Process of magma channel formation in the upper mantle

在上地幔橄榄岩中可识别出高温岩浆穿流上地幔形成通道的痕迹。造山带伊赫兹岩中的纯橄榄岩脉和蛇绿橄榄岩中的不整合纯橄榄岩-铬铁矿组合是上地幔岩浆通道的代表性样品。本文对北海道岩内岳橄榄岩和钟落橄榄岩杂岩中的通道样品进行了研究，分析了通道的产状、岩石结构和组成矿物。与土耳其西南部费蒂耶蛇绿岩和巴基斯坦北方马拉根德蛇绿岩的对比。取样是作为一个系统的一个跨岩浆通道，以检测引导岩浆/橄榄岩壁相互作用的影响。通过EPMA和LA-ICPMS分析了岩石中橄榄石、斜方辉石、单斜辉石和尖晶石等主要成分矿物的主量元素和微量元素，确定了岩石熔体的组成和通道岩浆的类型。岩内岳橄榄岩杂岩中的纯橄榄岩-铬铁矿通道具有极高的Mg#和Cr#特征，表明该水道建造的岩浆类型为高镁安山岩或板状安山岩岩浆。包裹铬铁矿层的纯橄榄岩可解释为岩浆在壁面辉橄榄岩中窜流时，熔体/壁面橄榄岩相互作用形成的交代纯橄榄岩。在寄主方辉橄榄岩和霍罗曼橄榄岩杂岩中的二辉橄榄岩中发现了大量纯橄榄岩脉作为岩浆通道群。在霍罗曼橄榄岩中发现了多种岩浆类型，如MORB型玄武岩和一种更难熔的玄武岩。

Traces of channels formed by high temperature magma channeling through the upper mantle can be recognized in peridotites derived from the upper mantle. Dunite dykes in orogenic Iherzolites and discordant dunite-chromitite associations in ophiolitic peridotites are representative samples of magma channels formed in the upper mantle.Mode of occurrence of channels, structures and textures of rocks, and the constituent minerals ware examined on the channel samples from the Iwanai-dake peridotite and the Horoman peridotite complexes in Hokkaido, together with those from the Fethiye ophiolite of SW Turkey and the Malakand ophiolite of Northern Pakistan. The sampling were performed as a systematic one across the magma channels to detect the effect of the channeling magma/peridotite wall interactions. The major constituent minerals such as olivine, orthopyroxene, clinopyroxene, and spinel, were analyzed by EPMA for major elements and also by LA-ICPMS for trace elements to detect the melt components and to characterize the type of the channeled magmas.The dunite-chromitite channels from the Iwanai-dake peridotite complex are characterized by minerals with extremely high Mg# and Cr# signatures, suggesting a high-Mg andesite or a baninite magma for the magma type of the channel formation. The dunite enveloping the chromitite layer can be explained as replasive dunite formed by a melt/wall peridotite interaction during the magma channeling in the wall harzburgite. A number of dunite dykes were found as magma channel swarm in the host harzburgite and Iherzolite in the Horoman peridotite complex. Several kinds of magma types such as MORB-type basalt and a more refractory basalt were detected for magmas channeled in the Horoman peridotites.