Creation and Destruction of Lithospheric Mantle beneath the North China Craton

Title of Dissertation: CREATION AND DESTRUCTION OF LITHOSPHERIC MANTLE BENEATH THE NORTH CHINA CRATON Jingao Liu, Ph.D., 2011 Dissertation directed by: Professor Roberta L. Rudnick, Geology Professor Richard J. Walker, Geology Mantle peridotites carried in volcanic rocks provide a window into the composition, age and origin of deep lithospheric mantle. Previous studies of mantle peridotites have shown that the central-eastern North China Craton experienced lithospheric reactivation in the past, which makes the craton an important location to investigate reactivation processes and mechanisms. In order to better understand the formation and evolution of the North China Craton, the petrology, major and trace element geochemistry of both whole rocks and minerals, whole rock Re-Os isotope systematics, and highly siderophile element (HSE) abundances, were carried out on peridotites entrained in Cretaceous to Tertiary lavas from the craton. Strontium-Nd-Hf-Pb isotopic compositions of clinopyroxenes separated from some of these rocks were also examined. Studies of these peridotites show that: 1) Peridotites from the western-central North China Craton record a north-south composition and age dichotomy. The northern portion of the central region of the craton experienced lithospheric mantle replacement via a ~1.8 Ga collision associated with amalgamation of the craton. The comparative Late Archean age between crust and lithospheric mantle in the southern portion of the central region suggests that the cratonization in this region occurred at ~2.5 Ga; 2) Lithospheric thinning and replacement beneath the northern edge of the eastern North China Craton occurred prior to ~100 Ma. Phanerozoic lithospheric thinning and replacement in the eastern North China Craton may have evolved from east to west, or from the margins to the interior of the continent with time in the Mesozoic; 3) Highly fractionated HSE patterns found in a majority of peridotite suites and characterized by Os, Pd and Re depletions relative to Ir were caused by recent sulfide breakdown via interaction with a S-undersaturated oxidizing melt/fluid; and, 4) The trace element characteristics and isotopic tracers (Sr-Nd-Hf-Pb) present in clinopyroxenes, combined with their whole-rock Re-Os data, record a multi-stage history of primary melt depletion and secondary overprinting processes. Lu-Hf isotopic compositions of pyroxenes may record the cooling age when peridotites cooled down below the closure temperature of the system. CREATION AND DESTRUCTION OF LITHOSPHERIC MANTLE BENEATH THE NORTH CHINA CRATON