chapter 4. ttg & genesis of the early continental crust

Chapter 4. TTG & Genesis of the Early Continental Crust

In Archean terrains, 3 types of granitoids are found in association with greenstone belts:

(1) gneiss complex: composition TTG(2) diapiric plutons : composition TTG(3) discordant plutons (late granite intrusions):

composition granite.

Question: What are TTG’s? and the relative ages between the greenstone belts and the surrounding granitic gneisses?

Ref. - Martin et al., 2005. Lithos 79, 1-24 (and refs therein). (This is an excellent review paper)

Introduction

Highly deformed

greenstone- granite terrain

Amphibolites and TTG gneiss (“grey gneiss”)

TTG’s - how do they look like?

TTG gneiss intruded by late granite (highly deformed)

灰色片麻岩

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

TTG is an acronym for “tonalite-trondhjemite-granodiorite”. 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.

TTG classification

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

Chemical definitionTTG = siliceous rocks, with

SiO2 = 65 - 75%Al2O3 >15% (SiO2 ≤ 70%), <14% (SiO2 ≥ 70%)(FeOT + MgO) < 3.4%FeOT/MgO = 2 - 3CaO = 1.5 - 3%Na2O = 4 - 5.5%K2O ≤ 2%; Na2O/K2O >1

at SiO2 = 70%, if Al2O3 >15%, then called high-Al TTGif Al2O3 <15%, then called low-Al TTG

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

(La/Yb)N vs (Yb)N plot

Q - why this kind of plot? its physical meaning?

Model calculations

(Yb)N

Melting experiments at low pressures

Melting experiments at medium pressures

Melting experiments at high pressures

Model calculations

(Yb)N

Melting in subduction zones

warm mantle-warm crust

warm mantle -cool crust

cool mantle- cool crust

cool mantle- warm crust

Melting curves and geothermal gradients

What is the geotectonic significance? Why TTG’s of HREE depletion typically occur in the Archean?

Comparison with sanukitoids & adakitic rocksAdakite (Defant & Drummond, 1990) - adakites form suites of intermediate to felsic rocks whose compositions range from hb-andesite to dacite and rhyolite; basaltic members are lacking. The rocks have: SiO2 >56%, Na2O = 3.5-7.5%, Na2O/K2O ≈ 2.REE highly fractionated with high (La/Yb)N >10.[Sr] content high (>400 ppm), high Sr/Y ratios.2 groups of adakites (Martin & Moyen, 2003):

HSA (high silica), SiO2 >60%,LSA (low silica), SiO2 <60%.

K-Na-Ca plots

Martin et al., 2005

TTG-ADK

element plots

MgO- SiO2 & MgO-K2O plots

Archean TTG’s and modern HSA (heavy dotted lined field) and LSA (light dash lined field)

black diamond - TTG <3.0 Gagrey diamond - TTG 3.0-3.5 Gawhite diamond - TTG >3.5 Ga

open circle - Archean sanukitoidssoild circle - Closepet-type granites (MgO & K2O > TTG)

TTG & ADK spidergrams

Sanukitoids & adakites

(LSA)

Where do they occur, the adakitic rocks?

1. in subduction zones with young and warm subducting oceanic lithosphere (e.g., Adak Island, Aleutian Islands; Kay, 1978).

2. in thickened continental crust in which the lower part of the crust was basaltic in composition, probably formed by underplated magma (e.g., South Tibet; Chung et al., 2003, Geology)

South Tibet

Tibetan adakites

Q - why do we use Sr/Y vs Y plot, instead of (La/Yb)N vs YbN plot?

Conclusions

Important questions:

1. TTG and basaltic rocks (greenstones) - which came first? (perpetual chicken and egg problem?)

2. No or little Eu anomalies in TTG REE patterns, why?3. Plate tectonics, did it work in the Archean? (have you

seen the subduction geochemical signatures?)4. Heat flow in the continental crust - higher in the Archean

than in the later times? (no clear evidence from granulite facies terranes)

5. Nature of the earliest CC? anorthositic crust?