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31.
SANTANU ACHARJEE JYOTISANKAR RAY PAYEL DEY DEBAPRIYA BHATTACHARYYA MOUSUMI BANERJEE BASAB CHATTOPADHYAY SHYAMAL SENGUPTA A K BHATT D CHOWDHURY A K DWIVEDI SANJOY MAHATO ARKA RANJAN JANA P B MAITHANI P V RAMESH BABU 《Journal of Earth System Science》2016,125(8):1681-1696
The area of investigation at and around Mashak Pahar, Bankura district, West Bengal, India comprises a number of rock types namely: granite gneiss, migmatized quartz tourmaline gneiss, quartz pebble conglomerate, ferruginous quartzite, quartz tourmaline veins (as veins) and graphite schists. Interestingly, the study area lies in the region extending South Purulia Shear Zone (~Tamar–Porapahar Shear Zone) which marks the boundary between two contrasting tectonic blocks of eastern India, namely, the Chhotanagpur Gneissic Terrane (CGC) to the north and Singhbhum Group of rocks to the south. The rocks of the study area are poly-phasedly deformed by three phases of folding, namely, F1, F2 and F3. All the tourmalines are classified to be of ‘Alkali Group’. Chemistry of tourmalines from migmatized quartz tourmaline gneiss and those from quartz tourmaline veins are in conformity with their relation to (earthquake induced) shear system evolution in this terrain. In general, the compositional evolution of tourmaline during prograde metamorphism (~400°–730°C) has been supported by both petrographic and chemical evidences. Assessment of mineral–chemical data of constituent tourmaline grains clearly suggests compositional variations across zonal boundaries within tourmaline that was controlled by changing metamorphic milieu in this terrane. Field and petrographic evidences clearly indicate activation of earlier and later shears in this region accompanied by infiltration of boron and formation of zoned tourmaline crystals. 相似文献
32.
RAY Durbar BANERJEE Ranadip MAZUMDER Anin PAROPKARI Anil L. MUKHOPADHYAY Subir BALAKRISHNAN Srinivasan 《海洋学报(英文版)》2018,37(4):22-33
Polymetallic sulfides from two hydrothermal chimneys and talus deposit from the Vienna Woods field of Manus Basin were studied for mineralogy, elemental composition, and S-isotope ratio to understand their evolution. The factors including the nature of source fluid, mineral paragenesis, and related geochemical processes have been discussed. Mineralogy and elemental concentration of Cu and Fe-rich large chimney at the central part of this hydrothermal field was completely different from the smaller Zn-rich peripheral chimney and Fe-rich talus deposit, suggesting the variable degree of alterations generate physico-chemically different source fluids responsible for these hydrothermal structures. Similarly, S-isotope ratios also indicate chemically diverse fluids and different modes of precipitation were involved in their evolution. Distinct mineral zonings and associated elemental and isotopic compositions within individual deposit confirm paragenetic shifts were involved during their growth process. 相似文献
33.
JYOTISANKAR RAY ABHISHEK SAHA SOHINI GANGULY V BALARAM A KESHAV KRISHNA SAMPA HAZRA 《Journal of Earth System Science》2011,120(3):459-473
The Mylliem granitoids of the Meghalaya Plateau, northeastern India, represent one of the disharmonic Neoproterozoic igneous
plutons, which are intrusive into low-grade Shillong Group of metasediments. Field studies indicate that the Mylliem granitoids
cover an area of about 40 km2 and is characterized by development of variable attitude of primary foliations mostly marked along the margin of the pluton.
Xenoliths of both Shillong Group of metasediments and mafic rocks have been found to occur within Mylliem granitoids. Structural
study of the primary foliation is suggestive of funnel-shaped intrusion of Mylliem granitoids with no appreciable evidence
of shearing. Petrographically, Mylliem granitoids are characterized by pink to white phenocrysts of prismatic microcline/perthite
and lath-shaped plagioclase (An20–An29). Groundmass material is characterized by quartz, microcline, plagioclase, muscovite and biotite. Sphene and apatite occur
as accessory minerals. Petrographically Mylliem granitoids have been discriminated as granite and granodiorite according to
IUGS system of classification. 相似文献
34.
Serpentinites from the inside corner high(6°38.5′S/68°19.34′E)from the Northern Central Indian Ridge(NCIR)are comprised mainly of high Mg-rich lizardite and chrysotile pseudomorphs with varying morphologies.‘Mesh rim',‘window',‘hourglass' and‘bastite' are the most common textures displayed by both chrysotile and lizardite.Numerous chrysotile veins in association with cross cutting magnetite veins indicate an advanced stage of serpentinisation.The relatively high abundance of chrysotile and lizardite suggest their close association and formation at a temperature below 250℃. Abundant‘mesh rim'and‘bastite'texture and variegated matrix reveal that the present serpentinites might have formed due to the interaction of harzburgite and seawater.Positive Eu anomaly(Eu/Eu~* up to 3.38),higher La/Sm(up to 4.40)and Nb/La(up to 6.34)ratios suggest substantial hydrothermal influence during the formation of the serpentinites. 相似文献
35.
Experimental Constraints on the Relationships between Peralkaline Rhyolites of the Kenya Rift Valley 总被引:13,自引:1,他引:13
Crystallization experiments on three comendites provide evidencefor the genetic relationships between peralkaline rhyolitesin the central Kenya rift valley. The crystallization of calcicclinopyroxene in slightly peralkaline rhyolites inhibits increasein peralkalinity by counteracting the effects of feldspar. Fractionationunder high fO2 conditions produces residual liquids that areless, or only slightly more, peralkaline than the bulk composition.In contrast, crystallization under reduced conditions (<FMQ,where FMQ is the fayalitemagnetitequartz buffer)and at high fF2 inhibits calcic clinopyroxene and yields residualliquids that are more peralkaline than coexisting alkali feldspar,whose subsequent crystallization increases the peralkalinityof the liquid. A marginally peralkaline rhyolite [molar (Na2O+ K2O)/Al2O3 (NK/A) = 1·05] can yield a more typicallycomenditic rhyolite (NK/A = 1·28) after 95 wt % of crystallization.This comendite yields pantelleritic derivatives (NK/A >1·4)after 25 wt % crystallization. Upon further crystallization,extreme peralkaline compositions (NK/A 相似文献
36.
Sea-level and tectonic control of middle to late Pleistocene turbidite systems in Santa Monica Basin, offshore California 总被引:1,自引:0,他引:1
Small turbidite systems offshore from southern California provide an opportunity to track sediment from river source through the turbidity‐current initiation process to ultimate deposition, and to evaluate the impact of changing sea level and tectonics. The Santa Monica Basin is almost a closed system for terrigenous sediment input, and is supplied principally from the Santa Clara River. The Hueneme fan is supplied directly by the river, whereas the smaller Mugu and Dume fans are nourished by southward longshore drift. This study of the Late Quaternary turbidite fill of the Santa Monica Basin uses a dense grid of high‐resolution seismic‐reflection profiles tied to new radiocarbon ages for Ocean Drilling Program (ODP) Site 1015 back to 32 ka. Over the last glacial cycle, sedimentation rates in the distal part of Santa Monica Basin averaged 2–3 mm yr?1, with increases at times of extreme relative sea‐level lowstand. Coarser‐grained mid‐fan lobes prograded into the basin from the Hueneme, Mugu and Dume fans at times of rapid sea‐level fall. These pulses of coarse‐grained sediment resulted from river channel incision and delta cannibalization. During the extreme lowstand of the last glacial maximum, sediment delivery was concentrated on the Hueneme Fan, with mean depositional rates of up to 13 mm yr?1 on the mid‐ and upper fan. During the marine isotope stage (MIS) 2 transgression, enhanced rates of sedimentation of > 4 mm yr?1 occurred on the Mugu and Dume fans, as a result of distributary switching and southward littoral drift providing nourishment to these fan systems. Longer‐term sediment delivery to Santa Monica Basin was controlled by tectonics. Prior to MIS 10, the Anacapa ridge blocked the southward discharge of the Santa Clara River into the Santa Monica Basin. The pattern and distribution of turbidite sedimentation was strongly controlled by sea level through the rate of supply of coarse sediment and the style of initiation of turbidity currents. These two factors appear to have been more important than the absolute position of sea level. 相似文献
37.
SOHINI?GANGULYEmail author JYOTISANKAR?RAY CHRISTIAN?KOEBERL THEODOROS?NTAFLOS MOUSUMI?BANERJEE 《Journal of Earth System Science》2012,121(1):91-108
Several basaltic lava flows have been identified in the study area in and around Linga, in the Eastern Deccan Volcanic Province
(EDVP) on the basis of distinctly developed structural zones defined by primary volcanic structures such as columnar joints
and vesicles. These basaltic lava flows are spatially distributed in four different sectors, viz., (i) Bargona–Gadarwara (BG)
sector (ii) Shikarpur–Linga (SL) sector (iii) Arjunvari–Survir Hill (AS) sector and (iv) Kukrachiman–Morand Hill (KM) sector.
A three-tier classification scheme has been adopted for the characterization and classification of individual lava flows.
Each lava flow consists of a Lower Colonnade Zone (LCZ) overlain by the Entablature Zone (EZ) and Upper Colonnade Zone (UCZ).
The LCZ and UCZ grade into a distinct/indistinct Lower Vesicular Zone (LVZ) and Upper Vesicular Zone (UVZ), respectively.
The LCZ and UCZ of the flows are characterized by columnar joints while the EZ is marked by multi-directional hackly jointing.
The geometry of different joint patterns corresponds to different styles of cooling during solidification of lava flows. Detailed
petrographic studies of the investigated lava flows reveal inequigranular phenocrystal basalts characterized by development
of phenocrystal phases including plagioclase, clinopyroxene and olivine, whereas groundmass composition is marked by tiny
plagioclase, clinopyroxene, opaque mineral and glass. Electron microprobe analyses indicate that the olivine has a wide range
∼Fo22 to Fo66 revealing a wide spectrum of compositional variation. Pyroxene compositions are distinctly designated as Quad pyroxenes.
Phenocrystal pyroxenes are mostly diopsidic, while the groundmass pyroxenes mainly correspond to augite with a minor pigeonite
component. Pyroxene phenocrysts are characterized by a prominent Ti-enrichment. Phenocrystal plagioclase grains are calcic
(An52.7–An72.9), whereas groundmass plagioclase are relatively sodic (An39.2–An61.6). Groundmass opaque minerals are characteristically found to be Ti–magnetite/ilmenite/pyrophanite. Pyroxene thermometry reveals
a temperature span of 850°C to 1280°C for the studied lavas while olivine–clinopyroxene thermometry yields a temperature range
from 1040°–1160°C. The variation of temperature for the lava flows is ascribed to their normal cooling history after eruption. 相似文献