Gravity Field and Its Significance to the Origin of the Bengal Anorthosite

The Bengal anorthosite is the only known major anorthosite occurrence in India outside the Eastern Ghats province. The characteristic Eastern Ghats gravity 'high', however, continues northward across the Bengal anothosite. Recent gravity mapping shows that this anorthosite is associated with a Significant positive anomaly of 20 mgal amplitude and 25 km width. The gravity high also extends far another 100 km westward into the Chhotanagpur gneissic terrain. Gravity interpretation suggests a more mafic phase of the anorthosite underlying its southern part, the mafic mass being of gabbroic composition. The mafic mass is inferred to have the form of an inverted cone, at least 3.3 km thick, below a thin veneer of anorthositic rocks, but its thickness reduces westward inside the Chhotanagpur gneisses where the mafic mass assumes a more tabular form. Two large granitic stocks, upto 1.8 km thick and 20km wide, are inferred to have intruded the country rocks to the immediate south of the anorthosite suite. The granites are supposedly derived by palingenesis of in situ leptynites.