Polyphase inclusions in the Shuanghe UHP eclogites formed by subsolidus transformation and incipient melting during exhumation of deeply subducted crust

Abstract

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=3.58p.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 3.5–4GPa. 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 ~1.5–2GPa 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.