川西塔公松林口岩体LA-ICP-MS锆石U-Pb年龄与地球化学特征

Zircon LA-ICP-MS U-Pb Age and Geochemical Features of the Songlinkou Pluton, Western Sichuan

松林口岩体出露于松潘—甘孜造山带东缘,紧邻甲基卡超大型稀有金属伟晶岩型锂矿床,为确定松林口岩体侵位时代与物质来源,探讨该岩体与甲基卡成矿岩体的地球化学和含矿性差异,本文对采集的11件岩体样品,采用X射线荧光光谱法、电感耦合等离子体质谱法(ICP-MS)、激光剥蚀电感耦合等离子体质谱法(LA-ICP-MS)和多接收电感耦合等离子体质谱法(MC-ICP-MS),对岩石的主量和微量元素组成、锆石U-Pb年龄和锆石Lu-Hf同位素进行分析。结果表明:松林口岩体中富含闪长质暗色包体,SiO2含量介于56.56%~68.99%之间,全碱含量3.78%~6.82%,K2O/Na2O=1.02~1.93,里特曼指数σ=1.01~1.93,Mg^#值总体为46.73~61.27,岩石属于高钾钙碱性Ⅰ型花岗岩系列,具有轻稀土富集的特点,LREE/HREE=2.67~8.31,La N/Yb N值为2.11~9.74,所有岩石样品均具有明显的负Eu异常;总体上松林口岩体及其包体富集LILE元素,Ta、Nb、P、Ti等HFSE元素相对亏损。松林口边部花岗闪长岩、中部二长花岗岩的锆石U-Pb年龄分别为212.6±1.0Ma(MSWD=0.55)、222.4±1.1Ma(MSWD=0.39),岩浆侵位时代为晚三叠世;二长花岗岩锆石Lu-Hf同位素结果εHf(t)介于-9.09^-6.86,二阶段模式年龄(TDM^C)在1524.874~1666.002Ma之间,岩体物质来源是中元古代扬子克拉通基底物质部分熔融,并混有部分富集上地幔物质。甲基卡S型花岗岩为上地壳部分熔融形成的,与松林口岩体属不同的物质来源,且岩浆演化程度和成矿构造条件不同,这可能是松林口岩体不具有锂稀有金属成矿的主要原因。

BACKGROUND:The Songlinkou pluton is located in the eastern margin of the Songpan—Ganzi orogenic belt,which is adjacent to the pegmatite-type lithium deposit of Jiajika.OBJECTIVES:To explore the similarities and differences between Songlinkou pluton and Jiajika mineralized intrusive rocks,and to study geological conditions of mineralization and magma sources of the pluton.METHODS:Major and trace elements were determined by X-ray fluorescence spectrometry(XRF)and inductively coupled plasma-mass spectrometry(ICP-MS),respectively.Zircon U-Pb age and Hf isotopes were analyzed by laser ablation-inductively coupled plasma-mass spectrometry(LA-ICP-MS)and multiple collector-inductively coupled plasma-mass spectrometry(MC-ICP-MS),respectively.RESULTS:The Songlinkou pluton contained diorite enclaves,and has SiO2 contents of 56.56%-68.99%,total alkali of 3.78%-6.82%,K2O/Na2O of 1.02-1.93,and Rittmann indices of 1.01-1.93.The rock has relative high Mg^#values(Mg^#=46.73-61.27).The rock belongs to the high-potassium calcium-alkaline type Ⅰ granite series showed light rare earth enrichment with LREE/HREE ratios of 2.67-8.31,LaN/YbN of 2.11-9.74.All samples have strongly negative Eu anomaly.It was rich in LILE,and depleted in Ta,Nb,P and Ti.The zircon U-Pb results of the granodiorite and monzogranite were 212.6±1.0Ma(MSWD=0.55)and 222.4±1.1Ma(MSWD=0.39),respectively.The εHf(t)of zircon in monzogranite ranged from-9.09 to-6.86,and TDM^C ranged from 1524.874Ma to 1666.002Ma.CONCLUSIONS:The Songliankou pluton is produced by the partial melting of the middle Proterozoic Yangtze craton basement mixed with the enriched upper mantle.The Jiajika S-type granite is formed by partial melting of the upper crust,and is different from Songliankou pluton in terms of magma sources,evolution and mineralization tectonic conditions;this may be the main reason why the Songlinkou pluton does not have lithium rare metal mineralization.