The Study On Geochemistry And Mineralization Age Of Nephrite From Panshi,Jilin,Southeast China

This paper takes nephrite produced in Panshi City,Jilin Province as the research object,using optical microscope,scanning electron microscope with energy dispersive spectrum(SEM & EDS),fourier infrared spectrometer,laser raman spectrometer,electron microprobe(EMP),laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS),secondary ion mass spectrometry(SIMS)and other instruments were used to observe the mineral composition,geochemical characteristics and mineralization age of the collected samples,and summarized the formation information of nephrite in some areas of China about the era.The research samples were divided into three groups: the nephrite group(C1-C5),the surrounding rock group(W1-W2),and the transition zone between the nephrite and surrounding rock(T1-T4).The samples of the nephrite group are yellowish green and fine nephrite;the samples of the wall rock group are grayish white with coarse particles,mainly carbonate minerals;the transition group shows the mixed characteristics of the first two groups.The relative density and refractive index of five samples in the nephrite group were measured.The relative density was 2.86-3.02,the average value was 2.95;the refractive index was 1.61-1.63,the average value was 1.62.The three grouped samples were made into optical thin sections,and observed with an optical microscope.The main mineral composition of the nephrite group was tremolite,and the mineral component of the surrounding rock group was calcite.In addition to tremolite and calcite,chlorite and diopside can be observed in the transition group.The samples of the transition group were tested with a scanning electron microscope equipped with an energy spectrometer,and it can be seen that the apatite,titanite,rutile and zircon accessory minerals with smaller particle sizes.Laser Raman spectrometer test contained minerals such as chrysotile,talc,dolomite and so on.The LA-ICP-MS test was used to study the geochemical characteristics of tremolite.The total amount of trace elements and rare earth elements in the tremolite minerals in the nephrite obtained by experiments is very low.The large ion lithophile element U is strongly enriched,and Ba exhibits a negative anomaly;the high field strength elements Nb and Ti exhibit an obvious negative anomaly.The change range of ?REE is 1.831-5.121 ppm,there is no obvious difference between light and heavy rare earth elements,Eu has positive anomalies and negative anomalies,Ce shows negative anomalies.Based on geochemical characteristics,it is speculated that nephrite formed under multi-phase intermediate-acid-acid magmatic activities,and the overall partial oxidation environment.U-Pb dating of titanite accessory minerals in nephrite using SIMS technology,the U-Pb age of titanite is 159 ± 9.2Ma.Through petrographic observation,titanite chemical composition test and the theoretical application of temperature and pressure environment,it is believed that titanite and tremolite are syngeneic minerals,so the mineralization age of Panshi nephrite is 159 ± 9.2Ma,corresponding to the Yanshanian period.The titanite U-Pb dating method has certain advantages in the study of nephrite chronology,therefore,the application of this method in the future is worth popularizing.Panshi nephrite was formed by contact metasomatism in multi-phase intermediateacid-acid magma intrusion into dolomite marble,among which acid granite in Yanshan period plays a key role in the final formation of nephrite.