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Much research on high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks has been done in order to understand deep subduction or collision, and exhumation processes. Basic petrological descriptions of HP and UHP rocks have revealed that HP and UHP minerals are often disrupted and found as sypmplectite or pseudomorph (e.g. Okamoto et al. 2000). In the worst case, they have been completely decomposed as low-pressure minerals (e.g. Terabayashi et al., 2002). In such cases, HP and UHP minerals (and fluids) are only preserved in harder minerals (e.g. garnet, zircon, diamond etc.) as inclusions (e.g. Katayama et al. 2000). Therefore, in order to unravel the deep-subduction and earlier exhumation processes of HP and UHP rocks, micro-scale analysis is quite useful (e.g. Hwang et al. 2005; Okamoto et al. 2006). Micro-scale petrofabric analysis as well as pressure–temperature (P–T) estimation and micro-scale age dating (e.g. SHRIMP zircon U–P analysis) have been investigated in many HP and UHP terranes (e.g. Ota et al. 2004; Okamoto et al., 2004, Yamamoto et al. 2004) in order to understand subduction, collision and exhumation processes.

This thematic section is an outcome of the symposium on ‘Frontier of micro-analysis on UHP rocks’ organized by K. Okamoto, M. Terabayashi, and H. Yamamoto at the 115th Annual Meeting of the Geological Society of Japan in 2008, where six oral presentations related to this topic were given during a half-day session. In response to a call for submission of papers for a special issue, four articles were contributed from the presentations given at the115th Annual Meeting of the Geological Society of Japan, and the guest editors adopted two articles not presented there.

In the first paper of this thematic section by Rehman et al. (2013), Metamorphic P–T evolution of the high-pressure Kaghan Valley, with eclogites from garnet growth and reaction textures, are demonstrated. Since the discovery of coesite from eclogites, gneiss, and HP and UHP rocks, the Kaghan Valley transect (e.g. Kaneko et al. 2003), northern Pakistan, has been investigated by many researchers. Rehman et al. (2013) show that garnets from the eclogite represent very simple zonal structures. The prograde P–T stage is recorded in cores, the peak high-pressure stage in mantles, and the retrograde exhumation stage is surmised due to collision in rims.

The second paper, by Okamoto et al. (2013), reports a redox state at UHP metamorphism using the Chinese Continental Scientific Drilling (CCSD) eclogites. In order to estimate the fluid composition, C-species of the CCSD eclogites are investigated. Graphite is always recognized with pyrite in the eclogites, suggesting their growth from the H2O-rich (CO2-poor) fluids released during metamorphism. The studied eclogites contain a vacancy (CaEs component = Ca0.50.5AlSi2O6) in clinopyroxene. Therefore, the Fe3+ content in clinopyroxene cannot be calculated based on their chemical composition as determined by electron probe micro analyzer (e.g. Okamoto & Maruyama 2004). Okamoto et al. (2013) estimated Fe3+/ΣFe based on Mössbauer analysis and calculated the P–T conditions from the assemblage of phengite-garnet-clinopyroxene-(coesite) as P = 3–4 GPa, and T = 650–780°C. The P–T conditions obtained in this study show lower values than previously reported elsewhere for the CCSD eclogites, suggesting that the effect of Fe3+ estimation is significant. Another important fact is that the estimated T conditions are below the wet solidus, or on the solidus in the case where there are any partially melted textures.

The third paper is a Micro-XANES determination of Fe3+/ΣFe in omphacite inclusion within garnet from the Dabie eclogite, East-Central China by Terabayashi et al. (2013). The Fe K-edge micro-XANES spectra of omphacite included within garnet from the Dabie eclogites, east-central China, were measured to estimate the oxidation state of iron in omphacite. The micro-XANES analyses of omphacite inclusions within garnet yielded higher Fe3+/ΣFe ratios than those calculated using Fe3+ and Fe2+ contents from the electron microprobe analyses. Along the north–south cross-section in the study area, the estimated temperatures based on the charge balance calculation of Fe3+ tend to be identical or higher than temperatures estimated based on the calculation of Fe3+ using the XANES technique. The estimated temperature conditions by a garnet-clinopyroxene-phengite geothermobarometer are lower than those of the solidus conditions.

The fourth paper, by H. Yamamoto et al. (2013a), reports Northward extrusion of the ultrahigh-pressure units in the southern Dabie metamorphic belt, east-central China. Detailed mapping revealed that the metamorphic sequence in this area can be subdivided into four sub-horizontal lithotectonic units. These units are denoted I, II, III and IV in the order from structural bottom to top. Those metamorphic rocks underwent penetrative ductile deformation that is denoted by gently south-dipping or sub-horizontal foliation and NNW-trending mineral lineation. Deformation microstructures in oriented samples from Units I and II indicate consistent northward displacements of hanging wall and those from Units III and IV indicate southward displacements. The sub-horizontal structure and opposite shear directions between Units I–II and Units III–IV suggest northward extrusion of UHP metamorphic units (Units II and III).

The fifth paper is a rheological contrast between glaucophane and lawsonite in naturally deformed blueschist from the Diablo Range, California, USA, by Kim et al. (2013). Developments of crystal-preferred orientations (CPOs) with small grain size, irregular grain boundary and high aspect ratio of glaucophane indicate deformation mechanism as recovery and dynamic recrystallization possibly accommodated by dislocation creep, while lawsonite deforms by rigid body rotation based on euhedral grains with angular or straight grain boundaries. Higher aspect ratios, lower angle to foliation, and stronger CPOs of both minerals in the glaucophane-rich layer rather than those in the lawsonite-rich layer suggest the strain localization into the glaucophane-rich layer. Fabric strength and seismic anisotropy are higher in the glaucophane-rich layer than that of the lawsonite-rich layer. All their results imply that the dominant role of glaucophane rather than lawsonite for rheological behavior and seismic anisotropy of blueschist.

The sixth paper, by Arakawa et al. (2013), is SHRIMP U–Pb dating of zircons related to the partial melting in a deep subduction zone – a case study from the Sanbagawa quartz-bearing eclogite, central Shikoku, Japan. The Sanbagawa high P/T metamorphic rocks have been considered as typical cold oceanic material subducted during the Cretaceous. However, an eclogite outcrop has been discorvered that exhibits the partial melting texture in the Sanbagawa metamorphic belt. In order to confirm the age of partial melting of Sanbagawa metamorphic rocks, zircons were dated from both the melted portion and the host eclogite using the U–Pb SHRIMP age-dating at the Korean Basic Science Institute. Zircons from the melted portion (SHT16&75) are rounded and have sector zoning. The core and mantle yield U–Pb age in the 130–113 Ma (average 120 Ma) range, and the rim ages are in the 115–104 Ma range. Zircons from the eclogite (SHT15&76) have homogenous core with thin mantle and rims. The U–Pb ages are concentrated at 123–112 Ma. These pieces of evidence suggest that the eclogite metamorphism occurred at about 120 Ma and the subsequent partial melting happened at about 110 Ma.

The final paper, by S. Yamamoto et al. (2013b), reports the recycled crustal zircons from podiform chromitites in the Luobusa ophiolite, southern Tibet. Spot analyses with a laser ablation microprobe-inductively coupled plasma mass spectrometer (LA-ICPMS) assisted by cathodoluminescence images gave a wide age range from Cretaceous to Late Archean (c. 100–2700 Ma). Laser-Raman spectroscopy analyses revealed that the zircons recovered from the chromitites contained crustal mineral inclusions, such as quartz and K-feldspar, but lack any mantle minerals, suggesting that they had a crustal origin. The results indicate that crustal zircons in chromitites had a xenocrystic origin and resided in the mantle peridotite for a long period before being entrained into the chromitite during its formation. This indicates that the mantle peridotite under the Neo-Tethys Ocean was affected by the crustal material contamination. Their results are consistent with the previous reports that mid-ocean ridge basalts in the Indian Ocean have the isotopic signature of crustal material contamination. From these results and previous isotopic studies on Gondwana geology, they conclude that ancient zircons from podiform chromitites could be evidence of crustal material being recycled through the upper mantle.

The above-mentioned papers in this thematic section of Island Arc are examples of the exciting findings by contributors on fluid-related phenomena and dynamics. They also indicate that the importance of different approaches and integration of them will improve our understanding of HP and UHP rocks.

Acknowledgements

  1. Top of page
  2. Acknowledgements
  3. References

We thank all the authors for their valuable contributions to this thematic section. We also express our sincere thanks to all the referees who spared their valuable time to provide thoughtful and careful reviews. We are grateful to Island Arc for accepting our proposal and undertaking the publication of this thematic section. We extend our thanks to the Editor-in-Chief, Prof. H. Maekawa and Executive Editor, Dr. H. Hara for their encouragement and support. K. Okamoto thanks Prof. Sunlin Chung and Prof. Borming Jahn for their kindness and encouragement because we have edited this special issue when K. Okamoto had been a visiting researcher at National Taiwan University from August 5 to November 2, 2012.

References

  1. Top of page
  2. Acknowledgements
  3. References