkali feldspar + plagioclase inclusion ranges 809-750oC. The differences in crystallization temperature calculated for the pairs alkali feldspar domains (megacryst) and plagioclase inclusions from successive inner rims or for the pairs alkali feldspar domains (megacryst) – plagioclase mantle varied within 100oC. Common lack of equilibrium for neighbouring pairs of plagioclase inclusion – alkali feldspar is noticeable. Growth morphologies of plagioclases (inclusions and mantle) and alkali feldspar have been proved by means of CL. Mostly the feldspars display signs of resorption and re-growth at many stages of their formation. The barium concentration in the feldspars also points to discontinuous growth. The crystallization path of the megacrysts and the formation of rapakivi texture were influenced by magma mixing, not by decompression processes.