Mapping lithospheric boundaries using Os isotopes of mantle xenoliths: An example from the North China Craton

The petrology, mineral compositions, whole rock major/trace element concentrations, including highly siderophile elements, and Re-Os isotopes of 99 peridotite xenoliths from the central North China Craton were determined in order to constrain the structure and evolution of the deep lithosphere. Samples from seven Early Cretaceous-Tertiary volcanic centers display distinct geochemical characteristics from north to south. Peridotites from the northern section are generally more fertile (e.g., Al2O3 = 0.9-4.0%) than those from the south (e.g., Al2O3 = 0.2-2.2%), and have maximum whole-rock Re-depletion Os model ages (T-RD) of similar to 1.8 Ga suggesting their coeval formation in the latest Paleoproterozoic. By contrast, peridotites from the south have maximum T-RD model ages that span the Archean-Proterozoic boundary (2.1-2.5 Ga). Peridotites with model ages from both groups are found at Fansi, the southernmost locality in the northern group, which likely marks a lithospheric boundary. The Neoarchean age of the lithospheric mantle in the southern section matches that of the overlying crust and likely reflects the time of amalgamation of the North China Craton via collision between the Eastern and Western blocks. The Late Paleoproterozoic (similar to 18 Ga) lithospheric mantle beneath the northern section is significantly younger than the overlying Archean crust, indicating that the original lithospheric mantle was replaced in this region, either during a major north-south continent-continent collision that occurred during assembly of the Columbia supercontinent at similar to 1.8-1.9 Ga, or from extrusion of similar to 1.9 Ga lithosphere from the Khondalite Belt beneath the northern Trans-North China Orogen, during the similar to 1.85 Ga continental collision between Eastern and Western blocks. Post-Cretaceous heating of the southern section is indicated by high temperatures (> 1000 degrees C) recorded in peridotites from the 4 Ma Hebi suite, which are significantly higher than the temperatures recorded in peridotites from the nearby Early Cretaceous Fushan suite (< 720 degrees C), and likely reflects significant lithospheric thinning after the Early Cretaceous. Combining previous Os isotope results on mantle xenoliths from the eastern North China Craton with our new data, it appears that lithospheric thinning and replacement may have evolved from east to west with time, commencing before the Triassic on the eastern edge of the craton, occurring during the Jurassic-Cretaceous within the interior, and post-dating 125 Ma on the westernmost boundary. (C) 2011 Elsevier Ltd. All rights reserved.