Petrologic Geochemistry And Isotopic Geochronology Of The Luotian Eclogites From The North Dabie Complex Zone,Central China

The Dabie orogen is a continental-continental collision-type belt and contains various tectonic rock units related to subduction and exhumation processes, including three eclogite-bearing rock units that called North Dabie complex zone (NDZ), Central Dabie middle-T/ultrahigh-pressure (UHP) metamorphic zone (CDZ) and South Dabie low-T eclogite zone (SDZ), respectively. Petrology, petrological geochemistry, isotopic geochronology and tectonics on UHP eclogites and related rocks from the Dabie orogen have been done for over twenty years, and a series of important results, including discovery of UHP index minerals such as coesite and diamond, determination of UHP metamorphic ages, foundation of tectonic regime in orogen, division of various petro-tectonic units as well as identification of continental subduction and exhumation processes, are obtained. However, the previous studies focused mainly on the SDZ and CDZ, but rarely on the NDZ. In addition, almost all exposed UHP metamorphic rocks are subducted upper and felsic lower continental crust with minor mafic boudins. Recently, eclogites from the Luotian dome in the NDZ were discovered and studied by means of petrology and zircon U-Pb geochronology, documented them to be transformed from the Neoproterozoic mafic lower continental crust involved in a multistage metamorphic evolution during the Triassic deep subduction and exhumation. Therefore, these eclogites provide an important probability for the studies on subduction and exhumation processes of the NDZ, elemental and isotopic behavior during continental collision as well as recycling of subducted mafic lower continental crust in the Dabie orogen. In this study, a combined study of petrologic geochemistry, geochronology, elemental and isotopic geochemistry are carried on eclogites and related garnet-bearing amphibolites from the Luotian dome in the NDZ. The results suggest that the eclogites underwent the Triassic UHP metamorphism, experienced a multistage evolution of subduction and exhumation processes, and have a mafic lower-crustal origin of their precursors. In addition, their elemental and isotopic behavior during multistage exhumation and cooling history are discussed. The main conclusions are addressed as the following.1. The study on metamorphic petrology implies that the eclogites in the NDZ experienced a multistage evolutional history, from UHP eclogite-to granulite-and amphibolite-facies retrogression conditions, resulting in common formation of unusual multistage decompression such as double symplectites and corona and needle exsolutions. New UHP metamorphic evidences such as coesite and quartz pseudomorphs after coesite are discovered in the eclogite from NDZ for the first time, combining with the occurrence of oriented needles such as quartz+sodic plagioclase+amphibole or quartz+hypersthene in clinopyroxene and clinopyroxene+rutile+apatite+amphibole in garnet as well as Na-enrichment garnet, indicating that the possibility of peak pressure of Luotian eclogite was about5-7GPa. Considering the presence of diamonds from both eclogite and gneiss in the northern part of the NDZ, it suggests that the entire NDZ experienced Triassic UHP metamorphism as a coherent unit. Furthermore, different from the CDZ and SDZ, the NDZ underwent a multistage high-temperature metamorphism from UHP eclogite-facies to granulite-facies overpriting, showing a rapid exhumation and slow cooling during the early stage of uplift. This study further provides new petrological evidences to constraint the different exhuamtional processes of the three UHP slices in Dabie orogen.2. This research for the first time constraints the precise timing of UHP and HP eclogite-facies metamorphism as well as granulite-facies retrogression for the NDZ in the Dabie orogen with the ages of226±2Ma,214±2Ma and207±4Ma, respectively, by SHRIMP U-Pb dating, minerals inclusion and trace element on the zircon from the Luotian eclogites. In addition, other separate age-groups of235±3±243±15Ma,194±8—199±2Ma and176±2—188±2Ma are also found in zircon from the Luotian eclogites, which may represent the prograde metamorphic timing prior the UHP metamorphism and two stages of amphibolite-facies retrogression, respectively. Although this peak metamorphic age of the NDZ is similar to that of the CDZ and the SDZ within error, the three eclogites-bearing UHP slices in the Dabie orogen have different metamorphic P-T-t paths and exhumation history as well as various geochronological records in the post-peak stage. It is further documented that there are several different slices formed by multiple decoupling within the subducted continental crust of the SCB during subduction to mantle depths and subsequent exhumation, mainly resulted from the difference of mechanic strength of rocks in various levels of the continental crust. This study provides more geochronological evidences of subduction and multistage exhumation for the NDZ.3. Major and trace elements as well as Sr-Nd-Pb-Hf isotopes indicate that the protoliths of Luotian eclogites are main gabbros and minor basalts. All the eclogites display low radiogenic Pb-isotope compositions with206Pb/204Pbi,207Pb/204Pbi and208pb/204pb. values of15.217-17.522,15.077-15.540and35.219-38.082, characterized by the lowest Pb-isotope compositions in the Dabie orogen and documenting a mafic lower-crustal origin of their protoliths. The εHf(790Ma) values of-9.5to+7.9for magmatic zircons and a whole-rockεNd(790Ma) values of+1.7as well as the occurrence of2.15Ga relic zircon from the Luotian eclogites, suggest that their precursors were derived from underplating of mantle-derived magma at the base of lower continental crust, contaminated by variable degrees of acient continental crustal materials during Neoproterozoic.4. The Luotian eclogites underwent a complex metamorphic evolution involving UHP and HP eclogite-facies metamorphism, granulite-facies overprinting and amphibolite-facies retrogression during the Triassic continental deep subduction and exhumation, showing variation of LREE and LILE from enrichment to pronounced depletion. Some eclogites show LILE enrichment or depletion relative to LREE, while some other eclogites display strong depletion in LILE and LREE, suggesting that the element mobility in the Luotian eclogites were modified not only by fluid activity but also by partial melting, probably due to metamorphic processes or characteristics of different stages during exhumation. The fluid activity during amphibolite-facies regression stage includes both the outer fluids from country-rock gneiss and inner fluids from eclogites itself, resulting in obvious enrichment or strong depletion of LILE relative to LREE, respectively. While depletion in both LILE and LREE of some eclogites are ascribed to decompression melting during granulite-facies overprinting at206±9Ma, defined by a whole-rock Sm-Nd isotopic isochron. Finally, this study obains the ages and geochemical evidences for decompress melting of Luotian eclogites for the first time.5. Ages of199±2Ma and174±1-173±7Ma were obtained by Sm-Nd mineral (garnet+clinopyroxene+whole rock) isochron from three Luotian eclogites, combining with the closed temperature of Sm-Nd isotopic system being changed under different conditions, these ages may record the Luotian eclogites uplifting to different crustal-level during amphibolite-facies retrograde stage。 Amphibole Ar-Ar dating and rutile U-Pb dating both yield Early Cretaceous ages of121.8±1.1-135.1±11.7Ma, indicating that the Luotian eclogite were affected by Yanshannian event. Combining with the zircon U-Pb ages of Luotian eclogites, a cooling curve of UHP rocks in NDZ is described. It shows the Luotian eclogites underwent a near-line isothermal decompressional stage in the early exhumation, followed by a rapid cooling and slowly uplift process, then experienced slowly cooling and decompression; they were affected by Yanshannian event during Early Cretaceous and further uplifted to surface at last.