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The dynamics and magma transport at the boundary between the upper and lower oceanic crusts (i.e., the dike–gabbro transition) are crucial for understanding the crustal accretion beneath mid-ocean ridges, which however have been studied at quite a few sites such as the East Pacific Rise and ophiolites like Troodos and Oman. Here we present detailed geological, petrological, and geochemical data for the dike–gabbro transition and associated basalts in the Yunzhug ophiolite, central Tibet, to constrain the complex magmatic processes in this specific horizon. The Yunzhug ophiolite contains a large (~ 20 km2) well-exposed sheeted dike complex, which is rooted in a dike–gabbro transition that consists of diverse lithologies, including diabase, gabbro, and minor porphyritic diabase. Petrographically, the Yunzhug gabbros could be grouped into the dominant Plg (plagioclase)-euhedral gabbros (euhedral–subhedral plagioclases enclosed in clinopyroxene oikocrysts) and a small amount of Cpx (clinopyroxene)-euhedral gabbros (with abundant euhedral clinopyroxenes). Plagioclases and their equilibrated melts of the two types of gabbros are similar, whereas clinopyroxenes and their equilibrated melts of the Cpx-euhedral gabbros are more primary and depleted than those of the Plg-euhedral gabbros. These petrographic and geochemical features suggest an earlier crystallization of clinopyroxene for the Cpx-euhedral gabbros, which is best explained by occasional water input in the magmatic system. Nevertheless, the modeled equilibrated melts of the two types of gabbros have compositions indistinguishable from the whole rock compositions of diabases and basalts, indicating a direct genetic linkage between these rocks. The unusual porphyritic diabases, on the other hand, provide evidence supporting for plagioclase accumulation and aggregation during magma upward migration, thus may have served as a unique way for magma to transport from the lower to upper crust. Studies of the Yunzhug ophiolite thus provide some key constraints on the complex magmatic processes in the oceanic dike–gabbro transition, regarding its dynamic accretion and magmatic plumbing mechanisms. [ABSTRACT FROM AUTHOR]

The nature (i.e., sub-oceanic, sub-arc or sub-continental) of ophiolitic mantle peridotites from the eastern Neo-Tethyan domain in southern Tibet has been hotly debated. This uncertainty limits our understanding of the history and evolution of the eastern Neo-Tethys Ocean. Here we present petrological, geochemical and Re–Os isotopic data for the mantle peridotites from the Dazhuqu ophiolite in the central segment of the Yarlung Zangbo suture zone, southern Tibet. Samples collected include both spinel lherzolites and spinel harzburgites. The lherzolites have spinel Cr# [Cr/(Cr + Al), ~ 0.3–0.4] comparable to those of typical abyssal peridotites. In contrast, the harzburgites have spinel Cr# (~ 0.3–0.7) overlapping with the ranges of both abyssal and fore-arc peridotites; two samples have spinel Cr# higher than 0.6, which is probably ascribed to intense melt–rock interactions. Clinopyroxene trace element modeling indicates that the Dazhuqu mantle peridotites have experienced 0–6% garnet-facies melting followed by 10–18% melting in the spinel stability field. This is similar to the degree of garnet-facies melting inferred for many abyssal peridotites and implies deep initial melting (> 85 km), which distinguishes the Dazhuqu mantle peridotites from fore-arc peridotites (commonly < 80 km in origin). The Dazhuqu peridotites have unradiogenic 187Os/188Os of 0.11836–0.12922, which are commonly lower than the recommended value of primitive upper mantle (PUM). All but one samples yield relatively younger Re depletion ages (TRD = 0.06–0.81 Ga) with respect to the only one sample having an older TRD age of 1.66 Ga. Re–Os isotopes and highly siderophile element (HSE) compositions of the Dazhuqu peridotites are similar to those of abyssal peridotites and the Oman southern massifs but are distinct from non-cratonic sub-continental lithospheric mantle (SCLM) xenoliths and sub-arc mantle. We emphasize the similarity between the Dazhuqu and Oman ophiolites, both representing Neo-Tethyan oceanic lithosphere and implying ridge–trench collision. [ABSTRACT FROM AUTHOR]