Archean Magmatism
I. Archaean specificities
A.What ?
The Archaean = the oldest period of Earth’s history for
which we’ve got direct geological evidences. It extends from the oldest known
rocks (today: 4.04 Ga Acasta gneisses in the Slave Province, Canada, plus older
zircons crystals in younger sediments, 4.4 Ga), to the Grat Dyke of Zimbabwe
(ca. 2.5 Ga). It therefore represents 1/3 to 1/2 of Earth’s history.
B. Where ?
The oldest Archaean terranes
- The Acasta gneisses in the Slave
province are just big enclaves in much younger rocks;
- The oldest coherent Archaean
block: SW Greenland (Isua Greenstone belt, Amitsoq gneisses)
- Pilbara (NW Australia) and
Kaapvaal (Barberton )
cratons formed between 3.5 and 3.2 Ga, they are probably the next oldest
blocks.
Present in many small blocks in all continents, both as
Archaean terranes, or as reworked/covered provinces.
C. Why ?
Most of the continental crust (up to 75 % ?) formed during
the Archaean.
Important economic resources associated with Archaean
terranes: gold (either primary deposits – cf mines around Barberton :
Sheba , Consort, Fairview —or reworked: Rand
gold), PGE-bearing sulfides, nickel…
Interesting petrological problems: different rocks and no
direct evidence of geodynamic context.
D. Geology of Archaean terranes
Three main components:
- “grey gneisses”= more or less
complex unit of orthogneisses,broadly of TTG (see below) composition;
- “greenstone belts”: synclines of
mafic/ultramafic lavas and detrical metasediments, commonly metamorphosed in
greenschist facies;
-
Late to post-tectonic granites.
E. Why was the Archaean different?
Most of the Earth’s heat production comes from
disintegration of radioactive nuclide; therefore, heat production decreases
exponentially. In the Archaean, possibly 2-4 times more heat produced than now.
Effects of higher heat production? Two end-members:
- “uniform” increase of the heat
fluxes: all parts of the Earth are hotter. This can result in hotter
intra-plate situations, and maybe blur the difference between within plate and
plate boundaries situations. This would correspond to a “non-plate tectonics”
model. Supported by geological evidences such as “dome and basin” structures (Zimbabwe ,
Pilbara)/
- “heterogen” increase, with more
hot zones (ridges) and less cold zones, maybe resulting in more numerous,
smaller plates (but still plate tectonics operating).
Anyway, there is no consensus on the nature of Archaean
tectonics – so, studying Archaean igneous rocks is more complicated because it
is not possible to make assumptions on the context of formation of the rocks.
More careful studies, cautiously moving from observations to interpretations,
are needed: a good case study to test our understanding of petrogenetic
processes.
Two types of Archaean igneous rocks are really different
from modern associations:
- komatiites
- TTGs
II. Komatiites
They are a class of ultramafic, magnesian lavas first
described in Barberton Greenstone Belt in 1969 (along the Komatii River
45-50 % SiO2 and 20-25 % MgO: this is very close to
peridotite (mantle) composition!
A. Structure of komatiite flows
Komatiites form small (1-5 m thick) lava flows, each with
the same succession:
1. Typical section
From top to bottom, komatiites are made of
-
Chilled top: Pillows and/or
breccias, glassy, subaquatic quenching.
-
Spinifex olivine texture: Large
needles of olivine, growing from the top. Named after an Australian grass
-
Euhedral olivine flow: “suspension”
of euhedral olivine crystals in a fine grained matrix
-
Chilled base: Emplacement
breccias, like in all normal lava flows.
2.Emplacement of komatiitic flows
-
Emplaced in subaquatic situations
(they form pillows) (important implication for climate, origin of life, etc.:
there was free liquid water on Earth’s surface).
-
Form lava tubes or tunnels, with
chilled top and bottom preserving lava flow inside
-
Fast cooling results in important
undercooling and fast growth rate, resulting in development of huge, euhedral
crystals with particular morphology (spinifex texture).
B. Origin of komatiite lavas
-Must form from the mantle at high
melt fraction (only way to get a composition close to the mantle…)
-Melting must therefore occur at
very high temperature (1600-1800 °C).
This is a very high temperature, much higher than the
hottest part of present’s day mantle (hotspots). Suggest that:
- Komatiites formed in mantle plumes
- Archaean mantle plumes were hotter
than modern mantle plumes
Komatiites and the Archaean mantle
Several groups of komatiites can be defined on the base of
their chemistry (Gd/Yb and Ca/Al ratios). This corresponds to (1) komatiites
formed from the “primitive” mantle; (2) komatiites formed from “non-primitive”
mantle.
This shows that differenciation of the mantle occurred very
early (before 3.5 Ga) in Earth’s history.
III. TTGs
Archaean grey gneisses = dominant component of Archaean continents.
Formed of more or less complex orthogneisses
A. The TTG series
The main component of the grey gneisses is made of
calc-alkaline granitoids (≈ I-types), with some differences from modern I-type
plutons:
- They are rich in Na, and
consequently in sodic plagioclage (albite) (whereas modern I-types are rich in
K and K-Spar). They therefore are made of tonalites, trondhjemites
(leuco-tonalites, quartz bearing) and granodiorites – therefore the name TTG.
- They have characteristic trace
elements signatures, marked with relatively low Y or HREE contents (elements
with strong affinity for garnet), and corresponding high La/Yb or Sr/Y. La/Yb
vs. Yb or Sr/Y vs. Y diagrams clearly show the difference.
B.Origin of TTG magmas
Sodic, intermediate magmas must be formed from a
plagioclase-rich source; experimental studies show that the most likely source
is amphibolite (metamorphosed basalt).
Melting is produced by dehydration-melting of amphibole: Amp
+ Pg = Melt + Grt/Opx; quite similar to
the reactions that form granites (Bt+Pg = M + Crd/Grt).
Low Y and Yb imply that Garnet was present in the residuum,
so pressures must have been above 10-12 kbar.
C. Geodynamic context
Two end-members (cf. discussion on Archaean tectonics
above):
- Melting of the subducted slab
- Melting of the base of a thick
crust –either a thick oceanic plateau or underplated basalts in a subduction
zone (cf. discussion on adakites/sodic plutons in modern subductions).
D.
Some lines of research and
debate
Are TTGs and adakites similar? Some think yes, some think
no.
– If they are: Adakites can be used as an indicator of the site of
TTG formation, but…
•
Are the adakites formed as slab
melts
•
.. Or as melts of underplated
basalts (Cordilera Blanca)?
– If they are not: they still are rather similar, how to explain
this?
It seems that TTGs younger than 3.0 Ga are rather similar to
adakites, but older TTGs are more different – the main difference is Mg, Ni, Cr
contents.
2. Interactions between TTGs and mantle wedge
TTGs are typically Mg, Ni and Cr richer than experimental
melts, suggesting some sort of “secondary” enrichement. This could be due to
interaction between TTG liquids and ultramafic mantle wedge during magma
ascent, and this implies a geometry with the basaltic source located below
peridotites. Can this be achieved in a situation other than a subducted slab?
Increasing Mg Ni Cr contents in TTGs from 3.5 to 2.5 Ga
suggest that the magnitude of these interactions increase from 3.5 to 2.5 Ga. This is consistent
with progressive cooling of the Earth and progressively deeper melting, until
the Earth becomes to cold to allow melting (at the end of the Archaean), except
in some specific situations (adakites).
Other rock types (“sanukitoids”) are known, that also show
important TTG-mantle interactions, again suggesting melting of a basaltic slab
located below peridotites.
See also