Mechanism and timing of lithospheric modification and replacement beneath the eastern North China Craton: Peridotitic xenoliths from the 100 Ma Fuxin basalts and a regional synthesis

摘要

Lithospheric thinning beneath the eastern North China Craton is widely recognized, but the mechanism and timing of the thinning are contentious. New data on peridotitic xenoliths from the Cretaceous (similar to 100 Ma) Fuxin basalts at the northern edge of the craton have been integrated with data from other localities across the craton, to provide an overview of the processes involved. The Fuxin peridotite xenoliths can be subdivided into three types which can also be recognized in other xenolith suites across the craton. The dominant Type 1, lherzolites with olivine Mg-# similar to 90, represents fertile mantle (5-12% partial-melt extraction) that makes up much of the Late Mesozoic-Cenozoic lithosphere beneath the craton. Type 2 consists of magnesian (olivine Mg-#> >92) harzburgites, interpreted as shallow relics of the Archean cratonic mantle. Type 3, minor lherzolite xenoliths with olivine Mg-# similar to 86 reflect the interaction of the lithosphere with magmas similar to the host basalts. In-situ Re-Os data on sulfides in xenoliths from Hebi (4 Ma, interior of the craton) and Hannuoba (22 Ma, northern edge of the Trans-North China Orogen within the craton) basalts give model ages of 3.1-3.0, 2.5, 2.2-2.1, 1.4 and 0.8 Ga, These correspond to the U-Pb ages of zircons from early Mesozoic (178 Ma) peridotitic xenoliths at the southern margin of the craton, and record events during which the Archean lithospheric mantle was modified. The dominance of fertile peridotite xenoliths in the 100 Ma Fuxin basalts indicates that the mantle replacement beneath the eastern North China Craton at least partly took place before that time. The regional synthesis suggests that Mesozoic-Cenozoic lithospheric thinning and mantle replacement was heterogeneously distributed across the North China Craton in space and time. Lateral spreading of the lithosphere, accompanied by asthenospheric upwelling and melt-peridotite interaction, is the most probable mechanism for the lithospheric thinning beneath the eastern part of the craton. Subsequent cooling of the upwelled asthenosphere caused some re-thickening of the lithosphere; this overall more fertile and hence denser lithosphere resulted in widespread basin formation. (c) 2007 Elsevier Ltd. All rights reserved.