Petrology and geochemistry of mafic rocks from me ´lange and flysch units adjacent to the Yarlung Zangbo Suture Zone, southern Tibet C. Dupuis a, * , R. He ´bert a , V. Dubois-Co ˆte ´ a , C.S. Wang b , Y.L. Li b , Z.J. Li b a De ´partement de Ge ´ologie et ge ´nie ge ´ologique, Universite ´ Laval, Que ´bec, Canada G1K 7P4 b Chengdu University of Technology, Chengdu, Sichuan, 610059, P.R. China Received 25 July 2003; accepted 25 October 2004 Abstract The Yarlung Zangbo Suture Zone (YZSZ), Southern Tibet, is marked by a continuous but tectonically disturbed ophiolitic belt. Three main me ´lange-type units lie immediately south of it:the Early Cretaceous ophiolitic me ´lange, the Mesozoic Yamdrock me ´lange 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 me ´lange 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 me ´lange are derived from tectonic disruption of the YZSZ ophiolites. Mafic rocks from the Yamdrock me ´lange 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 Re ´union 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 Re ´union 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 0009-2541/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2004.10.005 * Corresponding author. E-mail address: cedupuis@ggl.ulaval.ca (C. Dupuis). Chemical Geology 214 (2005) 287 – 308 www.elsevier.com/locate/chemgeo