Abstract
In the Donachui River area of the Santa Marta Mountains, Precambrian metamorphic rocks of amphibolite facies and three large Mesozoic masses are exposed. The metamorphism is considered to be associated with a large-scale granitization and probably superimposed on an earlier granulitic facies metamorphism. Many retrogressive minerals are also observed in the metamorphic rocks.
From the obliquities and optics of K-feldspar, the following three events are recognized in the metamorphic process. (1) a hair- or string-perthitic orthoclase (Δ=0, 2VX=59°-65°) was formed during the main phase of metamorphim; (2) this orthoclase was mostly transformed into maximum microcline (Δ=0.84-0.99, 2VX=76°-83°) with grading texture during the cataclastic movement; (3) some orthoclases have been transformed into intermediate microcline (Δ=0.77-0.81, 2VX=68°-72°) and the maximum microcline has been converted into “dirty” microcline (Δ=0.86-0.92, 2VX=76°-85°) showing ambiguous grating texture and containing abundant second generation perthitic albite during the retrogressive metamorphism.
Most K-feldspar in Mesozoic granites is orthoclase.
