云南建水煌斑岩年代学和地球化学及其构造意义

华南大陆内部燕山期的岩浆事件可分为2期:燕山早期180~150 Ma,燕山晚期140~80 Ma。而处于扬子板块西南缘的滇黔贵等地,仅见有第二期晚白垩世中酸性侵入岩与相关成矿作用,缺乏第一期岩浆活动的时间记录。本文在滇东建水地区首次发现了麦地村煌斑岩,对其进行高精度LA-ICP-MS锆石U-Pb定年和主微量元素测试分析。结果显示煌斑岩的锆石U-Pb年龄为(158.4±2.4)Ma(MSWD=4.2),为晚侏罗世的产物,补充了该区较为空白的第一期岩浆热事件。麦地村煌斑岩不相容元素明显高于原始地幔,表现出不同程度的富集,大离子亲石元素Rb、Ba、Ce等富集程度不明显,高场强元素中U、Ta、Pb等表现出一定程度富集,LREE相对富集,HREE相对亏损,不具有Nb-Ta-Ti负异常的俯冲型配分曲线特征,具有明显的OIB型洋岛玄武岩稀土元素配分特征,为板内伸展环境下的产物。滇东地区发育中生代时期的NNE构造,早期NNE向褶皱叠加有后期同方向正断层。早期褶皱表现为SEE-NWW挤压应力作用下的构造特征,后期断层表现出同方向张应力作用下的特征。煌斑岩正是由于该区发生挤压之后伴随着一期应力松弛调整,伸展减压作用诱发地幔物质部分融熔,来自于深处的软流圈地幔流体与岩石圈地幔发生交代作用,形成富集型地幔,交代富集的岩石圈地幔局部受热、部分熔融,发生上升侵位。煌斑岩的侵位时间即是滇东地区中生代从挤压向伸展转换的起始时间。

Geochronology, geochemistry and geological implications of the lamprophyre in Jianshui, east Yunnan Province

The magmatic event of Yanshanian period in South China has been divided into two stages:Early Yanshanian (180-150Ma) (J₂-J₃) and Late Yanshanian (140-80Ma) (K₁-K₂). However, there exist only the data concerning Cretaceous mineralization associated with acidic intrusive rocks, but the record of the early magmatic events on the southwest margin of the Yangtze Plate has not been seen. In this paper, the authors newly discovered Maidicun lamprophyre in Jianshui area of eastern Yunnan. The U-Pb age of the zircon is 158.4±2.4 Ma (MSWD=4.2), and hence it is obvious that the lamprophyre was formed in the Late Jurassic, corresponding to the first stage of the thermal event in southeast China. The values of incompatible elements are significantly higher than those of the primitive mantle of Maidicun lamprophyre, showing different degrees of enrichment. The LILE (Rb, Ba, Ce) enrichment is not obvious, HFSE (U, Ta, Pb) show some enrichment, and Nb, Ti, Zr, Hf don''t show significant enrichment and losses. LREE/HREE ratios are in the range of 5.65-6.07, averagely 5.89,(La/Yb)_N ratios are in the range of 7-12, indicating LREE enrichment and HREF relative losses. It has no negative anomalies of Nb-Ta-Ti and shows REE characteristics of OIB-type oceanic island basalts, and the Maidicun lamprophyre was formed under the condition of intraplate extension. NNE-trending structures were developed in the study area during the Mesozoic, and the early NNE-trending folds were superposed by late normal faults in the same direction. The fold axial surface is mainly SE-trending and partly NW-trending, indicating tectonic characteristics in response to SEE-NWW trending extrusion stress. Faults are mostly high-angle normal faults in the NW-SE direction, showing that they were influenced by EW-trending tensile stress. The lamprophyre''s intrusion was mainly controlled by the action of deep asthenosphere mantle, from which the fluid experienced metasomatism with lithosphere mantle in response to partial diapering of asthenosphere and the stretch of lithosphere in eastern Yunnan in the Late Jurassic period, followed by the formation of the enriched mantle. And this kind of enriched mantle was partially heated, melted and migrated upward for emplacement finally. The intrusion age of the lamprophyre (Late Jurassic 158 Ma) is the starting time of extension in the study area.