Malani Igneous Suite Research Articles

Terminal Proterozoic-Cambrian sequences in India: a review with special reference to Precambrian-Cambrian Boundary

The Terminal Proterozoic marine sequences, grading into Cambrian, are present in the Krol Basin (Lesser Himalaya), the Kashmir, Spiti-Zanskar and Kumaun Basins (Higher Himalaya) and in Marwar Basin (western India). These sequences postdate a major tectono-thermal event associated with Cadomian Orogeny and rest on eroded older successions which include well-dated Malani Igneous Suite (Malani Rhyolite 745 ±10 Ma; Siwana Granite 731± 14 Ma) in Western India and over the Salkhala Group with granites (745 ± 50 Ma) or the Simla Group in the Himalaya. This cycle of sedimentation terminated with Pan-African Orogeny in Late Cambrian. These sequences are dominantly siliciclastic in basins in Higher Himalaya while those in Krol Basin and Marwar Basin show development of thick carbonate-evaporite facies with or without phosphorite. From the Upper Vindhyan and Bhima Groups in central and south India, respectively a Chuaria-Tawuia Assemblage along with sphaeromorphida Acritarch of Early Neoproterozoic age has been recorded. The δ13C values range from +1.3 to +4.0 ‰ PDB and that of δ 18O from -5 to -9‰ PDB. These are, thus, not considered part of this sequence. In absence of age-determinating biota and radiometric dates from the basal part of the succession, the lower boundary of the Terminal Proterozoic cannot be delineated and dated. However, a significant depletion in values may be taken to mark the lower boundary.
 The upper boundary of the Terminal Proterozoic (Precambrian-Cambrian Boundary) cannot be demarcated in terms of GSSP due to the absence of trace fossils of Zone-1 (Harlaneilla podolika Zone) and Zone-II (Phycodes pedum Zone) in carbonate facies in Krol Basin and poor documentation of siliciclastic facies in Kashmir and Spiti-Zanskar Basins. The trace fossils correlative with ichnozone-III occur in all sections in the Himalayas and are useful for regional and global correlations. However, a significant depletion in δ13C values has been recorded in the carbonate facies of the Krol Basin between the horizons yielding Ediacaran and small shelly fossils of Meishucunian Zone-I. This has also been recorded in the Marwar Basin below the Phosphorite Bed. This depletion may be correlated with that recorded from the Precambrian-Cambrian transition in Siberian Platform, Anti Atlas Mountains, Morocco, China, etc. It may be taken into consideration to define and mark the boundary in the absence of trace fossils.

Petrochemistry and petrogenesis of the Malani igneous suite, India: Discussion and reply

Petrochemistry and petrogenesis of the Malani igneous suite, India: Discussion and reply

Petrochemistry and petrogenesis of the Malani igneous suite, India: Discussion and reply

Petrochemistry and petrogenesis of the Malani igneous suite, India: Discussion and reply

Petrochemistry and Petrogenesis of the Malani Igneous Suite, India

“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).

Petrochemistry and petrogenesis of the Malani igneous suite, India: Summary

Petrochemistry and petrogenesis of the Malani igneous suite, India: Summary