Two-stage, extreme albitization of A-type granites from Rajasthan, NW India


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Two-stage, extreme albitization of A-type granites from Rajasthan, NW India
Journal of Petrology
Kaur P., Chaudhri N., Hofmann A.W., Raczek I., Okrusch M., Skora S., Baumgartner L.P.
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Albitization is a common process during which hydrothermal fluids
convert plagioclase and/or K-feldspar into nearly pure albite; however,
its specific mechanism in granitoids is not well understood. The c. 1700
Ma A-type metaluminous ferroan granites in the Khetri complex of
Rajasthan, NW India, have been albitized to a large extent by two
metasomatic fronts, an initial transformation of oligoclase to nearly
pure albite and a subsequent replacement of microcline by albite, with
sharp contacts between the microcline-bearing and microcline-free zones.
Albitization has bleached the original pinkish grey granite and turned
it white. The mineralogical changes include transformation of oligoclase
(similar to An(12)) and microcline (similar to Or(95)) to almost pure
albite (similar to An(0 center dot 5-2)), amphibole from potassian
ferropargasite (X-Fe 0 center dot 84-0 center dot 86) to potassic
hastingsite (X-Fe 0 center dot 88-0 center dot 97) and actinolite (X-Fe
0 center dot 32-0 center dot 67), and biotite from annite (X-Fe 0 center
dot 71-0 center dot 74) to annite (X-Fe 0 center dot 90-0 center dot
91). Whole-rock isocon diagrams show that, during albitization, the
granites experienced major hydration, slight gain in Si and major gain
in Na, whereas K, Mg, Fe and Ca were lost along with Rb, Ba, Sr, Zn,
light rare earth elements and U. Whole-rock Sm-Nd isotope data plot on
an apparent isochron of 1419 +/- 98 Ma and reveal significant
disturbance and at least partial resetting of the intrusion age. Severe
scatter in the whole-rock Rb-Sr isochron plot reflects the extreme Rb
loss in the completely albitized samples, effectively freezing
Sr-87/Sr-86 ratios in the albite granites at very high values (0 center
dot 725-0 center dot 735). This indicates either infiltration of highly
radiogenic Sr from the country rock or, more likely, radiogenic ingrowth
during a considerable time lag (estimated to be at least 300 Myr)
between original intrusion and albitization. The albitization took place
at similar to 350-400 degrees C. It was caused by the infiltration of an
ascending hydrothermal fluid that had acquired high Na/K and Na/Ca
ratios during migration through metamorphic rocks at even lower
temperatures in the periphery of the plutons. Oxygen isotope ratios
increase from delta O-18 = 7 parts per thousand in the original granite
to values of 9-10 parts per thousand in completely albitized samples,
suggesting that the fluid had equilibrated with surrounding
metamorphosed crust. A metasomatic model, using chromatographic theory
of fluid infiltration, explains the process for generating the observed
zonation in terms of a leading metasomatic front where oligoclase of the
original granite is converted to albite, and a second, trailing front
where microcline is also converted to albite. The temperature gradients
driving the fluid infiltration may have been produced by the high heat
production of the granites themselves. The confinement of the albitized
granites along the NE-SW-trending Khetri lineament and the pervasive
nature of the albitization suggest that the albitizing fluids possibly
originated during reactivation of the lineament. More generally,
steady-state temperature gradients induced by the high internal heat
production of A-type granites may provide the driving force for similar
metasomatic and ore-forming processes in other highly enriched granitoid
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