Petrology And Metamorphic Evolution Of The Eclogites,Blueschists,and Metasedimentary Rocks In The Changning-menglian Orogenic Belt And Their Implications On The Subduction And Orogeny Of The Paleo-tethys

The Paleo-Tethys existed during a particularly important period in the geological evolution of the Earth.Investigations on the formation and tectonic evolution of the Paleo-Tethyan domain plays a critical role in the reconstruction of the continental blocks in the Southeast Asia.The Changning-Menglian orogenic belt(CMOB)in the southeastern Tibetan Plateau and central part of the Sanjiang area is an important Paleo-Tethyan subduction-accretionary belt.The belt separates the Gondwana-derived Baoshan-Tengchong Block from the Indochina-Yangtze-derived Lanping-Simao Block,and represents the remnant of the main Paleo-Tethyan Ocean.The CMOB provided the invaluable information of the complete tectonic process from seafloor spreading to subduction and closure of the ancient ocean,and exhumation and uplift of the high pressure(HP)-ultrahigh pressure(UHP)rocks.It has been accepted that the CMOB extends northwestward to the Longmu Co-Shuanghu suture zone(LCSS)in the northern Tibetan Plateau,southeastward from the southeastern Thailand to the Malaysia.In this study,we presented a comprehensive investigation of the eclogites,blueschists,and host metasedimentary rocks in the CMOB in terms of the petrology,mineralogy,thermodynamic modelling,geochemical compositions,Sm-Nd isotope,and multiple isotopic dating analyses.These new data,integrated with previous results,were studied in detail to constrain the protolith nature,geochronological framework,and metamorphic P-T-t paths for the HP-UHP rocks.Both the lawsonite-bearing eclogites and blueschists have been identified for the first time.We have clarified the occurrences and spatial distributions of the eclogites and blueschists in the CMOB,and determined the extent of the main Paleo-Tethyan suture in the Sanjiang area.The subduction-exhumation history of the Paleo-Tethyan oceanic crust,the exhumation mechanism of subduction channel model,and the tectonic evolution of the Paleo-Tethys have been proposed.The eclogites form an elongate N-S-trending metamorphic belt that is 200 km long and 50 km wide within the CMOB and are mainly exposed in the areas of Mengku,Genhenhe,Bangbing-Yakou,Heihe,Qianmai,and Jinghong from the north towards the south.They display a variety of mineral assemblages,which include the low-temperature eclogites,such as lawsonite-bearing retrograde eclogite(Grt + Omp + Lws + Amp + Pl + Rt/Ilm/Ttn + Qz),lawsonite-talc-phengite eclogite(Grt + Omp + Lws + Tlc + Ph + Amp + Rt + Qz),phengite/talc/epidote-glaucophane eclogite(Grt + Omp + Gln + Aln/Ep ± Ph ± Tlc + Pg + Rt + Qz),the middle-temperature eclogites,such as dolomite/magnesite-kyanite eclogite(Grt + Omp + Ky + Dol + Mgs + Rt + Pg + Aln/Ep +Qz),as well as phengite eclogite(Grt + Omp + Ph + Rt + Qz)and retrograde eclogite(Grt + Amp + Pl + Rt/Ilm/Ttn + Qz ± Ky ± Ph).The blueschists in the CMOB show similar spatial patterns to those of eclogites,and are distributed mostly in the areas of Gengma-Zhangtuo,Nanlang-Suyi-Ankang-Xiaoheijiang,Huimin,Xiding,and Bulangshan from the north to the south.Our study found that the blueschists in the Liyunxinzhai and Xinzhai areas contain abundant lawsonite grains.They consist of epidote-magnesioriebeckite schist(Mrbk + Ep/Aln + Chl + Ab + Ttn + Ph + Hem + Lws + Aug-Aeg + Qz)and garnet-ferroglaucophane schist(Grt + Fgl + Ttn + Lws + Omp + Ph + Qz ± Stp).Lawsonite has been regarded as an index of cold subduction of the oceanic slab.The coexistences of the lawsonite-bearing eclogite and blueschist are very rare within an orogenic belt.The Changning-Menglian orogenic belt represents the third belt,after the North Qilian orogenic belt and Southwest Tianshan orogenic belt in China,from which both lawsonite-bearing eclogite and blueschist are well-preserved.Therefore,the CMOB should represent a typical cold subduction-accretionary belt.In the eclogites and blueschists,lawsonite occurs as fine-grained inclusions within the garnet and sodic amphibole(magnesioriebeckite and ferroglaucophane),the presence of well-preserved lawsonite crystals indicates the rapid subduction and rapid exhumation histories for these HP-UHP rocks.Metasedimentary rocks are widespread in the CMOB.They are comprised mainly of phengite-quartz schist,albite-quartz-phengite schist,garnet-phengite schist,staurolite-kyanite-garnet-bearing mica schist,chloritoid-paragonite-phengite schist,and chlorite-glaucophane-albite schist.These rocks widely preserved HP mineral assemblages and are characterized by the presences of phengite as well as garnet,kyanite,chloritoid,paragonite,and glaucophane.As a result of these new discoveries,we defined a regional,elongate,and nearly N-S-trending HP–UHP metamorphic belt at least 400 km long and 50 km wide in the CMOB.Eclogites and blueschists have complex chemical compositions.They are geochemically similar to the OIB,E-MORB,and N-MORB,and have mainly positive(0.26~7.89)and some negative(-5.45~-0.04)values of ?Nd(t).The lawsonite-bearing retrograde eclogites and epidote-magnesioriebeckite schists show typical OIB affinities and have ?Nd(t)values of-0.35~2.98 and 2.29~4.40,respectively.Some of retrograde eclogites display trace and rare earth element distribution patterns compatible to those of N-MORB and have elevated ?Nd(t)values(7.89).The other(retrograde)eclogites and garnet-ferroglaucophane schists show E-MORB-like signatures and have variable ?Nd(t)values of-4.34~4.78 and-5.45~4.10,respectively.We inferred that the mafic protoliths of these HP-UHP rocks formed in an oceanic island-seamount setting or were derived from an enriched and depleted oceanic lithosphere mantle source with limited degree of crustal contamination.Moreover,the geochemical features and the Sm-Nd isotopic ratios of eclogites and blueschists are similar to those of the main Paleo-Tethyan Ocean-derived(meta-)basic rocks within the ophiolite suites and oceanic island-seamount sequences,which suggests that the protoliths of eclogites and blueschists in the CMOB belong to the Paleo-Tethyan oceanic crust.Abundant U-Pb age data of magmatic zircons have revealed that the eclogites and blueschists in the CMOB record a wide range of protolith formation ages varying from 451 Ma to 250 Ma.The lawsonite-bearing retrograde eclogites yielded the oldest magmatic U-Pb age of ~451 Ma,whereas the youngest age data of 256-250 Ma for the phengite-kyanite-bearing retrograde eclogites and garnet-ferroglaucophane eclogites.Magnesite-kyanite eclogite and retrograde eclogites provided protolith ages in a range of 317-262 Ma.All the protolith timing recorded by these meta-mafic rocks fits well within the time frame of the formation and evolution of the Paleo-Tethyan Ocean in the CMOB.The metasedimentary rocks display immature compositions similar to arkose and subarkose and are sourced from felsic rocks.They show uniform detrital zircon U-Pb age spectra that are dominated by two major age groups of 1150-850 Ma with a peak at 950 Ma,and 600-450 Ma with a peak at 550 Ma.The maximum depositional age of ~428 Ma was defined based on the youngest detrital age peaks.The metasedimentary rocks display complex material sources of a mixture of Precambrian sediments in the Lanping-Simao Block and Pan-African Orogeny-related magmatic rocks with minor Early Paleozoic Proto-Tethyan subduction-and volcanic rocks related to the onset of oceanic spreading of Paleo-Tethys.Therefore,the metasedimentary rocks should be the Early Paleozoic sedimentary sequence of the Lanping-Simao Block that have been affected by the tectonothermal events during the Neoproterozoic to Early Paleozoic periods.Both the eclogites and blueschsits consistently recorded a clockwise metamorphic P-T path.Their prograde subduction P-T trajectory evolved along a low geothermal gradient of 5-10 ?/km and were marked by the formation of lawsonite.The retrograde metamorphism follows an isothermal-or cooling-decompressive process,and is represented by the developments of low-pressure phases such as sodic-calcic amphibole(winchite and barroisite),calcic amphibole(actinolite and edenite),diopside,epidote,paragonite,and albite.However,these rocks show distinctly different peak metamorphic P-T conditions and the peak mineral assemblages.Both the epidote-magnesioriebeckite schists(Lws + Mrbk + Aug-Aeg + Ph + Aln + Ttn ± Chl + Qz)and garnet-ferroglaucophane schists(Grt + Fgl + Omp + Ph + Lws + Rt/Ilm + Qz)preserved lawsonite-blueschist facies peak-stage assemblages but yielded different peak P-T conditions of 12.4-16.8 kbar,350-406 ? and 19.5-22.6 kbar,490-510 ?,respectively,corresponding to the subduction depths of ~50-70 km.The lawsonite-bearing retrograde eclogites and phengite/talc-glaucophane eclogites show peak mineral associations(Grt + Omp + Gln + Ph + Aln ± Tlc + Rt + Qz)similar to those of garnet-ferroglaucophane schists but higher peak P-T conditions(23-26 kbar,520-610 ?)and subduction depths(75-80 km),consistent with lawsonite-eclogite facies metamorphism.In contrast,the dolomite/magnesite-kyanite eclogites are almost fluid-free with a peak mineral assemblage of Grt + Omp + Ky + Dol + Mgs + Ph + Rt + Qz,and they record the highest peak P–T conditions(29-32 kbar,675-754 ?)and a maximum subduction depth of 90-95 km that falls in the field of middle-temperature/ultrahigh pressure kyanite-eclogite facies.The peak metamorphic P-T conditions recorded by the metasedimentary rocks are different from those of eclogites and blueschists.The staurolite-kyanite-garnet-bearing mica schists show eclogite-facies peak metamorphic mineral assemblage of Grt + Ph + Ky ± Jd + Qz with the peak P-T conditions of 19-30 kbar,600-750 ?.The garnet-phengite schists yielded blueschist-facies to eclogite-facies peak P-T conditions of 18.8-21.6 kbar and 392-553 ?.The chlorite-glaucophane-albite schist records lower peak P-T conditions of 9-11 kbar,430-520 ? with peak mineral assemblage of Gln + Ab + Ph + Ttn + Qz.Abundant metamorphic zircon U-Pb ages combined with the phengite 40Ar-39 Ar isotopic dating constrained the metamorphic ages of eclogites and metasedimentary rocks to be 246-227 Ma and 238-231 Ma,respectively.The lawsonite-bearing retrograde eclogites with oldest protolith ages(~450 Ma)yielded the oldest metamorphic timing of 246-245 Ma,whereas the youngest protolith ages(256-250 Ma)recorded by the phengite-kyanite-bearing retrograde eclogites related to the relatively younger HP ages of 235 Ma.It was therefore suggested that the Paleo-Tethyan oceanic crusts with different formation ages have subducted continuously in the periods of 246-227 Ma.The prograde stage of the oceanic subduction occurred in the ~250 Ma.The oceanic slab underwent a process of cold and fast subduction(~4.5–6.0 mm/yr)along a low geothermal gradient of 5–10 ?/km.The minor differences between the timing of magmatic crystallization and the peak metamorphic age for the phengite-kyanite-bearing retrograde eclogites and garnet-ferroglaucophane schists suggested that the newly formed oceanic crust was probably abruptly dragged down by the old downgoing Paleo-Tethyan oceanic lithosphere after the protolith generation into the depths of ~50-80 km in the subduction channel,and formed the lawsonite blueschists and low-temperature eclogites.In the ~234 Ma,the oceanic crust subducted into the maximum depths of 90-95 km and formed the UHP dolomite/magnesite-kyanite eclogites.The rapid exhumation(3.2-4.2 km/myr)of the subducted slabs occurred at the time of 227-225 Ma.In the end of the Late Triassic(222-207 Ma),the HP-UHP rocks uplifted to shallow crustal levels and some of them were overprinted by the strong retrogressive metamorphism.The coeval HP metamorphic ages of the metasedimentary rocks to those of the eclogites and blueschists in the CMOB demonstrate that a significant volume of Lanping-Simao Block-derived continental sediments have been involved in the Paleo-Tethyan subduction event.The sedimentary rocks in the upper plate would have been off scraped by the downgoing Paleo Tethyan oceanic lithosphere and underwent HP metamorphism,and then experienced exhumation and uplift processes along with the eclogites and blueschists.The exhumation of the HP-UHP rocks correlates not only with the ascent of the low-viscosity metasedimentary rocks,but also with the subduction channel flow dynamics.The subducted slabs in the CMOB show a variety of material sources,which include both the older and younger oceanic crusts with the time span of 200 Ma,and the metasedimentary rock from the hanging wall blocks.The inhomogeneous P-T trajectories and peak P-T conditions for the HP-UHP rocks reflect a complex burial-exhumation history along the plate interface,such as the local coupling or the underplating process between the descending slab and the overlying mantle.The fluctuating profiles in some of HP mineral phases(e.g.garnets and ferroglaucophanes)are likely to be linked to variations in P-T conditions that were deduced from the short-lived burial-exhumation cycles in the subduction channel.The retrogressive P-T paths showing isothermal decompression or cooling plus decompression with no significant melting are indicative of the exhumation of HP-UHP rocks controlled by the channel flow.These characteristics are consistent with the model of subduction channel as the subduction-exhumation mechanism for the HP-UHP metamorphic mélange in the CMOB.The HP-UHP mafic and overriding sedimentary rocks from varying depths were mechanically intermingled within the subduction channel,and the return channel flow carried the subducted slabs upwards to undergo rapid exhumation and juxtaposition in a shallow-level of the crust.The compatible field occurrences,mineral assemblages,HP-UHP metamorphic ages,and geochemical signatures for eclogites and bluescists in the CMOB and the LCSS suggested that the CMOB can be associated with the LCSS in the northern Tibetan Plateau.The main suture zone of the Palaeo?Tethyan domain extends at least 2000 km in length from the CMOB to the LCSS in the Tibetan Plateau.The identification of the main Paleo-Tethyan suture zones has far-reaching implications for the deep geodynamic processes and complex tectonic evolution of the Paleo-Tethys.The detailed investigations of the eclogites,blueschists,and metasedimentary rocks in the CMOB provide significant insights into geothermal dynamics,fluid-rock interactions,element transport,and crust-mantle mass recycling in the subduction belt.