Pdf/17/2/626/5259763/626.Pdf 626 by Guest on 25 September 2021 Research Paper

Research Paper

GEOSPHERE Protolith affiliation and tectonometamorphic evolution of the Gurla Mandhata core complex, NW Nepal Himalaya GEOSPHERE, v. 17, no. 2 Laurent Godin1, Mark Ahenda1, Djordje Grujic2, Ross Stevenson3, and John Cottle4 1Geological Sciences and Geological Engineering, Queen´s University, Kingston, Ontario K7L 3N6, Canada https://doi.org/10.1130/GES02326.1 2Department of Earth and Environmental Sciences, Dalhousie University, 1459 Oxford Street, Halifax, Nova Scotia B3H 4R2, Canada 3GÉOTOP and Département des Sciences de la Terre et de l’Atmosphère–Université du Québec à Montréal, C.P. 8888, Succursale Centre-Ville, Montréal, Québec H3C 3P8, Canada 11 figures; 2 tables; 1 set of supplemental files 4Department of Earth Science, University of California, Santa Barbara, California 93106-9630, USA

CORRESPONDENCE: [email protected] ABSTRACT record metamorphism coeval with late Miocene extends several hundred kilometers from foreland extensional core complex exhumation, suggesting klippen (Antolín et al., 2013; Soucy La Roche et al., CITATION: Godin, L., Ahenda, M., Grujic, D., Steven‑ son, R., and Cottle, J., 2021, Protolith affiliation and Assigning correct protolith to high metamor- that peak metamorphism and generation of ana- 2016, 2018a, 2018b, 2019) to a series of metamorphic tectonometamorphic evolution of the Gurla Mandhata phic-grade core zone rocks of large hot orogens tectic melt in the core complex had ceased prior core complexes in the hinterland (Fig. 1A; Chen et al., core complex, NW Nepal Himalaya: Geosphere, v. 17, is a particularly important challenge to overcome to the onset of orogen-parallel hinterland exten- 1990; Lee et al., 2000; Murphy et al., 2002; Murphy no. 2, p. 626–646, https://doi.org/10.1130/GES02326.1. when attempting to constrain the early stages of sion at ca. 15–13 Ma. The geometry of the Gurla and Copeland, 2005; Thiede et al., 2006; Jessup et al., orogenic evolution and paleogeography of litho- Mandhata core complex requires significant hin- 2008, 2019; Quigley et al., 2008; Cottle et al., 2009a; Science Editor: Andrea Hampel Associate Editor: Valerio Acocella tectonic units from these orogens. The Gurla terland crustal thickening prior to 16 Ma, which is Langille et al., 2010, 2012, 2014; Larson et al., 2010a). Mandhata core complex in NW Nepal exposes the attributed to ductile HMC thickening and footwall Cenozoic granite- and migmatite-cored meta Received 2 August 2020 Himalayan metamorphic core (HMC), a sequence accretion of LHS protolith associated with a Main morphic complexes exposed in the Himalayan Revision received 2 November 2020 of high metamorphic-grade gneiss, migmatite, Himalayan thrust ramp below the core complex. hinterland include North Himalayan gneiss domes Accepted 20 January 2021 and granite, in the hinterland of the Himalayan We demonstrate that isotopic signatures such as in southern Tibet, the Ama Drime massif in southern orogen. Sm-Nd isotopic analyses indicate that Sm-Nd should be used to characterize rock units Tibet, the Leo Pargil dome in northwestern India, Published online 8 March 2021 the HMC comprises Greater Himalayan sequence and structures across the Himalaya only in con- and the Gurla Mandhata core complex in NW Nepal (GHS) and Lesser Himalayan sequence (LHS) rocks. junction with supporting petrochronological and (Fig. 1A). The North Himalayan gneiss domes lie Conventional interpretation of such provenance structural data. along the North Himalayan antiform (Fig. 1A); their data would require the Main Central thrust (MCT) domal configuration is interpreted to be associ- to be also outcropping within the core complex. ated with Eocene–Miocene orogen-perpendicular However, new in situ U-Th/Pb monazite petrochro- ■■ INTRODUCTION contraction, hinterland crustal thickening, and mid- nology coupled with petrographic, structural, and crustal ramp in the Main Himalayan thrust (MHT; microstructural observations reveal that the core Metamorphic core zones of large hot orogens Hauck et al., 1998; Grujic et al., 2002; Beaumont et complex is composed solely of rocks in the hang- such as the Himalayan-Tibet system provide valu- al., 2004; Lee et al., 2004, 2006; Larson et al., 2010a). ing wall of the MCT. Rocks from the core complex able insight into the thermal evolution of orogens. In In contrast, the Ama Drime massif and the Leo Pargil record Eocene and late Oligocene to early Miocene contrast, the high metamorphic grade and degree of dome are bounded to the east and west by normal monazite (re-)crystallization periods (monazite age partial melting within these orogenic cores greatly faults, and their domal geometry is associated with peaks of 40 Ma, 25–19 Ma, and 19–16 Ma) over- hamper protolith affiliation interpretation. Assigning late Miocene orogen-parallel extension (Thiede et printing pre-Himalayan Ordovician Bhimphedian correct protolith is a particularly important challenge al., 2006; Jessup et al., 2008; Cottle et al., 2009a; metamorphism and magmatism (ca. 470 Ma). The to overcome when attempting to constrain the early Kali et al., 2010; Langille et al., 2010). The Gurla combination of Sm-Nd isotopic analysis and U-Th/ stages of orogenic evolution and paleogeography Mandhata core complex shares structural character- Pb monazite petrochronology demonstrates that of its lithotectonic units. In the Himalaya, the green- istics with both classes of Himalayan domes, being both GHS and LHS protolith rocks were captured schist to granulite metamorphic-grade mid-crustal associated with orogen-parallel extension and the in the hanging wall of the MCT and experienced rocks of the Himalayan metamorphic core (HMC) are Karakoram fault system while also aligning with Cenozoic Himalayan metamorphism during exposed in an orogen-parallel, north-dipping crustal the trace of the North Himalayan antiform (Fig. 1A). south-directed extrusion. Monazite ages do not layer along the entire length of the orogen (Fig. 1A; Previous work in the Gurla Mandhata core This paper is published under the terms of the Hodges, 2000; DeCelles et al., 2001; Yin, 2006; Martin, complex focused on the Gurla Mandhata–Humla CC‑BY-NC license. Laurent Godin https://orcid.org/0000-0003-1639-3550 2017). In the tectonic transport direction, the HMC detachment system (GMH), its kinematic and

GEOSPHERE | Volume 17 | Number 2 Godin et al. | Gurla Mandhata core complex, NW Nepal Himalaya Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/17/2/626/5259763/626.pdf 626 by guest on 25 September 2021 Research Paper

MFT MBT MCT STD C Gurla A’ Lesser Cenozoic granite Tethyan Mandhata Ganga Sub- Himalayan sedimentary Basin Himalaya Himalayan China sequence metamorphic core sequence

30°N Simikot Lhasa Block Tibetan Lhasa30°N Almora Jumla IYZS Plateau NHA Dhaulagiri NHGD Him

29°N a lay Annapurna an F rontal Khula Thrust A Kangri

India Nepal Manaslu STD India Ama Drime E Pokhara

20°N

MCT 90° 0° 28°N Kathmandu Everest 7 80°E MBT Kanchenjunga MFT Thrust fault Peak Bhutan

Normal fault 27°N City 0 100 km Strike-slip fault India 82°E 84°E 86°E 88°E 90°E 92°E

Gurla Mandhata A Foreland klippe core complex A’ SSW NNE STD GMH 10 MFT MBT MCT ? 0

-10 LHS duplex MHT

Elevation (km) -20 Indian craton 10 km -30

Figure 1. (A) Simplified geologic map Gong Co La’nga Co Lhasa Block 31°N of the Nepal, Bhutan, and North Indian MapamKF Yo Co IYZS (undiff.) Himalaya modified after Murphy and Xiao Gurla Copeland (2005), Yin (2006), McQuarrie Barga Complex et al. (2008), Antolín et al. (2013), and Soucy La Roche et al. (2018a). (B) Cross Kiogar-Jungbwa STD Mapam section A–A′ of West Nepal, through the La’ nga Yum Co Gong foreland klippe and the hinterland Gurla ophiolite Co Co 30°30’N GMH Mandhata core complex, modified after GurlaGurla Mandhata Mandhata Xiao-Gurla (7728(7728 m) m) C’ Antolín et al. (2013) and Soucy La Roche et al. (2018a). Main Himalayan thrust B B’ (unmapped) trajectory is projected from Gao et al. Murphy (2007) (2016), and Lesser Himalayan sequence (LHS) duplex is projected from Robinson B B’ China GMCC Chuwa et al. (2006). (C) Compilation map of the Pulan Granite Gurla Mandhata core complex region, India GMCC modified from McCallister et al. (2014), Nepal Yakymchuk and Godin (2012), and Nagy Leuco- et al. (2015). Sm-Nd samples from Mur- Fig. 3 granite phy (2007) are indicated. Sections B–B′ GMH GMH Upper Karnali Fig. 3 Quaternary and C–C′ displayed in Figure 7B. Ab- Pulan Basin STD breviations: GHS—Greater Himalayan 30°N sequence; GMCC—Gurla Mandhata Miocene leucogranite Chuwa Ophiolites khola core complex; GMH—Gurla Mand- MCT C Simikot TSS hata–Humla fault; IYZS—Indus-Yarlung Chuwa Khola

Simikot 30°0’N Zangpo suture zone; KF—Karakoram GHS Murphy, 2007 Karnali Seti khola WNFS fault; MBT—Main Boundary thrust; LHS metamorphosed Karnali River WNFS MCT—Main Central thrust; MFT—Main 20 km N Detachment Fault Normal Fault Frontal thrust; MHT—Main Himalayan thrust; NHA—North Himalayan antiform; 81°E STD 82°E Thrust Fault Anticline, syncline corrugations NHGD—North Himalayan gneiss domes; MCT 10 km STD—South Tibetan detachment; Strike Slip Fault International Border TSS—Tethyan sedimentary sequence; WNFS—Western Nepal fault system. 81°30’E 82°0’E

geodynamical links to the Karakoram fault and the composed of Paleoproterozoic to early Cenozoic The LHS is similarly defined based on either nearby Indus-Yarlung Zangpo suture zone, and rocks from the pre-collisional northern margin of protolith or structural criteria. The LHS protolith in its accommodation of orogen-parallel extension the Indian continental plate, Cenozoic leucogran- western Nepal contains primarily Paleoproterozoic and exhumation (Fig. 1; e.g., Murphy et al., 2000; ites, and Neogene foreland basin sediments. The to early Mesoproterozoic clastic sedimentary, car- Murphy et al., 2002; Murphy and Copeland, 2005; Himalaya is divided into four major lithotectonic bonate, magmatic, and volcanic rocks, all deposited McCallister et al., 2014; Murphy et al., 2014; Nagy et units, from north to south, the Tethyan sedimentary and intruded proximal to the northern continental al., 2015). Although this focus has proved valuable in sequence (TSS), the GHS, the LHS, and the Sub-Hi- margin of the Indian plate, but also Paleozoic and furthering the understanding of crustal-scale shear malaya (Figs. 1A and 1B). These lithotectonic units perhaps even Mesozoic rocks (Brookfield, 1993; zones, orogen-parallel extension, and slip transfer in are cut by crustal-scale ductile shear zones and fault Upreti, 1999; DeCelles et al., 2000, 2004; Richards orogenic systems, only a cursory study of the met- systems. From north to south, these are the South et al., 2005; Kohn et al., 2010; Gehrels et al., 2011; amorphic rocks in the core complex has been done Tibetan detachment system (STD), the Main Central Long et al., 2011; Martin et al., 2011; Mottram et al., to date. Geologic mapping in the Gurla Mandhata thrust (MCT), the Main Boundary thrust (MBT), and 2014; Martin, 2017). Structurally, the LHS lies in the core complex has revealed amphibolite metamor- the Main Frontal thrust (Figs. 1A and 1B). The latter footwall of the MCT, is thrust along the MBT over phic-grade rocks throughout the dome consistent three are interpreted to merge at depth into the the Cenozoic foreland sediments, and has experi- with the structural and tectonometamorphic char- MHT, the basal detachment of the orogen (Fig. 1B; enced Cenozoic deformation in a foreland fold and acteristics of the HMC (Murphy et al., 2002; Murphy Schelling and Arita, 1991; Nelson et al., 1996). thrust belt style under low to upper greenschist-fa- and Copeland, 2005). However, Sm-Nd isotopic The GHS was initially defined as a sequence of cies metamorphism (Figs. 1A and 1B; DeCelles et al., analyses identified rocks with isotopic signatures orthogneiss, paragneiss, and granite in the hanging 2000; Hodges, 2000; DiPietro and Pogue, 2004; Yin, of both Greater Himalayan sequence (GHS) and wall of the MCT (Heim and Gansser, 1939; Gansser, 2006; Martin, 2017; DeCelles et al., 2020). Lesser Himalayan sequence (LHS) affinity (Murphy, 1964; Le Fort, 1975). The conflation of lithologi- In addition to these lithotectonic units, we also 2007). The presence of both isotopic signatures in cal, structural, and geographic characteristics in define a tectonometamorphic unit, the HMC (Cot- the dome introduces significant uncertainty to the the historic definition was problematic; so more tle et al., 2015). The HMC records two stages of underlying geometry, provenance, and tectonomet- recent definitions of the GHS focus on protolith Cenozoic metamorphism, an Eocene–Oligocene amorphic history of the core complex, because the age or structure (e.g., Hodges, 2000; Yin and Har- high-pressure (P) and moderate temperature (T) GHS and LHS have undergone significantly differ- rison, 2000; Yin, 2006; Searle et al., 2008; Martin, “Eohimalayan” phase (Inger and Harris, 1992; Van- ent tectonometamorphic evolutions (e.g., Garzanti, 2017). According to the protolith definition, the GHS nay and Hodges, 1996; Godin et al., 2001; Kellett et 1999; DeCelles et al., 2000, 2001; Godin et al., 2001; in central and western Nepal consists of a Neo al., 2014) and an Oligocene–Miocene high-T and Kohn et al., 2005, 2010; Goscombe et al., 2006, 2018; proterozoic to Ordovician sequence of metapelitic moderate-P “Neohimalayan” phase (Vannay and Yin, 2006; Kohn, 2008; Martin, 2017). kyanite-sillimanite-garnet-biotite schist and gneiss, Hodges, 1996; Godin et al., 2001; Streule et al., In this paper, we use Sm-Nd isotopic analysis hornblende-biotite orthogneiss, granitic augen 2010; Waters, 2019). These metamorphic phases and in situ U-Th/Pb monazite petrochronology gneiss, and calc-silicate gneiss, with Miocene overprint Ordovician Bhimphedian metamorphism coupled with structural field mapping and petro- leucogranitic intrusions (Searle and Godin, 2003; related to an Andean-type margin developed at the graphic and microstructural analysis to compare Gleeson and Godin, 2006; Yin, 2006; Larson et al., northern margin of the Indian continent following the tectonometamorphic and protolith histories of 2010b; Yakymchuk and Godin, 2012). GHS protoliths Gondwana assembly (DeCelles et al., 2000; Godin the Gurla Mandhata core complex. Our results pro- were deposited or intruded distal to the northern et al., 2001; Gehrels et al., 2003, 2011; Cawood et al., vide insight into the metamorphic evolution, and continental margin of the Indian plate (Garzanti et 2007; Martin et al., 2007; Stübner et al., 2017). The ultimately subsurface geometry and dome forma- al., 1986; DeCelles et al., 2000; Godin et al., 2001; Oligocene–Miocene metamorphism in the HMC is tion, of the core complex. Robinson et al., 2001; Myrow et al., 2003; Gos- associated with its southward extrusion by broadly combe et al., 2006; Yin, 2010; Gehrels et al., 2011; synchronous motion along the MCT and the STD, Martin, 2017). The GHS has also been structurally which terminated in western Nepal by ca. 15–13 Ma ■■ LITHOTECTONIC ARCHITECTURE OF THE defined as the package of rocks in the hanging wall (Godin et al., 2006; Yin, 2006; Cottle et al., 2009b, HIMALAYAN OROGEN of the MCT and the footwall of the STD exhibiting 2015; Stübner et al., 2014; Nagy et al., 2015; Soucy pervasive ductile deformation coeval with peak La Roche et al., 2016, 2018a, 2018b; Braden et al., The Himalayan orogen was formed by the metamorphic conditions and magmatism from the 2020). The HMC is therefore pervasively sheared collision of the Eurasian and Indian plates at ca. Eocene to the Miocene (Heim and Gansser, 1939; and confined between the MCT and STD (Fig. 1). 55–50 Ma and subsequent convergence (Najman Gansser, 1964; Le Fort, 1975; Hodges, 2000; Yin and The MCT is a top-to-the-south shear zone that et al., 2010, 2017; Hu et al., 2016). The orogen is Harrison, 2000; Searle et al., 2008). places high metamorphic-grade rocks over low

metamorphic-grade rocks (Searle et al., 2008; Lar- In western Nepal, the MCT and STD were active muscovite ± sillimanite quartz arenite layers. The son and Godin, 2009). In contrast to the “traditional” from the Oligocene to the mid-Miocene (Antolín et structurally higher metapelitic unit contains quartz definition that positions the MCT along the base of al., 2013; Stübner et al., 2014; Soucy La Roche et + feldspar + biotite + sillimanite ± muscovite ± gar- the GHS protolith, the structural boundary and the al., 2016; Braden et al., 2020). Although a strand net ± hornblende (Figs. 5A and 5B). The mineral protolith boundary of the GHS/LHS do not always of the MCT was reactivated as recently as <8 Ma assemblage is consistent with mid- to upper-am- coincide (e.g., Larson and Godin, 2009; Larson et (Braden et al., 2018), most of the shortening in the phibolite–facies metamorphic conditions. Garnet is al., 2010b; Tobgay et al., 2010; McKenzie et al., 2011; Himalayan orogen was transferred south toward only rarely present and is irregular, embayed, and Yakymchuk and Godin, 2012; Mottram et al., 2014; the foreland, the MBT, to the Western Nepal fault inclusion-filled (Figs. 5A and 5C). Migmatite restite Braden et al., 2018, 2020; Mukherjee et al., 2019; system (Murphy et al., 2014), and to a network of is rich in hornblende and biotite (Figs. 5D and 5F). Hopkinson et al., 2020). Therefore, to avoid con- imbricate thrusts and duplexes in the LHS by the Mappable leucogranite bodies with slightly fusing the conflicting structural and protolith GHS mid-Miocene (Meigs et al., 1995; DeCelles et al., elongate laccolith geometry and meter-scale definitions, the HMC is used here to describe the 2001; Soucy La Roche et al., 2016, 2018a; DeCelles leucogranite dikes and sills outcrop throughout rocks in the hanging wall of the MCT, which can et al., 2020). the core complex (Figs. 4B and 4D). These leu- include rocks of both “GHS” and “LHS” protolith cogranite bodies commonly display preferential affinities (Fig. 2; Braden et al., 2018). orientations of biotite and muscovite forming a ■■ GEOLOGY OF THE GURLA MANDHATA foliation sub-parallel to the dominant foliation of CORE COMPLEX the host rock. All units contain a penetrative east-dipping foli- Our mapping identified five rock units exposed ation and a southeast-plunging mineral lineation along a central transect through the Gurla Mand- defined by sillimanite or, in its absence, biotite hata core complex: a biotite metapelite, an augen aggregates and quartzo-feldspathic rods (Fig. 3). orthogneiss, a marble/calc-silicate/metasandstone Shear-sense indicators include abundant C–S fab-

GHS suite, a sillimanite-garnet-biotite metapelite, and rics and σ- and δ-type feldspar porphyroclasts, all

HMC several leucogranite bodies, including the larger consistent with top-to-the-northwest sense of shear Chuwa granite (Figs. 1C and 3). (Figs. 4A, 4F, and 5C–5E). Quartz in the metapelite The augen orthogneiss (quartz + feldspar + unit exhibits grain boundary migration recrystal- MCT zone biotite) is strongly foliated with elongated quart- lization texture, implying deformation T > 500 °C zo-feldspathic augen defining a southeast-plunging (Fig. 5G), as well as subgrain rotation recrystalliza- mineral lineation (Figs. 3 and 4A). Biotite- and horn- tion texture implying deformation T ≈ 400–500 °C

LHS blende-rich gneiss layers are present within the (Fig. 5H; Stipp et al., 2002a, 2002b; Law, 2014). High- augen gneiss in the south of the dome, and are er-T dynamic recrystallization textures are more LHS intruded by meter- to outcrop-scale leucogranite common in the center of the dome, and lower-T Structurally Protolith dikes and sills (Fig. 4B). textures are observed near the northern and south- defined defined The calc-silicate gneiss (calcite + quartz + phlo- ern flanks of the dome (e.g., Nagy et al., 2015). gopite + diopside) is present in the northern part Figure 2. Differences in protolith and struc- of the dome, and is interlayered with centimeter- tural definitions of the Greater Himalayan sequence (GHS) and Lesser Himalayan se- to meter-scale marble and metasandstone layers. ■■ METHODS quence (LHS). According to the structural The marble/calc-silicate/metasandstone package is definition, the Himalayan metamorphic commonly boudinaged and preserved in cores of Sm-Nd Isotopic Geochemistry core (HMC) is defined as all rocks in the minor tight, west-verging folds with N-S–trending hanging wall of the Main Central thrust (MCT) zone, which can incorporate rocks hinges (Figs. 3 and 4C). Studies conducted across the Himalaya provide from both GHS and LHS protoliths into the The metapelitic units range from schist to evidence that Sm-Nd isotopic analysis can distin- high-strain, high metamorphic-grade HMC. gneiss to migmatite (Fig. 4D), containing varying guish between GHS and LHS protoliths because According to protolith definitions, the GHS leucosome proportion up to diatexite with >20% they each have a characteristic range of εNd(0) val- and LHS are distinguished by their differing leucosome and schlieren with biotite-rimmed leu- ues (see Fig. 6 for compilation). These distinctive protolith age and chemistry independent of the position of the MCT zone. Modified cosome lenses (Figs. 4E and 4F). The structurally εNd(0) value ranges reflect different protolith ages of from Braden et al. (2017). lower garnet + biotite metapelitic unit contains the GHS and LHS, with the LHS and GHS protoliths

E81°30’00” E81°35’00” E81°40’00” E81°45’00”

Tethyan sedimentary sequence 14 TIBET (undifferentiated) N MA-21 GMH C’ Leucogranite MA-20 N30°25’00” N30°25’00” Biotite-sillimanite metapelitic 19

schist, gneiss, and migmatite GHS Calc-silicate gneiss and 5000 20

MA-14 core lu Kho metasandstone iso la Ta

MA-12 MA-13 Augen orthogneiss MA-11

19 32 Biotite metapelitic schist, gneiss LHS and migmatite

Himalayan metamorphic

Gyu N30°20’00” a Khola N30°20’00” la ho a K ky Sa Foliation (this study) Sm-Nd sample 5000 MA-22 19 U-Th/Pb sample 22 MA-06 5000 Foliation (Murphy & Copeland, 2005; 16 30 23 B B’ Nagy et al., 2015) Takchhe MA-03 Contact (observed) Village 24 5000 Contact (interpreted) River

19 a

5000 l International

o Contact (inferred)

h K a border hol n

u N30°15’00” K l N30°15’00” Antiform

akchhe a

T T

4000

Synform

Halii

MA-24 35 Detachment fault 21 29 25 Strike-slip fault 35

Base topographic map from Nepal Survey Department Projection: Modified Universal Transverse Mercator Origin: Longitude 81º E, Latitude 0º N 5000 11 Spheroid: Everest 1830 MA-26

N30°10’00” 4000 N30°10’00” 5000 MA-02 15

H 5 km

u Contour interval 1000m 5000 m MA-27 32 la

K a 23 r

a Foliation

N 28 a

d MA-01 E81°50’00” i 4000

30 5000 l a Mean pole 22 ho 4000 la 20 i K ho 28 ec h to foliation K S 36 4000 lli N30°05’00” Tumkot a N30°05’00” HK115 S 4000 18 23 22 Muchu Yangar 33 19 hola K r 42 HK09 u 29 Kermi g GMH n a n = 18 34 T 3000 25 41 HK105 35 4000 Elongation 3000 Hepka la lineation ho 45 36 K ka C 4000 p e H 5000 42 Dharapori N30°00’00” E81°30’00” E81°35’00” E81°40’00” 4000

05 N30°00’00” H u m

l a 08

K MA-31

r n a li Na Simikot di n = 25

E81°45’00” E81°50’00”

Figure 3. Detailed map of the central Gurla Mandhata core complex, complemented by information from Murphy and Copeland (2005) and Nagy et al. (2015). GMH—Gurla Mandhata–Humla fault. Stereographic projections were produced with the Stereonet (v. 9.8.3) program by R. Allmendinger. Contouring was done with the 1% area method and 5% contour intervals.

being primarily Paleo- to Mesoproterozoic, and NW SE NW SE Neoproterozoic to Ordovician, respectively (Par- Leucogranite rish 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). Bt-hbl orthogneiss Some authors have attempted to draw tectonic 5 cm 5 cm boundaries based on Sm-Nd isotopic data, assum- ing the protolith boundary of the GHS and LHS N S NW Leucogranite SE 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 Hima-

laya suggest that rocks with GHS εNd(0) values can Calc-silicate appear in the footwall of the MCT, although mostly boudins Metapelitic in the outer LHS rocks (upper LHS in McQuarrie schist et al., 2008) that have Paleozoic depositional ages. Conversely, rocks with LHS Nd values have also 1 m ε (0) 10 cm been observed in the hanging wall of the MCT (Chakungal et al., 2010; Tobgay et al., 2010; McKenzie NW Diatexite migmatite SE NW SE et al., 2011; Mottram et al., 2014; Mukherjee et al., 2019; Hopkinson et al., 2020).

Previously published εNd(0) values reported Leucosome Restite 1 PROTOLITH AFFILIATION AND TECTONOMETAMORPHIC EVOLUTION OF THE from the Gurla Mandhata core complex yield both 2 GURLA MANDHATA CORE COMPLEX, NW NEPAL HIMALAYA Mylonitic metapelite

3 LAURENT GODIN1†, MARK AHENDA1, DJORDJE GRUJIC2, ROSS STEVENSON3, AND JOHN COTTLE4 GHS (εNd(0) = −10.5 to −17.6) and LHS (εNd(0) = −21.3

4 1 Geological Sciences and Geological Engineering, Queen´s University, Kingston, Ontario, 5 Canada, K7L 3N6 to −23.4) signatures on the western perimeter of 6 2 Department of Earth and Environmental Sciences, Dalhousie University, 1459 Oxford Street, 7 Halifax, NS B3H 4R2, Canada 8 3 GÉOTOP and Département des Sciences de la Terre et de l’Atmosphère - Université du Québec the dome (location of samples on Fig. 1C; Murphy, 9 à Montréal, C.P. 8888, Succursale Centre-Ville, Montréal, QC, Canada H3C 3P8 10 4 Department of Earth Science, University of California, Santa Barbara, CA 93106-9630, USA 10 cm 11 2007). A complex structural model involving signif- E 5 cm 12 SUPPLEMENTARY FILE 1 icant thickening by duplex stacking of the LHS and 13 14 exposure of the MCT in the dome (i.e., a tectonic Figure 4. Field photographs of the main rock units and outcrop features of the Gurla Mandhata core 15 1A. Sm-Nd analytical procedures 16 1B. U-Th/Pb petrochronology and analytical procedures window) has been invoked to explain the occur- complex. (A) Augen orthogneiss unit with σ-type porphyroclasts and C–S fabrics indicating top-to- 17 1C. Supplementary back-scattered electron imagery and X-ray ion microprobe chemical the-northwest sense of shear (photo mirrored for consistency; N30°17.087′; E081°40.672′). (B) Biotite 18 maps rence of the putative LHS rocks in the Himalayan 19 1D. References (bt)- and hornblende (hbl)-orthogneiss intruded by leucogranite sill (N30°03.769′; E081°44.982′). 20 hinterland (Murphy, 2007). To test this model and to 21 (C) Boudinaged calc-silicate layer (N30°17.505′; E081°40.321′). (D) Common field relationship between constrain lithotectonic affinity, we undertook Sm-Nd metapelitic unit and leucogranite bodies. Small pockets of leucosome in the schist are folded and isotopic analysis of 19 samples collected in the cen- sheared, and the schist is intruded by two-mica granite dikes and sills (N30°21.534′; E081°39.861′). tral part of the Gurla Mandhata core complex. (E) Metapelitic unit metamorphosed to diatexite migmatite, with high degrees of partial melt, and rafts of restite in leucosome domains (N30°08.635′; E081°42.925′). (F) Sheared leucosome and my- Sm-Nd analyses were conducted on 16 samples lonitic metapelite, consistent with top-to-the-northwest sense of shear (N30°26.291′; E081°38.719′). collected along a N-S transect through the Gurla 1 Mandhata core complex and three samples from the upper Karnali River (Fig. 3 and Table 1; Nagy U-Th/Pb Petrochronology Monazite grains were identified in thin section et al., 2015). Detailed Sm-Nd analytical procedures and selected using a Mineral Liberation Analysis 1 Supplemental Material. Analytical procedures, 1 back-scattered electron imagery, X-ray ion micro- are provided in Supplemental File 1 . Bulk-rock We investigated the timing of metamorphism 650 field emission gun environmental scanning probe chemical maps, geochemistry results, and geochemical data for all samples were provided in the Gurla Mandhata core complex using in situ electron microscope at Queen’s Facility for Isotope U-Th/Pb analytical results. Please visit https://doi by Acme Labs at Bureau Veritas Mineral Labora- monazite U-Th/Pb petrochronology on two garnet- Research (Queen’s University, Kingston, Ontario, .org/10.1130/GEOS.S.13619120 to access the supplemental material, and contact [email protected] tories in Vancouver, Canada (Supplemental File 2 bearing samples and two samples without garnet Canada). Selected grains were chemically mapped with any questions. [footnote 1]). (Fig. 3 and Table 1). for U, Th, Y, Ca, and Si with X-ray wavelength

MA-26 MA-06 NW MA-21c SE Qz-Fsp domain Grt Fsp

Qz Ms Bt Bt Sill Qz-Bt domain Grt Qz

2 mm 2 mm 2 mm NW MA-21b SE NW MA-11 SE MA-27

Qz Hbl

Bt 2 mm E 2 mm 500 um MA-24a MA-21a MA-15

GBM SR Kfs Qz

G 500 um H 500 um 1 mm

Figure 5. Photomicrographs of selected samples in the Gurla Mandhata core complex. (A) Mineral assemblage of garnet-biotite metapelite unit, quartz + feldspar + muscovite + biotite + garnet. (B) Sillimanite-biotite metapelitic schist, quartz + feldspar + sillimanite + biotite ± muscovite ± garnet. (C) My- lonitic garnet-biotite metapelite unit with two distinct mineralogical domains displaying top-to-the-northwest shear-sense indicators concentrated in the quartz-biotite domain. (D) Mica fish in protomylonitic metapelite showing top-to-the-northwest sense of shear. (E) C–C′–S fabric in sillimanite-biotite schist, showing top-to-the-northwest sense of shear. (F) Restite domain of diatexite migmatite in the metapelite unit, with concentrated biotite and hornblende. (G) Quartz grain boundary migration (GBM) recrystallization texture that shows quartz dynamic recrystallization at >500 °C in the core of the dome. (H) Quartz subgrain rotation (SR) recrystallization texture that indicates that quartz dynamic recrystallization near the South Tibetan detachment occurred at ~400–500 °C. (I) Characteristic leucogranite bodies, with large K-feldspar grains and biotite. Sample locations from Figure 3. Mineral abbreviations after Whitney and Evans (2010).

0.5130 dispersive spectrometry on a JEOL JXA-8230 5 Gurla Mandhata dome electron microprobe, also at Queen’s Facility for Isotope Research. In situ U-Th/Pb and trace-ele- North Himalayan gneiss domes 0 ment data were acquired simultaneously using the 0.5125 Greater Himalayan sequence -5 laser ablation split stream system at the University Lesser Himalayan sequence of California, Santa Barbara, consisting of a Pho- -10 ton Machines 193 nm ArF Excimer laser ablation 0.5120 system connected to a multi-collector Nu Plasma ε Nd Nd -15 (U/Th-Pb data) and an Agilent 7700S Quadrupole 144 GHS (0) (trace-element data) inductively coupled plasma Nd/ -20

143 LHS 0.5115 mass spectrometer. Detailed U-Th/Pb petrochro-

-25 nology procedures and results are provided in Supplemental Files 1 and 3 (footnote 1). -30 Monazite grains in Himalayan rocks are typically 0.5110 Cenozoic and, due to their young ages, have low -35 207Pb accumulation and return imprecise 207Pb/235U dates (e.g., Soucy La Roche et al., 2016). Therefore, -40 208 232 0.5105 Pb/ Th dates are used for interpretation for all 0 0.05 0.10 0.15 0.20 0.25 data. Obtained dates are reported to ±2σ and are 147Sm/144Nd interpreted in light of complementary elemental 0.25 information from the dated monazite (Kyland- er-Clark et al., 2013). The term “date” is used to LHS GHS refer to the analytical results, and the term “age” is 0.20 Greater Himalayan sequence used to refer to the interpretation in a tectonometamorphic context. Data are excluded if they are Lesser Himalayan sequence more than 20% discordant or if the analytical spot is 0.15 astride two distinct chemical domains, particularly zones of differing Y concentration.

0.10

Relative Frequency ■■ RESULTS

0.05 Sm-Nd Isotopic Geochemistry

The εNd(0) values from the Gurla Mandhata core 0.00 -35 -30 -25 -20 -15 -10 -5 0 5 complex range from −10.5 to −22.4 (Fig. 7A and Table 2). Results fall into two distinct groups: struc- εNd(0) turally higher samples with GHS εNd(0) values and

Figure 6. (A) Compilation of Sm-Nd data for Himalayan rocks. (B) Same data set expressed in a relative frequency diagram. structurally lower samples with LHS εNd(0) values For clarity, rocks from the Main Central thrust (MCT) zone or equivalent are not included because of their wide range of (Fig. 7). Most samples have εNd(0) value higher than isotopic composition and inconsistent definition of the MCT location. Furthermore, data for the Miocene leucogranites and from the Tethyan sedimentary sequence are not presented, although they are very similar to the Greater Himalayan −19, suggesting GHS affinity. In contrast, an augen sequence (GHS) rocks (Zhang et al., 2004; Aikman et al., 2012; Zeng et al., 2019; Hopkinson et al., 2020). For consistency, gneiss (MA-01; εNd(0) = −22.4) and a metapelitic 143 144 the εNd(0) values were recalculated for present-day chondritic uniform reservoir (CHUR) Nd/ Nd = 0.5126380. The εNd(0) schist (MA-31; εNd(0) = −22.1) from the core complex value of −19 is used to mark the Greater Himalayan sequence–Lesser Himalayan sequence (LHS) protolith boundary. Data and the augen orthogneiss and metapelitic samples from Deniel et al., 1987; Inger and Harris, 1992; France-Lanord et al., 1993; Massey, 1994; Ayres, 1997; Prince, 1999; Ahmad from the Karnali River (HK09 εNd = −19.5; HK105B et al., 2000; Miller et al., 2001; Robinson et al., 2001; Argles et al., 2003; Zhang et al., 2004; Martin et al., 2005; Richards (0) et al., 2005, 2006; Liu et al., 2007; Murphy, 2007; Dai et al., 2008; Imayama and Arita, 2008; McQuarrie et al., 2008, 2013; εNd(0) = −24.8; HK115 εNd(0) = −24.5) are interpreted Chakungal et al., 2010; Kellett, 2010; Tobgay et al., 2010; Aikman et al., 2012; Mottram et al., 2014; Mukherjee et al., 2019. as having an LHS affinity. The latter samples are

TABLE 1. SAMPLE LOCATIONS TABLE 2. εNd(0) RESULTS

147 144 143 144 Sample Latitude Longitude Lithology Nd Sm Sm/ Nd Nd/ Nd 2σ error εNd(0) number N E (ppm) (ppm) MA‑01 3006.60′ 8142.637′ MA-01 Augen gneiss 38.4 7.74 0.1218 0.511490 0.000007 −22.4 MA‑02 3008.81′ 8143.22′ MA-02 Schist 25.8 4.75 0.1114 0.511796 0.000018 −16.4 MA‑03 3017.036′ 8141.177′ MA-03 Mylonite 7.94 1.63 0.1238 0.511817 0.000025 −16.0 MA‑06 3017.236′ 8140.711′ MA-06 Schist 34.9 6.51 0.1128 0.511776 0.000011 −16.8 MA‑11 3021.60′ 813.86′ MA-11 Schist 36.8 7.05 0.1159 0.511876 0.000010 −14.9 MA‑12 3021.674′ 8140.073′ MA-12 Schist 43.5 7.91 0.1100 0.511680 0.000009 −18.7 MA‑13 3021.6′ 8140.112′ MA-13 Schist 28.1 5.49 0.1179 0.511875 0.000007 −14.9 MA‑14 3021.787′ 813.724′ MA-14 Schist 32.6 6.53 0.1214 0.511896 0.000008 −14.5 MA‑20a 3024.20′ 8138.83′ MA-20a Schist 28.9 5.70 0.1191 0.511834 0.000008 −15.7 MA‑21b 3026.21 8138.71 ′ ′ MA-21b Mylonite 30.3 6.04 0.1206 0.511864 0.000013 −15.1 MA‑21c 3026.21′ 8138.71′ MA-21c Ultramylonite 17.8 4.30 0.1457 0.511931 0.000013 −13.8 MA‑22b 3018.648′ 8140.44′ MA-22b Schist 33.2 6.61 0.1202 0.511727 0.000007 −17.8 MA‑24a 3013.84′ 8142.336′ MA-24a Schist 31.8 6.24 0.1186 0.511876 0.000017 −14.9 MA‑26 300.20′ 8142.460′ MA-26 Schist 36.1 7.15 0.1198 0.512100 0.000009 −10.5 MA‑27 3008.36′ 8142.2′ MA-27 Migmatite 17.4 3.72 0.1297 0.512024 0.000010 −12.0 MA‑31 28.361′ 8148.632′ H0 3003.111′ 8137.78′ MA-31 Schist 57.3 10.9 0.1151 0.511505 0.000007 −22.1 H10B 3002.867′ 8142.380′ HK09 Augen gneiss 17.5 3.86 0.1331 0.511637 0.000004 −19.5 H11 3004.62′ 8136.087′ HK105B Schist 53.2 9.79 0.1112 0.511365 0.000006 −24.8 HK115 Schist 47.8 8.49 0.1074 0.511380 0.000019 −24.5 Note: World Geodetic System WGS 184 143 144 geodetic datum. Note: Error propagation from the Nd/ Nd analysis results in εNd(0) uncertainty of ±0.5.

from ~5 km structurally below the GMH shear zone the younger 26–20 Ma high-Y domains (Fig. 10). transect. Monazite grains are mostly randomly (Fig. 7). Monazite 28 contains xenotime in its core yielding oriented with respect to the foliation, though one dates of 31–25 Ma and a younger monazite over- monazite is an inclusion in an σ-type garnet porphy- growth yielding dates from 23 to 22 Ma. roclast, consistent with top-down-to-the-northwest U-Th/Pb Petrochronology Sample MA-11 is a sillimanite-biotite metapelitic mylonitic foliation (Monazite 3; Fig. 8). The chem- schist that does not contain garnet. Monazite grains ically mapped garnet displays uniform Mg and Fe Sample MA-06 is a sillimanite-biotite meta are only found in the matrix and are either oriented composition with a thin Ca-depleted rim, suggest- pelitic schist devoid of garnet (sample locations in randomly with respect to the matrix foliation or ing some growth zoning. The garnet rim is also Fig. 3; representative thin section photos in Fig. 5). parallel to it. Forty-five meaningful analyses from subtly depleted in Y and is high in Mn, indicative of Monazite grains are only found in the matrix and seven matrix monazite grains in sample MA-11 yield garnet resorption (Supplemental File 1 [footnote 1]). are randomly oriented with respect to the matrix dates from 22.6 ± 0.5 Ma to 15.9 ± 0.4 Ma (Fig. 9). Twenty-eight meaningful analyses from four grains foliation. Fifty-five meaningful analyses from Randomly oriented grains have low-Y cores and in sample MA-21 yield two populations of dates: three matrix monazite grains in sample MA-06 high-Y rims, but the dates between the core and rim typical Miocene dates from 21.3 ± 0.5 Ma to 17.6 yield dates from 41.2 ± 1.0 Ma to 19.7 ± 0.5 Ma. are only subtly different, and all range from 19 to ± 0.4 Ma and Ordovician–Silurian dates from 470 The low-Y core from the largest grain, monazite 1, 16 Ma (Figs. 8–10). Foliation-parallel grains, which ± 12 Ma to 42 ± 11 Ma (Figs. 8–10). The monazite yields dates from 41 to 40 Ma, while high-Y rims, display resorbed rims indicative of monazite break- inclusion in the σ-type garnet porphyroclast yields and other monazite grains with relatively uniform down, have high-Y cores that decrease smoothly dates from 447 to 442 Ma (Monazite 3; Fig. 8). high-Y content yield dates from 26 to 20 Ma (Fig. 8). toward the truncated rims and yield similar dates to Monazite 2 is randomly oriented in the matrix and Spots from the low-Y domain of monazite 1 sug- the randomly oriented grains. The Tb/Lu values in yields young dates from 21 to 18 Ma. Monazite gest two peaks in monazite growth at ca. 24 Ma MA-11 are highly variable and appear uncontrolled 2 contains a high-Y core with a small low-Y rim and ca. 21 Ma, with the ca. 24 Ma peak being the by any core and/or rim zonation (Fig. 10). that was too narrow to target with the laser beam. more significant (Fig. 9). The Tb/Lu value is rela- Sample MA-21 is a protomylonitic garnet-biotite Monazite 4 is oriented with its long axis parallel tively high in the old 41–40 Ma core and lower in metapelite at the structurally highest level of the to the foliation with quartz and biotite wrapping

LHS GHS 0 MA21b MA21c

-1000 MA20 GMH8 GMH2

MA11 -2000 MA12 MA13 MA14

GMH3 -3000 GMH1

GMH9 MA22b

MA03 MA26 -4000 GMH6 GMH5 MA06 GMH7 GMH4 MA24a MA27 MA02 HK115 MA31

Structural level below GMH-STD (m) below level Structural -5000 HK09

HK105 MA01 -6000 -30 -25 -20 -15 -10

εNd(0)

B STD MA11 B’ MA12 MA13 8 MA21b,c 8 GMH LHS-GHS protolith boundary MA14 7 GMH5 MA20a 7 6 GMH1 GMH9 6 MA06 5 GMH2 GMH4 5 GMH3 GMH6 4 GMH7 4

km (asl) 3 3 MA22b 2 MA03 MA24 2 1 1 0 0 5 km

C STD C’ 8 8 GMH 7 MA27 MA02 7 6 LHS-GHS MA26 MA24a 6 MA22b MA14 MA12 5 protolith boundary MA03 MA21b,c 5 MA31 MA01 MA13 MA11 MA20a 4 HK115 MA06 4 km (asl) 3 3 HK105b 2 HK09 2 1 1 0 0 5 km

Figure 7. (A) Sm-Nd results from the Gurla Mandhata core complex and the Karnali River plotted against structural level (dots), combined with results from Murphy (2007) (squares labeled GMH1 to GHM9) on samples collected at the western termination of the Gurla Mandhata core complex (see Fig. 1C for location). (B) East-west (B–B′; location Fig. 1C) and north-south (C–C′; location Fig. 3) cross sections used to calculate structural level of samples by projection below the approximate GMH/STD surface. Solid arrow below each section indicates

where cross sections intersect. Results are roughly correlated with structural level, with structurally lower samples yielding LHS εNd(0) values. Unit colors follow that of Figure 3. Abbreviations: GHS—Greater Himalayan sequence; GMH—Gurla Mandhata–Humla fault; LHS— Lesser Himalayan sequence; STD—South Tibetan detachment.

W MA-06 Monazite 1 E 1 - 25.5 ± 0.6 17 - 24.7 ± 0.6 30 - 24.7 ± 0.6 Figure 8. Selected monazite grains from each sample with 2 - 25.6 ± 0.6 18 - 24.7 ± 0.6 31 - 23.8 ± 0.5 17 46 6 47 3 - 25.1 ± 0.6 19 - 25.3 ± 0.6 32 - 25.0 ± 0.6 scanning electron microscope image, elemental yttrium 18 7 1 16 40 3 2 4 - 24.7 ± 0.6 20 - 25.3 ± 0.6 35 - 19.7 ± 0.5 X-ray map showing low-Y cores (blue and green) and high-Y 19 31 41 15 37 4 8 6 - 25.1 ± 0.6 21 - 24.6 ± 0.6 37 - 19.8 ± 0.5 20 42 14 43 rims (orange and red), and recorded dates (in Ma) from 35 7 - 21.9 ± 0.5 22 - 41.2 ± 1.0 40 - 21.4 ± 0.5 30 21 9 32 8 - 20.4 ± 0.5 23 - 40.0 ± 1.1 41 - 23.8 ± 0.6 each laser spot. Mnz 1 29 22 9 - 20.2 ± 0.4 26 - 25.9 ± 0.6 42 - 23.2 ± 0.5 28 23 27 14 - 20.5 ± 0.5 27 - 23.0 ± 0.5 43 - 20.9 ± 0.5 26 20 um 15 - 20.7 ± 0.5 28 - 20.6 ± 0.5 46 - 24.1 ± 0.5 1 mm 16 - 24.8 ± 0.6 29 - 23.5 ± 0.5 47 - 23.9 ± 0.5 around it and yields dates from 455 to 425 Ma. W E MA-11 Monazite 1 11 9 Monazite 9 preserves a low-Y core of 470–433 Ma

23 18 3 21 17 2 28 and a high-Y rim of 20–18 Ma (Fig. 8). 15 4 22 14

12 Sample MA-26 is a garnet-biotite metapelitic

50 um schist. Most monazite grains are oriented parallel

2 - 17.6 ± 0.4 17 - 18.0 ± 0.5 to foliation, although two are randomly oriented 3 - 17.6 ± 0.4 18 - 18.6 ± 0.4 with respect to the foliation. Large poikiloblastic 4 - 17.8 ± 0.4 21 - 17.7 ± 0.4 9 - 18.0 ± 0.4 22 - 19.6 ± 0.5 garnets with inclusions of biotite, quartz, monazite, Mnz 1 11 - 16.4 ± 0.4 23 - 19.7 ± 0.5 xenotime, ilmenite, and zircon appear through- 12 - 18.1 ± 0.4 28 - 17.9 ± 0.4 14 - 18.5 ± 0.4 1 mm out the sample. Monazite inclusions in garnet in 15 - 22.6 ± 0.6 sample MA-26 are too small to analyze (Fig. 8). SE NW 1 - 442.0 ± 11.4 Fifty-one meaningful analyses from six matrix MA-21 Monazite 3 3 - 447.1 ± 10.8 1 4 - 447.7 ± 10.9 monazite grains in sample MA-26 yield dates from 5 - 447.7 ± 10.5 3 22.0 ± 0.5 Ma to 16.4 ± 0.4 Ma (Fig. 9). Monazite 5 4 Mnz 3 grains all have distinct chemical zoning, with low-Y Garnet cores yielding dates from 22 to 19 Ma and high-Y rims yielding dates from 19 to 16 Ma (Figs. 8–10). Tb/Lu values are highest in spots at ca. 22 Ma and decrease sharply from 22 to 19 Ma, then continue to 1 mm 20 um decrease gradually from 19 to 16 Ma (Fig. 10). Two

1 - 19.9 ± 0.5 chemically mapped garnets show relatively uni- SE MA-21 Monazite 9 NW 2 - 19.5 ± 0.5 form distribution of Ca, Mg, Fe, and Y, preserving 3 - 18.4 ± 0.4 4 - 19.0 ± 0.5 no evidence for growth zoning but have a spike in 7 - 448.3 ± 10.7 13 24 1 9 - 17.9 ± 0.5 Mn content at the embayed rims, further suggest- 13 - 433.2 ± 10.6 23 2 ing garnet resorption. Mnz 9 9 19 - 464.0 ± 11.5 20 19 3 20 - 470.0 ± 11.7 7 4 23 - 462.8 ± 11.4 24 - 439.8 ± 11.0 ■■ INTERPRETATION AND DISCUSSION 1 mm 20 mm The MCT Is Not Exposed in the Gurla W E MA-26 Monazite 8 Mandhata Core Complex 10 9 13 2 8 3 Garnet 7 4 11 5 The structural style and mineral assemblages of our transect through the Gurla Mandhata core 520 um complex are relatively homogeneous. Pelitic Mnz 8 2 - 17.9 ± 0.4 10 - 17.4 ± 0.4 mineral assemblages across the core complex 3 - 19.9 ± 0.5 11 - 17.4 ± 0.4 4 - 19.5 ± 0.5 13 - 19.6 ± 0.5 are consistent with upper-amphibolite–facies 5 - 22.0 ± 0.5 7 - 16.9 ± 0.4 metamorphism typical of the HMC in central and 8 - 17.3 ± 0.4 western Nepal (Searle and Godin, 2003; Gleeson 1 mm 9 - 17.6 ± 0.4 and Godin, 2006; Corrie and Kohn, 2011; Yakymchuk

MA-06 MA-11 The LHS-GHS Protolith Boundary Is Not 16 20 Protracted crystallization of Protracted growth of cores Always a Structural Boundary 14 Peak ca. 24 Ma rims between 26-19 Ma 18 and rims from 19-16 Ma 16 12 All the observed metapelitic rocks along the 14 Gurla Mandhata core complex transect exhibit sim- 10 12 ilar deformation microstructures and experienced Peak ca. 21 Ma 8 10 Cenozoic deformation and magmatism at amphib- 8 6 Growth of xenotime olite metamorphic grade. Consequently, all rocks in between 31-28 Ma 6 the Gurla Mandhata core complex are inferred to be 4

Number of analyses 4 Number of analyses Early metamorphic growth Early crystallization of part of the HMC. However, these rocks have affinity 2 of cores at ca. 40 Ma 2 cores at ca. 22 Ma with both LHS and GHS protoliths as demonstrated 0 0 by Sm-Nd data. Yet, there is no MCT-type shear 15 16 17 18 19 20 21 22 23 24 5 10 15 20 25 30 35 40 45 50 zone at the isotopically defined GHS-LHS bound- 208 232 208 232 Th/ Pb date Th/ Pb date ary. Therefore, all rocks in the Gurla Mandhata core complex are interpreted to be in the hanging wall MA-21 (Miocene) MA-26 4 10 of the MCT. The MCT must then be at deeper struc- Protracted crystallization of Protracted growth of Protracted growth of tural level in the core of the dome. The correlation young rims between 22-17 Ma 9 rims from 19-15 Ma cores from 22-19 Ma 8 between structural level and εNd(0) values of the 3 7 samples suggests that a protolith boundary exists above the MCT, at the structural level represented 6 by εNd(0) ≈ −19, about four kilometers beneath the 2 5 projected trace of the STD (Fig. 7). 4 The protoliths of the LHS and GHS reflect older 3 1 proximal and younger distal sedimentation, respec- Number of analyses Number of analyses 2 tively, at the northern paleo-continental margin of 1 proto-India from the Paleoproterozoic to early Paleo- 0 0 zoic (McKenzie et al., 2011; Martin, 2017; Najman et 15 16 17 18 19 20 21 22 23 24 15 16 17 18 19 20 21 22 23 24 al., 2017). The LHS sedimentary rocks in this part 208Th/232Pb date 208Th/232Pb date of the Himalaya were derived primarily from the Paleo- to Mesoproterozoic Vindhyan Supergroup

High Y Low Y sedimentary sequence, with minor contribution (rims) (cores) from late Mesoproterozoic to early Cambrian units (McKenzie et al., 2011). GHS metasedimentary rocks Figure 9. Frequency diagrams for dated monazites. Light shaded areas represent low-Y cores and dark shaded areas represent high-Y rims. Only Miocene dates are plotted for sample MA-21. Sample MA-11 has overlapping rim and core are derived from Neoproterozoic ca. 880–800 Ma dates; so the area is graded. Diagrams constructed using Isoplot, v. 3.75. 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; and Godin, 2012; Iaccarino et al., 2015, 2017; Carosi metamorphic grade within the dome, i.e., a steep Cawood et al., 2007; Yin, 2010; Gehrels et al., 2011; et al., 2019). Despite searching for a shear zone sep- inverted temperature (metamorphic) gradient typ- Martin, 2017), and they may have some additional arating GHS and LHS protolith rocks as suggested ically associated with the MCT zone. Based on our sediment input from the Vindhyan Supergroup by Murphy (2007), we did not encounter a higher field observations, we therefore argue that the MCT (Chakrabarti et al., 2007). concentration of strain expected to be present at is not exposed in the Gurla Mandhata core complex. The LHS and GHS metasedimentary rocks, that contact. Additionally, sheared rocks in the We interpret the metamorphism and anatexis to by virtue of their different depositional ages and

Gurla Mandhata core complex display dominant be associated with crustal thickening in the late source rocks, yield different εNd(0) value ranges

top-to-the-northwest shear sense, in contrast to Oligocene to early Miocene, prior to overprinting with the εNd(0) ≈ −19 cut-off value according to

the top-to-the-south shear associated with the MCT. mid-Miocene E-W extensional deformation, dom- our compilation (Fig. 6). The εNd(0) values from We also did not observe any systematic change in ing, and exhumation. the Gurla Mandhata core complex fall on both

35000 300 sides of this cut-off value, suggesting that both 30000 250 LHS and GHS protoliths comprise the core com- 25000 200 20000 plex. Consequently, we infer a protolith boundary 150 Tb/Lu Y (ppm) Y 15000 in the core complex separating structurally higher 10000 100 GHS material from structurally lower LHS material 50 5000 MA-06 MA-06 (Fig. 7). Data are consistent with results from Mur- 0 0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 phy (2007), who reports samples with both GHS 208 232 208 232 Pb/ Th date (Ma) Pb/ Th date (Ma) and LHS εNd(0) affinity on the western flank of the

35000 Gurla Mandhata core complex (Figs 1 and 7). 160 30000 140 25000 120 The Gurla Mandhata Core Complex

Y (ppm) Y 20000 100

Tb/Lu 80 Experienced Protracted Crustal Thickening 15000 60 from 40 Ma to 16 Ma 10000 40 5000 20 MA-11 MA-11 In pelitic rocks, xenotime can strongly partition 0 0 0.0 0.0 5.0 10.0 15.0 20.0 25.0 5.0 10.0 15.0 20.0 25.0 Y and heavy rare-earth elements (HREEs) during 208Pb/232Th date (Ma) 208Pb/232Th date (Ma) metamorphism (Gratz and Heinrich, 1997; Pyle et al., 2001). Xenotime is only present in trace amounts in 35000 180 the analyzed thin sections, and while in the absence 30000 160 of garnet it may unduly influence the Y and HREE 25000 140 120 20000 budget, its scarcity makes it unlikely to be the dom- 100 Y (ppm) Y 15000 inant Y budget control in our samples. The ratio of

Tb/Lu 80 10000 60 Tb to Lu in analyzed monazite is used as a proxy 40 5000 to infer the overall HREE behavior. Because gar- MA-21 20 MA-21 0 0 net breakdown also releases HREEs, a decreasing 0.0 100.0 200.0 300.0 400.0 500.0 0.0 100.0 200.0 300.0 400.0 500.0 208Pb/232Th date (Ma) 208Pb/232Th date (Ma) Tb/Lu value in monazite suggests conditions consistent with garnet breakdown, while an increasing 80 35000 70 Tb/Lu value is consistent with garnet growth, and 30000 60 a highly variable Tb/Lu value may be indicative 25000 50 of melting or fluid-rich metamorphism (Zhu and 20000 40 O’Nions, 1999; Kelsey et al., 2008; Stepanov et al., Tb/Lu Y (ppm) Y 15000 30 10000 20 2012; Engi, 2017). 5000 Monazite Y-zoning in sample MA-26 is inter- MA-21 (Miocene only) 10 MA-21 (Miocene only) 0 0 preted to be garnet-controlled. We infer monazite 0.0 5.0 10.0 15.0 20.0 25.0 0.0 5.0 10.0 15.0 20.0 25.0 208Pb/232Th date (Ma) 208Pb/232Th date (Ma) growth in the presence of garnet prior to ca. 19 Ma, followed by monazite growth during garnet break- 25000 900 800 down from 19 to 16 Ma. Low-Y domains yield a date 20000 700 range of 22–19 Ma with a peak at 20 Ma, and high-Y 600 domains have a date range of 20–16 Ma with a peak 15000 500 at 18 Ma (Figs. 10 and 11). The date ranges are also

Tb/Lu 400 Y (ppm) Y 10000 300 reflected in the HREE ratio with high Tb/Lu prior to 5000 200 19 Ma and low Tb/Lu from 19 to 16 Ma, which, com- MA-26 100 MA-26 0 0 bined with an Y content increase, could indicate a 0.0 5.0 10.0 15.0 20.0 25.0 0.0 5.0 10.0 15.0 20.0 25.0 1–2 m.y. period of garnet breakdown and/or of melt 208Pb/232Th date (Ma) 208Pb/232Th date (Ma) (Fig. 10). Monazite inclusions in garnet, although Figure 10. Y and Tb/Lu versus 208Pb/232Th date for each analyzed sample. too small to target for U-Th/Pb petrochronology,

(A) 40-30 Ma N Approximate protolith Thickened depositional S N boundary Crustal thickening sequence of the northern Indian margin Younger distal protolith rocks Older proximal protolith rocks ? Deformation front Kyanite-grade metamorphism and limited partial melting

(B) 30-25 Ma N Onset of southward extrusion of HMC between MCT and STD STD Continually thickening incipient Himalaya

MHT/MCT

Foreland deformation front

Figure 11. Schematic temporal evolution of the (C) 25-16 Ma Exhumation and cooling Sillimanite-grade metamorphism, Himalayan metamorphic core (HMC) in the hin- extensive partial melting and LHS N of HMC in the foreland Future Karakoram- terland of west Nepal in cross section (left) and footwall accretion in the HMC hinterland MCT STD GMH fault trace map (right) views. Present Main Himalayan thrust (MHT) geometry, including hinterland ramp, is taken from Gao et al. (2016), and southward propagation of the foreland ramp is inferred from STD Mercier et al. (2017). Initiation of the Karakoram MHT Cold “plunger,” forcing HMC over MHT ramp MCT fault (KF) system and geometry of the incipient Gurla Mandhata core complex (GMCC) modified from Murphy and Copeland (2005). See text for (D) 16-7 Ma Initiation of E-W transtension, full description of temporal evolution. Modified KF Slow-to-inactive Karakoram and GMH faults Incipient N after Soucy La Roche et al. (2018a) and Braden OOST STD overprint the STD GMCC foreland MCT and STD et al. (2020). Abbreviations: GHS—Greater Hima- MCT STD STD layan sequence; GMH—Gurla Mandhata–Humla OOST fault; LHS—Lesser Himalayan sequence; MBT— Main Boundary thrust; MCT—Main Central thrust; STD OOST—Out-of-sequence thrust; STD—South Ti- MHT MCT betan detachment; TSS—Tethyan sedimentary sequence; WNFS—Western Nepal fault system. MBT

Exposed GHS and accreted (E) 7 Ma to present KF N LHS in GMCC OOST GMH GMH GMCC MCT Foreland klippe TSS klippe WNFS OOST

MHT Foreland klippe MCT MCT MBT TSS

Miocene leucogranite Eocene-Oligocene leucosome

HMC Protolith GHS Protolith LHS accreted to HMC

LHS Active Structure Sub-Himalaya Inactive Structure

suggest that monazite was present for at least in the GHS in central and eastern Nepal (Godin interpret the MCT to be located at depth in the some stage of garnet growth and prograde meta- et al., 2001; Gehrels et al., 2003, 2011). In the NW core complex. morphism or could even have been inherited from Himalaya, Stübner et al. (2014) considered contact U-Th/Pb monazite petrochronology presented the Ordovician Bhimphedian event. Outcrop-scale metamorphism the most likely cause of Ordovi- in this paper documents monazite growth from 40 and thin section–scale textural evidence suggests a cian monazite growth in metapelites. Monazite to 16 Ma, consistent with “Eohimalayan” and “Neo- degree of partial melting within the rocks, which we ages from 22 to 18 Ma are recorded in spots in himalayan” metamorphic ages across the orogen interpret to have been generated during the garnet high-Y domains with Y concentration comparable (Inger and Harris, 1992; Vannay and Hodges, 1996; breakdown stage and metamorphic retrogression to high-Y domains in sample MA-26 (Fig. 10). There- Godin et al., 2001; Streule et al., 2010). No recovered (e.g., Yakymchuk and Godin, 2012). fore, we interpret coeval growth of monazite and monazite analyses yield <15 Ma dates, which could Samples MA-06 and MA-11 have no visible garnet during top-to-the-northwest shear from 22 have indicated decompression melting associated garnet; Y-content in monazite therefore cannot to 18 Ma. with doming (e.g., Jessup et al., 2008; Cottle et al., be linked to garnet growth and/or breakdown or In summary, obtained U-Th/Pb monazite petro- 2009a). This contrasts with 11–7 Ma Th-Pb monazite correlated with metamorphic stages. Neverthe- chronology data indicate that rocks from the Gurla ages from samples collected at the western termi- less, sample MA-06 yields monazite with 40 Ma Mandhata core complex record Cenozoic metamor- nation of the Gurla Mandhata complex (Murphy et cores and rims that crystallized from 25 to 20 Ma, phism as early as 40 Ma, but primarily from 25 to al., 2002), which suggest that decompression melt- suggesting metamorphism in the Gurla Mandhata 16 Ma. Monazite growth in the presence of garnet ing may only be occurring proximal to the GMH rocks from as early as middle Eocene with onset of is interpreted to have occurred from at least 22 Ma extensional shear zones. Based on these results, significant monazite growth at ca. 25 Ma (Figs. 9 to 19 Ma coeval with top-to-the-northwest shear we suggest that the Gurla Mandhata core complex and 10). Sample MA-11 yields monazite dates rang- near the STD, and during garnet breakdown from experienced high-T metamorphism and anatectic ing from 19 to 16 Ma with ambiguous Y-zonation 19 to 16 Ma. Based on these results, we interpret melting during crustal thickening and southward and highly variable Tb/Lu ratios over that period the Gurla Mandhata complex to expose rocks that extrusion from 40 to 16 Ma, prior to the initiation (Figs. 9 and 10). Highly variable HREE ratios can are part of the HMC and have experienced Cenozoic of orogen-parallel extension and top-to-the-NW suggest an open system in which HREEs migrate Himalayan high-temperature metamorphism, with shearing at ca. 15–13 Ma (Fig. 11; Nagy et al., 2015). freely, such as during fluid-rich metamorphism a short period of retrograde metamorphism, possi- Eocene metamorphism in the HMC is recorded (Zhu and O’Nions, 1999; Kelsey et al., 2008; Ste- bly associated with melt or fluid activity beginning throughout the Himalaya and southern Tibet panov et al., 2012; Yakymchuk and Brown, 2014); ca. 19 Ma. This pattern of metamorphism is con- (Godin et al., 2001; Aikman et al., 2008; Cottle et however, this interpretation is tenuous due to the sistent with metamorphic stages recorded in the al., 2009b; Zhang et al., 2011; Kellett et al., 2014; lack of garnet in sample MA-11 since garnet exerts HMC in central and western Nepal and neighbor- Stübner et al., 2014; Larson and Cottle, 2015; Soucy the primary control on HREE concentrations in ing Kumaon Himalaya (e.g., Célérier et al., 2009a, La Roche et al., 2018a). In central and western Nepal, metapelites. Despite the lack of garnet, the date 2009b; Cottle et al., 2009b; Carosi et al., 2010, 2019; kyanite-grade metamorphism and a small volume ranges of samples MA-06 and MA-11 are consistent Larson et al., 2011; Patel et al., 2011; Singh et al., of partial melting occurred at mid-crustal depths with the two phases of monazite growth in sample 2012; Montomoli et al., 2013; Iaccarino et al., 2015; (≥37 km) during initial crustal thickening between MA-26, with complete garnet resorption in sample Larson and Cottle, 2015; Nagy et al., 2015; Gibson 40 and 30 Ma (Fig. 11A; Godin et al., 2001; Larson MA-11 by 19 Ma. et al., 2016; Braden et al., 2017, 2020). and Cottle, 2015; Soucy La Roche et al., 2018a). This The structurally highest sample MA-21 records kyanite-grade metamorphism is interpreted to have top-to-the-northwest sense of shear. We infer that caused melt-weakening of the middle crust, initi- sample MA-21 is from ~2 km south of the STD trace Temporal Evolution of the Thickened ating the southward mid-crustal flow of the HMC (Fig. 1C; Pullen et al., 2011; McCallister et al., 2014). Hinterland HMC and activation of the STD and MCT (Fig 11B; Grujic Garnet and monazite textural evidence in the sam- et al., 1996; Beaumont et al., 2001, 2004; Jamieson ple is consistent with syn-deformational monazite The domal geometry of the Gurla Mandhata et al., 2004, 2006; Godin et al., 2006; Hollister and growth during top-to-the-northwest shear (MA- core complex requires significant hinterland crustal Grujic, 2006; Braden et al., 2020). During this time, 21 Monazite 3 in Fig. 8). The sample also contains thickening of the HMC (Murphy, 2007; Antolín et the tip of the mid-crustal flow zone underwent cool- matrix monazite grains with Ordovician–Silurian al., 2013; Gao et al., 2016; Fan and Murphy, 2020). ing and extrusion driven by focused denudation (470–425 Ma) cores, which we interpret to be either Our observations also indicate that the transition in in the foreland, while the middle crust in the hin-

detrital grains derived from Bhimphedian Ordovi- εNd(0) values from LHS to GHS in the Gurla Mand- terland remained hot and pervasively deforming, cian granites (Cawood et al., 2007) or preserved hata core complex reflects a protolith boundary and underwent extensive anatectic melting and grains from Ordovician metamorphism recorded rather than a structural one, and we consequently related magmatism (Beaumont et al., 2001; Godin

et al., 2006; Grujic, 2006; Soucy La Roche et al., results suggest that sillimanite-bearing diatexite (Fig. 11D; see also Braden et al., 2018, 2020). In the 2018a, 2018b). migmatite rocks in the core complex were sub- cases of both the Gurla Mandhata core complex Geodynamic models suggest that fast extrud- jected to anatexis during crustal thickening only and the North Himalayan gneiss domes, anatexis ing mid-crustal channel, combined with ongoing prior to 16 Ma. The onset of east-west extensional precedes exhumation by tectonic denudation (Lee contraction of the entire orogen, causes a colder, GMH shear zone system at ca. 16 Ma is interpreted et al., 2006; Larson et al., 2010a). rheologically stronger lower plate in the hinterland to be linked with top-to-the-northwest shear in the mid-crust to develop a ramp at its leading edge. Gurla Mandhata core complex, likely overprinting This ramp acts as a “plunger” that deflects the an already established foliation from the Eocene– ■■ CONCLUSIONS melt-weakened HMC mid-crustal channel upwards Oligocene crustal thickening stage (e.g., Figs. 5D (Fig. 11C; Model HT111 discussed in Beaumont et and 5E; Murphy and Copeland, 2005; Nagy et U-Th/Pb monazite petrochronological ages al., 2004; Jamieson et al., 2006; Warren et al., 2008). al., 2015). coupled with structural mapping and microstruc- This new ramp-flat geometry of the MHT causes We interpret exhumation associated with oro- tural analyses on rocks of the Gurla Mandhata core deflection of the HMC and forces it up and over the gen-parallel transtensional deformation along the complex show evidence for Cenozoic (40–16 Ma) ramp, creating a dome in the mid-crust (Fig. 11C; GMH system to be coeval with out-of-sequence metamorphism, coeval with protracted crustal thick- Beaumont et al., 2004; Jamieson et al., 2006; hinterland thrusting in NW Nepal and NW India ening, anataxis, and southward extrusion between Warren et al., 2008, 2011; Grujic et al., 2011). The (Figs. 11D and 11E; Montomoli et al., 2013; Braden et shear zones with opposite kinematics, followed by ramp-flat geometry of the MHT in the mid-crust has al., 2017, 2018; Thiede et al., 2017). Out-of-sequence rock uplift and exhumation in a transtensional tec- been geophysically imaged across the Himalaya, hinterland thrusting isolates klippen of HMC in the tonic regime. These inferences, combined with field including below the North Himalayan antiform and foreland, such that HMC klippen preserve rocks observation and thin section petrography, imply directly below the Gurla Mandhata core complex with metamorphic ages and pressure-tempera- that the Gurla Mandhata core complex is part of (Hauck et al., 1998; Gao et al., 2016). ture-time-deformation (P-T-t-d) paths characteristic the HMC. However, Sm-Nd isotopic analysis reveals We propose that the HMC progressively incor- of “Eohimalayan” crustal thickening phase, allow- that the core complex retains isotopic signatures of

porated LHS protolith material from the MCT ing for ongoing hinterland deformation in the HMC both GHS and LHS protolith affinity ε( Nd(0) = −10.5 to footwall to the MCT hanging wall during the Oli- as late as ca. 8 Ma (Figs. 11D and 11E; Soucy La −22.4). Integration of structural mapping with Sm-Nd gocene to early Miocene, as a consequence of the Roche et al., 2016, 2018a; Braden et al., 2018, 2020). results reveals an isotopic protolith boundary in MCT cutting downward over time (Bollinger et al., The interaction of east-west transtension along the the HMC; yet, the traditionally assumed protolith 2006; Hopkinson et al., 2020), the development of GMH system, hinterland out-of-sequence thrusting, boundary, the MCT, is not outcropping within the an imbricate MCT thrust system (e.g., Larson et al., and foreland deformation requires significant strain Gurla Mandhata core complex. This demonstrates 2015) (Fig. 11C), or by arrival of the LHS paleogeo- partitioning between all these shear zone systems that Sm-Nd isotopic analysis cannot effectively dis- graphic domain into the mid-crust in the hinterland at the crustal scale (e.g., Styron et al., 2010, 2011; cern between the tectonometamorphic units of the and its accretion (“posterior accretion” instead of McCallister et al., 2014; Murphy et al., 2014; Silver HMC and the LHS or define the location of the MCT. standard basal accretion) into the source region of et al., 2015; Cannon et al., 2018). Results presented here imply significant crustal the HMC (Jamieson et al., 2006; figure 8 and the The geometry and tectonometamorphic history thickening in the HMC in the hinterland of the oro- related text). This LHS basal accretion to the HMC of the Gurla Mandhata core complex are similar to gen involving both GHS and LHS protolith material. is consistent with the change in melt source from that of North Himalayan gneiss domes in southern Analyzed monazite grains in the Gurla Mandhata GHS to LHS as documented in Bhutan (Hopkinson Tibet (Fig. 1A) and to the Xiao Gurla complex imme- core complex show no evidence for anatectic melt et al., 2020). We suggest that sillimanite-grade diately to the north (Fig. 1C; Pullen et al., 2011). The generation during the orogen-parallel extensional metamorphism of both LHS and GHS protolith rise of North Himalayan gneiss domes is explained phase of the core complex from 15 Ma to present. material, anatectic melt generation, and establish- by underthrusting of a major ramp in the MHT All reported U-Th/Pb monazite ages are older than ment of a domal geometry were occurring in the (Hauck et al., 1998; Jamieson et al., 2006; Lee et al., 16 Ma, before the transition from south-directed Gurla Mandhata core complex during this phase of 2006; Grujic et al., 2011; Gao et al., 2016). Crustal extrusion of the HMC to east-directed orogen-paral- hinterland HMC thickening at 25–16 Ma (Fig. 11C). thickening in the North Himalayan gneiss domes is lel transtension in the hinterland of the orogen (e.g., During the mid-Miocene, the already estab- also interpreted to have been associated with hin- Nagy et al., 2015). This implies that anatectic melt in lished dome was further exhumed through the terland out-of-sequence thrusts within the Tethyan the Gurla Mandhata core complex was not gener- upper crust by orogen-parallel transtension, which Himalaya (Burg et al., 1984; Lee et al., 2006; Larson ated due to mid-Miocene initiation of orogen-parallel started at ca. 16 Ma (Fig 11D; Murphy et al., 2002; et al., 2010a, 2010b), similar to our interpretation transtension. The Gurla Mandhata core complex is Murphy and Copeland, 2005; Nagy et al., 2015). Our involving an out-of-sequence thrust within the HMC a structural and metamorphic culmination that was

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