LA-ICPMS chemistry on zircons
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LA-ICPMS chemistry on zircons 0 500 1000 1500 2000 2500 3000 3500 4000 0 2000 4000 6000 8000 10000 12000 0 500 1000 1500 2000 2500 3000 3500 4000 0 500 1000 1500 2000 2500 3000 3500 LA-ICPMS zircon U-Pb geochronology and chemistry: implications to the evolution of the reworked Paleoproterozoic Quirino complex within the Neoproterozoic Ribeira belt, SE-Brazil. Claudia Sayão Valladares 1,2 and Cin-Ty A. Lee 2 1 UERJ-Brasil-CNPq scholarship / 2 Rice University-– email: [email protected] 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 2 Figure 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/Klippe ca. 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 207 Pb/ 206 Pb age inheritance. The outcrop- A mylonitic granitic hbl-bt- orthogneiss with Archean, Paleoproterozoic and Neoproterozoic zircons was selected. Motivation s: 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. 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 0 500 1000 1500 2000 2500 3000 3500 4000 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 ppm U 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 ppm Th ppm c) Ti-in-zircon thermometry 0 500 1000 1500 2000 2500 3000 3500 4000 0 200 400 600 800 1000 1200 1400 Why 8 of 12 U-Pb Neoproteroic zircons (interpreted by geochronology as metamorphic) show >800 ˚C apparent temperature of formation? Age Ma Temp ˚C 0 500 1000 1500 2000 2500 3000 3500 0 50 100 150 200 250 300 350 400 450 500 Fe ppm Ti ppm Allanite inclusion s The Neoproterozoic zircons show positive correlation Ce-Th and Ce- U Enrichment in Ti. Causes? Fe-Ti (ilmenite) and/or Ti- rich inclusions (rutile). 0 500 1000 1500 2000 2500 3000 3500 0 200 400 600 800 1000 1200 1400 Mg ppm Fe ppm Positive 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 c) REE pattern and ƒO2 RESULTS 2: Chondrite-normalized REE plot for LA-ICP-MS zircon data (CI chrondrite values from Anders and Grevesse, 1989). Most (32 of 35) Paleoprotero zoic zircons show expected pattern for zircons. Most of he Neoproterozoi c zircons (9 of 12) show unexpected pattern, probably caused by allanite inclusions. 0 5 10 15 20 25 30 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Eu/ Eu* 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 LuN 0.1 1 10 100 1000 10000 LaN CeN PrN NdN SmN EuN GdN TbN DyN HoN ErN TmN YbN LuN 0.10 1.00 10.00 100.00 1000.00 10000.00 0 50 100 150 200 250 300 350 400 450 500 0 500 1000 1500 2000 2500 3000 3500 0 50 100 150 200 250 300 350 400 450 500 0 2000 4000 6000 8000 10000 12000 Ce ppm 0 4 8 12 16 20 24 0.0 0.1 0.2 0.3 0.4 0.5 0.6 207Pb/235U 206Pb/238U data-point error ellipses are 2s Sample TRM71A, zircon 40µm n= 48 analyses Intercepts at 606 ± 22 & 2308.3 ± 9.2 Ma MSWD = 5.2 Grain #33 Intercepts at 606 ± 22 & 3492 ± 22 Ma 0.6 0.7 0.8 0.9 1.0 1.1 1.2 0.08 0.09 0.10 0.11 0.12 0.13 207Pb/235U 206Pb/238U data-point error ellipses are 2s
description
LA-ICPMS zircon U- Pb geochronology and chemistry: implications to the evolution of the reworked Paleoproterozoic Quirino complex within the Neoproterozoic Ribeira belt, SE-Brazil. Claudia Sayão Valladares 1,2 and Cin -Ty A. Lee 2 - PowerPoint PPT Presentation
Transcript of LA-ICPMS chemistry on zircons
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: [email protected]
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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1400Why 8 of 12 U-Pb Neoproteroic zir-cons (interpreted by geochronol-ogy as metamorphic) show >800 ˚C apparent temperature of forma-tion?
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).
0 500 1000 1500 2000 2500 3000 35000
200
400
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1200
1400
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
0 4 8 12 16 20 240.0
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0.6
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
0.6 0.7 0.8 0.9 1.0 1.1 1.20.08
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207Pb/235U
206P
b/23
8U
data-point error ellipses are 2s
