Spatial-temporal Patter And Geological Implications Of Early Paleozoic-Early Mesozoic Granitoids In The East Kunlun Orogenic Belt (Eastern Segment)

1.The East Kulun orogenic belt experienced a multi-phase,multi-stage and long complex tectonic evolution,accomponding activities of granitic magma.The granitoid studies on petrogenesis including source rock research for various-stage granites involved the formation of the East Kunlun orogenic belt,in particular the geodynamic processes and evolution of the each stages magmatic activity.Eight granite bodies in east section of the East Kunlun were selected to be studied in the thesis.These plutons were distributed on the entire structural zones.Using the methods of systematic geochemical,zircon geochronology and Hf isotope geochemistry,we divided stages of magmatic activity,and further clarified the material source and petrogenesis of granites and geodynamic significance.2.In this thesis we obtained ages from eight representative granite plutons by the method of LA-ICP-MS Zircon U-Pb.Combined with regional geological setting the granitoid magmagtism can be divided into early Paleozoic and late to Early Mesozoic periods according to zircon U-Pb ages.The first granitoid magatism period could be classified into three groups:the first group covering the time span from Early Silurian to Middle Silurian?450⁴33Ma?,is mainly comprised by early Silurian diorite-granodiorite and Middle Silurian monzegranite;the second group emplaced during late Silurian?424⁴17Ma?,consisting of two mica granite which exposed in the north of the south tectonic zone and granodiorite which exposed in the south of south tectonic zone;the third group is middle Devonian orthoclase.The late granitoid magmatism period could be classified into three groups also:the first group covering the time span from late Permian to Middle Triassic?260²40Ma?,consisting of granodiorite and monzegranite;the second group emplaced Middle Triassic consisting mainly of syenogranite porphyry;the third group is late Triassic granitoids?230²20Ma?with main lithology of monzegranite and granodiorite.3.Based on the elemental and isotopic compositions,the different groups are characterized by different rock and petrogensis.For the first period,the first group is metaluminous high-K calc-alkaline I-type granitoids,and exhibits the typical geochemical characteristics of adakites which were derived from partial melting of newly underplated basaltic crust.In the second group,the two mica granite is characterized by high Na₂O contents and strongly peraluminous,which was derived from partial melting of a source region amphibolite with subordinate metapelite.The granodiorite exposed in the south of south structural zone exhibit the typical geochemical characteristics of adakites,which was derived from partial of newly underplated basaltic crust too.The third group is similar the geochemical feature of A–type granites,which was derived from the partial of crust materials meta-greywacke source.For the second period of magmatism,the first group is metaluminous,high-K calc-alkaline I-type granitoids and exhibit the typical geochemical characteristic of adakite,which was derive from partial melting of lower crust mainly composed of Precambrian metabaslic basement rocks.The monzegranite is similar classic high-K calc-alkaline I-type granitoids,which derived from partial melting of a source of region from Neoprotozoic meta-greywacke and meta-amphibolite.The second group is exhibit the geochemical characteristic of A–type granitoids,which derived from partial melting of Mesoproterozoic meta-greywacke.The last group is metalumious high-K calc-alkaline I-type granitoids.The U-Pb dating results show indicated that the monzegranite formed earlier than the granodiorite.On the Harker diagram,the component of magma shows a change trend from acidic to moderate,implied the source region of the different batches of magma emplacement.4.Based on above studies,we discussed the tectonic setting of different stages granitic magmatism and obtained the following conclusions:?1?The Paleo-Kunlun Ocean located at the north side of the South Kunlun structural zone began to southward dipping at the late Ordovician and then the ocean is closed in late Silurian.The early to middle Silurian granitoids were formed in subuduction-related setting.?2?During the late Silurian,the terranes occurred collision between the south and north structural zone,and the two mica granite was formed in collision setting.The Proto-Tethys Ocean located at the south side of the South Kunlun structural zone was dominated by northward subduction during the late Silurian and persisted to Middle Devonian.The late Silurian granodiorite were form in subduction-relating setting and the Middle Devonian A-type granite were form in a tectonic extension setting related the northward subduction of the Proto-Tethys.?3?The Paleo-Tehtyan Ocean is closed in Middle Triassic,and the late Permian to Early Triassic granitoids were formed in subdution-relate setting.As early as the late of Middle Triassic,the tectonic setting transited into post collisional extension,the role of mantle derived magma strengthen again.During the late of the middle Triassic the mantle magma heated the lower crust inducing the partial melting of upper part felsic greywacke in the lower crust,with formation of the Middle Triassic A-type granite.In the late Triassic the deep mantle magma upwelled and underplated to the lower crust,leading the lower crust to melt4.The granite in the KEOB show a limit range of Sr-Nd isotopic component,suggested granitic magma derived from a source with stable isotopic composition.However,the Hf isotopic component exhibit a range from depleted source to enriched source implied a mixed with juvenile and ancient materials in the source.In this study,we consider that the basic magma underpalting and curst-mantle mixing is the main mechanism for the origin of large-scale granite.The ideal place of curst-mantle mixing is the crust-mantle transition layer.The process of a persisted mantel magma underplating and accompanying a partial melting of basaltic lower crust resulte a dynamic equilibrium.