华北克拉通东部岩石圈破坏与幔柱的成因

Lithosphere destruction and mantle plume genesis of eastern North China Craton

华北克拉通东部是指大兴安岭—太行山重力梯度带以东的地区。该地区与东北及华南地区一样,也存在着岩石圈减薄和岩石圈破坏的现象。华北克拉通古生代与新生代的橄榄岩包体地热值分别为40mW/m^2和80mW/m^2,地热增高是岩石圈破坏的重要因素。中国东部—朝鲜半岛的GRACE卫星布格重力异常呈巨型环状展布,可能是地幔亚热柱所致。区内郯庐断裂带呈NNE向纵贯切割,最新的地震反射剖面成果反映,该断裂带以走滑为主,兼有逆冲和正断层,所组成的大型"花状"复杂断裂构造带,切穿地壳、莫霍面,直至岩石圈地幔,起到破坏岩石圈的作用。S波速成像结果证实,在苏鲁造山带下方,分布的碎块状波速扰动带延伸>300km,应是中生代扬子板块走滑俯冲、断离、折返的产物;而在渤海湾盆地下方,下地幔也存在陡倾的破碎带,延伸>300km,在岩石圈76km处及其深部,S波速降低,扰动很剧烈,可能是地幔柱头部呈粗大状的反映,故可认为渤海湾盆地是由蘑菇云状的地幔柱形成。再从华北克拉通的大陆根来看,东部为A型反时针pT轨迹麻粒岩分布,而中西部则为B型麻粒岩,这是因为东部新太古代绿岩带中赋存有科马提岩,其岩浆温度可高达1 600℃,如用幔柱构造模型解释成因最为合适。通过研究近年大量地震反射P波和S波层析资料,华北克拉通的东部在晚侏罗-早白垩世及新生代期间,受太平洋板块俯冲和地幔热柱上隆的相互作用,深部的地幔流体常沿构造破裂带运移,促使岩石圈拆沉,并发生热侵蚀作用,形成酸性大火成岩省(SLIP)、裂谷型盆地的广泛分布和成矿成藏作用的"大爆发"。由此认为,建立区域构造模型将有利于指导深部找矿工作。

The eastern North China Craton refers to the east region of Daxin′anling-Taihangshan gravity gradient zone,where in fact there happened lithosphere thinning and destruction as in South China and the northeast of East China.The geothermal features of Paleozoic and Cenozoic peridotite xenoliths in North China craton are 40mW/m^2 and 80mW/m^2 respectively,the increased geotherm is undoubtedly an important factor of lithosphere destruction.The Bouguer gravity anomaly of East China-Korean peninsula from GRACE appears as a giant ring which might be the sign of sub-mantle plume.Tanlu Fault Zone runs along NNE direction across the region.According to the latest deep seismic reflection profile,it is a very large flower-like complex fault zone,mainly strike-slip faults and some thrust faults and normal faults cutting the crust,Moho and subcontinental lithospheric mantle and playing a destructive role to lithosphere.The velocity tomography of S-wave has confirmed that there exists fragmental velocity disturbance zone extending over 300 km beneath the Sulu orogen,which might be caused by the slip,subduction,dispersion and exhumation inversion of Mesozoic Yangtze plate.Under the Bohai Bay,there is a steeply dipping fracture zone over 300 km long in lower mantle and at 76 km in lithosphere and deeper place the reduction and disturbance of S-wave velocity are both strong,which might be the reflection of thick and big plume head and Bohai bay basin can be regarded as a product formed by mushroom cloud-like plume.In the continental root of North China craton,A-type granulites with anti-clockwise p-Tpath are distributed in the east while the B-type granulites are in the middle and west which can be explained by mantle plume according to the fact that in the east occurs komatite whose magma is at 1 600 ℃in Late Archean greenstone belt.The study on a lot of tomographic data of P-wave and S-wave in recent years shows that similar to the East China the eastern North China was formed by interaction of the subduction of Pacific plate and uplift of mantle plume during Late Jurassic-Early Cretaceous period and Cenozoic Era.The deep mantle fluids always migrated along structurally fractured zones and prompted the delamination of lithosphere and the thermal erosion,and caused the silicic large igneous province（SLIP）,widespread rift basins and the explosive development of mineral deposits and gas/oil reservoirs.Therefore establishing regional structural model will guide the deep exploration.