Geochemical Characteristics And Tectonic Significance Of Neoproterozoic Granites In Wanzai County,Jiangxi Province

Wanzai county is located in the northwest of Jiangxi Province and in the middle of the Jiangnan orogenic belt.It is one of the large-scale exposed areas of granite in South China and the exposed Neoproterozoic granite belongs to Jiuling granite.The Neoproterozoic granites studied in this survey belong to the part of the Jiuling rock mass exposed in the area.The author newly found that there are some fine-grained granite types?flowers?that invaded later in the Jiuling rock mass in the study area.The granite diorite,Yingyun diorite,diorthogranite,granite?porphyry?are distributed,so this paper systematically expands the research area from the aspects of its field contact relationship and petrological characteristics,chronology,geochemistry,etc.The comparative and comprehensive study of Proterozoic granites mainly discusses the petrological characteristics,formation age,tectonic environment,and the relationship between the intrusion order of different rock types of intrusions.The paper systematically collected different intrusion orders of the 38 rock samples,XRF method was used to analyze the major elements;ICP-MS method was used to analyze the trace elements and rare earth elements;LA-LCP-MS and SHRIMP methods were used to test the zircon U-Pb age.Mainly obtain the following knowledge:1.The Neoproterozoic-coarse-grained granodiorite zircon U-Pb age is 837Ma;the age of later-introduced fine-grained granodiorite?Guanyuanshan intrusion?zircon U-Pb age is 806 Ma,and the age of fine-grained granite zircon U-Pb age is 796 Ma.2.The Neoproterozoic granite in the study area is widely distributed and has many lithologies.Different rock types also show a certain intrusion order.By the field intrusion contact relationship,zircon dating results and previous data,this paper concludes that the formation of Neoproterozoic intrusive rocks in the study area has experienced at least two stages of"major intrusion"and"supplementary intrusion"magmatic intrusion.The main intrusion stage includes:medium coarse-medium grain granodiorite,medium grain-medium fine grain diorite,medium grain-medium coarse grain diorite in the order of successive invasion;The order includes:medium-fine-grain granodiorite,fine-grained diorite,fine-grain diorite,and fine-grain granite?porphyry?.3.Geochemical characteristics show that the Neoproterozoic granite intrusive rocks in the study area have mostly Si O₂content of 62%-70%,and a small amount of late granite?porphyry?rock content of 72%-75%.The content of Al₂O₃is generally high,ranging from 14.31%-20.59%,the average value is 15.69%,and it is inversely related to the content of Si O₂;the content of Na₂O+K₂O is between 3.59%-9.85%,?=0.51-1.94,aluminum saturation index A/CNK=between 1.07-4.09,with an average value of 1.66.The rock is generally rich in aluminum and alkali,relatively rich in potassium and low in sodium.The rock type belongs to a transition type of I-S granite.The characteristics of different types of granite are similar.In the trace element cobweb chart standardized by MORB,the overall curve is a right-inclined curve,showing the characteristics of homologous evolution,K-Rb-Ba-Th and Ce are peaks,and the Ta-Nb trough is obvious.In addition,Ba is slightly"losing"relative to Th and Rb,while P and Ti are slightly deficient,showing valleys,generally showing the characteristics of the lower crustal magma source.In the rare earth element distribution diagram,the overall appearance is a right-curved curve type,which obviously has the characteristics of enriching rare earth elements.The light-medium rare earth?La?Dy?part is steeply inclined to the right,while the medium-heavy rare earth?Dy?Lu?is relatively flat.The ratio of?LREE/?HREE?7.08-9.65?and?La/Yb?_N?7.98-13.20?of Yingyun diorite-granodiorite is relatively high,indicating that the light rare earth is more enriched and differentiated than the heavy rare earth the degree is more obvious;while the ratio of?LREE/?HREE?2.47-3.7?and?La/Yb?_N?3.38-3.69?of the monzonite granite to the fine-grained granite?porphyry?rock is relatively low.This also indicates that there is a roughly negative correlation between the?REE of the rock and the Si O₂content.?Eu is less than 1?0.22-0.56?,and diorite granites show moderate negative anomalies,while the delta Eu negative anomalies of monzonite granite and fine-grained granite?porphyry?are slightly increased?0.22-0.43?,It shows that plagioclase has undergone more obvious separation and crystallization during magma evolution.The research results show that the Neoproterozoic granites in the area are derived from sandy,sandy-muddy,and argillaceous source areas,and the composition of each intrusive body from early to late source area is changed from sandy to sandy-muddy,and then to muddy.The process may be a manifestation of the increasing role of regional continental crust extension.4.The study believes that the Neoproterozoic granites in the study area were formed in the post-collision tectonic environment and were the product of the post-collision evolution stage in the Jinning period.Comparing the characteristics of known deposits,it is believed that Neoproterozoic granite in the study area has a certain tungsten metallogenic potential,but the potential is not great.