Zircon, Monazite SHRIMP U-Th-Pb and Quartz Oxygen Isotopic Results from the Higher Himalayan Crystallines (HHC) of the Sikkim Himalayas

Author:

Prabha-Mohan Shashank12ORCID,Williams Ian S.3ORCID,Singh Sandeep1ORCID

Affiliation:

1. Department of Earth Sciences, Indian Institute of Technology, Roorkee 247667, India

2. Université Clermont Auvergne, CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, 63000 Clermont-Ferrand, France

3. Research School of Earth Sciences, Australian National University, Canberra, ACT 2601, Australia

Abstract

Migmatites and partial melts are exposed in both the lower and upper package of the Higher Himalayan Crystallines (HHC) thrust sheet within the Sikkim Himalayas. Zircon monazite and quartz oxygen isotopic ratios from Yumthang Valley, North Sikkim, and Rathong Chuu, West Sikkim, have been used to identify their sources and equilibrium conditions. Monazites show homogeneous growth, whereas zircons show growth rings. U-Th-Pb data on monazite only indicate the latest metamorphic event. However, zircons show metamorphic rim growth between 36 and 24 Ma over their detrital core with trailing growth from 22 Ma to 15 Ma. Pervasive fluids have been interpreted in coeval development during metamorphism, as shown by monazite and zircon c. 30 Ma. The Th/U ratio of zircon is higher and variable with weak residual zoning in the samples from higher elevations. Quartz–metamorphic zircon oxygen fractionation suggests Teq > 600 °C, while quartz–monazite fractionation shows the same or lower temperatures. Multiple sources of melts in the HHC (even along a single valley) have been observed by δ18O of 7‰ to 10‰ in zircon and 5‰ to 9‰ in monazite. Zircon and monazite generated in the same rock have similar δ18O values. Monazite grown ~20 Ma in the lower elevation sample had a low δ18O, suggesting interaction with an external fluid.

Funder

Future Research Talent (FRT) Travel Award

Publisher

MDPI AG

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