Carbonatite Bombs, Lapillus, Pisolites And Ashes In Semi- Unconsolidated Conglomerate Of Early Pleistocene From Thiruvalangadu, Tamil Nadu, India
نویسنده
چکیده
Occurrence of sediments hosted smooth surfaced flat / discoid black and pink coloured bimodal carbonatite bombs, lapillus, pisolites and ashes in semi-unconsolidated conglomerate covering an area of 90 sq.km is reported from Thiruvalangadu (13 o 10’36”N-76 o 44’38”E) area situated 60 km WNW of Chennai. The carbonatite materials are uniformly composed of flow-oriented fine-grained calcites <0.02 mm with vesicles < 1mm dimensions filled with recrystallized relatively coarse-grained calcites (~0.1mm). These rocks are essentially composed of calcite with accessories of apatite, augite, hornblende, biotite, wollastonite, skeletal sanidine, sodic oligoclase and corroded quartz. These rocks have significant amount of silica and alumina. Their Fe 3+ /Fe 2+ is greater than 1. The pink variety contains K2O> Na2O. Mafic minerals are alkalic in nature. The low values of ∂ 13 CPDB (from -8.2 to -5.5‰) in black carbonatite and in pink carbonatite (from -5.2 to 0.10‰) and high values of ∂ 18 OSMOW (from 23.1 to 23.7‰) for block carbonatites and (from 24.4 to 27.63‰) for pink carbonatites indicate extensive degassing and loss of volatiles and alkalies by exhalative eruption of these lavas.
منابع مشابه
Crystallization, Fractionation and Solidification of Co-Magmatic Alkaline Series Sequentially Emplaced in the Carbonatite Complex of Tiruppattur, Tamil Nadu, India
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