High P-T Polymetamorphism, Dehydration Melting, and Generation of Migmatites and Granites in the Higher Himalayan Crystalline Complex, Sikkim, India

Neogi, Sudipta; Dasgupta, Somnath; Fukuoka, Masato

doi:10.1093/petroj/39.1.61

Cited by 90 publications

(71 citation statements)

References 65 publications

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“…They found that clinopyroxeneplagioclase assemblage developed from garnets with about 38 mol% grossular content. Neogi et al (1998) reports similar observation from granulite facies calc-silicate rocks in the Higher Himalayan Crystalline Complex, Sikkim, India. They observed that garnet grains with high grossular content (51-63 mol%) are surrounded by a symplectite of clinopyroxene and plagioclase.…”

Section: Origin Of High-pressure Xenolithssupporting

confidence: 75%

“…This would mean a reaction of the garnet with external fluid or melt rich in the above elements. However the reaction products suggest the existence of previous grossular-rich garnet similar to those in the Szbk-239 and those described by Bhowmik and Roy (2003) and Neogi et al (1998). Geothermo-barometry and the presence of pressure induced garnet breakdown reactions in both the mafic garnet granulites and the high-pressure clinopyroxeneplagioclase xenoliths shows that they occurred at similar depths in the old thick crust and shared a common uplift history during crustal thinning.…”

Section: Origin Of High-pressure Xenolithssupporting

confidence: 63%

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On the origin and fluid content of some rare crustal xenoliths and their bearing on the structure and evolution of the crust beneath the Bakony–Balaton Highland Volcanic Field (W-Hungary)

Török

2012

Int J Earth Sci (Geol Rundsch)

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Rarely occurring clinopyroxene-plagioclase bearing, felsic granulite and skarn xenoliths were studied from the mantle and crustal xenolith-bearing alkaline basaltic and pyroclastic localities of the Bakony-Balaton Highland Volcanic Field (W-Hungary). Geobarometry and geothermometry of the xenoliths made it possible to categorise them in three groups according to their depth of formation. The first group formed in the lower crust together with mafic and metasedimentary granulites. The second group represents magmatic intrusions of the middle crust, and the third one comprises contact metamorphic rocks of relatively shallow origin. The calculated pressure difference from the core and rim compositions of plagioclase and clinopyroxene as well as garnet breakdown reactions in some xenoliths show evidence for pressure decrease due to crustal thinning both in lower crustal and middle crustal xenoliths during formation of the Pannonian Basin. Fluid inclusion studies reveal the dominance of the CO 2 -rich fluids in the whole crustal section in contrast with fluids found in mafic garnet-bearing xenoliths. Crustal stratigraphy was constructed for the periods prior to the extension and after the extension on the basis of geobarometry and geophysical data. On the basis of mineral stabilities and geothermo-barometry, we estimated that the pre-extensional thickness of the lower and upper crust may have been 27-34 and 26-28 km, respectively. Comparison of pre-extensional and present-day thickness of the lower and upper crust indicate that thinning affected both the lower and the upper portion of the crust but on a different scale. The calculated thinning factors are between 2.25 and 3.4 for the lower crust and 1.3-1.56 for the upper crust.

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Section: Origin Of High-pressure Xenolithssupporting

confidence: 75%

Section: Origin Of High-pressure Xenolithssupporting

confidence: 63%

On the origin and fluid content of some rare crustal xenoliths and their bearing on the structure and evolution of the crust beneath the Bakony–Balaton Highland Volcanic Field (W-Hungary)

Török

2012

Int J Earth Sci (Geol Rundsch)

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“…One of the most well-known discontinuities is the Main Central Thrust (MCT) which separates the Higher Himalayan Sequence in the north from the underlying Lesser Himalayan Sequence in the south. The Higher Himalayan Crystalline Sequence (HHCS) consists of quartzofeldspathic gneisses of both igneous and sedimentary parentage that suffered high-grade amphibolite and granulite facies metamorphism (Neogi et al 1998;Catlos et al 2001;Dasgupta et al 2004). In the Darjeeling-Sikkim area, the base of the HHCS is represented by a strongly deformed granitic body called the Lingtse Gneiss (Acharyya 1978).…”

Section: Geological Backgroundmentioning

confidence: 99%

Integrated very low-frequency EM, electrical resistivity, and geological studies on the Lanta Khola landslide, North Sikkim, India

et al. 2009

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Landslides are very common in high-altitude Himalayan terrains. Major roads in the Himalayas are frequently blocked due to heavy landslides and remain closed for long periods of time. Permanent mitigatory solutions to these landslides are required to keep the highways open. Lanta Khola, located 71.2 km north of Gangtok (capital of the Indian state of Sikkim), is one of the oldest landslides on the North Sikkim Highway and is active since 1975. The rock types on either side of the landslide are different (augen gneiss in the east and metapelitic schist in the west), and it is believed that the Main Central Thrust passes through the slide zone. Since the slide is invariably activated in the aftermath of heavy rainfall, it is important to identify the subsurface structures that channel water below the landslide surface in order to understand the triggers of slide activity. This can only be accomplished by geophysical survey; however, an appropriate geophysical technique that can be applied in such terrains must be identified. Very low-frequency (VLF) electromagnetic survey was performed over the Lanta Khola landside in order to delineate subsurface structures. Although a very limited number of VLF transmitters are available worldwide, it was possible to pick up VLF signals from a number of VLF stations even in this highaltitude mountainous terrain. VLF measurements along five profiles perpendicular to the geological strike were recorded, and a high conducting zone was delineated from the VLF observations. This conducting zone correlates with the low resistive zone identified from gradient resistivity profiling. The anomalies confirm that there is a water-saturated zone (soggy zone) even in the subsurface of the slide parallel to the geological gneiss-schist contact within the Lanta Khola slide. This indicates that the conductive feature correlates with a weak water-saturated debris layer that lies along the slide and is parallel to the geological contact. Resistive structures on either side of the landslide zone can thus be correlated with the stable ground. It is necessary to drain out water from the soggy zone to minimize slide activity since this zone appears to penetrate into the body of the slide.

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“…Vast appearances originating from partial melting are observed in HHC and in granulites in eastern Himalayan syntaxises. Geothermobarometric estimates for peak metamorphism conditions of the rocks indicate that the temperatures reached 800-850℃, and pressures varied from 0.8 to 1.2 GPa [31] , reflecting that the part of HHC rocks to some extent had undergone granulitic-grade metamorphism during which temperature and pressure were in excess of paragneiss solidus. Additionally, the post-collision anatexis and magmatism lasted from 23 to 4 Ma approximately in southern Tibet [32][33][34] .…”

Section: Effects Of Mineral Transitions On Vpmentioning

confidence: 99%

Vp of muscovite-biotite gneiss up to 950°C at 400 MPa: Constraints on the origin of abnormal seismic layers in continental crust

2007

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Here we present experimental results of compressional wave velocity (Vp) of muscovite-biotite gneiss from Higher Himalayan Crystallines (HHC) at the temperature up to 950℃ and the pressure of 0.1-400 MPa. At 400 MPa, when the temperature is lower than 600℃, Vp decreases linearly with increasing temperature at the rate of (∂Vp/∂T) p = −4.43×10 −4 km/s ℃. In the temperature range of 600-800℃, Vp drops significantly and the signal is degraded gradually due to the dehydration of muscovite and α-quartz softening. When the temperature rises from 800℃ to 875℃, Vp increases and the signals become clear again as a result of the temperature going through the β-quartz range. The experiments indicate that the duration has great influence on the experimental results when temperature is above the dehydration point of biotite. During the first 30 h at 950℃, the Vp decreases substantially from 5.9 to 5.4 km/s and the signal amplitude is attenuated by more than 80%. After the 30-h transition, the Vp and the amplitude of ultrasonic wave signals become steady. The decrease of Vp and attenuation of the signals at 950℃ are associated with the breakdown reactions of biotite. The experiments suggest that the breakdown of muscovite and/or quartz softening can contribute to the low seismic wave velocity in thickened quartz-rich felsic-crust such as what is beneath southern Tibet. Additionally, α-β quartz transition generates a measurable high seismic velocity zone, which provides a possibility of precisely constraining the temperature in the upper-middle continental crust. Our study also demonstrates that duration is a key factor to obtain credible experimental results. α-β quartz transition, dehydration of mica, experiments at high temperature and high pressure, intracrustal high or low wave velocity layers, temperature gradient Seismic exploration is one of the most important means to reveal the structures and compositions of deep crust, and to infer the crustal evolution. However, in order to interpret the seismic data, scientists need relations between seismic wave velocities, endogenic properties of rocks (such as mineral compositions, lattice-preferred orientation of minerals, pore and pore fluid), and exogenic environments (such as temperature and pressure) [1][2][3][4][5] . These relations are the foundation of constraining the interpretation [6][7][8][9][10][11] . In tectonically active area with the thickening of crust and rather large geothermal gradient such as Tibetan Plateau and Andean orogenic zone, it is irreversible that some minerals undergo transitions which will consequently influence the physical properties of rocks [12][13][14][15][16] . Substantial decrease

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High P-T Polymetamorphism, Dehydration Melting, and Generation of Migmatites and Granites in the Higher Himalayan Crystalline Complex, Sikkim, India

Cited by 90 publications

References 65 publications

On the origin and fluid content of some rare crustal xenoliths and their bearing on the structure and evolution of the crust beneath the Bakony–Balaton Highland Volcanic Field (W-Hungary)

On the origin and fluid content of some rare crustal xenoliths and their bearing on the structure and evolution of the crust beneath the Bakony–Balaton Highland Volcanic Field (W-Hungary)

Integrated very low-frequency EM, electrical resistivity, and geological studies on the Lanta Khola landslide, North Sikkim, India

Vp of muscovite-biotite gneiss up to 950°C at 400 MPa: Constraints on the origin of abnormal seismic layers in continental crust

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