ICP-MS Trace Elemental Analyzed Method: Applications For The Mesozoic Magmatic Rocks In Southern Anhui Province

Based on further understanding of the main structure and working principle of inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7500cx), a series of conditional experiments have been undertook in our new builded laboratory and a appropriated analyzed method has been established:the method of seals HF+HNO3 in Teflon Bomb to digest samples is used in order to ensure complete dissolution of the sample; corrected by matrix-matched standard solution which simulated content ratios of natural rocks, it can better suppress the matrix interference; using 115In-103Rh double internal calibration system, it can effectively monitor and correct signal analysis of the short and long term drift, also the matrix effects have a significant compensation; using double mode to analyze samples, it is in combination with Nogas mode and He mode. This method to analyze geological samples of micro, trace, ultra-trace elements has achieved results with high accuracy and good precision and can be widely used in geological samples with multi-elemental analysis. Meanwhile, the analyzed accuracy meets the generally researched demand.The southern Anhui province is located in the lower Yangtze block, tectonically belonging to the eastern part of the Jiangnan orogenic belt. Yanshanian magmatic plutons are widely distributed in this region, with dominate rock type of granitoids. In this work we select the Tanshan pluton, Taiping-Huangshan complex plutons, Yixian pluton and Chengan-Dayanchong complex plutons for major elemental, LA-ICPMS zircon U-Pb dating and trace elemental studies. Results of LA-ICPMS zircon U-Pb dating show that the granodiorites of Yixian pluton, Taiping pluton and Chengan pluton formed between 149.3±2.4 Ma to 138.3±1.4 Ma. The Tanshan monzogranite pluton formed between 128.5±1.7 Ma to 126.7±1.7 Ma. K-feldspar granites of the Tanshan pluton, Huangshan pluton and Dayanchong pluton formed between 134.9±2.6 Ma to 128.4±1.4 Ma, belonging to the three stages of the Yanshanian magmatisms in the southern Anhui province. Granodiorites of the early stage show characteristics of SiO2 contents of 64.27 wt% to 70.49 wt%, total alkali contents ranging from 6.38 wt% to 7.93 wt%, K20/Na20>1, A/CNK<1, A>NK, a geochemical signatures which belong to the high potassium calc-alkaline series and metaluminous I-type granite. Monzogranites of the middle stage have relatively high SiO2 contents of 72.44 wt% to 75.45 wt%, total alkali contents ranging from 8.26 wt% to 9.24 wt%, K20/Na20>1, A/CNK>1, A>NK, belonging to the high potassium calc-alkaline series and weakly peraluminous I-type granite. K-feldspar granite of the late stage is charactered by high SiO2 contents of 73.29 wt% to 76.58 wt%, lower CaO contents of 0.49 wt% to 0.78 wt%, A/CNK>1, A>NK, K2O/Na2O>1, also showing the high potassium calc-alkaline series affinities, but being A-type granite. From early to late stages, contents of REE and trace elements exhibited growing trend and the differentiation between LREE and HREE becomes smaller, but the inner differences gradually increased. HREE have a tendency to become flatten, Eu, Ba, Sr growing losses. All rocks show enrichment in the LILE elements such as Rb, Th and U, and deplation in the HFSE such as Nb, P and Ti. All those features and tendencies are consistent with that of the Yangshanian three-stage magmatisms in the southern Anhui province.The studies of the major elements and trace elements indicate that, the granodiorite and monzogranite plutons in the southern Anhui province were formed by melt mixing which separately derived from partial melting of the lithospheric mantle which metasomatized by the fluid/melts from the subducted slab, and the late Archaean-early Proterozoic crust which were induced to partial melting by the underplating basaltic magma, under a tectonic setting of the Pacific plate low-angle subduction. Associated with increase of the subducted angle and roll-back of the subducted slab, the tectonic setting in the southern Anhui Province transformed into back arc extension. The asthenospheric upwelling caused anatexis of the Mesoproterozoic basement which formed the K-feldspar granites under a condition of relatively low pressure and high temperature.