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Metamorphism
Michael Brown


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.

For this reason, the recent discoveries of coesite in eclogite in the Himalayas by O'Brien and others (Geology, v. 29, p. 435-439) and in the Erzgebirge (Germany) by Massonne (European Journal of Mineralogy, v. 13, p. 565-570) are important. Further, Searle and colleagues (Journal of Geology, v. 109, p. 143-153) have suggested that coesite eclogite and diamond-bearing gneiss most likely are being formed today along the Hindu Kush seismic zone, where graphitic shales and carbonates along the northwest Indian plate margin are being subducted together with the underlying continental lithosphere.

The oldest record of ultrahigh-pressure metamorphism known to the author was published last year by Jahn and co-authors (Chemical Geology, v. 178, p. 143-158), who retrieved ages of last year by Jahn and others (Chemical Geology, v. 178, p. 143-158), who retrieved ages of about 620 million years from omphacite-kyanite micaschist and mafic eclogite from the Pan-African belt in northern Mali. This is potentially significant: if UHPM is not recorded in orogenic belts older than the Pan-African event at the end of the Precambrian, a major change in global geodynamics may have occurred at that time. In contrast, the discovery of a circa 2,500-million-year-old ophiolite complex by Kusky and others (Science, v. 292, p. 1142-1145; see also Technical Comments by Zhai and co-authors and Kusky and Li, Science, v. 295, p. 923a) may also be significant because of the antiquity. Although the interpretation is controversial, the complex appears to include the major units found in typical Phanerozoic ophiolite complexes, and it is not dismembered or metamorphosed to an extent that prevents reconstruction as a representative section of oceanic lithosphere formed during the Archean-Paleoproterozoic transition.

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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Brown is director of the Laboratory for Crustal Petrology and a professor in the Department of Geology at the University of Maryland, College Park. E-mail

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