Field Photos: Dikes And Gneiss

My friends have been traveling across India and sending me pictures of landscapes and rocks. I am doing field work vicariously.

Posting below a few pictures that I have received.

Dikes Intruding Bundelkhand Gneiss. Pictures by Rajesh Sarde.

These two photos were taken at the Ken River gorge in Madhya Pradesh, near a gharial sanctuary. 

The dark rock making up the floor of the gorge is a dike. It has intruded the pink colored gneiss rock. Notice that the gneiss is fractured. Intrusions follow major weak zones in the gneiss.  Being softer than the gneiss, erosion over time has removed much of the dike, forming a narrow valley.

And in this picture, an arm of the dike known as an apophysis can been seen. It is almost at right angles to the gorge.

The Bundelkhand Gneiss ranges in age between 3.2 billion to about 2.5 billion years ago. The mafic dikes, igneous rocks rich in iron and magnesium silicate minerals, intruded later into the granite gneiss. Recent geochronological work on the dikes suggest two distinct events of dike emplacement, an early episode dated to about 2 billion years ago, and a later one at 1.1 billion years ago. Interestingly, the magnetic signatures frozen in these dikes have been used to make inferences about paleogeography. The results indicate that the north and south Indian crustal blocks which had independent origins were in close proximity by about 2.5 billion years ago. 

The magnetic signatures of the 1.1 billion year old dikes throw up a puzzle. They match those preserved in the Upper Vindhyan strata and intrusive rocks, seemingly constraining the age of the Upper Vindhyan sequence to around 1 billion years. However, recent fossil finds which I wrote about in a recent article for Nature India point to the Upper Vindhyans being much younger, about 550 million years old!

Dikes Intruding the Deccan Traps. Pictures by Rajesh Sarde.

These two photos were taken at the base of Tamhini Ghat, west of Pune, near a popular trekking spot known as Plus Valley. The rocks are about 66-65 million years old.

As in the previous example, the dike has eroded away faster than the host rock forming a narrow depression. Notice the closely spaced jointing pattern or cracks in the dike. 

 In this photo, the sharp boundary between the dike and the basalt rock can be clearly seen.

 Tonalite Trondhjemite Gneiss, Palolem Beach, Goa. Picture by Aneeha. 

These rocks, abbreviated as TTG, are relicts of early continental crust. They are about 3.4-3.2 billion years old. They represent Archaean age magmatism that formed the lighter continental crust. Such TTG's  are found all across India. They are the oldest component of cratons, the nucleus of the first continents. These magmas are generally granodiorites, rich in sodium and calcium feldspars and poor in potassium feldspars. They were deformed and metamorphosed subsequently in to a gneiss, in the process acquiring a characteristic banding. 

Next time hopefully pictures from my own field trips!

Field Photos: Dikes At Korlai, India West Coast

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.