龙门山北段水磨地区辉绿岩锆石U-Pb年龄及地球化学特征

龙门山北段的四川广元水磨地区零星分布北东-南西向的基性岩脉,主要侵位于中生代。前人对龙门山地区的基性岩脉研究较少,更缺乏高精度的年龄数据及地球化学报道。研究区辉绿岩锆石SHRIMP U-Pb年龄数据具双峰特点,结合区域构造及岩浆事件分析,采用185.7Ma为本区辉绿岩的成岩年龄。对于49.8±0.7Ma(MSWD=0.83)的年龄数据,可能指示了在喜山运动50Ma左右,本区经历过一次较强的热事件致锆石变质结晶。辉绿岩的Si O2含量相对较低,Ti O2含量较高。稀土元素总含量高,轻稀土元素相对富集,轻重稀土分异明显,配分模式为向右陡倾型。微量元素Zr/Y-Zr和2Nb-Zr/4-Y图解亦指示其具板内拉张环境特征。结合前人研究,认为水磨地区基性岩脉为印支运动后的晚三叠世-侏罗纪,由于后造山期挤压应力的逐渐消退,该区域曾一度进入挤压造山后的松弛调整阶段,致使幔源基性岩浆沿张性构造裂隙侵位而成。

Zircon U-Pb dating and geochemical characteristics of diabase in the Shuimo area, northern section of the Longmen Shan

In the northern section of the Longmen Shan, the Shuimo region of Guangyuan, Sichuan Province, scattered NE-SW trending mafic dikes, mainly emplaced in the Mesozoic. Analysis on samples of these dikes shows that the diabase SHRIMP U-Pb zircon ages have a bimodal characteristic. Combined with analysis of regional tectonic and magmatic events, this study adopts 185.7Ma as the diagenetic age of diabase for this area. The 49.8±0.7Ma (MSWD=0.83) age may indicate that in the Himalayan movement around 50Ma, this region experienced a strong heat-induced zircon metamorphic event. Diabase has relatively low SiO2 content and high TiO2 content. The total REE content is high, LREE is enriched relatively and obviously differentiated with HREE, and the distribution pattern is of a right-steep tilting type. Trace elements Zr/Y-Zr and 2Nb-Zr/4-Y diagrams imply an extensional environment within a plate. In conjunction with previous studies, this work suggests that the mafic dikes of the Shuimo region formed in the Late Triassic-Jurassic, after the Indo-China movement. Because post-orogenic compressive stress gradually decreased, this region had once entered the post-orogenic relaxation adjustment phase, resulting in mantle-derived mafic magma intrusion along extensional structural fissures and thus generating the mafic dikes.