Timescales of Partial Melting and Melt Crystallization in the Eastern Himalayan Orogen: Insights From Zircon Petrochronology

Abstract

Revealing the timescales of metamorphic and anatectic processes is central to our understanding of tectonic evolution of collisional orogens. High‐temperature migmatites and leucogranites are well exposed in the Himalayan orogenic core, making it an ideal region to study the timing and duration of partial melting and melt crystallization of the orogen. Here, we report an integrated and comprehensive data set of petrography, U‐Pb age, and trace element data for zircon from a pelitic granulite and associated leucosomes of the Greater Himalayan Sequence (GHS) in the Yadong area, eastern Himalaya. Zircon grains with complex internal structure retain variable ages ranging from 32 Ma to 13 Ma that correlate systematically with changes in the concentrations of Y, Th, U, Hf, Nb, Ta, and HREE, and ratios of Th/U, Eu/Eu*, and Nb/Ta. Combined with petrologic analysis, we conclude that the granulite witnessed high‐temperature metamorphism, melting, and melt crystallization over ∼20 Myr. Prograde, simultaneous increases in pressure and temperature and associated dehydration melting began at least by ∼32 Ma and lasted until ∼24 Ma. Subsequent quasi‐isothermal decompression‐melting occurred between ∼22 and 19 Ma, and late melt crystallization spanned ∼19 to 13 Ma. Large volumes of melt generated during prograde metamorphism could have triggered exhumation of GHS rocks, increasing melt fraction through a positive feedback between exhumation and melting. More comprehensive analysis of different rock types led to more complete and different interpretations for the timing of exhumation and melt crystallization in the Yadong‐Sikkim region and might enable alternate interpretations elsewhere in the Himalayas.

Tectonothermal transition from continental collision to post‐collision: Insights from eclogites overprinted in the ultrahigh‐temperature granulite facies (Yadong region, central Himalaya)

“…30 Ma from the hosting gneisses represent the time of eclogitization. However, their arguments are debatable because (1) the yttrium (Y) contents in the monazite stay low until 20 Ma or later, which means garnet continuously grew from 30 Ma to at least 20 Ma; (2) the GHC gneisses have undergone prolonged (from 32 Ma to at least 24 Ma) prograde partial melting (H. Ding et al, 2021). At c .…”

“…J. M. Wang, Lanari, et al (2021) argued about these traditional discriminant methods and proposed that zircon and monazite ages of c. 30 Ma from the hosting gneisses represent the time of eclogitization. However, their arguments are debatable because (1) the yttrium (Y) contents in the monazite stay low until 20 Ma or later, which means garnet continuously grew from 30 Ma to at least 20 Ma; (2) the GHC gneisses have undergone prolonged (from 32 Ma to at least 24 Ma) prograde partial melting (H. Ding et al, 2021). At c. 30 Ma, the hosting gneisses may start to melt probably under high pressure (e.g., >1.0 GPa), but no evidence proves they reached the peak-pressure stage ($2.0 GPa);…”

A modern pulse of ultrafast exhumation and diachronous crustal melting in the Nanga Parbat Massif

“…Petrologic studies suggest that the former midcrustal channel preserved in the exposed Greater Himalayan Sequence in the central Himalaya (Fig. 1) underwent brief periods of anomalously rapid exhumation approaching plate tectonic rates (6 to 10 mm/year) in Oligo-Miocene time, themselves thought to represent pulses of accelerated midcrustal flow (36)(37)(38). The syntaxial massifs of the orogen are currently the sites of the highest intracontinental exhumation rates on the modern Earth.…”

“…Moreover, the buoyancy increase associated with partial melting may assist exhumation (54). Oligo-Miocene pulses of rapid exhumation in the Greater Himalayan Sequence are thought to have been triggered by partial melting that occurred just before, or coeval with, the period of rapid exhumation (36)(37)(38)55). This was suggested to explain ultrafast exhumation in the NPM in (4), which proposed that the biotite breakdown melting reaction, crossed during decompression through ~0.5 GPa, sufficiently weakened the crust to trigger the onset of extremely rapid exhumation.…”

Tracing high-pressure metamorphism in the eastern Himalayan syntaxis using detrital zircon and monazite from modern stream sediments

“…In contrast, the eclogite-or HP granulite-facies metamorphic rocks from the central Himalayas (Fig. 1a; Kali Gandaki Valley, Ama Drime Massif, Arun River Valley, Diggye, Yadong, North Sikkim, and Jomolhari Massif) yield younger peak metamorphic ages of 38-13 Ma (Li et al, 2003;Groppo et al, 2007Groppo et al, , 2010Cottle et al, 2009;Corrie et al, 2010Corrie et al, , 2011Grujic et al, 2011;Kohn and Corrie, 2011;Rubatto et al, 2013;Kellett et al, 2014;Mottram et al, 2014;Iaccarino et al, 2015;Li et al, 2015;Wang et al, 2017a;Wang et al, 2017b;Li et al, 2019;Ding et al, 2021). The different P-T-t paths indicate that these HP/UHP metamorphic rocks formed in different structural positions along the Himalayas during India-Asian collision.…”