When Continents Collide

When Continents Collide: Geodynamics and Geochemistry of Ultrahigh-Pressure Rocks
a volume edited by B.R. Hacker and J.G. Liou
to be published in late 1998 by Chapman-Hall

This volume brings together current perspectives on geochemical, geophysical, and geodynamical processes active during ultrahigh-pressure (UHP) tectonics. It was once easy for most geologists to dismiss the couple of small coesite-bearing outcrops discovered by Chopin (1984) and Smith (1984) as rare tectonic freaks worthy of only idle speculation. In the last 10 years, however, the number of orogenic belts worldwide that contain diamond, coesite, or other indications of metamorphic pressures >2 GPa, has exploded to more than 15! It is time for all geologists to consider the manifold ramifications of this proof that continental blocks as large as 5 x 50 x 100 km were subducted to depths of 100-150 km commonly during the history of the Earth and may have played a significant role in the formation of most mountain belts. For example, there are three known areas where metamorphic rocks formed at depths >60 km during the last 100 m.y.--Oman, Tso Morari, Dora Maira.

The best-known modern examples of continental crust at UHP in the mantle today are Australia-Timor and the Pamir-Hindu Kush. In the first chapter of this volume, Lin and Roecker use seismic tomography, earthquake data, geodesy, and leveling to argue that Taiwan also belongs to this select group, showing that the title of this volume, When Continents Collid should not be taken too literally. Patino-Douce and McCarthy use an experimental perspective to show that the bulk of continental crust is unlikely to melt during subduction to UHP depths, but instead should undergo dehydration melting during decompression. Eclogites have often been thought of as strong rocks, but Stšckhert and Renner argue that eclogites are weak and that continental material at UHP is too soft to undergo exhumation as a coherent slab. Davies and von Blanckenburg study the detachment of oceanic lithosphere from continental lithosphere during continental collision, and show that the depth of breakoff depends strongly on the strength of the subducted continental lithosphere; breakoff in old lithosphere occurs along the passive margin, but breakoff in young lithosphere is within the continent. Grasemann et al. present simpler thermal calculations and suggest that UHP rocks should undergo two-stage cooling histories. They emphasize that the slowest cooling corresponds to the most rapid period of exhumation and the fastest cooling corresponds to the slowest period of exhumation. Blythe notes that few modern exhumation rates are as fast as exhumation rates commonly argued for exhumed high-pressure rocks, and suggests that this implies either special precipitation or glaciation patterns or that high-pressure rocks are exhumed more slowly than argued.

Excess ⁴⁰Ar has been a long-standing problem in high-pressure rocks, but Scaillet reveals how this can be turned to advantage to reveal fluid-rock interactions at UHP conditions. Eclogites and ultramafic rocks in UHP orogens might plausibly come from subducted oceanic lithosphere, but Jahn shows for the Dabie-Su-Lu orogen that the eclogites and ultramafic rocks were derived from continental intrusions and subcontinental upper mantle, respectively. UHP rocks in China have yielded astoundingly low stable isotopic ratios, which Rumble interprets as evidence that the protolith represents a high-latitude geothermal system. The spatial extent of the fossil geothermal system indicates that UHP metamorphism, without a pervasive free fluid, occurred over a length scale of 100 km. Tabata et al. report on Raman spectroscopy on mineral inclusions in paragneiss that show that the entire southern Dabie Shan of China was subducted to mantle depths. The rate and means by which volatiles are subducted control many facets of subduction zones. Ernst et al. note that mineral assemblages in exhumed metamorphic rocks conflict with conclusions drawn from recent experimental studies, and that mafic lithosphere carries more volatiles into the mantle than continental lithosphere. Realization of the important geodynamical effects of slow phase transformations is growing, and Austrheim shows that the rates of metamorphic reactions in the deep crust are controlled by fluid availability and can be sluggish at temperatures as high as 800°C.

Contents

Active crustal subduction and exhumation in Taiwan
Cheng-Horng Lin and Steven W. Roecker

Melting of crustal rocks during continental collision and subduction
Alberto E. Patino Douce and T.C. McCarthy

Rheology of crustal rocks at ultrahigh pressure
Bernhard Stšckhert and Jšrg Renner

Thermal controls on slab breakoff and the rise of high-pressure rocks during continental collisions
J. Huw Davies and Friedhelm von Blanckenburg

Exhumation of ultrahigh-pressure rocks: Thermal boundary conditions and cooling history
Berhard Grasemann, Lothar Ratschbacher, and Bradley Hacker

Active tectonics and ultrahigh-pressure rocks
Ann E. Blythe

K-Ar (⁴⁰Ar/³⁹Ar) geochronology of ultrahigh-pressure rocks
Stephane Scaillet

Geochemical and isotopic characteristics of UHP eclogites and ultramafic rocks of the Dabie orogen: Implications for continental subduction and collisional tectonics
Bor-Ming Jahn

Stable isotope geochemistry of ultrahigh-pressure rocks
Douglas Rumble

Tracing the extent of a UHP metamorphic terrane: a mineral-inclusion study of zircons in gneisses from the Dabie Shan
Hirokazu Tabata, Kazuhiro Yamauchi, Shigenori Maruyama, and Juhn G. Liou

H₂O recycling during continental collision: Phase-equilibrium and kinetic considerations
W. Gary Ernst, J. L. Mosenfelder, Mary L. Leech, and Jun Liu

Influence of fluid and deformation on metamorphism of the deep crust and consequences for the geodynamics of collision zones
Hakon Austrheim