Process of magma channel formation in the upper mantle

在源自上地幔的橄榄岩中可以辨认出高温岩浆穿过上地幔形成的通道痕迹。造山带辉橄榄岩中的泥质岩脉和蛇绿质橄榄岩中的不调和泥质-铬铁矿组合是上地幔岩浆通道形成的代表性样品。对北海道Iwanai-dake橄榄岩和Horoman橄榄岩杂岩、土耳其西南部的Fethiye蛇绿岩和巴基斯坦北部的Malakand蛇绿岩的通道样品进行了通道产状、岩石结构、结构和组成矿物的研究。为了检测通道岩浆/橄榄岩壁相互作用的影响，对整个岩浆通道进行了系统采样。主要组成矿物为橄榄石、正辉石、斜辉石和尖晶石，采用EPMA法对其进行主元素分析，采用LA-ICPMS法对其进行微量元素分析，以确定其熔体成分，并对通道岩浆的类型进行表征。岩内橄榄岩杂岩中褐铁矿-铬铁矿通道具有极高的Mg#和Cr#特征，表明该通道形成的岩浆类型为高Mg安山岩或baninite岩浆。包裹铬铁矿层的灰岩可以解释为岩浆在壁面辉锌矿中通道过程中熔融/壁橄榄岩相互作用形成的替代灰岩。在荷罗曼橄榄岩杂岩中，在寄主黑斑岩和黑斑岩中发现了大量的泥质岩脉，形成岩浆通道群。在霍罗曼橄榄岩通道岩浆中发现了morb型玄武岩和难熔玄武岩等多种岩浆类型。

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.