# Himalayan Leucogranites: An Experimental Petrology Perspective

Bruno Scaillet, Michel Pichavant

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Bruno Scaillet, Michel Pichavant. Himalayan Leucogranites: An Experimental Petrology Perspective. Elements, 2024, 20, pp.388-394. 10.2138/gselements.20.6.388. insu-04848648

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# Himalayan Leucogranites: An Experimental Petrology Perspective

Bruno Scaillet<fnref n="1" /> and Michel Pichavant<fnref n="1" />

1811-5209/24/0020-0388$2.50 DOI: 10.2138/gselements.20.6.388

The Bhagirathi peaks in the Garwhal Himalayas, showing part of Gangotri lenses. PHOTO: BRUNO SCAILLET.

The High Himalayan leucogranites (HHL) are produced by muscovite breakdown of a metapelitic source, at temperatures below 800 °C, with initial melt water contents of ~5–7 wt.%. The tourmaline-rich HHL variety is colder, possibly a fractionation product of the hotter two-mica HHL. HHL lack restites such as iron-rich garnet, which, when present, is Mn-rich, signaling fractionation processes. The low redox state of HHL mirrors that of their graphite-bearing source, yet there is evidence of a significant increase in fO₂ during crystallization of some HHL. Their relationships with regional deformation call for late emplacement of the main bodies, which must have cooled at 3–4 kb to allow muscovite crystallization, which in turn imposes stringent constraints on unroofing rates of the collisional chain.

KEYWORDS: leucogranites; petrology; experiments; Himalayas

## INTRODUCTION

Himalayan leucogranites are lenticular bodies of variable dimensions that crop out in a discontinuous fashion along the entire Himalayan range, i.e., for more than 2500 km of lateral extension. Although they represent a minor component (less than 3% in area; Le Fort et al. 1987) of all rocks exhumed since the Indo-Tibetan collision, they have been the focus of numerous petrological and geochemical studies mainly because their outcrops are unrivaled in terms of both their 3D quality and freshness. Their relatively young age (in general, younger than 25 Ma) and high altitude of occurrence have preserved them from the adverse effects of erosion and alteration that usually obscure or even erase the petrological and geochemical attributes pertaining to the magmatic stage of plutonic rocks. They are typically considered pure products of crustal melting processes, i.e., magmas produced without any mantle input (but see Zheng et al. 2016; Pichavant et al. 2024). Despite their occurrence along several thousands of km, this plutonic suite displays a remarkably narrow compositional range characterized by low FeO-MgO-CaO (generally below 1 wt.% each) and high SiO₂ (72–75 wt.%) and Al₂O₃ (14–15 wt.%) contents (TABLE 1), suggesting melting under chemically buffered conditions. Their strong peraluminous (i.e., excess of molar Al₂O₃ over Na₂O + K₂O + CaO) character is a distinctive feature of this suite of magmas. Mineralogically, peraluminosity is expressed by the occurrence of Al-rich minerals such as muscovite ± tourmaline ± garnet ± andalusite ± sillimanite ± kyanite ± cordierite. The frequent occurrence of tourmaline, along with muscovite, is often taken as evidence of fluid-rich conditions being reached at some stage during the magmatic evolution.

Constraining the P–T–H₂O–fO₂ conditions of stability of these minerals in leucogranite melts has been an important goal of petrological studies aimed at defining the origin and evolutionary paths of the magmatic precursor to these plutonic rocks. This is important not only for understanding the nature of the magmatic processes at work during granite formation but also because the HHL are the main, if not the sole, magmatic consequence (in the Himalayan chain) of the collision between Asia and India. Hence, knowledge about HHL petrogenesis provides unique information about the

temperature and pressure conditions prevailing in the continental crust during collision, and in turn advice on thermomechanical concepts and models simulating the Himalayan orogen (e.g., Nabelek 2020). By inference, this gain of knowledge serves to shed light on which conditions and processes operated in more ancient mountain belts that contain similar felsic peraluminous rocks (e.g., Nabelek 2020), such as the Hercynian chain in western Europe, or areas of more recent magmatic activity, such as in northern Tibet or in the Peruvian-Bolivian Andes (TABLE 1), for which there is no access to magma sources.

## SOURCE AND CONDITIONS OF LEUCOGRANITE FORMATION

The presence of Al-rich biotite and muscovite as the principal aluminous and ferromagnesian phases in leucogranites suggests that these minerals were key players during the formation process. Isotopic constraints (e.g., Deniel et al. 1987) indicate that HHL share the same signature of the underlying metapelites in the High Himalayan Crystallines (HHC), which very often display textural evidence of partial melting that hints at a genetic link between these two lithologies. Exposed migmatites (heterogeneous metamorphic rock preserving evidence of partial melting), however, have major and trace element abundance patterns significantly different from the overlying HHL (Barbey et al. 1996), hence showing that there is no direct connection between migmatites and extracted leucogranites. Similarly, minute melt inclusions (Bartoli et al. 2019) present in some of the metamorphic minerals have major element compositions unlike the HHL, presumably recording either the very early stages of melt production or some older events not directly linked to the main stage of magma formation. Early models invoked partial melting of Himalayan metapelites triggered by the influx of hydrous fluids coming from the dehydration of underlying crustal slices (Le Fort et al.

<footnote n="1">¹ ISTO₂, CNRS/Université d'Orléans/BRGM
Orléans/45071, France
E-mail: bruno.scaillet@cnrs-orleans.fr
E-mail: michel.pichavant@cnrs-orleans.fr</footnote>
1987), with the temperature of melting possibly lowered by the addition of boron (whose presence is suggested by the common occurrence of tourmaline in HHL). Later work based on trace element behavior concluded that rather than fluid-assisted melting, fluid-absent melting, involving predominantly muscovite (FIG. 1), was the controlling mechanism of melt production (Harris et al. 1993; Wang et al. 2022a). Experimental work has further constrained the role of water in generating HHL. Partial melting experiments of Himalayan metapelites performed under either fluid-absent or fluid-present conditions (Patiño Douce and Harris 1998) clearly show that water-present melting gives rise to silicic melts too rich in Na₂O compared with HHL (FIG. 1). This highlights the fact that water-rich conditions lead primarily to the consumption of plagioclase and, hence, to the production of trondhjemicit rather than granitic liquids. In contrast, dehydration (or fluid-absent) melting of Himalayan metapelites in the temperature range 700–800 °C at middle to lower crustal pressures (6–9 kb; FIG. 1) involves predominantly the breakdown of muscovite (with the participation of quartz + plagioclase; FIG. 1) and produces liquids that are almost identical to HHL (TABLE 1; Patiño Douce and Harris 1998).

These experiments further indicate that HHL melts were indeed rich in water, containing at least 5 wt.% dissolved H₂O at this stage. This observation is in agreement with constraints gained from phase equilibrium experiments indicating that water contents of HHL melts during emplacement in the middle crust were in the range of 5–7 wt.% (FIG. 1; Scaillet et al. 1995). An important outcome of partial melting experiments is that they indicate that the amount of melt produced in a metapelite is closely controlled by the amount of muscovite present in the source rocks, with a 1:1 correspondence (i.e., 1 g of muscovite yields 1 g of melt). Therefore, the fertility of the protolith is directly controlled by the modal proportion of muscovite, which

has implications for the rheological behavior of partially melted crust, because the bulk viscosity of partially melted rocks, among other things, depends on melt proportion. In terms of temperature, the generally low FeO contents of HHL (<1 wt.%) implies that biotite contribution to melt production was relatively minor, although in the central part of the orogen, some HHL may have FeO contents approaching, or even exceeding, 2 wt.%, which suggests biotite breakdown (Visonà et al. 2012). This is also borne out by phase equilibrium experiments carried out on HHL, which show that biotite is a near-liquidus phase in HHL, indicating saturation in or stability of that phase in the source region during melting (FIG. 2). But in these particular experiments, muscovite is a late-crystallizing mineral in HHL, which is in accord with exhaustion of muscovite in the source during partial melting. It is worth noting

<figure><img src="image_2.png" /><figcaption>**FIGURE 1** (A) The principal stability curves of micas in metapelites along with the wet solidus of haplogranite (dashed line) and the one corresponding to peraluminous silicic melts (red line). ADAPTED FROM PATIÑO DOUCE AND HARRIS (1998) AND VISONÀ ET AL. (2012). MBS and MS stand for muscovite-biotite micaschist and muscovite-schist lithologies as used by Patiño Douce and Harris (1998) (B) Composition of experimental glasses produced during partial melting of Himalayan metapelites compared to HHL. Note that only dehydration melting produces melts similar to HHL (FROM PATIÑO DOUCE AND HARRIS (1998)).</figcaption></figure>

<table><caption>TABLE 1 REPRESENTATIVE COMPOSITIONS OF HIMALAYAN LEUCOGRANITES AND THEIR VOLCANIC EQUIVALENTS.</caption><thead><tr><th></th><th>Glass from metapelite dehydration melting<fnref n="1" /></th><th>Ms-Bt leucogranite (Manaslu)²</th><th>Tur-Ms leucogranite (Gangotri)²</th><th>Bt-Ms leucogranite (Makalu)³</th><th>Bt-Ms rhyolite (N. Tibet)⁴</th><th>Bt-Ms rhyolite (Macusani)⁵</th></tr></thead><tbody><tr><td>SiO₂</td><td>74.66</td><td>73.04</td><td>72.94</td><td>70.80</td><td>73.13</td><td>72.60</td></tr><tr><td>Al₂O₃</td><td>15.63</td><td>15.32</td><td>15.57</td><td>15.69</td><td>14.49</td><td>14.90</td></tr><tr><td>TiO₂</td><td>0.14</td><td>0.13</td><td>0.06</td><td>0.23</td><td>0.23</td><td>0.18</td></tr><tr><td>FeO</td><td>0.93</td><td>0.89</td><td>0.75</td><td>1.98</td><td>1.44</td><td>1.22</td></tr><tr><td>MgO</td><td>0.27</td><td>0.20</td><td>0.14</td><td>0.29</td><td>0.26</td><td>0.24</td></tr><tr><td>MnO</td><td>0.04</td><td>0.01</td><td>0.01</td><td>0.04</td><td>0.02</td><td>0.03</td></tr><tr><td>CaO</td><td>0.73</td><td>0.85</td><td>0.57</td><td>1.00</td><td>0.67</td><td>0.92</td></tr><tr><td>Na₂O</td><td>3.75</td><td>3.85</td><td>4.56</td><td>3.87</td><td>3.21</td><td>3.54</td></tr><tr><td>K₂O</td><td>3.87</td><td>4.96</td><td>4.14</td><td>4.7</td><td>5.08</td><td>4.74</td></tr><tr><td>P₂O₅</td><td>nd</td><td>0.14</td><td>0.19</td><td>0.15</td><td>0.32</td><td>0.50</td></tr><tr><td>Total</td><td>94.40</td><td>100.06</td><td>99.52</td><td>99.57</td><td>99.65</td><td>98.93</td></tr></tbody></table>

<footnote n="1">1 Patiño Douce and Harris (1998); 2 Scaillet et al. (1995); 3 Visonà et al. (2012); 4 Wang et al. (2012); 5 Pichavant et al. (1988)</footnote>
<figure><img src="image_3.png" /></figure>

**FIGURE 2** Phase relationships of a Bt-Ms leucogranite and a Tur-Ms leucogranite from Scaillet et al. (1995). Note the restricted stability field of muscovite, which occurs close to solidus conditions. The two yellow paths indicate the crystallization sequence for each leucogranite as inferred from petrographical observations. Mineral abbreviations are: Bt: biotite; Ms: muscovite; Pl: plagioclase; Afs: alkali feldspar; Qz: quartz; Tur: tourmaline; Hc: hercynite; Crd: cordierite.

that HHL outcropping in the northern part of the chain (the so-called North Himalayan Leucogranites, NHL) have higher contents of Fe, Mg, and Ca than HHL in general, which suggest that production of these leucogranites may have reached higher temperatures than elsewhere and may have involved consumption of some biotite and more calcic plagioclase.

The pressure of magma production is difficult to constrain. On the one hand, the study of metamorphic assemblages in partially melted metapelites generally yields peak pressures of 7–12 kb (Wang et al. 2022a) that could be taken as the pressure of melt production in the Himalayas. Field observations show that, in general, the upper reaches of the main HHL bodies have clear intrusive relationships with their overlying host rocks. In some favorable cases, such as in the Gangotri area, the bottom part of the bodies is seen connected to feeder dykes (Scaillet et al. 1995), which suggests that magma rose from deeper levels in the crust than the paleodepth of pluton formation. In other sections, such as in the Everest area, the relationships between HHL and their host rocks seem to be more intricate, suggesting a closer depth proximity between source and granite (Visonà et al. 2012). It therefore seems that these intrusions are either near-source melts, which have amalgamated and crystallized in close proximity to their sources, or allochtonous bodies having experienced significant vertical transfer following extraction. Hence, although HHL occur at broadly the same structural level across the chain (atop the mid-crust), there is probably a diversity in distance between sources and emplacement levels of HHLs along strike. This possibly reflects differences in erosional levels in different N–S transects, but also in the local thermal regime reached along any particular transect, imparting different mineralogical assemblages and abundances of Fe–Mg rich phases, in particular biotite, in the crystallized bodies (e.g., andalusite may typify low-pressure crystallization as opposed to muscovite or kyanite, which record higher pressures). Gangotri plutons could thus represent colder structural levels of emplacement compared with those

seen in the Everest-Makalu area, which testify to a hotter environment. Interestingly, the latter area is close to the Ama Drime massif where exceptionally high temperatures (exceeding 900 °C) were reached in the Miocene (Wang et al. 2022a).

A fundamental petrological attribute of HHL is the occurrence of muscovite, whose clear magmatic origin (FIG. 3) requires pressures of magma crystallization of at least 3.5–4 kb (FIG. 1). Experiments performed by Benard et al. (1985) below 3 kb were unable to crystallize muscovite, showing that 3 kb is too low of a pressure for HHL crystallization. In contrast, experiments at 4 kb by Scaillet et al. (1995) crystallized this diagnostic phase (FIGS. 2 and 3). A pressure differ-

ence of up to about 8 kb therefore seems to exist between where melts are sourced and where they are emplaced in the Himalayas. The occurrence of andalusite in some HHLs, notably in the Everest-Makalu area (Visonà et al. 2012), indicates that some HHLs entered the stability field of this aluminosilicate phase during their crystallization. This requires not only pressures somewhat below 4 kb but also temperatures significantly lower than the water-saturated solidus of leucogranites. This could be due to the combined effect of excess Al₂O₃ and volatile elements, such as B and F, which may have locally depressed the solidus into the stability field of andalusite (FIG. 1). That this actually happens in nature is indicated by the occurrence of magmatic andalusite in the peraluminous Macusani volcanic rocks (southeastern Peru), which are compositionally equivalent to HHL (Pichavant et al. 1988), albeit produced in a more conventional subduction zone setting. In the Macusani volcanic rocks, andalusite and muscovite are clearly exclusive of each other (Pichavant et al. 1988), while that same evidence is less apparent in HHL.

Garnet is also frequently observed in HHL (FIG. 3), although less so than tourmaline (FIG. 3). When present, it seems to indicate quite fractionated HHL. Garnet-bearing HHLs lack biotite, and garnet is rich in MnO and poor in both CaO and MgO (Barbey et al. 1996) and may occur together with muscovite and/or tourmaline (FIG. 3). Garnet occurrence seems to be clearly related to the enrichment in Mn of the residual liquid, which is first-order evidence for fractionation processes to have occurred to some extent in the HHL, at least on a local scale. Although almandine-rich garnet is frequently described in leucosomes (the lightest-colored part of a migmatite) present in partially melted metapelites (Barbey et al. 1996), so far this phase has not been found in sizeable HHL bodies. Similarly, the aforementioned phase equilibrium experiments at 4 kb did not establish any stability field for almandine-rich garnet in HHL (Scaillet et al. 1995). This is at variance with the more mafic S-type granites (Clemens and Birch 2012), in which garnet is a near-liquidus phase at 5 kb, being replaced by cordierite at lower pressure. In HHL, cordierite displays a small high-temperature, near-solidus stability field (FIG. 2), which suggests that cordierite is unstable during the late stage of HHL crystallization. As for andalusite, occurrence of cordierite in HHL may signal specific conditions of magma production at relatively low pressure and high temperature (Visonà et al. 2012), and possibly drier conditions.
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>**FIGURE 3** (A–D) Back-scattered electron images of typical HHL granites showing the main Fe–Mg silicate phases crystallizing from such magmas. Note the euhedral habit of Pl, Tur, and micas. (E, F). BSE images of run products of crystallization experiments on HHL (Scaillet et al. 1995) showing a Pl–Qz-saturated melt (GB32, 803 °C, about 4 wt.% H₂O in melt) and a near-solidus charge saturated with tectosilicates and muscovite and biotite (GB43, 682° C, about 4 wt.% H₂O in melt). Phase abbreviations are: Bt: biotite; Ms: muscovite; Pl: plagioclase; Afs: alkali feldspar; Qz: quartz; Tur: tourmaline; Grt: garnet; Ap: apatite; Gl: glass.</figcaption></figure>

A last feature of interest regarding the petrology of HHL is their conspicuous lack of oxide minerals. Neither ilmenite nor magnetite have been described to occur in the main bodies of leucogranites, notably those in the High Himalayas. This reflects redox conditions characterized by low fO₂ at or below that defined by the quartz-fayalite-magnetite (QFM) buffer (Scaillet et al. 1995). Such low fO₂ is also a characteristic of their volcanic equivalent (the Macusani volcanic rocks), which nevertheless contain ilmenite (Pichavant et al. 1988), a phase not observed in HHL. This overall low fO₂ of HHL is largely thought to mirror the redox conditions prevailing in their source. Himalayan metapelites are rich in graphite (Le Fort et al. 1987), whose presence acts as a powerful buffer of the melt's fO₂ around QFM (Chu and Ague 2013). Crystallization experiments on

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

HHLs at or below QFM lack oxides (Scaillet et al. 1995), whereas they contain magnetite when carried out around the Ni–NiO redox transition (Benard et al. 1985). Note that the lack of oxides does not necessarily imply that redox conditions never increased during magma evolution: it may well be that oxidation occurred late, at a time when iron activity in the melt was low because most FeO was already sequestered in other Fe-bearing minerals (tourmaline, muscovite, biotite, garnet).

## FRACTIONATION PROCESSES IN HIMALAYAN LEUCOGRANITES

High Himalayan leucogranites have near-minimum melt compositions and very often do not display clear differentiation trends for either major or trace elements at the pluton scale. This, along with their strong Sr isotopic heterogeneity (Deniel et al. 1987), has popularized the idea that the precursor magmas underwent only little or no differentiation upon leaving their source. The protracted time interval of crystallization ages recorded by the large bodies (about 5–7 My, Deniel et al. 1987; Harrison et al. 1999; Cottle et al. 2019) suggests that magma emplacement was incremental, proceeding via the addition of a succession of small batches of magma, each crystallizing soon after emplacement, thereby leaving little opportunity for pluton-scale differentiation. The Manaslu intrusion in the Nepal Himalayas is a case in point: this 5-km-thick body was emplaced in about 5 My (Harrison et al. 1999), which corresponds to the addition of a magma layer 1 mm thick every year (or 1 km every million years), if occurring evenly across the time interval in question. At such a low rate, it is easily conceivable that any small intrusion will freeze almost instantaneously against previously crystallized injections. As a result, the geochemistry of HHLs has largely been considered to carry the fingerprint of their source (Le Fort et al. 1987), with biotite and tourmaline indicating slightly different conditions of magma production or source (Guillot and Le Fort 1995; Nabelek 2020).

Yet some intrusions do display clear geochemical trends for both major and trace elements, which are suggestive of fractionation processes occurring in situ at the emplacement level. An example of this type of HHL is the Gangotri lenses in the Garwhal Himalayas, in which tourmaline-bearing HHL are seen as fractionated products of the more mafic, biotite-bearing variety (Scaillet et al. 1990). Monazite and zircon thermometry yield temperatures
consistent with expected differentiation trends driven by plagioclase and biotite fractionation (i.e., regular cooling as FeO + CaO decreases) matching phase equilibrium constraints, altogether pointing to a temperature below 750 °C for the tourmaline-bearing variety, and in the range of 750–800 °C for the more common two-mica granites (Scaillet et al. 1990, 1995).

More recently, new isotopic tracers have been used to unravel whether existing geochemical trends reflect melting (i.e., source) or crystallization processes (Shi et al. 2021; Wang et al. 2022b). Depending on the isotopes used, however, the conclusions are seemingly contradictory: whereas K isotopes point to rather extensive crystal fractionation having affected the HHL (Wang et al. 2022), Fe isotopes suggest the opposite (Shi et al. 2021). This may simply reflect that both processes are at work in HHL, with their relative importance depending on the massif being studied. To a first order, on thermal grounds, large bodies (i.e., at least one km thick) can be expected to show evidence of in-situ fractionation, while small stocks or sills may better preserve source characteristics, owing to their faster cooling rates (depending also on the rate of intrusion and assuming fast upward transfer via dykes).

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>**FIGURE 4** (A) Crystallization path of a leucogranite magma with 5 wt.% H₂O dissolved in the melt at about 800 °C, calculated assuming a closed system based on data of Scaillet et al. (1995). (B) The melting-crystallization loop followed by a typical HHL, from its source to its emplacement level. THE MELT FRACTION TREND OF PARTIALLY MELTED METAPELITE IS FROM PATIÑO DOUCE AND HARRIS (1998).</figcaption></figure>

As with many plutonic rocks, the physical mechanism through which fractionation proceeds in HHL remains elusive. Settling of early crystallized minerals is one option, but other factors may also play a role, such as boundary layer fractionation, or compaction of a cumulate pile with expulsion of residual liquids. There is no textural evidence of massive restite entrainment in HHL, save some minor garnet or fibrolitic sillimanite/andalusite coring plagioclase in some HHL of inferred near-autochthonous character (Visonà et al. 2012). Near-liquidus phases such as plagioclase and biotite have compositions well reproduced by crystallization experiments at the level of emplacement. Almandine garnet, a likely stable phase in the source (Patiño Douce and Harris 1998), does not crystallize in HHL, and is thus unlikely to have played any significant role in driving observed chemical trends. The rather high P₂O₅ contents of HHL (compared with common metaluminous granites) have been shown to reflect the enhanced solubility of P in aluminous melts (Pichavant et al. 1992) rather than the restitic origin of apatite: apatite interstitial occurrence in HHL or as abundant inclusions in tourmaline is in agreement with relatively late crystallization of this mineral. Altogether, textural observations suggest that the crystal cargo of the magma entrained from its source was very small, except perhaps for HHL inferred to have crystallized near their sources (Visonà et al. 2012). Most HHL magmas are likely characterized by low crystal contents upon extraction and, following their emplacement at upper crustal levels, crystallize under a near-eutectic regime in which the main part of crystallization occurs some 10–20 °C above the solidus (FIG. 4).

Although the redox state of HHL seems to have been fairly low at the onset, there is mineralogical evidence for fO₂ change during crystallization of some HHL (e.g., Scaillet et al. 1995). In particular, the absence of biotite or its replacement by muscovite or tourmaline may record such a phenomenon (Scaillet et al. 1995; Pichavant et al. 2016). There have been various reports documenting cassiterite occurrence in HHL, most frequently in their cortege of pegmatites (e.g., Wu et al. 2020), which also calls for some late oxidation (Linnen et al. 1996). As for the fractionation mechanism, the physical process through which an HHL magma gets oxidized (e.g., settling of Fe²⁺-rich minerals, loss of fluid phases) remains to be identified.

## EMPLACEMENT PROCESSES

High Himalayan leucogranites are intimately linked to the collision between India and Eurasia and from a general viewpoint can be considered as syntectonic granites, being sandwiched between two major tectonic features of the chain, the Main Central Thrust (MCT) at the bottom and the South Tibetan Detachment (STD) at the top. There is a variety of cross-cutting relationships between HHL and their host rocks that reflects the duration of magmatic activity over several million years (Harrison et al. 1999). In detail, however, large plutons appear to be only weakly deformed or undeformed, and mineral fabric or preferred orientations are very often absent, except for magmatic layering
involving tourmaline that may occur in some outcrops. The intrusive relationships of HHL with their host rocks, notably in the upper reaches of the plutons where HHLs sharply crosscut the local metamorphic foliation/lineation without being deformed themselves, indicate that pluton growth proceeded late relative to the acquisition of the regional metamorphic fabric. In contrast, when observable, the bottom parts of large intrusions appear to display more conformable geometry with the regional fabric. Thus, while at the scale of the entire Himalaya chain, HHL production may be viewed as coeval with the N–S convergence and ensuing development of metamorphic fabric (i.e., syncol- lisional bodies), at the outcrop scale, this relationship is not always apparent.

Detailed geochronological work on large intrusions shows that crystallization ages recorded by monazite are increas- ingly younger towards the bottom of plutons (Cottle et al. 2019), supporting the view that the plutons have been built by successive additions of melt batches from the top down. Thermal modeling has also shown that in order to (i) prevent wholesale convection (as required by the strong Sr isotope heterogeneity), (ii) preserve the pristine character of magmatic muscovite (which, owing to its limited thermal stability, could be affected by thermal excursions arising from intrusions occurring in close successions), and (iii) develop only thin thermal aureoles in the top part of Manaslu, magmatic accretion must have proceeded via the slow stacking of small sills each about a hundred meters thick, also from the top down (Annen and Scaillet 2006). Interestingly, the latter size is about the dimension of the outcrop from which a unique Rb/Sr isochron has been obtained on an HHL (Deniel et al. 1987).

The concept of small batches amalgamating progressively to produce sizeable plutons, a popular notion nowadays to explain many if not all plutons in arc settings, was first clearly demonstrated for the Manaslu batholith (Deniel et al. 1987). This, along with the recognition of dykes seemingly feeding some HHL, has led to the model of magma moving as dykes through the crust on the way to form HHLs (Scaillet et al. 1996). Dyking requires low magma viscosity, which fits with the low crystal contents of injected melts, thus supporting the idea that the bulk of crystallization of HHLs is achieved at their final emplace- ment levels (FIG. 4). For the NHL, their structural relation- ships with their host rocks (coring regional metamorphic domes), has led to their common description as diapiric intrusions rather than sills (e.g., Wang et al. 2022a).

The relationships between HHL and the STD have long been debated due to their complexity (Kellett et al. 2018). A variety of cross-cutting relationships between HHL and their host rocks has been described, with some studies reporting HHL as predating the STD, while others report HHL cross-cutting the STD and, hence, postdating its movement (see Kellett et al. 2018). A difficulty in disen- tangling this relationship is that not every leucocratic layer in the metamorphic package crystallized from a true peraluminous melt; instead, the chemically analo- gous bodies outcropping up-section may have been partial melts formed by muscovite breakdown. Small dykes cutting across the STD may correspond to the waning stage of magma activity rather than the main episode of melt intru- sion that formed the largest HHL plutons. It is clear that

HHL and the STD are spatially closely related but whether HHL production favored the STD or vice versa is a chicken- and-egg dilemma (Weinberg 2016): anatexis and granite formation may have weakened the crust leading to the development of the STD either as an extensional response or due to the development of a channel between the STD and the MCT. Petrological considerations have led some authors to propose near-adiabatic decompression as a way to partially melt the Himalayan metapelites (Harris et al. 1993), in which case, the STD is temporally the first in the sequence, and the HHL should be viewed as a mere conse- quence of the extensional collapse of the thickened crust. In contrast, other studies have put forward petrological evidence arguing for a near-source emplacement along a near-isobaric prograde path (Visonà et al. 2012). Thermal modeling at the scale of the Himalayan chain has been used to argue that shear heating is an essential component in generating sufficient heat to partially melt the source region, hence calling for synkinematic magma production (Harrison et al. 1999). There is thus a diversity of opinions about the causes of melt production. However, if both the MCT and the STD were indeed synchronous and the STD played a critical role in HHL generation, why then are the largest HHL plutons essentially devoid of both magmatic and solid-state deformation? One possibility is the occur- rence of so-called strain localization, in particular once the HHLs were in the solid state and when surrounded by softer, schist-dominated host-rocks that could accommodate most of the deformation (Weinberg 2016). Clearly, however, the tops of HHL plutons are often affected by solid-state shear related to movement of the STD, but the bulk of petro- graphic and structural evidence suggests a post-kinematic emplacement of the largest leucogranite plutons outcrop- ping south of the STD. In addition, thermal modeling of HHL plutons has shown that in order to allow magmatic crystallization of muscovite in the thickest bodies, a very subtle balance between tectonic unroofing, erosion, and emplacement rates is required (Annen and Scaillet 2006). For a thick body such as Manaslu, coeval movement of the STD with magma emplacement implies an unroofing rate of about 1 mm/y and an initial injection depth of about 22 km. Because the plutons grew from the top downwards, this would imply that the first lenses were intruded into a quite deep and hot environment, which is not, however, what field observations show everywhere. Hence, the nature of the relationships between leucogranite genera- tion and the regional deformation regime still needs to be scrutinized in detail: the macroscopic evidence so far calls for HHL crystallization in a static medium after movement of the MCT but prior to movement of the STD. However, as mentioned above, part of this problem could be alleviated if strain partitioning preserved the main mass of HHL, in particular whenever they were intruded into softer rocks.

## ACKNOWLEDGMENTS

This work has received continuous support from CNRS-INSU. We also acknowledge support from both the LabEx VOLTAIRE (ANR-10-LABX-100-01) and the EquipEx PLANEX (ANR-11-EQPX-0036) projects. The authors thank Roberto Weinberg, Liu Zhi Chao, and Peter Nabelek for insightful reviews and Fuyuan Wu, Fang-Zhen Teng, and Janne Blichert-Toft for careful editorial handling.
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