Remotely India: Bundelkhand Mafic Dikes and Quartz Veins

Remotely India #14

Do you see anything striking (pun intended) about this geologic map of the Bundelkhand craton?

Source: Sarada Mohanty 2023- Proterozoic Basins of the Bundelkhand Craton

Notice that the green lines are predominantly oriented in a NW-SE direction. The pink lines are predominantly striking NE-SW. These are magmatic and fluid intrusions into the Bundelkhand granitic crust. The green lines represent mafic dikes (Mg and Fe rich basaltic magma), and the pink lines represent quartz veins. 

The Bundelkhand craton is an Archean age block of continental crust. Like other Archean age terrains, it has a long history of magmatism, volcanism, and sedimentation. The oldest rocks, a suite of granitic rocks going by the term 'tonalite–trondhjemite–granodiorite', and associated volcanics and chemical sediments are as old as 3.4 billion years. Through the Archean the crust grew by repeated injections of magma. Voluminous magmatism petered out by around 2. 4 billion years ago with the formation of the Bundelkhand granodiorite batholith, an enormous subsurface body of congealed magma. Granodiorite is a calcium feldspar bearing variant of granite. This younger rock type covers most of the surface area of this terrain.

Geologic activity continued for hundred of millions of years after the emplacement of this batholith with the intrusion of these impressive dike swarms and quartz vein clusters.

Staying true to the objective of this series on Indian geology as seen from satellite imagery, the emphasis here will be on the field features of these intrusive bodies.

Giant Quartz Veins:

Locality- Northeast of Mauranipur, Uttar Pradesh.

The quartz vein stands out as a high long ridge. Steep sided blocks of quartz make up the spine of the ridge. Weathered boulders shed from the quartz vein have formed the surrounding slopes. This distinctive landform is instantly recognizable in the imagery as you explore this region.  

Locality: Southeast of Mohangarh, Madhya Pradesh.

Here you can observe the intrusive relationship between the giant quartz vein and the older Bundelkhand granite (BG) which crops up as low hills made up of a light toned fractured rock. The linear vein can be traced cutting across the host rock.

Locality: Southeast of Mauranipur, Uttar Pradesh.

At this location you can observe an unusual feature. Two quartz veins have split to form a tuning fork shaped geomorphic feature.

These quartz veins intruded the crust around 2.15 to 2 billion years ago. The quartz crystals contain bubbles of gas and minuscule amounts of liquid trapped inside them. They inform us about the temperature and pressure during precipitation of the crystals and about the salinity of the fluid. There are also tiny crystals of other hydrous minerals like chlorite and epidote found inside the quartz. These reveal the source of the fluid. Such studies conducted by Duttanjali Rout and colleagues identify two distinct sources of fluids involved in the formation of these veins. A hot moderate salinity fluid derived from the Bundelkhand granodiorite mixed with meteoric water percolation downwards through fractures. The deeper fluids were sourced from not more than 5 km in the subsurface.

A drop in the temperature and pressure of the rising silica saturated fluid as it encountered the colder meteoric water resulted in decrease of silica solubility and the precipitation of quartz. The giant quartz veins are the product of a vigorous Proterozoic geothermal system that lasted tens of millions of years. The researchers have drawn a comparison with Broadlands-Ohaaki geothermal system in Northland, New Zealand, and the Kakkonda geothermal system in NE Japan. Both are in granitic terrains and could be loose analogs for the processes in operation during the formation of the Bundelkhand quartz veins.  

There are differences in what we can observe in these ancient and modern systems. In the Proterozoic example, the surface expression of the silica rich geothermal system, the hot springs and geysers, have long since eroded away. We can study only the subsurface plumbing system. In the modern settings, the surface processes are apparent and the underground patterns of fluid flow have to be inferred. 

Mafic Dikes:

Locality- Northeast of Lalitput, Uttar Pradesh,

A NNW-SSE trending dike is exposed near Tera village. The surface expression of mafic dikes is very different from the quartz veins. The dikes weather away faster and are exposed as low relief hills with extensive boulder fields derived from the weathering of the dolerite rock. In the satellite imagery, you can see the dark toned nature of the boulders hinting at its mafic composition. Due to the spread of boulders around the dike, the width of the intrusion appears far more that its true width. 

Locality- Mahoba , Uttar Pradesh

An ENE-WSW trending mafic dike is surrounded by Mahoba town. As with the NW-SE trending cluster, these E-W trending intrusions also appear as dark toned low relief boulder strewn hills.

Locality- Mahoba, Uttar Pradesh.

This is a synoptic view of the E-W trending dike, captured by ISRO Cartosat. The white rectangle in the lower left of the image is the bounding area covered by the previous imagery. It is quite an extensive intrusion, and to the eastern end it can be seen cutting across outcrops of the Bundelkhand granite. 

Geochronologic work on these mafic dikes shows that the NW-SE trending dike swarm intruded around 1.9 to 1.8 billion years ago. The E-W trending group of dikes are much younger, dated to about 1.1 billion years ago. 

The geochemistry of these dikes point to an upper mantle source of the magma. The dikes are a variety of thoeliitic basalt, not too much different from the basalts of the Deccan Traps in Maharashtra. Unlike the shallow sourced fluid of the quartz veins, the source magma of the dikes was generated at least 50 km down in the mantle lithosphere.

The crisscrossing lines you see on a geologic map of the Bundelkhand craton are a record of geologic activity that continued long after voluminous granitic magmatism ended. In rare exposures, mafic dikes are seen cutting across quartz veins, indicating that they are the younger of the intrusives. Most of the geochronology data collected so far supports this field observation. Studies of the spatial patterns of the dikes and quartz veins too hint that they represent two independent deformation events. The formation of both these systems required extensive fracturing and faulting  of the crust by extensional forces. Geologists are still working out the reasons for these crustal disturbances. 

In the case of the quartz veins, the fracture systems tapped relatively shallow sources of heat and fluids. In the subsequent reactivation of the crust, much deeper fracture systems cutting across the crust tapped upper mantle sources of heat,  providing conduits for the passage of mafic magma to shallower crustal levels. 

These deep crust penetrating fractures and Proterozoic mafic dike swarms tell another story about the strength of the crust and the advent of plate tectonics, but that is fuel for another post!

I am having fun resurrecting my Remotely India series. Stay in touch for more explorations of Indian geology on this blog.

Ganga Earthquake, Nile, Deep Sea Habitats

Some readings over the past few weeks:

1) An Earthquake Changed the Course of the Ganges. Could It Happen Again?  Sediment load carried by big rivers like the Ganga often choke up channels and force the river to cut another path. Sudden channel shifts can also occur due to tectonic movements. A recent survey of the Ganga about 100 km south of Dhaka, Bangladesh , identified an old channel of the Ganga. Exploring this area, researchers came across veins of sand cutting across the sediment layers. These veins or sand dikes were  injected into the surrounding sediment. They are a sign of major ground trembling triggered by a large earthquake. Ground motion may pressurize buried sand and inject it  upwards. Further studies showed that this event may have been a 7-8 magnitude earthquake that occurred 2500 years ago. Kevin Krajick writes about this discovery and the larger geologic context.

2)  Lessons from the Nile about rivers and society. The Nile river has sustained agriculture, human habitation, and royal dynasties for millennia. A Nature Geoscience editorial summarizes a collection of sedimentological and geomorphologic studies that track the evolution of the Nile through the Holocene. Phases of the river cutting a deep valley changed to phases of the river flooding laterally and building fertile floodplains. This geomorphological evolution driven by climate change and more recent dam building have influenced agriculture and social systems in the past as well as the present. 

3) Ocean Exploration: Meet The Deep. I highly recommend this site. National Ocean and Atmospheric Administration, U.S.A., has compiled some stunning photographs and videos of deep sea habitats and the diverse forms of life that inhabit this little understood world. Corals, sponges, brittle stars, molluscs, worm tubes- attached to the bottom, living around energy sources such as hydrothermal vents and cold seeps. It is an absolutely fascinating exploration of the biodiversity of the deep sea. 

Octocoral and Brittle Star: Source NOAA Ocean Exploration

As a bonus, these images make great wallpaper for your mobile phone!