Abstract

Assigning correct protolith to high metamorphic-grade core zone rocks of large hot orogens is a particularly important challenge to overcome when attempting to constrain the early stages of orogenic evolution and paleogeography of lithotectonic units from these orogens. The Gurla Mandhata core complex in NW Nepal exposes the Himalayan metamorphic core (HMC), a sequence of high metamorphic-grade gneiss, migmatite, and granite, in the hinterland of the Himalayan orogen. Sm-Nd isotopic analyses indicate that the HMC comprises Greater Himalayan sequence (GHS) and Lesser Himalayan sequence (LHS) rocks. Conventional interpretation of such provenance data would require the Main Central thrust (MCT) to be also outcropping within the core complex. However, new in situ U-Th/Pb monazite petrochronology coupled with petrographic, structural, and microstructural observations reveal that the core complex is composed solely of rocks in the hanging wall of the MCT. Rocks from the core complex record Eocene and late Oligocene to early Miocene monazite (re-)crystallization periods (monazite age peaks of 40 Ma, 25–19 Ma, and 19–16 Ma) overprinting pre-Himalayan Ordovician Bhimphedian metamorphism and magmatism (ca. 470 Ma). The combination of Sm-Nd isotopic analysis and U-Th/Pb monazite petrochronology demonstrates that both GHS and LHS protolith rocks were captured in the hanging wall of the MCT and experienced Cenozoic Himalayan metamorphism during south-directed extrusion. Monazite ages do not record metamorphism coeval with late Miocene extensional core complex exhumation, suggesting that peak metamorphism and generation of anatectic melt in the core complex had ceased prior to the onset of orogen-parallel hinterland extension at ca. 15–13 Ma. The geometry of the Gurla Mandhata core complex requires significant hinterland crustal thickening prior to 16 Ma, which is attributed to ductile HMC thickening and footwall accretion of LHS protolith associated with a Main Himalayan thrust ramp below the core complex. We demonstrate that isotopic signatures such as Sm-Nd should be used to characterize rock units and structures across the Himalaya only in conjunction with supporting petrochronological and structural data.

INTRODUCTION

Metamorphic core zones of large hot orogens such as the Himalayan-Tibet system provide valuable insight into the thermal evolution of orogens. In contrast, the high metamorphic grade and degree of partial melting within these orogenic cores greatly hamper protolith affiliation interpretation. Assigning correct protolith is a particularly important challenge to overcome when attempting to constrain the early stages of orogenic evolution and paleogeography of its lithotectonic units. In the Himalaya, the greenschist to granulite metamorphic-grade mid-crustal rocks of the Himalayan metamorphic core (HMC) are exposed in an orogen-parallel, north-dipping crustal layer along the entire length of the orogen (Fig. 1A; Hodges, 2000; DeCelles et al., 2001; Yin, 2006; Martin, 2017). In the tectonic transport direction, the HMC extends several hundred kilometers from foreland klippen (Antolín et al., 2013; Soucy La Roche et al., 2016, 2018a, 2018b, 2019) to a series of metamorphic core complexes in the hinterland (Fig. 1A; Chen et al., 1990; Lee et al., 2000; Murphy et al., 2002; Murphy and Copeland, 2005; Thiede et al., 2006; Jessup et al., 2008, 2019; Quigley et al., 2008; Cottle et al., 2009a; Langille et al., 2010, 2012, 2014; Larson et al., 2010a).

Cenozoic granite- and migmatite-cored metamorphic complexes exposed in the Himalayan hinterland include North Himalayan gneiss domes in southern Tibet, the Ama Drime massif in southern Tibet, the Leo Pargil dome in northwestern India, and the Gurla Mandhata core complex in NW Nepal (Fig. 1A). The North Himalayan gneiss domes lie along the North Himalayan antiform (Fig. 1A); their domal configuration is interpreted to be associated with Eocene–Miocene orogen-perpendicular contraction, hinterland crustal thickening, and mid-crustal ramp in the Main Himalayan thrust (MHT; Hauck et al., 1998; Grujic et al., 2002; Beaumont et al., 2004; Lee et al., 2004, 2006; Larson et al., 2010a). In contrast, the Ama Drime massif and the Leo Pargil dome are bounded to the east and west by normal faults, and their domal geometry is associated with late Miocene orogen-parallel extension (Thiede et al., 2006; Jessup et al., 2008; Cottle et al., 2009a; Kali et al., 2010; Langille et al., 2010). The Gurla Mandhata core complex shares structural characteristics with both classes of Himalayan domes, being associated with orogen-parallel extension and the Karakoram fault system while also aligning with the trace of the North Himalayan antiform (Fig. 1A).

Previous work in the Gurla Mandhata core complex focused on the Gurla Mandhata–Humla detachment system (GMH), its kinematic and geodynamical links to the Karakoram fault and the nearby Indus-Yarlung Zangpo suture zone, and its accommodation of orogen-parallel extension and exhumation (Fig. 1; e.g., Murphy et al., 2000; Murphy et al., 2002; Murphy and Copeland, 2005; McCallister et al., 2014; Murphy et al., 2014; Nagy et al., 2015). Although this focus has proved valuable in furthering the understanding of crustal-scale shear zones, orogen-parallel extension, and slip transfer in orogenic systems, only a cursory study of the metamorphic rocks in the core complex has been done to date. Geologic mapping in the Gurla Mandhata core complex has revealed amphibolite metamorphic-grade rocks throughout the dome consistent with the structural and tectonometamorphic characteristics of the HMC (Murphy et al., 2002; Murphy and Copeland, 2005). However, Sm-Nd isotopic analyses identified rocks with isotopic signatures of both Greater Himalayan sequence (GHS) and Lesser Himalayan sequence (LHS) affinity (Murphy, 2007). The presence of both isotopic signatures in the dome introduces significant uncertainty to the underlying geometry, provenance, and tectonometamorphic history of the core complex, because the GHS and LHS have undergone significantly different tectonometamorphic evolutions (e.g., Garzanti, 1999; DeCelles et al., 2000, 2001; Godin et al., 2001; Kohn et al., 2005, 2010; Goscombe et al., 2006, 2018; Yin, 2006; Kohn, 2008; Martin, 2017).

In this paper, we use Sm-Nd isotopic analysis and in situ U-Th/Pb monazite petrochronology coupled with structural field mapping and petrographic and microstructural analysis to compare the tectonometamorphic and protolith histories of the Gurla Mandhata core complex. Our results provide insight into the metamorphic evolution, and ultimately subsurface geometry and dome formation, of the core complex.

LITHOTECTONIC ARCHITECTURE OF THE HIMALAYAN OROGEN

The Himalayan orogen was formed by the collision of the Eurasian and Indian plates at ca. 55–50 Ma and subsequent convergence (Najman et al., 2010, 2017; Hu et al., 2016). The orogen is composed of Paleoproterozoic to early Cenozoic rocks from the pre-collisional northern margin of the Indian continental plate, Cenozoic leucogranites, and Neogene foreland basin sediments. The Himalaya is divided into four major lithotectonic units, from north to south, the Tethyan sedimentary sequence (TSS), the GHS, the LHS, and the Sub-Himalaya (Figs. 1A and 1B). These lithotectonic units are cut by crustal-scale ductile shear zones and fault systems. From north to south, these are the South Tibetan detachment system (STD), the Main Central thrust (MCT), the Main Boundary thrust (MBT), and the Main Frontal thrust (Figs. 1A and 1B). The latter three are interpreted to merge at depth into the MHT, the basal detachment of the orogen (Fig. 1B; Schelling and Arita, 1991; Nelson et al., 1996).

The GHS was initially defined as a sequence of orthogneiss, paragneiss, and granite in the hanging wall of the MCT (Heim and Gansser, 1939; Gansser, 1964; Le Fort, 1975). The conflation of lithological, structural, and geographic characteristics in the historic definition was problematic; so more recent definitions of the GHS focus on protolith age or structure (e.g., Hodges, 2000; Yin and Harrison, 2000; Yin, 2006; Searle et al., 2008; Martin, 2017). According to the protolith definition, the GHS in central and western Nepal consists of a Neoproterozoic to Ordovician sequence of metapelitic kyanite-sillimanite-garnet-biotite schist and gneiss, hornblende-biotite orthogneiss, granitic augen gneiss, and calc-silicate gneiss, with Miocene leucogranitic intrusions (Searle and Godin, 2003; Gleeson and Godin, 2006; Yin, 2006; Larson et al., 2010b; Yakymchuk and Godin, 2012). GHS protoliths were deposited or intruded distal to the northern continental margin of the Indian plate (Garzanti et al., 1986; DeCelles et al., 2000; Godin et al., 2001; Robinson et al., 2001; Myrow et al., 2003; Goscombe et al., 2006; Yin, 2010; Gehrels et al., 2011; Martin, 2017). The GHS has also been structurally defined as the package of rocks in the hanging wall of the MCT and the footwall of the STD exhibiting pervasive ductile deformation coeval with peak metamorphic conditions and magmatism from the Eocene to the Miocene (Heim and Gansser, 1939; Gansser, 1964; Le Fort, 1975; Hodges, 2000; Yin and Harrison, 2000; Searle et al., 2008).

The LHS is similarly defined based on either protolith or structural criteria. The LHS protolith in western Nepal contains primarily Paleoproterozoic to early Mesoproterozoic clastic sedimentary, carbonate, magmatic, and volcanic rocks, all deposited and intruded proximal to the northern continental margin of the Indian plate, but also Paleozoic and perhaps even Mesozoic rocks (Brookfield, 1993; Upreti, 1999; DeCelles et al., 2000, 2004; Richards et al., 2005; Kohn et al., 2010; Gehrels et al., 2011; Long et al., 2011; Martin et al., 2011; Mottram et al., 2014; Martin, 2017). Structurally, the LHS lies in the footwall of the MCT, is thrust along the MBT over the Cenozoic foreland sediments, and has experienced Cenozoic deformation in a foreland fold and thrust belt style under low to upper greenschist-facies metamorphism (Figs. 1A and 1B; DeCelles et al., 2000; Hodges, 2000; DiPietro and Pogue, 2004; Yin, 2006; Martin, 2017; DeCelles et al., 2020).

In addition to these lithotectonic units, we also define a tectonometamorphic unit, the HMC (Cottle et al., 2015). The HMC records two stages of Cenozoic metamorphism, an Eocene–Oligocene high-pressure (P) and moderate temperature (T) “Eohimalayan” phase (Inger and Harris, 1992; Vannay and Hodges, 1996; Godin et al., 2001; Kellett et al., 2014) and an Oligocene–Miocene high-T and moderate-P “Neohimalayan” phase (Vannay and Hodges, 1996; Godin et al., 2001; Streule et al., 2010; Waters, 2019). These metamorphic phases overprint Ordovician Bhimphedian metamorphism related to an Andean-type margin developed at the northern margin of the Indian continent following Gondwana assembly (DeCelles et al., 2000; Godin et al., 2001; Gehrels et al., 2003, 2011; Cawood et al., 2007; Martin et al., 2007; Stübner et al., 2017). The Oligocene–Miocene metamorphism in the HMC is associated with its southward extrusion by broadly synchronous motion along the MCT and the STD, which terminated in western Nepal by ca. 15–13 Ma (Godin et al., 2006; Yin, 2006; Cottle et al., 2009b, 2015; Stübner et al., 2014; Nagy et al., 2015; Soucy La Roche et al., 2016, 2018a, 2018b; Braden et al., 2020). The HMC is therefore pervasively sheared and confined between the MCT and STD (Fig. 1).

The MCT is a top-to-the-south shear zone that places high metamorphic-grade rocks over low metamorphic-grade rocks (Searle et al., 2008; Larson and Godin, 2009). In contrast to the “traditional” definition that positions the MCT along the base of the GHS protolith, the structural boundary and the protolith boundary of the GHS/LHS do not always coincide (e.g., Larson and Godin, 2009; Larson et al., 2010b; Tobgay et al., 2010; McKenzie et al., 2011; Yakymchuk and Godin, 2012; Mottram et al., 2014; Braden et al., 2018, 2020; Mukherjee et al., 2019; Hopkinson et al., 2020). Therefore, to avoid confusing the conflicting structural and protolith GHS definitions, the HMC is used here to describe the rocks in the hanging wall of the MCT, which can include rocks of both “GHS” and “LHS” protolith affinities (Fig. 2; Braden et al., 2018).

In western Nepal, the MCT and STD were active from the Oligocene to the mid-Miocene (Antolín et al., 2013; Stübner et al., 2014; Soucy La Roche et al., 2016; Braden et al., 2020). Although a strand of the MCT was reactivated as recently as <8 Ma (Braden et al., 2018), most of the shortening in the Himalayan orogen was transferred south toward the foreland, the MBT, to the Western Nepal fault system (Murphy et al., 2014), and to a network of imbricate thrusts and duplexes in the LHS by the mid-Miocene (Meigs et al., 1995; DeCelles et al., 2001; Soucy La Roche et al., 2016, 2018a; DeCelles et al., 2020).

GEOLOGY OF THE GURLA MANDHATA CORE COMPLEX

Our mapping identified five rock units exposed along a central transect through the Gurla Mandhata core complex: a biotite metapelite, an augen orthogneiss, a marble/calc-silicate/metasandstone suite, a sillimanite-garnet-biotite metapelite, and several leucogranite bodies, including the larger Chuwa granite (Figs. 1C and 3).

The augen orthogneiss (quartz + feldspar + biotite) is strongly foliated with elongated quartzo-feldspathic augen defining a southeast-plunging mineral lineation (Figs. 3 and 4A). Biotite- and hornblende-rich gneiss layers are present within the augen gneiss in the south of the dome, and are intruded by meter- to outcrop-scale leucogranite dikes and sills (Fig. 4B).

The calc-silicate gneiss (calcite + quartz + phlogopite + diopside) is present in the northern part of the dome, and is interlayered with centimeter- to meter-scale marble and metasandstone layers. The marble/calc-silicate/metasandstone package is commonly boudinaged and preserved in cores of minor tight, west-verging folds with N-S–trending hinges (Figs. 3 and 4C).

The metapelitic units range from schist to gneiss to migmatite (Fig. 4D), containing varying leucosome proportion up to diatexite with >20% leucosome and schlieren with biotite-rimmed leucosome lenses (Figs. 4E and 4F). The structurally lower garnet + biotite metapelitic unit contains muscovite ± sillimanite quartz arenite layers. The structurally higher metapelitic unit contains quartz + feldspar + biotite + sillimanite ± muscovite ± garnet ± hornblende (Figs. 5A and 5B). The mineral assemblage is consistent with mid- to upper-amphibolite–facies metamorphic conditions. Garnet is only rarely present and is irregular, embayed, and inclusion-filled (Figs. 5A and 5C). Migmatite restite is rich in hornblende and biotite (Figs. 5D and 5F).

Mappable leucogranite bodies with slightly elongate laccolith geometry and meter-scale leucogranite dikes and sills outcrop throughout the core complex (Figs. 4B and 4D). These leucogranite bodies commonly display preferential orientations of biotite and muscovite forming a foliation sub-parallel to the dominant foliation of the host rock.

All units contain a penetrative east-dipping foliation and a southeast-plunging mineral lineation defined by sillimanite or, in its absence, biotite aggregates and quartzo-feldspathic rods (Fig. 3). Shear-sense indicators include abundant C–S fabrics and σ- and δ-type feldspar porphyroclasts, all consistent with top-to-the-northwest sense of shear (Figs. 4A, 4F, and 5C–5E). Quartz in the metapelite unit exhibits grain boundary migration recrystallization texture, implying deformation T > 500 °C (Fig. 5G), as well as subgrain rotation recrystallization texture implying deformation T ≈ 400–500 °C (Fig. 5H; Stipp et al., 2002a, 2002b; Law, 2014). Higher-T dynamic recrystallization textures are more common in the center of the dome, and lower-T textures are observed near the northern and southern flanks of the dome (e.g., Nagy et al., 2015).

METHODS

Sm-Nd Isotopic Geochemistry

Studies conducted across the Himalaya provide evidence that Sm-Nd isotopic analysis can distinguish between GHS and LHS protoliths because they each have a characteristic range of εNd(0) values (see Fig. 6 for compilation). These distinctive εNd(0) value ranges reflect different protolith ages of the GHS and LHS, with the LHS and GHS protoliths being primarily Paleo- to Mesoproterozoic, and Neoproterozoic to Ordovician, respectively (Parrish and Hodges, 1996; Whittington et al., 1999; Yin, 2006; Martin, 2017). Our compilation of the Sm-Nd isotopic data across the Himalaya indicates that the GHS-LHS protolith boundary is at εNd(0) −19, such that rocks with εNd(0) ≥−19 are assigned to the GHS (Fig. 6).

Some authors have attempted to draw tectonic boundaries based on Sm-Nd isotopic data, assuming the protolith boundary of the GHS and LHS always coincides with the MCT (e.g., Ahmad et al., 2000; Martin et al., 2005; Murphy, 2007). However, data from NW India, Sikkim, and Bhutan Himalaya suggest that rocks with GHS εNd(0) values can appear in the footwall of the MCT, although mostly in the outer LHS rocks (upper LHS in McQuarrie et al., 2008) that have Paleozoic depositional ages. Conversely, rocks with LHS εNd(0) values have also been observed in the hanging wall of the MCT (Chakungal et al., 2010; Tobgay et al., 2010; McKenzie et al., 2011; Mottram et al., 2014; Mukherjee et al., 2019; Hopkinson et al., 2020).

Previously published εNd(0) values reported from the Gurla Mandhata core complex yield both GHS (εNd(0) = −10.5 to −17.6) and LHS (εNd(0) = −21.3 to −23.4) signatures on the western perimeter of the dome (location of samples on Fig. 1C; Murphy, 2007). A complex structural model involving significant thickening by duplex stacking of the LHS and exposure of the MCT in the dome (i.e., a tectonic window) has been invoked to explain the occurrence of the putative LHS rocks in the Himalayan hinterland (Murphy, 2007). To test this model and to constrain lithotectonic affinity, we undertook Sm-Nd isotopic analysis of 19 samples collected in the central part of the Gurla Mandhata core complex.

Sm-Nd analyses were conducted on 16 samples collected along a N-S transect through the Gurla Mandhata core complex and three samples from the upper Karnali River (Fig. 3 and Table 1; Nagy et al., 2015). Detailed Sm-Nd analytical procedures are provided in Supplemental File 11. Bulk-rock geochemical data for all samples were provided by Acme Labs at Bureau Veritas Mineral Laboratories in Vancouver, Canada (Supplemental File 2 [footnote 1]).

U-Th/Pb Petrochronology

We investigated the timing of metamorphism in the Gurla Mandhata core complex using in situ monazite U-Th/Pb petrochronology on two garnet-bearing samples and two samples without garnet (Fig. 3 and Table 1).

Monazite grains were identified in thin section and selected using a Mineral Liberation Analysis 650 field emission gun environmental scanning electron microscope at Queen’s Facility for Isotope Research (Queen’s University, Kingston, Ontario, Canada). Selected grains were chemically mapped for U, Th, Y, Ca, and Si with X-ray wavelength dispersive spectrometry on a JEOL JXA-8230 electron microprobe, also at Queen’s Facility for Isotope Research. In situ U-Th/Pb and trace-element data were acquired simultaneously using the laser ablation split stream system at the University of California, Santa Barbara, consisting of a Photon Machines 193 nm ArF Excimer laser ablation system connected to a multi-collector Nu Plasma (U/Th-Pb data) and an Agilent 7700S Quadrupole (trace-element data) inductively coupled plasma mass spectrometer. Detailed U-Th/Pb petrochronology procedures and results are provided in Supplemental Files 1 and 3 (footnote 1).

Monazite grains in Himalayan rocks are typically Cenozoic and, due to their young ages, have low 207Pb accumulation and return imprecise 207Pb/235U dates (e.g., Soucy La Roche et al., 2016). Therefore, 208Pb/232Th dates are used for interpretation for all data. Obtained dates are reported to ±2σ and are interpreted in light of complementary elemental information from the dated monazite (Kylander-Clark et al., 2013). The term “date” is used to refer to the analytical results, and the term “age” is used to refer to the interpretation in a tectonometamorphic context. Data are excluded if they are more than 20% discordant or if the analytical spot is astride two distinct chemical domains, particularly zones of differing Y concentration.

RESULTS

Sm-Nd Isotopic Geochemistry

The εNd(0) values from the Gurla Mandhata core complex range from −10.5 to −22.4 (Fig. 7A and Table 2). Results fall into two distinct groups: structurally higher samples with GHS εNd(0) values and structurally lower samples with LHS εNd(0) values (Fig. 7). Most samples have εNd(0) value higher than −19, suggesting GHS affinity. In contrast, an augen gneiss (MA-01; εNd(0) = −22.4) and a metapelitic schist (MA-31; εNd(0) = −22.1) from the core complex and the augen orthogneiss and metapelitic samples from the Karnali River (HK09 εNd(0) = −19.5; HK105B εNd(0) = −24.8; HK115 εNd(0) = −24.5) are interpreted as having an LHS affinity. The latter samples are from ∼5 km structurally below the GMH shear zone (Fig. 7).

U-Th/Pb Petrochronology

Sample MA-06 is a sillimanite-biotite metapelitic schist devoid of garnet (sample locations in Fig. 3; representative thin section photos in Fig. 5). Monazite grains are only found in the matrix and are randomly oriented with respect to the matrix foliation. Fifty-five meaningful analyses from three matrix monazite grains in sample MA-06 yield dates from 41.2 ± 1.0 Ma to 19.7 ± 0.5 Ma. The low-Y core from the largest grain, monazite 1, yields dates from 41 to 40 Ma, while high-Y rims, and other monazite grains with relatively uniform high-Y content yield dates from 26 to 20 Ma (Fig. 8). Spots from the low-Y domain of monazite 1 suggest two peaks in monazite growth at ca. 24 Ma and ca. 21 Ma, with the ca. 24 Ma peak being the more significant (Fig. 9). The Tb/Lu value is relatively high in the old 41–40 Ma core and lower in the younger 26–20 Ma high-Y domains (Fig. 10). Monazite 28 contains xenotime in its core yielding dates of 31–25 Ma and a younger monazite overgrowth yielding dates from 23 to 22 Ma.

Sample MA-11 is a sillimanite-biotite metapelitic schist that does not contain garnet. Monazite grains are only found in the matrix and are either oriented randomly with respect to the matrix foliation or parallel to it. Forty-five meaningful analyses from seven matrix monazite grains in sample MA-11 yield dates from 22.6 ± 0.5 Ma to 15.9 ± 0.4 Ma (Fig. 9). Randomly oriented grains have low-Y cores and high-Y rims, but the dates between the core and rim are only subtly different, and all range from 19 to 16 Ma (Figs. 810). Foliation-parallel grains, which display resorbed rims indicative of monazite breakdown, have high-Y cores that decrease smoothly toward the truncated rims and yield similar dates to the randomly oriented grains. The Tb/Lu values in MA-11 are highly variable and appear uncontrolled by any core and/or rim zonation (Fig. 10).

Sample MA-21 is a protomylonitic garnet-biotite metapelite at the structurally highest level of the transect. Monazite grains are mostly randomly oriented with respect to the foliation, though one monazite is an inclusion in an σ-type garnet porphyroclast, consistent with top-down-to-the-northwest mylonitic foliation (Monazite 3; Fig. 8). The chemically mapped garnet displays uniform Mg and Fe composition with a thin Ca-depleted rim, suggesting some growth zoning. The garnet rim is also subtly depleted in Y and is high in Mn, indicative of garnet resorption (Supplemental File 1 [footnote 1]). Twenty-eight meaningful analyses from four grains in sample MA-21 yield two populations of dates: typical Miocene dates from 21.3 ± 0.5 Ma to 17.6 ± 0.4 Ma and Ordovician–Silurian dates from 470 ± 12 Ma to 42 ± 11 Ma (Figs. 810). The monazite inclusion in the σ-type garnet porphyroclast yields dates from 447 to 442 Ma (Monazite 3; Fig. 8). Monazite 2 is randomly oriented in the matrix and yields young dates from 21 to 18 Ma. Monazite 2 contains a high-Y core with a small low-Y rim that was too narrow to target with the laser beam. Monazite 4 is oriented with its long axis parallel to the foliation with quartz and biotite wrapping around it and yields dates from 455 to 425 Ma. Monazite 9 preserves a low-Y core of 470–433 Ma and a high-Y rim of 20–18 Ma (Fig. 8).

Sample MA-26 is a garnet-biotite metapelitic schist. Most monazite grains are oriented parallel to foliation, although two are randomly oriented with respect to the foliation. Large poikiloblastic garnets with inclusions of biotite, quartz, monazite, xenotime, ilmenite, and zircon appear throughout the sample. Monazite inclusions in garnet in sample MA-26 are too small to analyze (Fig. 8). Fifty-one meaningful analyses from six matrix monazite grains in sample MA-26 yield dates from 22.0 ± 0.5 Ma to 16.4 ± 0.4 Ma (Fig. 9). Monazite grains all have distinct chemical zoning, with low-Y cores yielding dates from 22 to 19 Ma and high-Y rims yielding dates from 19 to 16 Ma (Figs. 810). Tb/Lu values are highest in spots at ca. 22 Ma and decrease sharply from 22 to 19 Ma, then continue to decrease gradually from 19 to 16 Ma (Fig. 10). Two chemically mapped garnets show relatively uniform distribution of Ca, Mg, Fe, and Y, preserving no evidence for growth zoning but have a spike in Mn content at the embayed rims, further suggesting garnet resorption.

INTERPRETATION AND DISCUSSION

The MCT Is Not Exposed in the Gurla Mandhata Core Complex

The structural style and mineral assemblages of our transect through the Gurla Mandhata core complex are relatively homogeneous. Pelitic mineral assemblages across the core complex are consistent with upper-amphibolite–facies metamorphism typical of the HMC in central and western Nepal (Searle and Godin, 2003; Gleeson and Godin, 2006; Corrie and Kohn, 2011; Yakymchuk and Godin, 2012; Iaccarino et al., 2015, 2017; Carosi et al., 2019). Despite searching for a shear zone separating GHS and LHS protolith rocks as suggested by Murphy (2007), we did not encounter a higher concentration of strain expected to be present at that contact. Additionally, sheared rocks in the Gurla Mandhata core complex display dominant top-to-the-northwest shear sense, in contrast to the top-to-the-south shear associated with the MCT. We also did not observe any systematic change in metamorphic grade within the dome, i.e., a steep inverted temperature (metamorphic) gradient typically associated with the MCT zone. Based on our field observations, we therefore argue that the MCT is not exposed in the Gurla Mandhata core complex. We interpret the metamorphism and anatexis to be associated with crustal thickening in the late Oligocene to early Miocene, prior to overprinting mid-Miocene E-W extensional deformation, doming, and exhumation.

The LHS-GHS Protolith Boundary Is Not Always a Structural Boundary

All the observed metapelitic rocks along the Gurla Mandhata core complex transect exhibit similar deformation microstructures and experienced Cenozoic deformation and magmatism at amphibolite metamorphic grade. Consequently, all rocks in the Gurla Mandhata core complex are inferred to be part of the HMC. However, these rocks have affinity with both LHS and GHS protoliths as demonstrated by Sm-Nd data. Yet, there is no MCT-type shear zone at the isotopically defined GHS-LHS boundary. Therefore, all rocks in the Gurla Mandhata core complex are interpreted to be in the hanging wall of the MCT. The MCT must then be at deeper structural level in the core of the dome. The correlation between structural level and εNd(0) values of the samples suggests that a protolith boundary exists above the MCT, at the structural level represented by εNd(0) ≈ −19, about four kilometers beneath the projected trace of the STD (Fig. 7).

The protoliths of the LHS and GHS reflect older proximal and younger distal sedimentation, respectively, at the northern paleo-continental margin of proto-India from the Paleoproterozoic to early Paleozoic (McKenzie et al., 2011; Martin, 2017; Najman et al., 2017). The LHS sedimentary rocks in this part of the Himalaya were derived primarily from the Paleo- to Mesoproterozoic Vindhyan Supergroup sedimentary sequence, with minor contribution from late Mesoproterozoic to early Cambrian units (McKenzie et al., 2011). GHS metasedimentary rocks are derived from Neoproterozoic ca. 880–800 Ma and Cambro-Ordovician ca. 510–460 Ma granites (Garzanti et al., 1986; DeCelles et al., 2000; Godin et al., 2001; Myrow et al., 2003; Goscombe et al., 2006; Cawood et al., 2007; Yin, 2010; Gehrels et al., 2011; Martin, 2017), and they may have some additional sediment input from the Vindhyan Supergroup (Chakrabarti et al., 2007).

The LHS and GHS metasedimentary rocks, by virtue of their different depositional ages and source rocks, yield different εNd(0) value ranges with the εNd(0) ≈ −19 cut-off value according to our compilation (Fig. 6). The εNd(0) values from the Gurla Mandhata core complex fall on both sides of this cut-off value, suggesting that both LHS and GHS protoliths comprise the core complex. Consequently, we infer a protolith boundary in the core complex separating structurally higher GHS material from structurally lower LHS material (Fig. 7). Data are consistent with results from Murphy (2007), who reports samples with both GHS and LHS εNd(0) affinity on the western flank of the Gurla Mandhata core complex (Figs 1 and 7).

The Gurla Mandhata Core Complex Experienced Protracted Crustal Thickening from 40 Ma to 16 Ma

In pelitic rocks, xenotime can strongly partition Y and heavy rare-earth elements (HREEs) during metamorphism (Gratz and Heinrich, 1997; Pyle et al., 2001). Xenotime is only present in trace amounts in the analyzed thin sections, and while in the absence of garnet it may unduly influence the Y and HREE budget, its scarcity makes it unlikely to be the dominant Y budget control in our samples. The ratio of Tb to Lu in analyzed monazite is used as a proxy to infer the overall HREE behavior. Because garnet breakdown also releases HREEs, a decreasing Tb/Lu value in monazite suggests conditions consistent with garnet breakdown, while an increasing Tb/Lu value is consistent with garnet growth, and a highly variable Tb/Lu value may be indicative of melting or fluid-rich metamorphism (Zhu and O’Nions, 1999; Kelsey et al., 2008; Stepanov et al., 2012; Engi, 2017).

Monazite Y-zoning in sample MA-26 is interpreted to be garnet-controlled. We infer monazite growth in the presence of garnet prior to ca. 19 Ma, followed by monazite growth during garnet breakdown from 19 to 16 Ma. Low-Y domains yield a date range of 22–19 Ma with a peak at 20 Ma, and high-Y domains have a date range of 20–16 Ma with a peak at 18 Ma (Figs. 10 and 11). The date ranges are also reflected in the HREE ratio with high Tb/Lu prior to 19 Ma and low Tb/Lu from 19 to 16 Ma, which, combined with an Y content increase, could indicate a 1–2 m.y. period of garnet breakdown and/or of melt (Fig. 10). Monazite inclusions in garnet, although too small to target for U-Th/Pb petrochronology, suggest that monazite was present for at least some stage of garnet growth and prograde metamorphism or could even have been inherited from the Ordovician Bhimphedian event. Outcrop-scale and thin section–scale textural evidence suggests a degree of partial melting within the rocks, which we interpret to have been generated during the garnet breakdown stage and metamorphic retrogression (e.g., Yakymchuk and Godin, 2012).

Samples MA-06 and MA-11 have no visible garnet; Y-content in monazite therefore cannot be linked to garnet growth and/or breakdown or correlated with metamorphic stages. Nevertheless, sample MA-06 yields monazite with 40 Ma cores and rims that crystallized from 25 to 20 Ma, suggesting metamorphism in the Gurla Mandhata rocks from as early as middle Eocene with onset of significant monazite growth at ca. 25 Ma (Figs. 9 and 10). Sample MA-11 yields monazite dates ranging from 19 to 16 Ma with ambiguous Y-zonation and highly variable Tb/Lu ratios over that period (Figs. 9 and 10). Highly variable HREE ratios can suggest an open system in which HREEs migrate freely, such as during fluid-rich metamorphism (Zhu and O’Nions, 1999; Kelsey et al., 2008; Stepanov et al., 2012; Yakymchuk and Brown, 2014); however, this interpretation is tenuous due to the lack of garnet in sample MA-11 since garnet exerts the primary control on HREE concentrations in metapelites. Despite the lack of garnet, the date ranges of samples MA-06 and MA-11 are consistent with the two phases of monazite growth in sample MA-26, with complete garnet resorption in sample MA-11 by 19 Ma.

The structurally highest sample MA-21 records top-to-the-northwest sense of shear. We infer that sample MA-21 is from ∼2 km south of the STD trace (Fig. 1C; Pullen et al., 2011; McCallister et al., 2014). Garnet and monazite textural evidence in the sample is consistent with syn-deformational monazite growth during top-to-the-northwest shear (MA-21 Monazite 3 in Fig. 8). The sample also contains matrix monazite grains with Ordovician–Silurian (470–425 Ma) cores, which we interpret to be either detrital grains derived from Bhimphedian Ordovician granites (Cawood et al., 2007) or preserved grains from Ordovician metamorphism recorded in the GHS in central and eastern Nepal (Godin et al., 2001; Gehrels et al., 2003, 2011). In the NW Himalaya, Stübner et al. (2014) considered contact metamorphism the most likely cause of Ordovician monazite growth in metapelites. Monazite ages from 22 to 18 Ma are recorded in spots in high-Y domains with Y concentration comparable to high-Y domains in sample MA-26 (Fig. 10). Therefore, we interpret coeval growth of monazite and garnet during top-to-the-northwest shear from 22 to 18 Ma.

In summary, obtained U-Th/Pb monazite petrochronology data indicate that rocks from the Gurla Mandhata core complex record Cenozoic metamorphism as early as 40 Ma, but primarily from 25 to 16 Ma. Monazite growth in the presence of garnet is interpreted to have occurred from at least 22 Ma to 19 Ma coeval with top-to-the-northwest shear near the STD, and during garnet breakdown from 19 to 16 Ma. Based on these results, we interpret the Gurla Mandhata complex to expose rocks that are part of the HMC and have experienced Cenozoic Himalayan high-temperature metamorphism, with a short period of retrograde metamorphism, possibly associated with melt or fluid activity beginning ca. 19 Ma. This pattern of metamorphism is consistent with metamorphic stages recorded in the HMC in central and western Nepal and neighboring Kumaon Himalaya (e.g., Célérier et al., 2009a, 2009b; Cottle et al., 2009b; Carosi et al., 2010, 2019; Larson et al., 2011; Patel et al., 2011; Singh et al., 2012; Montomoli et al., 2013; Iaccarino et al., 2015; Larson and Cottle, 2015; Nagy et al., 2015; Gibson et al., 2016; Braden et al., 2017, 2020).

Temporal Evolution of the Thickened Hinterland HMC

The domal geometry of the Gurla Mandhata core complex requires significant hinterland crustal thickening of the HMC (Murphy, 2007; Antolín et al., 2013; Gao et al., 2016; Fan and Murphy, 2020). Our observations also indicate that the transition in εNd(0) values from LHS to GHS in the Gurla Mandhata core complex reflects a protolith boundary rather than a structural one, and we consequently interpret the MCT to be located at depth in the core complex.

U-Th/Pb monazite petrochronology presented in this paper documents monazite growth from 40 to 16 Ma, consistent with “Eohimalayan” and “Neohimalayan” metamorphic ages across the orogen (Inger and Harris, 1992; Vannay and Hodges, 1996; Godin et al., 2001; Streule et al., 2010). No recovered monazite analyses yield <15 Ma dates, which could have indicated decompression melting associated with doming (e.g., Jessup et al., 2008; Cottle et al., 2009a). This contrasts with 11–7 Ma Th-Pb monazite ages from samples collected at the western termination of the Gurla Mandhata complex (Murphy et al., 2002), which suggest that decompression melting may only be occurring proximal to the GMH extensional shear zones. Based on these results, we suggest that the Gurla Mandhata core complex experienced high-T metamorphism and anatectic melting during crustal thickening and southward extrusion from 40 to 16 Ma, prior to the initiation of orogen-parallel extension and top-to-the-NW shearing at ca. 15–13 Ma (Fig. 11; Nagy et al., 2015).

Eocene metamorphism in the HMC is recorded throughout the Himalaya and southern Tibet (Godin et al., 2001; Aikman et al., 2008; Cottle et al., 2009b; Zhang et al., 2011; Kellett et al., 2014; Stübner et al., 2014; Larson and Cottle, 2015; Soucy La Roche et al., 2018a). In central and western Nepal, kyanite-grade metamorphism and a small volume of partial melting occurred at mid-crustal depths (≥37 km) during initial crustal thickening between 40 and 30 Ma (Fig. 11A; Godin et al., 2001; Larson and Cottle, 2015; Soucy La Roche et al., 2018a). This kyanite-grade metamorphism is interpreted to have caused melt-weakening of the middle crust, initiating the southward mid-crustal flow of the HMC and activation of the STD and MCT (Fig 11B; Grujic et al., 1996; Beaumont et al., 2001, 2004; Jamieson et al., 2004, 2006; Godin et al., 2006; Hollister and Grujic, 2006; Braden et al., 2020). During this time, the tip of the mid-crustal flow zone underwent cooling and extrusion driven by focused denudation in the foreland, while the middle crust in the hinterland remained hot and pervasively deforming, and underwent extensive anatectic melting and related magmatism (Beaumont et al., 2001; Godin et al., 2006; Grujic, 2006; Soucy La Roche et al., 2018a, 2018b).

Geodynamic models suggest that fast extruding mid-crustal channel, combined with ongoing contraction of the entire orogen, causes a colder, rheologically stronger lower plate in the hinterland mid-crust to develop a ramp at its leading edge. This ramp acts as a “plunger” that deflects the melt-weakened HMC mid-crustal channel upwards (Fig. 11C; Model HT111 discussed in Beaumont et al., 2004; Jamieson et al., 2006; Warren et al., 2008). This new ramp-flat geometry of the MHT causes deflection of the HMC and forces it up and over the ramp, creating a dome in the mid-crust (Fig. 11C; Beaumont et al., 2004; Jamieson et al., 2006; Warren et al., 2008, 2011; Grujic et al., 2011). The ramp-flat geometry of the MHT in the mid-crust has been geophysically imaged across the Himalaya, including below the North Himalayan antiform and directly below the Gurla Mandhata core complex (Hauck et al., 1998; Gao et al., 2016).

We propose that the HMC progressively incorporated LHS protolith material from the MCT footwall to the MCT hanging wall during the Oligocene to early Miocene, as a consequence of the MCT cutting downward over time (Bollinger et al., 2006; Hopkinson et al., 2020), the development of an imbricate MCT thrust system (e.g., Larson et al., 2015) (Fig. 11C), or by arrival of the LHS paleogeographic domain into the mid-crust in the hinterland and its accretion (“posterior accretion” instead of standard basal accretion) into the source region of the HMC (Jamieson et al., 2006; figure 8 and the related text). This LHS basal accretion to the HMC is consistent with the change in melt source from GHS to LHS as documented in Bhutan (Hopkinson et al., 2020). We suggest that sillimanite-grade metamorphism of both LHS and GHS protolith material, anatectic melt generation, and establishment of a domal geometry were occurring in the Gurla Mandhata core complex during this phase of hinterland HMC thickening at 25–16 Ma (Fig. 11C).

During the mid-Miocene, the already established dome was further exhumed through the upper crust by orogen-parallel transtension, which started at ca. 16 Ma (Fig 11D; Murphy et al., 2002; Murphy and Copeland, 2005; Nagy et al., 2015). Our results suggest that sillimanite-bearing diatexite migmatite rocks in the core complex were subjected to anatexis during crustal thickening only prior to 16 Ma. The onset of east-west extensional GMH shear zone system at ca. 16 Ma is interpreted to be linked with top-to-the-northwest shear in the Gurla Mandhata core complex, likely overprinting an already established foliation from the Eocene–Oligocene crustal thickening stage (e.g., Figs. 5D and 5E; Murphy and Copeland, 2005; Nagy et al., 2015).

We interpret exhumation associated with orogen-parallel transtensional deformation along the GMH system to be coeval with out-of-sequence hinterland thrusting in NW Nepal and NW India (Figs. 11D and 11E; Montomoli et al., 2013; Braden et al., 2017, 2018; Thiede et al., 2017). Out-of-sequence hinterland thrusting isolates klippen of HMC in the foreland, such that HMC klippen preserve rocks with metamorphic ages and pressure-temperature-time-deformation (P-T-t-d) paths characteristic of “Eohimalayan” crustal thickening phase, allowing for ongoing hinterland deformation in the HMC as late as ca. 8 Ma (Figs. 11D and 11E; Soucy La Roche et al., 2016, 2018a; Braden et al., 2018, 2020). The interaction of east-west transtension along the GMH system, hinterland out-of-sequence thrusting, and foreland deformation requires significant strain partitioning between all these shear zone systems at the crustal scale (e.g., Styron et al., 2010, 2011; McCallister et al., 2014; Murphy et al., 2014; Silver et al., 2015; Cannon et al., 2018).

The geometry and tectonometamorphic history of the Gurla Mandhata core complex are similar to that of North Himalayan gneiss domes in southern Tibet (Fig. 1A) and to the Xiao Gurla complex immediately to the north (Fig. 1C; Pullen et al., 2011). The rise of North Himalayan gneiss domes is explained by underthrusting of a major ramp in the MHT (Hauck et al., 1998; Jamieson et al., 2006; Lee et al., 2006; Grujic et al., 2011; Gao et al., 2016). Crustal thickening in the North Himalayan gneiss domes is also interpreted to have been associated with hinterland out-of-sequence thrusts within the Tethyan Himalaya (Burg et al., 1984; Lee et al., 2006; Larson et al., 2010a, 2010b), similar to our interpretation involving an out-of-sequence thrust within the HMC (Fig. 11D; see also Braden et al., 2018, 2020). In the cases of both the Gurla Mandhata core complex and the North Himalayan gneiss domes, anatexis precedes exhumation by tectonic denudation (Lee et al., 2006; Larson et al., 2010a).

CONCLUSIONS

U-Th/Pb monazite petrochronological ages coupled with structural mapping and microstructural analyses on rocks of the Gurla Mandhata core complex show evidence for Cenozoic (40–16 Ma) metamorphism, coeval with protracted crustal thickening, anataxis, and southward extrusion between shear zones with opposite kinematics, followed by rock uplift and exhumation in a transtensional tectonic regime. These inferences, combined with field observation and thin section petrography, imply that the Gurla Mandhata core complex is part of the HMC. However, Sm-Nd isotopic analysis reveals that the core complex retains isotopic signatures of both GHS and LHS protolith affinity (εNd(0) = −10.5 to −22.4). Integration of structural mapping with Sm-Nd results reveals an isotopic protolith boundary in the HMC; yet, the traditionally assumed protolith boundary, the MCT, is not outcropping within the Gurla Mandhata core complex. This demonstrates that Sm-Nd isotopic analysis cannot effectively discern between the tectonometamorphic units of the HMC and the LHS or define the location of the MCT.

Results presented here imply significant crustal thickening in the HMC in the hinterland of the orogen involving both GHS and LHS protolith material. Analyzed monazite grains in the Gurla Mandhata core complex show no evidence for anatectic melt generation during the orogen-parallel extensional phase of the core complex from 15 Ma to present. All reported U-Th/Pb monazite ages are older than 16 Ma, before the transition from south-directed extrusion of the HMC to east-directed orogen-parallel transtension in the hinterland of the orogen (e.g., Nagy et al., 2015). This implies that anatectic melt in the Gurla Mandhata core complex was not generated due to mid-Miocene initiation of orogen-parallel transtension. The Gurla Mandhata core complex is a structural and metamorphic culmination that was formed in the Oligocene to early Miocene during the crustal thickening stage before being overprinted by the transtensional Gurla Mandhata–Humla fault system in the mid-Miocene.

ACKNOWLEDGMENTS

This study was supported by Natural Sciences and Engineering Research Council of Canada (NSERC) discovery grants to L. Godin (RGPIN-2014-06209) and D. Grujic (RGPIN-2015-04311), and by an Ontario Graduate Scholarship in Science and Technology and a Geological Society of America Student Research Grant to M. Ahenda. Field work was made possible by the logistical support of Dawa Tamang and the crew from Argos Trekking Nepal. A. Dobosz, B. Joy, A. Poirier, and J. Godot are thanked for their technical assistance with the scanning electron microscope, electron microprobe, Sm-Nd clean lab, and thermal ionization mass spectrometer, respectively. Z. Braden and R. Soucy La Roche are thanked for their editorial comments on an earlier version of this manuscript. We thank an anonymous reviewer, M. Murphy, and Associate Editor V. Acocella for providing critical and constructive comments that helped clarify the presentation of our data and interpretations.

1Supplemental Material. Analytical procedures, back-scattered electron imagery, X-ray ion microprobe chemical maps, geochemistry results, and U-Th/Pb analytical results. Please visit https://doi.org/10.1130/GEOS.S.13619120 to access the supplemental material, and contact editing@geosociety.org with any questions.
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Associate Editor: Valerio Acocella
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