西藏班公湖-怒江缝合带中西段早白垩世洋岛火山岩岩石学及地球化学特征
The petrology and geochemistry of Early Cretaceous ocean island volcanic rocks in the middle-western segment of Bangong Co-Nujiang suture zone
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摘要: 仲岗洋岛位于班公湖-怒江板块缝合带中西段，前人对仲岗洋岛的研究主要集中在东段，中段洞错一带尚未有人研究。对仲岗洋岛的岩石组合及岩石地球化学进行研究，并对洋岛火山岩的源区及构造环境进行分析。在仲岗洋岛中段洞错北部的一条剖面上采集了8个玄武岩和8个玄武安山岩地球化学样品，主量元素特征表明，二者皆为具有富Ti特征的碱性玄武岩，微量元素特征显示二者富集Nb、Ta元素。稀土元素配分曲线和微量元素蛛网图与典型洋岛玄武岩曲线相似。化学成分指示，仲岗洋岛玄武岩与玄武安山岩可能来自于同一岩浆源区，且二者来源于具有洋岛玄武岩特征的地幔，相容元素Cr、Ni的亏损表明，成岩过程中发生了橄榄石和辉石的分离结晶作用。洋岛形成于大洋板块内以洋壳为基底的洋岛环境。Abstract: Zhonggang ocean island lies in the middle-western segment of Bangong Co-Nujiang suture zone. Previous researches mainly focused on the eastern segment of Zhonggang ocean island, with no attention paid to Dongco area in the middle segment. In this paper, the authors studied the rock association and whole rock geochemistry of Zhonggang ocean island and analyzed the source area and structural environment of the island volcanic rocks. Eight basalt and eight basaltic andesite geochemical samples were collected along a transect at the middle part of Zhonggang ocean island in the north of Dongco. The major element characteristics of these two kinds of rocks indicate that basalt is alkali basalt enriched in Ti, and the trace element characteristics show that both of them are enriched in Nb and Ta. The REE patterns and the trace element spider diagram are similar to features of typical OIB. Chemical compositions of Zhonggang ocean island basalts and basaltic andesites suggest that they probably came from the same magma source, and both of them had mantle source with OIB characteristics; the depletion of Cr, Ni indicates that fractional crystallization of olivine and augite occurred during the rockforming process. The ocean island was formed in an oceanic island tectonic environment on the base of the oceanic crust.
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