Bruce and all,
To reiterate Bruce's point about low-P, high-T rocks and their
uniqueness, how does our terminology constructively deal with a group
of rocks I've looked at in New York that contain sillimanite +
Al-spinel + Fe-Ti oxides + corundum + sapphirine +/- Al-rich
orthopyroxene +/- Mg-rich garnet (50% pyrope) +/- cordierite? The
sillimanite commonly contains abundant exsolved rods of corundum,
suggesting significant original solid solution toward mullite, and
there are thin quartzofeldspathic veins with a single ternary
feldspar (now perthitic) containing 45mol% Or, 40mol% Ab and 15mol%
An. Thermometric calculations indicate 1000 C+.
These rocks are very fine-grained (texturally fairly classic
hornfels) and in fact are partially melted and metsomatized former
pelitic-schist xenoliths (and rarely external contact-aureole rocks)
in mafic to ultramafic cumulates at the base of a small Silurian
layered intrusion. Regional thermobarometry, however, indicates the
current erosion level represents a pressure of about 7 kbar. There
is no doubt whatsoever that these are contact-metamorphic rocks yet
they lie far outside the P-T range that any textbook indicates for
very high-T contact-metamorphic facies (pyroxene hornfels, sanidinite
or some merged version of them). In fact, 1000-1050 C and 7 kbar
would put these somewhere out in the far reaches of the granulite
facies, as portrayed in virtually any text. Yet I suspect that
almost any of us confronted with an out-of-context hand sample, even
a thin section, from this occurrence would suggest that these are
classic hornfelses formed in a shallow (< 2 kbar) contact aureole,
e.g., the buchites from margins of Tertiary mafic plutons along the
west coast of Scotland. Parenthetically, I have seen thin sections
from low-P contact occurrences in Ardnamurchan (Scotland) and Morton
Pass (Laramie Anorthosite Complex in Wyoming) that are amazingly
similar to these rocks in all regards.
Were we to try to use the normal terminology for these rocks (based
either on the samples' assemblages and textures OR on the calculated
P-T conditions), without significant qualification of these with the
scientific and contextual details given above, we'd be likely to
greatly mislead, as Bruce and others have suggested. Based on
assemblages and textures they are pyroxene hornfels or sanidinite
facies rocks; based on P and T they are granulite facies.
Classifying them as either camouflages a critically important aspect
of their petrogenesis.
To complicate this situation even further, the terrane in which this
small pluton occurs also contains scattered localized occurrences
within pelitic schists of sillimanite-K-feldspar-spinel-hogbomite
(+/- corundum and cordierite) assemblages that are almost certainly
contact-related but for which the causative small sill-like gabbroic
plutons either have already been eroded away or haven't yet appeared
by erosion. With no obvious localized heat sources, do we classify
these as regional or contact? If they are sufficiently fine-grained
(as some are) are they hornfelses or not, even if they formed at
18-20 km depth? In NW Maine and other places (e.g., NE Scotland,
Whetstone Lake, the Arunta Block, SE Australia, the Sierra Pampeana
of Argentina) there are quadrangle-scale metamorphic zones that are
clearly the result of magmatic heating, commonly from occult plutons,
but are on much larger scale than classic contact-aureole isograds.
Admittedly these rocks are typicaly too coarse-grained to be called
hornfelses, but they also significantly blur and confuse our
nomenclatural distinctions between contact facies and regional facies.
The obvious way to deal with such occurrences, as Dugald, Jurgen and
others have suggested, is to recognize that there is what might be
called "regional-scale contact metamorphism" in terranes that were
magmatically active during or immediately following more typical
regional metamorphism. Isograds related to seen or unseen plutons
overprint older regional metamorphism of roughly similar grade,
significantly complicating interpretation of feature seen in both
quadrangle scale (e.g., isograds) and thin section scale (e.g.,
textures and mineral chemistry). This is actually quite common in
many collisional orogens and, along with Bruce's example of shallow
hydrothermally driven regional metamorphism (such as at the Salton
Sea), indicates the need to take older nomenclatural schemes where
contact = shallow, and regional = deep, with a large grain of salt
(excuse me, halite!).
And as we near the American Thanksgiving holiday, let's all give
thanks that this discussion may be approaching its conclusion (this
is my last contribution, I promise!).
Bob T.
>It is a sad comment on where Metamorphic Petrology has got to, that some of
>us do not appear to read the scientific content of each other's emails any
>more, we just worry about terminology. I attempted to make the scientific
>point, which Jorge Julian Restrepo has also picked up on, that, although we
>all agree that there are a wide range of P-T conditions in which either
>classic hornfelses or apparently normal regionally metamorphosed rocks may
>be produced, there are some extreme conditions of high-T, low-P contact
>metamorphism with no regional equivalents, in which some very distinctive
>assemblages are produced. The science behind why this should be the case is
>clearly of interest. The fact that I referred in passing to Turner's
>regional/honfels facies terminology, even though it was only to point out
>that it was not appropriate, may perhaps be of interest to lexicographers
>but it is not of scientific interest. I would like to propose that someone
>sets up a separate geo-lexicography mailing list for people who want to
>discuss matters of pure terminology.
>
>Bruce Yardley
>
>
>
>--------------------------------------------
>Professor Bruce Yardley
>School of Earth Sciences
>University of Leeds
>Leeds LS2 9JT
>UK
>
>Tel. 0113 233 5227 Fax 0113 233 5259
>---------------------------------------------
>
>GEOFLUIDS now exists! http://www.blackwell-science.com/gfl
--
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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)
|
