AGE FREQUENCY DISTRIBUTIONS OF THE PAST CONTINENTAL CRUST ESTIMATED THROUGH DETRITAL ZIRCON CHRONOLOGY

Continental crust composed of granitic rocks characterize the water-rocky planet, and its growth history is important for the evolution of the Earth. Although it is considered there were active formation of continental crust in the early part history of the Earth, continental crsut older than 2.0 Ga remains only 20 % of the total continental crust on the modern Earth surface. Thus, the geolgical information for temporal and quantitive discussion of the growth and destruction of the continental crust is unknown but critical.

This study proposes new compilation method and discussion, and estimated age frequency distributions of the past continental crust. Age frequency distribution of the existing cotinental crust is estimated through chlonology of detrital zircons from river sands (Rino et al., 2008 combined with surface geology; Maruyama et al., 2014). There are several global unconformities in the history of the Earth which can be regarded as the past continental margin, and clastic rocks on them can be regarded as the past river sands. Age frequency distributions of the past continental crust can be estimated at given age through detrital zircon from the clastic rocks above the global unconfirmity.

In this study, we analysed U-Pb ages of detrital zircons from clastic rocks covering unconformably on the Wyoming, Zimbabwe, Kaapvaal, and Pilbara Cratons through LA-ICPMS. Then, we estimated the age frequency distributions at 2.9, 2.6, 2.3, 1.0, and 0.6 Ga through compilation of the data with already reported data by studies. Comparing these compilations of the 5 periods and the compelation of current continental crust (Rino et al., 2008), it is delineated that the average life span is different between the early part and later part of the history of the Earth. The average life sapn of the continental crust is about 3 billion years in the later part including at current while in the early part the average life span was about 1 billion years. This suggests that the process decreasing continental crust by tectonic erosion dominantly in young Earth. Possible reason of this is the longer plate boundaries because of smaller size of plates generated by more active mantle convection in the early part of the history of the Earth.