COMPARISON:

1
Typical Analyses and CIPW Norms of WA-Wi
Amphibolites
COMPARISON Webster-Addie Amphibolites to Buck
Creek and Carroll Knob Amphibolites
Whole-Rock Geochemical Variations of WA-WI
Amphibolites Amphibolites from the Webster-Addie
and Willets areas differ markedly in their
compositions from the Buck Creek, Carroll Knob,
and other amphibolite exposures in the
southwestern North Carolina Blue Ridge. All the
WA-WI amphibolites are relatively high in SiO2
(52-60 wt), and are generally strongly quartz
normative. On classification diagrams such as
NaK vs. SiO2 or K2O vs. SiO2, the WA-WI samples
plot as more evolved igneous rocks (i.e, andesite
and dacite), and are distinctly less mafic than
like rocks from either Buck Creek or Carroll
Knob. On plots of MgO vs. Al2O3, and CaO vs.
Al2O3, it is clear that the WA-WI rocks cannot be
explained as simple mixtures of mafic minerals -
if these rocks had igneous protoliths, those
protoliths were most likely intermediate
composition lavas, and not mafic cumulate rocks.
Perhaps the most telling difference between
WA-WI amphibolites and others of the SW Blue
Ridge is evident in the AFM diagram, where these
rocks follow a distinct calc-alkaline trajectory,
suggesting that unlike the rocks of Buck Creek
and/or Carroll Knob, they are probably related to
subduction zone processes.
Constraints on the origin of Webster-Addie/Willets
amphibolites
Sample Preparation and Analytical Techniques  
On most tectonic classification diagrams, the
WA-WI amphibolites do not plot coherently. On
the V vs. Ti classification plot of Shervais
(1982) and the TiO2 vs. Fe plot of Serri
(1981), the WI-WA rocks plot largely in the
oceanic basalt fields. On the V vs. Fe/Mg
classification diagram of Desmos (1980), WA-WI
rocks fall largely between the calc-alkaline and
tholeiitic fields, and on the V vs. Cr plot of
Pearce (1973), most of the WA-WI samples fall
outside the field for MORBs. The WA-WI
amphibolites differ from calc-alkaline lavas in
that they have high concentrations of TiO2,
evidenced in the samples as abundant titanite,
ilmenite, and rutile. The process of partial
melting during migmatite formation sequesters
Fe-Mg-Ti bearing minerals such that on the scale
of hand samples, anomalously high (and low) Fe
and Ti concentrations are possible, depending on
whether the sample included more leucosome vs.
melansome. Interestingly, even if one takes the
mean values of our amphibolite samples, they
still plot anomalously on classification
diagrams. Options for explaining these patterns
include the extraction of anatectic melt, which
would shift the compositions of the rocks (and
would imply their protoliths were even more
felsic), or the WA-WI amphibolites may not, in
fact, represent igneous protoliths.
Summary of Observations and Inferences
WA-WI amphibolites occur as lenticular bodies
in the field, on a variety of scales, and may or
may not be in close association with ultramafic
rocks. Amphibolite fabrics and textures range
from strongly foliated to migmatitic. "Block in
Matrix" structures are commonly observed in the
field.  Mineral assemblages are Hbld Plag
Qz Bio Gt Titanite ilmenite rutile,
with chlorite, clinozoisite, and carbontes as
secondary metamorphic phases.   Major-element
chemistry of WA-WI amphibolites points to
probable igneous rock protoliths of intermediate,
calc-alkaline compostions. The processes of
metamorphism, migmatization, and melt extraction
have extensively redistributed even "immobile"
trace elements, preventing more detailed
assessments of tectonic affinities. In terms
of composition and probable protolith, the WA-WI
amphibolites are profoundly different from those
associated with the Buck Creek and Carroll Knob
complexes to the SW. The differences in the
amphibolites may indicate different origins and
histories for the rocks in these regions of the
Blue Ridge.