Eclogites of the Dabie Region: Retrograde Metamorphism and Fluid Evolution

地质学报：英文版 (2002)

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Abstract:

Based upon fluid effects, retrograde metamorphism of eclogites in the Dabie region can be divided into the fluid-poor, fluid-bearing and fluid-rich stages. The fluid-poor stage is marked by polymorphic inversion, recrystallization and exsolution of solid solutions, and is thought to represent eclogite-facies retrograde environments. The fluid-bearing stage is likely to have occurred at the late stage of ecologite-facies diaphthorosis and is represented by kyanite porphyroblasts, rutile, and sodic pyroxene in association with high-pressure hydrous minerals such as phengite and zoisite (clinozoisite) without significant amount of hydrous minerals such as amphibole, epidote and biotite. The fluid-rich stage might have commenced concomitantly with lower amphibolite-facies diaphthoresis and persisted all the way towards the near-surface environment. The product of this stage is characterized by plentiful hydrous and volatile-bearing phases.The dissemination-type rutile mineralizations in eclogites might have formed by preferential shearing-induced pressure solution of gangue minerals at the fluid-bearing stage. The accompanying vein rutile was precipitated from fluids of this stage after local transport and concentration, and may hence represent proximal mobilization of titanium from the eclogite. Therefore, rutile veins can be used as an exploration indicator for dissemination-type rutile deposits.

Keywords:

Amphibole

Phengite

Omphacite

Topics:

Geological and Geochemical Analysis

Pleistocene-Era Hominins and Archaeology

Geochemistry and Geologic Mapping

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Lithos (2013)

Qiang Liu Jörg Hermann Junfeng Zhang

Ultrahigh-pressure (UHP) layered eclogites from the Shuanghe area, Dabie Mountains display the peak assemblage garnet, Na-rich omphacite (Jd60), and minor amounts of coesite, rutile, Si-rich phengite (Si = 3.58 p.f.u) and apatite. Zr-in-rutile thermometry provides evidence for peak metamorphic conditions of ~ 650–700 °C, which combined with garnet–clinopyroxene–phengite barometry yields a peak pressure of 3.5–4 GPa. Three different types of polyphase inclusions that formed after peak conditions are identified in garnet. The first type consists of a symplectitic intergrowth of zoisite + quartz ± amphibole associated with epidote and exhibits a rectangular shape. These inclusions are interpreted to have formed during decompression of the UHP rocks as pseudomorphs after prograde to peak lawsonite. The second type of polyphase inclusions contains approximately equal amounts of K-feldspar and quartz. These inclusions are usually surrounded by radial fractures and exhibit regular negative crystal shapes. The observed gradual breakdown of peak metamorphic phengite resulting in these types of inclusions suggests that phengite melting at an advanced stage of exhumation at ~ 1.5–2 GPa and ~ 750–800 °C occurred. The third type of polyphase inclusions displays irregular shapes and preserves the retrograde mineral assemblage amphibole + plagioclase + quartz + biotite + K-feldspar, which likely formed during late stage fluid influx that is also responsible for the replacement of phengite by biotite as well as omphacite by amphibole + plagioclase. The trace element composition of K-feldspar + quartz inclusions has been determined in-situ by LA-ICP-MS and is characterized by a strong enrichment in LILE and a moderate enrichment in LREE with respect to HREE and a depletion in HFSE. The presence of melts assisted in the recrystallization of the eclogites and resulted in the segregation of garnet + quartz-rich and omphacite-rich domains. Field observations show that the melting is very limited and no interconnected network of partial melts formed. Thus, the low degree of partial melting did not result in a chemical differentiation of the subducted rocks but likely influenced the rheology of the eclogites during exhumation.

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Lithos (2003)

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