Post on 23-Feb-2016
description
LA-ICPMS chemistry on zircons
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LA-ICPMS zircon U-Pb geochronology and chemistry: implications to the evolution of the reworkedPaleoproterozoic Quirino complex within the Neoproterozoic Ribeira belt, SE-Brazil.
Claudia Sayão Valladares 1,2 and Cin-Ty A. Lee2
1 UERJ-Brasil-CNPq scholarship /2Rice University-– email: claudia.s.valladares@gmail.com
CONCLUSIONS
Geotectonic Setting
U-Pb geochronology
The Ribeira belt (Figure 1) resulted from the youngest lithospheric convergence of the Pan African-Brazilian collage. The Quirino complex represents the reworked Paleoproterozoic basement of the Paraíba do Sul terrane (Figure 2 and Figure 3).
Figure 2Figure 1
Paraiba do Sul Terrane- Terrane with Archean to
Paleoproterozoic basement units: relicts of older continental plates.
Figure 3- Transect A-B
Paraíba do Sul Terrane/Klippeca. 2.3-2.1 Ga Arc
ca. 3.4-2.7 Ga Archean inheritance
Figure 4- Geologic Map and sample location (dark blue arrow).
U-Pb geochronology reveals protolith crystallized at 2308 ± 9Ma (upper intercept) and metamorphosed at 606 ± 22 Ma (lower intercept). One Archean zircon (3388 ± 16 Ma), represents minimum 207Pb/206Pb age inheritance.
The outcrop- A mylonitic granitic hbl-bt-orthogneiss with Archean, Paleoproterozoic and Neoproterozoic zircons was selected.
Motivations:1) The ability of zircon to influence or record petrogenetic processes in igneous and metamorphic systems.
2) Interpret the geologic significance of multiple ages (Archean, Paleoproterozoic and Neoproterozoic), recorded by zircon growth in the Quirino complex, Paraiba do Sul terrane, Ribeira belt, SE Brazil.
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RESULTS 1:1) Conditions of formation:
Why 9 of 12 U-Pb Neoproterozoic zircons (interpreted by geochronology as metamorphic) show Th/U ratio> 0.5?
a) Th/U ratio.
Th/U
Age
Ma
Th p
pmU
ppm
Age Ma
Legend: Blue - Paleoproterozoic and Archean zircons; Red (F.F. 5˚) and green (Diamagnetic fraction) – Neoproterozoic zircons
They are enriched in U and Th.
U p
pmTh
ppm
c) Ti-in-zircon thermometry
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Age Ma
Tem
p ˚C
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Fe ppm
Ti p
pmAllanite inclusions
The Neoproterozoic zircons show positive correlation Ce-Th and Ce-U
Enrichment in Ti. Causes?Fe-Ti (ilmenite) and/or Ti-rich inclusions (rutile).
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Mg
ppm
Fe ppmPositive correlation Fe-Mg
(biotite inclusions).
1) The presence of inclusions, in this work figure out by the zircon chemistry, could lead to misinterpretation related to: genesis of zircon (igneous or metamorphic) and conditions of formation (Ti-in-zircon thermometer and ƒO2).
2) Most (9 of 12) of the Neoproterozoic grains grew after or simultaneously with the others grains of the matrix, in a solid state, including biotite, allanite and/or ilmenite-rutile being interpreted as metamorphic.
3) Ti-in-zircon thermometer for the Neoproterozoic zircons based on 3 grains of diamagnetic fraction with no inclusions (green symbols), reveals high-amphibolite facies metamorphic process under at 620-670˚C.
c) REE pattern and ƒO2RESULTS 2:
Chondrite-normalized REE plot for LA-ICP-MS zircon data (CI chrondrite values from Anders and Grevesse, 1989).
Most (32 of 35) Paleoproterozoic zircons show expected pattern for zircons.
Most of he Neoproterozoic zircons (9 of 12) show unexpected pattern, probably caused by allanite inclusions.
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Eu/E
u*
Ce/Ce*
Lack of positive correlation between Eu and Ce anomalies in 9 of 12 Neoproterozoic zircons (not related to ƒO2). The 3 grains of the diamagnetic fraction that show positive correlation reveal oxidant condition.
LaN CeN PrN NdN SmN EuN GdN TbN DyN HoN ErN TmN YbN LuN0.1
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LaN CeN PrN NdN SmN EuN GdN TbN DyN HoN ErN TmN YbN LuN0.10
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Ce ppm
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207Pb/235U
206P
b/23
8U
data-point error ellipses are 2s
Sample TRM71A, zircon 40µm
n= 48 analysesIntercepts at
606 ± 22 & 2308.3 ± 9.2 MaMSWD = 5.2
Grain #33Intercepts at
606 ± 22 & 3492 ± 22 Ma
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207Pb/235U
206P
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8U
data-point error ellipses are 2s