The Precambrian rocks near Pilar
Jeffrey A. Grambling
The metarhyolite in the Vadito Group near Pilar contains abundant,
conspicuous quartz-eyes that are thought to be relict phenocrysts. Whereas
Ortega Quartzite is generally 98% modal quartz, this schist has only
FIRST-DAY ROAD LOG 305
50-75% quartz and 25-50% muscovite. The rhyolite is not only metamorphosed
but also severely deformed. Flattening probably exceeds
100%, and textures suggest significant shearing.
A few hundred feet above the highway, the metavolcanics are pink
due to the reddish muscovite they contain. This coloration is from ferric
iron (up to 6%) and trace manganese in the muscovite. The deep-red
prismatic mineral appearing locally in the pink schist is piemontite, a
Approximately 500 ft above the highway is the Vadito/Ortega contact.
The contact is sharp and well exposed. At the contact is a viridinemuscovite–
quartz schist about 2 ft thick. (Viridine is a bright-green
variety of andalusite containing ferric iron and manganese.) This same
thin viridine-schist unit has been noted at the Vadito/Ortega contact in
about 30 or 40 localities in the Rio Mora area about 40 km southeast
of here. It may be evidence for weathering of the volcanic section prior
to deposition of the Ortega Quartzite, or, alternatively, it may represent
a manganous chemical sediment deposited prior to the deposition of
the Ortega Quartzite. This is the first reported occurrence of viridine
schist in the Picuris Range.
Above the viridine schist is Ortega Quartzite. The quartzite here is
typical: very pure quartzite with abundant and well-preserved crossbeds.
Near the contact some good crossbeds show that the section is not
The quartzite looks much less flattened and sheared than do the
underlying metavolcanics—especially the viridine schist just beneath
the contact. This is probably due to competency differences between
units during major deformation, and it may explain the apparent concentration
of shearing along the contact and in the uppermost part of
the metavolcanic section.
- Grambling, Jeffrey A., 1984, The Precambrian rocks near Pilar, in: Rio Grande rift--northern New Mexico, Baldridge, W. S.; Dickerson, P. W.; Riecker, R. E.; Zidek, J., New Mexico Geological Society, Guidebook, 35th Field Conference, pp. 305-306.