蛇绿岩研究之检讨与反思:以“双沟蛇绿岩”为例

"双沟蛇绿岩"是笔者最早研究的蛇绿岩之一,位于云南哀牢山带。双沟出露的岩石有二辉橄榄岩、辉长岩、辉绿岩、斜长花岗岩、玄武岩、硅质岩等。辉长岩亏损LREE,锆石U-Pb年龄为362～328Ma。玄武岩具N-MORB和E-MORB的特征,锆石U-Pb年龄为249Ma。研究认为,"双沟蛇绿岩"可能产于陆间小洋盆或裂谷或裂陷槽背景。但是,双沟没有可信的深海沉积和混杂堆积的记录,虽然岩石组合类似蛇绿岩,地球化学也具有MORB的特征,暗示双沟可能不是一个典型的蛇绿岩。如果双沟不是蛇绿岩,则晚古生代的哀牢山带就不存在一个有一定规模的洋盆,也不可能存在大陆碰撞的记录。双沟不是蛇绿岩是什么?可能是造山橄榄岩(Orogenic peridotite)。造山橄榄岩与蛇绿岩的岩石组合类似,蛇绿岩的橄榄岩产于洋壳之下;造山橄榄岩产于陆壳之下。检讨双沟蛇绿岩的研究,反思蛇绿岩的概念。笔者认为,斯泰因曼的"三位一体"概念是合适的,1972年彭罗斯会议的决议是正确的,1996年怀柔会议构造学家对蛇绿岩概念的理解是对的。考虑到混杂堆积对于蛇绿岩的重要性,建议将混杂堆积也作为与蛇绿岩相伴的一个重要指标加进来。如果这个想法合适,则一个完整的蛇绿岩组合将由三个要素组成:1)岩浆岩(包括地幔岩、堆晶岩、侵入岩和火山岩,代表大洋岩石圈的物质组成); 2)深海沉积(代表洋盆顶部的物质组成); 3)混杂堆积(代表洋盆消失、陆块碰撞的构造产物)。蛇绿岩不同于其他岩浆岩,其研究需要特殊的方法和思路,明白这一点,蛇绿岩研究才能走上正轨。双沟蛇绿岩研究遇到危机,中国其他一些蛇绿岩也可能需要重新审视。因此,检讨双沟蛇绿岩,对反思蛇绿岩的研究具有一定的意义。

Review and reflection on ophiolite research: Taking Shuanggou ophiolite as an example

Shuanggou ophiolite, located in the Ailao Mountain Belt, Yunnan Province, is one of the first ophiolites studied by the author. The rocks exposed in Shuanggou include peridotite, gabbro, diabase, plagiogranite, basalt, silicalite and so on. The gabbro loses LREE, and the zircon U-Pb age is 362~328Ma. Basalt has the characteristics of N-MORB and E-MORB, and the zircon U-Pb age is 249Ma. Research suggests that Shuanggou ophiolite may be produced in the intercontinental small ocean basin or rift trough backgrounds. However, Shuanggou has no credible records of deep-sea deposit and mélange. Although its rock assemblage is similar to ophiolite, their geochemistry features also have MORB characteristics, suggesting that Shuanggou may not be a typical ophiolite. If the rock assemblage in Shuanggou were not an ophiolite suite, there would be no ocean basin of a certain scale in the Ailaoshan belt of the Late Paleozoic, and there would be no record of a continental collision. If Shuanggou is not a ophiolite, it may be orogenic peridotite since the rock assemblage of orogenic peridotite is similar to that of ophiolite. Peridotite of ophiolite is produced under oceanic crust; orogenic peridotite is produced under continental crust. By reviewing the research of Shuanggou ophiolite and reflecting on the concept of ophiolite, the author believes that Steinman''s "Trinity" concept is appropriate, the resolution of the Penrose Conference in 1972 is correct, and the understanding of the concept of ophiolite by tectonic geologist at the Huairou Conference in 1996 is correct. Considering the importance of mélange for ophiolite, it is recommended to add mélange as an important indicator accompanying ophiolite. If this idea is appropriate, a complete ophiolite assemblage will consist of three elements, magmatic rocks (including mantle rocks, cumulite, intrusive rocks and volcanic rocks, representing the material composition of the oceanic lithosphere), deep-sea deposit (representing the material composition at the top of the ocean basin), and mélange (representing the tectonic products of the disappearance of ocean basins and collision of continental plates). Unlike other magmatic rocks, ophiolite requires special methods and special ideas. Only if we understand this point, ophiolite research can be on the right track. The research of Shuanggou ophiolite is in crisis, and some other ophiolites in China may also need to be reviewed. Therefore, reviewing the Shuanggou ophiolite may be meaningful to reflect on the research of ophiolite.