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The Altaids accreted around, and grew southward, from the Siberian craton, but the time of final amalgamation of this orogen is still controversial. The Eastern Tianshan in the southernmost Altaids is characterized by multiple, late, accreted arcs and thus is an ideal tectonic environment to answer the time of final amalgamation of the Altaids. In this study we report the results of new field-based lithological mapping and structural analysis on the Kanguer mélange in the Eastern Tianshan, which is composed of blocks of basalt, chert, limestone, and other rocks within a strongly deformed and cleaved matrix of sandstone and schist. Our geochemical and isotopic data of basaltic blocks from several parts of the Kanguer mélange show they are relics of Normal-Mid-Ocean-Ridge (N-MORB)-type oceanic lithosphere, and U–Pb ages and Hf isotopes of detrital zircons from the matrix sandstones indicate they were derived only from the Dananhu arc to the north. Accordingly, our interpretation is that the Kanguer mélange was part of an accretionary complex that fringed the Dananhu arc, and therefore the subduction polarity of the Kanguer Ocean was to the north (present coordinates). The maximum depositional ages (MDAs) of our three sandstone samples (08K01, 08K02, and 08K03) from the mélange matrix were 234 ± 14 Ma, 242.5 ± 1.3 Ma, and 236 ± 2.0 Ma respectively, indicating that the Kanguer Ocean was still being subducted at ca. 234 Ma, and the accretion of the Kanguer mélange must have lasted until that time, when the accretionary complex was still located opposite to the Yamansu-CTS accretionary complex to the south. Thus, the final amalgamation of the Dananhu and Yanmansu-CTS arcs took place by the welding of two accretionary complexes in the late Middle Triassic (Ladinian) in this part of the southern Altaids. Integration with relevant amalgamation histories throughout the Tianshan indicates that the time of terminal amalgamation in the southern Altaids was probably in the Middle-Late Triassic, which is much younger than previously envisaged. [ABSTRACT FROM AUTHOR]

Ophiolites in the Zagros orogen represent relics of the Neo‐Tethyan Ocean that existed between the Laurasia and Gondwana continents. There is much debate about the formation and evolution of the Neo‐Tethyan Ocean. Knowledge of its pre‐collisional tectonic evolution is essential in constraining the timing and process of the collision between the Eurasian and Arabian continents. We present a combined analysis of zircon LA–ICP–MS U–Pb ages and Hf–O isotopes of the Sardasht ophiolite from the NW Zagros orogen. Our new data demonstrate that the Sardasht ophiolite formed at ca. 50.6 Ma. Geochemistry of plagiogranite and diabase indicates that the Sardasht ophiolite probably originated in the mantle wedge, influenced by subduction fluids. The mafic rocks of the Sardasht ophiolite include boninites, arc basaltic rocks, and OIBs, which accreted together when the Neo‐Tethys finally closed. Finally, we argue that there was an intra‐oceanic fore‐arc extension and arc system during 50–24 Ma in the southern branch of the Neo‐Tethys in NW Zagros. [ABSTRACT FROM AUTHOR]

The Makran complex in southeast Iran provides a spectacular subduction-related accretionary complex to understand the mechanism of oceanic accretion and the evolution of subduction zones. In this paper, we present new major and trace element data as well as isotopic compositions of mafic volcanic blocks from the Makran ophiolitic mélange complex (OMC). Our aim is to assess the genesis of these rocks and discuss their implications on the evolution of Neotethys Ocean. These volcanic blocks are composed mainly of basalts with minor trachytes. The Makran lavas are occasionally interlayered with tuff layers. Zircons from these tuffs give U-Pb ages of 95 Ma, which is well in accordance with the reposted microfossil data for the interlayered pelagic limestones with pillow lavas. Makran basalts can be geochemically subdivided into four groups; normal to transitional MORB, enriched-MORB, Plume-type MORB and alkaline (-OIB-like) basalts. The OIB-like pillow lavas are represented by high values of Th/Tb (6.3–7.4) which are higher than other basalts (group 1 = 0.3–0.8; groups 2 = 0.7–1.6; group 3 = 1.58–1.36).143Nd/144Nd(t) ratios for basalts ranges from 0.51247 to 0.51292, whereas 87Sr/86Sr(t) isotopic composition of the OMC lavas varies from 0.704433 to 0.709466. The Pb isotopic composition of the lavas are quite high, ranging from 15.49–15.66 for 207Pb/204Pb(t), 18.09–19.12 for 206Pb/204Pb(t) and 37.80–39.23 for 208Pb/204Pb(t). The chemistry of these rocks suggests that they were formed most likely in an oceanic setting with clear plume-ridge interaction. These rocks can form from partial melting of a highly heterogeneous mantle source, which is extensively metasomatized with deep mantle OIB-type components. We suggest these rocks have been generated in an oceanic ridge with plume-ridge interaction, similar to the Iceland-Reykjanes Ridge, before being fragmented and accreted into the Makran accretionary complex. [ABSTRACT FROM AUTHOR]

The Tethyan Himalaya Sequence between the Yarlung‐Zangbo Suture and South Tibetan Detachment is a fundamental component of the Himalayan orogenic belt, traditionally considered to belong to the passive continental margin of India. However, the eastern Tethyan Himalaya Sequence is poorly understood, compared with that in the central and western Himalaya. To improve general understanding, we undertook north‐south geological‐structural profiles across all the litho‐tectonic units of the eastern Tethyan Himalaya Sequence and produced new isotopic‐geochemical data from all units, to constrain their mode of occurrence, structure, tectonic setting, and time of formation. The whole Tethyan Himalaya Sequence consists of a thrust‐imbricated mélange that contains blocks of ophiolites (such as Lang County, Yumen, and Lhunze), oceanic islands (e.g., Comei) with ocean island basalts‐type basalts, mafic greenschists, limestone olistoliths, and cherts in a turbiditic matrix of greywackes, siltstones, and slates in south‐directed thrust slices. Our findings suggest that the eastern Tethyan Himalaya Sequence represents the evolution of an accretionary prism, rather than a passive margin of the Indian Plate. The prism was created during northward subduction of Neo‐Tethys between ~130 and ~50 Ma. These data allow us to produce a new tectonic model for the structural development of the eastern Himalayan orogenic belt in relation to the India‐Asia collision. [ABSTRACT FROM AUTHOR]

The tectonic history and time of closure of the Palaeo-Asian ocean of the Altaids are issues of lively current debate. To address these issues, this paper presents detailed geological, petrological and geochemical data of the Liuyuan complex (LC) in the Beishan region in NW China, located in the southernmost Altaids, in order to constrain its age, origin and tectonic setting. The LC mainly comprises massive basalts, pillow basalts, basaltic breccias, gabbros and ultramafic rocks together with cherts and tuffs. Most prominent are gabbros and large volumes of basaltic lavas. These mafic rocks have high TiO2 contents, flat rare earth element (REE) patterns and show high-field-strength elements (HFSEs) similar to those of mid-ocean ridge basalts (MORB). The mafic rocks exhibit positive εNd(t) (6.6–9.0) values, representing magmas derived from the mantle. But these basic rocks are also enriched in Th relative to REEs, and are systematically depleted in Nb–Ta–(Ti) relative to REEs. There is also a large range in initial 87Sr/86Sr (0.7037–0.7093). All these variables indicate that mantle-derived magma was contaminated by fluids and/or melts from a subducting lithospheric slab, and formed in a supra-subduction zone (SSZ) setting. A gabbro intruded in the complex was dated by LA-ICP-MS on 20 zircons that yielded a 206Pb–238U weighted average age of 286 ± 2 Ma. Considering the fact that all these basalts are imbricated against Permian tuffaceous sediments and limestone, we propose that the LC formed as an ophiolite in a fore-arc in Carboniferous–Permian time. This indicates that the Palaeo-Asian ocean still existed at 286 ± 2 Ma in early Permian time, and thus the time of closure of the Palaeo-Asian ocean was in or after the late Permian. [ABSTRACT FROM PUBLISHER]