Contact Metamorphism
105
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Regional metamorphic rocks
Sillimanite
grade
Staurolite-kyanite
~
grade
~
Garnet grade
Biotite grade
Chlorite grade
Shale
Contact metamoq~bic rocks
r~'~'~·
~
.. """I.m. ""' 'ill_It,,~.1
~ Outer aureole
........,
~ IAndalusite and biotite zone)
a Outside aureo[e(Chlorite grade)
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4
6
8
10
[2
[4
If 180 %. (whole rock)
)6
18
Fig.36. 018/0lD ratios of pelitic rocks plotted against metamorphic grade for both
regional and contact metamorphism. (After SHIEH and TAYLOR, 1969a)
contact offer the possibility to investigate the extent of isotopic exchange
between the intrusive and its country rock.
In contrast to regional metamorphic rocks, the 180rO ratios in
contact metamorphic rocks remain exceedingly constant throughout the
entire contact aureole (see Fig. 36). Somewhat surprisingly, Jl80-values
in pelitic hornfels of a number of contact aureoles have been found to
remain very much the same as in the parent sedimentary material, except
within a meter or so of plutonic contacts (SHIEH and TAYLOR, 1969 a).
The fact that only a very narrow zone of country rock undergoes
isotopic exchange with the intrusive implies that horizontal outward
movement of water into the contact metamorphic aureole is almost
negligible during crystallization of the pluton.
This indicates that most of the water involved in contact metamorphism is derived from the country rocks themselves rather than from the
intrusive.
Based on oxygen isotope fractionation calibration curves, SHIEH and
TAYLOR (1969a) estimated the contact temperatures of granitic plutons
and country rock to be between 525 0 C and 625
0
C, which agrees