Nigel B. W. Harris

The Higher Himalayas in Sikkim consist of two-mica leucogranites (2mg), tourmaline leucogranites (Tg), and pegmatites. The leucogranites in North Sikkim intrude the Higher Himalayan Sequences (HHS). In this study, we present the first systematic dataset of whole-rock oxygen isotopic compositions for Higher Himalayan leucogranites from Sikkim, providing insights into their magmatic sources and evolution. Oxygen isotope measurement was accomplished using bulk fluorination and isotope ratio mass-spectrometry, and the oxygen isotope ratios (δ¹⁸O) were measured relative to Vienna Standard Mean Ocean Water (VSMOW). The analyses were calibrated against international standards NBS-28 (quartz). The two-mica leucogranites (7 samples) are characterized by biotite and muscovite, exhibit a mean δ¹⁸OW.R value of 9.6 ± 1.7‰, whilst tourmaline leucogranites (3 samples), characterized by the presence of tourmaline, yield a mean δ¹⁸OW.R value of 11.6 ± 3.9‰. The variations in δ¹⁸O values possibly reflect the originally distinct δ¹⁸O signatures of the source sediments, which were moderated by diffusive exchange during diagenesis and metamorphism (France-Lanord et al., 1988). The higher δ¹⁸O values observed in leucogranite samples may be attributed to the pelite-rich sediments, and the lower δ¹⁸O values can result from metagreywacke source or due to the presence of epidotized calc-silicates.

The nature, location, longevity and pressure-temperature conditions of different crustal melt reactions during orogenesis provide constraints on the structure, mechanical strength and exhumation of orogenic middle crust as well as element mobilisation and crustal differentiation. The Himalayan orogen offers a natural laboratory for studying crustal melting by exposing both migmatites and leucogranites in its structurally highest levels. We combine previous frameworks that link petrography or bulk geochemistry to melt reaction with in-situ trace-element analyses of large-ion lithophile elements in feldspar, mica, and garnet, U-Th-Pb isotopes in monazite and zircon and thermometry calculations in samples from the Badrinath region of the Garhwal Himalaya. Our samples naturally fall into three groups that we interpret as having formed by fluid-present melting of muscovite (Group 1, all migmatites; 650–750 °C), muscovite dehydration melting (Group 2, migmatites, leucosomes and leucogranites; 730–800 °C) and biotite dehydration melting (exemplified by a single leucogranite that contained zoned and inclusion-rich garnet and porpyroblastic K-feldspar). Geochronological data suggest that melting occurred over 20 Ma, with different samples experiencing different reactions and capturing different parts of the record at different times. Despite experiencing the same thermal history, individual outcrops typically only record one melting reaction instead of a progression through fluid-present melting followed by muscovite-dehydration melting. We interpreted this as being due to local compositional variations and availability of fluids. Our results show that petrographic observations and the mineral chemistry record are similar between (source) migmatites and (product) granites, but that fluid-present reactions are only documented in migmatites.

The Himalayan leucogranites are important for deciphering the role of crustal thickening and anatexis during the tectono-thermal evolution of the Himalayas. The Higher Himalayan Sequences (HHS) in Sikkim are intruded by Miocene leucogranites which comprise two-mica-bearing (2mg) and tourmaline-bearing leucogranites (Tg) both of which are peraluminous in nature, and are characterized by high 87Sr/86Sr ratios and low 143Nd/144Nd ratios, typical of crustal values. In the case of 2mg, the average (87Sr/86Sr) ratio is 0.792020, and the average (143Nd/144Nd) ratio is 0.511764 whilst for Tg, the average (87Sr/86Sr) and (143Nd/144Nd) ratios are 0.772874 and 0.511912 respectively. In comparison to 2mg, Tg has lower average ratios of (87Sr/86Sr) and slightly elevated average ratios of (143Nd/144Nd). The Sr-Nd isotope compositions for the granitic gneisses from the HHS have an average (87Sr/86Sr) ratio of 0.764026 and an average (143Nd/144Nd) ratio of 0. 511959. The present day average εNd values for 2mg, Tg, and granite gneiss are -17.1, -14.2, and -13.2 respectively. The granite gneiss samples all lie within the field defined by the HHS across the Himalayan orogen as do the Tg samples suggesting a source from pelitic rocks within the HHS. The 2mg samples are somewhat evolved towards more radiogenic Sr isotope ratios and slightly lower neodymium ratios suggesting either a source in metagreywacke rocks of the HHC or, alternatively, reflect minor melt contributions from anatexis of the Lesser Himalayan formations.Keywords: Sr-Nd isotopes, Source magma, Sikkim Himalayas, Leucogranites

The timing of high-pressure (HP) metamorphism in the eastern Himalayan syntaxis is important for understanding the India-Asia collisional processes, but it remains elusive. To reveal the metamorphic history of the eastern Himalayan syntaxis, we performed a study of geochronology, trace elements, and mineral inclusions of detrital zircon and monazite from modern stream sediments in the eastern Himalayan syntaxis. Detrital zircon comprise magmatic and metamorphic domains with different zoning. Inherited magmatic zircon domains have high Th/U, low (Dy/Yb)_N, and retain ages of 1798−360 Ma. Metamorphic zircon domains with low Th/U, high (Dy/Yb)_N, and inclusions of garnet, kyanite, and/or clinopyroxene probably formed under HP conditions. They yield age groups of 49−35 Ma, 33−17 Ma, and 12−7 Ma. The low Th/U and low (Dy/Yb)_N metamorphic zircon domains probably formed during retrogression and yield age groups of 27−16 Ma and 10−6 Ma. Detrital monazite yield age distributions similar to those of the low (Dy/Yb)_N metamorphic zircon except for the 821−402 Ma inherited cores. The (Dy/Yb)_N of 31.6−5.7 Ma monazite decreases with increasing Y content, which indicates that it likely formed under the retrograde stage during garnet breakdown. Based on the oldest metamorphic ages, the initial India-Asia collision occurred no later than 50−44 Ma in the eastern Himalayan syntaxis. The multimodal age patterns of the metamorphic zircon and monazite indicate that the Indian continent underwent multistage HP and retrograde metamorphism in the eastern Himalayan syntaxis. The nearly contemporaneous HP and retrograde metamorphism indicate that the Indian continent continued subducting while the earlier HP metamorphic slices detached and exhumed.

The petrogenesis of Himalayan leucogranites remains crucial for understanding the thermal and tectonic evolution of the Himalayan orogen. To understand whether they are largely pristine melts of crustal anatexis or have experienced a high degree of fractional crystallization (FC), we present Fe isotopic data of 30 representative Himalayan leucogranites and 9 local metasedimentary rocks. Excepting three garnet leucogranites with low δ⁵⁶Fe (−0.04‰–0.06‰) that are likely affected by garnet accumulation, tourmaline, and two‐mica leucogranites have largely homogeneous δ⁵⁶Fe from 0.13‰ to 0.24‰ irrespective of their highly variable SiO₂, MgO, and FeOt contents. Combined with observed mineral assemblages and available fractionation factors, this does not support a high degree of FC (with or without assimilation) in their petrogenesis. The elevated δ⁵⁶Fe relative to the supposed source rocks, represented by metasedimentary rocks and/or metabasite with a δ⁵⁶Fe value of 0.10‰, by ∼0.07‰, may reflect Fe isotope fractionation during crustal anatexis. This study indicates most leucogranites can provide robust constraints on the conditions of crustal anatexis and thus the thermal and tectonic evolution of the Himalayan orogen.

Asian Monsoon rainfall supports the livelihood of billions of people, yet the relative importance of different drivers remains an issue of great debate. Here, we present 30 million-year model-based reconstructions of Indian summer monsoon and South East Asian monsoon rainfall at millennial resolution. We show that precession is the dominant direct driver of orbital variability, although variability on obliquity timescales is driven through the ice sheets. Orographic development dominated the evolution of the South East Asian monsoon, but Indian summer monsoon evolution involved a complex mix of contributions from orography (39%), precession (25%), atmospheric CO₂ (21%), ice-sheet state (5%) and ocean gateways (5%). Prior to 15 Ma, the Indian summer monsoon was broadly stable, albeit with substantial orbital variability. From 15 Ma to 5 Ma, strengthening was driven by a combination of orography and glaciation, while closure of the Panama gateway provided the prerequisite for the modern Indian summer monsoon state through a strengthened Atlantic meridional overturning circulation.

The chemical compositions of magmatic zircon growth zones provide powerful insight into evolving magma compositions due to their ability to record both time and the local chemical environment. In situ U-Pb and Hf isotope analyses of zircon rims from Oligocene–Miocene leucogranites of the Bhutan Himalaya reveal, for the first time, an evolution in melt composition between 32 and 12 Ma. The data indicate a uniform melt source from 32 Ma to 17 Ma, and the progressive addition of an older source component to the melt from at least ca. 17 Ma. Age-corrected ε_Hf ratios decrease from between –10 and –15 down to values as low as –23 by 12 Ma. Complementary whole-rock Nd isotope data corroborate the Hf data, with a progressive decrease in ε_Nd(t) from ca. 18 to 12 Ma. Published zircon and whole-rock Nd data from different lithotectonic units in the Himalaya suggest a chemical distinction between the younger Greater Himalayan Series (GHS) and the older Lesser Himalayan Series (LHS). The time-dependent isotopic evolution shown in the leucogranites demonstrates a progressive increase in melt contribution from older lithologies, suggestive of increasing LHS involvement in Himalayan melting over time. The time-resolved data are consistent with LHS material being progressively accreted to the base of the GHS from ca. 17 Ma, facilitated by deformation along the Main Central thrust. From 17 Ma, decompression, which had triggered anatexis in the GHS since the Paleogene, enabled melting in older sources from the accreted LHS, now forming the lowermost hanging wall of the thrust.