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| Titel |
U-Pb ID-TIMS zircon ages of TTG gneisses of the Aravalli Craton of India |
| VerfasserIn |
Hiredya Chauhan, Ashima Saikia, Tatiana Kaulina, Tamara Bayanova, Talat Ahmad |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250105746
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| Publikation (Nr.) |
 EGU/EGU2015-5299.pdf |
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| Zusammenfassung |
| The crystalline basement of the Aravalli Craton is a heterogeneous assemblage dominated by
granitic gneisses and granites with sporadic occurrences of amphibolites and dismembered
sedimentary enclaves (Upadhyaya et al., 1992). This assemblage is known to have
experienced multiple deformation and metamorphic events followed by emplacement of
voluminous granites and basaltic dykes. Based on Sm-Nd whole rock data on the basement
Mewar orthogneisses of Jhamarkotra region (Gopalan et al., 1990) and Pb/Pb ages of zircon
from Gingla Granites which intrudes the basement (Wiedenbeck et al., 1996), it has been
inferred that the whole magmatic episode leading to the formation of the basement spanned
from 3300 to 2400 Ma and that the Aravalli cratonic block had broadly stabilized by
2500 Ma on which the younger Aravalli and Delhi Supergroup unconformably
deposited.
However, no comprehensive age data on the basement gneisses from the study area
spanning the entire magmatic episode is available. This work attempts to provide a time
frame work for evolution of the basement gneisses of the Aravalli Craton. We present here
U-Pb zircon ages from the Precambrian basement TTG gneisses of the Aravalli Craton of
north western India.
Pb and U were measured on multicollector Finnigan-MAT 262 mass spectrometer. The
temperatures of measurements were 1300ºC for Pb and 1500ºC for U. Pb isotope ratios
were corrected for mass fractionation with a factor of 0.10% per amu, based on
repeat analyses of the standard NBS SRM 982. The U analyses were corrected for
mass fractionation with a factor of 0.003% per amu, based on repeat analyses of
the NBS U 500 standard. Reproducibility of the U-Pb ratios was determined from
the repeated analysis of standard zircon IGFM-87 (Ukraine) and taken as 0.5%
for 207Pb/235U and 206Pb/238U ratios, respectively, at 95% confidence level. All
calculations were done using the programs PBDAT and ISOPLOT (Ludwig 1991,
2008).
Four zircon fractions corresponding to four zircon types from UD-16 sample yield a
U-Pb discordant age of 2680±30 Ma. Two zircon fractions from UD-17 sample
show discordant 207Pb/206Pb ages of 2506 and 2577 Ma. Zircons in our samples
have moderate to high U contents (180-770 ppm) with low Th/U ratios (0.2-0.5) in
the sample UD-16, characteristic for magmatic zircons from TTG rocks. Thus the
obtained age of 2680±30 Ma is interpreted as an age of magmatic crystallization of
tonalites.
Gopalan, K. et al., (1990): Precambrian Res., 48, 287-297.
Ludwig, K.R. (1991): PBDAT program. US. Geol. Surv. Open-file report 88-542, 38
p.
Ludwig, K. R. (2008): Isoplot/Ex, version 3.6, Berkeley Geochronology Center, Special
Publication no. 4.
Upadhyaya, R. et al., (1992): Current Sci., 62(2): 87-92.
Wiedenbeck, M. et al., (1996): Chem Geol. 129: 325-340. |
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