Trace elemental records of short-lived heating during exhumation of the CCSD eclogites

Abstract

Trace element compositions of garnet, omphacite and apatite in ultrahigh-pressure eclogites from the main hole of the Chinese Continental Scientific Drilling (CCSD) project were in situ analyzed by the LA-ICP-MS method. Although both garnet and omphacite have homogeneous major element compositions, their trace elements show zonations from core to rim in rare earth elements. In particular, middle rare earth elements in the garnet, heavy rare earth elements in the apatite and all rare earth elements in the omphacite increase from core to rim, respectively. Based on dependence of partition coefficients on temperature and pressure in these minerals, we suggest that the trace elemental zonations in these minerals may record a short-lived heating event during exhumation. eclogite, in situ analysis, trace element, mineral zoningThe Sulu ultrahigh pressure (UHP) metamorphic belt is an important part of the Qinling-Dabie Central Orogenic Belt in China. The main hole of the Chinese Continental Scientific Drilling (CCSD) is located at Donghai County, the southwestern part of the Sulu UHP terrain. Rocks drilled through by CCSD ever resided in the deep level of mantle beneath the convergent plate boundary, which is the best area to study the continental subduction and geodynamics [1,2] . Recent studies reveal that the eastmost Sulu UHP rocks suffered partial melting during exhumation [3] , and some eclogites in the eastern Shandong were overprinted by granulite-facies metamorphism [4][5][6][7] . Then, one important question is what causes the partial melting. To answer this question, decompression, increasing temperature and adding volatile were ever suggested. Yao et al. [7] attributed overprinting of granulite-facies metamorphism on eclogites to fluid infiltration during the exhumation. However, P-T path of coesite eclogite to granulite indicates that the overprinting of granulite-facies metamorphism was associated with heating, and the extent of temperature increasing was ~40℃ [4] . O'Brien and Vrána [8] suggested that a short-lived heating event during exhumation resulted in overprinting of granulite-facies metamorphism on the UHP eclogites in the Moldanubian Zone. The presence of corundumbearing mafic and ultramafic rock is an indicator of ultrahigh temperature [9][10][11][12] . In the Sulu UHP belt, corundum is found in eclogite enclosed in ultramafic rocks [13] and garnetite in garnet peridotite [14] . Especially, we recently found a unique mineral assemblage (garnet + corundum + spinel + quartz + feldspar) indicating (ultra-) high temperature nearby the CCSD. These observations imply that the eclogites in some parts of the Sulu UHP terrane could suffer a short-lived high-T stage during the exhumation.The thermodynamic theory of trace element partitioning between metamorphic minerals indicates that compared with major elements, trace elements are potentially more sensitive to chemical and thermobaromet-

Partial melting of deeply subducted continental crust and the formation of quartzofeldspathic polyphase inclusions in the Sulu UHP eclogites

“…Although fully crystallized, the polyphase Ab-or Kf-bearing inclusions in the Sulu eclogites are compositionally similar to silicate glass inclusions in the Pamir eclogite xenoliths, and so might also represent mica partial melting at HP or UHP conditions. Updated studies on the Sulu-Dabie UHP rocks demonstrated that the subduction and exhumation of this slab is characterized by dry subduction [8,9] and hot exhumation [4,[31][32][33][34] . Results from studies of petrography, stable isotopes, and fluid inclusions all suggested only restricted amounts of aqueous fluid were available to both prograde and retrograde metamorphic reactions in the Dabie-Sulu UHP eclogites [8,9,66] .…”

“…Earlier studies suggested that decompression from eclogite to granulite facies was nearly isothermal [50][51][52] , suggesting a rapid exhumation from ~230 Ma to ~215 Ma. However, updated studies suggested that the Sulu UHP rocks experienced a "hot" exhumation [4,34] characterized by a short pulse of heating and increase of temperature [31][32][33] .…”

“…However, identifying evidence of such melts present in formerly UHP rocks that have experienced various degrees of retrograde reactions is problematic. Petrographic observations and P-T estimates [31,32] , trace element and isotope geochemistry studies [33][34][35][36] have suggested that (1) the Dabie-Sulu UHP rocks experienced "hot" exhumation [4,33,34] , and partial melting; (2) fertile components were melted out to form quartzofeldspathic veins [35] or relatively large-scale granitic plutons [37] ; and (3) such melts infiltrated into the mafic eclogites and induced substantial geochemical anomalies [34] . In addition, polyphase inclusions within UHP phases might represent the presence of supercritical fluids [13] .…”

Element mobility in mafic and felsic ultrahigh-pressure metamorphic rocks during continental collision

“…Thus, an increase in temperature at decreasing pressure would occur at the initial exhumation of the deeply subducted continental crust. An in-situ study of trace elements in minerals from CCSD-MH eclogite also suggests short-lived heating during the exhumation (Zong et al, 2007). Therefore, the ''hot'' exhumation is evident for the UHP metamorphic rocks in the Sulu orogen.…”

Garnet–spinel–corundum–quartz-bearing titanohematite veins in eclogite from the Sulu ultrahigh-pressure terrane: Imprint of a short-lived, high-temperature metamorphic stage

“…Almost all of these studies have focused on UHP metamorphic processes, while only a few studies have focused on the thermal evolution of the Dabie-Sulu UHP terrane. Trace elemental zoning of garnet, apatite and omphacite as well as Ti-in-zircon geothermometers of some eclogites from the main hole of the Chinese Continental Scientific Drilling Project (CCSD-MH) recorded a shortlived heating event during exhumation of the terrane Zong et al, 2007). This is consistent with the overprinting of granulite-facies metamorphism on some eclogites in the Sulu UHP terrane (Banno et al, 2000;Nakamura and Hirajima, 2000;Wang et al, 1993;Yao et al, 2000), which has generally been attributed to high temperatures that prevailed during exhumation over a very short time scale (e.g., the Moldanubian Zone) (O'Brien, 1997;O'Brien and Vrána, 1995).…”