This slide is about how the crust is formed during the Archean time.

1. ARCHEAN CRUSTAL BUILDING
PROCESS
SUBMITTED BY SUBMITTED TO
SURAJ KUMAR SAHU DR. RAHUL MUKHERJEE
REG. NO.- 22MSCEGS08 DEPTT. OF GEOLOGY
CENTRAL UNIVERSITY OF PUNJAB

2. CONTENTS
1. What before archean ?
2. What is archean ?
3. Examples of archean rocks.
4. How did these nuclei evolve ?
5. Greenstone belt classification
6. Events involved in archean crust building process
7. References

3. WHAT BEFORE ARCHEAN ?
• Before Archean, Hadean eon (4.5-4.0 Ga) is there.
• There’s hardly any proof left that indicates about Hadean except few.
• So how do we come to know about the hadean eon ?
• Indicated by the zircon.
• Zircon – oldest terrestrial material on the earth.
• Two evidences indicating about the Hadean Eon –
1. Zircon in metamorphosed sandstone in the Jack Hill, Australia- 4.404±0.008By.
2. Acasta Gneiss in Slave Craton, Canada – 4.031± 0.003By.

4. WHAT IS ARCHEAN ?
• A time period in the Geological Time Scale.
• Archean ranges from 4.0 to 2.5 By.
• Archean Eon consists of 4 Eras:
1. Eoarchean Era- 4.0 to 3.6 By.
2. Paleoarchean Era- 3.6 to 3.2 By.
3. Mesoarchean Era- 3.2 to 2.8 By.
4. Neoarchean Era- 2.8 to 2.5 By.
Source:- From the geological time scale v
6.0

5. EXAMPLES OF ARCHEAN ROCKS
• Most continental crusts have the basement- archean.
• Crust building processes- liquid earth to solid earth.
• Study the rock types – tectonics operated in the Archean.
• Some examples of the Archean rocks:
1. In India, Gorur Gneiss in dharwar craton- 3.4-3.6by, some also in bastar craton,
singhbhum craton, etc.
2. Barberton hills, Kaapvaal craton in Africa – around 3.6 by.
3. Yilgarn (2.8by) and Pilbara (3.8-2.83by) cratons in Australia.
4. Baltic (3.1by) Craton in Russia.
5. Hudson Craton in Canada.

6. THESE ARE SOME INDIAN EXAMPLES OF THE
ARCHEAN ROCKS.
Source:- Simplified geological map of India showing
the Archaean cratons, Proterozoic mobile belts,
sedimentary basins and major Precambrian metallic
resources, modified after Ramakrishnan &
Vaidyanadhan (2008) and Sharma (2009). Available
radiometric ages in the Aravalli– Bundelkhand (a,
Weidenbeck & Goswami (1994), Roy & Kröner
(1996), Weidenbeck et al. (1996); b, Mondal et al.
(2002)) and Bastar (c) Rajesh et al. (2009) cratons
and the SSZ (d, Pal & Rhede (2013)) are also
indicated in boxes. ADFB, Aravalli Delhi Fold Belt;
CB, Cuddapah Basin; CGC, Chotanagpur Gneissic
Complex; CHB, Chhattisgarh Basin; CSZ; Chitradurga
Shear Zone; EDC, Eastern Dharwar Craton; IB,
Indravati Basin; MB, mobile belts; NSMB, North
Singhbhum Mobile Belt; PCSZ, Palghat–Cauvery
Shear Zone; SSZ, Singhbhum Shear Zone; VB,
Vindhyan Basin; WDC, Western Dharwar Craton.
4 cratons collectively called as archean
nuclei.

7. HOW DID THESE NUCLEI EVOLVE ?
• To explain the evolution (crustal building process) of archean – dharwar craton.
• Look in the dharwar craton- mostly igneous rocks- mostly basalt, mafic and
ultramafic rocks abundantly present- spinifex texture predominates.
• No sedimentary rock of archean age would be found in the dharwar craton- no
thick crust.
• To understand it, know about some rocks:- Peninsular Gneiss, TTG, Greenstone-
schist belt, Batholithic granite and charnockites.
• Before that let’s look at the map of dharwar craton-

8.  Greenstone-Schist Belt- zones of
metamorphosed mafic and ultramafic
volcanic rocks. Only found in archean
times.
 Peninsular Gneiss- actually TTG. Lots of
migmatites.
 TTG- stands for Tonalite Trondhjemite
Gneiss which is simply plag-rich Granite.
 Batholithic Granite- Granite which is in
batholithic form. Usually it is K-rich
granite.
 Charnockite- opx bearing granite.
 Cross section from EDC to CG- GB
synclinorium in nature.

9. GREENSTONE BELT CLASSIFICATION
• Greenstone belts – synclinorium in nature, both sides bounded by fractures, floating
in the PG and have conglomerates.
• On the basis of this, greenstone belts are classified into
WDC EDC
Chitradurga group (2700Ma)
BIF, greywacke, Mn-bearing, Felsic volcanics
CONGLOMERATE BEDS
Bababudhan group (3600Ma)
Metabasalt, Gabbro, Basic and ultrabasic rocks,
komatites, BIF
KOLAR GROUP- Champion Gneiss (felsic gneiss,
metabasalt, bimodal volcanics, komatites,
greywackes.
CONGLOMERATE BEDS
Peninsular Gneiss (3000Ma)
Sargur group
Ultramafic-mafic layered intrusives, komatites, BIF,
Quartzite(fuchistic) and Marble
Gorur Gneiss (TTG) (3400Ma)

10. EVENTS INVOLVED IN ARCHEAN CRUST BUILDING
PROCESS
1. Formation of Greenstone Belt
2. Development of Greenstone Belt
3. Emergence of TTG
4. Emplacement of potash rich Granite
5. Mantle degassing
6. Charnokitisation.

11. GREENSTONE BELT FORMATION
• First solidified crust formed around the earth in 4.0 By and Very thin- 3-5km – Primordial
crust. Composition- basaltic.
• This crust not preserved that much except in few places – earth very hot ~6000ºC.
• Molten mantle started to appear on the surfaces through various fractures in the crust.
• In the archean time, earth- very unstable, lots of volcanism happening back then.
• Everyday fracturing of the crust happening, hot molten lava came out and floods over the
surface – solidifies the flooded basalt- again fracturing happen- again flooding happened-
again solidified – so on.
• In this way, one flow after another flow came rapidly- in rapid succession they added up.
• Gradually, cooling happened of the top layers but the below layer still hot – sagging
happened- forms synclinorium and the layers called greenstone belt.
• Synclinorium floating in the primordial crust.
• In this way formation of greenstone belt happened.

12. DEVELOPMENT OF GREENSTONE BELT
• As we see, synclinorium type greenstone belt formed.
• Entire crust marked by many such depressions over the PC.
• Archean unstable- fracturing happened near these depressions- again flooding happened
but this time unable to make flood due to these depression and then settled into these
depressions.
• In this way, successive phase of greenstone eruptions took place and seat over the
previously formed layers.
• In another way greenstone belt can also form-
Fracturing can happen within the synclinorium and greenstone came out from this fracture
and seat over the previously formed greenstone belt.
• These depressions hold water- sediments like chert, greywacke, limestone and BIF formed.
• Similarly in the dharwar craton,
Sargur developed in 3.4 by followed by bababudhan in 3.0 by and lastly formed chitradurga
in 2.9 by.
• Younger greenstone belts- more sedimentary dominant and the older greenstone belts-
more komatiite dominant.

13. EMERGENCE OF TTG
• Each stages of fracturing produces new greenstone belt- added up and becoming
thick. This is called the accretion of greenstone belt. (5km to15km)
• Thickening of the crust induced melting of the basal part of the greenstone belt.
• This produced the Granite known as TTG which come in batholithic form.
• This TTG may undergo fracturing (following the fundamental fractures)and form
greenstone over the TTG.
• TTG become base for the younger Greenstone belt. That’s what we are seeing today
in the form of Gorur gneiss.
• Then the succession would look like this-
Greenstone belt- TTG- Greenstone belt- TTG- Greenstone belt-so on- Closepet
granite + dharwar granite+…….
• That’s why Archean tectonics are also known as Greenstone- TTG tectonics.

14. EMPLACEMENT OF POTASH GRANITE
• Along with GS belt melting, TTG- start melting to form Potash rich granite in 2.6By.
• 2.6By- mark the emplacement of potash-rich Granite.
CHARNOKITISATION
• All fractures sealed – granitic emplacement.
• Mantle degassing- process in which mantle fumes and volatile-enrich(CO2) layer
forms below the 15km crust.
• CO2 metasomatism- CO2 removes water from system and replace itself.
• H2O replaced by the CO2 transforming the hornblende and biotite into opx and
thus charnokite formed.
• Formation of charnokite- charnokitization.
• Released water – TTG- more potash granite.
• 2.7-2.5 – rapid crustal growth.

15. REFERENCES
• Geology of India, volume 1 by M. Ramakrishnan and R. Vaidyanandhan
• https://www.youtube.com/playlist?list=PLRFsXLaFH0Z_pN1Lnm1WTet3G31L2OqSg

16. THANK YOU