Monazite ID-TIMS U-Pb geochronology in the LAGIR laboratory, Rio de Janeiro State University: protocols and first applications to the assembly of Gondwana supercontinent in SE-Brazil

The chemical and spectrometric procedures of the U-Pb geochronology method on monazites, recently installed in the LAGIR laboratory, are described in detail. In addition, preliminary results on monazite samples from the Brasília and Ribeira belts are reported and discussed in the context of the regional geology. Several experiments for calibration of ion exchange chromatographic columns with the AG-1x8 resin, were performed with HCl, using dissolved natural monazite samples. The Pb blanks of reagents are ∼0.5 pg/g in acids and ∼1 pg/g in H2O. The total Pb blanks in chemical procedures were below 22 pg. Preliminary results are presented from three case studies related to Brasiliano orogenic belts of SE-Brazil, which correlate very well with previous age determinations from literature: two sub-concordant grains from an Araxá Group quartzite (southern Brasília belt) define a concordia age of 602.6 ±1.4 Ma; a -0.8% discordant grain from a quartzite of the São Fidelis Group (Costeiro Domain, central Ribeira belt) yielded a concordia age of 535.3 ± 2.4 Ma; two 0.4 % and 1.3 % discordant monazite grains from the post-collisional Itaoca Granite (Costeiro Domain, central Ribeira belt) define a concordia age of 476.4 ± 1.8 Ma.

本文详细阐述了近期在LAGIR实验室搭建完成的独居石（monazite）U-Pb地质年代学（U-Pb geochronology）方法的化学流程与光谱测试步骤。此外，本文还报道了采自巴西利亚造山带（Brasília Belt）与里贝拉造山带（Ribeira Belt）的独居石样品的初步测试结果，并结合区域地质背景展开讨论。研究团队以溶解后的天然独居石样品为实验材料，采用盐酸（HCl）作为洗脱介质，开展了多组针对AG-1x8树脂（AG-1x8 resin）离子交换色谱柱（ion exchange chromatographic columns）的校准实验。试剂的铅空白值在酸溶液中约为0.5皮克每克（pg/g），在去离子水（H₂O）中约为1皮克每克。化学分析流程的总铅空白值低于22皮克（pg）。本文还呈现了3项针对巴西东南部巴西利亚诺造山带（Brasiliano Orogenic Belt）的案例研究初步结果，其与已有文献报道的年代学测定结果高度吻合：采自巴西利亚造山带南部阿拉沙群（Araxá Group）石英岩的2颗次谐和独居石颗粒，给出的谐和年龄为602.6 ±1.4百万年（Ma）；采自里贝拉造山带中部科斯特罗区块（Costeiro Domain）圣菲德里群（São Fidelis Group）石英岩的1颗偏差率为-0.8%的非谐和独居石颗粒，给出的谐和年龄为535.3 ±2.4百万年；采自里贝拉造山带中部科斯特罗区块碰撞后期伊托卡花岗岩（Itaoca Granite）的2颗偏差率分别为0.4%与1.3%的非谐和独居石颗粒，给出的谐和年龄为476.4 ±1.8百万年。