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Ultrahigh‐pressure metamorphism in the forbidden zone: the Xugou garnet peridotite, Sulu terrane, eastern China
Author(s) -
Zhang R. Y.,
Liou J. G.,
Yang J. S.,
Ye K.
Publication year - 2003
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1046/j.1525-1314.2003.00462.x
Subject(s) - eclogite , geology , geochemistry , peridotite , protolith , metamorphism , kyanite , gneiss , metamorphic rock , ultramafic rock , felsic , mafic , petrology , partial melting , mantle (geology) , subduction , paleontology , tectonics
The Xugou garnet peridotite body of the southern Sulu ultrahigh‐pressure (UHP) terrane is enclosed in felsic gneiss, bounded by faults, and consists of harzburgite and lenses of garnet clinopyroxenite and eclogite. The peridotite is composed of variable amounts of olivine (Fo 91 ), enstatite (En 92−93 ), garnet (Alm 20−23 Prp 53−58 Knr 6−9 Grs 12−18 ), diopside and rare chromite. The ultramafic protolith has a depleted residual mantle composition, indicated by a high‐Mg number, very low CaO, Al 2 O 3 and total REE contents compared to primary mantle and other Sulu peridotites. Most garnet (Prp 44−58 ) clinopyroxenites are foliated. Except for rare kyanite‐bearing eclogitic bands, most eclogites contain a simple assemblage of garnet (Alm 29−34 Prp 32−50 Grs 15−39 ) + omphacite (Jd 24−36 ) + minor rutile. Clinopyroxenite and eclogite exhibit LREE‐depleted and LREE‐enriched patterns, respectively, but both have flat HREE patterns. Normalized La, Sm and Yb contents indicate that both eclogite and garnet clinopyroxenite formed by high‐pressure crystal accumulation (+ variable trapped melt) from melts resulting from two‐stage partial melting of a mantle source. Recrystallized textures and P–T estimates of 780–870 °C, 5–7 GPa and a metamorphic age of 231 ± 11 Ma indicate that both mafic and ultramafic protoliths experienced Triassic UHP metamorphism in the P–T forbidden zone with an extremely low thermal gradient (< 5 °C km −1 ), and multistage retrograde recrystallization during exhumation. Develop of prehnite veins in clinopyroxenite, eclogite, felsic blocks and country rock gneiss, and replacements of eclogitic minerals by prehnite, albite, white mica, and K‐feldspar indicate low‐temperature metasomatism.