Tuesday, June 20, 2017

Quiz: Spot The Granite Intrusion

I came across this glacially transported boulder in the Dugtu village valley near the Panchachuli Glacier in the Kumaon Himalaya.

It is a block of high grade gneiss intruded by a granite. Without scrolling beyond the first photograph, try to work out the contact between the gneiss and the granite.


The boulder is encrusted by moss. There is some mineral staining too. And sunlight falling on the rock gives it a speckled appearance.. All this reduces the contrast in color between the gneiss and the granite.

But there is a vital clue in the orientation of structures. Both the gneiss and the granite have a planar fabric imprinted on them.

The fabric of the gneiss is due to the orientation of platy minerals like micas stacked in layers, alternating with layers richer in quartz and feldspars. Assume this is the original disposition of the rock as well. The gneiss layering you see is due to the trace of horizontal planes of separation of different mineral layers. I have outlined some of this planar fabric in brown lines.

The granite has a planar fabric too, but this is due to near vertical fractures. The rock has been broken in to thin slabs  by fractures (red lines) which may have formed during the cooling of the magma. These fractures don't pass into the surrounding host gneiss. Two arms of the granite have penetrated between the gneiss layers forming mini sills.

You can see the contact (black line) between the gneiss and the granite roughly where my wallet is. Here, the horizontal planar fabric of the gneiss abruptly juxtaposes against the vertical planar fabric of the granite.


  1. Sir,
    Is this any different from a migmatite ? I saw similar structures somewhere between Ajmer and Pushkar.

  2. It can be hard to tell the difference. Partial melting forms a migmatite. Usually the melt segregates and forms concordant lighter layers and small veins and dykelets. You can generally make out in the field that the melt has segregated locally. But that melt can migrate and form smaller distinct intrusions too. In this case a very distinct leucogranite has intruded the Greater Himalayan Sequence forming massive sills and dykes. So it appears to have been derived from a larger zone of crustal melting and not a local partially melted host rock.