Timescales of crustal melting in the Higher Himalayan Crystallines (Sikkim, Eastern Himalaya) inferred from trace element-constrained monazite and zircon chronology

Abstract

The petrology and timing of crustal melting has been investigated in the migmatites of the Higher Himalayan Crystalline (HHC) exposed in Sikkim, India. The metapelites underwent pervasive partial melting through hydrous as well as dehydration melting reactions involving muscovite and biotite to produce a main assemblage of quartz, K-feldspar, plagioclase, biotite, garnet ± sillimanite. Peak metamorphic conditions were 8-9 kbar and *800 °C. Monazite and zircon crystals in several migmatites collected along a N-S transect show multiple growth domains. The domains were analyzed by microbeam techniques for age (SHRIMP) and trace element composition (LA-ICP-MS) to relate ages to conditions of formation. Monazite preserves the best record of metamorphism with domains that have different zoning pattern, composition and age. Zircon was generally less reactive than monazite, with metamorphic growth zones preserved in only a few samples. The growth of accessory minerals in the presence of melt was episodic in the interval between 31 and 17 Ma, but widespread and diachronous across samples. Systematic variations in the chemical composition of the dated mineral zones (HREE content and negative Eu anomaly) are related to the variation in garnet and K-feldspar abundances, respectively, and thus to metamorphic reactions and P-T stages. In turn, this allows prograde versus decompressional and retrograde melt production to be timed. A hierarchy of timescales characterizes melting which occurred over a period of *15 Ma (31-17 Ma): a given block within this region traversed the field of melting in 5-7 Ma, whereas individual melting reactions lasted for time durations below, or approaching, the resolution of microbeam dating techniques (*0.6 Ma). An older *36 Ma high-grade event is recorded in an allocthonous relict related to mafic lenses. We identify two sections of the HHC in Sikkim that traversed similar P-T conditions at different times, separated by a tectonic discontinuity. The higher structural levels reached melting and peak conditions later (*26-23 Ma) than the lower structural levels (*31-27 Ma). Diachronicity across the HHC cannot be reconciled with channel flow models in their simplest form, as it requires two similar high-grade sections to move independently during collision.

Tectonic interleaving along the Main Central Thrust, Sikkim Himalaya

“…2b), which is generally consistent with that presented by Rubatto et al (2013). Our study, which presents the first isotopic data from the rocks of the Sikkim Himalaya, demonstrates that rocks sometimes mapped as a separate lithological unit, the 'Main Central Thrust zone' (Fig.…”

Possible polymetamorphism and brine infiltration recorded in the garnet–sillimanite gneiss, Skallevikshalsen, Lützow–Holm Complex, East Antarctica

“…Estimating the timing and duration of high-grade metamorphism through zircon and monazite dating can potentially place a very important constraint on the tectonic framework of lower crustal processes during continental collision events (e.g., Johnson and Harley, 2012;Rubatto et al, 2013;Korhonen et al, 2013). Recent advance in understanding the growth/recrystallization timing of zircon and monazite during metamorphic cycle, especially the roles of partial melting and fluid infiltration events in formation/dissolution/recrystallization of these minerals enable to reconstruct the lower crustal evolution in more detail (e.g., Rubatto et al 2001;Rubatto, 2002;Harley and Kelly, 2007;Rubatto et al, 2013;Kawakami et al, 2013;Korhonen et al, 2013;Harley and Nandakumar, 2014).…”

“…Recent advance in understanding the growth/recrystallization timing of zircon and monazite during metamorphic cycle, especially the roles of partial melting and fluid infiltration events in formation/dissolution/recrystallization of these minerals enable to reconstruct the lower crustal evolution in more detail (e.g., Rubatto et al 2001;Rubatto, 2002;Harley and Kelly, 2007;Rubatto et al, 2013;Kawakami et al, 2013;Korhonen et al, 2013;Harley and Nandakumar, 2014).…”

Deformation structures and processes within the continental crust: an introduction

“…The duration of magmatic episodes at the scale of an orogen can last for tens of millions of years, albeit regionally the duration of magmatic events is down to millions of years (e.g. Rubatto et al 2013). On an even more detailed scale, considering just one single intrusive event, the duration does not commonly exceed 1 Ma.…”