广西北海涠洲岛第四纪晚期火山碎屑流中橄榄岩包体碎块的发现及其地质意义

Peridotite xenolith fragments from Late Quaternary pyroclastic flow in Weizhou Island,Beihai,Guangxi

广西北海涠洲岛是我国面积最大的第四纪火山岩海岛,岛内出露的最新一次火山喷发产物为碧玄质岩石,它覆盖于晚更新世湖光岩组(Q3h)层状凝灰岩地层之上。在该火山岩中发现有橄榄岩包体,罕见的是,包体是以火山角砾的形式见于碧玄质火山碎屑流之中,故应是被炸碎的包体碎块,而非原始态包体。这是迄今为止较为特殊的一种橄榄岩包体产状类型,其原始态包体的岩石类型可能为方辉橄榄岩和纯橄岩。岩石结构及探针数据均表明该橄榄岩包体来自岩石圈地幔。包体的温压估算结果为960～1040℃和1.3GPa,对应的稳定深度约40km。此外,由于包体来源深度远小于其寄主碧玄质火山岩浆的来源深度(大于99km),表明涠洲岛地区第四纪火山碎屑流中的橄榄岩包体是地幔柱来源的碧玄质岩浆在上升经过岩石圈地幔约40km深度时捕获的偶然包体,并在就位前经历过炸碎。

Weizhou Island in Beihai City of Guangxi is the largest Quaternary volcanic oceanic island in China. The latest eruptive product is basanitoid, which covers the tuffite strata of Late Pleistocene Huguangyan Formation （Q3h）. It is worth mentioning that many peridotite xenoliths were found in the strata. They are distributed in the basanitoid pyroclastic flow with the unique form of volcanic breccia, quite different from normal xenoliths. As the xenoliths have experienced blasting, they might be called xenolith fragments. Obviously, such xenoliths constitute one of the most peculiar types of peridotite xenolith occurrence. The original xenoliths might have been dunite and harzburgite. Both rock structure and microprobe data show that these peridotite xenoliths were derived from the oceanic lithosphere mantle. The estimated equilibrium p-T conditions of xenoliths are about 1.3 GPa and 960-1 04012, with the corresponding stable depth being 40 km. Besides, the xenoliths came from a place much shallower than their host rocks, suggesting that they were accidental xenoliths. And they experienced blasting before emplacement.