Metamorphism and geochronology of the exhumed Himalayan midcrust, Likhu Khola region, east-central Nepal: Recognition of a tectonometamorphic discontinuity

Understanding how convergence is accommodated in the midcrust during orogenesis is critical to elucidating the development of orogenic belts. While studying midcrustal evolution has led to insight into the processes active during the formation of the Trans-Hudson orogen (e.g., St-Onge, 1999, 2007; White et al., 2004), the Canadian Cordillera (e.g., Gibson et al., 1999; Gervais and Brown, 2011), the Grenville orogen (e.g., Slagstad et al., 2004; Jamieson et al., 2007), and the Himalaya (e.g., Searle, 1986; Cottle et al., 2007), among others, key questions remain about how those processes relate temporally and spatially to those recorded at other structural levels during orogenesis.

Current investigations of these key questions are now utilizing recent advances in analytical techniques, such as monazite petrochronology (e.g., Rubatto et al., 2013; Kylander-Clark et al., 2013; and many others), which allow for the collection of increasingly precise and spatially refined data. While previous studies have focused on orogen-scale compatibility between deformational processes occurring within the midcrust in the deep orogenic hinterland and those processes occurring at higher structural levels in the orogenic shallower foreland (e.g., Price, 1972; Simony and Carr, 2011), more recent studies have documented similar kinematic compatibility at much smaller scales (e.g., Larson et al., 2010, 2011, 2013; Yakymchuck and Godin, 2012). These types of studies, which integrate structural, metamorphic, and geochronologic data, provide a means by which to characterize the potentially distinct geologic histories of spatially adjacent rocks that may have evolved uniquely in initially separate regions of an orogen.

The evolution of the midcrustal core has been one of the most contentious issues in recent geologic research into the Himalayan orogen (e.g., Law et al., 2006, and references therein; Kohn, 2008; Cottle et al., 2009a; Larson et al., 2010; Carosi et al., 2010; Streule et al., 2010; Corrie and Kohn, 2011; Yakymchuk and Godin, 2012; Corrie et al., 2012; Montomoli et al., 2013; Larson and Cottle, 2014). There has been much debate over which of the “end-member” models proposed for the evolution of the Himalaya, commonly considered to be wedge-taper processes or channel flow, best describes or explains available geologic data (see Beaumont and Jamieson, 2010; Larson et al., 2013). However, thermo-mechanical orogenic simulations (e.g., Jamieson et al., 2004) implicitly predict a change from lateral midcrustal flow in the orogenic hinterland to wedge-taper processes in the foreland of the orogen (Beaumont and Jamieson, 2010). As more detailed, precise, spatially registered, field-based data are contributed to the general Himalayan knowledge base, it is becoming apparent that the proposed “end members” are not mutually exclusive (e.g., Beaumont and Jamieson, 2010; Larson et al., 2010, 2011, 2013). Indeed, field-based studies have begun to outline a temporal and spatial continuum between the different “end-member” models (Larson et al., 2010, 2011, 2013; Larson, 2012; Yakymchuk and Godin, 2012; Larson and Cottle, 2014). These studies describe discontinuities in the exhumed midcrust across which rocks record different deformational, metamorphic, and geochronologic histories. Rocks above these discontinuities, which generally coincide with the Main Central thrust in models of Jamieson et al. (2004), typically record hinterland-style deformation, vertical thinning, and horizontal elongation (e.g., Price, 1972), consistent with lateral midcrustal flow, whereas rocks below these discontinuities typically record foreland-style deformation more consistent with the vertical thickening and horizontal shortening expected in a wedge-taper environment (Larson et al., 2013). The existence of such discontinuities within the Himalayan midcrust provides potential insight into the processes that control convergence accommodation in large, hot orogens (e.g., Montomoli et al., 2013; Larson et al., 2013) and the way in which material is incorporated into the foreland. Identifying and understanding the distribution and characteristics of these structures represent a critical step in elucidating the history of Himalayan evolution and its potential application to other similar, older orogens.

Targeted mapping of exhumed midcrustal rocks was carried out along the Likhu Khola valley in east-central Nepal (Fig. 1B) to investigate the lateral continuity of a documented structural discontinuity within similar rocks in the nearby upper Tama Kosi valley (Larson, 2012; Larson et al., 2013) to the west. Mapping was integrated with thermobarometric estimates and in situ monazite geochronology to provide constraints on the pressure-temperature-time history of the rocks. Paired with previously published lithostratigraphy and structural interpretations (From and Larson, 2014), this study provides the data necessary to assess the lateral extent of the discontinuity from adjacent areas into the Likhu Khola region and interpret the role such a structure may play in accommodating convergence within the Himalayan kinematic framework.

The Likhu Khola is located in east-central Nepal ∼95 km east-northeast of Kathmandu (Figs. 1A and 1B). This study examines the region along the Likhu Khola valley from Tholo Priti in the south to Gyajo La in the north (Fig. 1C). The majority of the Likhu Khola region was mapped at a reconnaissance scale in the 1960s by Ishida (1969). Early observations, as summarized in Ishida and Ohta (1973), were augmented and reassessed by Schelling (1992), who also mapped parts of the Likhu Khola region as part of a larger study. These earlier works formed the basis for the more detailed assessment by From and Larson (2014). This study builds on that recent work and adds the first microanalytical-based pressure, temperature, and time data for the area.

Work in the adjacent Tama Kosi region to the west (Larson, 2012) has outlined a similar tectonostratigraphic sequence to that observed in the Likhu Khola region (From and Larson, 2014) that includes a dominantly metasedimentary package of exhumed midcrustal rocks intercalated at specific structural levels with orthogneiss. Moreover, Larson et al. (2013) documented a metamorphic, geochronologic, and deformational discontinuity between staurolite- and kyanite-grade metamorphic rocks in the Tama Kosi valley. The rocks above the discontinuity were interpreted to record lateral midcrustal ductile flow, while rocks below the discontinuity record foreland wedge-taper–related deformation. As discussed already, similar discontinuities, which are equivalent to the Main Central thrust in the thermo-mechanical models of Jamieson et al. (2004), have been described in other areas along the Himalaya (e.g., Groppo et al., 2009; Larson et al., 2010; Yakymchuk and Godin, 2012; Montomoli et al., 2013), indicating that they are laterally extensive, kinematically important features. Tracing the lateral continuity of these types of structures is necessary to meaningfully evaluate their regional significance and potential role in orogenic processes.

The structurally lowest units mapped in the southern portion of the Likhu Khola area (garnet zone I of Fig. 1C) are middle-greenschist-facies micaceous phyllitic schists with local chlorite. These give way up structural section northward to pelitic or semipelitic schist in the middle portion of the map area (garnet zone II of Fig. 1C) and finally to upper-amphibolite-facies aluminosilicate-bearing migmatitic gneiss in the structurally highest part of the study area (kyanite and sillimanite zones of Fig. 1C; From and Larson, 2014). Similar observations of increasing metamorphic grade up structural section are commonly reported from across the Himalaya (e.g., Mallet, 1875; Von Loczy, 1878; Oldham, 1883; Arita, 1983; Hodges, 2000; Searle et al., 2008; Dasgupta et al., 2009; and many others).

Pervasive top-to-the-south shear is recognized throughout the Likhu Khola study area. It is especially well developed in the southern (structurally lowest) portion of the map area, where it is characterized by delta-type garnet porphyroblasts, S and C′ fabrics, and asymmetric quartz lattice preferred orientation (LPO) fabrics (From and Larson, 2014). This is contrary to the previous reconnaissance-scale work, which mapped a portion of the current study area and inferred a number of thrust faults in the Likhu Khola region (Ishida, 1969; Ishida and Ohta, 1973). No discrete thrust-sense faults were observed during the field or subsequent microstructural analysis components of this study (From and Larson, 2014). The development of top-to-the-south shear is consistent with observations from many other sections of the exhumed Himalayan midcrust (e.g., Heim and Gansser, 1939; Le Fort, 1975; Larson and Godin, 2009).

Given the metamorphic grade and pervasive ductile deformation recorded across the field area, all rocks examined in the Likhu Khola study area are interpreted to be a part of the Himalayan metamorphic core. We use the term Himalayan metamorphic core herein to avoid confusion about whether these rocks should be classified as Greater Himalayan Sequence rocks or Lesser Himalayan Sequence rocks. Inconsistency in the way the Main Central thrust, which has been defined as separating Greater Himalayan sequence rocks in its hanging wall from Lesser Himalayan sequence rocks in its footwall in the Himalaya (Heim and Gansser, 1939; Gansser, 1964), is mapped has led to similar rocks being ascribed to different units based on their interpreted structural position in various studies (see summary in Searle et al., 2008). This has compounded the problems reconciling models for the midcrustal evolution of the orogen. For example, in the Likhu Khola region, the Main Central thrust has been previously drawn through the area at the approximate contact between garnet zones I and II (Schelling, 1992; discussed later herein) and south of the study area (From and Larson, 2014). Considering all rocks that record Cenozoic deformation and metamorphism together as part of the Himalayan metamorphic core helps avoid problems created by misinterpreted nomenclature and allows the focus to be placed on the implications of data extracted from the field area, not applied labels.

Petrographic analysis was conducted on 31 specimens collected from the Likhu Khola study area. The associated mineral assemblage and textural data allow the region to be separated into four metamorphic zones, as described next.

Garnet zone I extends over a horizontal distance of ∼8 km from the southernmost portion of the studied area to a distinctive quartzite layer northeast of Bhandar (Fig. 1C). This zone contains micaceous phyllitic schist, localized calc-silicate intercalations, and quartzite. The general assemblage of this zone is quartz + plagioclase feldspar + muscovite + biotite + garnet ± chlorite with accessory phases of tourmaline ± ilmenite ± epidote ± apatite. Garnet porphyroblasts occur throughout this zone, ranging from 0.5 mm to 3 mm in diameter with sigma- or delta-type tails developed locally (Fig. 4D inFrom and Larson, 2014). The anatexite content of this zone typically ranges from 0% to 15%. It is locally chaotic in form and quartz dominated, with subsidiary components of plagioclase feldspar and muscovite.

Garnet zone II extends over a horizontal distance of ∼7.5 km from the top of the quartzite layer near Bhandar to the first appearance of aluminosilicate minerals independent of anatexite (Fig. 1C). This zone typically contains pelitic schist with intercalated psammopelitic metasedimentary rocks and local lenses of calc-silicate. The general assemblage of this zone includes quartz + plagioclase feldspar + K-feldspar + muscovite + biotite ± garnet with accessory tourmaline ± zircon ± apatite ± monazite ± ilmenite. Garnet (0.75 mm to 2 mm diameter) in this zone also commonly occurs in anatexite lenses (Fig. 5C inFrom and Larson, 2014). Anatexite comprises 15% to 25% of total rock volume within this zone and is dominantly composed of quartz + plagioclase feldspar + muscovite ± biotite ± garnet.

The kyanite zone extends over a horizontal distance of ∼3 km from the first appearance of kyanite in paragneiss until it gives way to sillimanite at higher structural levels (Fig. 1C). The general assemblage of this zone is quartz + plagioclase feldspar + K-feldspar + biotite + muscovite (prograde and retrograde) + garnet + kyanite with accessory tourmaline ± zircon ± apatite ± monazite ± ilmenite. Garnet grains throughout this zone fluctuate in size. Quartz-rich layers generally contain grains with diameters between 1.5 mm and 3.5 mm, while quartz-poor layers typically contain smaller grains with diameters ranging from 0.1 mm to 1.5 mm. The anatexite in this zone occurs as two distinct phases, which together comprise up to 45% of the total rock volume. One phase of the anatexite occurs as foliation-parallel leucosomes containing quartz + feldspar and rimmed by biotite (Fig. 7C inFrom and Larson, 2014), whereas a younger pod-like phase, containing quartz + plagioclase and K-feldspar + muscovite (in the form of books) ± tourmaline, generally crosscuts both the foliation and the older, stromatic anatexite (Fig. 7F inFrom and Larson, 2014).

The sillimanite zone extends over a horizontal distance of more than 10 km from the disappearance of kyanite in favor of sillimanite to the structurally highest location reached in the study area at Gyajo La (Fig. 1C). This zone contains sillimanite-bearing gneiss and migmatite (rock with >45% of total rock volume composed of anatexite) with a general assemblage of quartz + plagioclase feldspar + K-feldspar + sillimanite + biotite + muscovite (retrograde) with accessory zircon ± monazite. The garnet grains throughout this zone vary in size and distribution similar to that observed in the kyanite zone. Both anatexite phases observed in the kyanite zone are also common in the sillimanite zone.

Ten specimens in total, representing all four metamorphic zones, were selected for detailed geothermobarometry to constrain the pressure-temperature (P-T) evolution of this section of the Himalayan metamorphic core exposed in the Likhu Khola. A Cameca SX100 electron microprobe housed at the Saskatchewan Research Council in Saskatoon was utilized for qualitative and quantitative analyses. Qualitative X-ray elemental maps were acquired for garnet grains representative of each metamorphic zone to investigate potential compositional zoning. Garnet grains were selected based on the potential of the grains to represent equatorial sections through, their proximity to, and textural relationship with biotite, muscovite, and plagioclase. Representative grains were imaged using backscattered electron microscopy and mapped for Fe, Ca, Mg, Mn, Al, K, Na, Si, Ti, Th, and Y by wavelength dispersive spectrometry with an accelerating voltage of 15 kV, beam current of ∼200 nA, and step size between 2–4 μm.

Quantitative major-element concentrations were obtained with the Cameca SX100 operating at 15–20 kV with a beam size of ∼10 nm and current of 20 nA. Line transects and single point data were extracted from multiple garnet, biotite, muscovite, and feldspar grains in each specimen. Compositional transect data, paired with the chemical maps, were used to determine which data to include/exclude for use in geothermobarometric calculations (see following). Because the geothermobarometric methods employed rely on garnet chemistry to estimate both pressure and temperatures, a detailed examination of garnet is a necessary first step in estimating metamorphic P-T conditions.

Representative transects of garnet grains from the specimens used in geothermobarometric analyses are presented in Figure 2 as mol% for almandine (Fe), spessartine (Mn), pyrope (Mg), and grossular (Ca) and Fe# (Fe/[Fe + Mg]). Descriptions of the garnet grains from specimens utilized for geothermobarometric analyses are summarized in the following. In general, garnet compositions with the lowest Mn coincided with the lowest Fe#. These compositions were targeted for P-T calculations because this is thought to represent the closest approximation of garnet composition prior to potential resorption and diffusion effects during a high-grade metamorphic overprint (e.g., Spear and Peacock, 1989; Kohn and Spear, 2000).

Garnet grains from garnet zone I (Fig. 1C) retain a variable record of prograde zonation with generally decreasing Mn, Ca, and Fe# from core to rim and increasing Mg and Fe from core to rim (e.g., Yardley, 1977; Spear et al., 1990; Figs. 2A and 2B). There is evidence for minor resorption (e.g., Florence and Spear, 1991; Kohn and Spear, 2000) in grains from higher structural positions within Garnet zone I, but a similar overall zonation is observed. For garnet grains in this zone, the compositional data near the rims of the garnet, avoiding any resorption rind present, are interpreted to represent peak or near-peak conditions (e.g., Hollister, 1966).

Garnet grains from garnet zone II (Fig. 1C) are typically characterized by relatively flat Mn, Mg, Ca, and Fe profiles in their core regions and rim compositions that show a sharp increase in Mn and Fe# paired with a decrease in Mg and, to a lesser degree, Fe concentration (Fig. 2C). The largely invariant chemical composition of the garnet grain is consistent with diffusional homogenization, while the sharp increase in Mn concentration along the grain rim is attributed to garnet resorption and the preferential retention of an Mn-rich resorption rind (as above; Florence and Spear, 1991; Kohn and Spear, 2000). There is one exception to the general characteristics in garnet zone II. In specimen 039, small garnet grains (∼500 μm) show a homogeneous chemical profile, while large garnet grains (>1000 μm) from the same location have an inclusion-rich core region that retains come chemical zonation consistent with prograde metamorphism (Fig. 2D; Yardley, 1977; Spear et al., 1990; Kohn and Spear, 2000). This prograde zonation in the large garnet grains may reflect incomplete diffusional homogenization. For garnet grains in this zone, the core compositions of the smaller grains and the near-rim compositions of the large grains, avoiding any resorption rind present, are interpreted to represent the peak or near-peak assemblage.

Garnet grains in the kyanite zone (Fig. 1C) present a similar profile to those in the structurally lower garnet zone II. Line transects across the grains show negligible variation in Mn and Ca, with minor irregularities in Mg, Fe, and Fe# near one of the rims (Fig. 2E). This is confirmed in the chemical maps, which show negligible zonation, an inclusion-rich core, inclusion-free rim, and locally embayed grain boundaries (Fig. 2E). For these garnet grains, the average of the transect data, excluding the rims, is used to represent the closest approximation of near-peak metamorphic conditions.

Garnet grains from the sillimanite zone (Fig. 1C) are characterized by abundant inclusions, irregular shapes, and locally embayed grain boundaries. Three of the specimens (051, 052, and 055) also have inclusion-free rims. The chemical distribution across those three grains is generally homogeneous, with evidence of a Mn-rich resorption rind (e.g., Kohn and Spear, 2000) variably preserved. For these grains, the average of the transect data, excluding the rims, is used to represent peak or near-peak metamorphic conditions. Garnet grains from the structurally highest specimen analyzed, 059, do not have an inclusion-free rim. Moreover, there is appreciable zonation in grains where Mg decreases from core to rim, while Fe, Mn, and Fe# increase toward the rims of the grains. The core compositions of these grains, where Mn and Fe# are the lowest, are taken to represent peak or near-peak metamorphic conditions.

In addition to the garnet zonation, the chemical distributions within plagioclase, biotite, and muscovite were examined in order to select the compositional data that best represent the equilibrium P-T estimates. Biotite was investigated with line transects across large grains and grain clusters with negligible zonation found. For specimens that showed diffusional growth zonation and the development of Mn-rich resorption rinds in garnet (032, 035, 039b, 044, 046, 055, 059), the average compositions of matrix biotite were used exclusively, because the retrogressive reactions are more likely to affect the biotite that is in contact or close proximity to garnet grains (Kohn and Spear, 2000). In the specimens that preserved growth zoning (013 and 039a), the average combined biotite composition in the matrix and in close proximity to garnet was used, because these two compositions were generally indistinguishable. Muscovite analyses were also conducted as line transects across small grains and large sheaths, with negligible zonation found. Compositions used for geothermobarometry follow the same principles as the selection for biotite. Finally, transects of plagioclase feldspar also revealed negligible zonation, and therefore the same procedure of data selection as for biotite and muscovite was employed.

The one exception to the aforementioned selection technique was specimen 013, which has a lack of available near-rim biotite. Therefore, because of specimen 013’s distinctive prograde growth zonation, the rim compositions of garnet were used with near-rim muscovite and matrix biotite.

The compositional data used as input for the THERMOCALC P-T geothermometer, using the internally consistent data set of Holland and Powell (1998), are presented in Table 1. The P-T results are presented in Table 2 and Figure 3.

The uncertainties associated with P-T estimates from specimen 044 are exceptionally large and warrant discussion. Calculated temperature and pressure estimates are 888 ± 288 °C and 10.4 ± 3.2 kbar, respectively. The small size and scarcity of garnet available for quantitative analysis in specimen 044 are thought to be the main reasons for the imprecision of the calculated P-T data; only two garnet grains were large enough to constrain zonational variation and use for geothermobarometric analysis. These garnet grains are significantly embayed and partially resorbed, and contain biotite disseminated in fractures, which indicate that the garnet may not be in equilibrium with the rest of the assemblage. The P-T data calculated from this specimen are not considered further.

Peak temperatures are within error across the mapped area from specimen 032 at 772 ± 37 °C to specimen 059 at 853 ± 58 °C (Fig. 3). This is in agreement with observations from across the Himalaya of consistently high temperatures across the upper portion of the Himalayan metamorphic core (e.g., Hodges et al., 1988; Searle et al., 1999; Fraser et al., 2000; Larson et al., 2010), which have been interpreted to indicate the effect of thermal buffering due to pervasive in situ melt production (Hodges et al., 1988). Moreover, these high temperatures may be minimum estimates, given the diffusional homogenization noted in most of the garnets analyzed in this study (see previous).

Pressure estimates at lower structural levels are indistinguishable within uncertainty (specimen 032 at 10.8 ± 1.2 kbar to specimen 039 at 11.8 ± 1.4 kbar). Most of the specimens that are within uncertainty, however, have similar mineral assemblages and were subjected to the same analysis on the same instrumentation. It is possible, therefore, that the absolute uncertainties may be an overestimate of the relative uncertainties between the specimens (e.g., Fraser et al., 2000), and the apparent increase in pressure up structural section may represent real differences. An up structural section increase in pressures would be in agreement with apparent pressure gradients from the same structural level in west-central Nepal (Larson et al., 2010), far west Nepal (Yakymchuk and Godin, 2012), and northwest India (Spencer et al., 2012), and consistent with the inverted metamorphic sequence of isograds commonly found across the Himalaya (e.g., Mallet, 1875; Von Loczy, 1878; Oldham, 1883; Hodges, 2000).

The calculated pressure estimates at structurally higher levels decrease significantly up structural section from 11.8 ± 1.4 kbar in specimen 039 to 8.6 ± 1.4 kbar in specimen 046 and 6.5 ± 1.3 kbar at specimen 055, which is indistinguishable within uncertainty from the structurally highest pressure of 7.0 ± 1.4 kbar from specimen 059 (Fig. 3).

U-Th/Pb and rare earth element (REE) monazite petrochronology was employed to provide chronologic constraints on the metamorphic history of the rocks examined in this study. Monazite grains were identified in six of the specimens used for geothermobarometry, with all grains found occurring in the matrix; no monazite grains were found in the structurally lowest garnet zone I. Grains were identified through whole thin section X-ray maps of Ce and P generated using the Cameca SX100 electron microprobe at the Saskatchewan Research Council in Saskatoon. An accelerating voltage of 15 kV was used with a beam current of ∼200 nA and a step size of ∼30 μm. Suitable monazite grains were then mapped individually at higher resolution (step size of 1 μm) to characterize spatial variation in U, Th, and Y in each grain. Monazite grains varied significantly in shape (Fig. 4) and zoning patterns; however, none of the analyzed grains yielded an internal chemical structure consistent with growth coeval with shearing.

Y zonation was closely monitored during the selection of U-Th/Pb spot analyses in an attempt to sample distinct age-domains and avoid cross-domain sampling and mixing of age domains. Monazite grains were dated using a split-stream laser-ablation inductively coupled plasma–mass spectrometry (ICP-MS) system at the University of Santa Barbara, California (UCSB), in which U-Th/Pb and REE data were collected simultaneously on a Nu Plasma multicollector (MC) ICP-MS and Nu Attom single collector (SC) ICP-MS. Analytical protocol is similar to that described by Cottle et al. (2012) and Kylander-Clark et al. (2013). See the GSA Data Repository for full description of analytical methods.¹

Because of the young age of the monazite investigated in this study, radiogenic Pb signals tend to be low (typically <1000 cps ²⁰⁷Pb). This results in relatively imprecise ²⁰⁷Pb/²³⁵U ages and difficulty in applying an accurate ²⁰⁴Pb correction. The data presented in this study are therefore presented on ²⁰⁶Pb/²³⁸U versus ²⁰⁸Pb/²³²Th diagrams. To avoid potential issues with excess ²⁰⁶Pb and reverse discordance from unsupported ²³⁰Th (Schärer, 1984), monazite age interpretations are based on the ²⁰⁸Pb/²³²Th dates. Age and REE data reduction, including corrections for baseline, instrumental drift, mass bias, down-hole fractionation, and age calculations, was carried out using Iolite version 2.1.2 (www.iolite.org.au). Full details of the data reduction methodology can be found in Paton et al. (2011) and Cottle et al. (2012). Age data were plotted using Isoplot v.3.7 (Ludwig, 2003).

The full U-Th/Pb data set is presented in Table 3, with associated REE data available in the Data Repository (see ^{footnote 1}). The ²⁰⁶Pb/²³⁸U versus ²⁰⁸Pb/²³²Th diagrams, with associated thin section images and representative monazite Y grain maps for each specimen, are presented in Figure 4. Moreover, plots of ²⁰⁸Pb/²³²Th age versus ΣHREE (Gd-Lu inclusive) and ²⁰⁸Pb/²³²Th age versus Gd/Yb (a proxy for the slope of the heavy [H] REEs) for each specimen are shown in Figure 5.

Most of the specimens analyzed show multiple, distinct domains within monazite grains. The interior regions of most monazite grains typically contain lower Y, lower total HREEs, and more variable Gd/Yb ratio trends than the rims. Individual spot dates from all domains within the specimens analyzed range from 27.4 ± 0.4 Ma to 15.3 ± 0.2 Ma, with older dates always found in the low-Y cores of monazite grains. Each specimen analyzed is discussed next from structurally lowest to highest position.

The structurally lowest specimen, 046, from the top of garnet zone II (Fig. 1C), yielded dates of 24.7 ± 0.4 Ma (core) to 15.7 ± 0.2 Ma (rim) from a total of 19 spot analyses within seven individual monazite grains (Fig. 4A). A representative monazite grain from this specimen, located in the matrix in close proximity to biotite, garnet, and plagioclase, contains a ca. 24 Ma, low-Y, low-HREE, high-Gd/Yb core surrounded by a ca. 16 Ma, high-Y, high-HREE, low-Gd/Yb rim (Fig. 5A).

Specimen 048, from the kyanite zone (Fig. 1C), yielded dates of 20.9 ± 0.2 Ma (core) to 15.3 ± 0.2 Ma (rim) from a total of 78 spot analyses within five individual monazite grains (Fig. 4B). Though the monazite grains in this specimen appear zoned, the total HREE concentrations and Gd/Yb ratios in this specimen do not show any discernible pattern with age (Fig. 5B).

Specimen 052 (Fig. 1C), from the sillimanite zone, yielded dates of 27.4 ± 0.4 Ma (core) to 16.7 ± 0.3 Ma (rim) from 29 spot analyses within nine individual grains (Fig. 4D). Monazite from this specimen displays distinctive ca. 27 Ma cores that are lower in Y, lower in total HREEs, and higher in Gd/Yb than ca. 18 Ma rims, which have generally higher Y, higher total HREEs, and lower Gd/Yb ratios (Fig. 5D).

Specimen 055, also within the sillimanite zone (Fig. 1C), yielded dates of 19.4 ± 0.4 Ma (core) to 16.4 ± 0.4 Ma (rim) from 31 spot analyses within four individual grains (Fig. 4E). Y zonation in this specimen is varied, with the oldest spot dates of 19.4 ± 0.4 Ma and 18.8 ± 0.4 Ma derived from small, irregular low-Y zones (Fig. 4E). Younger ages are weakly correlated with higher Y and HREEs and lower Gd/Yb ratios (Fig. 5E).

Specimen 059, from the structurally highest position reached in the sillimanite zone (Fig. 1C), contained only three monazites suitable for analyses that yielded spot dates ranging from 20.5 ± 0.3 Ma to 18.6 ± 0.2 Ma from five analyses (Fig. 4F). Monazite grains in this specimen are rare and generally small in size (<30 μm across; Fig. 4F). No distinct Y zonation was observed. Total HREEs and Gd/Yb ratios show no discernible pattern with age (Fig. 5F).

The distribution of Th and Y within monazite has been used to provide insight into mechanisms of monazite growth, dissolution, and re-precipitation in metamorphic rocks (e.g., Foster et al., 2002; Gibson et al., 2004; Kohn et al., 2005). Zonation of Y specifically, a proxy for HREEs, has been shown to correlate with age domains within single monazite grains (e.g., Foster et al., 2002; Gibson et al., 2004; Kohn et al., 2005; Cottle et al., 2009a, 2009b; Rubatto et al., 2013; Kellett et al., 2010; Larson et al., 2011; Stearns et al., 2013). Monazite growth in the presence of stable/growing garnet and/or xenotime should be relatively low in Y, since both garnet and xenotime have higher partition coefficients for Y and HREEs than monazite (Foster et al., 2002), whereas monazite growth and/or recrystallization prior to garnet/xenotime growth and/or during garnet/xenotime breakdown may be relatively high in Y and HREEs (Gibson et al., 2004; Kohn et al., 2005; Rubatto et al., 2006; Kellett et al., 2010). Xenotime was not found in thin section mapping; therefore, only garnet controls on the HREE budget are considered in the following.

Combining the trace-element data with the ages from different Y domains present in the monazite grains can provide insight into the metamorphic processes during monazite growth and/or recrystallization. The timing of monazite growth and/or recrystallization in the Likhu Khola region is summarized in Figure 6 and brackets the duration of partial melting. The low-HREE and low-Y core regions of most monazite grains analyzed yield dates between ca. 27 and 23 Ma (Fig. 6). The HREE and Y concentrations of these older core domains may indicate monazite growth coeval with garnet growth during prograde metamorphism. In contrast, the relatively high-Y and HREE-rich rim overgrowths around the older monazite cores appear to record a separate, distinct growth event ranging between ca. 22 and 18 Ma. This enrichment of HREEs is interpreted to be sourced from the breakdown of garnet, and possibly preexisting monazite, during partial melting (e.g., Spear, 1993; Kohn et al., 2005; Larson et al., 2011). Garnet observed in thin section is commonly characterized by embayed grain boundaries, a potential result of resorption during anatexis (e.g., Waters, 2001), and Mn-rich rims, which are also consistent with resorption. If partial melting of these rocks and development of significant anatexite are responsible for the high HREE concentrations available for inclusion in monazite grain rims, it must have occurred or been ongoing prior to, or during, growth of those high-HREE domains, but after the growth of the low-HREE monazite cores. The youngest monazite dates associated with high-HREE concentrations and low-Gd/Yb ratios are typically rim domains with dates between ca. 18 and 15 Ma, indicating that melt crystallization, or at least monazite growth during garnet breakdown and increased HREE availability, appears to have commenced ca. 22 Ma and continued until at least ca. 15 Ma (Fig. 6). This time span is consistent with interpreted melt crystallizationages ranging between ca. 22 and 14 Ma in the adjacent Tama Kosi valley (Larson et al., 2013) and other localities across the Himalaya (e.g., Viskupic et al., 2005; Cottle et al., 2009a).

The new metamorphic and petrochronologic data provide important constraints on the evolution of the Likhu Khola region. When combined with additional data from adjacent study areas, these data help inform the kinematic evolution of the now-exhumed midcrustal core of the Himalaya.

While metamorphic temperature estimates are essentially invariant (Fig. 3) across the mapped area, pressure estimates from geothermobarometric calculations are not. In the lower Himalayan metamorphic core, pressures are within error of each other (10.8 ± 1.0 kbar in specimen 032 and 11.8 ± 1.4 kbar in specimen 039; Fig. 3); however, structurally higher pressure estimates decrease toward higher structural positions to a minimum of 6.5 ± 1.3 kbar (specimen 055). The initiation of that decrease occurs abruptly between specimens 039 and 046. Metamorphic pressure trends similar to that described herein have been interpreted to outline tectonometamorphic discontinuities (e.g., Groppo et al., 2009; Larson et al., 2010; Yakymchuk and Godin, 2012). If the same interpretation were made in the Likhu Khola region, this change in pressures would mark the lateral continuation of the discontinuity mapped in the Tama Kosi valley at a similar structural level (Larson et al., 2013). The discontinuity itself is cryptically defined in the field, marked only by an increase in the volume of anatexite up structural section with no observed associated strain gradients.

The discontinuities mapped at a similar structural level to that outlined in this study (Figs. 1C and 3) have been correlated with the Main Central thrust in the thermo-mechanical models of Jamieson et al. (2004) (see Larson et al., 2010, 2013). Such structures mark the separation between rocks above, which were metamorphosed and deformed primarily as part of the midcrust within the orogenic hinterland, from rocks below, which were deformed and metamorphosed as part of the orogenic foreland (Larson et al., 2010, 2013; Yakymchuk and Godin, 2012). Data from this study agree with results of previous studies that demonstrate a protracted record of metamorphism (ca. 27–23 Ma) and anatexis (ca. 22–15 Ma), and deformation above the discontinuity in rocks from the midcrustal hinterland of the orogen (e.g., Larson et al., 2011, 2013). While geochronologic constraints were not extracted from below the interpreted discontinuity in the Likhu Khola region due to a lack of suitable material, data from the adjacent Tama Kosi valley indicate that metamorphism below the discontinuity occurred significantly later than that above at ca. 8–10 Ma (Larson et al., 2013). This diachronous metamorphism across the discontinuity may be explained by an overthrusting relationship (e.g., Long et al., 2011), where material in the hanging wall, above the structure, was juxtaposed above the material below it. The juxtaposition, whether it was accomplished through overthrusting (e.g., Long et al. 2011) or underplating (e.g., Webb et al., 2013), drove coeval metamorphism and deformation in the rocks below the discontinuity. In such an interpretation, the age of cooling and melt crystallization above the discontinuity, ca. 15 Ma in the Likhu Khola region, limits the maximum age of metamorphism and deformation in the rocks below as rocks in the hanging wall would be expected to have cooled as they were moved higher in the crust and laterally toward the south during burial of the footwall in response to continued convergence. This younger than 15 Ma age constraint is compatible with monazite growth at ca. 10–8 Ma below the discontinuity in the adjacent Tama Kosi region, which developed along a pressure-temperature-time path indicative of a burial-type metamorphism and deformation in the footwall of the thrust (Larson et al., 2013).

The discontinuity interpreted in this study and its equivalents typically occur between staurolite- and kyanite-grade or lower-grade rocks (e.g., Yakymchuk and Godin, 2012; Larson et al., 2013). However, discontinuities across the Himalaya have also been reported from higher structural levels in the Himalayan metamorphic core within sillimanite-grade rocks (e.g., Reddy et al., 1993; Fraser et al., 2000; Carosi et al., 2010; Martin et al., 2010; Corrie and Kohn, 2011; Rubatto et al., 2013; Montomoli et al., 2013; Larson and Cottle, 2014). In contrast to the structurally lower discontinuities, which can be correlated with the Main Central thrust of Jamieson et al. (2004), these structurally higher entities are not always recorded as having fundamental changes in apparent metamorphic conditions, but are more commonly recognized by differential timing of similar pressure-temperature paths (e.g., Carosi et al., 2010; Rubatto et al., 2013; Montomoli et al., 2013) or a break in deformation temperatures (Larson and Cottle, 2014). These structurally higher discontinuities are generally interpreted as ductile shear zones accommodating thrust sense (top-to-the-south) movement (e.g., Carosi et al., 2010; Montomoli et al., 2013); however, the timing of movement across these shear zones is not well constrained outside of the Dolpo region of western Nepal, where they were active sometime between ca. 26 and 17 Ma (Carosi et al., 2010; Montomoli et al., 2013). There, the overthrusting of the hanging wall is interpreted to have driven metamorphism in the footwall (Montomoli et al., 2013), much like the process interpreted for structurally lower rocks in this study. Similar structures described in west-central Nepal, however, where the Himalayan metamorphic core is considerably thicker than it is in the Dolpo region, are thought to be postpeak metamorphic (Fraser et al., 2000; Martin et al., 2010; Corrie and Kohn, 2011), as are those noted to the east in east-central Nepal (Larson and Cottle, 2014), Sikkim, and Bhutan (Warren et al., 2011; Grujic et al., 2011; Rubatto et al., 2013). Such a timing relationship with metamorphism may indicate that these discontinuities represent the initial record of foreland-style, vertical thickening and horizontal shortening, recorded in the Himalayan midcrust as it was exhumed, cooled, and incorporated into the foreland.

The nature of the discontinuities within the sillimanite-grade core has only recently become the subject of focused research. However, initial interpretations have called into question the potential applicability of thermo-mechanical orogenic simulations (e.g., Beaumont et al., 2004), which, as presented, have typically required a significantly thickened midcrust to initiate, in their current form (e.g., Montomoli et al., 2013). Such an argument, however, does not take into account all factors controlling the potential lateral movement of the midcrust, which include temperature, convergence rate, material viscosity, and the associated pressure gradient (Grujic, 2006). While other factors such as the lack of coeval movement on bounding structures (e.g., Carosi et al., 2013) may act as evidence against lateral midcrustal flow, the recognition that the exhumed midcrust may be significantly thinner than originally thought does not exclude the possibility of its ductile, large-scale lateral translation.

The existence of tectonic discontinuities of different types and ages indicates that the evolution of the Himalaya may be significantly more complicated than generally thought. Identifying these discontinuities and investigating their role in the development of the orogen are critical to elucidating the processes that have accommodated convergence-related deformation and assessing the validity of established hypotheses and models (e.g., Carosi et al., 2010; Rubatto et al., 2013; Montomoli et al., 2013; Larson and Cottle, 2014).

The pervasive deformation and moderate to high metamorphic grades found throughout the entire study area indicate that all the rocks in the studied area are part of the Himalayan metamorphic core. Pressure-temperature estimates from across the Likhu Khola region yield a pattern where temperatures are relatively invariant and pressures abruptly decrease partway up the structural section. This decrease in pressure is interpreted to mark a tectonometamorphic discontinuity that separates the Himalayan metamorphic core into lower and upper domains that record different tectonometamorphic histories. Protracted monazite growth in the upper portion of the Himalayan metamorphic core in the Likhu Khola region records a history of metamorphism and anatexis between ca. 27 and 15 Ma. While no geochronologic data were extracted from the lower portion of the Himalayan metamorphic core in this study, the youngest monazite dates from the upper domain are interpreted to have grown coeval with melt crystallization related to cooling during movement across the discontinuity while overriding the material in its footwall. This constrains the timing of metamorphism and deformation structurally below the discontinuity to younger than middle Miocene in age. This is compatible with monazite dates from similar rocks at the same structural level in adjacent regions (Larson et al., 2013).

This study of the Himalayan metamorphic core in the Likhu Khola region has demonstrated the lateral continuation of a tectonometamorphic discontinuity recognized in adjacent regions into the study area and confirms that such structures are regional in scale. The existence of this and other discontinuities like it at various structural levels within the Himalayan metamorphic core indicates that the kinematic evolution of the metamorphic core of the Himalaya is more complex than has been commonly recognized.

This study was funded by University of Saskatchewan Faculty Start-up and National Science and Engineering Research Council Discover grants to K. Larson and by the National Science Foundation under grant EAR-1119380 awarded to J. Cottle. Steven Creighton at Saskatchewan Research Council is thanked for providing support during Microprobe analyses. Logistical support in Nepal was provided by Teke, Pradap, Pemba (2), Manoj, Buddhiman, Manggalsine, Lal Bahadar, Man Bahadar, and Rajesh Tamang. Constructive reviews by R. Carosi and an anonymous reviewer and editorial handling by J. Goodge greatly improved the manuscript.