PETROGENESIS OF THE PRECAMBRIAN ROCKS OF NICHLI ODAN-MANDAK KA GURAH AREA, SOUTHWEST NATHADWARA, UDAIPUR, RAJASTHAN

Abstract

The area of investigation lies in the Aravalli mountain range of the precambrian of north western India. This thesis presents the historical development of exposed rocks in the locality. To help modeling of the petro-gene sis, field . and petrographic studies made on the litholo-gical units have been detailed. Chemical and thermal analysis has been done on the carbonate rock, in order to determine its composition. The rock types in the region are largely of metasediments with subordinate metavolcanics. The netasedimentary units generally are represented by gneisses,• quartzites, amphibole schist, carbonates and phyllites, which generally grade into one another,.without recognized unconformity between them. The amphibolite, which at places is amygdaloidal represents the metavolcanics. Field and petrographic studies reveals the following evolutionary trends of the rocks in the area. The sediments were laid down in a changing environment of deposition, of which started with a semi pelitic assemblage typical of the continental shelf region. On top of this ; carbonates, calcareous sand stone, arenaceous politic and arenaceous sediments were accumulated in a gradually regressing sea. Contemporaneously and/or lately there was instability in VII the region manifested by volcanic activity. Volcanism ceased and, a gradually transgressing sea allowed precipitation of a thick carbonate succession. During the final stage the sea was fluctuating to deposit calcareous pelitic, calcareous sand-stone and arenaceous pelitic .sediments. The whole pile of sediments and volcanics were subjected to deformation which has resulted in isoclinal recumbent folds of north westerly trending, axis. Metamorphism has accompanied. the deformation to. produce i garnet, tremolite, amphibole, plagioclase, talc., biotite, chlorite, whitemica, iron ore, and etc., in different layers depending upon the composition of the rock. Rising anatectic melt from deeper levels form signs of feldspathization and hydrothermal alteration in the near by metasediments. Later on there was a deformation giving rise to the prominent NE-SW foliation. The previously formed rocks were exposed to retrograde metamorphism such that epidote has been formed in different layers. The last episode of deformation is marked by crenulations and kink bands in the more foliated rocks. .1