医巫闾山中—晚侏罗世构造变形与同构造花岗岩的耦合关系

Relationship between Middle-Late Jurassic deformation and syn-tectonic granitoid emplacement in the Yiwulüshan area,North China

晚中生代期间医巫闾山地区经历了早期的高温韧性变形和晚期的低温脆-韧性变形,并最终形成了变质核杂岩。该变质核杂岩中发育多个中—晚侏罗世花岗岩体,通过分析花岗岩体的岩浆组构及围岩中变形组构的特点,确定该期次的岩浆活动与高温韧性变形同时发生,且这些岩体就位于中部地壳的高温韧性剪切带内。岩浆就位的过程包括三个阶段:部分熔融与垂向运移阶段(D1)、水平运移阶段(D2)和固态变形阶段(D3)。尖砬子岩体的主量、微量元素分析结果显示岩浆为埃达克质岩浆,并具有过铝质和由钙碱性向高钾钙碱性过渡的特点。岩石的Th/U比值和碱性组分含量较高,Mg、Cr、Fe_2O_3~T含量较低,岩浆锆石ε_(Hf)(t)值=-29.5～-23.3、T_(DM2)=2.51～2.18Ga。这些特点指示岩浆来源于古老的华北陆块下地壳的部分熔融,表明岩浆具有从地壳深部向地壳浅部运移的过程。中—晚侏罗世岩体的产出形态、岩浆组构及磁组构的优选方位,表明岩浆在就位于中部地壳的过程中受到围岩剪切作用的控制,发生强定向的水平流动,形成了NE-SW向延伸最长、垂向最短的扁纺锤状岩浆房。对比中—晚侏罗世和早白垩世两期同构造岩浆就位机制,发现岩浆就位机制与围岩构造环境关系密切。同为下地壳部分熔融并向浅部运移的两期岩浆活动,岩浆就位机制差异很大:中—晚侏罗世岩浆就位于中部地壳的强剪切部位,就位过程中岩浆以侧向流动为主、气球膨胀作用为辅;早白垩世岩浆就位于浅部地壳的弱剪切部位,岩浆以气球膨胀作用和底辟作用为主。岩浆就位过程中不但受到围岩剪切作用的控制,岩浆房的扩张也会挤压围岩,使围岩叶理的产状发生变化,进而影响对构造性质的判断。

The Yiwulüshan metamorphic core complex,located in northeastern North China Block,was formed by two stages of ductile deformation at high and low temperatures successively during the Late Mesozoic.Several Middle-Late Jurassic granitic plutons are exposed in the Yiwulüshan area.These plutons were emplaced in the high-temperature shear zone in the mid-crust during crustal sliding,as inferred from our analyses of magmatic fabrics in plutons and deformation fabrics across the shear zone.The emplacement occurred in three stages:partial melting and vertical ascent of the magma(D1),horizontal migration of the magma(D2),and solid-state deformation(D3).Major and trace elemental data for these rocks showed typical adakite characteristics,featuring peraluminous and calc-alkaline to high-K calc-alkaline transition.The plutons have high Th/U ratio and high alkaline and low Mg,Cr,and Fe2 O3 contents.TheεHf(t)values of magmatic zircons ranged from-29.5 to-23.3 and TDM2 ages were between 2.51 and 2.18 Ga.These results indicate that the adakitic magma was derived from partial crustal melting of the North China Block.The locations of the plutons and the preferred orientation of magmatic fabrics further indicate that emplacement in the mid-crust was controlled by a NE-SW extensional stress,resulting in the NE-SW orientated,oblate,spindle-like plutons with a short vertical axis.Granitoids were emplaced during both the Jurassic and Cretaceous,and a close relationship between the emplacement mechanism and structural environment in both periods was observed.While both stages of magmatism resulted from partial melting of the lower crust,emplacement style differs in each case.Middle-Late Jurassic emplacement occurred in a strongly deformed mid-crustal shear zone,and the emplacement mechanism was mainly horizontal flow with minor ballooning.In contrast,emplacement during the Early Cretaceous occurred in the weakly deformed upper crust,with ballooning and diapirism being the main emplacement mechanisms.Our research here suggests that deformation of country rocks could impact magmatic emplacement and viceversa,and emplacement could further influence deformation property evaluation.