Patrick Le Fort

Isotopic study of the Manaslu granite (Himalaya, Nepal): inferences on the age and source of Himalayan leucogranites

A detailed isotopic study of the Manaslu leucogranite was carried out. A U-Pb age of 25 Ma and a whole rock Rb-Sr age isochron of 18 Ma were obtained, suggesting that the magmatic activity lasted at least 7 Ma. Initial Sr isotopic ratios are very high (0.740 to 0.760) and initial Nd isotopic ratios are low (ɛNdin: −13 to −16), and they show the existence of large isotopic variations even at the metre scale. These are not the result of perturbations by fluids but rather they reflect the initial isotopic heterogeneity of the source material which has not been obliterated by magmatic processes (e.g. fusion, mixing by convection). These results also support the crustal origin of this leucogranite. The Tibetan slab paragneisses, whose Sr and Nd isotopic ratios are very similar to those of the granite at an age of 20 Ma, are the most probable parental material. Nd model ages for both the leucogranite and the gneisses are in the range 1.5–2 Ga. A model of formation of the Manaslu granite by coalescence of different batches of magma is in agreement with the present data.

Exhumation, crustal deformation, and thermal structure of the Nepal Himalaya derived from the inversion of thermochronological and thermobarometric data and modeling of the topography

duplex initiated at 9.8 ± 1.7 Ma, leading to an increase of uplift rate at front of the High Himalaya from 0.9 ± 0.31 to 3.05 ± 0.9 mm yr −1 . We also run 3‐D models by coupling PECUBE with a landscape evolution model (CASCADE). This modeling shows that the effectoftheevolvingtopographycanexplainafractionofthescatterobservedinthedatabut not all of it, suggesting that lateral variations of the kinematics of crustal deformation and exhumationarelikely.Ithasbeenarguedthatthesteepphysiographictransitionatthefootof the Greater Himalayan Sequence indicates OOS thrusting, but our results demonstrate that the best fit duplex model derived from the thermochronological and thermobarometric data reproduces the present morphology of the Nepal Himalaya equally well.

Heat sources for Tertiary metamorphism and anatexis in the Annapurna-Manaslu Region central Nepal

Following the collision of India with southern Tibet, crustal rocks of the leading edge of India (1) underwent regional metamorphism to upper amphibolite grade, (2) melted locally to produce anatectic granitoids, and (3) were sheared and thrust onto lower grade rock along the Main Central Thrust, yielding an inverted metamorphic sequence. This sequence is exemplified in the Annapurna-Manaslu region. We use simple physical calculations to examine the heat sources involved in the different phases of metamorphism. The regional metamorphism apparently is due to the burial of the northern edge of India beneath the accretionary prism along the southern edge of Tibet. The observed temperatures and pressures of this first phase of metamorphism are consistent with the thermal relaxation, during the 10–30 m.y. before slip on the Main Central Thrust began, of thickened continental lithosphere whose original surface heat flux was between 50 and 70 mW m−2. The release of water from the footwall of the Main Central Thrust apparently facilitated melting of the overlying crust in the second phase. Such melting could have occurred in the first million years or so of thrusting, if warm (550–650°C) crust in the footwall contained the necessary water. If melting did not occur in the earliest stages of slip on the Main Central Thrust, dissipative heating, with shear stresses of 10 to 100 MPa, is required for temperatures near the Main Central Thrust to have remained high enough to generate melting above the fault during the underthrusting of cold material. The thickness (6–8 km) of the zone of inverted isograds associated with the fault, if undisrupted and due solely to thermal diffusion, implies that the time required to carry the rocks preserving the inverted metamorphism from the surface to depths of 30–40 km was 4–8 m.y. The apparent inverted temperature gradients (about 10–25°C/km) in this zone can be understood as the combined result of two processes. Diffusion of heat from hot rock thrust over cold rock expunged the original temperature gradient near the fault and could have created an inverse gradient of 10–20°C/km. The peak temperatures in such a zone, however, would not have exceeded about 350°C without an additional source of heat. Dissipative heating at shear stresses of about 100 MPa can account for peak temperatures in excess of 600°C during slip on the fault and would have contributed as much as 13°C/km to the inverse gradient. Although inversion of metamorphic isograds could have occurred as a result of deformation within the Main Central Thrust zone, the high temperatures during slip on this zone still require dissipative heating, unless the duration of slip exceeded 25 m.y.

Badrinath-Gangotri plutons (Garhwal, India):petrological and geochemical evidence for fractionation processes in a high Himalayan leucogranite.

The Gangotri leucogranite is the western end of the Badrinath granite, one of the largest bodies of the High Himalayan Leucogranite belt (HHL). It is a typical fine grained tourmaline + muscovite ± biotite leucogranite. The petrography shows a lack of restitic phases. The inferred crystallization sequence is characterized by the early appearance of plagioclase, quartz and biotite and by the late crystallization of the K-feldspar. This suggests that, in spite of being of near minimum melt composition, the granite probably had long crystallization or melting interval, in agreement with previous experimental studies. Tourmaline and muscovite have a mainly magmatic origin. Even though the major element composition is homogeneous, there are several geochemical trends (when CaO decreases there is an increase in Na2O, Rb, Sn, U, B, F and a decrease in K2O, Fe2O3, TiO2, Sr, Ba, Zr, REE, Th) which are best explained by a fractionation process with early crystallizing phases. Experimental solubility models for zircon and monazite in felsic melt support a magmatic origin for these two accessory phases as well. Rb/Sr isotope data show this granite to have, like other HHL, heterogeneous isotopic values for Sr (initial 87Sr/86Sr ratios, calculated at 20 Ma, range between 0.765 and 0.785). Therefore no mixing (i.e. no convection) occurred between the different batches of magma. In contrast 18O data show little variation (13.04% ± 0.25), implying a source with homogeneous 18O values. Differences in timing between fluid infiltration and the onset of melting, related to differences in temperature of the source, could explain why source homogenization occurred for the Gangotri and not for the Manaslu granite. The use of experimental results for solubility and the position of the accessory minerals during melting, predict a low viscosity for the melt during its extraction. This in turn explains the lack of restitic phases (major and accessory) in the granite as well as some field features (lensoid shape, pronounced magmatic layering). Based on the petrographic and isotopic studies, it is suggested that the mechanism of ascent was not diapiric but rather that the melt ascended along several fractures and the level of emplacement was partialy controlled by the density contrast between the melt and host rocks.

An Early Pliocene thermal disturbance of the main central thrust, central Nepal: Implications for Himalayan tectonics

Since the beginning of the collision between India and Asia at about 50 Ma, the convergence in the Himalaya has largely been taken up along major thrust zones. In this study, samples of the Lesser Himalaya Formations, up to 10 km below the Main Central Thrust (MCT), and the Greater Himalaya Sequence, up to 12 km above the MCT, have been analyzed by the 40Ar/39Ar and U-Pb methods to investigate the thermal history of the MCT. The ages can be summarized as follows: (1) The youngest ages from muscovites (3.1 Ma), biotites (3.4 Ma), and hornblendes (4.1 Ma) all come from within 1 km of the MCT, (2) there is a marked asymmetry of ages between the footwall and the hanging wall of the MCT; the maximum mica age in the hanging wall (Greater Himalaya Sequence) is 13 Ma, whereas a muscovite 5 km below the MCT, in the footwall, has an age spectrum with a gradient from 400 to 1400 Ma, (3) five K-feldspars from −6.2 to 11.9 km above the MCT all give minimum ages in the range 3.0–6.4 Ma, and (4) a 206Pb/238U age on a brannerite from the Greater Himalayan Sequence is 4.8 Ma. Structural and petrologic observations preclude the possibility that this age-distance distribution reflects faulting within the Greater Himalaya Sequence and production of the necessary thermal energy by shear heating requires unrealistically high shear stresses. Infiltration of hot fluids through the MCT zone appears to be the best hypothesis to explain these data. Simple numerical simulations, which account for heat transfer by advection within the fluid flow zone and by conduction outside it, indicate that the observed age distribution could have been produced by infiltration of hot fluids through the MCT zone at circa 5 to 4 Ma within the following range of conditions: the fluids heated rocks to temperatures in the range 470 to 510°C for less than 1 million years in a region narrower than the entire MCT zone. The temperature of the thermal disturbance inferred from the 40Ar/39Ar data is consistent with petrologic data from albitic alteration assemblages in the Greater Himalaya Sequence and the U-Pb age of the brannerite is similar to the youngest 40Ar/39Ar hornblende age. This scenario, hydrothermal heating culminating at about 4 Ma, is similar to the model of Le Fort (1981) for the generation of the High Himalayan leucogranites in the Late Oligocene - Early Miocene by dehydration of the footwall rocks of an active thrust. In the younger instance, we interpret that fluids derived from the footwall rocks of the Main Boundary Thrust migrated upward through the Lesser Himalaya Formations and were subsequently channeled along the MCT, producing the thermal disturbance.

New constraints on the age of the Manaslu leucogranite: Evidence for episodic tectonic denudation in the central Himalayas

The Manaslu leucogranite of central Nepal transacts one segment of the South Tibetan detachment system, a major extensional feature that helped to moderate large topographic gradients in the Himalayan orogen in middle Tertiary time. 40 Ar/ 39 Ar ages for hornblendes from the northeastern contact aureole of the pluton indicate that intrusion occurred prior to 22-23 Ma, providing a minimum age for formation of the detachment system in the Manaslu region that is at least 2 m.y. older than previous estimates from other parts of the orogen. 40 Ar/ 39 Ar mica ages from the aureole, similar to previously published mica ages from the upper part of the leucogranite, indicate an episode of rapid cooling at ∼19-16 Ma. We attribute this cooling event to tectonic denudation of the pluton by movement on structurally higher exten-sional faults, and we suggest that gravitational collapse of the orogenic front occurred episodically over the late Oligocene-early Miocene interval in the Manaslu area.

Hydrogen and oxygen isotope variations in the high himalaya peraluminous Manaslu leucogranite: Evidence for heterogeneous sedimentary source

Abstract The Manaslu granite belongs to the High Himalaya leucogranitic belt which was produced by melting of the crust during postcollisional thrusting. δD and δ18O values have been determined for whole rock and coexisting minerals from the ~8 km thick Manaslu massif and its 50 km long dyke sheet, its country rocks and the Formation 1 (F1) paragneisses which are the source of the granite. For the granite, δDmusc range from −70 to −85%. and δ18O W.R. from 10.9 to 12.8%. H and O-isotope fractionations among minerals are consistent with high temperature equilibrium and, for oxygen, closed system evolution. A few samples, coming mainly from the dyke swarm, have very low δD values (down to −188%.) and biotite-muscovite H-isotope fractionations indicative of disequilibrium; the D H ratios of associated magmatic tourmaline are essentially unmodified. From the distribution of δD values in the granite and its country rocks, circulation of very low deuterium meteoric hydrothermal waters was extremely localized. Because these waters are depleted in deuterium by up to 50%. relative to modern meteoric waters, the Manaslu area was either at an altitude substantially higher than that of today (2500–6000 m for the analyzed samples) or a mountain chain once existed to the south. The F1 gneisses have δ18Oquartz between 12 and 14.3%. which confirms that the granite was generated from F1, but δD values are ≈20%. higher than in the granite. Such a difference can be a result of degassing of the magma and/or introduction of fluid in the melting zone. Infiltration of low δD fluid (≈−90%.) into the hot but dry F1 probably triggered partial melting; these fluids could have come from the dehydration of the Midlands sediments which are separated from the overlying F1 by the Main Central Thrust. The correlations among δ18O, ( 87 Sr 86 Sr) 20 Ma and ϵNd values in both F1 and the granite indicate that the variations of these isotopic ratios in the Manaslu are inherited from those in F1 at the time of melting. In turn, these ratios in F1 are related to the proportion of quartz and phyllosilicates for the isotopic ratios of Nd and O, and to the quantity of radiogenic Sr generated within the sediment, which is a function of age and Rb content (amount of phyllosilicate). Some other Himalayan leucogranites require either other crustal source rocks or the δ18O and 87 Sr 86 Sr ratios of F1 vary along the Himalaya.

Age and cooling history of the Manaslu granite: implications for Himalayan tectonics

Abstract The Manaslu granite is one of the High Himalayan leucogranites which are believed to be derived by partial melting of quartzofeldspathic gneissess of the High Himalayan Crystallines (HHC) and emplaced at the contact between the HHC and the Tethyan Sedimentary Series. We have analyzed a suite of muscovite (15), biotite (3), and alkali feldspar (10) by the 40 Ar/ 39 Ar method to help constrain the age and cooling history of the granite. 40 Ar/ 39 Ar ages ranges are 18.4 to 13.3 Ma, 17.0 to 14.7, and 16.4 to 3.4 for muscovite, biotite, and K-feldspar, respectively. Based on the muscovite analyses and a re-interpretation of a previously published U-Pb monazite age we conclude that the crystallization age of the Manaslu granite is everywhere — 20 Ma. The postcrystallization cooling history given by the micas and the feldspars suggests the presently exposed parts of the pluton were emplaced at depths of 8 to 15 km. These data place a minimum age of movement on the MCT in this area of 20 Ma. We estimate the time of magma segregation and transport to be no more than - 5 Ma.

Contact metamorphism and depth of emplacement of the Manaslu granite (central Nepal). Implications for Himalayan orogenesis

Abstract The Manaslu massif (central Nepal) provides a well-exposed example of a deeply eroded pluton: its contact aureole can be followed from the base to the top along medium- to low-grade (mesozonal to anchizonal) Tethyan metasediments. Contact metamorphic mineral assemblages and thermobarometric estimations suggest that the granite was emplaced at 18–21 km for the base and 9–13 km for the roof. Calculated temperatures within the aureole, at 550 ± 40°C, are compatible with the intrusion temperature of a leucogranitic magma. Microstructural evidence shows that the temperature remained high (> 500°C) at the base of the massif during and after granite emplacement, whereas towards the top of the granite deformation proceeded rapidly at lower temperature. Heating of the abundant calcareous rocks in the contact aureole released a local CO2-rich fluid, whereas a H2O- and boron-rich fluid seems to have pervaded the whole zone; this fluid is probably exsolved from the granitic melt during its crystallization. The depth of emplacement of the massif has an important implication for the reconstruction of the Himalayan geodynamic evolution, implying a burial of the Tethyan metasediments by a major refolding of the sedimentary cover or, more probably, by extensive development of the North Himalayan nappes towards the south for more than 70 km, before granite emplacement, i.e., before the Miocene. The young age of the granite and its depth of emplacement suggest a rapid tectonic denudation, in the order of 1 mm a−1, probably by normal faulting north of the massif during its cooling.

Magnetic properties of the High Himalayan leucogranites: Structural implications

The magnetic properties of the High Himalayan leucogranites have been investigated on 527 specimens in three plutons, Everest-Makalu (6 sites) and Manaslu (40 sites) in Nepal, and Gangotri (43 sites) in India. Susceptibility varies between 2 and 100 × 10 -6 SI, with an anisotropy ratio up to 1.16. High field and low-temperature magnetic measurements together with comparison with weight percent iron demonstrate that anisotropy of magnetic susceptibility is carried by paramagnetic biotite and tourmaline. The latter produces an inverse fabric, i.e. with the minimum axis parallel to mineral lineation. The magnetic fabric demonstrates complex patterns of stretching lineations during magmatic emplacement, and its usefulness in semi-quantitatively estimating petrofabric intensity is demonstrated for the biotite-bearing facies. Natural remanent magnetization was measurable at only two sites in Everest-Makalu, where there are well-defined reverse directions carried by titanomagnetite and pyrrhotite. Comparison of these preliminary results with predicted directions for stable India suggests northward tilting of about 10 ° and a small clockwise rotation of this massif.