华北克拉通岩石圈地幔置换作用和壳幔生长耦合的Re-Os同位素证据

测定了辽宁复县奥陶纪金伯利岩和河北汉诺坝与山东栖霞第三纪碱性玄武岩中产出的地幔包体的Re Os同位素组成。金伯利岩中地幔包体的Re贫化Os同位素模式年龄 (TRD)为 2 .5～ 2 .8Ga ,从Re Os同位素定年角度证明了华北克拉通确实存在太古宙岩石圈地幔。对汉诺坝二辉橄榄岩包体获得了 (1.9± 0 .18)Ga的Re Os同位素等时线年龄 ,表明现今保存在那里的地幔主要是古元古代时形成的。汉诺坝地区出露有大量新太古代岩石 ,表明曾存在太古宙地幔。由于缺乏太古宙年龄 ,说明由汉诺坝所代表的克拉通中部曾存在的太古宙地幔在古元古代时已被减薄 ,并被 1.9Ga的新生岩石圈地幔置换。该事件与华北克拉通中部广泛的古元古代碰撞造山过程导致的麻粒岩相变质作用的时代相同 ,说明有关的岩石圈置换作用可能主要与拆沉作用有关。栖霞地幔包体具有与现代对流地幔相同的Os同位素组成 ,且Os同位素组成与Re/Os比值没有明显相关性 ,表明年龄很新。结合其它地质地球化学证据 ,说明克拉通东部的太古宙岩石圈地幔的置换作用主要发生在中生代 ,且可能与三叠纪华北和扬子陆块的陆陆碰撞造山导致的岩石圈地幔和下地壳的拆沉作用有关。本研究表明华北克拉通岩石圈地幔置换作用在时空上的分布是十分不均一的。 2 .5～ 2 .8Ga与 1.9Ga不仅?

REMOVAL OF LITHOSPHERIC MANTLE IN THE NORTH CHINA CRATON: Re-Os ISOTOPIC EVIDENCE FOR COUPLED CRUST-MANTLE GROWTH

Mantle xenoliths from Ordovician kimberlite at Fuxian, Liaoning and from Neogene alkaline basalts at Hannuoba and Qixia of the North China craton were measured for Re-Os isotopic compositions. Xenoliths from the kimberlite yield Re depleted Os model ages (T_(RD)) of 2.5～2.8 Ga. This confirms the presence of Archean lithospheric mantle beneath the North China Craton. The Hannuoba xenoliths give a Re-Os isotopic isochron of (1.9±0.18) Ga, which demonstrates that the present-day lithospheric mantle preserved there is Paleoproterozoic in age. The surface of the Hannuoba area is dominated by Archean crustal rocks, suggesting former presence of Archean mantle. However, lack of Archean ages in the Hannuoba peridotite xenoliths show that the Archean mantle had been replaced by the newly formed Paleoproterozoic mantle. This event coincides with the widespread Paleoproterozoic granulite-facies metamorphism resulting from continental collision. This in turn implies that the Paleoproterozoic removal of the lithospheric mantle were associated with lithospheric delamination. The Qixia xenoliths have Os isotopic compositions similar to the present-day convecting mantle and show no correlations between Os isotopic and Re/Os ratios, which suggest that they are very young in age. Together with other lines of geological and geochemical evidence, these results point to Mesozoic removal of the lithospheric mantle in the eastern North China Craton, which is speculated to be associated with delamination of the lower crust and lithospheric mantle induced by the Triassic collision between the North China and Yangtze cratons. The present study documents that timing and location of removal of the lithospheric mantle vary from the western to the eastern parts of the North China craton. The 2.5～2.8 Ga and 1.9 Ga lithospheric mantle forming events agree with the two most important crustal growth episodes in the North China craton. Therefore, formation of the continental crust and lithospheric mantle is coupled in time in the North China craton.