The emplacement of the Manaslu granite of Central Nepal: field and magnetic susceptibility constraints

Geological Society, London, Special Publications

doi: 10.1144/GSL.SP.1993.074.01.28p413-428.

1993, v.74;Geological Society, London, Special Publications S. Guillot, A. Pêcher, P. Rochette and P. Le Fort Nepal: field and magnetic susceptibility constraintsThe emplacement of the Manaslu granite of Central

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The emplacement of the Manaslu granite of Central Nepal: field and magnetic susceptibility constraints

S. G U I L L O T 1, A. P I~CHER 1, P. R O C H E T T E 2 & P. L E F O R T 1

1 Labora to i re de G O o d y n a m i q u e des Chatnes A lp ines , Inst i tut D o l o m i e u , 38031

Grenoble , France

2 Gdoch imie et MagnOtisme des Roches , Facultd St J & o m e 13397 Marsei l le cedex 13,

France

Abstract: This paper investigates the relationship between the emplacement of the Manaslu leucogranite and the regional deformation. Structural data are derived from field measurements and from the anisotropy of magnetic susceptibility of oriented samples. The geometrical relationship between the internal structure of the granite, north vergent folds affecting the Higher Himalayan Sedimentary Series and the E-W dextral shear-zone, separating the Higher Himalayan Crystalline Series and its sedimentary cover, indicate that in the sedimentary series, the collapse structures and E-W dextral shearing are two distinct structural features. Emplacement of the granite post-dates the collapse structures but is related to the E-W dextral shearing. A model for the Manaslu granite emplacement is proposed which requires development of extensional fracturing at 45 ° to the shear direction, followed by intrusion of granitic magma in successive pulses which results in lateral expansion of the pluton towards the NE.

All along the Himalayan belt, the base of the Higher Himalayan Sedimentary Series (HHSS) is intruded by bodies of leucogranite of the Higher Himalayan leucogranite belt (HHL; Debon et al. 1986). These granites are Miocene in age and located at the interface between the Higher Himalayan Crystalline Series (HHCS) and its sedimentary cover. This interface appears to be a major structural break, the North Himalayan Normal Fault (NHNF) (Burg et al. 1984; Burchfiel & Royden 1985; Herren 1987; Mattauer & Brunel 1989; P6cher & Scaillet 1989), which appears to be of Miocene age (Hodges et al. 1992). The leucogranites could have been generated in response to thickening of the crust (Le Fort 1975; England et al. 1992), after collision between the Indian and Eurasian plates, around 50 Ma. They were produced by melting of the metasedimentary part of the Higher Himalayan Crystalline Series (HHCS) favoured according to Rb/Sr, Sm/Nd, U/Pb and O systematics (Le Fort 1981; Vidal et al. 1982; Deniel et al. 1987; France-Lanord & Le Fort 1988), by the percolation of fluids as demonstrated by oxygen and hydrogen isotopic studies (France-Lanord & Le Fort 1988), and some frictional heating in the Main Central Thrust zone (England et al. 1992).

While petrological models for these granites are now well constrained (review in Le Fort et al. 1987), their mechanism of emplacement is still rather poorly understood, despite current inter-

est in the relationship between tectonics and granite emplacement (Brun & Pons 1981; Bateman 1984; Hutton 1988; Lagarde 1989; Paterson et al. 1989; Bouchez & Diot 1990). Structural data is available for the leucogranite sheet of Nyalam, north of the main HHL belt (Burg et al. 1984; Burchfiel & Royden 1985), the Chhokang arm, an eastward extension of the Manaslu leucogranite, (P~cher & Bouchez 1987), the leucogranitic sills occurring in the Zanskar normal fault shear zone (Herren 1987), and the Garhwal Badrinath-Gangotri granites (Scaillet 1990). These studies have emphasised subsolidus strain. For example, a strong S-C type fabric has been observed in Zanskar and Manaslu areas (Chhokang arm): in Zanskar, it corresponds to normal faulting, whereas in the Chhokang arm it indicates a dominant dextral shearing component (Pacher & Bouchez 1987; P~cher et al. 1991). In Garhwal, earlier syn- magmatic longitudinal stretching has also been described (Scaillet 1990).

Granite emplacement structure is essentially determined by the measurement of the preferred orientation of early magmatic minerals, through field observations, or petrofabric studies in the laboratory. As granites are often poorly orientated, field and thin section observations must be implemented by a more sensitive technique: the anisotropy of magnetic susceptibility (AMS). Its principle is simple: preferred orientation of anisotropic magnetic

From TRELOAR, P.J. & SEARLE, M.P. (eds), Himalayan Tectonics Geological Society Special Publication, No. 74, pp. 413-428.

413

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CONSTRAINTS ON THE MANASLU GRANITE EMPLACEMENT 415

minerals (such as biotite, tourmaline, iron oxides) results in a detectable magnetic anisotropy of the rock (Hrouda 1982; Rochette et al. 1992). AMS appears to be a very rapid and sensitive way to determine the petrofabric once the magnetic minerals responsible for it are identi- fied. For iron oxides, such as magnetite, AMS is due to shape preferred orientations. Previous AMS studies in granites, including syntectonic leucogranites, have shown the importance of this technique for granite emplacement studies (Jover 1986; Bernier et al. 1987; Bouchez et al. 1987; Gleizes 1992; Rochette 1988; Scaillet 1990).

The aim of this paper is to present structural constraints on Manaslu granite emplacement, through field and magnetic anisotropy data.

G e o l o g i c a l s e t t i n g

The Manaslu granite pluton outcrops over an area of 400 km 2. It is 30 km in length, 13 km in width, elongated in a NW-SE direction. Its base is right above the normal fault/dextral shear zone, at the top of the HHCS. Its top intrudes lower Jurassic limestones (Fig. 1).

The grain size is millimetric to centrimetric, and the mineral assemblage is quartz (_+32%), Na-rich plagioclase (_+37%), K-feldspar (+21%), muscovite (_+7%) and biotite (+3%). The granite is usually tourmaline rich (few %), but nearly completely devoid of accessory minerals and enclaves. Only three types have been recognized (Le Fort 1991): scarce xenoliths of country rocks located in the margins of the

pluton, mica-rich enclaves and schlieren, and nodules or 'cocardes' of tourmaline.

Isotopic studies support a Miocene age for the Manaslu granite: Rb/Sr and U/Pb data by Deniel et al. (1987) suggest a crystallization age span- ning 25 to 18 Ma, whereas 4°Ar/39Ar systematics of Copeland et al. (1990) suggest a shorter time of emplacement and cooling, around 20 Ma.

There is no evidence of thermal disequilib- rium between the base of the granite and the surrounding rocks. But at the top of the intrusion there is a contact aureole a few tens of meters thick developed in the sedimentary host rocks. Thermobarometric investigations in the host rock indicate pressures of around 5.5 + 0.5 kbar and temperature greater than 550 ° C at the base of the granite, and around 3 kbar and 550-600 ° C at the upper contact aur6ole. These values suggest the granite was emplaced at a depth of between 20 and 12 km (Guillot et al. 1991a).

Field r e l a t i o n s h i p s

South of the massif, the eastern cliff of Manaslu displays a 4000m high section, in which the lower and the upper contacts of the granite are visible (Fig. 2).

The base of the pluton intrudes Lower Palaeo- zoic limestones of the Annapurna formation, above the orthogneisses (granites around 500 Ma old, Vidal et al. 1984) at the top of th6 HHCS. The contact appears to be as a whole rather fiat and parallel to the main metamorphic

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416 S. G U I L L O T ET AL.

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Fig. 4. Detail of the lateral contact of the granite: cross-cutting of the hinge of the Naike north vergent fold. (NW of Sama, altitude: 4050 m, view towards the south).




cleavage, with a transition zone, a few hundred meters thick, where the gneisses and marbles are cross-cut by a network of sills and dykes.

The top contact, also parallel to the main cleavage, forms a huge domal structure (Fig. 2). Thus the granite forms a large assymetric lens, which disappears southward. Another lens is developed in the Dakura area. The two lenses appear to be connected by a transition zone injected by sills of granite. Such a disposition is similar to the one observed by Scaillet (1990) in the Gangotr i-Badrinath leucogranitic lenses.

North of the Manaslu summit, the salient structural feature is the Naike kilometre-sized fold underlain by Palaeozoic series. This regional northward vergent fold, orientated N100°-l l0° E (Fig. 3), deforms the lithological boundaries SO, and the main metamorphic cleavage Sm. Here, the lateral and upper contact of the granite sharply cut the lower limb of the fold (Figs 2 & 4). Close to the pluton, the fold is reorientated to N70 ° E, 20 ° E (Fig. 3), so that the S0-Sm plane becomes parallel to the granite- host rock contact.

Further north, the upper contact of the granite locally defines a new well marked dome. Here the granite displays, close to the contact, a well marked flattening plane, dipping towards the south in the southern part and dipping towards the north in the northern part of the dome and oblique to the contact (Fig. 2). According to Gapais (1989a), the obliquity between the contact and the magmatic foliation can be used to determine the local shear-sense during granite emplacement. Following these criteria, the observed relationships, seen better in the southern limb of the dome, would indicate a local syn- emplacement expansion or ballooning of the laccolith (Fig. 2).

In the Glacier Blanc region (Fig. 1), the upper contact is tilted in the inverted limb of the north vergent folds. It dips gently at 20 ° towards the ENE, then becomes flatter in the Cheo area. East of the Larkya, where the granite intrudes the Triassic and Lower Jurassic Series, the sediments are strongly flattened close to the granite, dykes of granite display pinch and swell structures (axis of boudins from N20°W to N10 ° E), and are folded in east- to northeast- verging folds (Fig. 5). A new cleavage appears, striking NW-SE and dipping around 40 ° NE and superimposed by a late N-S strain-slip cleavage. The obliquity between the contact and the reorientated schistosity, and the assymetry of the late small folds, suggest relative upward motion of the granite and the sliding of its cover towards the ENE.

In the Larkya and Glacier Blanc area, the

Fig. 5. The Triassic pelites around twenty meters above the upper contact of the granite. The pegmatic dyke, with pinch and swell structures is folded by a northeastern vergent fold. The axial plane cleavage of this fold is parallel to the contact (NE of the Larkya pass, altitude: 5400 m, view towards the NW).

contact metamorphic aureole displays a strong lineation, marked by elongate minerals (biotite or oxides) or by pressure shadows around staurolite or garnet in the pelitic layers and around scapolite and pyroxene in the calc- silicate rocks. Orientation of this lineation is remarkably constant between N70°E and N80 ° E, plunging 10 ° E to 20 ° E (Figs 3 & 6a).

It should be noted that a similar stretching lineation, with the same direction, is also present south of the massif (Naike & Pong-Gien area), even farther from the contact (Fig. 6a), but is only very scarcely observed between the two areas.

Fabrics within the granite

The internal fabric of the granite, better ex- pressed by preferred orientation of ferromagnesian silicates (biotite, muscovite or



418 S. GUILLOT ET AL.

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tourmaline), is difficult to see in the field. Its development depends on the grain size and the mineralogy of the rock. A planar fabric may be distinguishable in the muscovite and biotite-rich facies, whereas the lineation is easier to detect in the tourmaline-rich facies.

When visible, the foliation is homogeneous at the outcrop scale, but may be locally cross-cut by muscovite rich shear bands with S-C ge- ometries.

In the southern part of the massif, near its base, the strike of the foliation is rather constant at Nl10 ° E (_+20°), and its dip is usually greater than 40 ° towards the north. In the core of the massif, the planar foliation has an irregular geometry, with a slight preferred ENE orientation, the dip being usually higher than 50 ° N. In the northeastern part (Larkya), the foliation gradually becomes N-S striking with a dip of between 20 to 50 ° to the East.

Due to the scarcity of tourmaline-rich facies, the mineral lineation is often difficult to observe in the field. When present, it is orientated E - W at the base, NE-SW in the core and N60°E to N80 ° E towards the eastern border (Fig. 6a). Its plunge is around 40 ° N, in the south and in the core of the massif, but only a few degrees to the NE in the Larkya-Glacier Blanc area.

Microstructural data

Country rocks

Microstructures in the contact aureole are one of the tools available to constrain the relationship between granite emplacement and regional deformation (Hutton 1988; Lagarde 1989; Pater- son et al. 1989; Vernon et al. 1989). Here an outline of microstructural data is summarised.

At the basal contact, in the lower and middle Palaeozoic rocks, cleavage and lineation are defined by high temperature minerals (pyroxene, scapolite, K-feldspar, phlogopite). In pelitic rocks, porphyroblasts of garnet are wrapped by a high temperature foliation defined by fibrous sillimanite (fibrolite) and two micas, which is deformed by the late strain-slip cleavage. Close to the roof of the granite, in the Triassic schists, new fabric related to granite emplacement is defined by biotite, muscovite and tourmaline wrapping the garnet and the staurolite. Staurolite grew during and after the new penetrative cleavage. In the Lower Jurassic limestones, the emplacement related cleavage postdates the minerals of the contact metamorphism (pyroxene and scapolite).

These relationships suggest that: (i) towards the base of the massif, temperature

(ii)

was high throughout the deformation related to emplacement of the granite, and the N70-80 ° E lineation is synchronous with the emplacement; towards the roof of the granite, the heat required for the high temperature deformation (more than 550 ° C) was limited to the vicinity of the granite, and decreased strongly in space and time within a few metres of the granite (Guillot et al. 1991 b).

Granitic microstructures: 'magmatic' versus

solid-state deformation

The fabric of a granite can be acquired either at 'magmatic stage', that is at an intermediate crystal-melt state, with less than 40% of the melt (Van der Molen & Paterson 1979; Hibbard 1987) or can be due to high-temperature solid- state deformation (Gapais & Barbarin 1986). Microscopic investigations have been carried out on more than 200 thin-sections to assess the magmatic or solid-state character of the fabric.

Fig. 7. Magmatic foliation defined by K-feldspar, plagioclase and magmatic muscovite alignment. Note how plagioclase twins are parallel to the long dimension of crystals. In the left and upper part of the figure, a submagmatic microfracture transects a plagioclase and is sealed by a large monocrystal of quartz. Broken line in quartz indicate prismatic subboundaries perpendicular to the magmatic lineation. (Sample XG 106, Larkya pass, thin-section cut perpendicular to the magnetic foliation and parallel to the magnetic lineation). Size: × 100.




In the core of the Manaslu granite (east and west of Bimtang), the planar fabric is defined by the preferred orientation of micas, as well as by the orientation of euhedral K-feldspar and plagioclase, without evidence of plastic deformation or recrystallization (Fig. 7). Such a fabric is typical of a predominantly 'magmatic' fabric (Shelley 1985; Hutton 1988; Vernon et al. 1989; Paterson et al. 1989). In places, imbrication of K-feldspar crystals is visible, implying non- coaxial magmatic flow (Blumenfeld 1983; Blumenfeld & Bouchez 1988; Ildefonse 1992). However, the scarcity of this imbrication does not allow a determination of the shear sense. Some K-feldspar crystals contain cracks perpendicular to the lineation, filled with quartz or K-feldspar. According to Bouchez et al. (1992), these indicate brittle deformation at the intermediate-state.

Towards the eastern rim of the granite, a late foliation defined by the growth of muscovite cross-cutting the magmatic foliation, indicates a top towards the east shear sense. Late muscovite is similar to magmatic muscovite in the magmatic foliation. Moreover, since there is no quartz recrystallization in the second foliation it can be inferred that this foliation also developed during deformation of a magma in an intermediate-state rather than at solid-state.

Quartz, which readily deforms plastically, is a sensitive indicator of solid-state flow (Vauchez 1980; Bouchez & P6cher 1981; Marre 1986). In the Manaslu granite, it often occurs as 1-2 cm long crystals with an elongated shape (axial ratio of 1/4 to 1/6 in xz section), but displays only weak plastic deformation structures (large slightly disorientated subgrains, without dynamic recrystallization and growth of new grains). Thus its shape fabric should also have been mainly acquired at magmatic stage.

A high temperature solid-state deformation increases towards the northeastern and southwestern rims of the granite. It is marked by the development of sub-grains and new grains at the rim of the elongated magmatic quartz, defining a solid-state foliation parallel to the magmatic o n e .

Thus deformation mainly occurred at the magmatic stage, although some low-temperature deformation can be observed in the core of the massif. Quartz shows prismatic bands sub- perpendicular to the foliation (a-slip at tempera- tures probably below 500 ° C, Gapais & Barbarin 1986; Gapais 1989b), and dynamic recrystallization with grain size reduction (Fig. 8). In the same samples, micas show undulatory extinc- tions and kink-bands, whereas feldspars are crosscut by millimetre-wide shear-bands con-

Fig. 8. Low-temperature solid-state deformation (probably below 500 ° C) underlined by the stretching of the quartz (in black and white), numerous subgrains in the quartz and kink-bands in the plagioclase. Muscovite recrystallizes into clay minerals at the rim of the K-feldspar (right corner of the figure). (Sample XL25, south-east of Bimtang, thin-section cut perpendicular to the solid-state foliation and parallel to the lineation). Size: X100.

taining quartz, chlorite and clay minerals. Quartz-tourmaline-bearing faults associated with this rather low-temperature deformation in the core of the massif (eastern part of Bimtang) strike at around N150 ° E 70 ° W.

Magnetic measurements

A n i s o t r o p y o f magne t ic susceptibi l i ty data

( A M S )

In the Manaslu granite, as in other leucogranites, it is often difficult to define the actual magmatic structure in the field or in laboratory due to the weak preferred orientation of the minerals and to the lack of strain markers (enclaves, schlieren planes, dykes). Measure- ment of the Anisotropy of Magnetic Suscepti- bility can then be useful in defining structure, even if the magmatic fabric is quite weak (Guillet et al. 1983).

To complement our field data, an AMS study has been carried out on 40 blocks, orientated in the field with a magnetic compass, then drilled in the laboratory. An average of seven specimens per sample was possible (total of 279 speci-




mens), each of them having a diameter of 2.5 cm and a length of 2.2cm. The anisotropy of magnetic susceptibility was measured in the I R I G M (Grenoble) using a Kappabridge KLY-2. The high sensitivity of this instrument (5.10 -8 SI) is usually necessary for weakly magnetic rocks such as leucogranite.

For each, 15 measurements along different axes were made (following the procedure of Jelinek 1980). From these measurements it is possible to obtain an estimate of the susceptibility ellipsoid, in magnitude and in direction. Principal axes are labelled K1 > / ( 2 > / ( 3 , and corrected for the diamagnetic term in the matrix (quartz and feldspar) with a value equal to -14 .10 -6 SI (Bernier et al. 1987; Rochette 1987).

The mean susceptibility Km (before diamagnetic correction) is low (5 to 42.10 -6 SI), but similar to other measurements made from leucogranites (Bernier et al. 1987; Rochette 1988). Actually, it is similar to the ones obtained by Rochette et al. (1992) in the Everest leucogranite, or by Scaillet (1990) in the Garhwal Badrinath-Gangotr i granite which have the same chemical composition as the Manaslu granite. Rochette et al. (1992) and Scaillet (1990) have demonstrated, using high field and low temperature magnetic measurements, as well as correlation between Km and iron amount, that Km of the H H L is essentially due to diamagnetic minerals (quartz and feldspar) and paramagnetic minerals (ferromagnesian silicates), while ferromagnetic contribution (iron oxides or sulphides) is negligible. Among these minerals, only phyllosilicates and tourmaline have a large intrinsic magnetic anisotropy. Therefore, the AMS will reflect the lattice preferred orientation of either tourmaline or biotite (and only rarely muscovite, due to its low iron content).

As seen above, the fabric of these minerals is of magmatic origin. Consequently, the magnetic fabric reflects the magmatic structure. For tourmaline and biotite crystals, the c-axes correspond to minimum susceptibility K3, with a revolution ellipsoid (/(1 = /(2) (Rochette et al. 1992). However, in terms of shape preferred orientation it results in a contrasted AMS.

(i) As the c-axis is the pole of the phyllosili- cate sheet, a biotite- (or muscovite-) bearing sample will yield a K3 axis parallel to the pole of the foliation and a K1 axis parallel to the zone axis of the biotite plane. This zone axis is interpreted as a stretching lineation;

(ii) On the contrary, tourmaline occurs as rods, elongated parallel to the c-axis: therefore in a tourmaline bearing sample, K3 is parallel to

the mineral lineation and K1 to the mineral foliation.

Amongst the collected samples, 31 are biotite dominant, 5 are tourmaline dominant and 4 correspond to samples without clear predominance of one mineral over the other. Reliability of AMS directions at the specimen level depends on L (for K1) and F (for /£3) values. All specimens have values above the detection limit of 1.002 (Table 1). At the site level, mean directions are obtained through the tensorial mean method of Jelinek (1982). On a stereoplot (Fig. 9a), the K1 and/(3 axes of specimens from the same block are usually well grouped, thus giving a small confidence angle on the average direction.

Four sites have been rejected because they show too high an angular dispersion in the various K1 and K3 values, due to difficulties of measurement, heterogeneous distribution of the magnetic grains in the rock or opposition of the contributions made by tourmaline and biotite (Fig. 9b).

For each site, the mean anisotropy parameters (Hrouda 1982; Jover 1986) have been estimated using values corrected for diamagnetic (see Rochette 1987):

the lineation parameter L = KI/K3 the foliation parameter F = K2/K3 the anisotropy degree P = KI/K3 the shape parameter T = 2(k~ - k3)/ (kl - k3) - I with ki = LogKi

Fig. 9. Stereoplot of AMS data in a biotite-rich sample (a). Maximum and minimum susceptibility axes are respectively open squares and circles. The black square and the black circle symbolize respectively the best pole of the maximum and minimum susceptibility. The angular dispersion is weak, then the directions of the lineation and of the foliation are well defined (block KG162, 9 specimens). Stereoplot of AMS data in a mixed sample (h) (tourmaline and biotite rich). The opposition of the contributions of the two minerals leads to very high angular dispersion: the sample is rejected. (block XP233, 7 specimens).



422 S. G U I L L O T ET AL.

T a b l e 1. Magnetic susceptibility data for the 40 sampling stations

Sample CN Min K K1

10-5 SI az.

K3 L F

inc. az. inc,

XG2 5 B 24.1 +2 .2 347 .7+9 .6 5 5 . 2 + 3 . 8 242 .9+6 .9 10+3 .5 1.0308+0.0119 1.0314+0.0049 XG48 6 T + B 18+3 .7 8 .7+32 .7 41 .9+13 .0 210 .3+19 .6 46+9 .1 1.0121+0.0027 1.0219+0.0091 XG52 6 B 21.6 + 0.8 60.1 + 28.4 37.1 + 5.6 301.1 4- 20.7 32.7 + 4.0 1.0397 4- 0.0086 1.0284 4- 0.0053 XG102 9 B 2 6 . 7 + 1 . 7 169.34-6.8 6 9 . 2 + 1 . 3 55 .9+2 .8 8.64-1.6 1.0311+0.0024 1.0552+0.0032 XG104 9 B 8 . 4 + 0 . 6 55.74-8.6 6 . 1 + 3 . 2 168+10.8 7 4 . 3 + 4 . 6 1.0111+0.0009 1.0105+0.0032 XG105 7 B 5.7 + 2.3 211.6 4- 9.7 20.9 4- 3.9 48 4- 5.7 68.2 + 3.9 1.0256 4- 0.0083 1.0278 4- 0.0101 XG106 6 B 12.34-2.2 247 .5+4 .7 2 3 . 5 + 3 . 8 5 .4+ 13.1 47.1 4-3.7 1.03474-0.0033 1.05584-0.0063 XG107 9 B 3 0 . 7 + 2 . 4 237.4_+7.7 2 6 + 1 . 8 24.14-9.1 59.84-1.8 1.0421+0.0013 1.0386+0.0018 XG108 10 B 27.9 _+ 2 43 _+ 2.6 17.1 + 1.3 226.8 + 2.6 72.8 + 1.4 1.0360 + 0.0020 1.0620 _+ 0.0037 X G I 4 7 8 T + B 22.7 + 4.7 172.2 4- 79.2 5.3 4- 20.2 273.3 4- 63.0 64.3 + 18.2 1.0119 + 0.0139 1.0161 + 0.005 XG148 7 T 26.54-1.4 314 .5+7 .4 2.64-5.5 4 5 . 9 + 6 . 9 26.8-+2.4 1.01204-0.0021 1.01494-0.0040 XG159 7 B 34_+3.2 75.74-11.9 2 3 . 1 + 5 . 7 270-+20.5 66 .2+5 .2 1.0320_+0.0025 1.0180_+0.0035 XG162 9 B 27.9 -+ 1.3 58.7 -+ 4.6 12.6 _+ 3.0 255.8 _+ 5.5 76.9 + 2.4 1.0183 -+ 0.0016 1.0749 + 0.0049 XL1 10 B 42,1 175.5 + 23.4 51.2 4- 1.9 303.7 4- 6.9 26.4 _+ 4.4 1.0528 4- 0.0023 1.0162 4- 0.0043 XL5 9 B 3 2 + 1.1 287.9_+7.7 6.4_+2.3 26.1 _+3.4 51.74- 1.3 1.02074-0.0034 1.0629+0.0033 XL8 9 B 36.4 + 0.6 109.5 4- 4.8 28.2 _+ 3.4 292.4_+ 12.9 61.7 4- 3.3 1.0243 _+ 0.0020 1.0346_+ 0.0023 XL9 8 B 34.4 _+ 1.4 87.1 _+ 4.7 8.1 -+ 1.7 248.7 _+ 15.3 81.5 + 2.0 1.0391 _+ 0.0067 1.0313 _+ 0.0027 X L l l 8 B 33.2-+1.8 31 .1+7 .8 6.44-5.6 125.4+21.9 33.94-2.5 1.04284-0.0035 1.03844-0.0066 XLI5 9 B 47 .6+ 1l 345.1 _+ 10.4 13_+6.3 91 _+ 14.5 49.84-5.4 1.03544-0.0088 1.04234-0.0120 XL20 8 B + T 38 .4+3 .7 322.2_+19.1 42.2_+10.7 23 43.24-10.5 1.0327+0.0140 1.07494-0.0106 XL22 6 T 34.54-2 90.64-23.9 3.54-4.6 181.1_+18.8 7.64-13.3 1.02554-0.0043 1.02464-0.0071 XL23 8 B 36.2 4- 2.3 288.3 -+ 134 10.2 _+ 3.4 38.3_+ 8.9 62.3 + 5.5 1.0203_+ 0.0034 1.0384 +_ 0.0045 XL24 7 B 31.4 _+ 0.9 105.3 + 9.4 68.1 _+ 7.8 347.6 _+ 27.1 10.6 -+ 4.2 1.0157 -+ 0.0039 1.0279 -+ 0.0068 XL25 5 B 33.54-1.4 115+27.1 2 7 . 9 + 4 . 8 226 .9+5 .8 35.24-3.3 1.0114_+0.0039 1.08004-0.0096 XL32 5 B 41.64-9.2 48.7_+22.3 7 . 5 + 7 . 6 273.5_+ 11.1 79.5-+5.7 1.03424-0.0088 1.0525-+0.0136 XL37 5 B 2 3 . 9 + 2 . 6 218.1_+15.4 70_+ 1.9 1.8_+6.7 16.4+ 1.3 1.0210-+0.0037 1.1146_+0.0132 XIA5 9 B 26.1 _+5 246.94- 11.6 65.6_+7.6 153_+ 11.8 1.8_+5.2 1.0251 -+0.0037 1.0290_+0.0072 XP96 8 B 26.14-6 56 .2+2 .8 2 0 . 4 + 1 . 8 191.54-1.9 62.44-1.3 1.04564-0.0022 1.11414-0.0109 XP233 7 T 2 1 . 4 + 3 . 7 9 .0_+43 .9 7 .0_+15.1 102.6_+64.9 2 7 . 3 + 3 0 . 8 1 . 0 1 8 7 + 0 . 0 0 3 2 1 .01724-0 .0139 XP265 9 T 26.2 + 1.5 43.4 _+ 6.2 41.6 + 3.1 237.9 + 5.1 47.7 -+ 3.7 1.0293 -+ 0.0060 1.0569 + 0.0103 XP270 9 B 29.9 + 5.7 349.2 + 32.9 32.9 -+ 4.4 234.9 -+ 5.4 32.5 -+ 3.5 1.0125 4- 0 .0046 1.1060 _+ 0.0202 XP271 5 B 21.6 4- 3.5 271.8 4- 18.7 41.6 + 12.0 42.3 4- 27.3 36.2 4- 7.1 1.0166 4- 0.0066 1.0623 4- 0.0187 XP307 8 B 27 .6+3 .2 214.1_+7.1 19.5_+1.7 353.3-+4.7 64.9_+1.7 1.01464-0.0047 1.09864-0.0062 XP310 6 B 4- T 17.2_+ 4.9 79.8_+ 58.1 23.3 4- 13.1 186.5 4- 45.9 33.6 + 13.0 1.0170 + 0.0084 1.0323-+ 0.0160 XP312 7 B 23.4 4- 0.9 243 4- 25.5 46.8 -+ 6.8 2 -+ 8.6 24.4 _+ 3.8 1.0277 4- 0.0068 1.0445 + 0.0117 DK72 4 B 14.8+1.1 106.84-20.7 19.44-4.4 285.94-12.9 70.6_+8.7 1.026_+0.005 1.019_+0.007 DK188 2 B 14.8 4- 0.1 87.7 _+ 29.9 4- 204.6 4- 38.2 -+ 1.027 4- 0.002 1.019 _+ 0.001 DK310 3 B 16.6_+1.7 111.9_+30.6 57 .5+26 .4 269.1-+79.3 30 .4+23 .4 1.016_+0.001 1.014_+0.005 DK289 3 B 15.1_+3.2 185.44-75.2 30.94-27.3 35.74-43.1 55.3_+31.1 1.012_+0.011 1.035_+0.026 DK314 4 T 15 .4+10.3 169.64-79.6 68.54-14.4 674-18.3 4.9-+10.6 1.010+0.008 1.015+0.015

K , m e a n s u s c e p t i b i l i t y ( b e f o r e d i a m a g n e t i c c o r r e c t i o n ) ; C N , c o r e n u m b e r ; M i n , p a r a m a g n e t i c m i n e r a l s (B , b i o t i t e , T , t o u r m a l i n e ) ; K1 , K 3 a r e r e s p e c t i v e l y m a x i m a l a n d m i n i m a l s u s c e p t i b i l i t y wi th K1 = K2. a z . , a z i m u t h in d e g r e e s , i nc . , p l u n g e . R e j e c t e d s a m p l e s a r e in b o l d .

1.12 ] • o blotite-rich o 1.06. "~ • samples

1.08 ~ o 1.04.

F d fieldof., o • F •

o4 t " , ,

1.00 ~ p o , fieldofconstriction | 1.01.

1 .00 1.01 1 .02 1 .03 1 .04 1 .05

t o u r m a l i n e - r i c h s a m p l e s

o

field of • c o n s ; / ~ l i o . / i L-F

o • ~ field of flattening

' ! !

1 .00 1.01 L 1'.02 . 0 3

Fig . 10. S h a p e p a r a m e t e r s L a n d F o f t h e m a g n e t i c e l l i p s o i d . B i o t i t e - a n d t o u r m a l i n e - r i c h s a m p l e s h a v e b e e n d i s t i n g u i s h e d . T h e s q u a r e s r e f e r to t h e s a m p l e s f r o m t h e r i m o f t h e g r a n i t e , t r i a n g l e s f r o m t h e i n t e r m e d i a t e l eve l s a n d c i r c l e s f r o m t h e c o r e o f t h e g r a n i t e .




The type and intensity of magnetic fabric are best demonstrated using an F versus L diagram. These diagrams can be interpreted in terms of petrofabric shape and intensity, taking into account the shape and intensity of the AMS of the magnetic mineral. Therefore, only samples from biotite- or tourmaline-bearing rocks can be compared with each other.

On the Fversus L diagram (Fig. 10), most of the samples from the biotite rich rocks fall in the flattening field (F i> L). However, in the tourmaline rich samples (Fig. 10), due to the inverse magnetic fabric of tourmaline, the oblate magnetic fabric characterizes a linear petrofabric.

Compar i son be tween A M S and f ie ld

measurements

In the Everest and Gangotri HHL, the good agreement between magnetic and petrofabric axes have been demonstrated by comparison of AMS and thin-section image analysis data (Ro- chette et al. 1992; Scaillet 1990). In the Manaslu granite, we observe a reasonable agreement between the field fabric and the AMS data (compare Fig. 6a and Fig. 6b and Fig. 11), which emphasises the ability of AMS to depict structures in only slightly deformed plutonic rocks, even if one must be cautious when the susceptibility is carried by two minerals with opposed fabrics (e.g. tourmaline and biotite).

The trends observed in the field are: a foliation plane striking WNW-ESE at the base and in the core of the pluton which trends NW-SE

towards the east; a lineation (Figs 6b & 11) constant at around Nl15 ° E plunging 10 to 70 ° E at the south of the massif (i.e. at its base), whereas at the northeast (i.e. at its roof) it is around N50-60 ° E, plunging 0-30 ° E. In the core the linear fabric is weak. The lineation seems to be quite dispersed, despite a rough N-S trend.

Discussion

The shape of a granitic body depends mainly on its tectonic environment (Pitcher 1979). Post- tectonic plutons tend to have subcircular con- tours with concentric internal structures (Pons 1983; Paterson & Tobish 1988). In contrast, syntectonic plutons record in their shape, internal structures and metamorphic aureole, the geometry and the kinetics of the regional deformation (Brun & Pons 1981; Hutton 1988; Lagarde 1989; Lagarde et al. 1990; Guineberteau et al. 1987; Vernon et al. 1989; Castro 1986). The Manaslu granite displays numerous features consistent with emplacement during regional deformation, but the tectonic control of its emplacement is poorly defined, particularly with regard to the North Himalayan normal fault.

Collapse structure versus dextral shearing

The Miocene deformation at the top of the HHCS and within the HHSS is characterized by normal faulting and northward gravity sliding of the HHSS and E-W dextral wrench shear (Caby et al. 1983; Burg et al. 1984; Brun et al. 1985;

L ASM Manaslu (A) N 36 mes I max =8

'i

N L mineral Manaslu (B)

I 28 mes m a x - 8

I

Fig. 11. Rose diagram of the orientation of the magnetic (A) and magmatic (B) lineations.



424 S. GUILLOT E T AL .

Burchfiel & Royden 1985; Herren 1987; P6cher & Scaillet 1989; P&her et al. 1991; Hodges et al. 1992).

In the upper part of the HHCS, where this deformation is best shown by sillimanite bearing S-C planes, the normal faulting and the dextral shearing appear to be coeval. The transport direction is defined by the same stretching/ sliding lineation, but its pitch varies from locality

to locality, with predominance either of dip slip or strike slip motion.

Higher in the tectono-metamorphic pile, in the sedimentary series, structures associated with the dextral shearing and the northward collapse are distinct, and the relative timing of the different stages of the deformation is clearer. Dextral shearing is visible in the lower part of the cover, marked by a longitudinal stretching line-

m m ? m m m

28°30

ooo o . . . . . . . . .

! i i ! i ! i ! ! : : i i i ! i i i i i i i : i : i ! i i l

::i ii::i::i: iiii:: i:i ?i:iii iii

D Sama

' ~ i i ~11 . 19 , , . iwm

85 °

I !!! " O i : : o : / : o :::::::::::::::::::::: : . . . . . . , .

28°30

G A N E S H

8 0 - -

N t j -~ - -5---------------- -_ _-

2 84!30 3 o

4 ~ 8 10 . . . . . . . . . . . 5 ~ 6 0007 "7/ 9 ~

28 °

Fig. 12. Stretching lineation patterns in central Nepal. (modified after P6cher 1991). 1, Higher Himalayan Sedimentary Series; 2, Higher Himalayan Crystalline Series; 3, Main Central Thrust (MCT); 4, Lesser Himalayan nappes; 5, Southern Himalayan sedimentary belt; 6, Manaslu leucogranite and its eastward extension (Chhokkang arm); 7, axial trace of north-vergent Annapurna-Naike fold; 8, late sillimanite lineation and stretching lineation in the sedimentary cover associated to the E-W dextral shears; 9, earlier lineation related to southward MCT thrusting; 10, the two shear-zones boardering the Manaslu granite.




ation, striking around N75°E (Fig. 3). A similar lineation pattern is also observed further to the north, in the Larkya-Gyala area. This suggests the existence of another E -W dextral shear-zone north of the granite (Fig. 12). The normal fault movement is marked by northward facing folds, visible in the whole sedimentary pile, and striking regularly N100 ° E, 20 ° E (Fig. 3). It appears that the stretching lineation has a remarkably constant direction around N75 ° E (Fig. 3), what- ever the position in the northward vergent folds, and is not folded about the hinges of the folds. Thus it can be inferred that the longitudinal shear must postdate the folds.

Geometric relationships between the

Manaslu granite and the north-vergent folds

As noted above, the north-vergent folds are cut by the granite. The fold axes are reoriented close to the granite contact and the metamorphic contact aureole is characterised by a new flattening plane, parallel to the granite contact and overprinting all the existing structures. This indicates that deformation related to emplacement of the granite postdates the collapse structures. This sequence is similar to that described by Hodges et al. (1992) north of Everest, where a HHL clearly crosscuts mylo- nites developed during normal faulting.

Several authors have suggested that the High Himalayan leucogranites predate the normal faulting (Burg et al. 1984; Burchfiel & Royden 1985; Herren 1987; Mattauer & Brune11989), or are coeval with the northwards collapse structures (Scaillet 1990). In the case of the Manaslu granite, there is no clear evidence that normal faulting and collapse structures contribute to emplacement of the granite. South of the pluton, in the Dakura and the Manaslu areas, the granite forms kilometric scale lenses similar to the crustal scale 'chocolate tablet' structure observed in Garhwal by Scaillet (1990). Such a structure could reflect the boudinage of the granitic melt during the northward sliding of the sedimentary cover. However, in the Manaslu granite, boudinage is only weakly developed, and is not consistent with the regularity of the roof contact to the north.

Emplacement o f the Manaslu granite in a crustal tension gash

The main features defining the structure of the Manaslu granite are its laccolithic shape with an axial ratio (length/width) of about 2.3, and the S shape of the magmatic lineation from the core to

the eastern rim of the pluton. Considering that the Manaslu granite is bounded to the south by a large E -W shear zone (NHSZ), and that another similar shear-zone may exist to the north, two models of emplacement can be proposed, both related to opening between two shear-zones at the ductile-brittle transition level: emplacement in a pull-apart system, parallel to the shear-zone, or emplacement in extensional fractures oblique to the shear direction.

The second model proposed by Castro (1986) for the Extremadura batholith in Spain is favoured, as it is more consistent with the obliquity (around 40 ° ) between the bulk elon- gation of the granite pluton and the shear-zones (Fig. 12), and with the internal magmatic lineation being orientated around N60-80 ° E in the northeastern part of the granite. Had the granite been emplaced in a pull-apart, the lineation should be broadly parallel to the shear plane (Guineberteau et al. 1987).

Conclusion

This study suggests that the Manaslu granite was emplaced in the following manner: initial opening (or rather creation of virtual opening fractures), which could be compared to a tension gash opening, directly related to the E -W dextral shear system at the top of the HHCS. The opening focuses the rise of granitic magma and determines the initial shape of the pluton. A feeder zone may correspond to the central and southwestern part of the massif, where the magmatic lineations and foliations are poorly orientated, and often steeply dipping. For- mation of the granite by successive pulses is strongly suggested by the geochemistry (Le Fort et al. 1987); Such pulses collect in the centre of the intrusion, which then grows by lateral expansion (a general mechanism proposed by Lagarde et al. 1990b) towards the NE and is accompanied by local ballooning and sliding of the cover. The base of the pluton (SW of the massif), is still sheared at high temperature during and after emplacement. Finally, N150 ° E dipping 70-80°W faults crosscut the granite, with an overprinting of the magmatic foliation by low- temperature shear bands.

This study emphasises once more the importance of the regional tectonics and particularly that of strike-slip systems in the control of the emplacement of granitic plutons, and introduces some additional constraints on the relationship between normal faulting, lateral shearing and granite emplacement in the High Himalaya granitic belt.




This paper was presented at the 7th Himalayan- Karakorum-Tibet workshop in Oxford, 1992. The authors wish to thank R.W. England, M.P. Searle, P.J. Treloar and an anonymous reviewer for their constructive comments. The magnetic data are available and can be obtained by writing to S. Guillot or P. Rochette. The financial assistance of CNRS (URA 69) and Commissariat h l'Energie Atomique are gratefully acknowledged.

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