Petrology and geochemistry of mafic rocks from mélange and flysch units adjacent to the Yarlung Zangbo Suture Zone, southern Tibet

Abstract

The Yarlung Zangbo Suture Zone (YZSZ), Southern Tibet, is marked by a continuous but tectonically disturbed ophiolitic belt. Three main mélange-type units lie immediately south of it:the Early Cretaceous ophiolitic mélange, the Mesozoic Yamdrock mélange and the Triassic flysch.The petrography, mineral chemistry and whole-rock chemistry of mafic rocks from these three geological units are reported in order to assess their tectonic settings. mafic rocks from the ophiolitic mélange are mainly gabbros and composed of primary plagioclase, clinopyroxene and locally brown amphibole. Except for a few coarse-grained gabbros that have plagioclase compositions ranging from An 80 to An 50, most plagioclase grains are albite pseudomorphs. Clinopyroxenes are augite to diopside. Reddish brown tschermakite to magnesiohornblende are probably of magmatic origin. Ilmenite and titanite are common. REE patterns are LREE-depleted with average (La/Yb) N of 0.5. Slight negative Nb–Ta and Ti anomalies suggest that these rocks formed in a back-arc basin. They show geochemical characteristics similar to mafic rocks of the Lau Basin. This study confirms that mafic rocks from the ophiolitic mélange are derived from tectonic disruption of the YZSZ ophiolites. mafic rocks from the Yamdrock mélange are mostly porphyritic and amygdaloidal basalts. Their primary composition consists of prismatic plagioclase, augite and ilmenite. A few samples also contain small euhedral crystals of brown magnesiohornblende, probably of magmatic origin. These rocks have been affected by low but pervasive hydrothermal metamorphism. REE patterns are LREE-enriched with average (La/Yb) N of 5.3.These rocks are of intraplate affinity, and are similar to volcanic rocks of the Indian Réunion hotspot. mafic rocks from the flysch are mostly gabbros. They are clinopyroxene-rich. Interstitial quartz is common. Ilmenite and titanite are abundant and relatively well preserved. REE patterns are the most LREE-enriched of any analyzed samples from the sedimentary units, with average (La/Yb) N of 6.9, while multi-element patterns show slight negative Nb–Ta and Ti anomalies. These last patterns are similar to patterns in volcanic rocks from the western Deccan Traps of India, whose geochemical behavior reflects mixing between an enriched mantle end-member of Réunion island-type and Indian continental lithospheric material. Hence, these rocks are interpreted to have been derived from an enriched intraplate mantle source, with additional crustal assimilation resulting from disaggregation of the Indian Plate during the opening of the Neo-Tethys ocean. AFC processes have been modelled and strongly suggest a felsic component analogous to Cambro-Ordovician granites as the major contaminant.