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A 15-cm-thick carbonate substrate encrusted with ferromanganese oxides from the Vityaz Fracture Zone, Central Indian Ridge was analyzed to reconstruct the paleoceanography of the region. Based on the calcareous nannoplankton assemblage, an early Pliocene age has been assigned to the calcareous substrate. Among the nannoplankton, discoasters outnumber coccoliths and show signs of dissolution. The presence of certain species of benthic Foraminifera such as Uvigerina, Lenticulina, Bulimina and Bolivina, indicates the infringement of the oxygen minimum zone during the deposition of the carbonates. The occurrence of a Reticulofenestra pseudoumbilica zone of early Pliocene age suggests a change in depositional conditions coinciding with the time of formation of the large depositional hiatuses documented in sediment cores from adjacent basins of the western Indian Ocean. These hiatuses resulted from the prevalence of intermediate subsurface currents such as the Somali Current or the Western Boundary Current. 相似文献
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A simple mathematical technique based on regressional analysis allows the transformation of dipolar, Wenner and two-electrode apparent resistivity sounding curves to Schlumberger ones. The algorithm is suitable for a programmable pocket calculator and the accuracy is very high. This has been demonstrated by comparing Schlumberger master curves with transformations of master curves for the other configurations for the same model. 相似文献
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Dwijesh RAY Ranadip BANERJEE Sridhar D IYER Basavaraju BASAVALINGU Subir MUKHOPADHYAY 《《地质学报》英文版》2009,83(6):1122-1135
Abstract: The glass and mineral chemistry of basalts examined from the northern central Indian ridge (NCIR) provides an insight into magma genesis around the vicinity of two transform faults: Vityaz (VT) and Vema (VM). The studied mid-ocean ridge basalts (MORBs) from the outer ridge flank (VT area) and a near-ridge seamount (VM area) reveal that they are moderately phyric plagioclase basalts composed of plagioclase (phenocryst [An60–90] and groundmass [An35–79]), olivine (Fo81–88), diopside (Wo45–51, En25–37, Fs14–24), and titanomagnetite (FeOt ~63.75 wt% and TiO2 ~22.69 wt%). The whole-rock composition of these basalts has similar Mg# [mole Mg/mole(Mg+Fe2+)] (VT basalt: ~0.56–0.58; VM basalt: ~0.57), but differ in their total alkali content (VT basalt: ~2.65; VM basalt: ~3.24). The bulk composition of the magma was gradually depleted in MgO and enriched in FeOt, TiO2, P2O5, and Na2O with progressive fractionation, the basalts were gradually enriched in Y and Zr and depleted in Ni and Cr. In addition, the SREE of magma also increased with fractionation, without any change in the (La/Yb)N value. Glass from the VM seamount shows more fractionated characters (Mg#: 0.56–0.57) compared to the outer ridge flank lava of the VT area (Mg#: 0.63–0.65). This study concludes that present basalts experienced low-pressure crystallization at a relatively shallow depth. The geochemical changes in the NCIR magmas resulted from fractional crystallization at a shallow depth. As a consequence, spinel was the first mineral to crystallize at a pressure >10 kbar, followed by Fe-rich olivine at <10 kbar pressure. 相似文献
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Mylonitic gabbro and altered gabbro were recovered from off-axis high and corner high locations at ridge-transform intersection,
adjacent to Vityaz transform fault of the slow spreading (32–35 mm/yr, full spreading) Northern Central Indian Ridge. Both
the varieties show signatures of extensive alteration caused due to interaction with sea water. Mylonitic gabbro represents
high temperature metamorphism (∼700–800°C) and comprised of hornblende mineral which exhibits well defined foliation/gneissic
appearance along with dynamically recrystallised plagioclase grains frequently intercalated with magnetite-ilmenite. Altered
gabbro from corner high generally includes low temperature greenschist grade (∼300°C) mineralogical assemblages: chlorite,
albite, quartz and locally magnesio hornblende. Crystal plastic deformation resulted in mylonite formation and often porphyroclasts
of plagioclase and clinopyroxene grains, while altered gabbro locally exhibits cataclastic texture. Presence of Vityaz transform
fault and adjacent megamullion at the weakly magmatic ridge-transform intersection and off-axis high locations prompted the
present scenario very much conducive for hydrothermal circulation and further facilitate the exhumation of present suite of
gabbro. 相似文献
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In the Central Indian Basin manganese nodule abundance was variable in all sediment types. Mean abundance varied from 1.5 in calcareous ooze to 10.2 kg/m2 in terrigenous-siliceous ooze sediments. Nodule grade and growth rates are positively correlated only up to 10 mm/My (million years), and grade shows no distinct relationship with abundance. Relationships between the morphochemical characteristics of the nodules and host sediment types are subtle. Both hydrogenetic and diagenetic nodules (with smooth and rough surfaces respectively) occur on almost all sediments, but in variable proportions. Thus, the overall distribution pattern shows that small nodules (<4-cm diameters) of lower grade (average value Ni+Cu+Co=1.21%) with smooth surfaces are more common on red clay, terrigenous, and terrigenous-siliceous ooze transition-zone sediments. By contrast, large nodules (>4-cm diameters) of higher grade (average value Ni+Cu+Co=1.80%) with rough surfaces are more prevalent on siliceous ooze, siliceous ooze-red clay, and calcareous ooze-red clay transition-zone sediments. This implies an enhanced supply of trace metals from pore waters to rough-surface nodules during early diagenesis. 相似文献
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A New Report of Serpentinites from Northern Central Indian Ridge (at 6°S)—An Implication for Hydrothermal Activity 总被引：1，自引：1，他引：0
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°C. 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/Sin (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
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Dwijesh RAY Sridhar D. IYER Ranadip BANERJEE Saumitra MISRA M. WIDDOWSON 《《地质学报》英文版》2007,81(1):99-112
1 Introduction The Northern Central Indian Ridge (NCIR) between 3° and 11°S latitudes is joined to the north with the slow spreading Carlsberg Ridge (CR; ~24–26 mm/a, full spreading rate) and to the south with the intermediate spreading Southern Central Indian Ridge (SCIR; ~50 mm/ a) (Fig. 1). Earlier petrological investigations of the Central Indian Ridge were concentrated either on or along the CR to the north and at the southern end of the CIR up to the Rodriguez Triple Junc… 相似文献