Journal of Geoscience and Environment Protection, 2014, 2, 170-180
Published Online June 2014 in SciRes. http://www.scirp.org/journal/gep
http://dx.doi.org/10.4236/gep.2014.23022
How to cite this paper: Viana, R. R., & Battilani, G. A. (2014). SHRIMP U-Pb and U-Pb Laser Ablation Geochronological on
Zircons from Monte Santo Alkaline Intrusive Suite, Westhern Araguaia Belt, Tocantins State, Brazil. Journal of Geoscience
and Environment Protection, 2, 170-180. http://dx.doi.org/10.4236/gep.2014.23022
SHRIMP U-Pb and U-Pb Laser Ablation
Geochronological on Zircons from Monte
Santo Alkaline Intrusive Suite, Westhern
Araguaia Belt, Tocantins State, Brazil
Rúbia Ribeiro Viana, Gislaine Amorés Battilani
Department of Mineral Resource, Federal University of Mato Grosso, Cuiabá, Brazil
Received April 2014
Abstract
The Monte Santo Alkaline Intrusive Suite (MSAIS) is an association syenite foid, nepheline syenite
and syenite, which are intruded in metapelites of the Rio do Coco meta-volcanic-sedimentary Se-
quence, presenting abundant pegmathoid veins cutting all of them. The ages obtained by Shrimp
(1051 ± 22 Ma, 1048 ± 11 Ma) are very close those younger age obtained by U-Pb laser ablation
(1056 ± 21), beeing interpreted as crystallization age. These dating reveal also that MSAIS rocks
were affected by common succession of younger events below 550 Ma ago, responsible by the later
rocky bodies of varying composition occurring in the region, including the alkaline pegmatites
hosted in the nepheline syenite of the MSAIS.
Keywords
Alkaline Rocks, Shrimp Dating, Tocantins Structural Province, Neoproterozoic, Brazil
1. Introduction
Ancient alkaline rocks exposure is not common. There are few alkaline complexes such as the Canadian Shield
(Superior Province), Greenland, Australia (Yilgarn Block) and South Africa is known. The oldest are found in
Kirkland Lake region, Canada, dated of 2.7 Ga, represented by tracytos and leucite fonolite. As the generation
the Neoproterozoic and Phanerozoic alkaline rocks occur in three main geodynamic settings: 1) continental rifts,
2) oceanic islands, and 3) subduction zones (peralkaline granites in back-arc zones). The Early Precambrian al-
kaline rocks formed at hotspots of the oceanic crust and are unknown in continental rifts (Sheth et al., 2002,
Bonin, 1998; Blichert-Toft et al., 1996).
Ancient alkaline rocks are difficult to understand due to most Archean rocks are metamorphosed and some
have undergone severe hydrothermal alteration, resulting in the destruction of the alteration-sensitive feld-
spathoids or alkaline mafic minerals that are diagnostic of alkaline magmatism. Alkaline lavas also may lose
part of their alkalis and as a result appear more like ordinary tholeiites (Blichert-Toft et al., 1996).

R. R. Viana, G. A. Battilani
171
Precambrian alkaline rocks are sparsely distributed and outcrop relatively small areas scattered from north to
south Brazil. These are not many studies about these rocks, being the main in alkalines rocks of Bahia (720 ± 9
to 732 ± 24 Ma Rosa et al., 2002, 2006; 2111 ± 13 Ma, Rios et al., 2007, 721 ± Ma Conceição et al., 2009), Rio
Grande do Sul (615 ± 99 and 611 ± 3 Ma, Soliani Jr et al. 2000, Philipp et al. 2002), Pará (580 ± 10 and 724 ±
30 Ma, Jorge-João, 1980; Villas, 1982), Paraíba (ca. 600 to 580 Ma, Holland et al., 2009) and Tocantins (ca.
1500 Ma, Kitajima, 2002; Iwanuch, 1991).
In the Tocantins State, center-west of Brazil, are known three alkaline suites named of Estrela, Peixe and
Monte Santo, therefore few detail studies of the petrogenetic and geochronological aspect were performed. This
work is an attempt to characterize the age of the magmatism of the Monte Santo Alkaline Intrusive Suite
(MSAIS), based in dating performed by Shrimp and laser ablation methods.
2. Geological Setting
The alkaline rocks studied in this work are positioned in the Tocantins Structural Province defined by Almeida
(1977) and placed between San Francisco and Amazon Cratons. According to Pimentel et al. (2000) the To-
cantins Structural Province represents a Brazilian orogen system characterized by belts of folds and thrusts
called Brasilia, Paraguay and Araguaia belts, resulting from the convergence and collision of three continental
blocks: the Amazon, San Francisco and Paranapanema cratons. In the study area neoproterozoic and basement
rocks are partially covered by phanerozoic sediments of the Parnaíba Basin (Fuck et al., 2001). Paleoproterozoic
basement rocks were partially reworked during the Brazilian orogeny (Pimentel et al., 2000).
The basement rocks in the area are represented by the core cratonic rocks with estimated ages between the
Archean and Paleoproterozoic. It is composed by a granite-gneiss terrain affected by medium to high metamor-
phic degree associated with a metavolcanic-sedimentary sequence of the greenschist facies. According to Frasca
& Araújo (2001) the cratonic unit represents the evolution of a portion of the rejuvenated crust re-mobilized and
stabilized during the Paleoproterozoic. Structural features suggest a crustal unit independent represented by the
Granite-Gneissic Rio dos Mangues Complex and by metavolcano-sedimentary Rio do Coco Sequence.
Frasca & Araújo (2001) reported that Monte Santo Alkaline Intrusive Suite is intruded in metapelites of the
Rio do Coco meta-volcanic-sedimentary Sequence and in the Baixo Araguaia Group while Estrela Alkaline
Suite outcrops accordingly with the rocks of the Rio dos Mangues Complex (Figure 1). Geochronological stud-
ies on zircons from syenitic gneisses by Pb-Pb method carried out by Souza & Moura (1996), indicated crystal-
lization minimum ages of 1011 ± 86 Ma, interpreted as evidence of the beginning of the rifting process that
generated the basin in which is deposited the Baixo Araguaia Group.
3. Analytical Techniques
In situ U-Pb analyses were performed on a SHRIMP-II instrument in the Center of Isotopic Research (CIR) at
VSEGEI, Saint Petersburg, Russia. The results were obtained with a secondary electron multiplier in peak-
jumping mode following the procedure described by Williams (1998). A primary beam of molecular oxygen
was employed to bombard zircon in order to extract secondary ions. A 70 μm Kohler aperture allowed focusing
of the primary beam so that the ellipse-shaped analytical spot had a size ca 25 μm × 20 μm, and the correspond-
ing ion current was 5 nA. The sputtered secondary ions were accelerated at 10 kV. The 80 μm wide slit of the
secondary ion source, in combination with a 100 μm multiplier slit, allowed mass-resolution M/ΔM ≥ 5000 (1%
valley); thus, all the possible isobaric interferences were resolved. One minute rastering over a rectangular area
of ca. 65 μm × 50 μm was employed before each analysis in order to remove the gold coating and any possible
surface common Pb contamination. The following ion species were measured in sequence: 196(Zr2O)-204Pb-
background (ca 204 AMU)-206Pb-207Pb-208Pb-238U-248ThO-254UO with integration time ranging from 2 s
to 14 s. Seven cycles for each analyzed spot were acquired. Apart from “unknown” zircons, each fourth meas-
urement was carried out on the zircon Pb/U standard TEMORA 1, which has an accepted 206Pb/238U age of
416.75 ± 0.24 Ma (Black et al., 2003). The 91500 zircon standard, with U concentration of 81.2 ppm and a
206Pb/238U age of 1062 Ma (Wiedenbeck et al., 1995) was applied as the “U-concentration” standard. The re-
sults collected were then processed with the SQUID 1.02 (Ludwig, 2001) and Isoplot/Ex 3.00 (Ludwig, 2003)
software, using the decay constants of Steiger and Jäger (1977). The common lead correction was done on the
basis of measured 204Pb/206Pb and modern (i.e. 0 Ma) Pb isotope composition, according to the model of Sta-
cey and Kramers (1975).

R. R. Viana, G. A. Battilani
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Figure 1. Geological sketch map of the Monte Santo Alkaline Intrusive Suite and adjacent rocks, Araguaia
Belt, showing sample locations (modified from Gorayeb et al. (1996)).
The LA-MC-ICPMS analyses were performed in the Geological Survey of Finland where the chosen zircon
grains were mounted in epoxy resin and sectioned approximately in half and polished. Back-scattered electron
images (BSE) and cathodo luminescence (CL) images were prepared for the zircons to target the spot analysis
sites. U-Pb dating analyses were performed using a Nu Plasma HR multicollector ICPMS in Espoo using a tech-
nique very similar to Rosa et al. (2009) except that a New Wave UP193 Nd: YAG laser microprobe was used.
Samples were ablated in He gas (gas flow = 1.0 l/min) using a low volume teardrop-shaped (<2.5 cm3) laser ab-
lation cell (Horstwood et al., 2003). Raw data were corrected for background, laser induced elemental fractiona-

R. R. Viana, G. A. Battilani
173
tion, mass discrimination and drift in ion counter gains and reduced to U–Pb isotope ratios by calibration to
concordant reference zircons of known age, using protocols adapted from Andersen et al. (2004) and Jackson et
al. (2004). Standard zircon GJ-01 (609 ± 1 Ma; Belousova et al., 2006) and an in-house standard A1772 (2711 ±
3 Ma/TIMS; 2712 ± 1 Ma/SIMS) were used for calibra-tion. For reference, either zircon A382 (1877 ± 2 Ma,
Patchett and Kouvo, 1986) or A1933 (TIMS/1641 ± 2 Ma, SIMS/1640 ± 4 Ma) was run as an unknown to check
the calibration. The calculations were done off-line, using an interactive spreadsheet pro-gram written in Micro-
soft Excel/VBA by Tom Andersen (Rosa et al., 2009).
Plotting of the U-Pb isotopic data and age calculations were performed using the Isoplot/Ex 3 program
(Ludwig, 2003). All the ages were calculated with 2σ errors and without decay constants errors. Data-point error
ellipses in the figures are at the 2 σ level.
4. Description of the Selected Samples
A total of three samples (named of ABX, HPO2 and HTOSN) were analyzed for the geochronology by Shrimp
with two them also analyzed by laser ablation (ABX and HTOSN). All the samples present similar zircons with
most of which preserved terminations. The backscattered electron (BSE) and cathodoluminescence (CL) imag-
ing showed that larger grains have normally euhedral forms and can to present more deformed internal structure
with a typical magmatic concentric or oscillatory zoning, while the smaller grains shows more altered blurry in-
ternal structure, sometimes presenting massive interior structure to zoning marginal domain (Figure 2).
5. Results and Discussion
Five grains of zircon from the sample ABX were analyzed by shrimp (Table 1 and Figure 3) and three grains
using MC-LA-ICP-MS (Table 2). The Shrimp analyses showed that from five grains analyzed two of them
(grains 2 and 3, Table 1) have two real ages of 1051 +/− 20 and 402 +/− 20 Ma (Figure 3(a)). All points of the
Figure 2. Cathodoluminescence images of selected zircon crystals separated from the studied rocks.

R. R. Viana, G. A. Battilani
174
(a)
(b)
Figure 3. SHRIMP Zircon U-Pb Concordia plots and recalculated weighted mean 206Pb/238U
ages (a) and MC-LA-ICPMS U-Pb (b) isotopic data for sample ABX from MSAIS rocks.
other grains analyzed showed ages close to those younger ages. The event of 520 Ma is not clearly registered. A
big grain (grain 05) with nine spots analyzed, illustrate event of 400 - 420 Ma old (Figure 3(a)). By MC-LA-
ICP-MS analyses, five analyses were done on the large grain and two on both the smaller grains, in which pro-
duced age of ~1100 Ma age for the two different types of zircon (Table 2, Figure 3(b)). In Table 2 is also ob-
served two discordant data points, it is not plotted in diagram.
From sample HTOS seventeen zircon grains were produced, in which thirteen were analyzed by shrimp (Ta-
ble 1) and only four by MC-LA-ICP-MS (Table 2). Most of the zircon show many metamictic portions. The

R. R. Viana, G. A. Battilani
175
Table 1. SHRIMP isotopic data for Zircon of the rocks from Monte Santo Alkaline Intrusive Suite.
Sample/
spot %
206Pbc
ppm
U
ppm
Th ppm
206Pb*
232Th
238U
(1)
206Pb
238U
Age
(1)
207Pb
206Pb
Age
%7
Dis-
7cor-
7dant
(1)
238U
206Pb*
±%
(1)
207Pb*
206Pb*
±%
(1)
207Pb*
235U
±%
(1)
206Pb*
238U
±%
Errcorr
ABX_1.1 0.41 853 508 63.1 0.62 532 ±7 581 ±43 +9 11.6 1.3 0.0594 2.0 0.70 2.4 0.086 1.3 0.6
ABX_2.1 0.73 49 81 3.79 1.73 560 ±11 601 ±175 +7 11.0 2.1 0.0599 8.1 0.75 8.3 0.091 2.1 0.2
ABX_2.2 0.09 124 235
18.8 1.96 1050 ±16 1050 ±42 -0 5.7 1.6 0.0743 2.1 1.81 2.7 0.177 1.6 0.6
ABX_3.1 0.39 143 236 16.5 1.71 813 ±12 848 ±72 +4 7.4 1.6 0.0673 3.5 1.25 3.8 0.134 1.6 0.4
ABX_3.2 0.36 112 594
17.1 5.48 1053 ±17 1051 ±62 -0 5.6 1.7 0.0744 3.1 1.82 3.5 0.177 1.7 0.5
ABX_4.1 -- 86 58 5.87 0.70 495 ±8 746 ±107 +35 12.5 1.8 0.0641 5.1 0.71 5.4 0.080 1.8 0.3
ABX_4.2 0.00 141 221
9.33 1.62 478 ±7 603 ±59 +21 13.0 1.6 0.0600 2.7 0.64 3.1 0.077 1.6 0.5
ABX_5.1 0.41 1475 340 69.5 0.24 344 ±5 493 ±52 +31 18.2 1.4 0.0570 2.4 0.43 2.7 0.055 1.4 0.5
ABX_5.2 -- 702 246
42.5 0.36 439 ±6 507 ±43 +14 14.2 1.5 0.0574 1.9 0.56 2.4 0.070 1.5 0.6
ABX_5.3 -- 678 231 46.4 0.35 495 ±7 598 ±38 +18 12.5 1.5 0.0599 1.8 0.66 2.3 0.080 1.5 0.6
ABX_5.4 0.34 403 115
24.2 0.29 436 ±13 497 ±78 +13 14.3 3.0 0.0571 3.5 0.55 4.7 0.070 3.0 0.7
ABX_5.5 0.17 553 134 31.9 0.25 419 ±6 485 ±56 +14 14.9 1.5 0.0568 2.5 0.53 2.9 0.067 1.5 0.5
ABX_5.6 1.28 118 49 6.69 0.43 411 ±8 153 ±312 -174 15.2 1.9 0.0491
13.3 0.45 13.4 0.066 1.9 0.1
ABX_5.7 0.00 100 38 5.52 0.39 403 ±7 563 ±95 +29 15.5 1.9 0.0589 4.4 0.52 4.7 0.065 1.9 0.4
ABX_5.8 0.26 352 165
20.3 0.49 420 ±6 591 ±71 +30 14.9 1.5 0.0597 3.3 0.55 3.6 0.067 1.5 0.4
ABX_5.9 0.00 470 192 26.4 0.42 408 ±12 423 ±55 +4 15.3 3.0 0.0553 2.5 0.50 3.9 0.065 3.0 0.8
HTOS_1.1 0.19 308 413
46.9 1.39 1052 ±17 1050 ±59 -0 5.6 1.7 0.0743 2.9 1.82 3.4 0.177 1.7 0.5
HTOS_2.1 0.17 384 461 57.8 1.24 1041 ±16 1126 ±30 +8 5.7 1.7 0.0772 1.5 1.87 2.3 0.175 1.7 0.7
HTOS_3.1 0.18 158 106
12.4 0.69 563 ±9 593 ±78 +5 11.0 1.6 0.0597 3.6 0.75 4.0 0.091 1.6 0.4
HTOS_4.1 -- 395 187 58.8 0.49 1029 ±16 1046 ±28 +2 5.8 1.7 0.0742 1.4 1.77 2.2 0.173 1.7 0.8
HTOS_5.1 0.59 80 19 9.39 0.25 821 ±14
921 ±99 +11 7.4 1.8 0.0697 4.8 1.31 5.1 0.136 1.8 0.4
HTOS_6.1 1.06 67 71 10.1 1.10 1051 ±19 1019 ±115 -3 5.6 1.9 0.0732 5.7 1.79 6.0 0.177 1.9 0.3
HTOS_7.1 -- 313 159
38.5 0.52 862 ±14 1001 ±76 +15 7.0 1.7 0.0725 3.7 1.43 4.1 0.143 1.7 0.4
HTOS_7.2 -- 241 21 17 0.09 508 ±7 582 ±73 +13 12.2 1.5 0.0594 3.4 0.67 3.7 0.082 1.5 0.4
HTOS_8.1 0.16 480 619
73.6 1.33 1058 ±14 1071 ±26 +1 5.6 1.4 0.0751 1.3 1.85 1.9 0.178 1.4 0.7
HTOS_8.2 0.51 610 342 43.3 0.58 512 ±7 550 ±53 +7 12.1 1.4 0.0585 2.4 0.67 2.8 0.083 1.4 0.5
HTOS_9.1 0.08 338 123
50.3 0.38 1031 ±14 1012 ±28 -2 5.8 1.5 0.0729 1.4 1.74 2.0 0.173 1.5 0.7
HTOS_9.2 -- 316 305 35.7 1.00 796 ±11 932 ±41 +15 7.6 1.5 0.0701 2.0 1.27 2.5 0.131 1.5 0.6
HTOS_10.1
-- 498 463 67.3 0.96 943 ±12 1044 ±21 +10 6.4 1.4 0.0741 1.1 1.61 1.7 0.157 1.4 0.8
HTOS_10.2 0.86 154 33 12.2 0.22 568 ±9 553 ±147 -3 10.9 1.7 0.0586 6.7 0.74 6.9 0.092 1.7 0.2
HTOS11.1 -- 340 298
51.8 0.91 1053 ±14 1114 ±33 +6 5.6 1.5 0.0767 1.6 1.88 2.2 0.177 1.5 0.7
HTOS_12.1 1.93 78 44 6.44 0.58 588 ±12 354 ±361 -69 10.5 2.2 0.0536 16.0 0.71 16.1 0.096 2.2 0.1
HTOS_13.1 1.34 195 255 15.8 1.35 581 ±38 719 ±121 +20 10.6 6.9 0.0633 5.7 0.82 8.9 0.094 6.9 0.8
HP002_2.1 2.66 39 81 2.83 2.15 523 ±18 170 ±783 -216 11.8 3.5 0.0495 33.5 0.58 33.7 0.085 3.5 0.1
HP002_6.1 29.37 3 19 0.132 6.24 301 ±89 3293 ±1213 +93 20.9 30.3 0.2677 77.3 1.76 83.0 0.048 30.3 0.4
HP002_6.2 0.40 525 122 30.4 0.24 421 ±6 683 ±55 +40 14.8 1.6 0.0623 2.6 0.58 3.0 0.068 1.6 0.5
HP002_4.1 3.48 398 696
24.1 1.81 440 ±7 605 ±201 +28 14.2 1.6 0.0600 9.3 0.58 9.4 0.071 1.6 0.2
HP002_4.2 0.96 305 81 22.7 0.27 535 ±8 540 ±109 +1 11.6 1.6 0.0583 5.0 0.69 5.2 0.086 1.6 0.3
HP002_1.1 -- 347 88 26.6 0.26 550 ±8 544 ±55 -1 11.2 1.5 0.0584
2.5 0.72 2.9 0.089 1.5 0.5
HP002_5.1 0.25 218 136 30.9 0.64 985 ±15 1020 ±46 +4 6.1 1.6 0.0732 2.3 1.67 2.8 0.165 1.6 0.6
HP002_3.1 0.00 356 146
53 0.42 1030 ±14 1098 ±25 +7 5.8 1.5 0.0761 1.3 1.82 2.0 0.173 1.5 0.8
Errors are 1-sigma; Pbc and Pb* indicate the common and radiogenic portions, respectively. Error in Temora1 Standard calibration was 0.50%. (1)
Common Pb corrected using measured 204Pb.

R. R. Viana, G. A. Battilani
176

R. R. Viana, G. A. Battilani
177
shrimp analyzed have a very good example of two ages in the single crystal, which is shown in grains 7, 8, 9 and
10 (Table 2). Both ages are concordant and are of 1048 ± 11 and 511 ± 10 Ma (Figure 4(a)). A total of ten
MC-LA-ICP-MS analysis were realized and revealed all the spots analyzed of the zircon 03 and one from zircon
01 showed highly discordant age (Table 2). The remaining analyses produced equivalent U-Pb age of approxi-
mating1060 Ma (Figure 4(b)).
The sample HPOO2 had their zircon analyzed only by Shrimp, in the total of six grains. The grain 4 is the
unique that revel two real ages of 1030 ± 14 e another of 535 ± 8, Ma (Table 2, Figure 5). The last event is
stronger and it was influenced for other younger (ca 420 Ma).
The ages obtained by MC-LA-ICP-MS and Shrimp method show ages very close. The studied samples were
characterized by common succession of events, with the Shrimp crystallization age varying of 1051 ± 22 Ma
and 1048 ± 11 Ma and MC-LA-ICP-MS ages varying of 1106 ± 10 Ma and 1056 ± 21 Ma, with subsequent re-
crystallization during much younger process below 500 Ma ago.
(a)
(b)
Figure 4. SHRIMP Zircon U–Pb Concordia plots and recalculated weighted
mean 206Pb/238U ages (a) MC-LA-ICPMS U-Pb isotopic data for sample
HTOS from MSAIS rocks.

R. R. Viana, G. A. Battilani
178
Figure 5. SHRIMP Zircon U-Pb Concordia plots and recalculated weighted
mean 206Pb/238U ages.
We interpret the concordant older ages as recording the time of crystallization of zircon in the Alkaline Monte
Santo Complex. Although some grains seems have been affected by cracks, no evidence from the analytical data
was found to be anomalous.
The crystals subsequently affected by one or more episodes of lead loss, suggesting that the nepheline syenitic
rocks have been involved in a thermo-tectonic episode, namely Brazilian Orogeny, in the end of the Neopro-
terozoic. These events are represented by concordant ages of 511 ± 10, 535 ± 8 and 402 ± 20 Ma and may have
been responsible for the granitegenesis described by Alvarenga et al. (2000) in domain of Estrondo Group and
also for the reactivation of numerous fractures occurred to south, as well as by the intense hydrothermal activity
and metasomatic alterations in which modify the primary mineralogy of the rocks of the Monte Santo Alkaline
Intrusive Suite. These episodes were also responsible by generation of the later bodies and including the alkaline
pegmatites present in the area.
Acknowledgements
This work was partially supported by the Foundation for Research Support of the State of Mato Grosso
(FAPEMAT) and National Scientific and Technological Development Council (CNPQ).
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