Cargill Township carbonatite complex, District of ... - Geology Ontario

R. P. SAGE
more euhedral when in contact with the olivine than with each other. The olivine
varies from fresh to completely altered. The alteration ocurs along grain bounda
ries and along fractures within the olivine. Serpentine, iddingsite, and bowlingite
appear to be the alteration products. Allen (1972, p.41) also reported the pres
ence of talc.
A very minor amount of olivine occurs as a narrow zone or band separating
magnetite and clinopyroxene. These bands are discontinuous and likely represent
intercumulus growth. Allen (1972) classified rocks displaying this feature as oli-
vine-rim magnetite clinopyroxenite. On the basis of microprobe data Allen
(1972, p.49) determined that the forsterite content of the olivines interstitial to
the clinopyroxene varied from 65.4 to 76.096. The narrow zones or bands of
olivine between the magnetite and clinopyroxene vary from 62.5 to 70.89fc
forsterite (Allen 1972, p.49).
Amphibole occurs as anhedral to rarely subhedral brown or greenish-brown
grains. It is common as irregular patches scattered throughout the clinopyroxene.
The patchy replacements are crystallographically compatible with, and merge in a
diffuse manner into the pyroxene and reflect late stage alteration of the
pyroxene. Crystallographic controlled structures within the pyroxene have obvi
ously controlled the replacement by the amphibole. Amphibole is also common
as rims of varying widths mantling the pyroxene grains. It is also common as a
narrow zone along magnetite-pyroxene contacts and greatly exceeds olivine
which also occurs as narrow zones along magnetite-pyroxene contacts.
One thin section contains a clot of carbonate into which euhedral amphibole
crystals project. The amphibole crystals lie along the edge of the pyroxene crystals
which enclosed the carbonate clot. The crystals have brown to greenish brown
bases and pale green rims. The amphibole crystals are strongly zoned and occur
with a small subhedral grain of sphene.
Microprobe analyses by Allen (1972, p.52, p. 138-146) indicate that the am
phibole is paragasite or ferroan paragasite and that the most sodic varieties are
hastingsite.
Allen (1972, p.46) considered the amphibole to be primary and the result of
crystallization of late-stage interstial residual liquids. The author partly concurs
with this interpretation on the basis of textures observed in thin section.
Primary amphibole is likely represented by those grains that lie in sharp con
tact with other mineral phases and do not display grain morphology or structures
that can be traced from pyroxene to the amphibole. Some amphiboles, which
display structures traceable from pyroxene into the amphibole across grain
boundaries that may be diffuse in appearance, are clearly secondary. This secon
dary amphibole may result from reaction of late-stage deuteric fluids with previ
ously crystallized pyroxene or may result from reaction of decanted fluids (fenit
ization) associated with the emplacement of the later carbonatite magma.
All samples contain 59fc or more magnetite, although the author did not use
magnetite as a modifier in naming the rocks.
Magnetite occurs as anhedral grains throughout the rocks. It is present as an
alteration of olivine and commonly displays an affinity for amphibole which may
be either primary or secondary in origin. Some of the grains are elongate, lobate,
and interstitial to the pyroxene. The textural relations suggest that a major portion
of the magnetite is intercumulus and primary in origin. Amphibole, and rarely,
olivine occur as thin rims between the pyroxene and magnetite and some of this
magnetite is likely secondary resulting from the breakdown of pyroxene or oli-
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