青藏高原北部鲸鱼湖地区中新世和更新世两期橄榄玄粗质系列火山岩

Miocene and Pleistocene shoshonitic volcanic rocks in the Jingyuhu area, north of the Qinghai-Tibet Plateau

鲸鱼湖地区识别出了两个时期橄榄玄粗质火山岩喷发,早期为中新世,晚期为更新世。早期的属低碱安粗岩类,晚期的主要为碱玄岩-高碱的安粗岩类。两者在斑晶类型、主元素和微量元素丰度和同位素比值等方面都存在区别,但显示了相同的稀土元素分配型式。岩石同位素的环境判别表明它们都源自EMⅡ型地幔,属有再循环地壳物质加入的富集地幔。鲸鱼湖产出两个时代火山岩表明不同时期的新生代火山活动在空间上是可以叠加的。鲸鱼湖的火山沿巨型昆仑山南缘左行走滑断裂带分布,主要受断裂带北侧展布的次级断裂控制。岩石圈深断裂起到了岩浆通道的作用,而两期火山喷发的存在则说明了断层的多期活动。

Two periods, Miocene and Pleistocene, of shoshonitic volcanic rocks were recognized in the Jingyuhu area ?northern Qinghai-Tibet plateau. The earlier one is intermediate trachyte series, and the later one is tephrite-basic trachyte series. The two series have different types of phenocryst, chemical composition, trace element pattern and ratio of isotopic element, but they show similar REE patterns. Isotopic characteristics suggest that these rocks were originated from EM II type of mantle, a kind of enriched mantle with the addition of recycled crustal material. The occurrence of the two periods and two series of volcanic rocks suggest that the different periods of Cenozoic volcanic activity on the plateau can be overlapped in space. The volcanic activities occurred along the giant sinistral strike-slip fault along the south margin of Kunlun Mountain, specifically constrained by the second-order faults to the north of the main fault. The deep-cut lithospheric faults here acted as a magma channel, and the occurrence of two periods of eruption represents the reactivity of the fault.