Feldspars of enderbite-charnockite complexes as indicators of alkalinity during the charnockitization of schists

Typochemical features and reaction textures of feldspars are examined as in dicators of the intensity of charnockitization of schists. The example of r ocks of the enderbite-charnockite complexes in East Antarctica is employed to distinguish five grades of transformations of schists into charnockites by means of magmatic replacement. Rocks of grade I (pyroxene-plagioclase sc hists transitional to enderbite) are characterized by an increase in potass ium activity in the fluid, a process that brings about plagioclase replacem ent by potassium feldspar, coupled with the crystallization of newly formed plagioclase of significantly more calcic (by 30% An) composition. The proc ess is described by Korzhinskii's reaction An . xAb + 0.5y (K2O in fluid) = yKfs + An . (x - y)Ab + 0.5y (Na2O in fluid). In the grade-II rocks (ender bites), the plagioclase has the composition NAn = 28-31 and contains large antiperthitic inclusions of potassium feldspar. The latter develops by the aggregation of newly formed potassium feldspar inclusions in plagioclase of grade-I rocks as a consequence of increasing potassium potential. The rock s of grade III (antiperthitic charnockites) are typically dominated by acid plagioclase (NAn = 0-13) with thread-like antiperthitic inclusions, which are spatially restricted to the surface of thin deformational twins. Grade IV is responsible for the development of two-feldspar charnockites with a h eterogeneous microtexture, which reflects both metasomatic processes and ma gmatic crystallization. The plagioclase of these rocks is more calcic (NAn = 21-30) than that in the grade-III rocks but more sodic than in the rocks of grade II. The potassium feldspar is intensely perthitized. The rocks of grade V (eutectic charnockites) have magmatic microtextures. The plagioclas e has the composition NAn = 44-49, the potassium feldspar is homogeneous. T he potassium feldspar, plagioclase, quartz, and orthopyroxene (eulyte) comp ose eutectic textures. Considered together with the massive structures of t he rocks, these features suggest that the rocks crystallized from melt.