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    华北克拉通古老岩石圈地幔及其组成的“逆向演化”

    Ancient lithospheric mantle and the “inverse evolution” of its components in North China Craton

    • 摘要: 提要:华北克拉通古老岩石圈地幔演化过程中有3种深部地质作用最为重要:①岩浆的萃取作用;②不同来源的熔/流体对岩石圈的交代作用;③软流圈及其熔体与岩石圈的相互作用。在蒙阴金伯利岩的捕虏体样品中发现有早晚两期斜方辉石,前者是早期粗粒结构橄榄岩残留的矿物;后者是晚期地幔发生塑性流变后形成的辉石。本文对比了蒙阴和复县两岩区以及南非两种典型结构橄榄岩捕虏体的主、微量元素,发现较晚期的具剪切结构的橄榄岩难熔程度反而低于较早期的具粗粒结构者。粗粒结构的Mg’值都大于0.90(0.902~0.93),Al2O3含量变化于0.22%~2.32%;而具剪切结构者Mg’值都小于0.90(0.86~0.88),Al2O3含量变化于1.69%~2.75%。表明地幔成分演化时不仅存在受第①种作用制约的地幔不断贫瘠/难熔的正向演化,而且还存在受第③种作用制约的地幔由贫瘠转向饱满的“逆向”演化趋势。经粗略对比后,推测在约3 Ga的这段地质历史时期内华北克拉通古老岩石圈地幔至少出现过2次岩浆萃取事件和两次“逆向演化”过程。与显生宙的相比,出现在古老岩石圈地幔中的第③种作用规模小,强度低,不均一性更为明显。

       

      Abstract: Abstract:The component evolution of ancient lithospheric mantle was constrained by three deep-seated geological processes, i.e., ① melt extraction, ② lithospheric mantle metasomatism of melt/fluid from different sources, and ③ interaction between lithosphere and asthenosphere /asthenospheric melt. Two stage enstatites (Py1 and Py2) were discovered in a sample of peridotite xenolith from Mengyin kimberlite. The enstatites of the early stage are residual minerals in the early peridotites with coarse-grained texture. However, the enstatites of the late stage were formed under the plastic rheological condition. A comparison of major and trace elements between the two kinds of peridotites from Mingyin, Fuxian and South Africa shows that the coarse-grained peridotites have higher Mg’ (0.902~0.93) and lower Al2O3 (0.22%~2.32%) values, whereas the peridotites with shear texture have lower Mg’ and higher Al2O3(1.69%~2.75%) values, suggesting that the late stage peridotites have lower refractory degree than the early stage peridotites. This phenomenon finds expression in the mantle component evolution process not only controlled by melt extraction but also by the interaction between lithosphere and asthenosphere / asthenospheric melt. The action caused the evolution of mantle composition from refractory back to fertile, and such an evolution trend is called “inverse evolution”. At least two magma extraction events and two “inverse evolution” processes occurred in ancient lithospueric mantle of North China Craton approximately during the 3 Ga geological period. Compared with the action ③ in Phanerozoic, it was characterized by smaller scale, lower strength and more obvious heterogeneity.

       

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