ABSTRACT
Whole-rock major and trace element compositions of one basalt, one
diabase, and 21 rocks from the magmatic-plutonic units of the
North Arm
Mountain massif, Bay of Islands ophiolite, Newfoundland, were
determined.
Mineral compositions of a subset of the plutonic rocks were also
determined.
The major and trace element compositions of the basalt and diabase
are similar to abyssal tholeiites, and this is consistent with the
REE
data of Malpas (1978) and Suen, et. al. (1979) that suggest the
lavas and
dikes formed from a depleted or slightly enriched abyssal
tholeiitic magma.
The alkaline nature of the magma proposed by several previous
investigators
based on major element chemistry is attributed to alteration. The
major
primary minerals of the plutonic rocks are approximately in
chemical equilibrium
with each other, and mineral zoning, where present, is normal.
This indicates
that the plutonic rocks formed mainly by in situ nucleation and
crystallization
on or near the margins of the magma chamber rather than by
homogeneous
nucleation and gravitational sorting.
Methods of estimating the amount of trapped liquid in plutonic
rocks
from incompatible trace element concentrations are discussed.
Textures
and estimates of trapped liquid indicate many of the plutonic
rocks from
the area of North Arm Mountain from where the rocks in this study
were
collected are mesocumulates. This and the thinning of the layered
units
and thickening of the isotropic gabbros suggest the plutonic rocks
in this
area formed under conditions of faster cooling than did those to
the northeast
and southwest.
Whole-rock and cryptic mineral variations with
pseudo-stratigraphic
height suggest the magma chamber was vertically zoned, with. the
degree
of differentiation increasing upward, but that the extent of
fractionation
was rather limited. Olivine Fo, plagioclase Ca#, and clinopyroxene
Mg#
varied by 9, 17, and 13 units, respectively, through a vertical
distance
of 6 km. through the transition zone and gabbroic units.
Mineral compositions determined in this study and others and the
gross
lithologic layering suggest the general crystallization order of
the North
Arm parent magma was olivine (± chromite) - clinopyroxene -
plagioclase
- orthopyroxene. This is inconsistent with phase relations of
abyssal tholeiites
in which clinopyroxene crystallizes after olivine and plagioclase.
No explanation
is suggested.
Most chemical and mineralogic features of North Arm Mountain
plutonic
rocks can be attributed to a combination of crystal fractionation
and repeated
mixing of newly injected parent magma in a large, steady-state
chamber
at an oceanic spreading center. This is consistent with the
geologic evidence
(Casey, 1980). However, other processes must be responsible for
some of
the minor variant mineral assemblages in the plutonic rocks.
Sullivan, J.W., 1981. Some chemical and mineralogical aspects of
plutonic
rocks from the North Arm Mountain Massif, Bay Of Islands
Ophiolite, Newfoundland.
Unpublished MSc. thesis, State University of New York at Albany.
149 pp.,
+xii
University at Albany Science Library call number: SCIENCE
Oversize
(*) QE 40 Z899 1981 S85
thesis (scanned text) - 6MB pdf file
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