LA-ICP-MS石榴子石U-Pb定年方法在异剥钙榴岩和矽卡岩年代学研究中的应用

Application of LA-ICP-MS Garnet U-Pb Dating Method on the Geochronological Study of Rodingite and Skarn

本文利用Coherent GeoLas HD型193 nm ArF准分子激光剥蚀系统和Agilent 7900型四极杆电感耦合等离子体质谱仪,建立了LA-ICP-MS石榴子石U-Pb定年方法。利用该方法,对采自冀北地区晚古生代镁铁质-超镁铁质混杂岩体中的异剥钙榴岩和闽西南马坑式铁矿含矿石榴子石矽卡岩这两种岩石中的石榴子石开展U-Pb定年研究。在冀北地区晚古生代镁铁质-超镁铁质混杂岩体中的异剥钙榴岩中,获得石榴子石下交点年龄为(387.6±5.4) Ma (D496-1, MSWD=1.1, N=30)和(409.3±7.8) Ma (D493-1, MSWD=2.0, N=60),在马坑铁矿石榴子石矽卡岩中,获得石榴子石下交点年龄为(128.6±2.1) Ma (ZK7921-b24, MSWD=2.0, N=60)和(128.7±3.2)Ma(ZK7922-b1,用锆石91500校正,MSWD=1.8,N=42);在潘田铁矿的石榴子石矽卡岩中,获得石榴子石的下交点年龄为(128.7±1.7)Ma (PT-b1, MSWD=1.7, N=30)和(132.1±1.3) Ma(PT-b1样品,用锆石91500校正,MSWD=1.6,N=30)(除了指明使用锆石标样91500校正石榴子石未知样品外,其他皆用石榴子石标样Willsboro校正石榴子石未知样品的U/Pb分馏)。以上结果与Sm-Nd等时线年龄及前人报道的锆石U-Pb年龄在误差范围内一致。对马坑式铁矿石榴子石矽卡岩U-Pb定年结果表明,利用石榴子石标样Willsboro和锆石标样91500作为外标样校正同一样品中石榴子石U/Pb同位素分馏,获得的下交点年龄一致,206Pb/238U年龄的加权平均值也一致,说明石榴子石与锆石之间的基体效应较小,在缺乏石榴子石标样时,可用锆石标样91500代替。在上述研究基础上分析了石榴子石U-Pb定年方法在矽卡岩型矿床成矿时代研究及异剥钙榴岩年代学研究中的应用潜力,认为石榴子石U-Pb定年方法在矽卡岩型矿床及异剥钙榴岩年代学研究中具有巨大的应用推广前景,具有重要的理论指导和实际应用意义。

Using the recently introduced GeoLas HD type 193nm ArF excimer laser ablation system and the Ag-ilent 7900 type quadrupole inductively coupled plasma mass spectrometer,we successfully established the method of in-situ garnet U-Pb analytical method by LA-ICP-MS.A U-Pb dating work was carried out on the rodingite rocks form the Late Paleozoic mafic-ultramafic mélange in the northern Hebei Province and the ore-bearing skarn from Makeng iron deposit in southwestern Fujian.It yielded lower intersection ages of(387.6±5.4)Ma(D496-1,MSWD=1.1,N=30)and(409.3±7.8)Ma(D493-1,MSWD=2.0,N=60)for 2 rodingite samples,and(128.6±2.1)Ma(ZK7921-b24,MSWD=2.0,N=60)and(128.7±3.2)Ma(ZK7922-b1,calibration with zircon 91500,MSWD=1.8,N=42)for 2 skarn samples from Makeng deposit.It yielded lower intersection ages of(128.7±1.7)Ma(PT-b1,MSWD=1.7,N=30)and(132.1±1.3)Ma(PT-b1,calibration with zircon 91500,MSWD=1.6,N=30)for a skarn sample from Pantian deposit(except for samples with instructions of calibration with zircon 91500,other samples are calibrated with garnet Willsboro).The garnet U-Pb ages above were consistent with the Sm-Nd isochron ages and previous zircon U-Pb ages within the allowable error range.Garnet U-Pb ages of the skarn samples collected from the Makeng-type deposit showed that the lower intersection and 206Pb/238U ages were consistent using the garnet Willsboro and zircon 91500 as standard samples respectively,which suggested that the matrix effect is small between zircon and garnet minerals.In case of an absence of the standard garnet sample in the laboratory,the zircon 91500 can be used instead.Based on the above research and understanding,an analysis on the application potential of the garnet U-Pb dating method in the geochronological study on skarn-type deposits and rodingite rocks was done.It is considered that the garnet U-Pb dating method has great application and promotion prospects in the chronology study of skarn-type deposits and rodingite rocks,and it is of great importance on the theoretical guidance and practical application.