Geoscience Reference
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7.1.2 Mount Albert Peridotite Example
The Mount Albert intrusion is the largest ultramafic mass (approx. 44 km 2 ) in the
Gasp´ (Quebec) portion of the so-called Appalachian ultramafic belt. It is probably
530 my old. The intrusion was mapped and sampled in 1959 by C.H. Smith and
I.D. MacGregor, who made their data available to the author for performing trend
surface analysis (Agterberg 1964 ). The petrography of the body is relatively simple.
Prior to serpentinization, it consisted of from 80 to 90 % olivine, the remainder
being primarily orthopyroxene with some chrome spinel (up to 1 %) and diopsidic
clinopyroxene. It was attempted to collect rock specimens from the nodes of a
rectangular grid with 1,000 ft. spacing. The actual number of specimens that could
be collected was less because of overburden. The number of mineralogical deter-
minations was further reduced by serpentine alteration. The following four vari-
ables were determined for as many collected specimens as possible: (1) cell edge
d 174 of olivine; (2) refraction index N z of orthopyroxene; (3) unit cell dimension of
chrome spinel; and (4) specific gravity of the whole rock. The first two variables
were converted into percentage magnesium in olivine and orthopyroxene, respec-
tively, and the results reported as mol. percent forsterite (Mg-olivine, cf . Fig. 4.2 )
and enstatite (Mg-orthopyroxene). Mineralogical and thermodynamic implications
of variations in the chrome spinel unit cell dimension were reviewed in MacGregor
and Smith ( 1963 ) and Agterberg ( 1974 ). The relationship between percent serpen-
tine and rock density in samples from the Mount Albert intrusion is approximately
linear. Serpentinization was a process that continued afterwards, when the ultra-
mafic body was already in place.
Trend surfaces for percentage forsterite in olivine, percentage enstatite in
orthopyroxene, and the chrome spinel cell edge are shown in Fig. 7.3 . The trends
for these three variables are similar in that they show an elongated minimum at
approximately the same location. This phenomenon was called cryptic zoning by
MacGregor and Smith ( 1963 ). The trend pattern is most pronounced for percentage
enstatite in orthopyroxene. The percentage explained sum of squares satisfies
ESS
39.2 % for the quadratic enstatite trend surface versus 6.3 and 16.0 % for
the quadratic olivine and chrome spinel trend surfaces, respectively. The origin of the
coordinate system with U pointing westward and V southward was set at a point
outside the study area. Unit of distance was 10,000 ft. Then, for example, the
quadratic equation of
¼
¼
the surface shown in Fig. 7.3b is: %Mg
79.116
2.2289 uv +1.9661 v 2 . Geometrically, this surface is
a hyperbolic paraboloid. By using elementary methods of matrix algebra, it can be
shown that this is equivalent to %Mg
0.0496 u 2
+5.806 u
1.245 v
0.5443 v 0 2 with the V 0
pointing WSW and U 0 NWN from the geometrical center of this quadratic surface.
The cubic trend surface for percentage enstatite in orthopyroxene suggests that
there occurs a %Mg maximum in the southeastern corner of the intrusion. Whether or
not this peak is real can be evaluated experimentally in several ways: Fig. 7.4 shows
the 95 % confidence belt on the entire cubic surface. The shape of this belt mirrors the
boundary of the intrusion because all observation points are contained within this
88.46 + 2.4608 u 0 2
¼
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