Metamorphic evolution of the East Tethys tectonic domain and its tectonic implications

The China Central Orogenic System(CCOS),extending in an east-west direction in the middle part of China,is composed of the Early Paleozoic Altyn-North Qilian-North Qaidam-East Kunlun-North Qinling-North Tongbai orogens in the west and the Late Paleozoic to Early Mesozoic South Tongbai-Hong'an-Dabie-Sulu orogens in the east.They were produced by oceanic subduction and continental subduction/collision during the closure of the Proto-Tethys and the Paleo-Tethys oceans,respectively.Different types of metamorphic rocks with various ages are extensively exposed in these orogens,and they were produced at different geothermal gradients in different stages during the tectonic evolution of convergent continental margins,making them ideal targets to reconstruct the spatiotemporal evolution of the Eastern Tethys tectonic domain.In this article,an integrated study of metamorphic temperature(T)-pressure(P)-time(t)records is presented for metamorphic rocks along the CCOS,aiming to ascertain the change of metamorphic T/P ratios in both time and space,and then shed light on the tectonic evolution of the East Tethys tectonic domain in association with the thermal state change of convergent continental margins.The results indicate that despite the difference in metamorphic ages,metamorphic rocks in different orogens show a common trend with clockwise P-T-t paths.With respect to plate convergence for subduction and collision,regional metamorphism is categorized into three stages:(1)an early convergent stage,corresponding to low T/P Alpine-type blueschist-to eclogite-facies high-P to ultrahigh-P metamorphism;(2)a later convergent stage,corresponding to the medium T/P Barrovian-type medium-P amphibolite to high-P granulite-facies metamorphism;and(3)a post-convergent stage,corresponding to the high T/P Buchan-type lowP amphibolite to MP granulite-facies metamorphism.Nonetheless,a few metamorphic rocks only record a two-sage metamorphic evolution,with an early Barrovian-type high-P granulite-facies metamorphism and a late Buchan-type low-P granulitefacies metamorphic overprinting.In modern convergent plate margins,Alpine-type metamorphism mainly occurs in the stages of oceanic subduction and continental collision,Barrovian-type metamorphism takes pace in both stages of crustal thickening during continental hard collision and slab exhumation when continental subduction zones have evolved from compressional to extensional regimes,and Buchan-type metamorphism occurs in intracontinental rifting stage after the plate convergence.Therefore,the tectonic evolution of convergent continental margins can be reconstructed by combining metamorphic T/P ratios with their corresponding metamorphic facies series and metamorphic timing of metamorphic rocks.Based on the reported metamorphic rocks of different types and ages along the CCOS,it appears that the continental subduction/collision occurred at 500–490 Ma in the Altyn-North Qinling-North Tongbai orogens but 450–430 Ma in the North Qaidam-East Kunlun orogens,and the intracontinental rifting occurred at 460–450 Ma in the Altyn-North Qinling-North Tongbai orogens but 410–400 Ma in the North Qaidam-East Kunlun orogens,respectively,in the western Proto-Tethys domain.For the eastern Paleo-Tethys domain,in contrast,the continental subduction/collision occurred at 250–220 Ma and post-collisional intracontinental rifting occurred at 140–120 Ma.Furthermore,metamorphic evolution from low T/P ratios in the subduction/collision stage to high T/P ratios in the intracontinental rifting stage needs 40–60 Myr in the Proto-Tethys domain but about 110 Myr in the Paleo-Tethys domain.For the two different orogenic domains,therefore,the convergent continental margins underwent a common tectonic evolution from warm collision/cold subduction to hot rifting,which starts from continental subduction/collision characterized by the formation of medium-P amphibolite to high-P granulite facies series or high-P to ultrahigh-P eclogite facies series in compressional regimes,through exhumation of the deeply subducted crustal rocks,and terminates with intracontinental rifting featured by highT to ultrahigh-T granulite facies series in extensional regimes.

Meso-Archean to Mid-Paleozoic Granitoids in Oulad Dlim Massif(the Pericratonic Terrane of the Reguibat Shield,West African Craton,Morocco):Petrology,Geochemistry,Geochronology and Geological Implications

The Oulad Dlim Massif,adjacent to the Reguibat Shield in South Morocco was considered up to now as part of the Variscan belt(Mauritanides)with a polyphase geologic history and a complex geodynamic evolution implicating oceans closures and accretion of exotic terranes(Avalonian and Meguman)during the Variscan-Alleghanian orogeny.The use of modern technology to characterize the petrology,the geochemistry and the geochronology of the lithological units forming this region,combined to field surveys has led to an updated geological architecture and different geological history.The Oulad Dlim Massif is mainly a deformed Archean terrane,as recorded by its eastern and western sectors,dominated in its central part by a bimodal felsic-mafic magmatism forming the Ediacaran sector.The study of these magmatic complexes supports strongly the intracontinental origin of this bimodal magmatism vs.the oceanic origin published before in literature.The exploration of this massif conducted also to the identification of a Silurian-Devonian sector in the western part.Therefore,up to date,different magmatic events lasting from the Meso-Archean to the Cretaceous are recorded in the Oulad Dlim Massif rocks,among them different generations of granitoids are reported.New data on granitoids from the Ediacaran sector are presented in this paper.This recent data demonstrates that Oulad Dlim Massif has been affected by the main Ediacaran–Cambrian extensional event widely documented in other structural domains of Morocco and other parts of North Gondwana.Additionally,the study of the Silurian-Devonian sector rocks highlighted the presence of a Caledonian tectonic event challenging the ideas about the paleogeography of this part of northwestern Africa and its geological evolution during the Paleozoic.However,despite the significant contribution of this extensive survey and the abundance of data on the Oulad Dlim Massif,more studies are required to reconstruct the puzzle at plate tectonic scale.