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| Titel |
\textbf{New geochronological and isotopic constraints for the 1.7 Ga albitised A-type granitoids in NE Rajasthan, India: implications for Palaeoproterozoic crustal evolution of the Aravalli orogen} |
| VerfasserIn |
Parampreet Kaur, Naveen Chaudhri, Albrecht W. Hofmann, Ingrid Raczek, Alfred Kröner  , Martin Okrusch |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250031791
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| Zusammenfassung |
| Granitoid plutons of the Khetri complex have been studied for zircon U-Pb and Pb-Pb
thermal ionisation mass spectrometry methods along with the whole-rock geochemistry to
provide new constraints on the Palaeoproterozoic magmatic activity in the Aravalli orogen of
northwestern peninsular India. The rocks are metaluminous to weakly peraluminous,
largely ferroan and intraplate A-type granites. All the intrusives, but for one, show
evidence of moderate to extreme albitisation forming microcline-albite granite and
albite granite, respectively. These granites have been albitised by two metasomatic
fronts, an initial transformation of oligoclase to nearly pure albite, and a subsequent
replacement by albite, with sharp contacts between the microcline-bearing and
microcline–free zones. The mineralogical changes associated with progressive albitisation
include transformation of oligoclase (~An12) and microcline (~Or95) to almost
pure albite (~An0.5-2), ferropargasite to hastingsite and actinolite, and biotite to
magenesio-hornblende/actinolite. Whole-rock mass balance calculations show that, during
albitisation, the granites experienced major gain in Na, whereas K, Mg, Fe and Ca were lost
along with Rb, Ba, Sr, Zn, LREE and U. At the scale of individual intrusions, the narrow
ranges in ÉNdivalues (-1.3 to -2.9 and -4.9 to -6.2), 147Sm/144Nd ratios (0.1024 to
0.1236 and 0.1359 to 0.1492) and mean crustal residence ages (TDM = 2.28-2.17 Ga
and 2.4-2.5 Ga) in moderately albitised rocks show that these parameters were not
strongly affected by albitisation whereas the completely albitised granites exhibit
relatively high 147Sm/144Nd ratios and low apparent ÉNdivalues. Severe scatter
in the whole-rock Rb-Sr isochron plot reflects the high mobility of both Sr and
Rb during albitisation. Extreme Rb loss in the completely albitised samples has
effectively frozen 87Sr/86Sr ratios in the albite granites at very high values, indicating a
considerable time lag (estimated to be at least 300 Ma) between original intrusion and
albitisation. The albitisation likely took place at sub-solidus temperatures of ~350 to
400Ë C. It was caused by the infiltration of an ascending high-δ18O hydrothermal
fluid, which had acquired high Na/K and Na/Ca ratios during migration through
metamorphic rocks at even lower temperatures in the periphery of the plutons. The regional
comparisons make evident that the northern Aravalli orogen records a widespread event of
sodium metasomatism. The confinement of the albitised granites along the NE-SW
trending Khetri lineament and the pervasive nature of the albitisation suggest that
the albitising fluids possibly originated during the continual reactivation of the
lineament. The U-Pb zircon age data for most of the plutons cover a time span of
1732-1682 Ma, while the Pb-Pb zircon evaporation data of some intrusives indicate
minimum ages between 1671 and 1537 Ma. Furthermore, a regional survey of late
Palaeoproterozoic ages in the Aravalli orogen provide considerable evidence for a
geographically widespread ca. 1700 Ma extensional-related event in the northwestern
Precambrian Indian shield. The record of comparable ages and magmatic history reported
in parts of North America and North China craton indicates the significance of
this event in understanding not only the Precambrian evolutionary history of the
Aravalli orogen, but also the breakup tectonics of the supercontinent Columbia. |
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