Continents form the platform on which our species has evolved, and nearly all of our natural resources derive from the continents. Understanding how continents form and evolve and what makes them stable is, therefore, of importance for humans. The oldest portions of continents, those regions that formed before 2.5 billion years ago and termed 'cratons', are typically the most stable surface features on Earth. They experience no active volcanism or earthquakes, and the presence of a thick and stiff rock layer beneath the continental crust, extending down to depths of 200-250 km, is generally considered key for the stability of these ancient regions. The ~3.8 billion year old North China Craton is an exception to this general picture. Some of the most devastating earthquakes in human history, as well as relatively recent volcanism and ore deposit formation have occurred in this craton. There is strong evidence that the thick rock layer, which was originally present beneath the crust of the North China Craton, was removed and replaced with a thinner and weaker rock layer several hundred million years ago, but the timing and process of this transformation is uncertain. The project outlined in this proposal seeks to determine when and how the thick rock layer was replaced beneath the North China Craton by studying pieces of this deep layer sampled by volcanoes that erupted over the last 400 million years, a time period that spans the replacement event.
The research proposed involves the use of petrography, Re-Os and Lu-Hf isotope geochemistry, as well as measuring the abundances of the platinum group elements (PGE) in mantle xenoliths to unravel the history of the mantle lithosphere beneath the North China Craton during the Mesozoic and Cenozoic. Large, statistically significant data sets for Re-Os will be collected at targeted locations in order to distinguish post-Archean lithospheric ages from the Re-Os spectrum observed in the convecting upper mantle. Clinopyroxene from select xenoliths will be measured for Lu and Hf isotopes in order to compare age systematics between the two systems and evaluate the utility of the Lu-Hf system for determining lithosphere formation ages. PGE data will be used to evaluate the veracity of Os model ages for individual samples, and will also provide information on processes, such as refertilization and melt-rock interaction, that may have acted on the mantle xenoliths. The results are expected to clarify the present-day age distribution of lithospheric mantle beneath different portions of the North China Craton, how the Mesozoic aged lithosphere compares with older and present-day lithospheric mantle, and how this information can be linked with available seismological images in order to deduce the deep architecture of the craton and determine the lateral and vertical extent of lithospheric mantle replacement. This, in turn, will inform models for the processes involved in the transformation of a stable craton into one of the most tectonically active regions of the world today.
