Field traverses were carried out in granitic terrain forming a part of the eastern Dharwar Craton in the areas of Narsimhulapet (17°30’33.
39″N 79°48’17.75″E), Bodlada (17°32’39.08″N 79°44’39.28″E), Pedda Nagaram (17°26’42.
19″N 79°49’40.04″E) and Kausalyadevipalli (17°31’36.48″N 79°50’20.03″E), on the Khammam-Warangal road.
The objective of this investigation is to classify and characterise the granitoids of the study area. The granites and gneisses of the area previously generalised as Peninsular Gneissic Complex (PGC) are reclassified into two divisions as (i) Tonalite-Granodiorite- Monzogranite (TGM) suite and (ii) Monzosyenite (MS) suite. The granitoids are intruded by dolerite dykes, some of which are porphyritic at places. The study area is predominant in monzogranite, which is coarse grained composed of sub-equal amounts of potash feldspar, quartz and plagioclase. The mafic minerals present are hornblende and biotite with magnetite, zoisite, sphene, zircon and apatite occurring as accessory minerals.
The mafic and accessory minerals constitute about 5 to 15% of the rock. At places, it is porphyritic with phenocrysts/megacrysts of potash feldspar ranging in size from a few centimeters to as large as 5 cm in length. Medium grained equigranular variant of the monzogranite is locally exposed as small bodies within the coarse porphyritic variety with sharp to gradational contact. The TGM suite of rocks constitute to a major portion (80%) of the rocks in the area, which are represented by tonalite, granodiorite and monzogranite. The contacts between these three granitoids are gradational. Tonalites which are melanocratic, dark grey to black in colour, composed mainly of plagioclase with minor potash feldspar and mafics, observed to be major rock types in some parts of the study area. Granodiorite is coarse grained, grey coloured, consisting of mainly plagioclase, potash feldspar, biotite and mafics.
These rocks are exposed around Jaya Tanda, Pedda Nagaram and east of Sitaram Tanda. Numerous thin epidote veins are noticed intruding the granodioritic rock at several places. A characteristic feature of the TGM suite granitoids is presence of ubiquitous mafic-rich microgranular enclaves (MMEs) in the form of spindle to cuspate shaped lenses of magmatic origin. The MMEs are rounded to oblongated shaped (50 cm to 1.5 meter), dark grey in clour, fine to medium grained, range in composition from diorite to granodiorite and are invariably more mafic than the host granitoid, showing smooth, lobate and gradational to sharp margins with the host.
They are made up of plagioclase, quartz, hornblende, biotite, minor potash feldspar and opaques. At places, xenocrysts derived from the host rock are found within the MMEs, indicating the presence of both mafic and felsic magmas simultaneously. This inturn indicates magma mixing and mingling processes in semisolid state. The granitoids belonging to MS suite are observed near Narsimhulapet and Viraram (17°26’28.83″N 79°53’29.
33″E). These are grey, white and pink in colour, medium to coarse grained and leuco to mesocratic granitic rocks composed of quartz, K-Feldspar and plagioclase. Biotite is the dominant mafic mineral. These rocks show hypidiomorphic granular texture.
Unlike the TGM suite granitoids they are compositionally restricted with very little variation in K-feldspar/plagioclase ratio. They are also distinguished from TGM suite of granitoids by absence of MMEs, lower mafic mineral content (< 5%), absence of hornblende and more leucocratic appearance.