Constraints On Neoproterozoic S-type Meiling Granites In Northern Jiangxi And Their Geological Significances

During the Neoproterozoic,the Yangtze Block and the Cathaysia Block were merged together along the Jiangnan Orogenic Belt and formed a unified South China Block.The convergent history and evolution process of the two plates has been widely discussed by academic circles at home and abroad.Jiuling rock mass is located in the eastern part of the Jiangnan Orogenic Belt.It is the Neoproterozoic granite rock mass with the largest distribution area in South China.Meiling rock mass is located at the southeastern edge of Jiuling rock mass.Thus,the Meiling and Jiuling rock masses can provide important constraints on the Yangtze and the Cathaysia Blocks.In this thesis,the integrated petrology,major trace element geochemistry,zircon U-Pb chronology,and zircon Hf isotope have been carried out on the Meiling rock mass.On basis of these results,genesis types and magmatic material sources of the Meiling rock mass and its geodynamic background have been further explored.The Meiling rock mass is a complex rock mass with biotite granodiorite as the main body,which is invaded by biotite monzogranite and a small amount of tonalite.The biotite granodiorite and biotite monzogranite samples are mainly composed of feldspar,quartz,and biotite,which have been partially altered.Both plagioclase and potash feldspar have been partially sericified.The appearance of cordierite and garnet indicate that the rock mass is S-type granite,which is further confirmed by their geochemical features.The total alkali(ALK)contents of the Meiling Neoproterozoic granite samples range from5.94%to 7.98%.Biotite granodiorite is mostly plotted into the granodiorite area on the TAS diagram,with only one sample plotting into the granite area,while biotite monzogranite is plotting into the granite area.In the Si O₂-K₂O diagram,all the samples fall into high potassium calcium alkaline series.All samples fall into the aluminum area in the A/CNK-A/NK diagram.The A/CNK and A/NK values of the samples ranged from 0.93 to 1.17 and 1.03 to 1.50,respectively.The samples are enriched with large ionic lithophiles elements Rb,Th,and K,and relatively deficient in high field strength elements Nb and Ta,with obvious negative anomalies of Ba,Nb,Sr,Ti,and Eu.Biotite monzogranite has a lower total rare earth content,and the content of each rare earth element is basically the same.Both are enriched with light rare earth,the degree of light and heavy rare earth fractionation is high,the negative Eu anomaly is obvious,and the distribution diagram is a right-inclined seagull type.Meiling biotite granodiorite LA-ICP-MS zircon U-Pb age is(806.4±8.3)Ma,(810.3±9.7)Ma,and(813.2±7.8)Ma respectively,The?810 Ma formation age of Meiling rock body is consistent with Jiuling rock mass,suggesting that Meiling rock body is also the product of Neoproterozoic magmatic activity.Biotite granodiorite zircon Hf model ages T_DM1 is mainly concentrated in 1110?1311 Ma and 1044 Ma?1520 Ma,T_DM2 is mainly concentrated in 1279Ma?1589 Ma and 1176 Ma?1684 Ma,with some older points(such as 2152 Ma and 3139Ma).The range of?_Hf(t)is relatively scattered.The two samples are(-8.2?+6.7)and(-33.8?+8.4),which are mainly positive values,with some points being relatively large negative values.The distribution characteristics are similar to their of Jiuling rock mass.Although the source area is mainly composed of younger materials,it may also receive a certain supply of mature continental crust.In combination with geochemical characteristics,this thesis indicates that the Meiling rock mass should be an extension of the Jiuling rock mass along the southeast margin.Comprehensive analysis shows that the source area of biotite granodiorite is a clay-depleted sandstone and the source area of biotite monzogranite is a muddy upper crust.The two lithologies of the Meiling rock mass display the characteristics of island arc magma,and all fall into the syn-collision and post-collision area in the structural analysis diagram.This paper believes that the Meiling rock mass should be formed at the end of the collisional orogeny between the Yangtze and the Cathaysia Blocks,at the time that the orogeny is changing from the syn-collision phase to the post-collision phase.