Khursheed Dinshaw (India)
Belur and Halebidu are part of the Dharwar Craton and are situated in the western part of the craton. (A craton is large portion of a continental plate that has been relatively undisturbed since the Precambrian Era.) The rock formations of this region belong to Dharwar supergroup which has three major components including the Sargur group of rocks dating back to 3,000 to 3,300Ma.
The Peninsular Gneissic Rocks here comprise of two orders, which serve as basement rocks for the deposition of the Sargur group and post 3,000Ma my gneissic rocks, which serve as basement rocks for the deposition of Dharwar group of rocks, which date from 2,800 to 2,500Ma.
Dr H S M Prakash, Deputy Director General (retd) of the Geological Survey of India, Bengaluru explained to me when I spoke to him recently that:
Intruding into this thick pile of rock formations is the intrusive granitic bodies outcropping here and there in the gneissic rocks with intrusion ages of 2,500 to 2,400Ma. Thus, Belur and Halebidu area has a cross section of the older geological formations of the geology of the state of Karnataka”
I was speaking to Dr. Prakash to get a better understanding about the formations of Belur and Halebidu.Dr Prakash has studied the area in detail and is based in the same state where Belur and Halebidu are located.
The oldest rock recorded so far in Karnataka is the Gorur Gneiss in Hassan District. Volcanological, sedimentological and metamorphic features, which are unique in nature, especially the ultramafic components with high magnesium, calcium, iron, chromium, nickel and cadmium values, are characteristic features of Sargurs.
The talc chlorite schistose rocks with typical bottle green colour are very attractive when they are polished. They have a uniform texture, which is an ideal property for their use as material for sculpting. With a hardness of 2 to 3 on Mohs scale of hardness, they can be easily carved for any type of intricate designs. This is the reason for their use in the world-famous stone temples at Belur and Halebidu of Hoysala Dynasty dating back to the twelfth century (Figs 7 to 11).
The main stone used in the construction of both temples is soapstone, which is a metamorphic rock comprising mainly of talc and other minerals like chlorite, amphiboles and carbonates. Soapstone is heat resistant and it was used because it is soft owing to the amount of talc in it and hence easy to sculpt into. Exposed to the atmosphere, the stone hardens. It is quarried extensively in nearby areas for use in these temples specially for carving statues (Figs. 1 to 6).
The Peninsular Gneissic Complex (PGC) rocks comprise of PGC-1 occurring as basement rocks for Sargurs. They are made up of multiple mafic and acidic components of different order periods of crustal thickening and crustal evolution, with several periods of deformation leading to various fold structures and deformation fabrics. These rocks are hard enough to be used as construction material for Temples (Figs. 10 and 11).
“The Dharwar group of rocks overlie the PGC-2 rocks which are younger to Sargurs. PGC-2 rocks show higher potash content compared to PGC-1 rocks which are more sodic in nature. Dharwar’s are subdivided into two groups viz younger Chitradurga group of rock formations and the older Bababudan group of rocks with a distinct unconformity between the two sets of rocks,” Dr Prakash also explained.
The Bababudan group of rocks is represented by impersistent high magnesium basalts, orthoquartzites, thin impersistent conglomerates, grit rocks, meta argillites and fuchsite quartzite. They host some important magnetite ore resources in other districts of Karnataka. The overlying Chitradurga group of rocks are represented by quartzites, shale, iron formations, meta basalts and limestone.
“These rocks host some of the best resources of iron, manganese, limestone, dolomites, gold and copper deposits. The metabasalts are pillowed in nature indicating submarine volcanism,” Dr Prakash went on to explain. The Dharwar super group of rocks is also known as a greenstone belt, due to greenish minerals such as chlorite, hornblende and fuchsite. They form persistent elongated belts running in a NW-SE direction up to 450km.
These belts are parallel to each other with intruding linear granitic batholiths in between. These granites are collectively called the Closepet group of granites. They are sodic to potassic (that is containing sodium or potassium) in composition, with adamellites (granitic rock), monzonites (igneous rock), granodiorites (igneous rock), diorites (igneous rock), syenites (igneous rock) and pink granites. They are quarried extensively for ornamental polished slabs. Intruding into all these rocks, we find thick and long dolerite, gabbro dykes trending in ENE-WSW and NNW-SSE directions.
All photographs are by Khursheed Dinshaw.