Petrogenesis And Tectonic Setting Of Paleozoic Felsic Intrusions In The Eastern Tianshan

The Central Asian Orogenic Belt?CAOB?is one of the largest Phanerozoic accretionary orogenic belts in the world.It was experienced long-lived?Early Neoproterozoic to Late Paleozoic?subduction–accretion evolution involving multiple accretions of island arcs,accretionary prisms,seamounts,ophiolites and microcontinents within the Paleo-Asian Ocean.It is generally accepted that the subduction–accretion evolution process of the southern part of the CAOB was later than the northern part,and the initial subduction of the Paleo-Asian Ocean in the West Junggar?southern part of the CAOB?might occur before the Early Cambrian?>515Ma?.To the south of Junggar,Eastern Tianshan?or Eastern Tienshan and Eastern Tianshan Orogenic Belt in some literatires?is the southernmost part of the CAOB and also has experienced very complex geodynamic evolution.It has recorded the latest stage of the subduction–accretion evolution process of the CAOB.Therefore,the Early Paleozoic is a key stage of subduction initiation in this area.Because the closure of the Paleo-Asian Ocean in the Eastern Tianshan might occur in the Late Paleozoic,geological evolution of the Eastern Tianshan in the Late Paleozoic is important to understanding the multiple accretions and final formation of the southern CAOB.In this dissertation,we present new LA–ICP–MS zircon U–Pb ages,whole–rock geochemistry,and Sr–Nd isotopic data for the Early Paleozoic diorites and granites in the northern Eastern Tianshan?Harlik and Dananhu belt?,as well as for the Late Paleozoic diorites and granites in the southern Eastern Tianshan?Yamansu belt and eastern Central Tianshan Block?.The results provide new constraints on not only the Early Paleozoic tectonic setting of the Eastern Tianshan,but also the Late Paleozoic geodynamic evolution of the Eastern Tianshan and the final formation of the southern CAOB.The two dioritic plutons in the northern Eastern Tianshan were generated in the Late Ordovician?452±4 Ma?and the Early Silurian?442±3 Ma?,respectively.Diorites from the two plutons are characterized by enrichments in large ion lithophile elements?LILE?and highly incompatible elements,with depletions in high field strength elements?HFSE?displaying typical geochemical features of a subduction-related origin.They have positive?_Nd?t?values?+5.08–+6.58?,relatively young Nd model ages(T_DM=0.71–1.08 Ga),with Ta/Yb?0.05–0.09?and Nb/Ta ratios?12.06–15.19?similar to those of depleted mantle,suggesting a juvenile mantle sources origin.The dioritic samples have relatively low Ce/Th?4.57–14.7?and Ba/Th?47.8–235?ratios,suggest an input of subducted sediment-derived melts.Meanwhile,they show relatively high Ba/La?13.3–35.9?and low Th/Yb?0.72–2.02?ratios,indicating a magmatic source metasomatized by slab-derived aqueous fluids.Therefore,these diorites were probably produced by partial melting of a depleted mantle wedge metasomatized by both subducted sediment-derived melts and slab-derived aqueous fluids.Zircon U–Pb dating of a granitic pluton in the northern Eastern Tianshan yielded a Late Ordovician intrusion age of 447±5 Ma.Granites from this pluton show calc-alkaline compositions with geochemical characteristics of I-type granites.They also show positive?_Nd?t?values?+6.49 to+6.95?and young Nd model ages(T_DM=0.69–0.87 Ga),indicating that the granites were most likely derived from juvenile lower crust.Our new dating results on the dioritic and granitic plutons suggest that arc-type magmatism in the northern Eastern Tianshan began prior to the Late Ordovician?452–442 Ma?.In addition,north-dipping subduction of the Kangguer oceanic lithosphere may account for the arc-type magmatism and the geodynamic process of the Eastern Tianshan in the Early Paleozoic.Late Paleozoic,however,is a key period for the accretion and collision of the southern Central Asian Orogenic Belt.Here,we present new zircon U–Pb ages,whole-rock geochemistry and Sr–Nd isotopic data for four Late Paleozoic felsic plutons in southern Eastern Tianshan?Yamansu belt?in order to constrain the tectonic evolution of the southern CAOB.The granodioritic pluton and its dioritic enclaves were synchronously formed in the Early Carboniferous?336±3 Ma and 335±2 Ma,respectively?.These rocks are depleted in Nb,Ta and Ti,and enriched in Rb,Ba,Th and U related to the primitive mantle,which show typical features of arc rocks.They both have similar Sr–Nd isotopic ratios to those granitic rocks from the eastern Central Tianshan Block and have the latest Mesoproterozoic two stage Nd model ages(T²_DM)?1111–1195 Ma for the granodioritic pluton and 1104–1108 Ma for the enclaves,respectively?,indicating that their source magmas may have been derived from the Mesoproterozoic crust.The albitophyric pluton was also emplaced in the Early Carboniferous?333±3 Ma?.Rocks of this pluton have similar?_Nd?t?values?–0.69 to–0.37?and T²_DM ages?1135–1161 Ma?to those of the granodioritic rocks,suggest similar crustal source for both types of rocks.In contrast,the K-feldspar granitic and monzonitic plutons were emplaced in the Early Permian?292±3 Ma and281±2 Ma,respectively?.Samples of the K-feldspar granitic pluton have high K₂O+Na₂O,FeO/MgO,Ga/Al,HFSE?e.g.,Zr and Hf?and low CaO,Sr and Ba,exhibiting characteristics of A₂-type granites,which probably emplaced in a post-collisional extension environment.The monzonitic granites are metaluminous to weakly peraluminous with A/CNK ranging from 0.93 to 1.05,and have very low P₂O₅,indicating characteristics of I-type granites.They have higher?_Nd?t?values?+2.22 to+3.27?and more juvenile T²_DM ages?799–878 Ma?than the Early Carboniferous plutons,suggesting that they were derived from relatively younger crustal sources.Based on an isotopic mixing simulation,significant mantle contributions were added to the magma source of both the Early Carboniferous and the Early Permian felsic rocks.The mantle contribution changes from⁶0%in the Early Carboniferous to⁷5%in the Early Permian.The remarkably increasing of mantle materials in the magma source of the felsic rocks in the Aqishan-Yamansu belt was most likely induced by the tectonic transition from an Early Carboniferous continental arc to an Early Permian post-collisional extension environment.Besides,we report new whole–rock geochemical and Sr–Nd isotopic data,and in situ zircon U–Pb ages for three typical A-type granitic intrusions?syenogranite,K-feldspar granite and monzonitic granite?and one dioritic pluton in the eastern Central Tianshan Block,NW China,in order to constrain their petrogenesis,tectonic setting and geodynamic evolution.Zircon LA-ICP-MS U–Pb ages indicate that the syenogranites and K-feldspar granites were both emplaced during a short time interval in the Early Permian?284–286 Ma?,whereas the diorites were crystallized at 293±6Ma.The monzonites were generated at 307±3 Ma,earlier than the other three granitic intrusions.The three kinds of granites?syenogranite,K-feldspar granite and monzonitic granite?have high HFSE,HREE and Y/Nb ratios?1.68–2.84?,and remarkably low Sr and Ba contents,with the absence of aluminium-rich minerals and show metaluminous to weakly peraluminous natures,indicating geochemical characteristics of aluminous A₂-type granites.These granites show negative?_Nd?t??–3.32 to–4.40?values and Mesoproterozoic two stage Nd model ages(T_DM²=1.36 to1.43 Ga),which suggest a relatively old crustal source.The Sr–Nd isotopic compositions imply that the Mesoproterozoic metamorphic basement of the Central Tianshan Block can be considered as a potential magmatic source for these aluminous A₂-type granites.In addition,they have geochemistry characteristics similar to the experimental melt that was derived from partial melting of crustal metaigneous rocks at the depth of middle to lower crust levels.Therefore,the granites were most likelyproduced by partial melting of metaigneous rocks in a post-collisional extensional setting.The diorites have low SiO₂ and high MgO,with uniform(⁸⁷Sr/⁸⁶Sr)_i values?⁰.7055?,negative?_Nd?t?values?–4.11 to–4.59?and Mesoproterozoic two stage Nd model ages(T²_DM=1.41 to 1.45 Ga).They have higher Ti/Zr?42.8–46.7?and Ti/Y?280–359?ratios than continental crust and show high Ta/Yb ratios?0.23–0.27?,Nb/Yb ratios,and negative Nb-Ta-Ti anamalies,which can be explained by partial melting of a relatively fertile mantle modified by involvements of old crustal materials of the Central Tianshan Block.The formation of the dioritic pluton was induced by an upwelling asthenosphere in an extensional geodynamic setting.Based on the zircon saturation temperatures of these A-type granites and the distribution of Late Carboniferous to Early Permian mantle-derived intrusions,a slab breakoff model is suggested to interpret the formation of the studied A-type granites and diorites.We infer that the slab breakoff was initiated in the Late Carboniferous and completed in the Early Permian.