Multiple Episodes Of Anatexis And Mountain-root Collapse In The North Dabie Complex Zone,Central China

Anatexis(i.e.partial melting),one of the most popular area in the earth science fields,plays an important role during the formation and evolution of continental collision orogens,and is of great significance to the development of plate tectonics.Crustal anatexis is a predominant factor controlling the continental crust differentiation,deeply subducted terrane exhumation and granite formation,which also significantly influences the post-collisional mountain-root collapse.The Dabie orogen in the Central China is a typical continental collision belt.It consists of five fault-bounded lithotectonic units with various metamorphic grades and evolutional histories.Among them,the North Dabie complex zone(NDZ)is one of the three eclogite-bearing ultrahigh-pressure(UHP)units of the Dabie orogen,and underwent a complex evolution including deep subduction,ultrahigh pressure metamorphism,multistage exhumation and post-collisional collapse as well as multiple episodes of anatexis.Various anatectic products such as igneous intrusions and migmatites are widespread in the NDZ,and provide an ideal natural laboratory to study the crustal anatexis and post-collisional collapse.Therefore,this PhD thesis deals on an integrated study including field investigations,petrographic observations,zircon U-Pb geochronological dating,elemental and isotopic analyses of metadiorite and various leucosomes in migmatites in this region.The results provide new constraints on the episodes,ages and mechanisms of anatexis as well as elemental and isotopic behaviors of leucosomes and metadiorites during high-T decompression of the initial exhumation and post-collisional mountain-root collapse,and bring new insights into the subduction-exhuamtion and post-collision tectonic evolutions of the Dabie orogen.The study of melanosomes and various leucosomes in the migmatites from the NDZ suggests multiple episodes of anatexis had occurred in this region.The leucosomes within the migmatites can be subdivided into four groups based on the mineral assemblages and deformation structures:(1)Grt-bearing leucosome;(2)Amp-rich leucosome;(3)Amp-poor leucosome and(4)Kfs-rich leucosome.Zircon morphological and U-Pb geochronological analyses of the Grt-bearing leucosomes suggests they were crystallized in the late Triassic(209 ± 2 Ma),responding to the decompression fluid-absent anatexis under granulite-facies conditions during the initial stage of the subducted NDZ terrane exhumation.These leucosomes were intensively deformed and display high heavy rare earth elements(REE)contents and Rb/Sr ratios,low Th/U ratios and distinct Eu anomalies while the peritectic garnets therein contain rounded multiphase mineral inclusions.Geothermometer and geobarometer calculation results suggest the Grt-bearing leucosomes were crystallized under P-T conditions of T=872-941? and P=8.2-10.0 kbar,exceeding the critical point of biotite decomposition,indicating that they were derived from the fluid-absent decompression anatexis due to biotite dehydration.In addition,Amp-rich,Amp-poor and Kfs-rich leucosomes were formed at 133 ± 3 Ma,124 ± 3 Ma and 114 ± 7 Ma,respectively,corresponding to the post-collision collapse time of the Dabie orogen in Cretaceous.Generally,the leucosomes are weekly deformed to undeformed.Continuous variations of mineral assemblages and elemental characters among the above three groups of leucosomes might suggest they were products of different stages of fractional crystallizations of anatectic melts.The Amp-bearing leucosomes with low Rb/Sr and Rb/Ba ratios were formed at P=2.3-4.4 kbar and T=665-789? which is lower than the critical point of biotite decomposition.Meanwhile,coarse-grained amphiboles therein occur as peritectic hydroites,demonstrating the external heating and fluid influx during the post-collision collapse and asthenosphere upwelling led to large-scale partial melting of rocks in the depth of middle crust level and resulted in the widespread migmatites with Amp-rich,Amp-poor and Kfs-rich leucosomes in the NDZ.Moreover,all groups of leucosomes display similar Sr-Nd isotopic compositions in consistent with the ultrahigh pressure metamorphic(UHPM)rocks such as eclogites and granitic orthogneisses in this region,suggestive of genesis relationships.The metadiorites in the NDZ are enriched in light REEs and large ion lithophile elements(LILEs),depleted in heavy REEs and high field strength elements(HFSEs),and display distinct Ba positive anomalies.Meanwhile,the whole-rock characters exhibit moderately enriched initial 87Sr/86Sr ratios(with range of 0.707582-0.708099),low ?Nd(t)values(with range of-15.3--20.4)and low initial 206Pb/204Pb,207Pb/204Pb and 208Pb/204Pb ratios(with ranges of 16.0978-16.8452,15.3167-15.4547 and 37.1778-37.8397,respectively).The features above are consistent with that of typical lower continental crust and of the UHPM rocks in this region.U-Pb dating and cathodoluminescence images of zircons therein are suggestive of two-episode high-temperature(HT)events due to their core-rim texture.Zircon cores with weight mean ages of ca.130 Ma are characterized by evident oscillatory zoning with high Th/U ratios and U contents,suggestive of igneous genesis.Whereas,the rims are homogeneous and no-zoning with lower Th/U ratios and U contents,generally replaced the void of corroded cores(embayed structures),yielding mean ages of ca.125 Ma,indicating HT metamorphism.The(meta)diorites were crystallized under P-T conditions of 725-806? and 4.7-6.2 kbar calculated with amphiboles and plagioclases compositions in the matrix.Zircon cores and rims share consistent ancient Archean two-stage Hf model ages,however,older than that of migmatites and UHPM rocks in this region.Hence,partial melting of only the lower crustal rocks could hardly produce the precursor of(meta)diorites.According to the elemental and isotopic characters of metadiorites and the tectonic evolution model of post-collision collapse in the Dabie region,we speculate the precursor of the samples were derived from the interaction between the dominantly Triassic subducted mafic lower crust and minor partially melted lithospheric mantle in the early Cretaceous.In addition,coarse-grained K-feldspar augen porphyroblasts generally wrapped by the foliation of the metadiorites,and exhibit well-preserved crystal shapes,indicating they were crystallized from melts approach to the age of deformation at T=583-599? and P<4.7 kbar.In conclusion,the NDZ terrane suffered two episodes of important anatexis.The first occurred during the early stage of subducted terrane exhumation(ca.209 Ma)under fluid-absent conditions.High-T rapid decompression resulted in biotite decomposed and released a few free H2O into country rocks and led to anatexis with limited melts,produced the Grt-bearing leucosomes.The second took place during the post-collision mountain-root collapse in the Cretaceous(110-130 Ma).With external heating and aqueous fluids injected into the overlying rocks resulted from thickened lower crust delamination and astheno sphere upwelling,the UHP metamorphic rocks in the NDZ terrane with depth of middle-crust level partial molten and produced Amp-rich,Amp-poor and Kfs-rich leucosomes.Meanwhile,melts derived from the anatexis of lower crustal mafic rocks interacted with minor mantle-derived materials and migrated upward under buoyancy,produced various intrusions including(meta)diorites.