1. Crust formation in the early Earth prior to the onset of subduction
an analogue study from
the Oman ophiolite
Hugh Rollinson
Sultan Qaboos University, Oman

2. Given that
The Earth’s earliest crust was mafic, and
The Earths early continental crust was
TTG in composition

3. Given that
The Earth’s earliest crust was mafic, and
The Earths early continental crust was
TTG in composition
It is proposed that the mechanisms whereby
felsic rocks (trondhjemites) form from mafic
crust in a geologically recent oceanic
environment is a powerful analogue for the
formation of felsic crust in the Hadean and
early Archaean

4. Given that
The Earth’s earliest crust was mafic, and
The Earths early continental crust was
TTG in composition
It is proposed that the mechanisms whereby
felsic rocks (trondhjemites) form from mafic
crust in a geologically recent oceanic
environment is a powerful analogue for the
formation of felsic crust in the Hadean and
early Archaean
A consequence of this proposal is that such
crust was not created in a subduction setting

5. Historically this idea had been neglected, because
Ophiolitic trondhjemites
thought to be the product
of crystal fractionation
and so of minor importance

6. Historically this idea had been neglected, because
Ophiolitic trondhjemites
thought to be the product
of crystal fractionation
and so of minor importance
LILE and LREE depleted
Ophiolitic and Archaean
trondhjemites have
different compositions

7. So what has changed …….? New models of oceanic magma chambers
large long-lived
are out

8. So what has changed …….? 2. New experimental evidence for water
saturated partial
melting of oceanic
gabbro at low
pressures
(data from Koepke et
al. 0.2 Gpa)

9. So what has changed …….? 3. Field evidence for partial melting
- the occurrence
of hornblende gabbros
- abundant agmatites

10. So what has changed? 4. Geochemical evidence for high temperature
Elevated
Sr-isotope
ratios
4. Geochemical
evidence for
high temperature
sea water
interaction
with mafic
and felsic melts
Elevated
O-isotope
ratios
Oman amphiboles have
elevated chlorine
(Coogan 2003)
Plagioclase-amphibole temperatures C (Bosch et al 2004)
Ophiolite data from Bosch et al. (J. Pet.,2004) with Trondhjemite data from Gregory and Taylor (JGR, 1981)

11. Geochemical similarities…………
Low Sr; high Y
Oman trondhjemites overlap
Limpopo belt TTGs, and to
a lesser extent the classical TTG trend (eg Lewisian)

12. Geochemical similarities…………
Low Sr; low MgO
Similar to early Archaean
but not late Archaean TTGs
Low Sr; high Y
Oman trondhjemites overlap
Limpopo belt TTGs, and to
a lesser extent the classical TTG trend (eg Lewisian)

13. Can this work for early Archaean felsic rocks at Isua ?
(Kamber et al. 2005)
Unit B1 – includes
felsic volcanic rocks
Abundant
mafic
rocks
Enriched in LILE, K, LREE & Pb
Low Sr, Ta, Ti
NE limb of Isua Greenstone belt, W Greenland

14. So how does/did it work ? Oman Partial melting of low K
gabbro section of
oceanic crust
Access of water
(seawater)
Heat source – active
magmatism of ridge
Melting in presence of
plagioclase and amphibole
absence of garnet

15. So how does/did it work ? Oman Early Earth Partial melting of low K
gabbro section of
oceanic crust
Access of water
(seawater)
Heat source – active
magmatism of ridge
Melting in presence of
plagioclase and amphibole
absence of garnet
Early Earth
Partial melting of ?high K
mafic crust
Access of water
Heat source –
? Radioactive heating
Melting in presence of
plagioclase +/- garnet

16. That’s it !