The Triassic Ultrahigh Temperature Metamorphism In The North Dabie Terrane,China

Ultrahigh temperature?UHT?metamorphism has significantly enhanced our understanding of the thermal properties of the crust in geodynamic modeling.It is crucial for understanding the nature and structure of the deep crust,evolution,and geodynamics of the collisional orogens.The Dabie–Sulu orogen is a typical continental collisional orogenic belt best known for its ultrahigh pressure metamorphic rocks.However,studies focused on the UHT metamorphism in continental collisional orogens are still very lacking.In this dissertation,the North Dabie Terrane?NDT?has carried on a detailed field investigation.According to the study of petrology,zircon internal structure,in situ LA–ICP–MS and SIMS U–Pb geochronology,in situ Hf–O isotope,in situ inclusions and electron probe microanalysis?EPMA?of the inclusions,UHT metamorphism has been recognized in the Dabie orogen.This work provides new insight into the thermal structure of the Dabie orogen during continental collision.A sample of migmatitic gneiss,composed of melanosome and K-feldspar-rich pegmatitic leucosome layers,was collected from the NDT.The leucosome layers are composed mainly of K-feldspar,plagioclase,and quartz with minor amphibole and biotite.The melanosome is dominated by amphibole with minor and variable amounts of clinopyroxene that is usually included within amphibole.Zircon grains in the migmatite generally show core–rim or core–mantle–rim structures in cathodoluminescence?CL?images.Core–rim structured zircons are common,whilst only a few grains display a core–mantle–rim structure.In the latter zircon type,a small relict core is characterised by oscillatory zoning and high CL brightness,which likely represent inherited magmatic zircon that underwent later modification.The surrounding mantle domain which is identical to the core of grains with core–rim structure,is broad,generally grey and weakly-zoned or cloudy,and is interpreted to have formed during metamorphism.The zircon rim is broad with clear oscillatory zoning and relatively weak CL luminescence.The metamorphic zircon domains contain mineral inclusions of garnet+clinopyroxene+plagioclase+quartz+rutile,consistent with a granulite-facies mineral assemblage.The rims contain a different suite of inclusion minerals,including K-feldspar,amphibole,and quartz.These minerals are found in the leucosome assemblage in the migmatite.The zircon cores have normal U and high Th concentrations with very high Th/U ratios?0.19–1.24?.They are enriched in heavy rare earth element?HREE?,resulting in steep HREE patterns with strong positive Ce and negative Eu anomalies,which are typical for magmatic zircon.The metamorphic zircon domains have normal U and low Th concentrations with low Th/U ratios?0.02–0.18?.They display flat HREE patterns with positive Ce anomalies and variable Eu anomalies,which are typical for high-grade metamorphic zircon.The rims have similar Th contents to the metamorphic zircon domains but higher U contents,and thus lower Th/U ratios?0.004–0.13?.Compared with the metamorphic domains,the zircon rims are enriched in HREE,resulting in steep HREE patterns with strong positive Ce and negative Eu anomalies.They are similar with anatectic zircon.Zircon LA–ICP–MS data gave the ²⁰⁶Pb/²³⁸U ages between 273±6 Ma and 685±9Ma for the inherits cores,with an upper intercept age of 798±53 Ma.Zircon SIMS U–Pb data gave one age of 613±9 Ma for the core.These suggest that the protolith of the migmatite may be the Neoproterozoic magmatic rocks.Zircon LA–ICP–MS data show that the metamorphic domains crystallized at 218±4 Ma?n=20,MSWD=4.2?.Zircon SIMS U–Pb data show that the metamorphic domains formed at 204±5 Ma?n=18,MSWD=7.8?.They imply that the Triassic metamorphism mainly occurred during204–218 Ma.Zircon LA–ICP–MS data reveal a weighted mean age of 138±2 Ma?n=18,MSWD=4.9?for the rims.And two peak ages of 167±4 Ma?n=3,MSWD=0.91?and 140±2 Ma?n=8,MSWD=4.6?)in the anatectic rims were obtained by SIMS U–Pb analysis.These suggest that the beginning of anatexis at¹85 Ma,and the anatexis is concentrated in two periods of¹70 Ma and 130–140 Ma.For the metamorphic domains,the pressure of 11–12 kbar was obtained using garnet–clinopyroxene–plagioclase–quartz geobarometer and the temperature of 927±38°C was calculated using Ti-in-zircon?TIZ?thermometer.The temperature of anatexis is about 811±77°C.Temperatures of the metamorphic zircon domains were also calculated using Zr-in-rutile?ZIR?thermometry.The results of 789±7?,784±7?and 783±7?are much lower than the temperatures of TIZ thermometry,implying decouple of TIZ and ZIR thermometry during Triassic UHT metamorphism.This thesis suggests that the subsolidus resetting of Zr compositions caused by diffusion is the factor that affects the result of ZIR in the NDT UHT metamorphism.And the diffusion is related to the distribution of elements in minerals,the grain size of rutile and duration of the UHT condition.Thus,UHT metamorphism exists in the Dabie orogen during Triassic,and the heat source of the UHT metamorphism was not from the anatexis during post-collisional collapse.Zircon Hf–O isotope data show that the?¹⁸O values of the metamorphic domains ranging from-4.36‰to-0.30‰,which are far below the level of normal mantle value?5.3±0.6‰?.They are in consistent with the O isotopic characteristics of the Yangtze block,implying the Triassic UHT metamorphism has a tectonic affinity of the Yangtze block.The?¹⁸O values of the zircon rims vary widely with a weighted average of-0.8±1.8,implying the tectonic affinity with the Yangtze block and affected by the mantle fluid.The metamorphic zircon domains have initial ¹⁷⁶Hf/¹⁷⁷Hf ratios between 0.282516and 0.282726,and variable?_Hf?t?values?-4.1–3.6,Mean=0.6±2.7?.These indicate the rocks belong to the Yangtze block underwent UHT metamorphism during the Triassic orogeny,and the crystallization of the Triassic metamorphic zircon was affected by the mantle fluid.The rims have initial ¹⁷⁶Hf/¹⁷⁷Hf ratios between 0.282131 and 0.282698,with variable?_Hf?t?values?-19.8–0.8,Mean=-13.2±3.4?.Thes indicate the Cretaceous anatectic magma may have originated from partial melting of the ancient crust,and their highly variable Hf isotopic composition suggests that mantle material was added.To sum up,UHT metamorphism exists in the Dabie orogenic belt.In the early stage of the collisional orogeny,some subducted Yangtze continental crust materials stayed somewhere in the granulite-facies lower crust during Triassic.The asthenosphere mantle may be upwelling along the subduction interface after slab break-off.Then those Yangtze continental crust materials got the additional heat and suffered from UHT metamorphism.In the stage of the post-collisional collapse,the migmatite formed caused by the extensive and intense anatexis during Late Jurassic–Early Cretaceous.According to the UHT metamorphism during the continental collisional orogeny and the anatexis during the collapse of the orogen in the NDT,the migmatite provides an important scientific evidence for the thermal structure and thermal evolution of the continental collisional orogenic belt.