冀东地区早侏罗世岩浆岩成因及其意义

Petrogenesis and implications of Early Jurassic igneous rocks from eastern Hebei Province

冀东地区位于华北克拉通东部,区内中生代岩浆活动强烈,形成了大量不同时代的侵入岩,成为研究华北克拉通破坏过程的重要研究对象。本文在冀东青龙地区对蛇盘兔花岗岩和龙须门闪长岩进行了锆石离子探针定年,分别获得了192.0±2.4Ma和190.5±1.3Ma的年龄,结合前人在冀东其它地区获得的190~199Ma的不同岩体的锆石年龄,表明冀东地区早侏罗世是一重要的岩浆作用时期。青龙地区的早侏罗世岩浆岩由经历了不同程度结晶分异的中-酸性岩组成,它们的锆石εHf(t)明显高于华北古老下地壳的范围,说明可能并不是完全由古老下地壳重熔产生。另一方面,蛇盘兔花岗岩和龙须门闪长岩的锆石εHf(t)均具有较小的变化范围,结合它们极低的Cr、Ni含量,可以排除壳幔岩浆的直接混合。鉴于该地区在三叠纪岩浆岩中存在亏损地幔物质的加入,推测冀东青龙地区早侏罗世岩浆岩可能来源于三叠纪底侵作用形成的新生地壳与古老地壳的混合源区的部分熔融。

The eastern Hebei area,which is located at the eastern part of the North China Craton,preserves voluminous Mesozoic igneous rocks that are well indicators of destruction of the North China Craton.Based on SIMS zircon U-Pb dating,we obtained 192.0±2.4Ma and 190.5±1.3Ma for the Shepantu granite and Longxumen diorite in Qinglong region.In combination with a number of ages ranging from 190Ma to 199Ma previously published,it demonstrates that Early Jurassic is an important period of magmatism in eastern Hebei area.The Early Jurassic igneous rocks in Qinglong region consist of intermediate-felsic rocks with various degrees of crystal fractionation.Their zirconεHf(t)values are higher than that of the ancient lower crust of the North China Craton,precluding complete derivation from melting of the ancient lower crust.On the other hand,their zirconεHf(t)values show a limited variation,combined with excessive low Cr and Ni content,arguing against direct mixing of crustal-and mantle-derived magmas.Given that addition of depleted mantle materials was involved in the Triassic igneous rocks,it is suggested that the Early Jurassic igneous rocks were derived by partial melting of both juvenile lower crust formed by Triassic underplating and ancient lower crust.