TTG TTGs how do they look like TTG

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TTG

TTG

TTGs - how do they look like? TTG gneiss intruded by late granite (highly

TTGs - how do they look like? TTG gneiss intruded by late granite (highly deformed)

A piece of grey gneiss from the Narryer Complex, Yilgarn Craton

A piece of grey gneiss from the Narryer Complex, Yilgarn Craton

Highly deformed greenstone - granite terrain Amphibolites and TTG gneiss (“grey gneiss”)

Highly deformed greenstone - granite terrain Amphibolites and TTG gneiss (“grey gneiss”)

What are TTGs? TTG is an acronym for “tonalitetrondhjemite-granodiorite”. These granitoids are a typical

What are TTGs? TTG is an acronym for “tonalitetrondhjemite-granodiorite”. These granitoids are a typical feature of all Archean terrains.

Trondhjemite = leuotonalite Q Q = Quartz A= Alkali-feldspar P= Plagioclase IUGS Classification Based

Trondhjemite = leuotonalite Q Q = Quartz A= Alkali-feldspar P= Plagioclase IUGS Classification Based on Modal Composition What is Mode? ? A P

Normative vs Modal mineralogy

Normative vs Modal mineralogy

TTG classification O’Connor’s classification - based on normative feldspar composition (valid for rocks with

TTG classification O’Connor’s classification - based on normative feldspar composition (valid for rocks with normative Q ≥ 10%)

K-Na-Ca plots Martin et al. , 2005

K-Na-Ca plots Martin et al. , 2005

Chemical definition TTG = siliceous rocks, with Si. O 2 = 65 - 75%

Chemical definition TTG = siliceous rocks, with Si. O 2 = 65 - 75% Al 2 O 3 >15% (Si. O 2 ≤ 70%), <14% (Si. O 2 ≥ 70%) (Fe. OT + Mg. O) < 3. 4% Fe. OT/Mg. O = 2 - 3 Ca. O = 1. 5 - 3% Na 2 O = 4 - 5. 5% (SODIC) K 2 O ≤ 2%; Na 2 O/K 2 O >1 at Si. O 2 = 70%, if Al 2 O 3 >15%, then called high-Al TTG if Al 2 O 3 <15%, then called low-Al TTG

REE patterns highly fractionated with HREE depletion; commonly no Eu anomaly; negative Nb-P-Ti anomaly

REE patterns highly fractionated with HREE depletion; commonly no Eu anomaly; negative Nb-P-Ti anomaly in spidergrams

Melting in subduction zones

Melting in subduction zones

Conclusion • TTG = sodic granitoids, low K/Na, high Na/K, Strongly fractionated REE patterns,

Conclusion • TTG = sodic granitoids, low K/Na, high Na/K, Strongly fractionated REE patterns, depleted HREE = garnet in residue • Most acceptable model = partial melting of basaltic crust in which garnet and/or amphibole are residual phases • Subduction zone, partial melting of subducted slab

Conclusions TTG is an acronym for “tonalite-trondhjemitegranodiorite”. These granitoids, together with greenstone belts, are

Conclusions TTG is an acronym for “tonalite-trondhjemitegranodiorite”. These granitoids, together with greenstone belts, are a typical feature of all Archean terrains. In addition, a rarer feature of Archean terrains is the occurrence of high-Mg diorite-tonalite-granodiorite rocks, collectively called “sanukitoids”. TTG’s, sanukitoids and modern adakites have something in common. They are all interpreted to represent to have a direct or indirect link to partial melting of basaltic protolith, and implicitly related to some form of plate subduction.

Question • Why TTG confined in the Archaean? • Why not found in the

Question • Why TTG confined in the Archaean? • Why not found in the post Archaean?

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