Petrography of Willits and northeastern W-A Amphibolites

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Amphibolite petrography and mineral chemistry
Petrography of Willits and northeastern W-A
Amphibolites

• Amphibolites found in the WI/WA areas show three
  distinct fabrics
• Amphibolite bands within mica gneisses of the
  Tallulah Falls Formation
• Pods (10 cm - 1 meter in diameter) within mica
  gneiss, or within a siliceous matrix. Near
  Webster-Addie ultramafic rocks -- commonly
  saprolitic
• Migmatitic bodies with spidery-looking leucosome
  textures. Most common in the Willits area
• Amphibolites found near the Webster-Addie
  ultramafic bodies were interspersed within the
  surrounding mica gneisses (interpreted as
  Tallulah Falls or Otto Formation by Quinn).
  Amphibolite exposures vary from Amphibole-rich
  bands to outcrop-scale pods. Larger scale
  amphibolite bodies show block in matrix
  structures with pervasive migmatitic textures,
  that range from lt1 m to gt15 m across. Near
  Webster-Addie, outcrops mica gneiss and
  amphibolite exposures are generally highly
  weathered, but changes in fabric and structures
  are still evident.
• In the Willits area only migmatitic amphibolite
  was observed in outcrops ranging from 3 to 100 m
  across. Changes in fabrics and textures were
  easily observed.

The common minerals in the amphibolites are
hornblende, plagioclase, quartz, and biotite.
Accessory minerals include garnet, titanite,
clinozoisite, apatite, chlorite, pyrite, and
pyrrhotite. Hornblende is compositionally zoned,
with a pleochroic scheme of light-brown to
dark-brown to blue-green and ranges from fine- to
coarse-grained. Amphibolite textures vary from
migmatitic to homogeneous and granoblastic.
Migmatitic amphibolites have a bimodal texture
quartz-plagioclase leucosomes surround
amphibole-biotite melanosomes. A later
metamorphic event appears to have produced
chlorite from biotite without restructuring the
rock. Biotite defines a lineation in some
samples.
AMPHIBOLITE MINERAL CHEMISTRY
Mineral Chemistry Determinations by Electron
Microprobe   Preliminary results for amphibolites
in and around the Webster-Addie ultramafic body
indicates a primary mineral assemblage of
hornblende quartz andesine biotite
garnet, with ilmenite, titanite, and rutile all
present in varying proportions. Retrograde
metamorphic effects are indicated by the presence
of chlorite replacing biotite and amphibole
(clinozoisite, though identified
petrographically, is not present in the thin
sections analyzed to date). The amphibolite
assemblages are consistent with the upper
amphibolite facies metamorphic conditions that
have been inferred for this region.
Mineral compositions of selected mafic and
ultramafic rocks collected during the 2001 REU
summer season were determined via electron
microprobe analysis using the JEOL JXA 8900R
Superprobe instrument at the Florida Center for
Analtyical Electron Microscopy (FL International
University). Initial studies were done during a
two-day analytical trip during the summer
program subsequent analyses were conducted in
real time from USF-Tampa via remote operation
protocols over an Internet 2-based operational
link. WDS analytical conditions were as follows
a 15 KeV analyzing beam with a 2 x 10-8 amp
current and a spot size 2 µm. All five
wavelength dispersive spectrometers were utilized
in analyzing ten elements Si, Al, Fe, Mg, Ca,
Mn, Ti, Na, K, and Cr. Counting times ranged from
10-30 seconds/element. A ZAF correction package
provided by JEOL was used for data reduction. A
variety of mineral standards (comprising some 15
different minerals) were used to generate "best
fit" calibrations for the various minerals
encountered in our analyses.