Origin of different dunites in the Luobusa ophiolite, Tibet
西藏罗布莎蛇绿岩中不同产出的纯橄岩及成因探讨

Three types of dunite occur in the Luobusa ophiolite, a thick layer of transition zone dunite, thin envelopes of dunite around podiform chromitites and small bands and lenses of dunite in the harzburgites, respectively. The transition zone dunite is about 700~1000m thick, and consists chiefly of high magnesium olivine (Fo93-95),low alumina and high magnesia clinopyroxene (Al2O3=0.47%~0.85%; Mg#=95~97),and high chrome and low magnesia Cr-spinel (Cr#=~77; Mg#=~51). Cr spinels in layers of disseminated chromite hosted in these dunites are similar in composition to those of the host rock but are slightly more magnesian (Mg#=~59) The dunite envelopes around the podiform chromitites are similar in composition to the transition zone dunite (olivine Fo92-94; Al2O3 in clinopyroxene <1% and Mg#=95~97; Cr# in Cr-spinel=71 and Mg#=52). The massive chromitites associated with the dunite envelopes have Cr#s averaging 79 and Mg#s averaging 68. The dunite lenses in harzburgite have olivine with somewhat lower Fo (91~92) and Cr-spinel with lower Cr#s (about 60), but clinopyroxene with higher Al2O3 (1.41%~1.71%). The mineral compositions in these lenses overlap those in the host harzburgite, with a gradual transition between them. The transition zone dunite in the Luobusa ophiolite is distinctly different from classic ultramafic cumulates, and most likely formed by reaction between tholeiitic basalt melt and mantle peridotite. Dunite lenses in the harzburgite may reflect either inhomogeneous partial melting of the host peridotites or in-situ melt-rock reaction. The dunite envelopes around the podiform chromitites are thought to have formed in the same way as the transition zone dunite, but the origin of the massive chromitites is not clear.