Composition And Evolution Of The Dunhuang Orogenic Belt

The Dunhuang orogenic belt,located in the east of the Tarim Craton,is northward to the Beishan orogenic collage and eastward to the Alxa block,and is bounded by the Altyn fault with Qilian orogen to the south.It is the tectonic junction of Tarim Craton,North China Craton,Paleo-Asian Ocean tectonic domain and Tethys tectonic domain,recorded a long and complicated geological evolutionary history.In this study,detailed field investigation,systematic petrological,geochronological and geochemical analyses were conducted on different rock units in the Dunhuang tectonic belt.Then,Temporal and spatial distribution of magmatic-metamorphic events was established,and crustal evolution and geodynamic settings of different stages of tectono-thermal events and their implications for regional tectonic evolution were discussed.Filed investigation,petrological and geochronological study reveal that the Dunhuang region has experienced tectono-thermal events in Archean,Proterozoic and Phanerozoic.The Archean tectono-thermal events are characterized by a series of magmatic activities;the Proterozoic tectono-thermal events are featured by magmatic activities and sedimentation-metamorphism of late Paleoproterozoic and magmatic activities of early Mesoproterozoic;while the Phanerozoic tectono-thermal events are marked by early Paleozoic magmatism-metamorphism and late Paleozoic magmatic activities.Accordingly,we set up the geological composition and evolution history of the Dunhuang terrane and proposed that it belongs to an orogen of the southernmost margin of the Central Asian Orogenic Belt(CAOB),rather than a stable Precambrian crystalline basement.The Archean magmatic activities produced TTG rocks at?3.1 Ga,?2.7 Ga?2.6 Ga and?2.6 Ga?2.5 Ga,and high-K granite at?2.5 Ga.The granodiorite formed at?3.1 Ga is the oldest rock discovered in study area so far.In addition,Zircon Hf isotope compositions of this rock suggest that there may be Eoarchean continental crustal materials in Dunhuang region.Both the tonalite-granodiorite formed at?2.7 Ga?2.6 Ga and tonalite formed at?2.6 Ga?2.5 Ga were derived by partial melting of the subduction-related juvenile oceanic crust.The relatively high Mg#values,Cr and Ni contents imply addition of juvenile materials.Besides,zircons of these TTG rocks show large portion of positive ?Ht(t)values with two-stage Hf model ages approximite to their crystallization age,indicating crustal growth.Whereas,small portion of nagative ?Hf(t)values suggest reworking of ancient continental crust.The high-K granite formed at?2.5 Ga belongs to calc-alkline I-type granite,origanated from partial melting of thickened lower crust in subduction zone and showing signatures of island arc rocks.The similar zircon Hf isotope compositions of this rock to those of tonalite formed at?2.6 Ga?2.5 Ga suggest similar crustal evolution processes.The Paleoproterozoic magmatic activities and sedimenatry-metamorphic events produced a series of complicated metamorphic complex.The metamorphosed magmatic complex includes tonalitic gneisses formed at-2.0 Ga,high-K granitic gneisses formed at?1.95 Ga,trondhjemitic gneisses formed at 1.88 Ga?1.82 Ga and metabasaltic rocks formed at 1.88 Ga?1.82 Ga.The trondhjemite and tonalite,formed under an island arc setting in active continental margin of subduction zone,were derived from partial melting of subducted oceanic crust.The high-K granite were produced by partial melting of thickened lower crust in subduction zone.The basaltic rocks were formed at the island arc and back-arc setting.The Paleoproterozoic metasedimentary rocks were deposited in a basin adjacent to a continental arc,with provenance of clastic sedimentary rocks dominated by Archean-Paleoproterozoic felsic to intermediate rocks in the Dunhuang orogenic belt.These metasedimentary rocks probably have the maximum depositional age of ca.1.86 Ga and then metamorphosed at ca.1.83-1.80 Ga.The high-pressure mafic granulite,only reported in the Shuixiakou area,displays a clockwise P-T-t path related with subductional-collisional orogenesis.The above rock assemblages imply that the Dunhuang orogenic belt was located in an active continental margin environment and underwent the convergent-collisional orogeny in Paleoproterozoic.Zircon Hf isotope compositions reveal that the Paleoproterozoic tectono-thermal event was characterized by reworking of Archean continental crust,accompanied with certain degree of crustal growth.The Mesoproterozoic magmatic activities are represented by high-K granites formed at 1.8 Ga?1.7 Ga and basic rocks formed at?1.6 Ga.The high-K granites are geochemically A-type granite,originated from partial melting of lower crust during post-orogenic evolution.Zircons from high-K granites mostly show nagative ?Hf(t)values,with small portion showing positive?Hf(t)values,indicating that this magmatic process is dominated by reworking of ancient continental crust,with addition of a small volume of juvenile materials.The basic rocks,formed under post-orogenic tectonic setting of compression to extension,were produced by upwelling of mantle materials.Zircons from these basic rocks mostly have positive ?Hf(t)values,with only a very few displaying negative ?Hf(t)values,implying that this magmatic activity is mainly of a custal growth process,mixed with minor amount materials of contimental crust.In the field,these Mesoproterozoic post-orogenic magmatic rocks are very dispersed and limited,representing the ending of the orogeny during Paleoproterozoic-Mesoproterozoic and the final formation of the Precambrian crystalline basement in the Dunhuang orogenic belt.The early Paleozoic magmatic activities and metamorphic events produced a large amount of intermediate-acidic intrusive rocks with formation age of-450 Ma-410 Ma and metabasic rocks with metamorphic age of 440 Ma?400 Ma,respectively.The intermediate-acidic intrusive rocks are characterized by a suite of gabbro diorites,diorites,granodiorites and granites,belonging to calc-alkline I-type granitic rocks of island arc setting.They were generated by partial melting of subducted oceanic crust or partial melting of the overlying continental crust above subduction zone.The oceanic crust-derived melts underwent varying degrees of interaction with the mantle peridotite when migrated upward to the mantle wedge.Zircon Hf isotopes and field distribution of those Paleozoic intrusive rocks reveal that the Silurian magmatic activities predominantly represent crustal growth process,accompanied by different degrees of reworking of Archean-Mesoproterozoic continental crust.Based on mineral assemblages,the metabasic rocks are chiefly of amphibolites,garnet-bearing amphibolites,garnet-clinopyroxene amphibolite and garnet-mica amphibolites.The protoliths of these metabasic rocks are mostly of MORB-like tholeiitic basaltic rocks.However,the metabasic rocks exposed in Archean TTG gneisses display signatures of island arc tholeiitic basaltic rocks.These metabasic rocks generally experienced high-amphibolite-granulite facies metamorphism,with individual rocks even underwent eclogite facies metamorphism.The entirely clockwise P-T-t evolution paths indicate a completed subduction-collision-exhumation orogenic process,corrresponding to the island arc magmatic activities in early Paleozoic.The late Paleozoic magmatic activities are cahracterized by intermidiate-acidic intrusive rocks crystallized in late Devonian(370 Ma?360 Ma)and alkali-rich granites crystallized in middle Carboniferous(335 Ma?315 Ma).The late Devonian intermidiate-acidic rocks,chiefly consisting of diorites,monzodiorites,granodiorite and granites,were formed under island arc setting of subduction zone,showing features of island arc calc-alkline I-type granitic rocks.Zircon Hf isotope compositions reveal that the late Devonian magmatic activities represent crustal growth processes in the Dunhuang tectonic belt,accompanied by reworking of various proportions of Archean-Neoproterozoic continental crust.The middle Carboniferous alkali-rich granites,possibly formed in a transitional tectonic setting from compression to extension,were produced through partial melting of thickened lower continental crust.Zircons from these alkali-rich granites have negative ?Hf(t)values,implying that the middle Carboniferous magmatic activity represents crustal reworking event.The late Devonian subduction-related intermediate-acid intrusive rocks and middle Carboniferous post-collisional granitoids indicate an orogenic cycle during late Paleozoic.Based on the above findings and regional tectonic evolution history,compositions of the Dunhuang orogenic belt can be divided into two basic rock units of Precambrian crystalline basement rocks(Dunhuang Complex)and Phanerozoic magmatic-metamorphic complexes.The former includes metamorphosed magmatic rocks(TTG rocks and high-K granites)and metasupracrustal rocks,whereas the latter includes the widely exposed magmatic complexes-high-grade metamorphic complexes of early Paleozoic and magmatic complexes of late Paleozoic.These results reveal that since the formation of Precambrian crystalline basement,there has existed nearly 1.2 Ga shutdown of tectono-thermal event in the Dunhuang tectonic belt until the beginning of Silurian(?450 Ma?440 Ma),without any tectono-thermal events and sedimentary events during this quiet interval.The Dunhuang region has experienced the Proterozoic and Phanerozoic orogenic processes.The Proterozoic orogeny is represented by island arc magmatic complex,metasupracrustal rock series and high-pressure mafic granulite of Paleoproterozoic(2.0 Ga?1.8 Ga)and post-orogenic magmatic complex of early Mesoproterozoic(1.8 Ga?1.6 Ga).The Phanerozoic orogeny is represented by the widely exposed intermidiate-acidic intrusive rocks of Silurian-Carboniferous and metabasic rocks of Silurian.Furthermore,the Dunhuang tectonic belt stretches as a long strip terrain along the NEE-SWW direction,corresponding to the strike of regional tectonic line or/and regional gneissosity.Additionally,the Precambrian crystalline basement rocks generally underwent the early Paleozoic metamorphism in 440 Ma?400 Ma.These geological facts also provide evidence for the Paleozoic orogeny.Studies show that the Dunhuang tectonic belt is a Paleozoic accretionary orogenic belt of the southermost margin of CAOB,rather than a stable block.First,the clockwise P-T-t paths for the Paleozoic metabasic rocks indicate a completed subduction-collision-exhumation orogenic process.In addition,Paleozoic tectono-thermal events in its adjacent tectonic belts reveal that the Dunhuang orogenic belt has undergone the similar tectonic evolution history to the South Tianshan Belt.Therefore,we propose that the Dunhaung orogenic belt belongs to Paleozoic accretionary orogen,having experienced the early Paleozoic orogeny during Silurian and the late Paleozoic orogeny during Devonian-Carboniferous.The early Paleozoic orogeny is featured by calc-alkaline I-type granitic rocks formed in Silurian and high-amphibolite-granulite facies metabasic rocks metamorphosed contemporarily.The late Paleozoic orogeny is cahracterized by calc-alkaline I-type granitic rocks formed in Devonian and post-orogenic alkali-rich granites formed in Carboniferous.This accretionary orogen represents the southernmost margin of the CAOB between the Tarim Craton and the North China Craton,and tectonically extends northward to the Beishan orogen and westward to the eastern South Tianshan Belt.Zircon Hf isotope analyses,together with whole rock geochemical data,reveal that continental crust of the Dunhuang orogenic belt was predominantly formed in Archean.However,crustal growth events in the Proterozoic and Phanerozoic orogenic processes are also significant and considerable,especially in oceanic crust subduction stage and post-orogenic basaltic magmatic process.Given the above,the crustal evolution history in Archean and subductional-collisional orogenic process in late Paleoproterozoic of the Dunhuang orogenic belt are similar to those of Tarim and North China Cratons.However,since the final formation of the Precambrian crystalline basement,the Dunhuang orogenic belt has experienced distinct tectonic evolution history with Tarim Craton and North China Craton.There were no any tectono-thermal events and sedimentary events recorded during the period of 1.6 Ga?0.45 Ga.Until the beginning of Silurian(?450 Ma?440 Ma),the Dunhuang tectonic belt was strongly involved into and took part in the accretionary orogenic process related to the closure of the southern margin of Paleo-Asian Ocean,having formed series of orogeny-related intermediate-acidic intrusive rocks and high-amphibolite-eclogite facies metabasic rocks.This Paleozoic orogenesis actually reflected basin-mountain coupling relation between Tarim Craton and CAOB,and joint of Paleo-Asian tectonic domain with Tethyan tectonic domain.