Kai Ye

The possible subduction of continental material to depths greater than 200 km.

Determining the depth to which continental lithosphere can be subducted into the mantle at convergent plate boundaries is of importance for understanding the long-term growth of supercontinents as well as the dynamic processes that shape such margins. Recent discoveries of coesite and diamond in regional ultrahigh-pressure (UHP) metamorphic rocks has demonstrated that continental material can be subducted to depths of at least 120 km (ref. 1), and subduction to depths of 150–300 km has been inferred from garnet peridotites in orogenic UHP belts based on several indirect observations. But continental subduction to such depths is difficult to trace directly in natural UHP metamorphic crustal rocks by conventional mineralogical and petrological methods because of extensive late-stage recrystallization and the lack of a suitable pressure indicator. It has been predicted from experimental work, however, that solid-state dissolution of pyroxene should occur in garnet at depths greater than 150 km (refs 6,7,8). Here we report the observation of high concentrations of clinopyroxene, rutile and apatite exsolutions in garnet within eclogites from Yangkou in the Sulu UHP metamorphic belt, China. We interpret these data as resulting from the high-pressure formation of pyroxene solid solutions in subducted continental material. Appropriate conditions for the Na2O concentrations and octahedral silicon observed in these samples are met at depths greater than 200 km.

Large areal extent of ultrahigh-pressure metamorphism in the Sulu ultrahigh-pressure terrane of East China : New implications from coesite and omphacite inclusions in zircon of granitic gneiss

Abstract Coesite and omphacite inclusions have been identified for the first time as minute inclusions in zircon from amphibolite-facies granitic orthogneiss in the Sulu ultrahigh-pressure (UHP) metamorphic terrane of eastern China by Raman spectroscopy and microprobe analyses. The occurrences of these minerals in the voluminous granitic gneiss of Sulu support a regional and pervasive UHP metamorphic event that predated regional amphibolitic retrogression. Taking into account the widespread discoveries of coesite in other lithologies, we thus conclude that a substantial crustal component in the Sulu UHP metamorphic terrane appears to have shared a common history of Triassic subduction to mantle depths and later exhumation.

Mineral Inclusions in Zircon from Gneisses in the Ultrahigh‐Pressure Zone of the Dabie Mountains, China

Mineral inclusions in zircon from gneisses in the ultrahigh‐pressure (UHP) zone of the Dabie Mountains, China, are studied by using laser Raman spectrophotometry. We identified numerous mineral inclusions including coesite, quartz, clinopyroxene, phengite, barite, anhydrite, and feldspar. Coesite is found to occur as zircon‐hosted inclusions in all types of gneisses, indicating that these gneisses had shared the ultrahigh‐pressure metamorphic history with the UHP eclogite blocks that reside in them. The occurrence of barite and anhydrite probably implies the existence of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

A transitional eclogite- to high pressure granulite-facies overprint on coesite–eclogite at Taohang in the Sulu ultrahigh-pressure terrane, Eastern China

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Overpressures induced by coesite-quartz transition in zircon

\end{document} ‐bearing fluids during the UHP metamorphic stage. We also found no simple ways to correlate growth zones of zircon as shown by cathodoluminescence images with metamorphic conditions determined from inclusion in any specific zircon grains. Of particular interest is the discovery of coesite included in zircons from gneiss of the “cold” eclogite zone that was previously considered to have not experienced UHP conditions.

Ultrahigh-pressure (UHP) low-Al titanites from carbonate-bearing rocks in Dabieshan-Sulu UHP terrane, eastern China

SIMS data demonstrate Archean (2.9 to 2.7 Ga) and Paleoproterozoic (1991 ± 43 Ma) ages for rounded detrital zircon cores and unzoned rims respectively in a graulite sample collected from Huangtuling, Dabie-Sulu orogenic belt, China. The granulite rocks in this locality have pelitic compositions and represent an exposure of the northern margin of the Yangtze Craton. The rounded zircon cores show oscillatory zoning and have Th/U ratios (0.17 − 0.78) systematically higher than those of the unzoned luminescent rims (0.005 − 0.03). The age of the detrital zircon cores and whole rock Nd isotope model age suggest that the Huangtuling granulite was possibly derived from a provenance dominated by Archean rocks similar to the Kongling Complex exposed 200 km in the southwest. Multiple phases of metamorphism have been identified for the granulites, with the dominant granulite-facies mineral assemblage, defining an early clock-wise metamorphic P-T path. Orthopyroxene, plagioclase and whole-rock material define a linear trend in a 147Sm/144Nd − 143Nd/144Nd diagram, corresponding to an age of 1926 ± 146 Ma. This result and the U-Pb data for the zircon rims suggest that a collisional event may have occurred along the northern margin of the Yangtze Craton at ∼2.0 Ga, inviting further study on whether the Yangtze Craton was involved in the assembly of the recently-proposed Paleo-Mesoproterozoic supercontinent Columbia.

Cogenetic relationship of the Yangkou gabbro-to-granite unit, Su-Lu terrane, eastern China, and implications for UHP metamorphism

Abstract A transitional eclogite- to high-pressure granulite-facies paragenesis (Omp+Pl+Qtz±Grt) after peak coesite–eclogite facies metamorphism and predating the later amphibolite-facies overprint is identified in coesite–eclogite from the Taohang area of the Sulu ultrahigh-pressure (UHP) terrane in eastern China. These minerals were equilibrated at 17 kbar and 820°C. This reveals that fluid infiltration might activate retrograde recrystallisation even at a deep level during the exhumation process of the UHP rocks. The tectono-metamorphic significance of the unusually high pressure overprint is also discussed.

Chlorine-rich amphibole in Yangkou eclogite, Sulu ultrahigh-pressure metamorphic terrane, China

Through the use of transmission electron microscopy and infrared spectroscopy, hydrous components have been identified as hydroxyl and free water (H 2 O) in elongate garnet of ultrahigh-pressure eclogite from the Dabie Mountains, eastern China. The H 2 O occurs as clusters of water molecules and as hydroxyl in garnet; the clusters of water molecules are interpreted to have promoted elongation of garnet through hydrolytic weakening. The presence of hydrous species in ultrahigh-pressure garnet from continental-crust protoliths indicates that such protoliths are able to carry water into the mantle during the subduction of continental lithosphere.