Meso-to Neoarchean Granitoids And Lithospheric Thermal State In The Eastern North China Craton

Thermal state and crust-and mantle-derived magmatism are crucial to understanding the geodynamic regime of early formation and evolution of continental crust,and scholars have proposed a variety of Archean crust-mantle dynamic mechanism based on the hypothesized or inferred thermal state.There are two major controversies about the late Archean geodynamic regime in the Eastern North China Craton: mantle plume and plate tectonics.The key to solve these scientific problems is to effectively define the lithospheric thermal state and crust-mantle interaction processes in different periods of the Archean.The North China Craton is one of a few Archean cratons with Meso-to Neoarchean rock records,which is an important window to study the late Archean crustal growth and evolution and explore the transition of geodynamic regime.In this thesis,we chose multiple periods of Archean tonalitetrondhjemite-granodiorite（TTG）gneisses and potassium-rich granitoids as the main research object,and carry out comprehensive studies of geology,petrology,mineralogy,geochemistry,chronology,zircon Lu-Hf isotopes and thermodynamics.The study aims to explore the petrogenesis of Meso-to Neoarchean granitoids and their inferred lithospheric thermal states and crust-mantle dynamics.The primary innovative achievements are listed below:（1）The 2525-2564 Ma diorites-quartz diorites-monzodiorites-granodiorites in the North Liaoning Block are attributed to Archean low-Ti sanukitoid suites.Petrogenetic studies reveal that most sanukitoids were derived from partial melting of metasomatized mantle,and only a few samples underwent varying degrees of fractional crystallization of clinopyroxene and amphibole in magmatic evolution.We found that Nb/Zr,Nb/Ta and Zr/Hf can effectively distinguish different metasomatism agents,such as oceanic slab-derived melts,sedimentderived melts and subducted fluids.Accordingly,the primary magmas of the sanukitoid diorites-quartz diorites-monzodiorites-granodiorites originated from partial melting of a lithospheric mantle wedge that had been enriched by subducted fluids,further metasomatized by slab-and sediment-derived melts.The complex mantle metasomatism is an essential condition for the diversification of Archean granitic magmatism.（2）Diverse late Neoarchean igneous rocks are widely exposed in the southwest of the Western Shandong Province.According to geological relationships,lithological features,magmatic zircon U-Pb-Lu-Hf isotopes and whole-rock chemical characteristics,eight main lithological assemblages are identified,and their petrogenesis is determined.The ～2.55-2.50 Ga potassic granites show the highest SiO₂ contents and were sourced from partial melting of metamorphic greywackes;the ～2.56-2.52 Ga TTG gneisses are rich in Al₂O₃,Na₂O and poor in MgO,which are the products of partial melting of the thickened lower mafic crust;the ～2.53 Ga quartz syenites exhibit the highest alkali contents and the magmas were mainly sourced from mantle-derived basaltic rocks,and the recycled sediments have been involved in the magmatic sources;the ～2.53-2.52 Ga quartz diorites-quartz monzodiorites belong to lessdifferentiated sanukitoids,which were generated by the melting of subduction-related fluid/melt metasomatized mantle wedge;the ～2.54-2.52 Ga high-Mg granodiorites（differentiated sanukitoids）were formed by crust-mantle magma mixing;the ～2.53 Ga hornblendes are cumulates from the earlier host quartz dioritic magma;the ～2.51-2.49 Ga gabbro magma originated from partial melting of fluid-metasomatized mantle;the ～2.54-2.53 Ga high-Mg andesites（boninites）formed from partial melting of refractory mantle that experienced the extraction of basaltic magma at earlier stage and was then enriched in large ion lithophile elements（LILEs）and light REEs（LREEs）by slab-derived melts.Integrating with the petrogenesis of these late Neoarchean diversified intrusive and volcanic rocks,the crust-mantle interaction and partial melting of the metasomatized mantle are the dominant thermo-dynamic processes for the generation of intermediate-mafic melts,then underplating of the mantle-derived magmas caused the partial melting of crustal materials,further led to the mixing of crust-mantle magmas.（3）We establish a dataset of 397 analyses（previously published and our latest data）of Meso-to Neoarchean TTGs from the Eastern North China Craton.Statistical results show that the magmatic crystallization ages of TTGs are almost continuously from ca.3.0 Ga to 2.5 Ga,with the three peaks occurring at ～2.91 Ga,～2.72 Ga and ～2.53 Ga,and reaching the maximum peak at ～2.53 Ga.The ～2.9 Ga TTGs are sporadically distributed in Jiaodong,Huoqiu and Eastern Hebei terranes,and ～2.7 Ga ages TTGs exhibit more field outcrops in North LiaoningSouth Jilin,Jiaodong,West Shandong,Dengfeng-Taihua,Zanhuang and Fuping terranes,whereas the ～2.5 Ga TTGs widely occur all over the Eastern North China Craton.（4）Among a large number of Archean TTG gneiss samples,we select the TTGs that were most likely originated from partial melting of basaltic rocks at the bottom of thickened crust.Based on the thermodynamic and trace element modelling of crust-sourced TTG gneisses,and in combination with the one-dimensional heat conduction model,we propose a new method for quantitatively estimating the Archean crustal thickness and thermal state.Whole-rock thermodynamic and trace element simulations demonstrate that the formation P-T conditions of crustal-derived TTG melts are 810-880 ℃ and 0.9-1.1 Gpa for ～2.9 Ga TTG gneisses,760-860 ℃ and 1.1-1.8 Gpa for ～2.7 Ga TTG gneisses,and 750-830 ℃ and 1.1-1.7 Gpa for ～2.5 Ga TTG gneisses,respectively.The pressure（P）conditions of crustal mafic rock-derived TTG gneisses may be used to estimate the minimum crustal thickness,whereas the temperature（T）conditions can represent the lower limit of Moho surface temperature.Based on the newly obtained crustal thickness,Moho surface temperature,and the classic geotherm models,the thermal structure of the Meso-to Neoarchean continental lithosphere can be reconstructed by mathematical modelling.（5）Combining our constraints on the Meso-to Neoarchean crustal thicknesses and thermal states with previous numerical experiments,petrological and structural studies,a systematic three-stage evolution of the Archean geodynamic regime has been identified in the Eastern North China Craton.During the ～2.9-2.8 Ga,the crustal thickness（30-39 km）was relatively thin,Moho geothermal gradient（21-31 ℃/km）and basal heat flow（46-80 mW/m²）were relatively high,and these observations reflect higher mantle potential temperature and thinner lithospheric thickness.Together with the development of contemporaneous komatiites and extensional tectonics,indicating that vertical tectonic regime related to mantle-plume played a dominant role in the late Mesoarchean;The crustal thickness began to increase rapidly from the late Mesoarchean（～2.8 Ga）and reached its maximum thickness of ～33-62 km by the early Neoarchean（～2.7 Ga）.During this crustal thickening period,the Moho geothermal gradient decreased from ～18-31 to ～7-24 °C/km,and the basal heat flow also decreases significantly,reflecting rapid cooling of the convecting mantle and/or thickening of the lithosphere.This transition from the Mesoarchean into the Neoarchean is also characterized by a change from komatiite to calc-alkaline volcanic rock,and a switch in structural style from extension to compression,and has been related to plate tectonics being the important geodynamic mechanism;A crustal stabilization period from ～2.7-2.5 Ga is marked by crustal thickness of 33 to 59 km,Moho geothermal gradient of 8 to 22 °C/km,and basal heat flow of 20-50 mW/m².These values have decreased slightly from those of the preceding period and are considered to represent a likely continuation of the tectonic regime dominated by hot subduction.Furthermore,the thermal state of continental crust is spatially zoned,which is similar to the thermal structure of present-day continental subduction zones,indicating that large-scale tectonic belts that resemble those produced by modern plate subduction have already formed.（6）At the end of Archean（～2.5 Ga）,a large amount of K-rich granitoids was emplaced into the North China Craton,and undeformed mafic dike swarms are widely developed throughout the craton.These geological phenomena,combined with relatively low mantle potential temperature and the thicker continental lithosphere,suggest that the lithosphere gradually stabilized and completed cratonization at the end of the Archean.