| The Yidun Island arc belt is located in the southwest Sanjiang tectonic belt,which is sandwiched between the Ganzi Litang suture zone and the Jinsha River suture zone.Along with the intense plate tectonic movement,the magmatic activity is more frequent,exposing a large amount of granite,High economic value of the deposit,is currently found in China is still the most complete preservation of the ancient island arc structure belt,so its in-depth study of the Southwest Sanjiang regional tectonic and metallogenic background and national economic support,has a very important significance.At present,most of the previous studies on the area are concentrated in the volcanic rocks,and little research is done on the Mesozoic granites in the Yidun Island arc belt.The method of determination of the formation time is relatively simple,and the LA-ICP-MS U-Pb Based on the zircon U-Pb dating,the LA-ICP-MS method was used to study the Mesozoic magmatic rocks in the Yidun Island arc belt.Study on the Characteristics of Hull Isotopes of Zircon.In this paper,the typical rock mass in the eastern,central and western granites of the Yidun Island arc belt is studied.The field geological investigation,petrological characteristics,zircon U-Pb age,main element,trace element and zircon Hf The study on the formation,source and tectonic evolution of the Mesozoic magmatic rocks in the Yidun Island arc belt is carried out in the light of the detailed research of isotopes,and the following conclusions are obtained:1)The U-Pb age of 286 magmatic zircons in Yidun Island arcs shows that the age of Mesozoic magmatic rocks in the Yidun Island arc belt is between 216 Ma and 93 Ma,mainly in the Triassic and Cretaceous ages,The Mesozoic Jurassic strata are absent and no visible,and they are divided into two distinct magmatic activities.One of them is in the eastern Yanshanian arc belt magmatic zone and the central porphyry zone,magma age For the 208 Ma to 216 Ma,for the Indosinian late;another major development of the western granite,magma age of 100 Ma or so,for the late Yanshan.2)The granite body in the eastern part of the Yidong Island arc belt is mainly monzonitic granite.The Nb-Ta = 12.35,which is close to the ratio of Nb / Ta to the alkaline mantle,which is a high-level,Granite rocks,indicating that the granite is mainly composed of aluminum-rich crust material remelting,while the mantle-derived magma contribution.In summary,it can be seen that the source area of the eastern granite of the Yidun Island arc belt is crust and mantle type.3)The granite porphyry in the middle of the Yidun Island arc belt is mainly exposed to granodiorite porphyry and granite porphyry.Which is mainly composed of quasi-aluminum-aluminous rocks,suggesting that the magmatic origin of the lower crust or mantle indicates that a large part of the porphyry magma in the middle of the Yidun Island arc is derived from the crust or mantle Nb / Ta = 12.35,which is close to the ratio of Nb / Ta in the alkaline mantle,showing the mantle-source characteristics.The rare earth distribution pattern is from the range of As a whole,it shows a significant right-to-right type,heavy rare earth relative to light rare earth obvious loss,Eu shows a relatively weak negative anomaly,according to its characteristics can be judged as mantle source.In the high field strength element,the relative loss of Nb can indicate that the source region is the mantle.In summary,it can be seen that the source area of the eastern granite of the Yidun Island arc belt is crust and mantle type.4)The granite in the western part of the Yidong Island arc belt is dominated by high-kalcium-alkaline-potassium-metamorphic and aluminous-aluminous(over-aluminum)rocks,indicating that the granite has rich aluminum crust material remelting Contribution,which originated in the continental collision tectonic environment.It can be seen that the distribution curve of the granite in the west is the right of the seagull,the Ba content is high and the Sm / Nd of the rare earth element is 0.2 ~ 0.3,which is lower than the average of the continental crust.The degree of light and heavy rare earth fractionation Indosinian granites are low and have a significant negative Eu anomaly.It can be seen from the Na2O-K2 O diagram that the granite in the western part of the Yidong Island arc belt is mainly A-type granite and the granite source area is the mantle source.5)In summary,the granite of the western part of the Yidong Island arc belt has the characteristics of mantle magma evolution and is mixed with different degrees of crust.The U-Pb age of the granite and the central granite porphyry in the eastern part of the granite,the central porphyry and the western granite in the eastern part of the Yidun Island arc are concentrated around 200 Ma.However,Most of the granites in the eastern part of the granite meteorite evolution line near and below the CHUR,the source area is mainly the more ancient crust;and the central granite porphyry in the chondritic evolution line CHUR and depleted mantle evolution line are There is a distribution,indicating that the central granite porphyry source area both loss of mantle melting,but also the contribution of the more ancient crustal composition.The TDM2 peak is 1.20 Ga,which represents a strong crust-mantle differentiation event during this period.The U-Pb ages of the western granites are mainly concentrated at about 100 Ma,and their distribution in the zircon U-Pb age-?Hf(t)is similar to that of the central granite porphyry.The evolution of the chondrite meteorite CHUR and the depleted mantle Near the line are distributed,representing the Yidong Island arc belt western granite granite granite.This is characterized by a high degree of similarity in the zircon H5 and isotopic composition of the granite zircon in the Shiwandashan area.It can reflect to some extent that the granite of the Yidong Island arc belt is mainly derived from the remodeling of the crustal material,and only a few parts of the depleted mantle Material participation. |
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