Sharing a few pics of dikes intruding the Deccan Volcanics at Korlai, a small village south of Mumbai. I had taken a group of nature lovers and science enthusiasts on a traverse from Pune to the west coast last weekend. We stopped at the Western Ghat escarpment to take in majestic views of massive lava flows and then went to the coast to observe dikes and silica geodes.
Several dikes are exposed along the rocky coastline. Most of them are oriented in a N-S to NNW-SSE direction.
Here is one with a shape of a serpent
A close up of a dike. Notice the clear contact between the dark colored dike and the brown/grey looking basalt lava flow.
Another large dike with closely spaced fractures.
And this one, intruded along en echelon fractures, showing a sinistral or left handed offset.
A view of the rocky wave cut platform from top of Korlai fort. Arrows point to two dikes.
One interesting feature of these dikes is that many of them contain tiny fragments (2 -20 cm in diameter) of the lower crust, incorporated by the basaltic magma as it ascended. These fragments or 'xenoliths' are composed of granulite, a common rock type formed in the high temperature - high pressure environments of the lower crust. Work done by A.G Dessai and colleagues suggest that these granulites are present at depths ranging from 15 km to 40 km below the surface. Dike chemistry suggests that the original composition of the parent basaltic magma, formed at even greater depths in the uppermost parts of the mantle, was modified due to reaction with and assimilation of this lower crustal granulite.
That was a point of great interest to the people who participated in this field trip. They were awestruck that they were looking at solidified sheets of magma that extended to depths of more than 40 km below the surface.
Several dikes are exposed along the rocky coastline. Most of them are oriented in a N-S to NNW-SSE direction.
Here is one with a shape of a serpent
A close up of a dike. Notice the clear contact between the dark colored dike and the brown/grey looking basalt lava flow.
Another large dike with closely spaced fractures.
And this one, intruded along en echelon fractures, showing a sinistral or left handed offset.
A view of the rocky wave cut platform from top of Korlai fort. Arrows point to two dikes.
One interesting feature of these dikes is that many of them contain tiny fragments (2 -20 cm in diameter) of the lower crust, incorporated by the basaltic magma as it ascended. These fragments or 'xenoliths' are composed of granulite, a common rock type formed in the high temperature - high pressure environments of the lower crust. Work done by A.G Dessai and colleagues suggest that these granulites are present at depths ranging from 15 km to 40 km below the surface. Dike chemistry suggests that the original composition of the parent basaltic magma, formed at even greater depths in the uppermost parts of the mantle, was modified due to reaction with and assimilation of this lower crustal granulite.
That was a point of great interest to the people who participated in this field trip. They were awestruck that they were looking at solidified sheets of magma that extended to depths of more than 40 km below the surface.
Fascinating, Suvrat. How many years of upwelling of magma would it have taken to build up layers 40 km deep? In other words how long were the Deccan traps in the making, and from when to when? All before the collision of the Indian plate with Asia?
ReplyDeleteThanks Theo- the magma layers are not 40 km deep. Magma originated at depths greater than 40 km and ascended through the crust to erupt at the surface. The lava pile is 1-2 km thick. The eruptions lasted from ~ 68 million years ago to ~ 63 million years ago, although the bulk of the Deccan Traps erupted in a 'shorter' span of about 300 thousand years between 66 to 65 million years ago. Yes, all this before the collision of the Indian plate with Asia.
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