Eric seems to forget that a facies is defined as "a set of metamorphic
mineral assemblages..." (Turner 1981, Metamorphic Petrology 2nd ed p.54;
Spear 1993, Metamorphic Phase Equilibria... p.14). The P-T range of a
facies is not part of its definition, but is strictly a matter of inference
and is always open to revision in the light of new experimental data or
whatever. Accordingly, we may be certain Frank Turner would never have
agreed with Eric that "The facies designation to use is whatever P-T you get
for the contact rocks and then consulting Turner's facies diagram." Indeed,
if we already know the P-T of a rock, what would be the purpose of
designating its metamorphic facies?
Metamorphic zones and isograds also are mineralogically defined, and I
was careful to define metamorphic bathozones and bathograds in the same
manner (Carmichael 1978, AmJSci 278), so that they would be independent of
any subsequent revisions to the petrogenetic grid, and independent of the
distinction between regional and contact metamorphism. But Turner's
classification of metamorphic facies, unfortunately, did not remain
independent of this distinction.
In 1962 Henk Zwart advocated that Turner's hornblende-hornfels facies be
abandoned because it was not mineralogically distinct from the regional
low-P amphibolite facies in the Bosost area nor the Buchan area of Scotland.
Turner first tried to meet this criticism by defining a "Hornblende-Hornfels
Facies in Regional Metamorphism" (1968, op cit 1st ed p.218). But in 1981
he assigned Bosost, Buchan and other areas to a "low-pressure amphibolite
facies series" that would be distinguishable from the hornblende-hornfels
facies only by its field setting:
"Is there a real difference between the hornblende-hornfels facies and the
amphibolite facies in the low-pressure environment? ...there are no
distinctive mineralogical criteria... And mineralogical criteria are by
definition the very basis of facies. If rock specimens or chemical
equilibria were the subject of this book, both classes of rocks could be
placed in a single facies (as indeed they were treated in my earlier work).
What justifies, even compels, separate treatment... is the completely
different tectonic environments and scales of development of the
hornblende-hornfels and the low-pressure amphibolite facies." (op cit 2nd ed
p.375).
Thus did Turner violate his own definition of facies, as well as demolishing
the inference that each facies has a distinct, non-overlapping range of P
and T - Eskola must have retrograded in his grave! Small wonder that some
of us consider metamorphic zones and bathozones to be necessary, sufficient,
and preferable!
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