The Tectonic Evolution Of Convergent Continental Boundaries

The formation and evolution of collisional orogens involve the tectonic transformation of orogenic systems from oceanic subduction for accretionary orogeny,through continental subduction for collisional orogeny,to continental rifting for rifting orogeny.The results are deformation and metamorphism of crustal rocks as well as contemporaneous magmatic activities along convergent continental boundaries.Therefore,collisional orogens are key targets to study continental dynamics and develop the theory of plate tectonics.Regional metamorphic rocks,as the products of tectonism in response to changes in temperature(T)and pressure(P),may form in a variety of tectonic settings and show different P-T paths and thermobaric ratios.Thus,they may provide a crucial record for reconstructing the formation and evolution of collisional orogens.The Tongbai orogen,located at the west-central part of the China Central Orogenic Belt(the Qinling-Tongbai-Hong'an-Dabie-Sulu orogen,is a composite orogen with two aspects of identity.On the one hand,it resembles the Qinling orogen in its western extension,recording the oceanic subduction through continental collision to postcollisional reworking in the Early Paleozoic.On the other hand,it resembles to its eastern extension comprising the Hong'an-Dabie-Sulu orogens,recording the Triassic subduction of the South China Block(SCB)benenth the North China Block(NCB)for collisional orogeny.This PhD thesis presents a combined study of metamorphic petrology and isotope geochronology for granulite and eclogite from the Tongbai orogen.The results not only shed light on the geodynamic processes from collisional thickening to rifting extension and their associated high pressure(HP)metamorphism overprinted by long duration ultrahigh temperature(UHT)metamorphism in the Early Paleozoic,but also clarify the tectonic evolution from continental subduction to early collision which led to short-term high temperature(HT)metamorphism superimposing on the early HP metamorphism in the Early Mesozoic.In addition,whole-rock geochemistry is combined with relict zircon U-Pb ages for eclogite and garnet amphibolite from the Tongbai orogen to reconstruct the origin of premetamorphic protoliths in the northern part of the Yangtze craton.This provides insights into the tectonic evolution from oceanic subduction for amalmagation of supercontinent Columbia to continental rifting for the attempted breakup of this supercontinent in the Paleoproterozoic.The Tongbai granulites were investigated by means of petrography,mineral inclusions,zircon U-Pb ages and trace elements,and pseudosection calculation.The results indicate two periods of metamorphic evolution for them,one in ca.490-450 Ma for collisional orogeny and the other in ca.440-405 Ma for rifting orogeny,each comprising two stages of regional metamorphism.The first period is characterized by a clockwise P-T path for metamorphic evolution.The mineral inclusion assemblage of rutile+kyanite+K-feldspar in the garnet mantle indicates a HP granulite-facies metamorphic stage(M1)at 785-815? and 10-14 kbar.The symplectitic assemblage of orthopyroxene+plagioclase+biotite surrounding garnet suggests a decompressional heating path to medium pressure(MP)granulite-facies metamorphic stage(M2)at 840-880? and 8-9 kbar.Metamorphic ages recorded by zircon cores and dark mantles indicate this period occurred from 490 to 450 Ma.The second period defines an anticlockwise P-T path for metamorphic evolution.The continuous heating lead to peak UHT metamorphism(M3)at 890-980? and 9-11 kbar,which are constrained by Xca isopleths of garnet mantles and porphyroblastic plagioclases.It is followed by decompressional cooling to amphibolite-facies metamorphic stage(M4)at 740-880? and 7-9 kbar.Metamorphic ages for bright mantles and dark rims of zircon grains place constraints on this period to occur from 440 Ma to 405 Ma.Therefore,the Tongbai orogen experienced a similar tectonic evolution to the Qinling orogen in the early Paleozoic,including not only the continental collision for lithosphere thickening and HP metamorphism in the Cambrian but also continental rifting for lithosphere thinning and UHT metamorphism in the Silurian.The long-term HT metamorphism makes the geological records of continental collision only preserved in the refractory zircons and mineral inclusions.The Tongbai eclogites were investigated by means of petrology,pseudosection modelling,multi-equilibrium thermometries,zircon U-Pb dating and Zr-in-rutile thermometry.The results indicate four stages of metamorphism during the continental collision between the SCB and NCB in the Triassic.The mineral assemblage of epidote,amphibole,albite,titanite and quartz included in garnet cores indicates an early stage of epidote amphibolite-facies metamorphism(M0).The composition of garnet cores provides constraints on a late stage of prograde metamorphism(M1)at 455-510? at 21-25 kbar,and XCa[Ca/(Ca+Mg+Fe)]isopleths of garnet rim together with the Zr-in-rutile thermometric result define a peak metamorphic stage(M2)at 580-625? and 26-27 kbar.Sodic-calcic amphibole compositions place constrains on an early stage of retrograde metamorphism(M3)at 580-655? and 14-17 kbar.Coronal or symplectitic mineral assemblage including titanite,plagioclase,hornblende/barroisite and epidote qualitatively constrains a late stage of retrograde metamorphism(M4)at 420-550? and 5-9 kbar.Therefore,the Tongbai eclogites underwent the prograde metamorphism with a rapidly isothermal compression followed by mainly isobaric heating and the retrograde metamorphism with decompressional heating initially followed by decompressional cooling.Zircon U-Pb dating yields two groups of metamorphic ages.The first group is clustered at 245 Ma for zircon domains showing relatively high contents of Th,U,Nb,Ta and MREE,indicating their growth at the peak metamorphic stage through glaucophane dehydration.The second group is clusterd at 240 Ma for zircon domains exhibiting low abundances of Th,U,Nb,Ta and MREE,suggesting their formation during amphibole-facies retrogression with extensive hydration.Considering the reported muscovite Ar/Ar age of 238 Ma for the retrograded eclogite,the duration of amphibolite-facies retrogression is estimated to be very short.Along with available observations for the China Central Orogenic Belt,the continental collision between the SCB and the NCB would be composed of at least two stages.First is the initial collision of the northeastern SCB leading edge with the southeastern NCB margin at about 245 Ma,resulting in diamond-bearing eclogite-facies metamorphism at pressures of>33 kbar in the Dabie-Sulu orogens but coesite-bearing eclogite-facies metamorphism(27-30 kbar)in the Tongbai-Hong'an orogens.The second stage would start with a clockwise rotation of the SCB after the initial collision and proceed with a gradual collision from east to west with the southern NCB margin in the Qinling orogen at ca.240-210 Ma.This along-strike difference in subduction depth is ascribed to the difference in the identity of overriding NCB margins.Whereas the ancient craton in the eastern part is responsible for the deeper subduction in the Dabie-Sulu orogens,the juvenile arc terranes in the middle and western parts are responsible for the shallower subduction in the Tongbai-Hong' an orogens.The protolith age and composition of eclogite and garnet amphibolite in the Tongbai orogen were investigated by means of relict zircon U-Pb dating and whole-rock geochemistry.The results analysis reveal two episodes of magmatism and one episode of metamorphism from ca.1.96 Ga to 1.63 Ga.The whole-rock geochemical analysis obtains that the 1.96 Ga metabasite shows typical island arc basalts(IAB)-like trace element distribution patterns with LREE enrichment and negative anomalies in HFSE,whereas the 1.63 Ga metabasites are characterized by ocean island basalts(OIB)-like trace elemental distribution patterns with LREE enrichment but without negative anomalies in HFSE.In addition,a 1.84 Ga granulite-facies metamorphism is recorded by sector zoned domains in zircons from the 1.96 Ga metabasites,which show flat HREE patterns with markedly negative Eu anomalies and a high Ti-in-zircon temperature of 825?.Integrating the available data from previous studies of geology for the northern Yangtze craton,we suggest that the 1.96 Ga metabasite was produced by decompressional melting of the metasomatic mantle domain,the 1.84 Ga granulite-facies metamorphism was induced by asthenospheric upwelling in response to post-collisional extension,and the 1.63 Ga magmatism was associated with lithospheric extension triggered by continental rifting.Therefore,the Paleoproterozoic metabasites in the Tongbai orogen record a composite tectonic evolution from oceanic subduction through continental collision to continental rifting,indicating that the northern margin of the Yangtze craton was involved in the assembly and breakup of Columbia supercontinent.In comparison to the coeval rift-related igneous rocks in the other blocks,the geochemical features of 1.63 Ga metabasites in the northern Yangtze are similar to those in the northern Australia,but are significantly different from those in the southeen Siberia and the northern Laurentia.This indicates a possible connection between the northern Yangtze and the northern Australia before the Columbia breakup.