Implantação da metodologia U-Pb em monazita por LA-ICP- MS no Laboratorio de Geologia Isotópica da UFPA (Pará-Iso): aplicação em rochas de alto grau metamórfico da região central do Amapá, Sudeste do Escudo das Guianas

Abstract

The objective of this dissertation was the implementation of the experimental procedure of the U-Pb methodology on monazite by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) at the Pará-Iso/UFPA laboratory. Monazite is an orthophosphate of light rare earth elements which occurs in several types of igneous and metamorphic rocks. Its resistance to metamictization and high closure temperature for Pb (~750-900°C - Parrish 1990; Cherniak et al. 2004) make it an important tool to provide timing of high-grade metamorphic events using U-Pb geochronology. Sample preparation procedure for monazite separation and concentration was adapted from that used for analyzes in zircon, taking into consideration the monazite specifities (mainly magnetic susceptibility). After crystal selection and making the mounts with epoxy resin, Backscattered Electron (BSE) images were obtained to assess internal structures and to select the sites in the monazite for isotopic analyzes by Neptune Thermo Finnigan ICP-MS equipped with a CETAC Nd: YAG 213 nm laser microprobe model LSX-213 G2. The raw data reduction was processed using an inhouse Excel spreadsheet, adapted for monazite data processing. Age calculations were performed with the Isoplot/EX. Initially, we analyzed two international reference materials (Bananeira and Diamantina monazites) to evaluate precision, accuracy and reproducibility of the analytical data. In addition, we evaluate as a potential reference material (RM) a sample of monazite from a Neoproterozoic pegmatite from eastern portion of the Araguaia Belt, state of Tocantins. Bananeira and Diamantina monazite analyzes by LA-ICP-MS provided ages of 510 ± 5 Ma (207Pb*/235U 238U weighted average age, n = 27, 95% conf., MSWD = 0,089) and 495 ± 2 Ma (206Pb*/238U weighted average age, n = 47, 95% conf., MSWD = 0,995) respectively, similar to the ages of 508 ± 1 Ma and 495 ± 1 Ma obtained by ID-TIMS, LA-QICP- MS and LA-SF-ICP-MS for these respective RMs. The higher intensity of the analytical signal of the Bananeira monazite got it elected as the primary MR and the Diamantina monazite as the secondary MR. BSE images and compositional maps highlighted the homogeneity of the Xambioá monazite. Analyzes by LA-ICP-MS provided 206Pb*/238U weighted average age of 514,3 ± 2,3 Ma (n = 27, 2σ, MSWD = 0,56), compatible with the regional geological framework. These data need to be validated by inter-laboratory comparison, however this monazite showed potential as an internal laboratory MR considering the size of the crystal available (7 cm length, ~180 g). Next, a first application of the U-Pb methodology on monazites by LA-ICP-MS supplied a reliable age for the highgrade metamorphism of Paleoproterozoic rocks from the Tartarugal Grande granulitic complex (CGTG), in central state of Amapá, southeastern Guiana Shield. For interlaboratory comparison, a sample was also analyzed at the LA-ICP-MS Laboratory of the Research Unit Géosciences-Montpellier at the University of Montpellier, France. Monazites from two Rhyacian garnet-biotite gneisses from CGTG (HP-17 and HP-04) and neosome derived from them (HP-09C) were analyzed. Studies estimated granulite facies conditions with temperature of 800° ± 20 °C and pressure of ~7 Kbar for the metamorphism of these rocks. U-Pb isotopic analyzes on monazites from sample HP-17 provided upper intercept ages of 2058 ± 19 Ma n = 51, MSWD = 0,64; Pará-Iso) and 2037 ± 4 Ma (n = 15, MSWD = 0,098; Montpellier) therefore similar within the error. The monazites from sample HP-09C provided a 207Pb/206Pb weighted average age of 2058 ± 7 Ma (n = 30, 2σ, MSWD = 0,15). These ages are interpreted as age of the granulitic metamorphic peak. The monazites of sample HP-04 presented a spreading of the analytical points along Concordia curve, with 207Pb/206Pb dates ranging from 2096 to 2056 Ma. These dates may depict a prolonged growth interval for the monazite during metamorphism or a partial to total reseting of the U-Pb system of monazite from the magmatic protolith during the metamorphic event. Data integration of U-Pb on monazite from this work with previous geochronological results from CGTG rocks and surrounding magmatic units indicate the occurrence of intense granitic magmatism between ~2.10 and 2.08 Ga, followed by a high temperature and intermediate pressure metamorphic event. The peak of metamorphism occurred around 2.06-2.04 Ga, and metamorphic cooling between ~2.04 and 1.96 Ga when it reached a temperature below 300°C, indicated by ages 40Ar-39Ar in biotite. These data confirm that high-grade late-orogenic metamorphism evidenced in central Amapá and identified in the Bakhuis Granulitic Belt (Suriname) are coeval. The ages obtained successfully demonstrate the feasibility of carrying out the U-Pb methodology in monazite by LA-ICP-MS at the Pará-Iso Laboratory that can be routinely made available to users.