Origin and composition of the lower crust of Europe

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Origin and composition of the lower crust of
Europe

• Hilary Downes
• School of Earth Sciences
• Birkbeck University of London

With many thanks to my friends and
colleagues Gabor Dobosi and Antal Embey-Isztin
(Hungary), Pamela Kempton and Andrew Markwick (UK)
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Examples of lower crustal rocks in the field
Outcrops of layered gabbros in the Ivrea Zone
(Italy) are examples of tectonically emplaced
lower crustal rocks
Granulite xenoliths in a Tertiary volcanic tuff
in Bournac (Massif Central, France) are examples
of lower crustal fragments entrained in alkaline
volcanic eruptions
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Hand specimens of lower crustal granulite
xenoliths from Bournac, Massif Central, France
Note the common banding, intrusive contacts and
even contacts between different lithologies e.g.
metasedimentary and metaigneous
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Having found our xenoliths and undertaken our
petrographic studies, we need to work on
geochemistry in order to determine their origin.
This can include EMP (e.g. for pressure and
temperature determinations), bulk rock analyses
(to determine protolith type), trace elements,
radiogenic and stable isotopes (to understand the
origin and evolution of the lower crust). Also,
we may be able to date the lower crust, using
zircons and/or Nd isotopes
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The European continent was constructed over 3.5
Ga from many different terranes. Precambrian
cratons include the Baltic Shield and East
European craton. The Caledonian, Variscan and
Alpine-Carpathian orogenies added fragments
derived mainly from Gondwana.
What can we learn about the lower crust of these
different regions?
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I will concentrate on two examples of lower
crustal xenoliths from regions of extremely
different ages
Archaean-Proterozoic Baltic shield (e.g. Kola)
where the crust is thick, cold and old

Tertiary Alpine Carpathian orogen (e.g. Pannonian
Basin) where the crust is thin, hot and young
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Case Study 1 Baltic Shield (Kola peninsula,
Russia)
Examples of mafic garnet granulite xenoliths (gt,
plag, cpx, rutile)
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Bulk rock analyses of granulite xenoliths from
Baltic Shield (Kola)
Field of primitive mafic melts
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Mafic cumulates
Melt compositions
Felsic calc-alkaline
Three types of REE patterns in Kola granulite
xenoliths indicate three varieties of protolith
(1) those with melt-like compositions (no Eu
anomaly), (2) cumulates with positive Eu
anomalies, and (3) a rare group with
calc-alkaline compositions and negative Eu
anomalies.
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The three groups differ in MgO content the
felsic calc-alkaline group may be formed by
melting of pre-existing crust (anatexis) or
remelting of mafic lower crust
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How can we date the lower crust?
Nd model ages for Kola granulite xenoliths
TDM Nd model ages (Ga)
This suggests that the Kola lower crust is early
Proterozoic in age
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Proterozoic crust
Phanerozoic crust
176Hf/177Hf
Archaean crust
143Nd/144Nd
Baltic shield xenoliths (purple squares)
generally fall in the Proterozoic crustal field
in agreement with zircon dating
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Zircon dating of lower crustal xenoliths from
Baltic Shield
Two mafic xenoliths were dated using zircons and
revealed essentially two ages
The older event at 2.4 Ga is recorded in igneous
zircons and is thought to be the main intrusion
age of a mafic underplate into pre-existing
Archaean (2.9Ga) crust
A younger event at around 1.7Ga is also recorded
in some metamorphic zircons.
(in Finland, lower crustal zircon ages reach
3.5Ga)
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Comparisons of Kola lower crustal xenolith suite
(in red) with various groups of igneous rocks
found in the northern Baltic Shield
Comparison with 2.4 Ga old intrusive rocks (part
of a Large Igneous Province)
Comparison with extrusive rocks dated at
2.4-1.9Ga, including ferropicrites
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Provisional conclusions from the Baltic Shield
xenoliths

• Lower crust is dominated by metaigneous rocks
  that were mafic melts and cumulates, i.e. igneous
  underplating
• Age of protoliths is probably 2.4 Ga (zircon age
  known age of mafic plutonism) but some may be
  as young as 1.9 Ga (age of ferropicrite lavas)
• Rare calc-alkaline group may result from
  melting of pre-existing crust or re-melting of
  mafic granulites

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Case Study 2 The Pannonian Basin
We will look more closely at an example from the
lower crust of a young thin hot region of
lithosphere in Europe
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A mafic metaigneous granulite xenolith from the
Pannonian Basin
Another mafic metaigneous granulite xenolith with
amphibole and kelyphite around garnet
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M3013 metasedimentary granulite xenolith from the
Pannonian Basin such types of xenoliths are not
known from the Baltic Shield
M3068 composite xenolith (with meta-igneous and
metasedimentary portions)
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SiO2/Al2O3 - Mg- plot of granulite xenoliths
from Pannonian Basin
pyroxene accumulation
0.9
primitive basalt
0.8
mafic 1
0.7
mafic 2
0.6
plagioclase accumulation
intermediate
0.5
Mg-number
mafic 3
0.4
felsic
0.3
garnet accumulation
0.2
Calc-alkaline fractionation trend
0.1
tholeiitic
alkali
ilmenite
1 2 3 4 5
6 7 8
SiO2 / Al2O3
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REE composition of metaigneous granulite
xenoliths from the Pannonian Basin
No petrographic difference between the two groups
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Pannonian Basin granulite xenoliths are depleted
in LREE contents compared with granulite
xenoliths worldwide
Comparative data Africa Hoggar Europe
Eifel N-Hessia
Massif Central
Scotland America Geronimo Australia
Chudleigh McBride Asia
Mongolia
La (N)
Pannonian xenoliths
Yb (N)
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Sr and Nd isotope ratios of Pannonian Basin lower
crustal granulite xenoliths
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The Pannonian Basin granulite xenoliths are
depleted in their Sr-Nd isotope compositions
compared with lower crust worldwide
143Nd/144Nd
87Sr/86Sr
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Oxygen isotope ratios of Pannonian Basin
granulites are also unusual
- Suggests a mixing hyperbole - Small changes in
Sr and Nd isotope ratios accompany large changes
in O isotope ratio (LREE-depleted xenoliths
red squares) - Some xenoliths have lower O
isotope ratios than the average mantle
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A comparison of Pannonian Basin granulite
xenoliths with lower crust of other regions
worldwide
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Lets try an igneous underplating and
hybridization model
The failure of the model is obvious. Assuming
that the data are OK, there are two possible
reasons - the model is not correct, or the
enriched component is not correct.
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Remember most rocks are composed of 50 oxygen!
If the oxygen isotope data are correct, then any
model needs to explain them (particularly the
values lt mantle values).
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Oceanic drillings Ophiolite complexes
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Origin of metasedimentary xenoliths Strong
spatial relationship between the metaigneous
and metasedimentary xenoliths (composite
xenolith) do they share a similar
origin? Pelitic protolith, but does not have
typical pelagic composition. Rather, it contains
material of continental origin, giving it an old
Nd model age - formed in a back-arc basin?
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Mixing hyperbolae for Hf and Nd isotope ratios
Mechanical mixing?
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Summary of the processes
Mixing in the pillow lava series
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Summary of Pannonian Basin lower crust
the protoliths of granulite xenoliths have an
oceanic crustal origin (back arc
basin?) tectonic emplacement into the lower
crust probably during the Alpine orogen (similar
to other Mediterranean ophiolites) granulite
facies metamorphism but under what conditions?
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Composition of clinopyroxene and garnet in
Pannonian basin granulite xenoliths
Ca
Ca
clinopyroxene
garnet
metaigneous
metasedimentary
Mg
Fe
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Estimation of pressure and temperature for
Pannonian Basin lower crust
Metaigneous xenoliths 760 - 900 oC 10.6 - 13.4
kbar Metasedimentary xenoliths 850 - 990
oC 10.8 - 11.6 kbar (Harley 1984, Perkins
and Chipera 1985)
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25
80
Plag out
Peridotite xenoliths
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60
90 mW/m2
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120 mW/m2
P (kbar)
Depth (km)
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mantle
10
crust
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Gt in
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olivin tholeiite solidus
1300
600
700
800
900
1000
1100
1200
T (oC)
Calculated P and T suggest a thicker crust at the
time of granulite facies metamorphism (followed
by recent extension)
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Crustal thicknesses in the Carpathian-Pannonian
Region (values in km)
42.5
45
50
30
25
32.5
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27.5
35
32.5
30
30
32.5
30
35
25
27.5
30
35
30
35
32.5
27.5
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Can we date the lower crust in the Pannonian
Basin?

• Yes we have metasedimentary granulite xenoliths
  that contain hundreds of zircons!
• (but what exactly are we dating??)

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Zircon U-Pb geochronology is one method of
determining the age of events in the xenoliths
and hence the age of the lower crust
Examples of zircons in BSE (above) and CL
(right), with laser pits from the U-Pb analysis
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Concordia plot
206Pb/ 238U
4Ma
207Pb/235U
A single metasedimentary xenolith from the
Pannonian Basin yields a wide range of zircon
ages from 360 Ma to a peak of ages as young as 4
Ma! Metamorphism is continuing in the hot lower
crust of the Alpine orogen even today. These are
hot zircons!
0
100
Relative probability plots
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Final remarks

• The lower crust of the two regions we have
  discussed differs in composition, age and origin
  
• Continental lower crust can be dominated by
  mafic rocks, or by a combination of mafic and
  felsic rocks (or even, only felsic rocks, as is
  the case in central Spain)
• Mafic rocks can be emplaced during major igneous
  events (e.g. in Baltic Shield, they are the same
  age and composition as a major LIP event)
• Or they can be tectonically accreted during
  collision, as seen in the Pannonian Basin

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MORB
Proterozoic crust
Phanerozoic crust
176Hf/177Hf
Archaean crust
143Nd/144Nd
Ages and origin of lower crust in Europe