| Massive and quite strong magmatism occurred in the western margin of the Yangtze block during the Neoproterozoic period,and a large number of magmatic rocks continuously distributed along the Kangdian rift were formed at the same time.Although a series of achievements such as petrology,geochemistry,and zircon U-Pb chronology have been achieved,the academic community still holds different ideas on the genetic mechanism and tectonic environment of granitoid and their related igneous rocks,such as“column mode”and“island arc mode”.In this paper,the formation age of the Neoproterozoic granitoid in Luding area was determined through comprehensive study of petrology,geochemistry and LA-ICP-MS zircon U-Pb chronology for granites,and the characteristics of the Neoproterozoic granitoid in this area were summarized;the genetic type of the granitoid and the characteristics of the magma source area are analyzed in order to explore the tectonic environment of the granitoid in Luding area.According to petrology research,the rock types of granitoid in this area are mainly quartz diorite,Yingyun diorite,monzonitic granite,granodiorite and the like.Through zircon U-Pb geochronology analysis,zircons have developed magmatic rhythmic annulus structures in five samples with high Th/U ratio?0.302.07?,indicating magmatic zircon.The test results of five samples were 754±4Ma?PM27-7-1?,738±6Ma?PM27-12-1?,725±12Ma?PM27-18-1?,753±5Ma?PM37-8-1?and 752±15Ma?PM37-64-1?respectively,showing that the granitoid in Luding area is formed in the Neoproterozoic.In terms of geochemical characteristics,the content of SiO2 in the Na-rich granitoid is 59.83%71.50%,the K2O content is 1.58%2.36%,the total content of ALK?K2O+Na2O?is 5.0%7.22%,Na2O/K2O content is 1.213.25,aluminum saturation index?A/CNK?is 0.850.96,and Na-rich granitoid belongs to quasi-aluminum calc-alkaline type.Compared with the Na-rich granitoid,the K-rich granitoid is characterized by high silicon?SiO2 content:71.79%78.6%?,low sodium?Na2O/K2O content:0.461.14?,and high potassium?K2O content:3.32%5.48%?,Aluminum saturation index?A/CNK?is 0.981.00,and K-rich granitoid belongs to quasi-aluminum high potassium calc-alkaline type.In terms of trace elements,the total amount of rare earth elements??REE?in Na-rich granitoid is 33.45×10-6146.79×10-6,and?La/Yb?N=3.3311.59 with an average of 6.31.?Eu=0.681.10 with an average of 0.82,showing a weak negative Uranium anomaly,and slightly inclined to the right.The large ion lithophile elements Rb,Ba,Th,K and the like are relatively enriched,the high field strength elements Zr and Hf are relatively enriched,and the high field strength elements Nb,Ta,P,Ti and the like are relatively bare.The Na-rich diorite inclusions and the host rocks are similar in partitioning patterns for rare earth element and trace element.The total amount of rare earth elements??REE?of the K-rich granitoid is91.02×10-6156.70×10-6,and?La/Yb?N=9.2525.06 with an average of 16.77.?Eu=0.641.15 with an average of 0.85,showing a weak negative Uranium anomaly,and inclined to the right in whole.Large ion lithophile elements such as Rb,Ba,Th,K and the like are enriched in K-rich granitoid,high field-strength elements Nb,Ta,P,Ti and the like are quite rare.The results indicate that the Neoproterozoic Na-rich granitoid and the K-rich granitoid in Luding area are characterized by typical island-arc magmatism.Combined with the characteristics of mineralogy,geochemistry and inclusion,the Neoproterozoic Na-rich granitoid in Luding area belongs to I-type granite,and the source may be from the partial amphibolite melt;the Neoproterozoic K-rich granitoid belongs to I-type granite,and the source may be from the partial mixed sandstone melt.In summary,we believe that the western margin of the Yangtze block is in a subduction environment in the middle of the Neoproterozoic.The magmatic activity in this area is related to the insertion of the oceanic crust into the Yangtze block eastward.The genetic mechanism of the Na-rich granitoid and the K-rich granitoid tend to support the“island arc mode”,and the granitoid in Luding area are formed in the continental marginal volcanic arc tectonic environment. |
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