Anatexis Of Accretionary Wedge: Perspectives On Silurian-Devonian Granitoids Petrogenesis And Crustal Evolution In The Chinese Altai

The Central Asian Orogenic Belt?CAOB?is the largest Phanerozoic accretionary orogenic system on Earth.The Altai,located in the hinterland part of the CAOB,evolved from an Ordovician accretionary wedge into a mature Devonian orogenic belt,and can provide important clues regarding transformation of accretionary wedge sediments into stable and mature continental crust in active continental margins.However,the exact mechanism of crustal differentiation and formation of mature continental crust in the Altai remains obscure.In the Chinese Altai,voluminous Silurian-Devonian granitoids?more than 45%of the exposed outcrop?intruded a greywacke-dominant Ordovician wedge sedimentary succession?the Habahe Group?,the latter is the oldest and most extensive sedimentary sequence in the region.It has been shown that the Habahe Group rocks underwent extensive anatexis during Devonian,and exhibited geochemical features similar to local Silurian-Devonian granitoids.The possibility that the local granitoids were derived from the regional anatexis of the Ordovician accretionary wedge warranted further investigation.The study could shed light on the role of crustal melting in formation of mature continental crust in accretionary orogens.To fully understand the Devonian magmatic evolution of the Chinese Altai,detailed field geological observations,phase equilibrium modelling and comprehensive geochemical studies were carried out in the current work.It is shown that the Habahe Group is mainly consisted of a felsic terrigenous component and a volcanogenic component.The Habahe Group underwent high temperature metamorphism at Devonian times resulting in extensive anatexis of them in the region.Geological studies revealed a close spatial relationship between the regional anatexis of the Habahe Group and the formation of the granitoids,which is characterized by a gradual textural evolution from the partial molten the Habahe Group rocks to the granitoid bodies.Geochemical studies of the Habahe terrigenous components reveals that they are chemically immature and compositionally similar to metagreywacke.These components are rich in large ion lithophile elements and depleted in high field strength elements,comparable to arc-related rocks.In contrast,the Habahe volcanogenic components have compositions varied from basalt to basaltic andesite in terms of their SiO₂ concentrations?46.30%⁵2.56%?.They have relatively high MgO?5.32%⁸.18%?and Na₂O+K₂O?3.13%⁵.35%?contents,and moderate-high Al₂O₃?13.69%¹7.15%?contents,and the trace element compositions are similar to those of the terrigenous components,but have weak Eu anomalies and flat REE patterns.Systematic investigations show that the Silurian-Devonian granitoids are mainly consisted of biotite-granodiorite and granite?about 85%?,associated with subordinate hornblende-bearing granodiorite and tonalite?about 15%?.Both of them show calc-alkaline and peraluminous-metaluminous geochemical characteristics.These granitoids exhibit obvious arc-related features,including strong enrichment of large-ion lithophile elements relative to many of the high field strength elements.Most granitoids in the Chinese Altai are compositionally comparable to partial melts derived from immature sediments dominated by metagreywacke.It is worth noting that the Habahe Group rocks are compositionally similar to metagreywacke.Nd isotopic comparison shows that the?_Nd?400Ma?of the Habahe Group terrigenous components vary from-6.1 to+2.6,and T_2DM vary from 0.9 to 1.6 Ga,overlapping with,or somewhat less radiogenic than,most local granitoids.In contrast,the?_Nd?400Ma?values of the Habahe volcanogenic components range from+4.1 to+9.1,chemically more primitive than the granitoids.The?_Nd?400Ma?values of Silurian-Devonian granitoids range from-4.2 to4.8,with T_2DM varying from 0.8 to 1.5 Ga.The Nd isotopic data of the granitoids exactly fall in the gap between those of Habahe terrigenous and volcanogenic components.It is obvious that the Nd isotopic signatures of nearly all granitoids can be attained by mixing in the source of terrigenous and volcanogenic components in various proportions,supporting the view that the Habahe Group rocks could potentially be the source of the local Silurian-Devonian granitoids.The thermodynamic phase equilibrium of volcanogenic components of the Habahe Group show that,partial melting of these intermediate-basic components can produce voluminous granodioritic to tonalitic melts?up to 40 vol.%?.The compositions of partial melts are comparable with local hornblende-bearing granodiorite and tonalite.Modelled trace element compositions of the melts are obviously similar to those of hornblende-bearing granodiorite and tonalite.In summary,there is an obvious spatial-temporal correlation between the anatexis of the Habahe Group and the Silurian-Devonian granitoids;they show similarities in their geochemical compositions;the Nd isotopic comparison also shows that anatexis of the Habahe Group rocks could account for the isotopic characteristics of the local granitoids rocks.The thermodynamic phase equilibrium results indicate that felsic components of the Habahe Group could produce melts compositionally similar to the dominant biotite-granodiorite and granite,and partial melting of the volcanogenic components could contribute to the formation of subordinate hornblende-bearing granodioritic to tonalitic melts.Therefore,a new genetic model for granitoids in the area is proposed in this paper,that is,magmatic recycling of the fertile Habahe Group rocks.Accordingly,the Nd-Hf decoupling of the Silurian-Devonian granitoids in the study area is re-examined in this work.Combined with pre-existing data,our newly obtained Nd-Hf isotopic data of the Habahe Group show significant Nd-Hf isotopic decoupling in the Habahe Group rocks.In other words,the Nd-Hf decoupling in the Habahe Group rocks is the primary causative factor leading to the prevailing Nd-Hf isotopic decoupling of the Silurian-Devonian granitoids in the Chinese Altai.In conclusion,the current study further shows that young accreted wedge metasediments in the Chinese Altai can also be a source of the"arc-like"Silurian-Devonian granitoids in the region.Our work shows that melting and subsequent crustal differentiation can transform accretionary wedge sediments into mature and stable continental crust.This may be another important mechanism contributing to the peripheral continental stabilization and growth in accretionary orogenic belts worldwide.