Geochemical Tracing Of Continental Rift Zones

Continental rifts are tectonic zones where regional extension modifies the thermal regime,structure and composition of the entire lithosphere.Substantial material and heat are transferred from the asthenospheric mantle into the lithospheric mantle and the crust during continental rifting.In addition,extension results in the extensive development of normal faults and fractures in the crust,accounting for the infiltration of surface fluids into deep crust.Therefore,continental rifts are excellent sites for major geological processes such as surface water-rock interaction,high-temperature metamorphism and bimodal magmatism.Studying fossil continental rifts are vital for understanding plate tectonics and continental crust evolution.The northern margin of the South China Block was a continental rift zone during the breakup of Rodinia supercontinent in the Middle Neoproterozoic,although its eastern part was subducted beneath and collided with the North China Block to generate the Dabie-Sulu orogenic belt in the Triassic.Records of continental rift processes are still preserved not only in low-grade metamorphic rocks(such as the Beihuaiyang meta-granite),but also in some refractory minerals in ultrahigh-pressure(UHP)metamorphic rocks(such as zircon).In addition,the crust experienced extensive anatexis during the continental subduction and exhumation.This PhD thesis focuses on low-grade meta-granite and meta-basalt samples from the Beihuaiyang zone in the northern part of the Dabie orogen,and UHP granitic gneiss and migmatite samples from the Weihai area in the northeastern part of the Sulu orogen.A combined study of petrology and geochemistry was conducted for these rocks.The results not only reveal surface water activity,high-temperature metamorphism and magmatism in the upper crust during continental rifting,but also emphasize the importance of peritectic minerals in studying crustal anatexis and geochemical differentiation in continental subduction zones.These provide new insights into geochemistry of continental rift zones.A combined study of petrology and geochemistry was conducted for low-grade meta-granite and meta-basalt samples from the Beihuaiyang zone.The results indicate that regional high-temperature(HT)/low-pressure(LP)metamorphism occurred in the northern margin of the South China Block in the Middle Neoproterozoic,and testify the hypothesis that continental rifts are a proper site for regional HT/LP metamorphism.It is for the first time that typical HT/LP metamorphic rocks are identified in a Neoproterozoic continental rift zone.Characteristic minerals for HT/LP metamorphism including andalusite and sillimanite both occur in these rocks.Andalusite and magmatic zircon have 8180 values of-11.7‰ and 4.8-6.2‰,respectively.It suggests the aluminosilicates were products of the HT/LP metamorphism and the metamorphic protoliths were hydrothermally altered rocks by continental glacial melt-water.Pseudosection calculations indicate that the HT/LP metamorphism occurred under conditions of 1.0-3.5 kbar and 560-660 ?.U-Pb dating of metamorphic titanite yields ages of ca.750 Ma for the HT/LP metamorphism,consistent with the timing of Rodinia breakup.Pre-metamorphic protoliths show bimodal lithochemistry and arc-like geochemical signatures,suggesting that they are rift magmatic rocks.The Rodinia assembly generated Grenvillian accretionary orogens along which continental rifting generally occurred before they became tectonically stable.Therefore,mafic magmatic rocks that were produced by continental rifting show geochemical features of preexisting metasomatized mantle source.The continental rifting is coupled with lithospheric stretching,accounting for the high heat flow in the upper crust.Heat was mainly from the asthenospheric mantle.It is supported by the comparison between heat flow provided by heat-producing elements in the meta-granites and that required to form the metamorphic peak mineral assemblages.Therefore,continental rifts are an important site for generating high thermal gradients and regional HT/LP metamorphism.Petrography and mineral geochemistry are combined with in-situ garnet oxygen(O)isotopes,in-situ zircon U-Pb ages and O isotopes to study garnet-bearing meta-granite samples from the Beihuaiyang zone.The results reveal hydration and dehydration of the crustal rocks in the Neoproterozoic continental rift zone.In the meta-granites,three types of garnet are distinguished in terms of petrographic textures and mineral chemistry,including hydrothermal Garnet-I,metamorphic Garnet-? and metasomatic Garnet-?.They were produced during hydrothermal alteration,metamorphic dehydration and albitization,respectively.All of the garnet types are characterized by negative ?18O values varying from-19.3‰ to-14.5‰,in contrast to magmatic zircon in the same sample with positive ?18O values of 3.3‰ to 6.2‰.The extremely negative ?18O values of hydrothermal Garnet-?require the participation of continental glacial melt-water during continental rifting,whereas zircon ?18O values indicate that it grew during emplacement of normal ?18O magma and thus prior to the glacial-hydrothermal alteration.Once the rocks were hydrothermally altered,the 18O depletion signature would occur in all later products,such as metamorphic Garnet-?.In addition,garnet compositional zoning reveals the albitization occurred at a later stage.The inheritance of O isotopes generally occurs in a closed system through local mineral reactions.The mineral geochemistry records not only the temporal sequence of continental rift magmatism,water-rock interaction and regional metamorphism,but also the evolution of temperature and water activity in the crust during the continental rifting.A combined study of in-situ zircon O isotopes and U-Pb ages was conducted for granitic gneiss and migmatite samples from the Weihai area.The results testify the existence of negative ?18O magmatism in the northern margin of the South China Block in the Middle Neoproterozoic.Zircon U-Pb dating yields protolith ages of 753±15 Ma to 780±13 Ma and metamorphic ages of 209±3 Ma to 244±7 Ma.The Neoproterozoic cores with concordant U-Pb ages exhibit a wide ?18O range from-11.0‰ to 5.8‰,which is nearly the same as that of cores with discordant U-Pb ages.The Triassic rims of some samples have homogeneous?18O values of around-10‰ whereas the rims of the other samples show a wide 8180 range from-9.8‰ to 5.0‰.The ?18O values as negative as-11.0‰ for zircon cores with concordant Neoproterozoic U-Pb ages are reported for the first time,representing the primary record of negative ?18O magmatism in the Middle Neoproterozoic.Despite the rocks experienced continental subduction-zone high-pressure to UHP metamorphism in the Triassic,metamorphic dehydration and partial melting did not erase the abnormal ?18O record in the protolith zircon cores.A conservative estimate suggests that the hydrothermal fluid reacted with the rocks should have ?18O values lower than-10‰,corresponding to meteoric water in cold paleoclimate or melt-water of local continental glaciation.The O isotope composition of Neoproterozoic zircon cores shows spatial variations,which are a manifestation of the O isotope heterogeneity in an extinct hydrothermal-magmatic system.The spatial heterogeneity of hydrothermal alteration in the Middle Neoproterozoic was recorded by the wide ?18O range in the zircon rims during the Triassic metamorphism.The intensive O isotope exchange between surface water and deep rocks requires high temperature and high water-rock ratios,which could be achieved in the continental rift zone formed due to splitting of the South China Block from the Rodinia supercontinent.A combined study of petrography,mineral inclusions,mineral major and trace element compositions,zircon and titanite U-Pb ages and zircon Hf-O isotopes was conducted for granitic gneiss and migmatite samples from the Weihai area.The results constrain the timing,P-T conditions and mechanisms of multiple crustal anatexis in the continental subduction zone.Field and petrographic observations identify the manifestations of crustal anatexis at different scales,including(1)outcrop scale:folded leucosomes,(2)thin section scale:phengite relics with irregular shapes,cuspate K-feldspar and quartz,and melt films along grain boundaries,and(3)mineral grain scale:multiphase crystal inclusions in phengite,zircon and garnet.The peritectic zircon and garnet were identified based on the occurrence of multiphase crystal inclusions and the characteristics of mineral chemistry.They were produced by two episodes of phengite dehydration melting.The first one occurred under UHP conditions with pressures of>3.5 GPa at 237±3 Ma during the initial exhumation stage.The second one took place at the transition from high-pressure eclogite to amphibolite facies with pressures around 2.3 GPa.The peritectic amphibole and titanite were also recognized by comparing their petrographic textures with natural samples and experimental products.They were produced by hydration melting under conditions of 650-729 ? and 600 MPa at 217±7 Ma to 222±2 Ma.Because it is the precursor minerals at local scales that participate in peritectic reacations,the peritectic minerals generally inherit geochemical characteristics from the reactants.However,they commonly underwent dissolution during later anatectic processes when the P-T conditions deviate significantly from those of their initial formation,which is supported by the dissolution features of peritectic zircon and garnet and by the highly anhedral peritectic amphibole.As a consequence,the behavior of peritectic minerals significantly influences the geochemical differentiation of crustal anatexis.