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The temporal and spatial distribution of enriched source components at sites of continent–continent collision provides critical insights into mantle dynamic processes associated with subduction- and collision-related events. However, determining the origin of such enriched components remains a significant challenge. We report a comprehensive dataset of the Linzizong volcanic succession (LVS) from four locations with varying distance across-strike to the Indus–Yarlung suture in southern Tibet, which marks the exposed surface expression of the India–Asia collision zone. The LVS rocks in this study can be divided into two groups: a calc-alkaline Group 1 (69–55 Ma), mainly including basaltic–andesitic varieties, and a shoshonitic Group 2 (52–50 Ma), consisting predominantly of silicic rocks with minor mafic compositions. Group 1 samples are likely derived from the fractional crystallization of primitive basaltic melts as a result of the partial melting of a metasomatized mantle wedge. These samples are decoupled in Nd–Hf isotopic compositions, suggesting an incorporation of subducting sediment-derived melts into the mantle wedge. The influence of sediment-derived melt on the mantle source increases away from the suture zone toward Asia (i.e. from the south to the north) as indicated by the more enriched Sr, Nd, Pb, and Hf isotopic compositions, as well as elevated Th/La and La/Sm ratios. The heavy δ26Mg values, and high Ba/Th and Sr/Th ratios of samples close to the suture coincide with the dehydration of the subducting Neo-Tethyan slab. Group 2 mafic samples have depleted and coupled εNd–εHf isotopic compositions and are characterized by elevated (La/Yb)N and Dy/Yb ratios as well as low Zr/Nb ratios, indicating an origin of enriched garnet-bearing lithospheric mantle with contributions from asthenosphere-derived materials. Group 2 silicic samples are isotopically enriched both near and far away from the suture, which can be attributed to the involvement of ancient lower crust-derived melt from Tethyan Himalaya and central Lhasa subterrane basement, respectively. Our results show that the LVS are the magmatic response to late subduction (Group 1), and to initial India–Asia collision and slab breakoff (Group 2). Negative trends in the whole-rock Nd and zircon Hf isotopic compositions at ~51 Ma should be treated with caution for geodynamic interpretations, depending on the distance between the samples and the India–Asia suture. [ABSTRACT FROM AUTHOR]

Amphibole plays an important role in the petrogenesis and evolution of arc magmas, but its role is not completely understood yet. Here, a field, petrological, geochronological and geochemical study is carried out on ultramafic–mafic arc cumulates with textural and chemical heterogeneities and on associated host diorites from the eastern Gangdese Batholith, southern Tibet to explore the problem. The cumulates occur as a large body in diorite host-rocks. The core of the body consists of coarse-grained Cpx hornblendite with a porphyritic texture. Towards the contact with the host diorite, the coarse-grained Cpx hornblendite grades to relatively homogeneous fine-grained melagabbro. Zircon U–Pb dating indicates that they all crystallized at 200 ± 1 Ma. Textural features and whole-rock and mineral chemical data reveal that both the Cpx hornblendite and the melagabbro are mixtures of two different mineral assemblages that are not in equilibrium: (1) brown amphibole and its clinopyroxene inclusions; (2) matrix clinopyroxene + green amphibole + plagioclase + quartz + accessory phases. Clinopyroxene and brown amphibole from the first assemblage are enriched in middle rare earth elements (MREE) relative to light REE (LREE) and heavy REE (HREE), and are weakly depleted in Ti, whereas clinopyroxene and green amphibole from the second assemblage are characterized by LREE enrichment over MREE–HREE and more marked Sr and Ti depletion. The higher Mg#, MgO and Cr of the late-formed green amphibole than the early formed brown amphibole suggest that the two assemblages are not on the same liquid line of descent. Given the close relations of the three rock types in the exposed crustal section, the cumulates are interpreted to have formed in an open system, in which an ultramafic cumulate body consisting of the first assemblage reacted with the host dioritic melt to form new clinopyroxene and amphibole of the second assemblage. The melt calculated to be in equilibrium with the first mineral assemblage resembles an average continental arc basalt, which is less evolved than the host dioritic melt, responsible for the second mineral assemblage. On the basis of whole-rock Sr–Nd–Hf isotopic similarity of the cumulates and a host diorite sample, we argue that the host diorites were formed through crystal fractionation from the parent melt of the first assemblage. Results of least-squares mass-balance calculations suggest that the quantities of the host dioritic melts, involved in the generation of these modified cumulates, vary from ~25 to ~44 %. The presence of magmatic epidote in the host diorites and Al-in-Hb geobarometry indicate that the reaction that occurred when the dioritic melts percolated through the cumulate body was at ~6 kbar. Both the brown and green amphiboles are enriched in MREE relative to HREE, and can impart to residual melts a strong geochemical signature of amphibole fractionation (low Dy/Yb). Thus, we conclude that fractional crystallization and melt–rock reaction are two mechanisms by which amphibole controls arc magma petrogenesis and evolution. [ABSTRACT FROM AUTHOR]