New evidence for two sharp replacement fronts during albitization of granitoids from northern Aravalli orogen, northwest India

Abstract

We present new evidence of infiltration metasomatism in granitoids that were albitized in a process that produced two sharp replacement fronts, both of which are clearly visible in the field. The two fronts advanced through the original granite simultaneously, but at different rates. Here we focus mainly on the Ajitgarh intrusive in the northern Aravalli orogen of northwest India. This intrusion shows geographically well-defined metasomatic zones on the outcrop scale as well as a large volume of original ferroan granite, both of which were poorly preserved in most of the previously studied Khetri granites. Stage I metasomatism transformed the grey original granite to pink microcline–albite granite, and stage II converted the microcline–albite granite to white albite granite. Both these reaction fronts are sharp and are easily recognized in the field by their different colours. The mineralogical and chemical changes during the first stage are expressed by transformation of original oligoclase to albite, biotite (annite-rich) and hastingsite (amphibole) to hastingsite with low X_Fe values, dehydration, gain in Na, and losses in Fe and Rb. The second stage of metasomatism caused almost complete conversion of microcline to albite and complete or nearly complete disappearance of amphibole. Chemically, these changes are manifested by substantial gain in Na and extreme losses in K, Rb, Ba, Ca, Sr, Fe, and Mg. Depending on the modal abundances of amphibole, stage II albitized rocks are depleted in light rare earth elements or heavy rare earth elements or both, signifying that rare earth elements are principally hosted by mafic phases. The disparity in whole-rock δ¹⁸O values during both stages of albitization is related to the variations in modal amounts of Si-bearing phases. The replacement microstructures are in accord with the fluid-mediated phase transformations by a coupled dissolution–precipitation mechanism. The albitizing event took place at low temperatures of 350–400 °C and the fluid was metamorphic in nature.

Keywords:

• albitization
• ferroan granites
• infiltration metasomatism
• metasomatic zoning
• Ajitgarh
• northwest India

Acknowledgements

We thank Martin Okrusch, two anonymous reviewers, Guest Editors Vinod Singh and S.P. Verma, and Editor-in-Chief Robert J. Stern for their valuable comments, which improved the manuscript. We also thank Sanjeet Verma for providing IgRoCS and an unpublished version of the TecDIA software. His help in using this software is gratefully acknowledged. We express our gratitude to D. Rameshwar Rao for carefully performing EMP analyses and to Nusrat Eliyas for help rendered in the field work and sample preparation.

Additional information

Funding

We express our gratitude to the Department of Science and Technology, New Delhi [DST, SR/S4/ES-388/2008], for the financial assistance.