Eur. J. Mineral. 2006, 18, 289–297
Evidence for Mio-Pliocene retrograde monazite in the Lesser Himalaya, far western Nepal LAURENT BOLLINGER1 and EMILIE JANOTS2,* 1Laboratoire
de d´etection et de G´eophysique, CEA, BP 12, F-91680 Bruy`eres-le-Chˆatel, France Corresponding author, e-mail:
[email protected] 2Laboratoire de G´ eologie, UMR 8538 du CNRS, Ecole normale sup´erieure, 24 rue Lhomond, F-75005 Paris, France *Now at: IGS, University of Bern, Baltzerstrasse 3, CH-3012, Bern, Switzerland
Abstract: A multichronometric study involving 40Ar/39Ar and 208Pb/232Th ion-microprobe dating was performed on the two northernmost windows of Lesser Himalayan rocks in far western Nepal. Both regions were sampled for their monazite-rich series. Metamorphic peak temperatures range from 540°C to 370°C. In the highest-grade rocks (Tmax ' 540°C), 40Ar/39Ar chronology on hornblende, biotite and muscovite gives ages of 12.9 ± 1.9 Ma, 8.9–11.7 Ma and 4.8 ± 0.4 Ma, respectively. Monazite grains yield two different 208Pb/232Th age populations of 9.3–11.4 Ma and 3.3–5.8 Ma range, respectively. The oldest monazites are found in garnet-rich samples whereas the youngest monazite grains texturally replace allanite in sample retrogressed under greenschist-facies conditions. The lowest-grade sample (Tmax ' 370°C) bears also young monazites at 9.0 ± 1.0 Ma, as replacement products of allanite. The chronological results as well as the clear textural relationships between allanite and monazite (which furthermore show identical REE patterns) indicate a monazite growth at the expense of allanite at low temperature (< 370°C) during exhumation. This study shows that young Mio-Pliocene Himalayan monazite should not be considered systematically as a prograde or metamorphic-peak mineral. Key-words: geochronology, monazite, allanite, ion-microprobe dating, Lesser Himalaya, black shales.
Introduction The Lesser Himalayan series (LH) consists in sediments of probably Proterozoic ages that have been scraped from the underthrusting Indian crust and accreted to the mountain range over the last 20 Ma. They have undergone pervasive thrust shearing and low-grade metamorphism. At the top of the LH, the Main Central Thrust fault zone (MCT) separates the LH from the overlying High Himalayan Crystalline (HHC) units. This large shear zone exhibits the strong inverse thermal imprint of the HHC overthrusting on the LH. Numerous studies have focused on the chronology of this major shear zone and its hanging-wall (e.g. Copeland et al., 1991). Th/Pb monazite ages have been used to date either the High Himalayan Crystalline sequences (Catlos et al., 2001; Simpson et al., 2000) or its intrusive leucogranite (Edwards & Harrison, 1997; Noble & Searle, 1995). Footwall monazites also yield young (late Miocene) and consistent ages along strike from the main shear zone in Garhwal (India) (Catlos et al., 2002a) to central western (Catlos et al., 2001; Harrison et al., 1997) and eastern Nepal (Catlos et al., 2002a). These ages were interpreted as the result of young transient out-of-sequence thrusting on the Main Central Thrust. Although this simple picture leads to a history that is consistent along strike, it appears that the cooling history reDOI: 10.1127/0935-1221/2006/0018-0289
vealed by several 40Ar/39Ar data is locally inconsistent with that interpretation. In this study we investigate the thermal history of the upper Lesser Himalaya formations accreted to the Himalayan range in far western Nepal, a region that shows a gap of chronological data as well as an interesting structural setting. For this purpose, far western Nepal samples for which peak temperatures were known were analyzed using 40Ar/ 39Ar and Th/Pb methods. Their structural and petrographic settings are documented here to show that the series might have encompassed passive roof thrust exhumation and interaction with a retrograde fluid.
Geological setting The tectonic wedge of far western Nepal exhibits a complex duplex structure (DeCelles et al., 2001). The structural evolution in this Himalayan region preserved several klippe relics, in contrast to central western Nepal (Fig. 1). This structural context, described along the Indo-Nepalese border since Heim & Gansser (1939), allows one to sample the same LH formations several times along a cross-section from the northernmost MCT trace at the front of the Himalayas to the present-day southern edge of the crystalline 0935-1221/06/0018-0289 $ 4.05
ˇ 2006 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart
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b
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FW0023 FW0024 Sird an g ▲ win ▲ do w
FW0025
Granites
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Chiplakot klippe Undifferentiated Lesser Himalayas
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Anticlinal axis Synclinal axis
Askot klippe
Askot -Chi p
Sample location
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ua hibo chi r t lite sts zi s te / b l
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Fig. 1. (a) Sketch map of central and western Nepal. Box shows location of Fig. 1b. (b) Generalized geological map of far western Nepal [modified after e.g. Shresta et al. (1987); Sinha (1989); Valdiya (1980)] showing sample locations, major drainages and topography (USGS SRTM30 digital elevation model). Stars, monazite-rich samples studied; circles, additional samples referred in the text for their Ar closure ages. (c) Simplified cross sections A-A’ and B-B’ modified from Sinha (1989).
nappes. Along the Mahakali River, three windows cut through sequences of HHC affinity exposing the underlying LH series: the small Sirdang window to the North and two larger windows separated by the Askot klippe, the northernmost one being referred here as the Askot-Chiplakot window from the name of the two klippes bordering it. The Sirdang metasedimentary window, previously described by Heim & Gansser (1939), Fuchs & Sinha (1978) or in Sinha
(1989), comprises dolomitic formations near its core and an overlying alternation of quartzite, black slate, schist and amphibolite, which is a characteristic upper LH formation along the MCT zone. Elsewhere in far western Nepal this litho-structural unit is usually referred to as the Ramgarh thrust sheet (DeCelles et al., 2001). The base of this thrust sheet crops out farther South, under the Askot klippe for example. This thrust sheet as well as the underlying Lesser Hi-
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Evidence for retrograde monazite in the Lesser Himalaya Table 1. Th/Pb ages for monazites FW0023, -25 (Sirdang window) and FW0144 (Askot-Chiplakot window). Sirdang metasedimentary window FW0023 garnet grain 6-1 9a-1 8a-1 8b-1
Average (Ma)
Askot-Chiplakot window
Age (1 c )
FW0023 matrix grain
Age (1 c )
10.2 9.4 11.4 9.3
12-1 12-2 11a-1 11b-1
9.5 10.3 10.4 9.6
(0.3) (0.2) (0.4) (0.6)
9.9 (1.3)
(0.2) (0.5) (0.2) (0.2)
FW0025 matrix grain Ds1 Ds1-1 Cs1 Cs1-1 Bs1 13s1 m2s1 m12s1 m10s1
9.9 (0.7)
malayan series encompass metasedimentary series which contain carbonaceous material (CM). The degree of organisation of the CM determined quantitatively by Raman spectroscopy has been used as a geothermometer (referred to as the RSCM method: Beyssac et al., 2002) to evaluate the metamorphic peak temperatures in the area (Beyssac et al., 2004). This study revealed a decreasing temperature from 540°C at the top of the Ramgarh thrust sheet down to 330°C in the core of the LH windows, as elsewhere along the LH in Nepal.
Petrological and thermochronological data Analytical techniques Samples were obtained from an upper section of the Lesser Himalaya, from the top of the Ramgarh thrust sheet in the Sirdang window down to its base under the Askot klippe farther south. The RSCM metamorphic peak temperatures reported here have all been determined by Beyssac et al. (2004) on the same samples as those described here for their monazite. Each temperature is reported with a standard deviation calculated on the population of spectra recorded, which reflects the within-sample structural heterogeneity. The accuracy of the calibration of the RSCM method is estimated to be ± 50°C in the 330–650°C range (Beyssac et al., 2002). Monazite grains and their textural relationship were characterized using scanning electron microscopy (SEM) in backscattered-electron mode (BSE) and electron microprobe (EMP). The EMP quantitative analyses were achieved with a CAMECA SX50 (CAMPARIS, Jussieu, Paris) at 10 nA and 15 kV conditions. We used monazite and doped glasses (REE, Y), apatite (P, F), diopside (Ca, Si, Mg), Fe2O3, MnTiO3 and orthoclase (Al) as standards with a ZAF correction. Actually, EMP analysis alone is not sufficient to distinguish monazite from rhabdophane, (LREE)PO4.H2O, for which the difference in analytical total is in the uncertainty range. Therefore, monazite was confirmed independently using Raman microspectrometry. Rhabdophane spectra were collected with a Renishaw spectrometer ( † = 514 nm) in the 300–4000 cm-1 range on a sample from the
Age (1 c ) 5.0 5.8 3.3 4.0 13.9 5.1 5.3 3.9 2.2
(0.9) (0.7) (0.5) (0.6) (1.3) (0.8) (1.2) (0.7) (1.4)
FW0144 matrix grain m1 m5 m6 m7 m8
4.6 (1.7)
Age (1 c ) 8.1 10.1 8.6 11.2 8.3
(0.8) (1.7) (0.8) (3.5) (6.5)
9.0 (1.0)
Table 2. 40Ar/39Ar total-gas ages (TGA), weighted mean ages (WMA), inverse-isochron and plateau ages for samples from the Sirdang and Askot-Chiplakot windows.
Biotite FW0022 Biotite FW0023 Hornblende FW0022 Muscovite FW0023 Muscovite FW0144 Muscovite FW0147 Muscovite FW0140
TGA
WMA
Inverse isochron
Plateau age
11.9 ± 0.8
11.7 ± 0.4
11.6 ± 0.4
11.6 ± 0.4
9.0 ± 0.6
8.9 ± 0.3
8.9 ± 0.5
8.7 ± 0.2
15.6 ± 2.4
12.9 ±1.9
10.6 ± 1.0
12.5 ± 0.4
5.7 ± 1.0
4.8 ± 0.4
4.7 ± 0.4
4.7 ± 0.2
28.7 ± 2.0
17.7 ± 2.6
16.9 ± 2.7
16.1 ± 0.4
26.2 ± 0.8
24.4 ± 2.5
23.4 ± 3.2
No
19.3 ± 0.9
17.7 ± 1.9
16.8 ± 2.2
No
type-locality (Cornwall, England), donated by the Mus´eum National d’Histoire Naturelle de Paris (no. 107.844). Rhabdophane displays a distinctive band centred around 3470 cm-1, i.e. in the OH-stretching region, which is absent from our monazite spectra. Thin-section portions with monazite grains suitable for ion microprobe Th-Pb dating (i.e. monazite grains limited to regular shapes and sizes over the typical 15–25 µm diameter of the O– primary beam) were mounted in epoxy along with five polished grains of monazite standard 554 (Harrison et al., 1997). Ion-microprobe Th-Pb analyses were performed using the IMS Cameca 1270 instrument at UCLA following the methods of Harrison et al. (1995) and procedures described in Catlos et al. (2002b). The Th/Pb monazite ages are given in Table 1. Biotite, muscovite and hornblende from the same samples and from adjacent amphibolite and micaschist were separated for 40Ar/39Ar chronology. Following their irradiation in the McMaster reactor, high-purity mineral concentrates were analysed with a VG3600 automated mass spectrometer (UCLA). Their 40Ar/39Ar ages were determined using the
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analytical procedure of McDougall & Harrison (1988). The 40Ar/39Ar data are presented in Table 2, Fig. 3 and 4 and as auxiliary material in a repository data file, which can be obtained from the authors or from the Editorial Office of the journal. Although plateau ages or isochron ages have been regarded individually as more representative of particular sample ages, Weighted Mean Ages have been reported systematically in the text for consistency. The Sirdang metasedimentary window Closely spaced samples (FW0022 is adjacent to FW0023, and structural distances between FW0022 and 0024, FW0024 and 0025 are 0.6 and 1.5 km, respectively) were studied in the Sirdang window which exposes the classical metasedimentary alternation of the upper Lesser Himalaya (Fig. 1). A peak temperature of 541 ± 50°C was determined for the black slate formation (FW0024), a result consistent with the mean temperature of 542 ± 16°C determined for the MCT zone in Nepal (Beyssac et al., 2004). The Ar-Ar dating method has been applied to hornblende (12.9 ± 1.9 Ma total gas age) and biotite (11.7 ± 0.4 Ma) from a hornblende-biotite-albite-quartz rich amphibolite, FW0022 (Fig. 3). An adjacent micaschist (FW0023) bears a garnet-biotitechlorite-muscovite-ilmenite assemblage, in which centime-
a
tre-size garnet grains are altered, sheared and fractured, and contain tiny inclusions (< 20 µm) of monazite and ilmenite in contact with fractures filled with retrograde phases (Fig. 2a). Monazite occurs also in the matrix as 15–20 µm elongate grains. Ages determined from the monazite grains included in the garnet rims as well as in the matrix yield highly consistent ages comprised in the 9.3–11.4 Ma range (Table 1, Fig. 3). The biotite weighted mean age of 8.9 ± 0.3 Ma for that sample is younger than those obtained on adjacent FW0022 (Fig. 3). However, the muscovite spectrum reveals a younger thermal history at 4.8 ± 0.4 Ma. Sample FW0025 is a “soapy” green schist mainly composed of quartz, chlorite, and muscovite with accessory monazite and apatite grains (15–20 µm). Monazite occurs in FW0025 along corroded, up to 1 mm allanite crystals (Fig. 2b). They are associated with quartz, chlorite and muscovite as replacement products of allanite (Fig. 2c). Electron-microprobe data indicate a similar REE distribution in allanite, Ca1.4REE0.5Mg0.2Fe0.5Al2.3(SiO4)3OH, and monazite, (REE)PO4, grains in mutual contact (e.g. XLa= 0.28 and XNd= 0.11 for both minerals with XREE defined for a given rare earth as XREE = REE/ 7 REE). Therefore no significant REE fractionation has occurred as a result of allanite breakdown. Apart from grains m10s1 at 2.2 ± 1.4 Ma and Bs1 at 13.9 ± 1.3 Ma considered as poor in radiogenic 208Pb* or outlying, monazite ages in FW025 range from 3.3 to 5.8 Ma (Table 1, Fig. 3).
b
Mnz
Chl
Aln Qtz
Mnz
Gt
Chl
Mnz
Mnz
50 µm
d
c
40 µm
e
Qtz
Chl/Ms Mnz Qtz Chl/Ms
Aln
Qtz
Ms
Mnz
Mnz Qtz Chl
25 µm
Ms
Qtz
Aln 10 µm
10 µm
Fig. 2. Textural relationships between monazite and allanite. (a) FW0023 monazite (Mnz) included in garnet (Gt). (b) FW0025 monazite and a millimetre-size allanite (Aln) crystal. (c) FW0025 allanite extensively replaced by chlorite (Chl), muscovite (Ms) and monazite (Mnz). (d) FW0144 monazite along euhedral allanite and (e) replacement of idioblastic FW0044 allanite by monazite and quartz (Qtz).
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Evidence for retrograde monazite in the Lesser Himalaya
In the Ramgarh thrust sheet under the Askot klippe To complement this upper Lesser Himalaya picture in far western Nepal, the Ramgarh thrust sheet was sampled farther south under the remnants of the Askot klippe (Fig. 1). The peak temperature in this region decreases from ~500°C to ~350°C as a function of the structural distance to the top of the Lesser Himalayan series (Beyssac et al., 2004). FW0144, sampled near the core of the Askot-Chiplakot window, yields a peak temperature of 373 ± 50°C which is consistent with the temperature distribution in that region. This sample is mainly composed of quartz, carbonaceous material and mica and hosts also allanite and monazite in close association. Monazite grains rim allanite crystals (Fig. 2d), or show partial or complete replacement of idioblastic allanite (Fig. 2e). The Th/Pb data obtained on monazite yield homogeneous ages around 9 Ma, in the 8.1–11.2 Ma range. This monazite age is younger than those obtained by Ar-Ar on FW0144 muscovite, at 17.7 ± 2.6 (Fig. 4). Although the age spectrum of FW0144 muscovite is not flat, the inverse isochron age is marginally consistent with those obtained on nearby samples (FW0140 and FW0147) as well as with ages reported at the same distance from the front of the high range (i.e. 40–50 km) in central Nepal (Bollinger et al., 2004).
Discussion Age interpretation In the Sirdang window, the peak temperature reached by the samples approximately corresponds to the estimated closure temperature for hornblende, i.e. around 525° C (McDougall & Harrison, 1988). Therefore the older Ar-Ar hornblende age of 12.9 ± 1.9 Ma is interpreted as dating the peak temperature event or incipient exhumation. Note that this age is younger than those published for an equivalent structural position in easternmost settings (Copeland et al., 1991; Hubbard & Harrison, 1989; Macfarlane, 1993). Assuming that biotite ages date the cooling through the closure temperature of around 325–350°C (McDougall & Harrison, 1988),
the similarity of hornblende (12.9 ± 1.9 Ma) and biotite ages (11.7 ± 0.4 Ma and 8.9 ± 0.3 Ma) suggests that the samples have undergone rapid cooling (> 50°C·Ma-1). However, although the closure temperature of muscovite is usually higher than that of biotite, FW0023 muscovite yields a youngest Ar-Ar age of 4.8 Ma. This inversion is common in the Himalayan setting (Edwards, 1995; Macfarlane, 1993) and has been discussed elsewhere (e.g. Stuwe & Foster, 2001). It may reflect late muscovite crystallization in a fluid-rich shear zone, a scenario preferred hereafter, or excess Ar in biotite (e.g. Kelley, 2002), a scenario implying that the biotite age has no geological significance. In this window, monazite Th/Pb dating yields two age populations. The first group of monazite at 9–11 Ma is found in sample FW0023. The ages span a similar range whatever the textural position of the grains, in the garnet (11.4 to 9.3 Ma) or in the matrix (10.4 to 9.5 Ma). Since these ages overlap hornblende and biotite Ar-Ar ages in the range of uncertainties (Table 1 and Table 2), they may record peak metamorphism as well as exhumation conditions. Although monazite armoured in garnet often records conditions of garnet growth (e.g. DeWolf et al. 1993; Montel et al., 2000), monazite in sample FW0023 occurs along the fractures of the garnet. Pb loss via diffusion must be extremely limited at the metamorphic peak temperature of 541 ± 50°C reached by this sample (Cherniak et al., 2004), but it could nevertheless be achieved via dissolution/recrystallisation processes (Seydoux-Guillaume et al., 2002). Therefore, monazite could have formed (or reequilibrated) contemporaneously with the garnet growth or after it (i.e. during exhumation) along the fractures in the presence of a fluid phase. The second group of Th/Pb ages at 4.6 ± 1.7 Ma was obtained on monazite rimming or replacing allanite in sample FW0025. This age is much younger than the monazite age obtained for FW0023 whereas it is quite similar to its muscovite Ar-Ar age at 4.8 ± 0.4 Ma (Fig. 3). The textural relationship between monazite and allanite shows that monazite formed at the expense of allanite (Fig. 2c). This reaction is clearly not isochemical at the mineral scale and it is likely to have involved a fluid phase. This very young age is consistent with a monazite formation during exhumation, as a retrograde breakdown product of allanite.
20
15 FW0023
FW0022 FW0022
(Matrix)
FW0023 (Garnet)
10
10
FW0023
FW0025 (Matrix)
FW0023
0
5
0 0
20
40
60 39
80
Cumulative % Ar released
100
40
40 35
30
30 25
FW01-47
20
20
FW01-40
15
FW01-44
FW0144 (matrix) 10
10 5
0
0
Monazite
Fig. 3. Age spectra for hornblende, biotite and muscovite from Sirdang window samples (FW0022, -23 and -25). Monazites Th/Pb ages, in matrix and garnet, are also given on the right-hand side with their standard deviation.
Th/Pb age (Ma)
15
45
Apparent Age (Ma)
Muscovite
5
50
50
Biotite
Th/Pb age (Ma)
Apparent Age (Ma)
Hornblende 20
0
20
40
60 39
80
Cumulative % Ar released
100
Monazite
Fig. 4. Age spectra for muscovite under Askot klippe samples (FW0140, -44 and -47). Average monazite Th/Pb age for FW0144 is also given on the right-hand side with its standard deviation.
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Under the Askot klippe, the FW0144 muscovite age is poorly constrained, as shown by the shape of its spectrum and the difference of the isochron and weighed mean ages. The minimum age is a plateau age corresponding to about 50 % of the total 39Ar release (cf. repository data material). However, it seems consistent with muscovite ages obtained along the same section up to the contact with Askot klippe (Bollinger, 2002). It is expected from the derived peak temperature (373 ± 50°C) that this sample has reached temperatures near the closure of muscovite to argon, thereby either diffusing a large amount of its inherited Ar, or having preserved its growth age (Fig. 4). Its plateau age of 16.1 ± 0.4 Ma might correspond to the age of the temperature peak in Askot-Chiplakot window. The texture of monazite (Fig. 2d and 2e) and its Th/Pb age at 9.0 „ 1.0 Ma preclude a detrital origin for monazite. The peak temperature of this sample (373 ± 50°C) sets a maximum temperature for monazite formation (or reequilibration). This formation temperature appears to be fairly low since monazite is usually considered as being formed during prograde metamorphism through the breakdown of allanite at temperatures higher than 525°C (Kingsbury et al., 1993; Smith & Barreiro, 1990; Wing et al., 2003). Moreover, in this sample, textural relationships indicate that monazite has replaced allanite (Fig. 2d and 2e). This is evidence that monazite can form from allanite at temperatures around 370°C or below. Again, the presence of a fluid phase may have triggered that replacement. Allanite breakdown into monazite This chronological study coupled to textural observations shows that monazite could form at the expense of allanite during exhumation in the Sirdang (sample FW0025) and the Askot-Chiplakot (sample FW0144) windows. The reaction of monazite formation in sample FW0023 is not addressed in the following discussion since allanite was not identified as precursor for monazite in garnet or in the matrix. The presence of rhabdophane instead of monazite was suspected in our samples, since it was already described as alteration product of monazite in metapelites at low-temperature conditions (Nagy et al., 2002). Raman microspectrometry confirms that ion-probe dating was performed on monazite in this study (see Analytical techniques). Although monazite is often assumed to be unstable until 525°C (Kingsbury et al., 1993; Smith & Barreiro, 1990; Wing et al., 2003), the growth of monazite under low-grade metamorphic conditions is not a new finding since it has been documented in shales (Rasmussen et al., 2001). Retrograde growth of monazite has also been reported for similar temperature conditions (below ca. 450°C) in HP-LT metapelites from the Rif, Morocco (Janots et al., 2006). Previous descriptions of LREE mineral assemblages in the literature suggest that the stability of monazite is mainly limited by allanite growth under greenschist, amphibolite and eclogite facies metamorphic conditions. But in the absence of thermodynamic data for most of the LREE solid phases, the relative stability of monazite and allanite remains difficult to predict in metamorphic series. Monazite is often found to disappear to form allanite at temperatures around 400°C
(Smith & Barreiro, 1990; Wing et al., 2003) and to reappear with increasing metamorphism at amphibolite-facies conditions (e.g. Fitzsimons et al., 2005; Smith & Barreiro, 1990; Wing et al., 2003). However, recent studies point out that the P-T conditions of monazite formation and breakdown could be drastically controlled by whole-rock composition, especially by Ca, Mg and Al contents (Fitzsimons et al., 2005; Wing et al., 2003). Even if monazite growth assisted by a retrograde fluid has already been described (Lanzirotti et al., 1996; Janots et al., 2006), this study is the first to provide clear evidence of textural replacements of allanite by monazite during exhumation. This textural relationship confirms the potential of allanite breakdown to monazite under retrograde conditions, first proposed by Pan (1997). The similarity between the young Miocene Th-Pb age of monazite (sample FW0025) and Ar-Ar age of muscovite in a nearby sample (FW0023) suggests that both minerals crystallised simultaneously thanks to the presence of a retrograde fluid. The monazite-forming reaction remains difficult to establish since the replacement of allanite by monazite is not isochemical and requires an open system (fluids) at the micrometre scale. Considering that chlorite texturally replaced allanite (Fig. 2c), this mineral is assumed to be involved in the allanite breakdown. Monazite also occurs with quartz, chlorite and muscovite assemblages (Fig. 2b, 2c), which appear to replace biotite since this mineral is widespread in the Sirdang window but absent from FW0025. We propose therefore that allanite might react with fluids and biotite to form monazite, chlorite, muscovite and quartz assemblages. This allanite destabilization is the (retrograde) back-reaction of the monazite breakdown into allanite described by Smith & Barreiro (1990) and Wing et al. (2003) with increasing metamorphism at the biotite isograd in metapelites. Inversely to the prograde breakdown of monazite, its retrograde formation associated to chlorite is a hydration reaction. This is why the presence of a fluid phase is required to crystallise monazite during exhumation. It may simply be pore fluid and does not need to be a streaming hydrothermal fluid. In any event, we assume that monazite is chemically stable at temperatures below the biotite isograd, as already proposed by Janots et al. (2006). In the AskotChiplakot window, the reaction of allanite destabilisation may differ because biotite was not observed in any sample. However, monazite formed from allanite (most likely through a fluid-assisted reaction) below the maximum temperatures of 373 ± 50°C reached by sample FW0144. These results demonstrate that monazite has the potential to record late-stage events as well as the commonly assumed prograde or peak metamorphic conditions. Therefore, monazite ages must be interpreted on the basis of monazite textural relationships; they should not be ascribed by principle to the metamorphic peak, especially in areas showing evidence of interactions with late-stage fluids. Geodynamical implications Although this new chronological dataset is not sufficient to constrain a thermo-kinematic solution for far western Nepal, it shows that this region exhibits a relatively similar
Evidence for retrograde monazite in the Lesser Himalaya
thermal history as elsewhere along the front of the High Himalayan range (e.g. Bollinger et al., 2004; Catlos et al., 2001; Catlos et al., 2002a). The Sirdang metasedimentary window shows metamorphic peak temperatures (~540°C) and ages (from 15 to 5 Ma), consistent with the thermochronological record of samples in similar structural positions along the Himalayan range (e.g. Beyssac et al., 2004; Catlos et al., 2001, 2002a, 2004; Copeland et al., 1991; Hubbard & Harrison, 1989). These ages as well as the metamorphic conditions exclude the strong inherited thermal imprint proposed by Sinha & Bist (1986) on the basis of wholerock K/Ar dating. However, although the youngest monazite ages (4.6 ± 1.7 Ma for FW0025) are comparable to the youngest ages previously acquired in the MCT zone (ranging between 3.3 and 5.4 Ma from western Nepal to Gharwal: Catlos et al., 2001, 2002a), the way they control the tectonic evolution model differs: the early Pliocene monazite growth conditions are restricted here to episodes of fluid injection along the retrograde path at low temperatures, whereas monazite growth is considered to mark the end of the prograde path or the metamorphic peak temperature in western Nepal or Gharwal. The latter scenario requires recent, dramatic out-of-sequence thrusting on the Main Central Thrust with velocities as high as >1 cm·y-1 – a velocity that must be compared to the ~2 cm·y-1 present-day and Holocene shortening rates accommodated on the MHT/MFT system, as recorded by GPS (e.g. Jouanne et al., 2004) and fluvial terraces across the Siwaliks Hills (Lav´e & Avouac, 2000). However, the high spatial variations in monazite and muscovite Ar closure ages determined along the western Nepal MCT might rule out localised powerful out-of-sequence thrusting events that must generate thermal imprint over large areas. This high frequency variation in Ar closure age has previously been ascribed to interactions with hot fluids channelled along the MCT at ~5 Ma (Copeland et al., 1991). A similar scenario, involving either fluids in equilibrium or such large fluid circulations, might be proposed for our far western samples FW0025 and FW0144, as textural relationships provide evidence for an interaction between allanite and fluid. It supports the view that late Miocene-Pliocene monazite should have formed under retrograde conditions in the area while the Ramgarh thrust sheet was passively exhumed to the surface. Although poorly documented, the fluid phase might have circulated through the fractured accreted slivers during their exhumation. This mechanism is consistent with fluid-inclusion data available along the Himalayan arc (Craw, 1990; Pˆecher, 1979; Sachan et al., 2001; Sauniac & Touret, 1983) as well as with chlorite geothermometry (Bollinger, 2002), which show a large post-peak PT range of fluid injections from 1 to 5 kbar for 100 to 500°C. These PT conditions as well as the nature of the fluids encompass large, structurally controlled variations, the CO2 contents of the fluid inclusions increasing up to the MCT (Craw, 1990) while their mean homogenization temperature tends to rise from the bottom of the Ramgarh thrust sheet to its top (Sachan et al., 2001). This paleofluid percolation might still be present at depth as documented by the large mid-crustal conductivity anomaly (Lemonnier et al., 1999), given to be generated near the tip of the brittle-ductile transition zone on the Main Himalayan Thrust.
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The interpretation developed here shows that young MioPliocene ages should no longer be systematically considered as characterising a footwall/prograde crystallisation. Out-of-sequence thrusting mechanisms, that should be necessary for wedge equilibration in far western Nepal thinskinned tectonics environment, cannot be ruled out. However, their magnitude might be lower than the late (within 5– 0 Ma) catastrophic MCT slips of up to 2 cm·yr-1, i.e. around the total shortening accommodated through the Himalaya, proposed in western Nepal to explain young monazite ages. Furthermore, this picture of the retrograde fluid interaction and its timing seem here consistent with the continuous underplating and passive roof thrust evolution proposed for the Lesser Himalaya evolution (Bollinger et al., 2004), a process that should have dominated the Himalayan range evolution since 8 Ma, given the large amount of accreted Lesser Himalaya (Robinson et al., 2003). Acknowledgements: We are grateful to E.J. Catlos, T.M. Harrison and M. Grove for help and facilities with ion-microprobe analysis and Ar spectrometry at University of California Los Angeles. This study has also been supported by the SNF N°200020-101826/1. P.J. Chiappero (MNHN, Paris) provided us with the rhabdophane sample. We thank F. Brunet and C. Chopin for corrections and comments on the manuscript, O. Beyssac for his help with the Raman microspectrometer at ENS and for discussion. We finally thank D. Waters and an anonymous referee for helpful comments. Field work was organized with the friendly assistance of our colleagues from Department of Mines and Geology (DMG/ NSC) in Kathmandu.
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