中生代华北克拉通岩石圈减薄的证据——以河北武安固镇杂岩体为例

本文对位于华北克拉通中部的固镇侵入杂岩体(闪长岩、二长岩、石英二长岩和正长岩)进行了野外地质调查、岩相学、矿物学、地球化学、锆石U-Pb定年、Sr-Nd同位素和Lu-Hf同位素研究,进而探讨其形成时代、源区和成因,以期得到关于华北克拉通中部中生代岩石圈减薄研究的新启示。研究区岩体形成时代和岩石结构有一定关联性,总体来看,等粒结构的岩体形成时代早于似斑状结构岩体,研究区岩浆活动可分为两个期次:第一期次岩浆活动形成等粒结构的深成岩体,包括中细粒闪长岩、二长岩、石英二长岩和正长岩,形成时代约为130～135Ma;第二期次岩浆活动形成浅部的似斑状岩体,包括似斑状闪长岩和石英二长岩,形成时代约为127Ma。结合野外观察、岩相学、矿物学、地球化学等特征,讨论分析了固镇杂岩体的成因:固镇闪长岩富镁,高Mg~#值,Cr、Ni含量较高,富集LILE和LREE,亏损HFSE,铕异常不明显,具有高镁安山岩(HMA)性质,是与其相对应的侵入岩。固镇高镁闪长岩是被俯冲板片熔体和流体交代后的地幔橄榄岩部分熔融的产物,岩浆在上升途中与地壳发生了较弱的同化混染。固镇正长岩形成于加厚陆壳的部分熔融,地壳厚度约为40～67km。固镇二长岩是由区内正长岩和高镁闪长岩混合形成。研究区石英二长岩是区内高镁闪长岩与另一种更富硅的岩浆混合形成。俯冲板片物质(流体+熔体)对固镇高镁闪长岩的形成有重要贡献,显示出古太平洋板块俯冲作用的影响也涉及到了NCC中部,但受影响的程度小于NCC东部;正长岩形成于加厚陆壳部分熔融,显示出拆沉作用在中生代NCC中部岩石圈减薄过程中发挥了重要作用。结合角闪石温压计的计算结果和前人研究成果可以推测,NCC中部大规模拆沉作用发生的时间节点为127Ma。

The evidence for lithospheric thinning of Mesozoic North China Craton: Taking Guzhen intrusive complex as an example

This paper takes the Guzhen intrusive complex which is located in Hebei Province in the central part of the North China Craton (NCC) as a research object. The author conducted detailed field geological survey, combined with indoor research, including petrography, mineralogy, geochemistry, Sr-Nd isotope, Lu-Hf isotope and U-Pb zircon chronology analysis, so as to explore the formation age, source area, petrogenesis of Guzhen intrusive complex, in order to draw new inspiration for the Mesozoic lithospheric thinning in the central NCC. In this paper, it is believed that the formation age of the Guzhen complex is related to the structure of the rock. The formation age of the rock with equal grain structure is earlier than that of the rock with porphyritic structure. Magmatic activity can be divided into two phases:the first stage (130~135Ma) magmatic activity forms plutonic bodies with equal grain structure, including medium-fine diorite, monzonite, quartz monzonite and syenite; the second stage (127Ma) magmatism forms hypabyssal bodies, including porphyritic diorite and porphyritic quartz monzonite. Combined with field observation, petrography, mineralogy and geochemical characteristics, the petrogenesis of the Guzhen complex is analyzed. The Guzheng diorite has HMA properties. Guzhen diorite is a product of partial melting of the mantle peridotite metasomatized by the subduction slab melt and fluid, and the magma has weak assimilation and contamination with the crustal material on the ascending route. The Guzhen syenite is a product of partial melting of the thickened continental crust with the depth of 40~67km. The Guzhen monzonite is formed by the mixing of syenite and diorite in the study area. The Guzhen quartz monzonite is formed by mixing of Guzhen diorite and another silicon-rich magma. The subduction material (fluid+melt) has an important contribution to the formation of Guzheng high-Mg diorite, which shows that the influence of the subduction of the Paleo-Pacific plate also involves the central NCC, but the degree of the impact is less than that of the eastern NCC. The Guzhen syenite is formed by partial melting of the thickening continental crust, which shows that the delamination plays an important role in the thinning process of the NCC lithosphere in the Middle Mesozoic. Combined with the calculation results of the amphibole thermobarometer and the previous research results, it can be inferred that the time node of the large-scale delaminatiom of the central NCC is 127Ma.