Petrology and geochemistry of the Saga and Sangsang ophiolitic massifs, Yarlung Zangbo Suture Zone, Southern Tibet: Evidence for an arc–back-arc origin

Abstract

The Saga and Sangsang ophiolites are located about 600 and 450 km west of Lhasa and represent a western extention of the central portion of the Yarlung Zangbo Suture Zone (YZSZ) ophiolite belt. The Saga massif comprises fresh mantle lherzolite and cpx-harzburgite, an ophiolite mélange (± amphibolite), metamorphosed mafic crustal rocks (meta-gabbro, meta-basalts and amphibolites) and a sequence of uppermost crustal rocks (chert, basaltic lavas, diabase sills and dikes). The Sangsang ophiolite consists of an ophiolite mélange (harzburgite) and upper mantle harzburgite with minor lavas and gabbro. Peridotites from both massifs show variable degrees of serpentinization. Their Mg# varies between 0.89 and 0.91. All peridotites show distinct flat REE (rare earth elements) patterns with La/Yb N ratios close to 1, probably indicative of a refertilized mantle. The Olivine-Spinel equilibrium and the spinel chemistry for the Saga (Cr# ~ 0.10–0.22) and Sangsang (Cr# ~ 0.30–0.55) peridotites suggest that the Saga peridotites have a deeper mantle provenance (> 20 kbar) and have undergone lower degrees of partial melting (5–12%) than the Sangsang peridotites (< 15 kbar; 17–30%). The composition of the Saga peridotites is similar to the composition of pre-oceanic peridotites while peridotites from the Sangsang massif resemble abyssal and subduction-related peridotites. Mafic rocks from both ophiolites have basalt and basaltic andesite compositions. They are slightly depleted in light REE with respect to the heavy REE, with La/Yb N between 0.5 and 0.8 and with a small negative Ta-Nb anomaly suggesting the presence of a subduction component. The abundances of incompatible elements in these mafic rocks are similar to N-MORB (mid-ocean ridge basalt) or back-arc-basin basalts (BABB). Our data suggest that the Saga and Sangsang ophiolites belong to an intraoceanic suprasubduction zone segment as postulated for other ophiolitic massifs in the eastern portion of the YZSZ. However, the geochemistry of mantle rocks from these two massifs is different compared to each other (fertile vs. refractory) and compared to other YZSZ ophiolites suggesting different petrogenetic histories. Field relationships and geochemical data furthermore suggest that the Saga and Sangsang ophiolites were formed in a complex arc–back-arc setting where at least two subducting slabs must have been active. This study places one more piece in the YZSZ puzzle and lead to a better understanding of the morphology of the convergence zone before the final stage of collision which led to the present configuration of the suture zone.