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Excitement in
metamorphism remains focused on the extremes. For example, in ultrahigh-pressure
metamorphism (UHPM), the important questions are: how deep can the upper part
of the continental lithosphere subduct, what volume of upper continental lithosphere
has been subducted to these depths, and how far back in Earth history is subduction
of upper continental lithosphere to these depths recorded? The extent of UHPM
in Phanerozoic orogenic belts tells us something fundamental about how Earth has
worked for the past 600 million years - just as our improved understanding of
former oceanic realms was built during the past 30 years on the realization that
ophiolite complexes are remnants of oceanic lithosphere. Thus, in the case of
both UHPM and ophiolite complexes, documentation of their spatial and temporal
extent throughout Earth's history contributes significantly to understanding the
global geodynamic behavior of Earth through time.Ultrahigh-temperature metamorphic
rocks
Next, there is the question of the hottest crustal rocks. Minimum temperatures
of 1, 100 degrees Celsius were reported by Hokada (American Mineralogist,
v. 86, p. 932-938) and Motoyoshi and Hensen (American Mineralogist, v.
86, p. 1404-1413) from the Napier Complex in Antarctica, confirming research
published the previous year. Asthenospheric mantle must be involved at shallow
depth to generate the extreme thermal conditions, which suggests some form of
slab break-off or lithosphere delamination event. Once again, there is the question
of how far back in time we find ultrahigh-temperature metamorphic (UHTM) rocks.
The paper by Carson and co-authors (Precambrian Research, in press) suggests
that the UHTM in the Napier complex occurred around 2,480-2,450 million years
ago, overprinting protoliths that crystallized approximately 2,626 million years
ago. In other regions, UHTM appears to have occurred close to the Archean-Paleoproterozoic
transition, for example, in the Lewisian Complex of northwest Scotland.
The oldest known UHPM is late Neoproterozoic and the oldest known UHTM is late
Archean-early Paleoproterozoic. These findings suggest that the long-standing
fundamental distinction between the Archean, Proterozoic, and Phanerozoic eras
is important from a geodynamic perspective. Yet the significance remains illusive
since researchers have reached no consensus on the geodynamic behavior of Earth
in the Archean.
High-pressure granulites have also become topical. These rocks are characterized
by the key mineral associations of kyanite-K-feldspar in metapelites and felsic
compositions and garnet-clinopyroxene-plagioclase in mafic compositions, which
commonly record pressure-temperature conditions of 1.0-2.0 gigapascals (Gpa)
and more than 800 degrees Celsius. These granulites were the focus of a Special
Session at the Geological Association of Canada-Mineralogical Association of
Canada 2001 Joint Annual Meeting in St. John's, Newfoundland.
Although we don't know how far back in Earth history a record of extreme metamorphic
conditions exists, what is becoming clear that some of Earth's continental crust
was formed very early. Zircon ages reported last year by Wilde and others (Nature,
v. 409, p. 175-178) and Mojzsis and co-authors (Nature, v. 409, p. 178-181)
suggest the existence of continental crust as far back as 4,400 million years
ago. Both papers report oxygen isotope data consistent with the presence of
water on the early planet's surface.
With respect to quantitative petrology, we see the increasing use of pseudosections
(two-dimensional sections of multidimensional space, the most common examples
of which are the pressure-temperature diagram for an average bulk composition
and the temperature-composition diagram for fixed pressure). Pseudosections
are constructed from results of petrological calculations based on an internally
consistent thermodynamic data set and are made using one of several mathematical
methods for which computer programs are available. This methodology has allowed
significant advances in understanding detailed pressure-temperature paths of
metamorphism, as shown by Zeh (Journal of Metamorphic Geology, v. 19,
p. 329-350), and in understanding the effect of bulk composition on phase equilibria,
as shown by Tinkham and co-authors (Geological Materials Research, v.
3, p. 1-42). Recently, it has been possible to incorporate data for granite
melt in such thermodynamic data sets and, therefore, to extend the use of pseudosections
into the melting regime. Examples of this approach include Johnson and co-authors
(Journal of Metamorphic Geology, v. 19, p. 99-118) and White and co-authors
(Journal of Metamorphic Geology, v. 19, p. 139-153).
Crustal melting
Melting of the continental crust and segregation, migration, ascent, and emplacement
of melt remain important areas of research. This theme and the relationship
of crustal melting to orogenic processes featured prominently at the Annual
Meeting of the Geological Society of America in Boston and was the focus of
the volume edited by Brown and co-editors on "Crustal melting and granite
magmatism: Causes and behaviours from pores to plutonic belts in orogens"
(Physics and Chemistry of the Earth, v. 26, p. 201-367). Other significant
papers on the theme include those by Milord and co-authors (Journal of Petrology,
v. 42, p. 487-505), Sawyer (Journal of Metamorphic Geology, v. 19, p.
291-309) and Solar and Brown (Journal of Petrology, v. 42, p. 789-823).
Studies of melting have also extended to unusual protolith compositions. First,
Mavrogenes and co-authors (Economic Geology and The Bulletin of the
Society of Economic Geologist, v. 96, p. 205) investigated partial melting
of the Broken Hill galena-sphalerite ore, based on experimental studies. Second,
Osinski and Spray (Earth and Planetary Science Letters, v. 194, p. 17-29)
presented evidence for the melting of dolomite-rich rocks and an impact structure
in the Canadian high Arctic.
Integration among specialties has become commonplace in the earth sciences.
In the United States, this may be driven in part by opportunities through the
National Science Foundation's Continental Dynamics Program. One dramatic example
involves studies in the Himalayas, particularly the active metamorphic massif
at Nanga Parbat, described by Meltzer and co-authors (Geology, v. 29,
p.651-654), and the relationship between erosion, geodynamics and geomorphology,
reported by Zeitler and co-authors (GSA Today, v. 2001, p. 4-9). Two
other examples are those by Ducea (GSA Today, v. 2001, p. 4-10) concerning
the Jurassic-Cretaceous history of the California arc, and the CD-ROM Working
Group (GSA Today, v. 2002, p. 4-10), concerning the structure and evolution
of the lithosphere beneath the Rocky Mountains.
In another fascinating paper from last year, Martinez and co-authors (Nature,
v. 411, p. 930-934) suggested that some metamorphic core complexes may form
by density inversion and lower-crust extrusion. The Annual Meeting of the Geological
Society of America in Denver in October will address this issue in the Topical
Session (T124) on "Thermal and Mechanical Significance of Gneiss Domes
in the Evolution of Orogens." There is also the question of why some orogens
collapse and others do not, which is discussed in relation to fluids in the
lower crust by Leech (Earth and Planetary Science Letters, v. 185, p.
149).
Everyone, it seems, wants to use an electron probe microanalyzer for chemical
dating of monazite, although significant issues of standardization and/or calibration
remain to be resolved. Nonetheless, as a reconnaissance tool, chemical dating
of monazite holds promise. The real value of rapid in situ dating of
an accessory mineral such as monazite in petrological studies, however, lies
in our ability to identify monazite-forming reactions potentially to date specific
points on the P-T-t path.
In the realm of fluids, Kerrick and Connolly (Nature, v. 411, p. 293-296;
Earth and Planetary Science Letters, v. 189, p. 19-29) addressed the
issue of metamorphic devolatilization of subducted marine sediments and oceanic
metabasalts and the implications with regard to seismicity, arc magmatism, volatile
recycling, and the transport of volatiles into Earth's mantle. Guiraud and co-authors
(Journal of Metamorphic Geology, v. 19, p. 445-454) published a fascinating
paper on the behavior of water in metamorphism and whether the metamorphic volatile
phase can be retained in the equilibration volume to enable retrogression during
exhumation; this is also topical at higher temperatures with respect to retention
of melt, as discussed by White and co-authors (Journal of Metamorphic Geology,
v. 19, p. 139-153) and Brown (Journal of Metamorphic Geology, v. 20,
p. 25-40).
The retirement of Jacques Touret, doyen of metamorphic-volatile-phase research
and fluid-inclusion studies, was an event commemorated in a special volume last
year (Lithos, v. 55, p. 1-321). And although Ron Vernon formally retired
some years ago, his attendance at meetings and publication of scientific papers
have not diminished and his books have now begun to appear. Vernon's career
contributions were celebrated at the 15th Australian Geological Convention in
Sydney in July 2000, and a selection of papers from that event was published
at the beginning of this year (Journal of Metamorphic Geology, v. 20,
p. 1-213).
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