Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

metamorphism and is locally migmatitic. The regular fine-scale gneissic layering
suggests a possible layered protolith and the mineralogy suggests an intermediate
composition.
Xaa Amphibolite agglomerate (Early Proterozoic) – The amphibolite agglomerate
is a minor and localized, but very distinctive, unit that was found in three zones in the
southwest corner of the Maysville quadrangle. A fourth zone is present about 1,000 ft
southeast of the southwest corner of the quadrangle. The zones consist of interlayered
amphibolite agglomerate and predominantly amphibolite gneiss with minor biotite felsic
gneiss (Xbfg) and muscovite felsic schist (Xmfs). The zones range from about 100 to
2,000 ft long and 100 to 600 ft thick. Individual layers of amphibolite agglomerate are up
to 25 to 30 ft thick and can be traced for up to 300 ft along strike. The amphibolite
agglomerate generally makes good outcrop with small knobs and ledges.
The amphibolite agglomerate is a dark-gray to black, fine- to medium-grained peculiar
rock exhibiting a well-preserved remnant agglomerate or breccia structure (figs. 29 and
36). The term agglomerate is used because of the typical subrounded clast shapes and a
suspected volcanic protolith. They are essentially metavolcanic breccias with
predominantly subrounded clast shapes. The remnant textures suggest poor to locally
moderate sorting of clasts and a high proportion of clasts to matrix. Clasts range from ½
inch to 2.0 ft in size and from rounded to angular in shape. Most clasts are 2 to 6 inches
in size and are subrounded. Clast compositions are variable, ranging from monolithic
agglomerate with just amphibolite and amphibolite gneiss clasts to heterolithic breccias
with mixed clasts of amphibolite, felsic metavolcanic (), biotite felsic gneiss,
microdiorite, medium-grained gabbro, and quartzite-metachert (). The gneissic layering
in amphibolite gneiss clasts is randomly oriented from clast to clast, suggesting that the
gneissic structure is a primary layering feature in the clasts. The breccia is generally
matrix supported and the matrix is usually recrystallized, fine-grained amphibolite. The
breccias generally lack foliations, but large areas up to hundreds of feet long display
evidence of plastic deformation where the clasts have been stretched and drawn out
parallel with the breccia layer contacts (fig. 36).
Estimated modes of amphibolite agglomerate (from one thin section and hand
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