| In this dissertation, a combined study of zircon U-Pb ages, whole-rock chemistry, Sr-Nd isotopes and in situ Lu-Hf isotopic ratios in zircons, was carried out on Permian monzogranites, granodiorites, mafic microgranular enclaves(MME) as well as coeval massive gabbros in the northern Alxa block, China to constrain magma sources and petrogenetic processes involved in their generation.The mafic, gabbroic- rocks similar in composition to some enclaves form layered appear forming layered synplutonic intrusions several metres in thickness and more than one hundred metres in lateral extent. A mixing test based on mass balance for whole-rock major element compositions reveal that mixing was an efficient process between two coeval magmas. Although monzogranites and enclaves may be derived from distinct magma sources, they share similar isotopic signatures, pointing to interaction processes in the source region. The combination of information from geochronology, petrology, whole-rock geochemistry and isotopic composition lead us to conclude that the process of hybridization and magma mixing were effective at both the level of emplacement in the shallow crust and the source region of magmas at depth. Hybridization in a source region within the lithospheric mantle, involving mantle and crustal source rocks, produced magma bimodality with strong geochemical affinities between end-members and clear calc-alkaline arc signatures, compatible with subduction.Based on the U-Pb age and Hf isotopic composition of zircon xenocrysts from igneous rocks of the northern Alxa and its nearby regions, combined with published data, several conclusions can be drawn: Abundant zircon xenocrysts within Permian igneous rocks of the Yabulai-Nuoergong-Honggueryulin(YNH) zone difine age populations at ca. 2.6- 2.1 Ga, 1.8- 1.6 Ga, 930- 750 Ma, and 460- 300 Ma, and old Hf model ages(TDM2, mainly 3.2- 1.4 Ga); while those from the ZongnaishanShalazhashan(ZS) zone show younger age populations(mainly around 1400 Ma, 600 Ma and 350 Ma), and younger model ages of 1.8- 0.5 Ga. Futhermore, the ages and Hf isotope of these zircon xenocrysts indicate that the basements rocks beneath the ZS zone are young and similar to these of the southern CAOB, whereas those beneath the YNH zone are old and similar to the Alxa Block. The southernmost boundary of the CAOB with the Alxa Block is located along the boundary between the ZS and the YNH zones, rather than along the Enger Us ophiolite belt as previously considered. And this study also indicates that statistical analysis(or isotopic mapping) of zircon xenocrysts is an effective method to trace the nature of the deep continental crust and to separate between newly-formed orogenic domain and ancient cratons.On the basis of the discussion of petrogenesis, a plausible new petrogenetic model, “subduction erosion plus relamination model”, which differs from the classical models based on mafic magma underplating, is proposed. Accordingly, the Yamatu monzogranites were generated from granitic melt segregated and ponded in buoyant silicic diapirs formed by subducted mélanges melted in the lithosphere mantle, which finally relaminated to the lower crust. They represent the melts that metasomatized the mantle region during a pre-Permian subduction event. The massive gabbros were formed by decompression melting of the previously-metasomatized mantle. The gabbroic enclaves and gabbro layers of the Yamatu pluton are interpreted as magmas formed by decompression melting of this modified hydrated mantle. The dioritic enclaves represent hybrid liquids generated from reaction between granitic melts, which are derived from subducted mélanges, and the hydrated mantle or melts derived from it. This makes further progress in our understanding of the processes involved in the generation of coeval felsic and mafic intrusions. |
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