Petrogenesis And Tectonic Setting Of Middle Palaeozoic Basic Intrusive Rocks In Kuruktag Area,Xinjiang

At present,there are still disputes about whether active continental margins existed in the northern margin of Tarim on the southern side of the South Tianshan Ocean Basin during the Paleozoic,which to some extent restricted people's accurate understanding of the tectonic evolution process of the South Tianshan orogenic belt.The Kuruktag area in the northeastern margin of the Tarim craton,the tectonic location is very important.It is affected by the superimposition of multiple tectonic movements,which makes the magmatic activity frequent and the intrusive rocks widely distributed in the area.However,previous studies have paid less attention to the Paleozoic basic intrusive rocks in this area.As a result of partial melting of the mantle,basic magma can better constrain the tectonic setting of its formation and itself is also a direct product of crustal growth,which is of great significance for exploring the chemical properties of the mantle and tracing the deep process of the lithosphere.In this paper,the basic intrusive rocks?dikes?in the western Kuruktag section are selected as the research objects.On the basis of detailed field work,systematic LA-ICP-MS zircon U-Pb chronology,whole rock geochemistry and Sr-Nd-Hf isotope studies are carried out to determine their genesis,and their tectonic background is discussed in combination with the existing geological data in the region.It is expected to provide new evidences and constraints for the tectonic evolution of the southern Tianshan orogenic belt and the crustal growth in the northern margin of Tarim.The main achievements are as follows:?1?The ages of three basic intrusive rocks?dikes?in the western Kuruktag section are433.4±1.6Ma?olivine gabbro dike?,411±5Ma?gabbro dike?and 410.3±5.7Ma?olivine gabbro rock?respectively,all of which are the products of mantle-derived magmatism in the middle Paleozoic.Based on the regional research results of predecessors,it can be revealed that there was a significant magmatic activity in the Kuruktag area in the northern margin of Tarim in the middle Paleozoic.?2?The olivine gabbro rocks?dikes?has relatively low content of SiO₂,relatively high content of MgO and Mg^#value?72.27-79.01?.Compared with olivine gabbro rocks?dikes?,the content of SiO₂ in gabbro rocks?dikes?increases significantly,while the content of MgO and Mg^#value?46.34-62.44?decrease significantly.At the same time,with the increase of SiO₂ content,the other major elements of rocks?dikes?also show a significant positive and negative correlation with SiO₂.In the trace element distribution diagrams,the olivine gabbro rocks?dikes?and gabbro rocks?dikes?shows the characteristics of enrichment of LREE and LILE,depletion of HREE and HFSE.Eu shows the trend of change from positive to negative anomalies.Besides,the one-stage Hf model age of olivine gabbro rocks and gabbro rocks?dikes?is basically the same.The above characteristics indicate that the basic intrusive rocks?dikes?may be formed by the evolution of homologous magma.In addition,the olivine gabbro rocks and gabbro dikes have a wide range of variation(⁸⁷Sr/⁸⁶Sr)_i values?0.703663-0.707838?and a range of positive and negative values of?_Nd?t??-2.99-+4.98?,as well as a positive and wide range of?_Hf?t??+3.1-+12.5?,suggesting that the magma of the basic intrusive rocks?dikes?originated from a relatively depleted mantle source area.Combined with the Lu-Hf isotope of zircon,the main and trace elements and Sr-Nd isotope characteristics of the whole rock,it is shown that the basic intrusive rocks?dikes?originated from partial melting of the depleted mantle wedge,which had undergone metasomatism induced by the fluid from the subducted plate.The parental magma has suffered a low degree of crustal contamination and weak crystallization differentiation during its evolution process.?3?In the middle Paleozoic,the Southern Tianshan Ocean experienced a stage of southward subduction.There was an active continental margin magmatic belt in the Kuruktag section of the northern margin of Tarim during Silurian-Devonian period.The olivine gabbro rocks?dikes?-gabbro rocks?dikes?are also a part of this magmatic belt.It is also the product of magmatic activity on the active continental margin and together constitutes the middle Paleozoic intrusive rock assemblage in Kuruktag area,i.e.olivine gabbro-gabbro-quartz diorite-granodiorite-granite.Based on the results of previous studies in the region,this paper holds that the genesis of the suite of intrusive rock assemblages can be explained as follows:during the process of subduction of the South Tianshan Ocean to the Tarim craton in the middle Paleozoic,the fluid released from the subduction plate enriched and metasomatized the overburden depleted mantle wedge,and partially melted to form basaltic magma.The water-rich basaltic magma intruded into the middle-lower crust or underlying the bottom of the lower crust to form the mixture of granitic magma and basaltic magma,forming crust-mantle mixed source magma.Finally,two groups of magma with different components were formed by crystallization differentiation and ascending emplacement.The crust-mantle mixed source magma was caused by mantle-derived magma undermining the crust,which reflects that the Kuruktag area on the northern margin of Tarim had significant vertical growth of the continental crust in the middle Paleozoic,and also indicates that the Kuruktag area on the northern margin of Tarim had the characteristics of active continental margin during this period.