Ancient collisions that built mountains

An ancient collision between the Indian and Asian continental plates created the Himalayas. However, the exact time of the collision is controversial.

PhD student Adrianna Rajkumar and her supervisor Prof. Geoffrey Clarke, at the University of Sydney (USyd), are attempting to clarify this mystery using Indian rocks.

Previously, researchers dated these rocks by crushing them to produce isolated zircons. The zircons were then dated, but without context to where in the rock they came from, which gave ages that were unexpectedly old (e.g. 60 million years).

Ms Rajkumar and Prof. Clarke recognised that the rocks record multiple stages of mineral growth. Dating the zircon grains while they were still in the whole rock – and, therefore, still associated with specific minerals – would help to date the mineral growth stages. This would inform a more accurate rock age.

Ms Rajkumar and Dr Pat Trimby at the AMMRF (now Microscopy Australia) at USyd scanned whole sections of rock, picking out zircons in backscatter electron images and confirming their composition using energy dispersive X-ray spectrometry. This method can correlate the age of the zircons with the rock’s history. Their results explain some of the anomalous dates previously reported, which are likely to be related to zircons from the original parent rocks formed before the start of the India–Asia collision.

Refining the date of collision and the ages of the parent rocks, and determining the depth of the rocks when the zircons grew, will shed light on the complex geological history of the India–Asia collision.