Abstract
Migmatites of Higher Himalayan Crystallines (HHC) along different river valleys are present above the Vaikrita Thrust in the upper package. They have experienced anatexis and protracted growth of in situ melt between ca. 46 Ma and ca. 20 Ma. Melt-filled conjugate set of extensional crenulation cleavage (ECC) of late-stage deformation indicating extension parallel to the preexisting foliation (Sm) and shortening normal to foliation plane. The field evidence indicates that ECC was formed during partial melting and later filled by the melt. During partial melting, the deformation got enhanced because of the accumulation of strain due to the prolonged presence of melt. That caused the increase in the melt pressure eventually leading to crossing the critical limit and forming extensional crenulation cleavage and making it the conduit for the emplacement of melt within the migmatite zone. The phenomenon of melt-filled ECC is due to melt-enhanced deformation occurring at the time of orogen parallel extension. The presence of episodic fluxing of melt as evident from U-Pb ages of zircon is very well be correlated with the building up of partial melt leading to an increase and drop of the melt pressure due to draining followed by rising of melt from the system along with the newly formed ECC until the next event. This episodic nature of fluxing appears to be a convenient mode to extract melt from partially melted terrains of migmatites leading to chaotic structures.
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Acknowledgements
SS thanks B. Mahabaleswar for inviting to write this contribution. SS also thanks to the Department of Science and Technology, New Delhi, Ministry of Earth Sciences (MoES/P.O. (Geosci)/27/2014; and MoES/P.O.(Geo)/101(g)/2016) for financial support over the years. Understanding of the subject has improved through discussions with A.K. Jain at various stages. Discussion with Pradeep Srivastava, Rajarshi Chakravarty, S P Pradhan and Sandeep Bhatt improved the understanding of the problem. AK thanks the PMRF fellowship for financial support for his Ph.D. work. The authors thank Sajeev Krishnan for the critical review.
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Singh, S., Kushwaha, A. Melt Enhanced Deformation in Migmatites of Higher Himalayan Crystallines (HHC), India. J Geol Soc India 99, 9–12 (2023). https://doi.org/10.1007/s12594-023-2261-9
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DOI: https://doi.org/10.1007/s12594-023-2261-9