Dehydration melting of UHP eclogite and paragneiss in the Dabie orogen: Evidence from laboratory experiment to natural observation

Abstract

Dehydration melting of subducted continental crust is significant during exhumation, and its study from both experimental and petrological observations is of great importance to our understanding of continental geodynamics. Dehydration melting experiments were carried out on ultrahigh-pressure (UHP) eclogite from Bixiling in the Dabie orogen using a piston cylinder at 1.5-3.0 GPa and 800-950°C to investigate partial melting of eclogite induced by phengite breakdown. The phengite-bearing eclogite started to melt at T800-850°C and P=1.5-2.0 GPa and produced about 3% granitic melt. The products of dehydration melting vary with temperature and pressure. Such results provide valuable constraints on the micro-texture related to partial melting of UHP rocks in the Dabie-Sulu orogenic belt. Three types of polyphase inclusions were identified in garnet from the Shuanghe UHP eclogite. K-feldspar and quartz inclusions are interpreted to represent the products of segregation and crystallization of minor amounts of melt that formed during dehydration melting of phengite by the inferred reaction Phengite+Omphacite±Quartz→ Amphibole±Garnet+Melt (K-feldspar+Quartz±Plagioclase). Polyphase inclusions of phengite and K-feldspar+Quartz inclusions were also found in zoisite/clinozoisite and garnet from the Shuanghe garnet-bearing paragneiss. These polyphase inclusions provide evidence for a continuous process from sub-solidus dehydration to partial melting within the UHP gneissic rocks. The compositional variation of garnets demonstrates that breakdown of epidote-group minerals may have played a crucial role during dehydration melting reaction of phengite. The Ti-in-zircon thermometry and Si content of phengite in zircon suggest that partial melting would occur at 783-839°C and 2.0-2.5 GPa. Therefore, both experimental results and petrological observations indicate that dehydration melting and fluid activity within the Dabie UHP rocks at micro-scale are controlled by the breakdown of phengite. dehydration melting, phengite, experimental petrology, polyphase inclusions, ultrahigh-pressure metamorphism, Dabie orogen Citation:Liu Q, Wu Y. Dehydration melting of UHP eclogite and paragneiss in the Dabie orogen: Evidence from laboratory experiment to natural observation. Chin Sci Bull, 2013Bull, , 58: 43904396, doi: 10.1007 Recent studies from experimental petrology [1-3], geochemical analyses [4][5][6] and geochronological data [7][8][9] have indicated that deeply subducted continental crust experienced partial melting during exhumation. Such anatexis can change mechanical and chemical behaviours of ultrahigh-pressure (UHP) metamorphic rocks. It has significant effects on facilitating the deformation mechanisms (or strain localization) of UHP rocks [10,11], increasing the buoyancy of melted rocks and promoting the segregation of partial melts. Thus, partial melting of subducted continental crust is of great significance for understanding the melting event and fluid activity at great depths as well as the rheological property of UHP ro...

Zircon U-Pb Dating for Paragneiss in the Xinxian Area in the Dabie–Sulu Orogenic Belt and Its Geochemical Characteristics

“…As Gu found LILEs and LREEs were depleted in eclogite (Gu et al, 2013), it suggests that the paragneiss in Xinxian county might be partially melted, like the felsic magma formed under high pressure. The continuous process from sub-solidus dehydration to partial melting within the UHP gneissic rocks is also evidenced by experimental and petrological observations in Shuanghe paragneiss (Liu and Wu, 2013). According to the geochemical characteristics of the samples, the protolith is likely to be sedimentary rock that has been partially melted during the subduction and retreat of the Yangtze block (Wallis et al, 2005).…”

Evidence for UHP anatexis in the Shuanghe UHP paragneiss from inclusions in clinozoisite, garnet, and zircon

“…It is important to note that only minor amounts of the inclusions display such breakdown textures (e.g. Liu & Wu, ), whereas most of the phengite in the matrix is not reacting to K‐feldspar+kyanite.…”

“…To harvest the information about the formation of polyphase inclusions and associated metamorphic equilibria in subducted crustal rocks, a combination of detailed petrographic observations and geochemical studies is required (e.g. Liu & Wu, ). Here, we report different types of single and polyphase inclusions in clinozoisite, garnet, and zircon from a garnet‐bearing paragneiss from the Shuanghe UHP locality in the Dabie orogen.…”

Partial melting due to breakdown of an epidote‐group mineral during exhumation of ultrahigh‐pressure eclogite: An example from the North‐East Greenland Caledonides

“…The first melting curve crossed during exhumation of the studied eclogite is the phengite breakdown curve (Liu et al, 2009;Skjerlie & Patiño Douce, 2002). Phengite melting may occur with matrix minerals of omphacite and quartz as reactants, and forms kyanite, plagioclase, and melt with or without garnet (Figure 12; Liu & Wu, 2013;Liu et al, 2009).…”