Zircon geochemistry records the action of metamorphic fluid on the formation of ultrahigh-pressure jadeite quartzite in the Dabie orogen

Abstract

A combined study of mineral inclusions, U–Pb ages, trace elements and Hf-O isotopes was carried out for zircons from a coesite-bearing jadeite quartzite in the Dabie orogen. The results provide insights into the action of ultrahigh-pressure (UHP) metamorphic fluids during continental deep subduction to a mantle depth and thus constraints on the origin of the jadeite quartzite in the continental subduction zone. The zircons mostly show core-rim structures in cathodoluminescence images. The overgrown rims contain rare mineral inclusions, and exhibit concordant U–Pb ages of 225 to 246Ma and flat HREE patterns with negligible Eu anomalies. In contrast, the relict cores contain UHP metamorphic mineral inclusions such as coesite, jadeite and rutile, and show discordant U–Pb ages ranging from 983 to 2045Ma and steep REE patterns with significant negative Eu anomalies. The U–Pb isotope data for the all cores and rims define an apparent discordia line with upper and lower intercept ages of 2000±43Ma and 234±18Ma, respectively. We interpret the rims with Triassic ages as the new growth of metamorphic origin and the cores with Precambrian ages as the protolith relics of magmatic origin. The relict magmatic zircons underwent two subtypes of metamorphic recrystallization, i.e., solid-state transformation and metasomatic alteration. The solid-state recrystallized zircons exhibit slightly discordant U–Pb ages close to the protolith age, steep MREE–HREE patterns, and almost unchanged Hf isotope ratios. These observations point to the lowest degree of resetting to the geochemical composition of protolith zircons. In contrast, the metasomatically recrystallized zircons exhibit partial resetting in protolith zircon REE composition and U–Pb and Lu–Hf isotopic systems. All of the zircon domains, regardless of the rims and cores, show relatively consistent δ18O values of 4.0±0.2‰. Such a consistency indicates not only that the metamorphic fluids are of internal origin from the deeply subducted continental crust but also that the oxygen isotope composition of protolith zircons was reequilibrated with the UHP metamorphic fluids of Triassic age. The metasomatic recrystallization of protolith zircons is indicated by the occurrence of UHP inclusion minerals such as coesite, rutile and jadeite in sealed microcracks. In this regard, the fluid metasomatism firstly took place along fractures of the relict zircons during prograde subduction of the continental crust and then experienced the metamorphic recrystallization to result in sealing of the fractures under the UHP conditions. As such, the metasomatic recrystallization has heterogeneously reset the U–Pb and Lu–Hf isotope systems of protolith zircons. The composition of inclusion minerals within the relict zircon cores suggests that the metamorphic fluids were rich in Si, Ti, Na and Al. These elements would be acquired by the metamorphic fluids through metasomatic reaction of metagreywackes overlying the granitic orthogneiss. Therefore, the jadeite quartzite would be precipitated from the UHP metamorphic fluids that were derived from dehydration of the underlying basement orthogneiss but reacted with the metagreywackes during the continental subduction-zone metamorphism.