Tres Piedras Granite
— Reinhard A. Wobus

The foliated Proterozoic granite that forms the conspicuous rounded
outcrops (more than three) for which this village was named is representative
of the older (ca 1,700 m.y.) plutonic episode of the Tusas
Range. The Tres Piedras Granite seen here, and its slightly more mafic
precursor, the Maquinita Granodiorite, bear the structural imprint of
the same period of regional metamorphism that deformed their metasedimentary
and metavolcanic wall rocks. The Tres Piedras is a highsilica
(about 76% Si02), low-calcium granite (quartz monzonite in pre-
JUGS terminology) with K20: Na20 of about 5 : 3. It is the same rock
that underlies most of the Tusas Box, but is not the same as the granite                                                                                    stock underlying Tusas Mountain, with which it was formerly correlated.
The latter rock, named the Tusas Mountain Granite (Wobus and
Hedge, 1982), is a post-tectonic granite locally enriched in fluorine and
a suite of trace elements indicative of late vapor-phase activity. Rb—Sr
whole-rock analyses and U—Pb dating of zircon from the Tusas Mountain
Granite indicate an age between 1,500 and 1,420 m.y.