麻豆传媒

Podiform chromitites of the Luobusa ophiolite in the Indus-Yarlung Zangbo Suture of southern Tibet are typical of the high-Cr variety. This ophiolite is composed of a mantle sequence, a transition zone and a melange zone, and is tectonically bounded by Triassic flysch on the south and Tertiary molasse deposits and the Gangdese Batholith on the north. The podiform chromitites in Luobusa occur in the mantle sequence. The mantle peridotites are chiefly harzburgites and Di-harzburgites composed of Ol (Fo=90-92), Opx (En=87-92), Cpx (Mg#=45-53) and chromite (Cr#=18-66). They have porphyroclastic textures, relatively uniform bulk-rock compositions (Mg#89-91), and unfractionated, chondrite-normalized PGE patterns. These rocks are essentially residua left after extraction of MORB magmas in a mature spreading centre. The chromitites display nodular, massive, disseminated or banded textures and typically have dunite envelopes that grade into the surrounding harzburgite and Di-harzburgite. They consist of relatively uniform chromite (Cr#=74-82) and low-Al pyroxenes and have strongly fractionated PGE patterns depleted in Pt and Pd. They are believed to have formed from a boninitic magma produced by a second stage of melting above a subduction zone. Dunites contain accessory chromites intermediate in composition between those of peridotites and chromitites and are the products of reaction between the boninitic magmas and MORB peridotites. The melt/rock reaction removed pyroxenes from the peridotites and precipitated Ol, forming dunite envelopes. The melts thus became more boninitic in composition and chromite-saturated. The interplay of melt/rock interaction, chromite fractionation and magma mixing would presumably lead to many fluctuations in melt composition, producing both massive and disseminated chromitites as well as phase layering within individual podiform bodies.

High-Cr and high-Al chromitites are believed to have formed by high and low degrees of partial melting and both are precipitated by interaction of newly-formed magmas with old lithospheric mantle peridotites in island arc environments and nascent spreading centres, respectively.

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Supervisor:聽 Paul Robinson