Leaving aside organ meats for the moment (which Americans are 
generally inclined to do), perhaps I could sum up messages from Frank 
Spear, Bruce Yardley, Chuck Guidotti and Juergen Reinhardt as 
indicating three fundamental problems with so-called triple-point 
assemblages (or, better stated, triple point so-called assemblages): 
spatial non-contiguity at time of origin of all three minerals, 
chronological non-contiguity (polymetamorphism), and nucleation or 
reaction problems.  Undoubtedly all three occur, and both Bruce and 
Chuck have eloquently described reasons for the latter.

I have checked on a number of reputed triple-point assemblages and 
all but one of them turned out to be problems with polymetamorphism: 
the one exception (noted by Frank) is rocks from north-central New 
England (e.g., Mt. Moosilauke) that are in a nappe pile which 
represents low-P, high-T rocks of Buchan type (and+sil) thrust over 
lower-T, intermediate-P rocks (ky+sil).  The timing of metamorphism 
may be roughly the same (possible but not proven), but the rocks were 
not contiguous (nor at similar crustal levels) when metamorphosed.  I 
suppose this could be regarded as a form of polymetamorphism.  A more 
common form of polymetamorphism appears to be typified by late quartz 
veins cutting Buchan-type pelites containing and+sil.  These veins 
commonly have ky either along their margins or within them.  I have 
seen this in a reputed triple-point location at Wheatland, Wyoming, 
and also in various New England localities and in metapelites from 
the Blue Ridge in Virginia and North Carolina.  Undoubtedly there are 
also cases of metamorphic heating events narrowly or widely separated 
in time that have affected the same rocks - either younger and older 
regional events or a contact metamorphism superimposed on older 
regional metamorphic rocks.  I think this latter may actually be 
fairly common, especially in the form of a late regional-scale 
thermal disturbance associated with late felsic plutonism.  A 
regional-scale contact metamorphism, if you will.

So the coexistence of single or pairs of Al-silicate polymorphs , but 
lack of all three, typically would in no way imply the rocks did not 
experience a P-T trajectory that passed through or near a P-T 
location where all three could stably coexist, wherever that might be 
(that's language that sounds like it could have come from a lawyer!). 
Conversely, the coexistence of all three either in a limited outcrop 
area or even in the same hand sample in no way proves they formed 
stably at the same time and in the same place (and typically they 
didn't, in my experience).

Finally, responding to Chuck Guidotti's challenge, I'd say that I 
know of not a single incontrovertible triple point assemblage, though 
I doubt that we'll stop looking.

Bob T.
-- 
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Dr. Robert J. Tracy
Professor of Geological Sciences
Virginia Tech
Blacksburg VA 24061-0420

540-231-5980
[log in to unmask]
(FAX: 540-231-3386)

"Capital letters were always the best way of
dealing with things you didn't have a good answer to."
-- Douglas  Adams


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%