Journal of Petrology Volume 42 Number 12 Pages 2175-2195 2001
© Oxford University Press 2001
Differentiation Processes of Deccan Trap Basalts: Contribution from Geochemistry and Experimental Petrology
1EARTHQUAKE RESEARCH INSTITUTE, THE UNIVERSITY OF TOKYO, TOKYO 113-0032, JAPAN
2INSTITUTE FOR GEOTHERMAL SCIENCES, KYOTO UNIVERSITY, NOGUCHIBARU, BEPPU 874-0903, JAPAN
3PLOT NO. 89, FRIENDS HOUSING SOCIETY LAYOUT-4, DEENDAYALNAGAR, NAGPUR 400 022, INDIA
4FACULTY OF GEO-ENVIRONMENTAL SCIENCES, RISSHO UNIVERSITY, KUMAGAYA 360-0194, JAPAN
5DEPARTMENT OF EARTH SCIENCES, NIHON UNIVERSITY, TOKYO 156-0045, JAPAN
The Deccan Traps basalts can be divided into sub-groups based on the inferred type and/or amount of contamination. The elemental characteristics of Ba, Sr, TiO2 and Zr/Nb are used to classify the sub-groups; the least-contaminated group has Ba contents <100 ppm, Sr 190–240 ppm and TiO2 2·0–4·0 wt %, and the most-contaminated group has TiO2 contents <1·5 wt % and Zr/Nb 
15. Analyses of 325 basalts, which were collected from 27 well-distributed sections through the Deccan Traps, demonstrate that the least- and most-contaminated groups are distributed widely. To understand the shallow-level fractionation of the Deccan Trap magmas, melting experiments were conducted at atmospheric pressure (100 kPa) at both fayalite–magnetite–quartz (FMQ ) and nickel–nickel oxide (NNO) oxygen fugacities for three Mg-rich basalts, one of which belongs to the least-contaminated group. The results indicate that the phenocryst assemblage and the chemical trend of the least-contaminated basalts can reasonably be explained by fractional crystallization in shallow chambers under FMQ-buffered conditions. The inferred fractional crystallization process also reproduces the chemical trend of the most-contaminated basalts, implying that crustal contamination was not accompanied by the shallow-level fractional crystallization.
KEY WORDS: Deccan Traps; fractional crystallization; crustal contamination; melting experiment; tholeiitic magma
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