Late Mesozoic-Eocene Mantle Replacement beneath the Eastern North China Craton: Evidence from the Paleozoic and Crnozoie Peridotite Xenoliths

摘要

Xenolith-bearmg Paleozoic kimbeiiites and Cenozoic basalts from the eastern part of the North China craton provide unusual insights into intraplate processes and Phanerozoic lithospheric evolu-tioit. Paleozoic peridotite xenoliths represent samples of ancient cratonic mantle; P-T estimates show that a thick (-230 km), cold (ca 40 mW/m2) lithosphere existed beneath the craton during mid-Ordovician time. However, xenoliths from Tertiary basalts sample a thin (< 90 km), hot (mean geotherm ca 80 rnW/m~2), compositionally heterogeneous lithosphere beneath the same area in Cenozoic time. Fertile, spinel-facies mantle makes up much of the Cenozoic lithosphere beneath the eastern North China craton, especially in regions along the translithospheric Tanlu fault. However, refractory spinel-facies xenoliths are found locally along the north-south gravity lineament in areas far away from the Tanlu fault. These refractory xenoliths are interpreted as derived from shallow' relics of the cratonic mantle embedded in more fertile Cenozoic lithosphere. The increasing incidence of fine-grained, sheared microstmctures in xenoliths from the north-south gravity lineament progressively toward the Tanlu fault suggests that the translithospheric fault system played an important role in the Mesozoic-Cenozoic replacement of pre-existing lithospheric mantle by more fertile material. Modification of cratonic mantle beneath the eastern North China craton involved irregular replacement of old lithosphere by cooling products of weakly depleted asthenosphere welling up along major shear systems. This lithosphere replacement was accompanied by an elevated geolherm and a shallower asthenosphere-lithosphere boundary.