The Neoarchean-Paleoproterozoic Geological Evolution Of The Zanhuang Complex In Central Segment Of The North China Craton

It is widely accepted that the North China Craton?NCC?was mainly amalgamated by a few micro-blocks along collisional belts.However,the number of micro-blocks,and when and how they were assembled to form a coherent basement remain controversial.Various models for the tectonic subdivision and amalgamation of the craton have been proposed,while none of them is widely accepted yet.The Zanhuang Complex,located in the eastern-most margin of central section of the Trans-North China Orogen?TNCO?of the NCC,and mainly crops out in the east part of the southern Taihang mountain.Like other Precambrian basements in the NCC,it is mainly composed of late Archean TTG gneisses,potassic to monzogranitic gneisses,and Archean-Paleoproterozoic supracrustal rocks?metavolcanic and metasedimentary successions?.Recent studies imply that the Zanhuang Complex is a key area to clarify the Archean-Paleoproterozoic convergent and collisional orogenesis and structural evolution history of the central part of the NCC.In this thesis,detailed field work,whole rock geochemistry and geochronology study of the Neoarchean-Paleoproterozoic granitoids and supracrustal rocks from the Zanhuang Precambrian basement were presented.The main aim of the thesis is to shed light on the Neoarchean-Paleoproterozoic tectonic events and implications of the crustal evolution history of the Zanhuang Complex,as well as the tectonic evolution of the TNCO.A variety of Neoarchean granitoids were cropped out in the Zanhuang Complex,which record typical characteristics of the critical transition of crust evolution during this period.These granitoids could be subdivided into four groups,i.e.TTGs,sanukitoids,crustal-sourced granitoids,and hybrid granitoids.The TTGs in the Zanhuang Complex formed at a longer and earlier period during².7 Ga to².5 Ga as the first stage,while the other three groups of granitoids formed at a shorter and later period as the second stage.The TTG gneisses show typical TTG characteristics and were interpreted to have formed in a subduction-related environment.The sanukitoid sample shows high MgO and Mg#and other geochemical signatures of typical Archean sanukitoids.The sanukitoid formed at the end of subduction,and the magma derived from partial melting of a hydrous mantle peridotite source which metasomatized by slab-derived melts or fluids.The crust-sourced granitoids were formed by melting/reworking of the dominant crustal lithologies,namely TTGs and meta-sedimentary rocks.The thickening of crust and the mantle upwelling provided enough heat for the large-scale crustal melting.The hybrid granitoids represented by the Haozhuang granitoid?2511 to2528 Ma?are characterized by enrichment in both incompatible?LILE and LREE?and compatible?Mg,Ni,Cr?elements,and may have formed by mixing/contamination of multiple magmas or sources.The temporal and spatial distributions of these groups of granitoids indicate that the Zanhuang Complex experienced a subduction-collision event between the Fuping arc terrane?including the Western Zanhuang Domain?and the Eastern Block of the NCC?including the Eastern Zanhuang Domain?at the end of Neoarchean.Detailed study of the supracrustal rocks?metavolcanic and metasedimentary successions?of the Guandu Group indicate that the so-called Guandu Group is not a single and unitary tectonic-lithological unit,but consists of two different units,namely the Archean unit?Unit 1?and the Paleoproterozoic unit?Unit 2?,which formed at different times and pieced together by later tectonic event?s?.The study reveals that the metamorphosed mafic rocks in the Guandu Group?or CZD?can be subdivided into two types by petrological,geochronological,structural and petrogenetic differences.Type 1 consists of deformed epidosite lenses within an amphibolite matrix interpreted as a pillow basalt formation,located at the west part of the Guandu Group;type 2 dominated by amphibolites and interbedding with various sedimentary rocks with epidotite lenses locally formed by deformation and alteration of later mafic dikes,which located at the central part.Type 1 mafic rocks?epidosite sample D0527-2?yielded SHRIMP zircon U-Pb ²⁰⁷Pb/²⁰⁶Pb age of 2501±12 Ma as crystallization age and constrains it formed at Neoarchean.Although the precise formation age of the type 2 mafic rocks is not obtained yet,the interbedded sedimentary rocks with detrital zircons younger than 2.1 Ga,and the much younger Nd T_DM model ages?2.2-2.3 Ga?of the amphibolites from type 2,together suggest the type 2 mafic rocks and associated sedimentary rocks may have formed at Paleoproterozoic.The?Nd?t?values of type 1 amphibolite samples range from+0.43 to+1.47,with T_DM model ages ranging from 2.75 to2.97 Ga,which clearly different with those of type 2.The Archean unit?Unit 1,mainly the type 1 mafic rocks?may has formed at the oceanic basin between the Eastern Block and Fuping arc terrane at the end of Neoarchean,and the Paleoproterozoic unit?Unit 2,mainly the type 2 mafic rocks and associated sedimentary rocks?may has formed at the oceanic basin by extension and rifting of the continent at Paleoproterozoic.The Neoarchean-Paleoproterozoic tectonic history of the Zanhuang Complex and the TNCO is far more complex than previously thought.By integrating previous published data and those obtained in this work,a new two-period tectonic events model is proposed here.Before 2.5 Ga,the western margin of the Eastern Block deposited a series of volcanic-sedimentary rocks?type 1?.At².5 Ga,the Fuping arc terrane?including the Western Zanhuang Domain?amalgamated with the Eastern Block by arc-continent subduction-collision,accompanied with huge amount of granitic rocks.After a quiescent period,it underwent².1 Ga extension and rifting with the emplacement of Xuting A-type granites.Later,the rift evolved into a small ocean basin with the deposition of volcanic-sedimentary sequences?type 2?,then closed through west-dipping subduction and collision at 1.9-1.85 Ga,resulting in the final amalgamation of the NCC and the widespread deformation and metamorphism in the Zanhuang Complex as well as the whole TNCO.