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Earth Matters: Geology of Guyana

Earth Matters: Geology of Guyana

The Trans Amazonian Orogeny
In the Lower Proterozoic between 1.9 and 2.1 billion years ago, a geological event occurred which led to severe structural deformation of the oceanic crust in the northern geological province parts of Guyana which were later preserved as the three greenstone belts.
At that time two continental plates of Archean age (2.5 billion years old) began to converge toward each other on a collision course with most of Guyana, Suriname French Guiana occurring as the ancient oceanic crust between the two plates.
As the two behemoths  one from the north west (the Venezuelan Archean plate) and the other from the south east (the Brazil Archean plate) reached closer to collision the softer oceanic crust in the northern province in between began to buckle.
Under the force of a relentless compression from the two converging plates, the oceanic crusts of the greenstone belts folded and became deformed and began to breakdown and sink downwards and were sub-ducted.
These disruptive movements, frictions and melting of the oceanic crust allowed immense quantities of hot liquid (magma) deep in the interior of the earth, to escape to the surface.
Some of the escaping magma cooled just under the crust to form plutonic (underground) rocks.
Some oozed up to the surface and spread laterally as lava beds and some exploded violently upwards and outwards as volcanoes.
The plutons and the volcanic rocks on the surface thickened the oceanic crust even more.

When the two slow moving continental plates  eventually collided trillions of tons of rocks at their boundaries crumpled upwards just like the fronts of two vehicles involved in a head on collision.
The two giant land masses braced hard against each other for quite a while: make that tens of millions of years.
In the ongoing collision, the relentless pressure and friction on the oceanic crust generated red hot rocks at the edges of the tectonic plates.
These rocks broke off cooled and became granite.
Finally one of the continental plates slid under the other.
This process caused the thickness of the crust to be doubled in the collision zone.
In addition, the intense magmatic activity heated the beleaguered crust causing it to become less dense, more buoyant and then under pressure from the ongoing collision, to rise to form mountains.
Thus, the greenstone belts of northern Guyana which  were formed  by volcanoes on the oceanic crust, between 2.3 and 2.2 billion years ago, over a period of a hundred million odd years became part of a large mountain range (or orogenic belt) caused by the pressure of convergent tectonic plates.
Not all of the greenstone belts were sub-ducted.
They are at present contained between the granitic bodies of the Northern Geological province.
Geologists refer to the event which caused the transformation as the Trans Amazonian Orogeny.
Orogeny , as explained in a previous article, being a convergent tectonic plates process causing intense folding and deformation of rocks at plate boundaries, crustal thickening, uplift of the crust and deep plutonic or underground rock forming activity and mountain building activity.
That orogeny is also referred to as the Trans Amazonian Tectonothermal Episode (TATE).
The early Proterozoic TATE was a major collisional orogeny which shaped the geology of Guyana as well as other countries within the Guiana shield.
This upheaval impacted on the topography of not only the Northern province of Guyana but the Southern Province as well..
The granitic rocks which intrude the modern landscape of the greenstone belts and can be found in many other areas of Guyana were all formed during the TATE.
The large Omai gold mine and many smaller gold deposits now found in greenstone belts were formed in the dying stages of the TATE.
The early Proterozoic TATE was followed by two other massive compressional episodes shaping the local landscape, namely a reactivation of the TATE around 1.8 billion years ago and the Nickerian Orogeny dated between 1.1 and 1.3 billion years ago.
The reactivation period of the Trans Amazonian orogeny, at approximately 1.8 billion years ago led to the formation of the Essequibo-Corentyne and Southern Guyana Complexes of granitoid rocks and associated Kanuku and Kwitaro gneisses( gneisses being metamorphic rocks formed from formerly sedimentary and igneous rocks).

This reactivation signified the end of the Trans Amazonian event .
There has been no known recurrence of such tectonic plate activity since then.
Approximately 20% of the Iwokrama Forest reserve is formed on a buried base of granitoids (granites and associated coarse-grained, silica-rich rocks and of felsic volcanic rocks)  formed during the TATE of the late Proterozoic age (1.8 billion years ago.).
Like Guyana, most of Suriname and French Guiana rest atop granitic rocks formed during the TATE of two billion years ago.
And what of the greenstone beltmountains pushed up by this orogeny?
Most were torn down by the relentless forces of erosion and weathering.

NEXT WEEK : Northern Province cont/d.

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