Electron microprobe investigations of metamorphic reactions and mineral growth histories, Kwoiek area, British Columbia
<p>The Kwoiek Area of British Columbia contains a pendant or screen of metamorphosed sedimentary and volcanic rocks almost entirely surrounded by a portion of the Coast Range Batholith, and intruded by several dozen stocks. The major metamorphic effects were produced by the quartz diorite...
| Summary: | <p>The Kwoiek Area of British Columbia contains a pendant or screen
of metamorphosed sedimentary and volcanic rocks almost entirely
surrounded by a portion of the Coast Range Batholith, and intruded by
several dozen stocks. The major metamorphic effects were produced by
the quartz diorite batholithic rocks, with minor and later effects by
the quartz diorite stocks. The sequence of important metamorphic
reactions in the metasedimentary and metavolcanic rocks, ranging in
grade from chlorite to sillimanite, is:</p>
<p>1. chlorite + carbonate + muscovite → epidote + biotite</p>
<p>2. chlorite + carbonate → actinolite + epidote</p>
<p>3. chlorite + muscovite → garnet + biotite</p>
<p>4. chlorite + epidote → garnet + hornblende</p>
<p>5. chlorite + muscovite → garnet + staurolite + biotite</p>
<p>6. chlorite + muscovite → aluminum silicate + biotite</p>
<p>7. muscovite + staurolite → garnet + aluminum silicate + biotite</p>
<p>8. staurolite → garnet + aluminum silicate</p>
<p>Continuous reactions, occurring between reactions 5 and 7, are:</p>
<p>A. chlorite + (high Ti) biotite + Al<sub>2</sub>O<sub>3</sub> (from plagioclase?)→
garnet + staurolite + (low Ti) biotite + O<sub>2</sub></p>
<p>B. muscovite (phengitic) → garnet + staurolite +muscovite (less
phengitic) + O<sub>2</sub> (?)</p>
<p>Detailed electron microprobe work on garnet, staurolite, biotite,
and chlorite shows that:</p>
<p>(1) The garnet porphyroblasts are zoned according to a depletion
model, called the Rayleigh depletion model, which assumes equilibrium
between the edge of a growing garnet and the minerals which are
unzoned, notably biotite, chlorite, and muscovite, but which assumes
disequilibrium within the garnet.</p>
<p>(2) The staurolite porphyroblasts are also zoned, and from their
zoning patterns reactions A, B, and 5 are documented. Progressive
reduction of iron with increasing grade of metamorphism is also
inferred from the staurolite zoning patterns.</p>
<p>(3) During a late period of falling temperature garnet continued
to grow and the biotite and chlorite reequilibrated. The biotite,
chlorite, and garnet edge compositions can vary from point to point in
a given thin section, indicating that the volume of equilibrium at the
final stage of metamorphism was only a few cubic microns.</p>
<p>(4) The horizon within the garnet that grew at maximum temperature
can be identified. The Mg/Fe ratio of this horizon, if the garnet
composition is a limiting composition in the Al<sub>2</sub>O<sub>3</sub> - K<sub>2</sub>O - FeO - MgO
tetrahedron, increases systematically with increasing metamorphic
grade. Biotite and chlorite compositions also show a general increase
in Mg/Fe ratio with increasing metamorphic grade, but staurolite
appears to show the reverse effect.</p>
<p>(5) The Mg/Fe ratio at the maximum temperature horizon of the
garnet porphyroblasts is a function of its Mn content as evidenced
from the study of five garnet-bearing rocks, collected from one outcrop
area, with the same assemblage but with differing proportions of
minerals.</p>
<p>An important implication of zoned minerals is that the effective
composition of a system in a phase lies on the join between
the homogeneous minerals (if there are two) and not within three-or-
four-phase fields when a zoned mineral, such as garnet or staurolite,
is present in the assemblage.</p>
<p>Study of the three aluminum silicates found in the Kwoiek Area
showed that a constant pressure change in polymorphs from andalusite
to kyanite to sillimanite took place with increasing temperature.
This transition series is best explained by the metastable formation
of andalusite.</p>
<p>Photographic materials on pages 15, 121, 160, 162, and 164 are
essential and will not reproduce clearly on Xerox copies. Photographic
copies should be ordered.</p> |
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