Model Forming of Mesoarchean Gridino Mafic Dyke Swarm during Subduction "Mid-Ocean Ridge – Continent"

The high-pressure metamorphosed Gridino dyke swarm comprises a major group of Mesoarchean 2.87-2.82 Ga mafic dykes intruded within the Mesoarchean continental crust of the Kola craton(the Belomorian tectonic province

An Intra-Oceanic Subduction System Influenced by Ridge Subduction in the Diyanmiao Subduction Accretionary Complex of the Xar Moron Area,Eastern Margin of the Central Asian Orogenic Belt

This study focuses on the geology,geochemistry,Sr-Nd isotopes and their tectonic settings of three types of basalts in Diyanmiao ophiolite in the Xar Moron area located on the eastern margin of the Central Asian Orogenic Belt.Type I basalts are oceanic tholeiites with a depleted light rare earth element(LREE)pattern,which are similar to the typical N-mid-oceanic ridge basalt(MORB)and suggests that they were formed at a mid-oceanic ridge.The initial 87Sr/86Sr ratios of Type I basalts range from 0.703966 to 0.705276 and theεNd(t)values are from 16.49 to 17.15,indicating that they were derived from a depleted mantle source.Type II basalts belong to the medium-potassium calc-akaline series and have the geochem-ical characteristics of Nb-enriched basalt(NEB)with high Nb content(14.5 ppm)and strong enrichment in LREEs,implying that they were created by the partial melting of mantle wedge peridotite that previously metasomatized by slab melts.Type III basalts are high-Al basalt(HAB)with high-Al contents(Al_(2)0_(3)=16.75 wt.%-18.00 wt.%),distinct Nb depletion and high Th/Yb ratios.Thus they were likely gen-erated in a normal island-arc setting.Therefore,the association of MORB,NEB,and HAB in the study area may be due to the subduction of a mid-oceanic ridge,and the Diyanmiao ophiolite is proposed to be formed in the forearc setting of a mid-oceanic ridge subduction system.

Key geodynamic processes and driving forces of Tethyan evolution

Tethys tectonic system has experienced a long-term evolution history,including multiple Wilson cycles;thus,it is an ideal target for analyzing plate tectonics and geodynamics.Tethyan evolution is typically characterized by a series of continental blocks that separated from the Gondwana in the Southern Hemisphere,drifted northward,and collided and accreted with Laurasia in the Northern Hemisphere.During this process,the successive opening and closing of multistage Tethys oceans(e.g.,Proto-Tethys,Paleo-Tethys,and Neo-Tethys)are considered core parts of the Tethyan evolution.Herein,focusing on the life cycle of an oceanic plate,four key geodynamic processes during the Tethyan evolution,namely,continental margin breakup,subduction initiation(SI),Mid-Ocean Ridge(MOR)subduction,and continental collision,were highlighted and dynamically analyzed to gather the following insights.(1)Breakup of the narrow continental margin terranes from the northern Gondwana is probably controlled by plate subduction,particularly the subduction-induced far-field stretching.The breakup of the Indian continent and the subsequent spreading of the Indian Ocean can be attributed to the interactions between multiple mantle plumes and slab drag-induced far-field stretching.(2)Continental margin terrane collision-induced subduction transference/jump is a key factor in progressive Tethyan evolution,which is driven by the combined forces of collision-induced reverse push,far-field ridge push,and mantle flow traction.Moreover,lithospheric weakening plays an important role in the occurrence of SI.(3)MOR subduction is generally accompanied by slab break-off.In case of the considerably reduced or temporary absence of slab pull,mantle flow traction may contribute to the progression of plate subduction.MOR subduction can dynamically influence the overriding and downgoing plates by producing important and diagnostic geological records.(4)The large gravitational potential energy of the Tibetan Plateau indicates that the long-lasting India-Asia continental collision requires other driving forces beyond the far-field ridge push.Further,the mantle flow traction is a good candidate that may considerably contribute to the continuous collision.The possible future SI in the northern Indian Ocean will release the sustained convergent force and cause the collapse of the Tibetan Plateau.Based on the integration of these four geodynamic processes and their driving forces,a“multienginedriving”model is proposed for the dynamics of Tethyan evolution,indicating that the multiple stages of Tethys oceanic subduction provide the main driving force for the northward drifting of continental margin terranes.However,the subducting slab pull may be considerably reduced or even lost during tectonic transitional processes,such as terrane collision or MOR subduction.In such stages,the far-field ridge push and mantle flow traction will induce the initiation of new subduction zones,driving the continuous northward convergence of the Tethys tectonic system.