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• Detailed mapping and structural analysis were conducted on the newly identified mélange belt. • This mélange belt includes dismembered ophiolite, accreted OPS, and forearc sediments. • Geochronological and geochemical data unveil a Paleozoic intra-oceanic subduction system. • New tectonic model for the evolutionary history of the North Tianshan Ocean. [Display omitted] The North Tianshan Belt occupies a pivotal geological position within the southwestern Central Asian Orogenic Belt, making it an ideal location to unravel the long-term evolutionary history of the North Tianshan Ocean. However, how, and when the Tianshan oceanic basin was finally consumed and closed remains highly debated. To resolve these questions, here, we conducted a comprehensive investigation of the different litho-tectonic assemblages of the Dacaotan composite mélange belt in the North Tianshan Belt. Using detailed geological mapping, structural analyses, rock associations, and new geochemical and geochronological data, we divide the Dacaotan composite mélange belt into three litho-tectonic assemblages: (1) the southern sedimentary forearc (ca. 423–398 Ma) assemblage that is characterized by relatively coherent and well-bedded flysch sequences; (2) the central supra-subduction zone-type ophiolitic mélange assemblage (ca. 468–461 Ma) consisting of petrogenetically linked upper mantle and oceanic crustal components, which are exposed as tectonic slices or blocks within a serpentinite and sedimentary matrix; and (3) the northern accreted ocean plate stratigraphy mélange assemblage (ca.448–398 Ma) that is composed of accreted trench-fill turbidites, seamounts (OIB-like basic rocks) and ocean floor rocks (MORB-like basic rocks) dispersed in a sandy to pelitic matrix. Post-kinematic pluton and dyke reveal that the final accretionary event was completed prior to ca. 371 Ma. In conjunction with the spatial–temporal configurations of the different litho-tectonic assemblages, we conclude that the North Tianshan Ocean underwent bi-directional subduction and intra-oceanic subduction events during the Early Paleozoic. Our findings suggest the final consumption of the North Tianshan Ocean basin occurred before the Latest Devonian, resulting in the formation of the ultimate suture between the Central Tianshan Block and the North Tianshan Belt along the Kanggguertag-Dacantao composite mélange belt. [ABSTRACT FROM AUTHOR]

In this study, we integrated extensive field, petrographic, whole-rock major and trace element, and Nd–Pb–Sr–O isotope, and zircon U–Pb ages, trace element and Lu–Hf isotope data from the Neoarchean Shawmere Anorthosite Complex and surrounding gneisses to unravel their petrogenetic origin and tectonic history. The ~2765 Ma Shawmere Anorthosite Complex is interpreted to have been emplaced into a sequence of interlayered greywacke and basalt deposited in an intra-continental arc rift system above a north-dipping subduction zone. The complex consists mainly of anorthosite, leucogabbro, gabbro, and hornblendite that were emplaced as several batches of magmas and crystal mushes originating from sub-arc mantle sources. In contrast to the previous studies, our field and petrographic data suggest an igneous origin for the most hornblende in the complex, implying hydrous parental magmas. A hydrous magma origin is also consistent with the high-anorthite content (mostly 70–90%) of the plagioclase in the complex. Percolation of hydrous basaltic melts through gabbroic cumulates in crustal magma chambers led to extensive (>50%) replacement of igneous clinopyroxene by igneous hornblende. Continued subduction resulted in the closure of the intra-arc rift system and the intrusion of the complex by tonalite, granodiorite and diorite between 2765 and 2680 Ma in an Andean-type margin. The complex and surrounding gneisses underwent hornblende granulite-facies metamorphism mainly between 2680 and 2620 Ma, overlapping with mid-crustal east-west extension between 2660 and 2640 Ma. The granulite-facies metamorphism is recorded by the replacement of hornblende, plagioclase and clinopyroxene by garnet and the development of a garnet-orthopyroxene-plagioclase metamorphic assemblage with a granoblastic texture. Tectonic rebounding of mid-crustal rocks to upper crustal levels after 2620 Ma led to the formation of an extensive network of extensional fractures and retrograde metamorphism. Migration of CO2-rich hydrous fluids along the extensional fractures and grain boundaries resulted in the precipitation of many metasomatic minerals mainly at the expense of hornblende and plagioclase, including epidote, clinozoisite, tremolite, actinolite, paragonite, margarite, titanite, quartz, calcite, sillimanite, dolomite, and chlorite. Prevalent replacement of hornblende by garnet during prograde metamorphism and metasomatic replacement of hornblende and plagioclase by retrograde mineral assemblages disturbed the Sm–Nd, U–Th–Pb, and Rb–Sr isotope systems. [ABSTRACT FROM AUTHOR]

Detailed field observations indicate that Neoarchean S-type granites were emplaced along and (or) proximal to some terrane (tectonic) boundary zones in the western Superior Province, southeastern Manitoba. These S-type granites are characterized by the presence of at least one diagnostic indicator mineral, such as sillimanite, cordierite, muscovite, garnet, and tourmaline. They are medium- to high-K calc-alkaline, moderately to strongly peraluminous (ANKC >1.1), and contain >1% CIPW normative corundum. Compared with more voluminous, older I-type granitoids in tonalite-trondhjemite-granodiorite suites in the region, the S-type granites occur as relatively small intrusions and have high (Si[O.sub.2] >72 wt.%) contents with a small silica range (Si[O.sub.2] = 72.2-81.2 wt.%), but a large range of Zr/Hf (17.1-43.8) and Nb/Ta (0.3-16.0) ratios. These geochemical characteristics suggest that the S-type granites were derived from partial melting of heterogeneous sedimentary rocks deposited as synorogenic flysch that underwent burial and crustal thickening during terrane collision. Although the S-type granites display geochemical variations in individual and between different plutons, their low Sr ( Des observations de terrain detaillees indiquent que des granites neoarcheens de type S ont ete mis en place le long de zones de limite de terranes ou a proximite de telles zones dans l'ouest de la province du Superieur du sud-est du Manitoba. Ces granites de type S sont caracterises par la presence d'au moins un mineral indicateur diagnostique, comme la sillimanite, la cordierite, la muscovite, le grenat ou la tourmaline. II s'agit de granites calcoalcalins a teneur moyenne ou elevee en K, moyennement a fortement hyperalumineux (ANKC >1,1) dont la norme CIPW presente >1 % de corindon. Comparativement aux granitoides de type I plus volumineux et plus vieux presents dans les series a tonalite-trondhjemite-granodiorite dans la region, les granites de type S forment des intrusions relativement petites et ont de fortes teneurs en silice (Si[O.sub.2] >72 % en poids) defmissant une fourchette restreinte (Si[O.sub.2] = 72,2-81,2 % en poids), mais des rapports Zr/Hf (17,1-43,8) et Nb/Ta (0,3-16,0) defmissant de grandes fourchettes. Ces caracteristiques geochimiques donnent a penser que les granites de type S sont derives de la fusion partielle de roches sedimentaires heterogenes deposees sous forme de flysch synorogenique ayant ete assujetti a l'enfouissement et l'epaississement crustal durant la collision de terranes. Bien que les granites de type S presentent des variations geochimiques au sein de plutons et entre eux, leurs faibles teneurs en Sr (, Introduction Granites derived from partial melting of sedimentary rocks in the Paleozoic Lachlan fold belt of southeastern Australia are termed 'S-type' granites by Chappell and White (1974). S-type granites are [...]

This special issue is dedicated to Ali Mehmet Celal Sengor for his outstanding contributions to plate tectonics and history of geology. His studies have unraveled several mysteries on the origin and deformation of continents and formation of orogenic belts in many parts of the world. We received 22 articles for the special issue, 11 of which are published in this issue. The rest of the articles will be published in the next issue. The articles in this issue mainly focus on geological processes in the Alpine-Himalayan orogenic belt and on the history of the theory of plate tectonics. Le present numero special est dedie a Ali Mehmet Celal Sengor pour sa contribution exceptionnelle a l'etude de la tectonique des plaques et a l'histoire de la geologie. Ses travaux ont leve plusieurs mysteres concernant l'origine et la deformation des continents et la formation de ceintures orogeniques dans de nombreuses regions du monde. Vingt-deux articles ont ete soumis pour ce numero special, dont 11 sont publies dans le present numero de la revue; les autres seront publies dans le numero suivant. Les articles du present numero s'interessent principalement aux processus geologiques dans la ceinture orogenique Alpes-Himalaya et a l'histoire de la theorie de la tectonique des plaques., Introduction This special issue of the CanadianJournal of Earth Sciences, entitled 'Understanding tectonic processes and their consequences', celebrates the contributions of Professor Sengor to earth sciences. It is a great [...]