A researcher studying the Chilka rocks

Geologists scouring the Chilka Lake region in Orissa have come up with new evidence to suggest that certain igneous rocks here crystallized 983 million years ago, about 200 million years earlier than previously thought1.

The scientists used Uranium-Lead zircon dating to come to the conclusion that the Chilka anorthosite, a plutonic igneous rock crystallized from mantle-derived magma that intruded the doubly deformed crust. Both crustal deformations took place before the anorthosite crystallized 983 million years ago.

Some previous workers had determined the age of the Chilka ferrodiorite, a rock usually associated with anorthosite, as 792 million years and inferred that the anorthosite was also of the same age. "We directly determined the age of the anorthosite and found that the anorthosite is actually about 200 million year older," says lead researcher Nilanjan Chatterjee.

The team interpreted the 792 million year age of ferrodiorite to correspond with an event related to the close approach of the Eastern Ghats-Rayner block and Australia believed to have happened 792-655 million years ago. The ferrodiorite may have originated with the anorthosite around 983 million years ago and metamorphosed around 792 million years ago. "Or it may have originated around 792 million years ago. This needs further investigation," Chatterjee says.

The finding is significant to how massif type anorthosite-ferrodiorite associations formed. Anorthosite massifs are unique geological features that mostly formed more than one billion years ago. In the Adirondocks of New York, anorthosite massifs and ferrodiorite seem to have formed simultaneously. But at Chilka, there may be around a 200 million year interval. "We did not investigate this aspect. Anorthosite massif formation could be related to plate tectonics the way it operated one billion year ago. But very little is known on this subject," he adds.

By determining the timing and cause of multiple deformations and accompanying metamorphic events and correlating them with similar events on other continental blocks, geoscientists attempt to reconstruct the relative positions of different continental blocks at different times in the earth's history.