Geology Of The Moxie Pluton In The Moosehead

28 GEOLOGY, MOXIE PLUTON, PISCATAQUIS COUNTY, MAINE
thousand feet below the surface are inferences rather than facts.
Depths of the pluton floor are based on the gravity interpretations
by Kane, which depend upon the assumption that the mafic
rocks of the Moxie pluton have a density of 3.0 and the metasedimentary
country rocks a density of 2.8, resulting in a difference
of 0.2. Although these average density values are very reasonable
assumptions, measured densities of both types range
widely. In the Greenville quadrangle (Espenshade and Boudette,
1967, tables 2, 3, and 5), the densities of mafic rock range from
2.78 to 3.30 and of metasedimentary rocks from 2.62 to 2.93.
Extensive areas of quartz diorite east of Moosehead Lake probably
range in density from 2.80 to 2.85 in contrast to troctolites
that range from about 3.00 to 3.10 (table 1). Thus, there may be
sizeable areas where the actual difference between the densities
of the pluton and the country rock varies appreciably from the
assumed value of 0.2. Kane (written commun., Feb. 4, 1970)
pointed out that in such areas the actual thickness of the pluton
would vary inversely with the difference, so that, for example,
where the difference was greater than 0.2, the actual thickness
of the pluton would be less than that shown in the sections of
plate 2. An additional factor of uncertainty comes from the uneven
distribution of gravity survey stations and the resulting
conjectural location of the gravity contours in areas of few stations
(pl. 3).
Differentiation
The present surface area of about 145 square miles indicates
that the original volume of the Moxie pluton must have been
several hundred cubic miles, quite possibly more than 500 cubic
miles, and the original vertical extent at least 3 or 4 miles. A
mafic 'intrusion of such enormous size obviously must have undergone
much differentiation during its intrusive and cooling history.
Differentiation is clearly shown by the wide range in chemical
and modal composition of the rocks and in the composition of the
principal minerals-plagioclase, olivine, and orthopyroxene.
Visher (1960) found that the most mafic rocks are in the circular
bulge surrounding Moxie Mountain at the southern end of
the pluton (fig. 1). Here, most plagioclase composition is Anso-9o,
and olivine composition is mostly Foi4-9o, in contrast to the more
sodic plagioclases and high-iron olivines that are so widespread
in the Greenville quadrangle and to the northeast. Compositional
layering of troctolite and dunite is common at the southern end of