Mesozoic Magmatism In The Southeast Margin Of North China Craton:Constraints On The Initiation Of Craton Destruction And Mg Isotopic Fractionation During Magmatism

As the most important medium of matter and energy between spheres of deep Earth, magmatism is of great importance in the generation, growth and evolution of continental crust. Furthermore, magmatic rocks can be used as "natural probes" of compositions, structures and even processes in interior Earth, e.g. plenty of information of tectonics and magma chambers’formation and evolution can be accessed by the spatial and temporal distribution of magmatic rocks. Magmatism isotope is a newly developed geotracer and has been used in tracing recycled sediments and sedimentary carbonate. However, little is known about the isotopic behavior during high silica magmatism. This dissertation would focus on the two aspects:a) using the spatial and temporal distribution of magmatic rocks to give constraints of initiation of lithosphere thinning; b) trying to understand the isotopic behaviors of Mg during the crustal magmatism.The first research project in this dissertation is to understanding the connections between significant lithospheric thinning of the North China Craton, large-scale strike-slip movement along the Tan-Lu fault, and regional-scale magmatism with associated metallogeny by investigating the chemical compositions (including major and trace elements), geochronology and oxygen isotopic compositions of Mesozoic magmatic rocks along the Tan-Lu fault.The characteristic spatial and temporal distributions of high-Mg adakitic rocks along the Tan-Lu fault with emplacement ages of 130-134Ma suggest a strong structural control of the emplacement of these intrusions, with magma generation possibly associated with the subduction of the Pacific plate in the early Cretaceous. The low-Mg adakitic rocks (127-120Ma) in the Su-Lu orogenic belt were formed later than the high-Mg adakitic rocks, whereas in the Dabie orogen belt, most of the low-Mg adakitic rocks (129-143Ma) were formed earlier than the high-Mg adakitic rocks. Based on available data, we suggest that the large scale strike-slip tectonics of the Tan-Lu fault in the Mesozoic initiated cratonic destruction at the south-eastern margin of the North China Craton, extending down to the lower continental crust within areas near the fault. This process resulted in crustal fragments sinking into the asthenosphere and reacting with peridotites, which increased the Mg# of the adakitic melts, generating the high-Mg adakitic rocks. The gravitationally unstable lower continental crust below the Tan-Lu fault in the Su-Lu orogenic belt triggered larger volume delamination of the lower continental crust or foundering of the root. The preliminary investigation to Mg isotopic compositions of Mesozoic magmatic rocks in this area suggest that Mg isotope is not a good tracer to distinguish high-Mg adakitic rocks from low-Mg adakitic rocks. And these results also suggest the lower crust of the southeast margin of North China Craton has a unique Mg isotopic composition.The other project is trying to give a comprehensive understanding of Mg isotopic recycling during the continental subduction and crustal melting by analyzing Mg isotopic compositions of various rock types and mineral separates from a well-studied granite (Jingshan granite) located in the southeast margin of the North China Craton (Bengbu Uplift) as well as elemental geochemistry data.In general, the leucogranite shows a large variation in Mg isotopic compositions (δ26Mg=-0.41 ～1.51‰) and a light Mg isotopic nature (<-0.41‰). By calculations on both Mg and O isotopes, certain percentage of light Mg isotopic carbonates should have been added to the gneissic magma source of the Jingshan-Tushan granite, resulted in the light Mg isotopic nature of the rock. Co-existing biotites and garnets displayed large inter-mineral fractionation from 1.10-1.43‰ (Δ26Mgbiotite-garnet).The fractionations were controlled by the processes of growth/resorption of garnets. The observed Mg isotopic variation of the Jingshan-Tushan granite was most probably generated in the melting stage of the magmatism due to the inter-mineral fractionation. Crustal melting/anataxis usually happens with/after burial metamorphism and at different metamorphic phases, along with formation and break-down of new minerals (micas or garnet). Melting in different temperature/depth would undergo different melting reactions, with different residual minerals. Our results here indicate that micas and garnets hold large inter-mineral fractionations during granitic magmatic processes. Thus the melts produced in different reactions (different T and P) would display different Mg isotopic compositions. Our results show that Mg isotopes are probable to fractionate during crustal melting and even crustal magmatisms (especially for those S-Type granitic magmatism with garnets involved), which is of great importance in understanding the Mg isotopic systematics of continental crust.