Thursday, November 14, 2013

What A Porphyry Copper Ore Body Tells Us About How India Was Assembled

Nature Geoscience has some interesting articles on giant magmatic ore deposits  ( 1 , 2 ) with a focus on porphyry copper- molybdenum deposits which occur within magmatic arcs above subduction zones.

Ever since I found that copy of Tyrrell I've been reminiscing a bit about my early days in geology. These papers on copper ores started another chain of thought. We were preparing for our first year M.Sc. field trip which is really supposed to be a tour to learn field mapping. So the area selected is usually one where rock bodies are exposed clearly, have lateral continuity, where relations and contacts between geological units can be observed, basically an area where principles of field mapping are relatively easy to learn. As it happens our department at Pune University had gotten a big grant from ONGC to do a reconnaissance of Gondwana rift basin sediments of Carboniferous-Permian age just north of Itarsi in Madhya Pradesh. Our department chair organized our field trip to this area, reasoning that we could use this for training as well as contribute to the project.

Unfortunately, it was a disaster. The area was thickly forested, rock exposures limited to few stream cuttings and occasional road cuts, just not what you want for a rigorous training in mapping. The one bright spot was the copper mine we visited at Malanjkhand. This is an open pit mine.

Google Interactive Map of Malanjkhand Copper Mines:

View Larger Map

We were allowed to walk right up to the exposed walls of the pit and observed the stringers of copper and molybdenum sulphide ore embedded in networks of quartz veins. The host rock was a granodiorite. It was altered to various clay assemblages but you could make out blobs of relatively unaltered textures. Overall, after two weeks of tramping through forests it was great to be looking at massive walls of rock and glistening ore!

Ok, so what does this copper ore body have to do with ideas of how India was assembled and what does that even mean?

The Malanjkhand deposit is what is known as porphyry copper ore. This means that the ore minerals are within a rock which is made up of large crystals of quartz and feldspar surrounded by a very fine crystalline matrix. This texture occurs when magma spends time at great depths cooling slowly. During this time some minerals grow to a large size. At a point geological conditions change, a route for the magma to ascend to shallower depths opens up and the magma rises rapidly. The temperature of the magma is lowered and much of the rest of the molten material very rapidly solidifies in to a fine crystalline mesh. Ascending with the magma are gaseous components which expand as they encounter lower and lower pressures. This expansion shatters the solidified rock body into networks of cracks and fractures called a stockwork. Mineralizing fluids deposit concentration of copper, molybdenum and other economic minerals in this stockwork.

Such type of magmatism and style of ore concentrations commonly occur in subduction settings where one tectonic plate pushes against and slides under another tectonic plate. The subducting plate heats up, and may undergo some melting. The melts and associated fluids rise and cause more melting in the overlying plate. Magmas produced in these settings made up of continental crust are granitic to granodioritic in composition. The classic example today are from the Andean mountains where magmatism is occurring in the setting where the Nazca plates and the northern edges of the Antarctica plate are sliding under the South American plate.

But take a look at the location of Malanjkhand in the map below. It occurs right in the middle of India just northeast of the city of Nagpur . Did subduction take place there once?

Source: Bhowmik et al 2012

If it did would it not mean that what is now a continuous block of continental crust was earlier separate blocks that collided with each other? In fact all the geological indicators point to such a scenario not just for the Malanjkhand area but for other parts of the India as well. In the Hadean, the Archean and the early Proterozoic from about 3.0 to about 2.0 billion years ago smaller crustal blocks collided and welded together to form what is recognizable the Indian continent. There are indications that a large crustal block making up what is today India south of the Narmada formed by the joining of smaller blocks assembled separately from blocks north of the present day Narmada. Eventually these two larger blocks collided and welded  along a gigantic suture known as the Central Indian Tectonic Zone (CITZ) which roughly parallels and encompasses what is today the Narmada valley. So the structure of this ancient suture controls the flow paths and direction of the Narmada and Tapi rivers.

The Malanjkhand ore mineralization which is dated to about 2. 4 billion years ago is thought to represent one of the early encounters of assembly between the southern blocks and the northern blocks, a northward subduction of a small tectonic plate producing granodiorite magmatism and ore concentrations. In the map above the Malanjkhand mine occurs at the northern edge of the southern Bhandara craton just south of CIS or Central Indian Suture which is considered the southernmost major fault of the CITZ system.

In other areas, there are other types of geological indicators that continental assembly took place in the distant past. For example very recently a small microcontinent named the Coorg block has been discovered as an entity that collided with the larger Dharwar cratonic block. The Coorg block is made up of subduction arc typical magmatic rocks and the welding is thought to have taken place around 1.2 billion years ago (thanks @sumitakale for the tip!).  The Dharwar cratonic block is surrounded by a garland of younger potassium rich granitic rocks. These types of magmas occur during the latter stages of continental collisions as has happened in the younger Himalayan mountain chains. Another important indicator is that these continental nuclei are surrounded by deformed and metamorphosed belts of younger sedimentary and igneous rocks. Such terrains are known as mobile belts and they point to the formation of basins due to bending of the crust at the margins of older cratonic nuclei during subduction and rifting and eventually the closing of these basins during collisional events in which these rocks are metamorphosed and even partially melted.

So, the story in the southern Indian crustal regime is one of three large continental nuclei, the Karnataka or Dharwar craton, the Bastar- Bhandara craton and the Singbhum craton made up of 3.8 - 2.6 billion year old tonalite-tronjhemite gneisses assembling over the Archean and Early Proterozoic into one rigid continental block and over time colliding and suturing with a northern Bundelkhand craton to form a larger pan Indian craton. These events did not take place sequentially. Even as assembly of the southern cratons was happening, the northern edges of the southern cratons were interacting with the Bundelkhand craton nuclei which makes up the core of the northern Indian continental crust. The Malanjkhand ore deposit dated to 2.4 billion years ago situated at the northern edge of the southern Indian cratonic blocks indicates this. Eventually though a continuous rigid southern block did form perhaps by around 2 billion years or so and merged over a time period of perhaps a billion years! with the Bundelkhand craton. The broad zone of assembly is marked by the Central Indian Tectonic Zone (CITZ) that I mentioned earlier.

The CITZ is an ENE-WSW trending feature about 800 km long and 400 km wide. The map below depicts its extent, overall structural trend and the location of continental cratonic nuclei north and south of it. The internal structure of the CITZ is complex and contains geological terrains of different ages, major faults, shear zones, metamorphic belts and chains of magmatic bodies all pointing to long repeated episodes of continental assembly involving subduction, deformation, metamorphism and continent collisions. The location of Malanjkhand is marked by the letter M.

 Source: K. Naganjaneyulu  and M. Santosh 2010

The CITZ contains the following geological indicators that it represents an ancient subduction-accretion-collisional complex; 1) metamorphosed mafic and ultramafic bodies 2) metamorphosed carbonate and iron and manganese formations, 3) high pressure and high temperature metamorphic belts namely the Sausar and Satpura metamorphic mobile belts 4) tonalite-granodiorite magmatic bodies and 5)  potassium rich granites. The mafic bodies and carbonate and iron and manganese formations point to a deep sea between two continental blocks. This setting is characterized by an oceanic crust made up of mafic basalt rocks. Fine grained carbonates, chert and iron-manganese sediments accumulate on the sea floor. Eventually, this ocean crust and overlying sediments get caught up in the subduction process and gets accreted (plastered) onto the sides of the continental nuclei  and metamorphosed. Distinct temperature and pressure gradients develop along the subducting slab resulting in discrete "paired metamorphic belts" composed of a suite of either high pressure metamorphic minerals or an assemblage of high temperature minerals. These belts represent the exhumed cores of the orogens developed at roughly right angles to the direction of compressive stresses. Such metamorphic belts of which Sausar mobile belt is one example along with  tonalite granodiorite plutons representing melting of the crust also indicate a subduction-collisional setting. Potassium rich granites point to magmatism associated with the latter stages of continental assembly wherein two blocks of continental crust collide.

The superficial exposed geology of the CITZ tell us a lot about the assembly of India. But what about what lies deeper in the crust? Recently a number of geophysical surveys  has successfully peeked at the deeper structure of the lithosphere underneath the CITZ and has confirmed what the exposed rocks are telling us.

For example K. Naganjaneyulu  and M. Santosh 2010 discuss gravity, magnetotelluric and seismic reflection surveys along the CITZ in a number of N-S traverses from west to east of the CITZ. The gravity and magnetotelluric data suggests the presence of a uniformly thick layer of mafic crust under cratons on both sides of the suture. This has been interpreted as the remnants of subducted slabs, although some caution is needed as underplating of the Indian crust by mafic magma related to the late Cretaceous Deccan Basalts has also complicated the crustal structure in this region.

Seismic reflection data show a northern dip in the southern parts suggesting northward subduction of the Dharwar/Karnataka craton. Interestingly the northern Bundelkhand craton show a southerly dip indicating a southward subduction of that craton. So, there could be a double sided subduction history along the CITZ. Such a situation does exist in more modern settings.. for example in the western Pacific where the 100-200 million year old Pacific plate subducts in a SW direction while the Indo-Asian plate subduction towards the  NE.  However, researchers warn that this interpretation is tentative and will need more data from the northern parts of the CITZ to assess whether southerly reflectors can be traced well into the Bundelkhand craton. The map below summarizes the interpretations of subduction polarity from geophysical data and the analogous situation in the western Pacific. The location of Malanjkhand (c) is marked by the letter M.

Source: K. Naganjaneyulu  and M. Santosh 2010

The overall picture of the deeper crustal architecture of the CITZ is one of a long protracted history through the Archean and Early-Mid Proterozoic of smaller continental cratonic nuclei assembling through subduction, accretion (plastering of intervening slices of crust ) and collision beginning around 2.5 billion years ago, resulting in formation of younger basins and subsequently their deformation and mobile belt formation with the final continent continent collisional events occurring - based on recent data from the Sausar orogenic mobile belt-  as late as 1 billion years ago. We see India as one rigid crustal entity. It is really a complex collage that assembled over time.

Starting from one copper deposit I guess this post has gone a little overboard in my attempt to draw a larger picture. But rambling away like this helps one draw from a wide range of readings I have been doing on Indian geology lately. This picture of India too is only a small part of the larger continental assembly going on throughout the globe during the Archean and the Proterozoic. I don't want to push my luck and write about that too in this post. I just want to point out that the CITZ is considered a median segment of a much larger orogenic belt and suture system that includes the Aravalli Belt to the west and Eastern Ghat orogenic belt in India as well as the including the Vestfold Hills orogenic complex in what is now East Antarctica. This speaks of a supercontinent assembly in the Archean -Early Proterozoic of which the assembly of what is now India was one component.

Off course when I first saw the Malanjkhand copper ore body all those years ago I was not thinking of this grand story. At that time I was just glad to be out of those mosquito ridden forests and to be staring at a giant wall of solid rock and metal.

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