云南德钦鲁春-红坡牛场上叠裂谷盆地“双峰式”火山岩的Rb-Sr年龄值

The Rb-Sr age determinations of the “bimodal” volcanic rocks in the Luchun-Hongponiuchang superimposed rift basin, Deqen, Yunnan

关键词:  上叠裂谷盆地  “双峰式”火山岩  Rb-Sr年龄值  鲁春-红坡牛场  云南德钦

Key words:  Luchun-Hongponiuchang superimposed rift basin  bimodal volcanic rock  Rb-Sr age determination  Deqen in Yunnan

摘要:

云南德钦鲁春-红坡牛场上叠裂谷盆地是金沙江构造带内中生代的重要赋矿盆地,位于金沙江结合带与昌都稳定陆块之间的活动边缘火山岩带中。由于该区火山岩的形成时代一直没有确切的年龄值,从而造成对火山岩的性质和构造背景认识上的差异。笔者采用Rb-Sr同位素年代学方法对鲁春-红坡牛场上叠裂谷盆地中玄武岩和流纹岩的年龄进行了测定,获得了"双峰式"火山岩的平均年龄值为224Ma,其形成于晚三叠世早期。玄武岩的⁸⁷Sr/⁸⁶Sr初始值为0.7065~0.7194,流纹岩的⁸⁷Sr/⁸⁶Sr初始值为0.7074~0.7199,二者十分相似,表明盆地中的玄武岩与流纹岩为同一岩浆源,从早期的基性岩浆演化为晚期的酸性岩浆。玄武岩和流纹岩组合的"双峰式"火山岩形成于金沙江弧-陆碰撞后的地壳伸展构造环境。

Abstract:

The Luchun-Hongponiuchang superimposed rift basin, Deqen, Yunnan is situated in the active marginal volcanic zones intermediate between the Jinshajiang suture zone and Qamdo landmass, and interpreted as an important Mesozoic host basin along the Jinshajiang tectonic zone. The whole-rock Rb-Sr isotopic ages are determined for the basalt and rhyolite from the rift basin. The average age for the bimodal volcanic rocks equals to 224 Ma, suggesting that they were formed during early Late Triassic time. The initial Sr⁸⁷/Sr⁸⁶ values for the basalt range from 0.7065 to 0.7194, whereas those for the rhyolite from 0.7074 to 0.7199. This implies that both the basalt and rhyolite were derived from the same magmatic source, i. e. the basic magmas in the early stage and the acidic magmas in the late stage, and that they were emplaced in the crustal extensional settings following the Jinshajiang arc-continent collision.