Magmatism in the southwestern Central Asian orogenic belt : implications for tectonic, metallogenic and environmental evolution

Abstract

The southwestern Central Asian Orogenic Belt (CAOB) was formed due to the long-lived subduction of the Paleo-Asian Ocean, with amalgamation of varied tectonic units (island arc and micro-continent, etc.). Numerous magmatic rocks recorded regional tectonic, metallogenic and environmental evolution of the western CAOB. Although many studies have been conducted, a series of controversies remain, especially on mechanism of oroclinal bending, origin of arc magmas, petrogenesis of adakite and intraplate magmatism, relationship between anoxic ocean and ore deposits, and link between magmas redox state and atmospheric oxygen. This Ph.D. project attempts to address the above-stated scientific issues by integrating new petrologic, geochronological, and geochemical results, as well as previously published data. Firstly, this study present systematic geochemical and isotopic data for granitic samples from the Northern Yili Block (NYB) in the southern Kazakhstan collage system. These data suggest that the subduction zone transformation and subsequent trench retreat may account for oroclinal bending in the southern limb of the Kazakhstan Orocline in the Late Carboniferous. Secondly, the systematic geochemical and isotopic data for magmatic rocks are combined to decipher their petrogenesis and tectonic evolution in the NYB. This study proposes that arc magmas were formed by melting of mélange materials in the mantle wedge. Thirdly, this study suggests that the high Sr/Y and La/Yb ratios, as well as high K2O contents of adakites may not indicate of high-pressure melting. Low MgO basaltic sources fluxed by fluids can generate either sodic or potassic adakites at mid- to lower-crustal depths during water-present melting and the K2O/Na2O values of the melt will be primarily controlled by the source composition, gently modulated by pressure and water content. It should be treated with caution when using geochemical signatures (e.g., Sr/Y and La/Yb ratios) of adakites to quantitatively calculate the crustal thickness. Fourthly, systematic major and trace elements, Mg-Ca-Sr-Nd-Pb isotopic compositions have been analyzed for the Cenozoic intraplate basalts from the Tianshan Orogenic Belt. The basalts possess significantly lower δ26Mg (–0.50 to –0.37 ‰) and δ44/40Ca (0.48 to 0.86 ‰) values, demonstrating incorporation of recycled marine carbonates in the mantle source. This study provides solid evidence that carbon within the recycled marine carbonates can be transferred into upper mantle and mantle transition zone, and then return to the atmosphere through intraplate magmatism. The high fluxes of intraplate magmatic degassing CO2 may have been a critical driver of the atmospheric CO2 variations and paleoclimatic changes in the Cenozoic. Fifthly, the CAOB contains abundant porphyry Cu (Au) deposits, but most of them are concentrated in the Kazakhstan and Tuva-Mongolia micro-continents, limited deposits have been identified in the Circum Junggar Collage. This study proposes that this infertility resulted from the anoxic Junggar ocean and deposition of reduced sediments, which promoted the generation of reduced arc magmas that inhibited the formation of porphyry Cu deposits. Sixthly, this study investigated the temporal variations of detrital zircon oxygen fugacity (ΔFMQ), which can be used as an igneous oxybarometer to track magma redox states over Earth’s history. A close link between magma redox state and atmospheric oxygen content is demonstrated by this study, the increase of zircon ΔFMQ may record the rise of atmospheric oxygen.