早白垩世华北克拉通东部岩石圈减薄过程和机制:来自河北西石门杂岩体的证据

The processes and mechanism of lithospheric thinning in eastern North China Craton during Early Cretaceous: Evidence from Xishimen Complex,Hebei Province

白垩纪华北克拉通岩石圈经历了巨大的减薄事件,但减薄过程及机制仍存在较大争论。早白垩世的西石门杂岩体是华北克拉通东部与岩石圈减薄相关的重要杂岩体。根据西石门杂岩体形成时代和岩石结构特征将其划分为两套岩石组合。早期岩套由二长闪长岩-二长岩-正长岩系列组成,具等粒结构,形成时代约为135.6Ma,岩石地球化学成分为SiO2=51.72%~68.56%、MgO=0.17%~4.88%,少部分样品表现出C型埃达克岩的特征;晚期岩套由斑状二长岩-斑状正长岩系列组成,具似斑状结构,形成时代约为125.3Ma,岩石地球化学成分为SiO2=57.00%~67.51%、MgO=0.61%~3.37%,其中还存在大量年龄约2500Ma的继承锆石。两套侵入岩均为高钾钙碱性系列但早期岩套多具埃达克质岩特征,晚期岩套岩石中锆石具有更负的εHf(t),显示其具有更多的壳源物质组成。角闪石温度计、压力计显示,早期岩套的定位环境为T=883.66℃、P=194.95MPa,相当于7.36km深处;晚期岩套的定位环境为T=642.34℃、P=26.81MPa,相当于1.01km深处。从135.6Ma至125.3Ma,地壳快速抬升了约6.35km。研究认为,在约135.6Ma时陆内岩浆作用开始影响研究区,软流圈地幔上涌促使地壳物质熔融、混合形成了早期岩套,同时大规模的岩浆活动及地壳褶皱变形导致陆壳加厚。之后岩石圈地幔发生拆沉作用,地壳快速抬升。在约125.3Ma后,软流圈地幔继续上涌,回弹的陆壳开始发生大规模的熔融形成了晚期岩套。据此认为拆沉作用是中生代华北克拉通岩石圈减薄的主要机制。

The lithosphere of the Cretaceous North China Craton experienced tremendous thinning events,but the processes and mechanism of the thinning are still controversial.The Early Cretaceous Xishimen Complex is an important complex related to lithospheric thinning in the eastern North China Craton.According to its formation ages and rock structure characteristics,the Xishimen Complex can be divided into two sets of rock suites.The early suite is composed of monzodiorite-monzonite-syenite series with equigranular structures,whose formation age is about 135.6Ma,with contents of SiO2=51.72%~68.56%and MgO=0.17%~4.88%.Furthermore,a few samples from this suite show the characteristics of a C-type adakite.The late suite is composed of porphyry monzonite-porphyry syenite series with porphyry-like structures,and its forming age is about 125.3Ma,with contents of SiO2=57.00%~67.51%and MgO=0.61%~3.37%,and a large number of inherited zircons with an age of about 2500Ma occurred in this suite.Both two suites have intrusive rocks of high potassium calc-alkaline series,but most of the early suite rocks have the characteristics of adakitic rocks.The zircons in the late suite rocks have a negativeεHf(t),which indicates that they have more crustal material compositions.The calculated data by the amphibole thermometer and barometer show that the early suite is formed under a PT condition of T=883.66℃and P=194.95MPa,which is equivalent to an environment of 7.36km depth underground.While the late suite is formed under a condition of T=642.34℃ and P=26.81MPa,which is equivalent to a depth of 1.01km underground.From 135.6Ma to 125.3Ma,the crust rapidly uplifts about 6.35km.Thus,we believe that the intracontinental magmatism begins to affect the study area at about 135.6Ma.The melting and mixing of crustal materials caused by the upwelling of asthenosphere mantle form the early suite,and the large-scale magmatic activity and crustal fold deformation result in the thickening of continental crust.Then,the lithospheric mantle detaches,and the crust uplifts rapidly.After about 125.3Ma,the asthenosphere mantle continues to upwell,and the rebounding continental crust begins to melt in large scale to form the late suite.In a word,delamination is the main mechanism of lithospheric thinning in the Mesozoic North China Craton.