The light colored minerals are dominantly plagioclase of andesine
composition and may include sodalite, and nepheline.
Euhedral plagioclase (< 3.0 mm) displays core compositions ranging from andesine
to labradorite ([An.sub.33-48]; Table 2, Fig.
Plagioclase with andesine
to labradorite and clinopyroxene with augitic composition are predominant phases in these volcanic rocks.
to oligoclase ([An.sub.27-30]) predominate over sodic sanidine and scarce clinopyroxene and titanomagnetite in the matrix of fresh trachyandesite facies.
These data indicate that the plagioclase has An values of 23.02-36.15 and that the granodiorite contains oligoclase (An = 23-28) and andesine
(An = 30-36) plagioclase (Fig.
Plagioclase is cloudy to saussuritized in some rocks especially in the more calcic cores but where fresh it is of intermediate anorthite content (calcic oligoclase to andesine
Most of them are classified as labradorite and andesine
Tabular plagioclase shows weak zoning with more sodic rim (oligoclase to andesine
) and more calcic cores (basic andesine
In vogesite its composition ranges from andesine
to bytownite, and minor albite occurs in the groundmass of monchiquite (Lindgren and Ransome, 1906; Loughlin and Koschmann, 1935; Thompson, 1992).
Other plagioclase grains have distinct cores of higher An content (of andesine
composition), which are commonly seriticized (Fig.
Euhedral to subhedral zoned oligoclase to andesine
plagioclase (30% to 40%) and anhedral quartz (20% to 30%), in small pods, are the primary constituents of the rock.
Zoned plagioclase with average composition of andesine
is the most abundant mineral, with less abundant interstitial K-feldspar and quartz.