Origin of peraluminous minerals (corundum,spinel, and sapphirine) in a highly calcic anorthosite from the Sittampundi Layered Complex,Tamil Nadu,India     

Shreya?KarmakarEmail author  Subham?Mukherjee  Sanjoy?Sanyal  Pulak?Sengupta 《Contributions to Mineralogy and Petrology》2017,172(8):67

The highly calcic anorthosite (An_>95) from the Sittampundi Layered Complex (SLC) develops corundum, spinel and sapphirine that are hitherto not reported from any anorthositic rocks in the world. Petrological observations indicate the following sequence of mineral growth: plagioclase_matrix → corundum; clinopyroxene → amphibole; corundum + amphibole → plagioclase_corona + spinel; and spinel + corundum → coronitic sapphirine. Phase relations in the CaO–Na₂O–Al₂O₃–SiO₂–H₂O (CNASH) system suggest that corundum was presumably developed through vapour present incongruent melting of the highly calcic plagioclase during ultra-high temperature (UHT) metamorphism (T ≥ 1000 °C, P ≥ 9 kbar). Topological constraints in parts of the Na₂O–CaO–MgO–Al₂O₃–SiO₂–H₂O (NCMASH) system suggest that subsequent to the UHT metamorphism, aqueous fluid(s) permeated the rock and the assemblage corundum + amphibole + anorthite + clinozoisite was stabilized during high-pressure (HP) metamorphism (11 ± 2 kbar, 750 ± 50 °C). Constraints of the NCMASH topology and thermodynamic and textural modeling study suggest that coronitic plagioclase and spinel formed at the expense of corundum + amphibole during a steeply decompressive retrograde P–T path (7–8 kbar and 700–800 °C) in an open system. Textural modeling studies combined with chemical potential diagrams (μSiO₂–μMgO) in the MASH system support the view that sapphirine also formed from due to silica and Mg metasomatism of the precursor spinel ± corundum, on the steeply decompressive retrograde P–T path, prior to onset of significant cooling of the SLC. Extremely channelized fluid flow and large positive solid volume change of the stoichiometrically balanced sapphirine forming reaction explains the localized growth of sapphirine.  相似文献   

Shake Table Tests and Numerical Modeling of Liquefaction of Kasai River Sand     

Raj Banerjee  Sanku Konai  Aniruddha Sengupta  Kousik Deb 《Geotechnical and Geological Engineering》2017,35(4):1327-1340

As a part of the seismic safety evaluation of several bridges and other hydraulic structures located on Kasai River bed in India, the liquefaction potential of Kasai River sand is studied in 1-g shake table in laboratory and numerically using a commercial software FLAC 2D. The surface settlement, lateral spreading, predominant frequency, amplification of the ground motion and pore water pressure development in Kasai River sand in dry and liquefied states have been studied when subjected to sinusoidal motions of amplitude 0.35 g at a frequency of 2 Hz. The nonlinear curves used to represent shear strain dependency of stiffness and damping ratio of Kasai River sand are obtained from cyclic triaxial tests. Reasonably good agreement between the experimental and the numerical results is observed. It is found that the settlement and lateral spreading for the liquefied sand is 2.60 and 2.50 times than those of the sand in the dry state. The volumetric strain of the liquefied sand is found to be around 4%, which is significantly higher than 1.53% observed in the dry sand. It is observed that the amplification of the peak ground acceleration for the saturated sand is 1.08 and 1.32 times higher than that for the dry sand from theoretical and experimental results, respectively. The shear strain developed in the liquefied sand is 1.17 times more than that for dry sand. The fundamental and higher modal frequencies of dry sand are found to be 1.13, 1.117 and 1.119 times more than those for the saturated sand, respectively.  相似文献