High Temperature Metamorphism,Partial Melting And P-T-t Path Of The Eclogites From The North Dabie Complex Zone,Central China

The Dabie collisional orogen is well-known for the widespread occurrence of numerous various ultrahigh-pressure(UHP)metamorphic rocks including coesite-and diamond-bearing eclogites.It is generally subdivided into five subduction-and collision-related rock units with varied rock compositions and different metamorphic evolutional histories,including three eclogite-bearing units:the North Dabie high-T UHP complex zone(NDZ),the Central Dabie mid-T UHP metamorphic zone(CDZ)and the South Dabie low-T eclogite zone(SDZ).In the last three dacades,numerous investigations have been carried out on the UHP eclogites and related rocks in the Dabie orogen,obtaining a variety of significant achievements.For example,the discovery of UHP evidences like index minerals of coesite and diamond,the constraints of the UHP metamorphic ages,the establishment of the tectonic framework of the orogen(including the division of different tectonic units)and the proposal of several related subduction-exhumation models.However,these investigations focused mainly on the CDZ and SDZ,whereas the studies on the NDZ are relatively rare that many problems still exist,especially in the aspect of petrology.Significantly different from the metamorphic evolution histories of the CDZ and SDZ,the NDZ experienced granulite-facies overprinting during exhumation,as well as post-orogenic migmatism.As a result,the prograde and peak mineral assemblages and their compositions are very difficult to be preserved in eclogites and related rocks,and thus set obstacles for estimating their metamorphic P-T conditions.A few versions of P-T paths of the NDZ eclogites previously constructed suggested high-T decompression processes,but few involved in the prograde stage of the P-T path.Moreover,the timing of the post-peak stages of the UHP rocks in the region was not well constrained.Previous studies indicate that,the high-T decompression segment of the P-T path of the NDZ eclogites crosses over the phengite-melting curves,thus the rocks may have undergone associated partial melting.Nevertheless,partial melting of the NDZ UHP rocks during this stage is rarely reported,not to say direct petrological evidences.The lack of fluid in their protoliths may be responsible for the low degree of decompression melting,and additional reasons include the modification of the UHP rocks by the post-orogenic migmatism.The Luotian area located in the southwest portion of the NDZ contains rare eclogite lens/blocks of mafic lower-crustal genesis.The eclogites are characterized by the widespread occurrence of a variey of granulite-and amphibolite-facies retrograde mineral assemblages,while the prograde and peak minerals mainly occur as rare inclusions within garnet or clinopyroxene.This contribution carried out systematic petrological,mineral chemical and zircon geochronological investigations on theeclogites from the Luotian area,the NDZ.Moreover,we applied modern petrological methods,i.e.,mineral trace element thermometers and phase equilibrium modeling method,on the eclogites,which contributed to the reconstruction of their metamorphic P-T-t path.In addition,according to the reconstructed P-T path,we confirmed decompression partial melting of the rocks during early exhumation,and provided rigid petrological evidence.The main contents and achievements are addressed as follows:1.Ti-in-zircon and Zr-in-rutile thermometers were applied to the NDZ eclogites to estimate their peak and post-peak metamorphic temperatures,demonstracting that they experienced multistage high-T(>850 0C)isothermal decompression and subsequent near isobaric cooling processes,and finally reached thermodynamic re-equilibrium at amphibolite-facies conditions(650±50 ?).In addition,we reconstructed the peak and post-peak P-T-t path of the eclogites,by combining the Ti-in-zircon temperatures with zircon U-Pb ages.Besides,we investigated the applicability of the two thermometers in high-T UHP rocks and pointed out some problems need to be taken care of in practice:only a small quantity of zircons and rutiles can preserve their original temperature records,whereas others usually yield lower temperatures due to the modification of later retrogression,therein rutile inclusion within zircon is the least affected.2.Phase equilibrium modeling combined with detailed petrographic observations revealed that the eclogites experienced granulite-facies overprinting and at least two stages of amphibolite-facies retrogression.In addition,we estimated the P-T conditions of the prograde metamorphic stage of the eclogites,based on the compositions of mineral inclusions within garnet,and thus filled the blank of this segment of P-T path.Besides,the discovery of coexisted orthopyroxene + plagioclase in fractures of amphibole porphyroblast demonstrates that the eclogites experienced the late granulite-facies overprinting after the amphibolite-facies retrogression.It for the first time provides direct petrological evidence and constraints on the formational conditions of the large-scale mantle-sourced magma upwelling and partial melting resulted from the post-collision mountain-root removal of the Dabie orogen at Cretaceous.According to new zircon U-Pb dating,CL images,mineral inclusions,trace element features,and plagioclase volume variation modeled by phase equilibrium modeling and the zircon Eu anomalies,the ages of each metamorphic stage of the eclogites,from prograde to peak and then to granulite-facies and amphibolite-facies,have been again constrained.The age results are 23 8±3 Ma(prograde),224±6 Ma(eclogite-facies),214±3 Ma(granulite-facies),204±4 Ma(amphibolite-facies I)and171±4 Ma(amphibolite-facies ?),respectively.Based on the above P-T path and zircon U-Pb dating,we for the first time reconstructed the comprehensive and detailed P-T-t path of the NDZ Luotian eclogites,and thus provided important foundation for further investigating the metamorphic evolution of the high-T eclogites,as well as the collision orogenic processes of the Dabie orogen.4.By comparing the high-T decompression segement of the P-T-t path obtained in this study with the calculated P-T pseudosection,the NDZ Luotian eclogites are proposed to have experienced decompression partial melting during early exhumation stage.Based on detailed petrographic observations on the eclogites,we recognized biotite + plagioclase intergrowth in matrix,which is consistent with the product of phengite decompression melting.Moreover,we excluded the possibility that melt-absent phengite decomposition led to the intergrowth formation,based on the relationship between the phase equilibrium modeling result and the P-T path.This is the first time that rigid petrographic evidence of decompression melting during early exhumation was recognized in the NDZ eclogites.Besides,we discovered petrographic evidence,i.e.,biotite + plagioclase intergrowth within amphibole porphyroblast,for the post-orogenic heating melting of the Luotian eclogites.