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Metamorphism
Michael Brown


Excitement in metamorphism remains focused on extremes, with two international meetings in Beijing, China, a workshop on "Geophysics and Structure, and Geology of UHPM Terranes" September 20-21, and a Penrose Conference on "Precambrian High-Pressure/High-Temperature Granulite-Facies Metamorphism" September 23-30 (UHPM is metamorphism of continental and oceanic rocks at pressures greater than about 85 kilometers depth, within the stability field of coesite, the dense polymorph of silicon dioxide). Additionally, a special issue, "Towards the Upper Limits of the Granulite Facies" was published in the Journal of Metamorphic Geology (v. 21, p. 1-120).

Photomicrograph to show resorbed crystals of andalusite (And) surrounded by cordierite (Crd) and altered former melt (scale bar = 100 m), from a residual graphitic metapelite xenolith (Mazarron, SE Spain). Andalusite contains numerous inclusions of granitic glass (arrows), trapped during partial melting of the rock and growth of peritectic andalusite. Analysis of the glass in the inclusions by Cesare and co-authors (Geology, v. 31, p. 573-576) has allowed refinement of phase relations applicable to high-temperature, low-pressure metamorphism of metapelites. Photomicrograph by Bernardo Cesare (Universitá di Padova, Italy).

During 2002, more than 1,100 articles were published that are identified by keywords metamorphic, metamorphism or metamorphic petrology or geology; of these, about 200 are concerned with metamorphic fluids, including melts.

Quantifying fluid flow in contact metamorphic aureoles (an aureole is the zone around an igneous intrusion in which the host rock exhibits the effects of metamorphism) remains an important first milestone, because one of the variables, pressure or depth, is fixed and another, the thermal structure or temperature distribution, is simply modeled.

In a seminal paper published more than 40 years ago, Graham Alan Chinner (Journal of Petrology, v. 1, p. 178-217) argued that the restriction of rocks of varying oxidation ratio to well-defined layers in metamorphosed sediments suggested that differences in oxygen content were premetamorphic, supporting the view that rocks behave as narrowly defined units buffered by respective mineral assemblages. Recently, Bruce Yardley and J.T. Graham (Geofluids, v. 2, p. 249-256) suggested that salinities of metamorphic fluids might reflect original presence or absence of highly saline formation waters and/or evaporites in protoliths, with variable high-fluid salinities in sediments deposited in shallow marine environments and low-fluid salinities in oceanic and accretionary prism sequences.

Fluid flow

Chinner further suggested that in contact metamorphic aureoles it was more common for the fluid phase to overwhelm buffering capacity of mineral assemblages. Fluid flow in aureoles remains topical because of uncertainty about direction of fluid flow (Ferry and co-authors, Contributions to Mineralogy and Petrology, v. 142, p. 679-699). Also uncertain is whether the commonly assumed uniform and unidirectional fluid flow is correct or whether fluid flow depends strongly on time and space (Cui and co-authors, Geological Society of America Bulletin, v. 114, p. 869-882).

In an innovative paper, Boswell A. Wing and John M. Ferry (Geology, v. 30, p. 639-642) argued that first-order control on gross geometry of peak metamorphic fluid flow during medium-pressure metamorphism is regional structure, and that cross-layer transport is secondary to terrane-scale fluid flow in driving prograde decarbonation reactions. Of particular importance are timing of fluid flow and source(s) of fluid. By studying oxygen-18 and Y zoning in garnet, A. Skelton and co-authors (Journal of Metamorphic Geology, v. 20, p. 457-466) argued for two pulses of metamorphic fluid flow, the first during deformation and a second following deformation. They suggested that fluid was sourced in neighboring calcareous pelites, thus requiring cross-layer transport.

Dehydration of hydrothermally altered oceanic crust at the top of subducting lithosphere is an important source of fluids. This fluid may be tracked because it will add silica and alkalis to metasedimentary rocks of an overlying accretionary prism. Such an example has been documented from the Otago Schist, New Zealand, by Christopher M. Breeding and Jay J. Ague (Geology, v. 30, p. 499-502), where increasing metamorphic grade from prehnite-pumpellyite facies to greenschist facies is accompanied by 10 to 30 percent volume quartz veins. These authors conclude that transfer of silica from subducting slabs into accretionary prisms is a plausible mechanism to increase silica content of continental crust beyond that generated by magmatic differentiation in arcs.

Fluids in UHPM

Concentration of energy at the extremes of metamorphism has led to interesting discoveries concerning fluids attending UHPM. For example, Yilin Xiao and co-authors (Journal of Petrology, v. 43, p. 1505-1527) have documented a difference in fluid and metamorphic evolution between the South Dabie Terrane and the North Dabie Complex, China. Fluid inclusions in rocks from South Dabie are mainly aqueous with varying salinities in rocks with low oxygen-18 values, indicating meteoric (atmospheric, not metamorphic) water-rock interactions before UHPM. In contrast, rocks from North Dabie have "normal" oxygen-18 values that preclude meteoric water-rock interaction. This conclusion is consistent with excellent results from seismic reflection profiling across the Dabie Shan (Xue-Cheng and co-authors, Geology, v. 31, p. 435-438), in which South Dabie is clearly identified as the subducted unit, whereas North Dabie represents the facing plate.

In a spectacular example of the role of "water" in deformation of minerals under UHPM, Wen Su and co-authors (Geology, v. 30, p. 611-614) report the presence of hydrous species (clusters of water molecules and hydroxyl ions) in elongate garnet from continental crust. Apparently, hydrolytic weakening enabled deformation of garnet, a feature that suggests that subducted continental crust may carry water into the mantle.

Fluids in granulite metamorphism

One controversy in granulite facies metamorphism concerns whether the "dry" mineral assemblages are residue after extraction of melt or whether such assemblages are consequent upon infiltration metasomatism by carbonic fluids. Unfortunately, both models predict a preponderance of carbonic fluid inclusions associated with peak metamorphic conditions. Writing in the May 2002 Journal of Petrology (v. 43, p. 769-799), Daniel E. Harlov and Hans-Jürgen Förster suggest an alternative to melting for producing the anhydrous (pyroxene-bearing) mineral assemblages in rocks of the granulite facies (high-temperature crustal metamorphism). They argue that fluid metasomatism - mass transfer by advective fluid flow resulting in change in bulk-rock composition - can explain grain boundary microstructures in mafic granulites of the Ivrea-Verbano Zone in Italy. These authors argue for propagation of a fluid front, with an initial fluid dominated by carbon dioxide becoming diluted with water as it moves up the rock column. However, the evidence Harlov and Förster adduce in favor of metasomatism potentially can be explained by a small amount of partial melting (a few percent by volume), consistent with more extensive melting (20 to 40 percent by volume) of metasedimentary rocks to create the associated stronalites (melt-depleted granulites, named after their occurrence in Val Strona). Very high-density carbonic fluid inclusions captured during maximum pressure-temperature conditions were identified from ultra-high temperature metamorphic — UHTM: metamorphism of crustal rocks at temperatures greater than 900 C {emdash} terranes in Antarctica's Napier Complex (Toshiaki Tsunogae and co-authors, Contributions to Mineralogy and Petrology, 2002, v. 143, p. 279-299); southern India (M. Santosh and Toshiaki Tsunogae, Journal of Geology, 2003, v. 111, p. 1-16); and the Eastern Ghats (Sushmita Sarkar and co-authors, Geology, January 2003, v. 31, p. 51-54). It is unambiguous that the very high-density carbon dioxide fluid inclusions provide a strong case for the presence of carbon dioxide-rich fluids during ultra-high temperature metamorphism, but it remains unclear whether this outcome requires carbon dioxide infiltration.

Melting

One issue in all experimental petrology involving natural samples at extreme metamorphic conditions is the question of what constitutes an appropriate starting material. Rajeev Nair and Thomas Chacko (Journal of Petrology, v. 43, p. 2121-2142) argue it is important to select natural samples from granulite facies rocks immediately below the solidus in order that the phase compositions correctly represent those stable prior to melting. Under these circumstances, melting due to biotite breakdown in rocks of semi-pelitic composition is delayed until approximately 875 degrees Celsius, due to higher titanium and fluorine in biotite in the starting materials, which stabilize the materials to higher temperatures. These authors suggest that temperatures of at least 875 to 1025 degrees Celsius are required to stabilize orthopyroxene under volatile phase absent conditions at lower-crustal depths, which is consistent with recently recalibrated temperature estimates of granulite facies metamorphism by David R. Pattison and co-authors (Journal of Petrology, v. 44, p. 867-900).

Melting and the distribution of residual melt remain topical. Examples include controls on distribution of small amounts of melt (a few percent by volume) in contact aureoles (Nathalie Marchildon and Michael Brown, Journal of Metamorphic Geology, May 2002, v. 20, p. 381-396); in granulites by Souad Guernina and Edward W. Sawyer (Journal of Metamorphic Geology, February 2003, v. 21, p. 181-201); and in experiments by C.W. Holyoke and Tracy Rushmer (Journal of Metamorphic Geology, June 2002, v. 20, p. 493-512). The distribution of melt is important, because it relates to retrogression by reaction between peritectic phases formed at the metamorphic peak and melt during cooling (Richard W. White and Roger Powell, Journal of Metamorphic Geology, September 2002, v. 20, p. 621-632). Such a process may be responsible for limited occurrence of UHTM mineral assemblages within "common" (apparent peak metamorphic temperature below 900 degrees Celsius) granulite terranes (Renato Moraes and co-authors, Journal of Petrology, September 2002, v. 43, p. 1673-1705). Also, studies of "melt" inclusions, in which the melt is represented by an appropriate crystallized mineral assemblage, are likely to be a more important part of future research (see figure).

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Brown is director of the Laboratory for Crustal Petrology and professor in the Department of Geology at the University of Maryland, College Park. E-mail: mbrown@geol.umd.edu.

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