Genesis Of Mafic-ultramafic Rocks In The Beishan Orogen And Their Implications For Deep Magmatic Processes

The accretionary orogenic belt is an important site for deep earth geochemical and dynamic processes such as continental crust growth,element and material recycling,and crust-mantle interaction in the evolution of the Earth.Arc magmatism plays a crucial role in the evolution of accretionary orogenic belts.The Central Asian Orogenic Belt（CAOB）,as the most important representative of accretionary orogenic belts,experienced extensive arc magmatism during the Paleozoic,resulting in significant crustal growth.It is closely related to the subduction of the Paleo Asian Ocean,including complex processes of subduction,collision,and accretion of island arcs,seamounts,and microcontinents.However,the crustal growth related deep magmatic processes,mantle properties,mechanism and scale are still controversial.Significant crustal growth is accompanied by the production of a large number of magmatic rocks.Among them,mafic-ultramafic rocks from deep-seated sources,which complement to shallow-level intermediate-felsic rocks,can provide unique perspectives on crustal growth,deep magmatic processes,and the evolution of the CAOB during the Paleozoic.Accurately constraining the geochemical characteristics and genesis of these mafic-ultramafic rocks is crucial to understanding the above scientific issues.Therefore,this thesis is based on field geological investigations and systematic petrographic and geochemical studies of a series of peridotites,pyroxenites,and basalts on the southern margin of the CAOB to explore the mantle properties,crustal growth mechanism,and evolution of the CAOB and the Paleo Asian Ocean,and to determine the closure time of the ancient Asian Ocean in the area.The Beishan orogen is a key area for studying the formation and evolution of the CAOB and the Paleo Asian Ocean.This study investigates the peridotites and pyroxenites in the largest early Paleozoic ophiolitic mélange belt in the Beishan orogenic belt using petrography,conventional major and trace elements,and radioactive Re-Os isotopes.The peridotites and pyroxenites in this region are characterized by low Al₂O₃,high Mg#（>90）,and high Cr and Ni contents,representing true mantle rocks.The rocks have very low rare earth element contents and are depleted in high field strength elements such as Nb,Ta,Zr,and Hf,indicating a highly depleted mantle source.The correlation between light rare earth elements and high field strength elements（La-Nb,Gd/Lu-Th）suggests that these ultramafic rocks may have undergone melt-rock reaction.The enrichment of fluid-mobile elements（Pb,Li,Sr,and U）and the depleted Os isotopes in the ultramafic rocks are similar to those in forearc mantle peridotites.Combining with that the mafic rocks developed above them have MORB characteristics,indicating that these rocks may have formed in a forearc extensional basin.The Re-Os isotopes of the bulk rock record the occurrence of ancient melt extraction events(T_RD=1.0–1.8 Ga,¹⁸⁷Os/¹⁸⁸Os=0.117–0.132).Using Dy-Ti and low HREE content estimates,it is suggested that these ultramafic rocks underwent multiple stages of up to30%melt extraction.The Re depletion model age suggests that the ultramafic rocks were formed during the Proterozoic.The predominant distribution of the Re depletion model age(T_RD=1.0–1.2 Ga)is consistent with the formation of the Paleo Asian Ocean and the onset of subduction during the Proterozoic.This implies that large-scale lithospheric melting events may have been triggered in response to the initiation of subduction.Moreover,the Re depletion model age of the mantle peridotites,which are widely distributed in the southern region of the CAOB,agrees with the age recorded by ancient crust in this area.The extremely depleted Os isotope composition of these rocks may indicate that they represent microcontinents,suggesting its crucial role in the formation and evolution of the Paleo Asian Ocean.However,the present-day decoupling between the Paleozoic crust and the lower Paleoproterozoic mantle might be attributed to the pronounced magmatic activity,tectonic movement,and other processes that occurred during the Paleozoic,which might have obscured the evidence of the existence of microcontinents.This study also presents bulk rock and mineral major and trace elements and U-Pb geochronology of the peridotites and pyroxenites in the western part of the above ophiolitic mélange belt.The results reveal that the ultramafic rocks,which were traditionally considered to be mantle components,actually have cumulate origins.Petrological evidence shows that these rocks have typical mesocumulate and adcumulate structures.The significant correlation between Mg O content and other major elements in the bulk rocks,as well as the weak positive correlation between Dy/Yb and Si O₂,represent fractional differentiation processes mainly involving olivine and clinopyroxene.The significantly lower Ni O content and Mg#in olivine and orthopyroxene deviate from the typical mantle ultramafic rock sequence,pointing to cumulate origins.Enrichment in large ion lithophile elements（Ba,Pb,Sr）and depletion in high field strength elements（Nb,Ta,Zr,Hf,Ti）show typical island arc characteristics,indicating that these rocks were formed by arc magmatic cumulates.Furthermore,the covariant relationship between high Mg#and low Si O₂is consistent with the evolution trend of hydrous arc ultramafic cumulates in typical island arc profiles,such as Kohistan and Talkeetna,which is in agreement with the estimated crystallization temperature and pressure conditions of 0.9–1.2 GPa and 890–950°C,respectively.All the available evidence indicates that the majority of ultramafic rocks found in the ophiolite mélange belt located in the middle of the Beishan orogenic belt are subarc cumulates.Therefore,they cannot represent the mantle portion of the ophiolite mélange and are not indicative of the suture.Through crystallization differentiation simulation,it has been demonstrated that these ultramafic rocks possess complementary characteristics to the intermediate felsic magma that evolved in the Beishan orogenic belt.This suggests that long-term subduction and accretion of the Paleo Asian Ocean led to the crystallization of arc magma in the deep arc,which significantly promoted large-scale crustal growth and compositional evolution in Central Asia during the Phanerozoic.Late Paleozoic basalts that developed in the southern part of the largest ophiolitic mélange belt,which is critical for constraining the subduction and death of the Paleo Asian Ocean.This study conducts bulk rock major and trace elements and Sr-Nd-Pb-Hf isotopic and zircon U-Pb-Lu-Hf isotopic analyses on the basalts and gabbros to constrain the origin of the basalts and discuss the closure time of the Paleo Asian Ocean.The zircon U-Pb age of gabbro is approximately 288-294 Ma,representing the formation time of the basaltic belt.The basalt shows MORB-like rare earth element characteristics,with bulk rockε_Hf（t）andε_Nd（t）values of 11.0–15.4 and 4.6–9.2,respectively,indicating that it originated from a depleted mantle source.However,some basalt exhibits positive anomalies in Pb,negative anomalies in Nb-Ta,high Th/Yb ratios,and evolved Sr-Nd-Pb-Hf isotopic compositions,suggesting varying degrees of contributions from continental crustal materials.Based on the covariation relationship between bulk rock Pb anomalies（Pb*=2×Pb _N/（Ce _N+Pr _N））and Sr-Nd-Pb-Hf isotopic compositions,it is inferred that the enrichment characteristics of Liuyuan basalt are attributed to crustal contamination during magma eruption,rather than the presence of recycled materials in the source area.The imbalance between zircon Hf isotope of gabbro and whole rock Hf isotope indicates the depletion of the source and the effect of crustal contamination.The Sr-Nd isotopes and Th/Yb-Nb/Yb simulation of two end-member mixing model between depleted mantle and continental upper crust demonstrate that basaltic magma underwent approximately 10%continental upper crust mixing,which effectively explains the enriched trace elements and the evolved Sr-Nd-Pb-Hf isotope characteristics.Therefore,it is more likely that the large-scale pillow basalts of the Permian in this region were formed in an intracontinental extension environment,rather than a subduction environment.The presence of Liuyuan basalt indicates that this basalt belt does not serve as a suture,implying that the Paleo Asian Ocean had already closed before the Permian,and the final closure location was not in this aera.During the Permian,the southern CAOB entered an intracontinental extension environment.In this thesis,we have identified two types of ultramafic rocks of different origins in the Early Paleozoic ophiolite mélange in the Beishan orogenic belt on the southern CAOB.A small part of these rocks represents highly depleted lithospheric mantle remnants of the Proterozoic,which provided the material sources for the formation and subduction initiation of the Paleo Asian Ocean.The majority of the ultramafic rocks were formed through deep crystallization of arc magma,which significantly promoted large-scale crustal growth and evolution in Central Asia during the Phanerozoic.Additionally,we discovered the geochemical characteristics and origin of the late Paleozoic large-scale pillow basalt belt.This emphasizes that the Liuyuan basalt has no suture significance and provides data supporting the final closure of the Paleo Asian Ocean is before Permian.