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.
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