Anasagar gneiss: A folded granitoid pluton in the Proterozoic South Delhi Fold Belt, central Rajasthan

The Anasagar gneiss was emplaced as a concordant sheet like body along the contact of quartzite and pelitic/semipelitic schist horizons in the norther n part of the South Delhi Fold Belt. It is typically a granite gneiss conta ining megacrysts of K-feldspar set in a recrystallised foliated matrix. The megacrysts are in general converted to granular aggregates, often retainin g their crystal outline. Garnet, sillimanite (fibrolite) and rarely staurol ite are the metamorphic minerals in the gneiss; these are also present in t he enveloping supracrustal rocks. Both the gneiss and the supracrustal rock s are involved in polyphase deformation. F-1 isoclinal folds are present on ly on minor scale in the supracrustal rocks. F-2 major and minor folding ha ve affected both the gneiss and the supracrustal rocks. These are asymmetri cal folds with alternate flat and steep, locally overturned, limbs and have consistent easterly vergence. F-3 folds are upright and coaxial with F-2. F-4 puckers and large scale warps have E-W to ESE-WNW subvertical axial pla nes. The gneiss is exposed in the core of an F-3 arch on the flat limb of a major F-2 antiform whose axial trace is bent by an F-4 fold. The intrusion was pre-F-2 and late-tectonic with F-1. U-Pb zircon dating suggests a crys tallization age of 1849 +/- 8 Ma. Hence the Anasagar gneiss is older than t he late- to post-tectonic "Erinpura-type" granite in the South Delhi Fold B elt.