地壳与弱化岩石圈地幔的相互作用:以燕山造山带为例

燕山造山带中生代发育4期钙碱性火山活动,它们的源区组成都是受壳幔相互作用的制约,其中髫髻山组和义县组分布广泛,具有代表性．髫髻山组岩性比较单一,地球化学参数变化范围小,岩浆的AFC作用不强烈,源区成分不复杂．依据Kay et al．(1991)的方法,估算了早-中侏罗世燕山地区的地壳厚度为40-45 km.髫髻山组粗安岩是在加厚的地壳 (40-45 km)条件下,源区是含角闪石的石榴石麻粒岩 底侵的基性岩的壳幔过渡带熔融形成．义县组火山岩的源区为下地壳 岩石圈地幔,地幔组分较髫髻山组增加．研究区中生代早期地壳开始加厚,发生下地壳拆沉,进入流变学性质改变了的“弱化的岩石圈地幔”,二者发生作用．岩石圈地幔在中生代晚期受到流体、熔体、地幔矿物中活化的分子水、剪切构造作用,以及温、压条件改变的影响,导致岩石圈地幔发生不均一的局部弱化,为容纳拆沉的下地壳提供了优化场所．推测弱化岩石圈地幔出现于135 Ma以后燕山地区发育的小型拉伸盆地之下,以及对应的小型软流圈底辟体之上．上述模型可以与俯冲带的楔形地幔与俯冲洋壳的相互作用相对比．

Interaction between Crust and Weakening Lithospheric Mantle:Take the Yanshan Orogenic Belt as an Example

Four-period calc-alkaline volcanic rocks are widespread in the Yanshan erogenic belt. The components of magma sources were controlled predominantly by the interaction between the continental crust and upper mantle near MOHO surface. The Tiaojishan and Yixian formations are the representative types of volcanic activity. The petrological features of Tiaojishan Formation volcanic rocks are relatively uniform, geochemical parameters varies at a narrow range and the AFC processes of magma are not strong enough, which indicates that the components of the magma source were not very complex. The estimated thickness of the crust was 40 - 45 km during the Early to Middle Jurassic. We infer that Tiaojishan tra-chyandesite magma was melted from amphibole-bearing garnet granulite underplating basalts at thickening crust condition. A possible model, delaminating lower continental crust into the underlying weakening lithospheric mantle and interaction between them, has been proposed as one means to explain the genesis of basic-middle basic magma in the Yixian Formation. The inferred source of these magmas was lower crust weakening lithosphere. Rheological behavior of the lithospheric mantle has been heterogeneously translated from rigid to ductile. This translation process was induced by the effects of fluid, melt, water released from mantle minerals, shear stress and increasing temperature in the lithospheric mantle beneath the small extension basin and above the uplifted asthenospheric diapir after approximately 135 Ma These local areas were the preferable sets to hold the delaminated lower crust. The interaction between the foundering continental lower crust and the underlying weakening lithospheric mantle may be compared with the interaction between the subducting ocean crust and mantle wedge at a subduction zone