Red and violet gahnite (spinel) occurrences in Ganesh Himal (central Nepal): their chemistry, inclusions and microthermometry

The red and violet gahnites occur within dolomite marbles of medium grade amphibolite facies in the Ganesh Himal area, central Nepal. The dolomites are part of the Higher Midland Formation and lie at the northern limb's eastern end of the Kunchha-Gorkha anticline within the MCT zone of Le Fort (1975). Both gahnites (ZnAl2O4) show a chemical zonation with various contents of Cr, Mg and Fe. The red colour is most probably caused by chromium. The inclusions containing fluids have ideal negative crystal shapes or are irregular. Beside them, various solid inclusions are also found. The fluids of the two- or three-phase inclusions contain H2O and CO2 Two generations of inclusions varying in the content of CO2 indicate a trapping at different PT condition or a water loss of the high CO2 inclusions during some tectonic event.

Download Full-text

Proterozoic geological evolution of the northern Vestfold Hills, Antarctica

Geological Magazine ◽

10.1017/s0016756800017581 ◽

1991 ◽

Vol 128 (4) ◽

pp. 307-318 ◽

Cited By ~ 20

Author(s):

C. W. Passchier ◽

R. F. Bekendam ◽

J. D. Hoek ◽

P. G. H. M. Dirks ◽

H. de Boorder

Keyword(s):

Large Scale ◽

Structural Evolution ◽

Shear Zones ◽

Granulite Facies ◽

Strike Slip ◽

Amphibolite Facies ◽

Tectonic Event ◽

Vestfold Hills ◽

Two Phases ◽

Brittle Faulting

AbstractThe presence of polyphase shear zones transected by several suites of dolerite dykes in Archaean basement of the Vestfold Hills, East Antarctica, allows a detailed reconstruction of the local structural evolution. Archaean and early Proterozoic deformation at granulite facies conditions was followed by two phases of dolerite intrusion and mylonite generation in strike-slip zones at amphibolite facies conditions. A subsequent middle Proterozoic phase of brittle normal faulting led to the development of pseudotachylite, predating intrusion of the major swarm of dolerite dykes around 1250 Ma. During the later stages and following this event, pseudotachylite veins were reactivated as ductile, mylonitic thrusts under prograde conditions, culminating in amphibolite facies metamorphism around 1000–1100 Ma. This is possibly part of a large-scale tectonic event during which the Vestfold block was overthrust from the south. In a final phase of strike-slip deformation, several pulses of pseudotachylite-generating brittle faulting alternated with ductile reactivation of pseudotachylite.

Download Full-text

Geology along the Arniko Highway between Barabise and Kodari (China-Nepal Boarder) area, central Himalaya

Journal of Nepal Geological Society ◽

10.3126/jngs.v42i0.31447 ◽

2011 ◽

Vol 42 ◽

pp. 41-50

Author(s):

Santa Man Rai

Keyword(s):

Amphibolite Facies ◽

Retrograde Metamorphism ◽

Mica Schist ◽

Lower Section ◽

Main Central Thrust ◽

Central Nepal ◽

Pelitic Schist ◽

Deformation Event ◽

Augen Gneiss ◽

Black Schist

The area between Barabise and Kodari in central Nepal along the Arniko Highway is geologically located into Higher Himalayan Crystallines (HHC) and Lesser Himalayan Sequence (LHS) that is separated by the Main Central Thrust (MCT). The HHC consists of amphibolite facies rocks (pelitic schist, psamitic schist, pelitic gneiss and quartzite), while LHS is comprised by green schist to amphibolite facies rocks (phyllite, calcareous phyllite, garnet-mica schist, black schist, quartzite and augen gneiss) in uppermost section and carbonate (dolomite and limestone) with phyllite, and metasandstone in lower section. The MCT in the area is oriented in E-W direction with about 30° dip due north and S-C structure preserved in augen gneiss of LHS characterizes the top-to-south sense of shearing, which could be related to the movement along the MCT. Mineral lineation marked by stretched mica indicates N to NNE direction in both HHC and LHS. Metamorphism of inverse grade from biotite at stratigraphically lower most section of Kuncha Formation to garnet at the uppermost section having schist and augen gneiss is obvious close to the MCT in the section. However, the Kuncha Formation contains tiny crystals of garnet in the rocks of greenschist facies. Kyanite and sillimanite isograds are developed in pelitic and psamitic schists, and pelitic schists appeared at the basal part of HHC above the MCT. The transformation of garnet to chlorite at the margin and fractures and formation of chlorite within bulk rocks of the MCT zone and HHC are the indicators of traces of retrograde metamorphism because of dropping in pressure-temperature probably related to post deformation event.

Download Full-text

Lithostratigraphy of the Nawakot Complex (Lesser Himalayan Sequence) from Malekhu area (south-west) to Syabrubensi area (north-east) along the Trishuli River, central Nepal Himalaya

Journal of Nepal Geological Society ◽

10.3126/jngs.v42i0.31451 ◽

2011 ◽

Vol 42 ◽

pp. 65-74

Author(s):

Santa Man Rai

Keyword(s):

Amphibolite Facies ◽

Kathmandu Valley ◽

Main Central Thrust ◽

Nepal Himalaya ◽

North East ◽

Central Nepal ◽

South West ◽

Facies Metamorphism ◽

Area North ◽

Stratigraphic Sequences

All formations (Kuncha Formation, Fagfog Quartzite, Dandagoan Phyllite, Nourpul Formation, Dhading Dolomite of the Lower Nawakot Group and Benighat Slate, Malekhu Limestone and Robang Formation of the Upper Nawakot Group) of the Nawakot Complex are well exposed along the Malekhu-Thopal Khola and Galchi - Kaljeri section s, west of the Kathmandu Valley. The Phalangu Khola and Syabrubensi sections, north of the Kathmandu Valley do not show the exposures of all formations (exposures of the Kuncha Formation, Fagfog Quartzite, and Benighat Slate only). The stratigraphic sequences from the Kuncha Formation to Dhading Dolomite (Lower Nawakot Group) along the Malekhu-Thopal Khola section are overturned, showing the younging strata to the deeper sections shown by the sedimentary structures (ripple marks, mudcracks, etc.) and stromatolites. The se overturned sequences could be related to the development of the post Main Central Thrust (MCT)/Mahabharat Thrust (MT) deformational structure (Gorkha - Pokhara anticlinorium, Likhu Khola anticline). The stratigraphic sequences are normal in the formations of the Upper Nawakot Group along the Malekhu-Thopal Khola section. This normal sequence can be also observed along the Galchi - Kaljeri, Phalangu Khola and Syabrubensi sections. In the northern part of the study area along the Likhu Khola, Tadi Khola, Phalangu Khola and Syabrubensi sections, the rocks of the Benighat Slate just below the MCT are highly sheared, deformed, metamorphosed with the development of the garnet and staurolite (medium amphibolite facies metamorphism) due to the movement of the MCT. However, these minerals are not developed in the Robang Formation (immediate below the MT) along the Malekhu-Thopal Khola and Galchi - Kaljeri sections. This formation could be caused by shallow depth metamorphism (green-schist to lower amphibolite facies) along the movement of the MT.

Download Full-text

Inclusions of Amorphous and Crystalline SiO2 in Minerals from Itrongay (Madagascar) and Other Evidence for the Natural Occurrence of Hydrosilicate Fluids

Geosciences ◽

10.3390/geosciences12010028 ◽

2022 ◽

Vol 12 (1) ◽

pp. 28

Author(s):

Daniil V. Popov ◽

Richard A. Spikings ◽

Théodore Razakamanana

Keyword(s):

Low Temperature ◽

Direct Evidence ◽

Experimental Studies ◽

Low Pressure ◽

Natural Occurrence ◽

Natural Systems ◽

Crystal Shapes ◽

Wide Range ◽

Negative Crystal ◽

Hydrothermal Veins

Experimental studies increasingly often report low-temperature (200–800 °C) and low-pressure (0.05–3 kbar) hydrosilicate fluids with >40 wt.% of SiO2 and >10 wt.% of H2O. Compositionally similar fluids were long suggested to potentially exist in natural systems such as pegmatites and hydrothermal veins. However, they are rarely invoked in recent petrogenetic models, perhaps because of the scarcity of direct evidence for their natural occurrence. Here we review such evidence from previous works and add to this by documenting inclusions of hydrosilicate fluids in quartz and feldspar from Itrongay. The latter comprise opal-A, opal-CT, moganite and quartz inclusions that frequently contain H2O and have negative crystal shapes. They coexist with inclusions of CO2- and H2O-rich fluids and complex polycrystalline inclusions containing chlorides, sulphates, carbonates, arsenates, oxides, hydroxides and silicates, which we interpret as remnants of saline liquids. Collectively, previous studies and our new results indicate that hydrosilicate fluids may be common in the Earth’s crust, although their tendency to transform into quartz upon cooling and exhumation renders them difficult to recognise. These data warrant more comprehensive research into the nature of such hydrosilicate fluids and their distribution across a wide range of pressure and temperature conditions and geological systems.

Download Full-text

Electron Microscopy of Silicon Deposits in Foliar Idioblasts of Magnolia Grandiflora

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100091858 ◽

1976 ◽

Vol 34 ◽

pp. 374-375

Author(s):

Michael T. Postek

Keyword(s):

Electron Microscopy ◽

Water Loss ◽

Normal Development ◽

Magnolia Grandiflora ◽

Plant Body ◽

Normal Metabolism ◽

Hydrated Oxide

Silicon occurs naturally in plants in the form of its hydrated oxide (SiO2.nH2O) commonly called silica. Silica has been shown to be a necessary element in the normal development of many plants, playing an array of roles including strengthening, protection, and reduction of water loss. Deposition of silica in various portions of the plant body, especially the leaves, may also be viewed as a way for the plant to dispose of any excess silica taken up beyond that necessary for normal metabolism.Studies of this “opaline” silica have thus far been limited to species of the Cyperaceae and Gramineae known to possess significant quantities of silica. Within the Magnoliaceae, certain “glistening” idioblast cells at the foliar veinlet termini and vein sheaths of Magnolia grandiflora (1) have been suspected to be siliceous in nature.

Download Full-text