Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

Xal Lineated amphibolite (Early Proterozoic) – Lineated amphibolite is present in
one main area associated with the triangular-shaped lithologic-structural domain with the
muscovite-sillimanite gneiss (Xmsg) in the eastern part of the Proterozoic terrane south
of the South Arkansas River. The lineated amphibolite makes up a 4,000 ft by 4,000 ft by
5,000 ft triangular area in the eastern part of this domain that extends across Green Creek
and beyond the southern edge of the quadrangle. It is poorly exposed with minor small
outcrops and commonly found as concentrated float.
The lineated amphibolite is dark gray to black and generally fine grained. The
mineralogy is dominated by amphibole and plagioclase; the rocks lack quartz and micas
and are generally nonmagnetic. Estimated modes are 50 to 70 percent hornblende and 30
to 50 percent plagioclase. In contrast to the well-layered amphibolite gneiss, the lineated
amphibolite occurs as thick massive zones suggesting a more homogeneous protolith.
The amphibolite is characterized by a moderate to strong penetrative lineation where the
amphiboles are aligned, discontinuous rods (fig. 37). When viewed normal to the
lineation the rock appears to have a strong foliation and when viewed parallel with the
lineation the rock appears to be equigranular and non-foliated. The orientation of the
lineation is consistently to the north-northeast (N10° to 35°E) with generally moderate,
but locally shallow plunges (20° to 64°) to the northeast.
Along the western contact with the muscovite-sillimanite gneiss a transitional
zone of amphibolite gneiss (with minor interlayered hornblende intermediate gneiss and
biotite-muscovite felsic gneiss) with gneissic layering is concordant to the contact and to
the moderate to strong foliation in the muscovite-sillimanite gneiss (Xmsg). The gneisses
in the transitional zone are generally oriented north-south with moderate dips (54° to 70°)
to the east.
In the central part of the lineated amphibolite sub-domain minor areas of the
amphibolite are interlayered with hornblende intermediate gneiss (Xhig), biotite felsic
gneiss (Xbfg), and calc-silicate gneiss (Xcs); these interlayers become more abundant and
in larger sized areas to the southeast. The eastern boundary of the lineated amphibolite
sub-domain is a major northwest trending fault zone that merges with or is truncated by
the Willow Creek fault bounding the western edge of the South Arkansas graben. The
distribution of Berthoud-type granite and pegmatite intrusions also changes across the
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