京北云蒙山地区四合堂韧性逆冲型剪切带的变形特征及早期变形时限约束

云蒙山杂岩是华北克拉通内记载了晚中生代构造演化的重要构造单元之一。云蒙山地区区域构造格架主要由四合堂背斜推覆体、四合堂逆冲型韧性剪切带、云蒙山背形、河防口正断层及水峪伸展型韧性剪切带等组成。对于四合堂逆冲型剪切带的活动时限及其与水峪剪切带之间的关系,是长期争论的课题之一。在四合堂韧性剪切带中广泛发育有剪切演化各个阶段就位的花岗质岩脉。本文研究云蒙山四合堂地区剪切变形特征及广泛发育的岩脉与构造变形之间关系,将岩脉厘定为构造期前(剪切前)岩脉、同构造期(同剪切)岩脉和构造期后(剪切后)岩脉等6期。不同类型的同构造岩脉锆石U-Pb定年获得了岩脉结晶年龄为170~150Ma。从构造-岩浆关系分析角度考虑,本文认为四合堂剪切带韧性逆冲作用早期的活动始于约170Ma,并持续到约150Ma。

Structural deformation characteristics and geochronological constraints on the early deformation of Sihetang ductile thrust shear zone in Yunmengshan, northern Beijing

The Yunmengshan complex is a typical structural unit that records the complicated Late Mesozoic tectonic evolution of the North China Craton. The major structural elements of the complex mainly include the Sihetang anticlinal nappe, the Sihetang ductile shear zone, the Yunmengshan antiform, the Shuiyu ductile shear zone and the Hefangkou normal fault. The timing of thrust shearing along the Sihetang shear zone and its relationship with the Shuiyu shear zone has been debated for decades. Granitic dykes of various stages are widespread along the Sihetang shear zone. Based on studies of the shear deformation characteristics and the relationship between the granitic dikes and tectonic deformation in the Sihetang shear zone, we grouped these dikes into three categories: pre-shearing dikes, syn-shearing dikes and post-shearing dikes. Zircon U-Pb dating reveals that the granitic dykes have crystallization ages from ca. 170Ma to 150Ma. From the structural relationships of the dykes with wall rocks we would argue that the early evolution of the Sihetang ductile shear zone may date back to ca. 170Ma and lasted to 150Ma.