Geochemical Characteristics And Tectonic Significance Of Late Paleozoic Igneous Rocks In The Yagan Region Of Alxa

Alxa massif is located in the intersection of Central Asia orogenic belt,Tarim plate,North China plate and Qilian orogenic belt.The geotectonic location is very important.It is an important area to study the final closure process of the Paleo-Asian Ocean.The YaGan fault belt is the northernmost tectonic boundary of the Alxa massif.YaKan fault belt is YaKan structural belt in the north and Zhusiling-Hangwula structural belt in the south.The predecessors have few researches on the structural attributes of this area.So this paper takes the basic igneous rocks and granites as study objects.Through the methods of petrology,mineralogy,zircon U-Pb chronology and geochemistry,the magmatic emplacement environment of the rock mass,the genesis of the rock,and the approximate age of the collisional orogeny are revealed,and some basis for the Carboniferous-Permian tectonic evolution are provided.The main ideas of this paper are as follows:The exposed strata in the YaGan area are Carboniferous,Permian and Triassic.The magmatism are mainly monzogranite,gabbro,diabase,granodiorite and K-feldspar granite,it is lack of neutral rock and has intrusive contact with the stratum.The zircon U-Pb ages of diabase,biotite monzogranite and gabbro near the Yagan fault belt are respectively 260.5±8.8Ma（n=8,MSWD=3.1）,257.5±5.4Ma（n=10,MSWD=1.8）and 257.1±2.8Ma（n=16,MSWD=0.64）.They are almost the same age.The zircon U-Pb ages of the medium and coarse-grained K-feldspar granite in Zhusiling-Hangwula structural belt is 373.5±3.8Ma（n=17,MSWD=0.91）.The medium and coarse-grained K-feldspar granite has high SiO₂（70.60%⁷3.27%）,total alkali,low MgO（7.74%⁸.77%）,and poor P₂O₅（0.08%⁰.14%）,and belongs to quasi-aluminum high-K calc-alkaline granite.Rb,Th,K,Pb,and Nd are enriched,Zr and Hf are slightly enriched,Ti is strongly decreased,Nb,Ta,Ba,Sr,and P are decreased,and the light rare earth elements such as La and Ce are decreased.In the distribution diagram of rare-earth element,it shows right leaning with"Seagull".Eu is strong negative anomaly（δEu is 0.45⁰.60）.It is I-type granite with high anomaly.The rock originated from the Late Mesoproterozoic,which formed by the partial melting of the depleted mantle in the late Mesoproterozoic,and part of the product occurred again in the late Paleozoic under the heating of basaltic magmatic intrusion.In terms of chemical composition,diabase belongs to quasi-aluminum low-K speckle series.The diabase has relatively low values of Mg^#（47.67⁴8.92）and MgO（6.01%⁶.07%）,and the values of Mg^#and MgO decrease with the increase of SiO₂.In the spider web of trace elements,the large ion lithophile elements such as Rb,Sr and Ba are enriched,the high field strength elements such as loss Nb,Ta and Ti are decreased,and the elements of Zr and Hf have weak positive anomalies.In the distribution diagram of rare-earth element,the trend of change is highly consistent,which shows that light rare earth is relatively enriched,heavy rare earth is relatively decreased with slight"right-leaning",the slight anomaly of Eu is slight（δEu=0.92⁰.95）,andεNd（t）（+0.89⁺2.68）andεHf（t）（+7.09⁺8.59）are both positive values.The rock originated from the product of partial melting of garnet diorite,which was formed in the tectonic environment extending in the plate.Biotite monzogranite has high K₂O（3.22%³.80%）,Na₂O（3.91%⁴.73%）and CaO（4.07%⁴.85%）,low SiO₂（63.14%⁶4.25%）,TiO₂（0.46%⁰.50%）and Al₂O₃（15.27%¹6.02%）.The light rare earth is relatively enriched.The slight anomaly of Eu（δEu=0.76⁰.83）belongs to the I-type granite of quasi-aluminum high-K calc-alkaline series.The negative value ofεNd（t）（-2.61^-2.40）and slightly negative value ofεHf（t）（-2.04⁺0.26,average value-0.49）indicate that they are the mixed source of crust and mantle and formed in the raised environment after collision.The rock originates from the result of partial melting of basaltic rocks in the lower crust under the heating of the mantle basalt magma underflow.The tectonic system conversion stage of thickened crustal collapse and extension.Gabbro has relatively low content of Na₂O（3.01%³.21%）,TFe₂O₃（7.50%⁷.72%）and TiO₂（0.59%⁰.74%）,relatively high content of MgO（6.45%⁸.19%）and CaO（9.84%¹2.68%）,and belongs to quasi-aluminum calcium-basic basic rocks.The value of Mg^#is about 63,Pb is strongly enriched,the elements of Rb,Sr,K and Nd are enriched,Nb,Ta,Ti and P are decreased.In the distribution diagram of rare-earth element,light rare earth is enriched,heavy rare earth is decreased with"right-leaning".Eu is moderate negative anomaly（δEu is0.78⁰.85）,and the value ofεHf（+2.63⁺3.36）is positive,which indicate that the source area is decreased mantle.The rock originated from the partial melting of the depleted mantle,and was contaminated by a greater degree of continental crust material during the ascending evolution of magma.Based on the above characteristics,it is inferred that the Zhusiling-Hangwula structural belt was subducted southward by the oceanic crust of the the Paleo-Asian Ocean branch which is represented by the Yagan fault during the Late Devonian;the ancient Asian branch where YaKan was located had been closed before the Middle Permian,the arc and land collided,and it entered the evolution stage of the post-orogenic intracontinental extension tectonic.