摘要
早中生代的扬子大陆向华北大陆的深俯冲碰撞作用以及中、新生代的华北东部岩石圈减薄作用是国际大陆动力学问题研究的两个热点。然而,把它们有机联系起来,探讨深部岩石圈演化动力学过程的研究还很少。报道了中国大陆科学钻探先导孔(CCSD-PP1)橄榄岩的矿物岩石化学分析结果。这些橄榄岩亏损玄武质组分(如低w(CaO Al2O3)、高Mg#等),并经历了中元古代来自软流圈的碳酸岩熔体的交代作用和早中生代的超高压再平衡过程。结合已发表的华北捕虏体橄榄岩资料,对这一橄榄岩的原始地幔属性进行了分析。发现CCSD-PP1橄榄岩与华北古老岩石圈地幔组成相似,是早中生代来自华北岩石圈的构造侵入体。早中生代华北岩石圈的伸展减薄与苏鲁超高压变质岩石的折返提供了早期软流圈物质上涌的构造体制。
Garnet peridotites from the Chinese Continental Scientific Drilling project(CCSD-PP1)have less magnesian olivine(Fo91.5),and lower-Cr garnet than the cratonic mantle xenoliths reflected by those from the Paleozoic Mengyin kimberlites(garnet-facies)and the Cenozoic Hebi basalts(spinel-facies).The CCSD-PP1 diopsides have low HREE contents and sinusoidal to LREE-enriched patterns.These features,and their high Mg/Si,and low CaO and Al2O3 contents indicate that the CCSD-PP1 peridotites represent a moderately refractory mantle protolith.Details of mineral chemistry indicate that this protolith experienced complex metasomatism by asthenosphere-derived melts/fluids in Mesoproterozoic,and subsolidus re-equilibration involving fluids/melts derived from the subducted Yangtze continental crust during ultrahigh-pressure(UHP)metamorphism in Early Mesozoic.Tectonic extension of the subcontinental lithospheric mantle(SCLM)of the North China Craton and exhumation of the Sulu UHP rocks in Early Mesozoic induced the early upwelling of asthenosphere.