Petrochemistry and Petrogenesis of the Malani Igneous Suite, India

Abstract

“Malani Beds” was the name given by W. T. Blanford (1877) for a series of volcanic rocks in western Rajputana (now Rajasthan) especially well developed in what was then the Malani district of Marwar (Jodhpur) State, India (Krishnan and Jacob, 1956). Subsequently, these rocks were described under the terms “Malani Volcanic Series” (La Touche, 1902), “Malani System” (Coulson, 1933), “Malani Granite and Volcanic suite” (Pascoe, 1959), and they are now being included under the name “Malani igneous suite,” to group all of the lavas and the associated rocks. While the helmet name “Malani” is non-existent now in the Survey of India topo sheet, and it appears only on the degree sheet 40 0 as Anti Mallani RS (Railway Station) (25°43i 45°N; 71°21iE), 6.4 km. west-southwest of Barmer city, Barmer, district, Rajasthan, these felsic rocks dominate western Rajasthan (Fig. 1). Their extent is about 51,000 km 3 ; possibly 100,000 km 2 could be underlain by sedimentary rocks of Cambrian and Jurassic ages (Pareek, 1980a). The Malani igneous suite may thus have had a total spread of more than 151,000 km 2 , with the possibility of extension of these rocks into the northern part of Gujarat. The Malani igneous suite had an initial volcanic phase, made up of extrusive felsic lava outpourings—Malani Tuff and Rhyolite, followed by plutonic intrusions—Siwana and Jalor Granites, which are comagmatic. The gap between the effusives and intrusives was not much separated in time (La Touche, 1902; Coulson, 1933). The general order of formation was (1) Malani rhyolites, tuffs, and sedimentaries; (2) granite; (3) porphyries; (4) and quartz veins (Pascoe, 1959). The effusives and intrusives are cut by a number of felsic, intermediate, and mafic dikes. These have been divided into five phases in the Sankra area; the first phase is rhyolite and rhyolite porphyry with ash beds; the second phase is of intrusives, the granites; the third phase is of agglomeratic acid flow; the fourth phase is of felsic rocks; and the fifth phase is of basic eruption (Bhushan, 1973). The Malani igneous suite thus is composed of (1) an extrusive phase—rhyolites and associated lavas, tuff, welded tuff, and mafics; (2) an intrusive phase—made up of granites; and (3) a dike phase which produced dolerite, diorite, gabbro, basalt, rhyolite, trachyte, andesite, granite, felsite, and quartz veins; this sequence completes an “igneous cycle,” as outlined by Hatch and others (1961).