Geological Setting, Petrography, and Petrochemistry of the Darla Trap, and its Comparative Study with Mandl and Panjal Traps

Abstract

Studies on the Darla trap have not been made previously. The Darla trap is overlain by the rocks of Basantpur Formation, and underlain by those of the Sundernagar Formation. It forms the core of an antiform, and its northern extremity is in thrust contact with the Shalis, the volcanics trending along this contact as a thinned outcrop south-easterly to Tattapani and beyond.

Microscopically, the Darla trap comprises saussuritised plagioclase felspar microlites and laths, altered clinopyroxene of diopsidic-augite range, pseudomorphs of olivine, granular and massive or skeletal iron ores, glass, palagonite, chlorite, epidote, apatite, and spinel. The amygdaloidal variety is the most common, and includes arnygdules of quartz, chalcedony, zeolite, chlorite, and quartz being surrounded by a thick rim of pink chalcedony. The other common variety has porphyritic chlorite in ophitic texture with plagioclase felspar. The chief rock types are basalt, palagonite-basalt, diabase, spilite, actionlitic-ferrunginous schistose rock, and rhyolite, the volcanic outpourings being thus of essentially basic rocks.

Chemically, the Darla trap shows gradation between tholeiitic and alkali basalts. The analytical data has been plotted on alkali-silica, Or-Ab-An, Fe₊-(Na+K)-Mg, DI-Oxides, SI-Oxides variation diagrams.

The Mandi trap forms part of the Darla-Mandi volcanics, and these traps are considered to be represented by the Panjal trap in the Simla area by Heron. Comparative studies have been made between the Darla, Mandi, and Panjal traps, and inferences drawn.