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STEPHEN A. KISH
Figure 1a. Rare earth chondrite normalized plot for C-10 granite. Chondrite normalization values from Henderson, 1984 (Table 1.5,
Column 1).
Figure 1b. Ocean ridge granite (ORG) normalized geochemical plot for the C-10 granite. Modified from Pearce and others, 1984.
The elements Ta and Hf have been omitted from the plot.
nearly constant 10x enrichment relative to a chondritic average
(Fig. 1a).
Trace element discrimination diagrams for granites
(Pearce and others, 1984) can be utilized to evaluate the possible
tectonic setting associated with granite formation. Utilizing
these diagrams (e.g. Fig. 1b) the C-10 granite would
be best characterized as a volcanic arc granite, however
these diagrams are not unambiguous. For instance, post-collisional
granites can have trace element signatures similar to
volcanic arc granites.
GEOCHRONOLOGY
In order to place some limits on the age of the C-10
granite, Rb-Sr isotopic analyses were performed on wholerock
and potassium feldspar (Table 2). Both biotite (mostly
chloritized) and plagioclase proved unsuitable for dating due
to the highly altered nature of these minerals.
A date obtained from a whole-rock and potassium feldspar
pair yields a value of 220+4 Ma (2 sigma). Because the
granite appears to have been metamorphosed and altered,
this age does not represent the time crystallization. Based
upon K-Ar and Rb-Sr dating studies of K-feldspar elsewhere
in the Appalachians, this age may represent the time when
the rock cooled to temperatures below 100 o – 150 o C. This
condition could be associated with extensive erosion that
took place following the end of Paleozoic orogenic activity.
A whole-rock model age may be calculated for the sample.
For such a calculation an assumed initial 87 Sr/ 86 Sr value
is used. For granitic rocks of the Piedmont (Fullagar, 1971;
Fullagar and Butler, 1979; Kish, 1983) the most common
range in 87 Sr/ 86 Sr initial ratio is 0.7040 to 0.7150. If the
upper value (0.7150) is used in the age calculation a model
age of 440 Ma is obtained; using the lower 87 Sr/ 86 Sr initial
ratio the model age is 550 Ma. Since most granites in the
Piedmont have relatively lo 87 Sr/ 86 Sr initial ratios, the
older age (~550 Ma) is preferred as an estimate for the age of
crystallization.
Table 2. Rb-Sr isotopic data for core sample C-10-1734.
Rb (ppm) Sr (ppm) 87 Rb/ 86 Sr 87 Sr/ 86 Sr
Whole-rock 165.7 150.0 3.206 0.73826
K-Feldspar 353.0 161.0 6.374 0.75474
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