新疆克拉玛依中基性岩墙群形成力学机制及其构造意义

Mechanism of the Karamay Basic-Intermediate Dyke Swarm from Xinjiang and Tectonic Implications

克拉玛依岩墙群产出在克拉玛依北岩体及其围岩之中,具有不连续分段岩墙和连续分段岩墙两种类型,空间分布受区域张性构造裂隙控制,总体走向以北西-南东(280°～300°)为主,还有少量北东-南西(30°～50°)走向的岩墙。克拉玛依岩墙群所受最大主压应力方向垂直于地表,最小主压应力方向与岩墙扩容作用方向平行,并垂直于北西-南东走向的分段岩墙。岩浆沿构造裂隙贯入、充填形成岩墙,其发育程度与裂隙和区域各主压应力之间的夹角有关。岩墙分段的力学原因是岩浆在流动过程中,遇到与最小主压应力斜交的先存裂隙时,应力强度因子瞬间降低所致。估算的主伸长度为2.7%。克拉玛依岩墙群的大量出现进一步表明,二叠纪时,区域构造已经由碰撞挤压体制转变为伸展体制。

Distribution of the Karamay dyke swarm is strictly controlled by the pre-existing extensional fractures, mainly with a striking of 280°-300°. The minimum principal compressive stress was normal to these dykes, and the maximum principal compressive stress was assumed to be vertical. The dykes did not propagate as a single sheet but rather as a set of discontinuous or continuous segments. The segmentation is partly attributable to the pre-existing fractures （30°-50°） that are oblique to the minimum compressive stress. The segments, which are oblique or parallel to the minimum compressive stress, tend to be thin or nearly absent. It is thus suggested that the intruding dykes activated these oblique fractures and were segmented by the instantaneous decrease in the stress intensity factor at the tip of dyke segments as they reached these fractures. The major extension or elongation of the Karamay dyke swarm is 2.7%. During Permian, northern Xinjiang was in a postcollisional setting and was characterized by significant addition of mantle-derived magmas and their differentiates to the crust. The Karamay dyke swarm was produced in an extensional regime and therefore may be one of the indicators of vertical growth of the continental crust.