唐古拉山北坡中新世霓辉石正长岩的地球化学特征、时代及构造意义

阿多霓辉石正长岩是分布在唐古拉山北坡的碱性侵入岩,岩石中含有大量的霓辉石、白榴石、霞石等碱性矿物。侵入体具有明显的分带性,其中心的白榴石霓辉石正长岩为典型的碱性岩,而边部或呈岩脉状出现的霓辉石正长斑岩为酸性程度较高的霓辉石正长岩,岩石具有富钾、高硅和低铁镁的特点,稀土元素配分型式呈轻稀土元素富集型,无Eu异常。霓辉石正长岩侵入于早—中侏罗世雁石坪群中,上新世曲果组不整合于其上。在黑云母霓辉石正长斑岩中获得了10.71Ma±0.08Ma和11.26Ma±0.16Ma正长石和黑云母的Ar39/Ar40坪年龄,其侵位时代为中新世。构造环境分析结果显示其形成于后造山构造环境。这表明在10Ma左右青藏高原处在构造体制转换时期,这些碱性岩是造山带崩塌早期阶段的产物。

Geochemical characteristics, age and tectonic significance of Miocene aegirine-augite syenite on the north slope of the Tanggula Mountains, China.

Abstract: Aegirine-augite syenite in the Adoi area is alkaline intrusive rock distributed on the north slope of the Tanggula Mountains. It contains a great deal of alkaline minerals such as aegirine-augite, leucite and nepheline. The intrusion shows distinct zoning. The leucite-aegirine-augite syenite in the center is typical alkaline rock, while the aegirine-augite syenite-porphyry occurring at edges or as dikes is more acid aegirine-augite syenite. The rock has the K-rich, high-Si and low-Fe and -Ma features and shows LREE-enriched distribution patterns, without Eu anomalies. Aegirine-augite syenite is intruded into the Early-Mid Jurassic Yanshiping Group and overlain by the Pliocene Quguo Formation. Orthoclase and biotite from the biotite aegirine-augite syenite-porphyry yielded Ar39/Ar40 plateau ages of 10.71±0.08 and 10.26±0.16 Ma respectively, indicating a Miocene emplacement age. Analysis of the tectonic environment suggests that the Qinghai-Tibet Plateau was in a period of transition of the tectonic regime at ~10 Ma. These alkaline rocks are the product of collapse of the orogenic belt in the early stage.