I. Archaean specificities
A. What ?
The Archaean = the oldest menstruation of Earth’s history for which we’ve got straight geological evidences. It extends from the oldest known rocks (today: 4.04 Ga Acasta gneisses inwards the Slave Province, Canada, addition older zircons crystals inwards younger sediments, 4.4 Ga), to the Grat Dyke of
Zimbabwe (ca. 2.5 Ga). It so represents 1/3 to 1/2 of Earth’s history.
B. Where ?
The oldest Archaean terranes
– The Acasta gneisses inwards the Slave province are simply large enclaves inwards much younger rocks;
– The oldest coherent Archaean block: SW Greenland (Isua Greenstone belt, Amitsoq gneisses)
– Pilbara (NW Australia) in addition to Kaapvaal (
Barberton) cratons formed betwixt 3.5 in addition to 3.2 Ga, they are likely the side past times side oldest blocks.
Present inwards many modest blocks inwards all continents, both every bit Archaean terranes, or every bit reworked/covered provinces.
C. Why ?
Most of the continental crust (up to 75 % ?) formed during the Archaean.
Important economical resources associated amongst Archaean terranes: gilt (either primary deposits – cf mines to a greater extent than or less
Barberton: Sheba, Consort, Fairview—or reworked: Rand gold), PGE-bearing sulfides, nickel…
Interesting petrological problems: dissimilar rocks in addition to no straight bear witness of geodynamic context.
D. Geology of Archaean terranes
Three primary components:
– “grey gneisses”= to a greater extent than or less complex unit of measurement of orthogneisses,broadly of TTG (see below) composition;
– “greenstone belts”: synclines of mafic/ultramafic lavas in addition to detrical metasediments, normally metamorphosed inwards greenschist facies;
– Late to post-tectonic granites.
E. Why was the Archaean different?
Most of the Earth’s rut production comes from disintegration of radioactive nuclide; therefore, rut production decreases exponentially. In the Archaean, perhaps 2-4 times to a greater extent than rut produced than now.
Effects of higher rut production? Two end-members:
– “uniform” increment of the rut fluxes: all parts of the public are hotter. This tin number inwards hotter intra-plate situations, in addition to maybe blur the departure betwixt within plate in addition to plate boundaries situations. This would fit to a “non-plate tectonics” model. Supported past times geological evidences such every bit “dome in addition to basin” structures (
– “heterogen” increase, amongst to a greater extent than hot zones (ridges) in addition to less mutual coldness zones, maybe resulting inwards to a greater extent than numerous, smaller plates (but however plate tectonics operating).
Anyway, in that location is no consensus on the nature of Archaean tectonics – so, studying Archaean igneous rocks is to a greater extent than complicated because it is non possible to brand assumptions on the context of formation of the rocks. More careful studies, cautiously moving from observations to interpretations, are needed: a skillful illustration written report to examine our agreement of petrogenetic processes.
Two types of Archaean igneous rocks are actually dissimilar from modern associations:
They are a shape of ultramafic, magnesian lavas offset described inwards Barberton Greenstone Belt inwards 1969 (along the
). Komatii River
45-50 % SiO2 in addition to 20-25 % MgO: this is really unopen to peridotite (mantle) composition!
A. Structure of komatiite flows
Komatiites shape modest (1-5 m thick) lava flows, each amongst the same succession:
1. Typical section
From move past times 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 later an Australian grass
– Euhedral olivine flow: “suspension” of euhedral olivine crystals inwards a fine grained matrix
– Chilled base: Emplacement breccias, similar inwards all normal lava flows.
2. Emplacement of komatiitic flows
– Emplaced inwards subaquatic situations (they shape pillows) (important implication for climate, source of life, etc.: in that location was gratis liquid H2O on Earth’s surface).
– Form lava tubes or tunnels, amongst chilled move past times in addition to bottom preserving lava stream inside
– Fast cooling results inwards of import undercooling in addition to fast growth rate, resulting inwards evolution of huge, euhedral crystals amongst detail morphology (spinifex texture).
B. Origin of komatiite lavas
– Must shape from the drape at high melt fraction (only means to larn a composition unopen to the mantle…)
– Melting must so occur at really hotness (1600-1800 °C).
This is a really high temperature, much higher than the hottest business office of present’s 24-hour interval drape (hotspots). Suggest that:
– Komatiites formed inwards drape plumes
– Archaean drape plumes were hotter than modern drape plumes
Komatiites in addition to the Archaean mantle
Several groups of komatiites tin hold upwardly defined on the base of operations of their chemical scientific discipline (Gd/Yb in addition to 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 drape occurred really early on (before 3.5 Ga) inwards Earth’s history.
Archaean gray gneisses = dominant element of Archaean continents.
Formed of to a greater extent than or less complex orthogneisses
A. The TTG series
The primary element of the gray gneisses is made of calc-alkaline granitoids (≈ I-types), amongst some differences from modern I-type plutons:
– They are rich inwards Na, in addition to consequently inwards sodic plagioclage (albite) (whereas modern I-types are rich inwards K in addition to K-Spar). They so are made of tonalites, trondhjemites (leuco-tonalites, quartz bearing) in addition to granodiorites – so the advert TTG.
– They convey feature line elements signatures, marked amongst relatively depression Y or HREE contents (elements amongst rigid affinity for garnet), in addition to corresponding high La/Yb or Sr/Y. La/Yb vs. Yb or Sr/Y vs. Y diagrams clearly demo the difference.
B. Origin of TTG magmas
Sodic, intermediate magmas must hold upwardly formed from a plagioclase-rich source; experimental studies demo that the nearly probable source is amphibolite (metamorphosed basalt).
Melting is produced past times dehydration-melting of amphibole: Amp + Pg = Melt + Grt/Opx; quite similar to the reactions that shape granites (Bt+Pg = 1000 + Crd/Grt).
Low Y in addition to Yb imply that Garnet was introduce inwards the residuum, in addition to so pressures must convey been higher upwardly 10-12 kbar.
C. Geodynamic context
Two end-members (cf. give-and-take on Archaean tectonics above):
– Melting of the subducted slab
– Melting of the base of operations of a thick crust –either a thick oceanic plateau or underplated basalts inwards a subduction zone (cf. give-and-take on adakites/sodic plutons inwards modern subductions).
D. Some lines of query in addition to debate
1. TTGs in addition to adakites
Are TTGs in addition to adakites similar? Some intend yes, some intend no.
– If they are: Adakites tin hold upwardly used every bit an indicator of the site of TTG formation, but…
• Are the adakites formed every bit slab melts
• .. Or every bit melts of underplated basalts (Cordilera Blanca)?
– If they are not: they however are rather similar, how to explicate this?
It seems that TTGs younger than 3.0 Ga are rather similar to adakites, but older TTGs are to a greater extent than dissimilar – the primary departure is Mg, Ni, Cr contents.
2. Interactions betwixt TTGs in addition to drape wedge
TTGs are typically Mg, Ni in addition to Cr richer than experimental melts, suggesting some variety of “secondary” enrichement. This could hold upwardly due to interaction betwixt TTG liquids in addition to ultramafic drape wedge during magma ascent, in addition to this implies a geometry amongst the basaltic source located below peridotites. Can this hold upwardly achieved inwards a province of affairs other than a subducted slab?
Increasing Mg Ni Cr contents inwards TTGs from 3.5 to 2.5 Ga advise that the magnitude of these interactions increment from 3.5 to 2.5
Ga. This is consistent amongst progressive cooling of the public in addition to progressively deeper melting, until the public becomes to mutual coldness to permit melting (at the destination of the Archaean), except inwards some specific situations (adakites).
Other stone types (“sanukitoids”) are known, that besides demo of import TTG-mantle interactions, ane time again suggesting melting of a basaltic slab located below peridotites.