Evaluating the role of tectonic setting in new continental crust formation by Pb isotopic ratios

How new continental crust formed remains a matter of debate. A key to solve this issue is to determine which tectonic setting(s) are involved in new crust formation. Modern mantle-derived magmas that formed in intra-continental extension settings (OIB-like U/Pb = -0.28-0.37) and in subduction settings (IAB-like U/Pb =-0.1-0.16) have distinct mean U/Pb ratios, furthermore, the mean U/Pb (-1.8-2.0) of depleted mantle (DM) is mainly between the OIB-like and IAB-like U/Pb ratios. Therefore, the Pb isotopic ratios (e.g., 206Pb/204Pb) of subduction-related mantle-derived magmas develop below the lead isotopic evolution curve of depleted mantle (DM), whereas those ratios of mantle-derived magmas generated in intracontinental extension settings evolve above the DM evolution curve. Here we used published Nd-Pb isotope compositions from 75 I-type granitoid samples to calculate the time of new crust formation (TDM) and 206Pb/204Pb ratios [(206Pb/204Pb)t] of new crust at t (i.e., time of new crust remelting) in the Songliao Block from East Asia to determine tectonic settings in which new crust formed have changed with time. Our results show that (206Pb/204Pb)t ratios corresponding to TDM of -1.55-1.2 Ga plot below the DM evolution curve, indicating a subduction setting of new crust formation, whereas, the (206Pb/204Pb)t ratios corresponding to TDM of -1.05-0.70 Ga plot above the DM evolution curve, indicating an intracontinental extension setting of new crust formation. Hence, here two distinct tectonic settings of new crust formation are recognized in the Songliao Block. In addition, this method is also applied to well determine the tectonic settings of new crust formation in the Jibei area of North China and the Qinling area of Central China during the Paleoproterozoic (ca. 2.2-1.8 Ga). Therefore, our study demonstrates that the calculation of TDM and (206Pb/204Pb)t ratios of new continental crust from the bulk-rock record may constitute a potential approach to better constrain how new continental crust formed and the Precambrian tectonic evolution of continents.