太行山阜平杂岩中麻棚-赤瓦屋岩体的时代、侵位深度及构造意义

Geochronology,emplacement depth and tectonic implications of the Mapeng-Chiwawu granitic pluton in Fuping Complex of the Taihang Mountains

中生代岩浆活动在太行山北段形成了一系列花岗岩体,其中的麻棚-赤瓦屋岩体侵位于穹窿状的阜平杂岩中,具有贫硅、富碱、高铝的钙碱性I型花岗岩特征。岩体分带明显,由边部向核部可以划分为石英闪长岩、花岗闪长岩、似斑状花岗岩3个相带,它们的锆石LA-ICP-MS U-Pb年龄分别为126.4±2.4 Ma、125.4±2.0 Ma和126.2±2.0 Ma,在误差范围内一致,指示岩体在早白垩世期间快速侵位、结晶。各相带样品的角闪石全铝平均值分别为1.018、0.880和0.871,对应的角闪石平均结晶压力分别为0.183、0.118和0.114 GPa,均值为0.141 GPa,对应的侵位深度约5 km,明显小于区域内元古界至侏罗系的地层厚度,表明在岩体侵位之前,阜平杂岩就已经具有穹窿状构造特征。结合相关资料,估计太行山早白垩世以来的平均隆升速率为0.07~0.08 km/Ma。

It is known that Mesozoic magmatism generated a series of granitic plutons in the northern segment of the Taihang Mountains. Among others, the Mapeng-Chiwawu pluton just crops out in the central part of the Neoarchean-Paleoproterozoic Fuping Complex, showing I-type calc-alkaline affinity, with low SiO2 and high A1203 and alkali content. The pluton consists of quartz diorite, granodiorite and granite from edge to core, and these intrusions yielded LA-ICP-MS zircon U-Pb ages of 126.4± 2.4 Ma, 125.4 ± 2.0 Ma and 126.2± 2.0 Ma, indicating that the pluton was emplaced in a short time around 126 Ma （Early Cretaceous）. The total amount of A1 in hornblende is 1. 018, 0.880 and 0.871 for quartz diorite, granodiorite and granite, and accord- ingly the crystallization pressure of hornblende is 0.183, 0.118 and 0.114 GPa, with an average of 0.141 GPa, indicating a depth of ca. 5 km. This is significantly smaller than the thickness of Proterozoic to Jurassic sequences around the Fuping Complex, suggesting a dome-shaped structure for the Fuping Complex before the Early Cretaceous. Combined with other data, an average uplift rate of 0.07--0.08 km/Ma can be obtained for the Taihang Mountains since the Early Cretaceous.