The Life of Zircon and Monazite in a Cenozoic Polymetamorphic Terrane - Higher Himalayan Crystallines (HHC)

Inverted metamorphism during the Cenozoic continental collision between India and Eurasia formed high grade ortho- and paragneisses, the Higher Himalayan Crystallines, between the Main Central Thrust and the South Tibetan Detachment. Zircon and monazite in the west (Bhagirathi and Dhauliganga Valleys, Uttarakhand) and central (Yumthang Valley and Rathong Chuu, Sikkim) Himalayas record episodic metamorphism from c. 55 to 15 Ma. In Uttarakhand, the events were discrete, c. 35 and 20 Ma. In Sikkim, mineral growth at c. 30 Ma was followed by trailing growth from c. 20 to 15 Ma. In each Sikkim sample, the 𝜹18O of different generations of zircon is the same, but differs between samples. 𝜹18O in the Uttarakhand samples is more varied. The same is the case for the monazite from both locations. Zircon and monazite formed in the same rock at the same time generally match in their 𝜹18O, but where they differ, the monazite 𝜹18O is lower. In all samples, even metasediments, the quartz 𝜹18O, although different between samples, is homogenous, consistent with isotopic re-equilibration.

Oxygen in zircon from orthogneisses with a Cambro-Ordovician protolith in both locations is > 9‰, consistent with a weathered lower crustal magma source. The difference in 𝜹18O between quartz and igneous zircon is generally not consistent with magmatic equilibration. The oxygen fractionation between quartz and metamorphic zircon indicates T < 400˚C in Uttarakhand and > 600˚C in Sikkim. The Quartz-Monazite fractionation indicates temperatures the same or lower. In Sikkim, the c. 30 Ma metamorphism recorded in both zircon and monazite involved a pervasive fluid—Th/U in the metamorphic zircon is very low and uniform, reflecting coeval growth of monazite. Protracted mineral growth during the period < 25 Ma was perhaps mostly solid state recrystallization, particularly in the highest elevation sample—the zircon Th/U is higher and varied and the zircons have only weak remnant zoning. In the lowest elevation sample, monazite grown during the period < 25 Ma has a distinctly low 𝜹18O, indicating interaction with an external fluid, probably groundwater.