Chemical composition of minerals and rocks from the Sierra Leone segment of the Mid-Atlantic Ridge and from the Markov depression, Central Atlantic

Silicic Fe-Ti-oxide magmatic series was the first recognized in the Sierra Leone axial segment of the Mid-Atlantic Ridge near 6°N. The series consists of intrusive rocks (harzburgites, lherzolites, bronzitites, norites, gabbronorites, hornblende Fe-Ti-oxide gabbronorites and gabbronorite-diorites, quartz diorites, and trondhjemites) and their subvolcanic (ilmenite-hornblende dolerites) and, possibly, volcanic analogues (ilmenite-bearing basalts). Deficit of most incompatible elements in the rocks of the series suggests that parental melts derived from a source that had already been melted. Correspondingly, these melts could not be MORB derivatives. Origin of the series is thought to be related to melting of the hydrated oceanic lithosphere during emplacement of an asthenospheric plume (protuberance on the surface of large asthenospheric lens beneath MAR). Genesis of different melts was supposedly controlled by ascent of a chamber of hot mantle magmas thought this lithosphere in compliance with the zone melting mechanism. Melt acquired fluid components from heated rocks at peripheries of the plume and became enriched in Fe, Ti, Pb, Cu, Zn, and other components mobile in fluids.

硅质铁钛氧化物岩浆岩系（Silicic Fe-Ti-oxide magmatic series）是首个在北纬6°附近大西洋中脊（Mid-Atlantic Ridge, MAR）塞拉利昂轴段被识别的岩系。该岩系包含侵入岩（方辉橄榄岩、二辉橄榄岩、古铜辉石岩、苏长岩、辉长苏长岩、角闪铁钛氧化物辉长苏长岩、辉长苏长岩-闪长岩、石英闪长岩以及奥长花岗岩）、次火山岩（钛铁矿-角闪石辉绿岩），并可能存在对应的火山岩类（含钛铁矿玄武岩）。该岩系的岩石普遍亏损多数不相容元素，表明其母岩浆源自已发生过熔融的源区，因此这类岩浆并非洋中脊玄武岩（MORB）的派生岩浆。研究认为该岩系的成因与大西洋中脊下方大型软流圈透镜体表面凸起的软流圈柱就位过程中，含水大洋岩石圈的熔融作用相关。不同岩浆的成因被认为受控于软流圈高温岩浆房沿该岩石圈上涌，并遵循区域熔融机制。岩浆从软流圈柱外围的受热岩石中获取流体组分，进而富集铁、钛、铅、铜、锌等流体活动性组分。