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Transcript
Chapter 4. TTG & Genesis of
the Early Continental Crust
Introduction
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)
Highly
deformed
greenstone
- granite
terrain
Amphibolites and TTG
gneiss (“grey gneiss”)
TTG’s - how do they look like?
灰色片麻岩
TTG gneiss intruded by late
granite (highly deformed)
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 definition
TTG = siliceous rocks, with
SiO2 = 65 - 75%
Al2O3 >15% (SiO2 ≤ 70%), <14% (SiO2 ≥ 70%)
(FeOT + MgO) < 3.4%
FeOT/MgO = 2 - 3
CaO = 1.5 - 3%
Na2O = 4 - 5.5%
K2O ≤ 2%; Na2O/K2O >1
at SiO2 = 70%,
if Al2O3 >15%, then called high-Al TTG
if 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
cool mantlewarm crust
cool mantlecool crust
warm
mantle cool crust
warm
mantlewarm 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 rocks
Adakite (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
TTGADK
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 Ga
grey diamond - TTG 3.0-3.5 Ga
white diamond - TTG >3.5 Ga
open circle - Archean sanukitoids
soild 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?