To clarify a point made in Solomon Buckman's query, UHP rocks HAVE undergone extensive retrograde metamorphism in all the UHP terranes I know about. Most of the well-preserved UHP rocks are volumetrically very small relics in terranes dominated by amphibolite or granulite grade rocks. There are several interesting physical models around at the moment attempting to explain how these rocks might have surfaced (e.g those by Chemenda or Hynes and their co-workers). However, testing these models in the field is very difficult due to the extensive retrograde metamorphic and structural overprint, which probably formed very late in the exhumation process when much of the exhumation had already happened. As a result, most of the interesting early exhumation fabrics have probably been obliterated.
I'm not sure if I understand Falk's evidence for melting in the Norwegian Western Gneiss Region, but I do agree that there is evidence for widespread, though limited melting of eclogites and orthogneisses during exhumation in parts of this terrane. Even so, one might expect more wholesale melting and plutonism during exhumation of such deeply subducted continental crustal rocks (I think the same applies to Dabie Shan). The limited extent of melting suggests that some sort of "refrigeration" mechanism is required.
At risk of being guilty of a "plug" some of these issues were covered in the Exhumation of Metamorphic Terranes meeting in Rennes this summer, the proceedings of which will be published late next year.
Cheers,
Simon
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>>> Falk H. Koenemann <[log in to unmask]> 11/10 12:42 am >>>
Solomon Buckman schrieb:
> Talking of metamorphic rocks, does anybody know how ultrahigh metamorphic
> rocks, as seen in the Dabie Shan China, are transported from mantle depths
> back to the surface without undergoing extensive retrograde metamorphism or
> even melting as a result of the rapid decrease in pressure? The ultrahigh
> metamorphism must be related to continental collision but what is the
> mechanism of relaxation that allows these rocks to move so rapidly towards
> the surface?
In Norway the western margin of the Western Gneiss Region went from UHP facies
(coesite, diamond) via hi-P granulite into amphibolite facies. I think the
felsic rocks did undergo wide-spread melting even if I have not made friends
with this opinion. Reviewers asked for geochemical evidence; in my view, tracing
a marble band from its original environment within the mafic eclogite through
the boundary to felsic, rather nondescript grey gneisses with lots of reworking
was sufficient evidence to me that the mafic rocks had been widely resorbed by
chemical mixing, making the marble a "chemical xenolith" in the felsic rocks.
These marble lenses which are largely unaccounted for, are fairly common between
Molde and Asesund. The original host rock to the marbles was the Blafjellet
unit, the uppermost nappe in the lower plate. Its type locality is near Dombas,
it has largely vanished along the coast. I think it was resorbed.
While the continental slab was "down there", pods of mantle material dropped
into it from the mantle wedge above it. The contacts between the UM-pods and
their felsic country rock is not tectonic where I have seen it (e.g. on Utroy,
west of Molde), but marked by swirls, schlieren and all sorts of weird chaotic
structures which I took for evidence for partial chemical resorption of the
UM-margin by the felsic rocks. No foliation developed. The felsic rock must have
been at least partly melted.
_______________________________________________________________
Falk H. Koenemann Aachen, Germany [log in to unmask]
http://home.t-online.de/home/peregrine/hp-fkoe.htm
_______________________________________________________________
Falk H. Koenemann Aachen, Germany [log in to unmask]
http://home.t-online.de/home/peregrine/hp-fkoe.htm
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