Transcript Lu-Hf isotopes in zircon

Zircon U-Pb (& Lu-Hf) isotope
geochronology of the
Hidaka Metamorphic Belt,
Hokkaido, NE Japan
Tony KEMP, Toshiaki SHIMURA
Department of Geology, Niigata University
Collaborators
• Tomokazu Hokada, Daniel Dunkley
National Institute for Polar Research, Tokyo, Japan
• Richard Hinton
Edinburgh Ion Microprobe Facility, Edinburgh University, UK
• Chris Hawkesworth
Department of Earth Sciences, University of Bristol, UK
Micro-chronology of zircon
U-Pb isotopes
Lu-Hf isotopes
Ion microprobe
Laser
Ablation
Crystallisation age
Source age
Hidaka Metamorphic Belt
Geological Map of the Hidaka Metamorphic Belt
Hidaka Metamorphic Belt
‘an exposed crustal slab derived
upper metamorphic rocks
lower metamorphic rocks
from an immature island arc’
43°N
upper granitoid
143°E
middle tonalite
(Komatsu et al. 1986; Osanai et al. 1991)
lower tonalite
basal tonalite
gabbro & diorite
peridotite
Poroshiri Ophiolite
0
10
20
km
Hokkaido
42°N
143°E
Cape Erimo
30
uppe r
Non-meta
sediments
Very low grade
metasediments
5
200
granodio ri tegranite
(modified from Komatsu et al. 1986)
P res sure (M P a)
Idealised succession of the
metamorphic and igneous
rocks in the ‘Hidaka Crust’
T hickness (km )
Hidaka Metamorphic Belt
Nakanogawa
group
Zone I-a
semi-schists
Zone I-b
10
Zone II
400
middle tonalite
Hbl-Bt gn.
15
meta-Gb
Grt-Crd-Bt gn.
amphibolite
600
Zone III
Ol-Gb
20
granulite
Zone IV
 Crustal evolutionary
processes in arcs
Bt-Ms gneiss
low e r ton alit e
2. Magmatic differentiation
mechanisms?
gabbro-di ori te
1. What is the temporal
framework?
schists
basal tonalite
Hidaka Main Thrust
800
Grt-Opx
granulite
25
Grt 2-pyroxene
mafic granulite
lowermost crust
(not exposed)
Gabbro
MOHO?
peridotite?
?
?
Upper Mantle?
Previous age studies
Most K-Ar and Rb-Sr mineral ages cluster
around 40-28 Ma and 20-15 Ma
• Owada et al 1991 : peak metamorphism and anatexis at
56 ± 6.1 Ma (WR Rb-Sr isochron)
• Usuki et al 2002 : granulite facies metamorphism at
23-18 Ma (SHRIMP U-Pb on thin zircon rims)
• Maeda et al. 1990: Gabbro-diorite-granite K-Ar WR
ages of 19.6-17 Ma. Cooling/exhumation ages (??)
Nissho
Upper S-type granite
Sample localities
Geological Map of the Hidaka Metamorphic Belt
Pankenushi
2-Px Gabbro
Hidaka Metamorphic Belt
Pankenushi
Basal S-type tonalite
& Enclave
upper metamorphic rocks
lower metamorphic rocks
43°N
upper granitoid
143°E
middle tonalite
Pipairo
Middle S-type tonalite
lower tonalite
basal tonalite
gabbro & diorite
Niikappu
Basal S-type tonalite
Basal I-type tonalite
Lower S-type tonalite
peridotite
Poroshiri Ophiolite
Satsunai
Middle S-type tonalite
Middle I-type tonalite
0
10
20
km
Horobetsu
Grt-Opx granulite
Opx-Crd-Ged granulite
Opx mafic granulite
Hokkaido
Nupinai
Upper I-type granite
42°N
Horoman
Mafic layer in peridotite
143°E
Cape Erimo
30
Basal I-type tonalite
Qtz + pl + opx + hbl + bt
Basal S-type tonalite
Qtz + pl + opx + grt + bt
Contrasting
sources
Mafic granulite
Metased. granulite
Middle ‘I-type’ tonalite
Centre of body
Margin of body
Assimilation of metasedimentary rock at the periphery
I-type zircons
Upper bt-hbl granite
Middle hbl-bt
tonalite
Basal
opx-hbl
tonalite
I-type samples #1
Concordia diagram
Average Pb/U age
Data at 2
BASAL I-TYPE TONALITE (Niikappu)
I-type samples #2
Concordia diagram
Average Pb/U age
Data at 2
Data at 2
MIDDLE I-TYPE TONALITE (Satsunai)
I-type samples #3
Concordia diagram
Average Pb/U age
Data at 2
UPPER I-TYPE GRANITE (Nupinai)
Basal S-type zircons
19 Ma
53 Ma
42 Ma
Basal S-types
Grt-opx tonalites - Pankenushi & Niikappu
Satsunai ms tonalite
Pipairo crd tonalite
71 Ma
37 Ma
52 Ma
All ca. 37 Ma
53 Ma
46 Ma
38 Ma
37
Ma
112
38 Ma
Middle S-types
Ms-bt-crd tonalites - Satsunai & Pipairo
Upper S-type
Biotite granite - Nissho
Metased. granulites
Grt-opx-crd granulite
Grt-opx granulite
18 Ma
19 Ma
19 Ma
21 Ma
54 Ma
Metased. granulites
S-type vs granulite zircons
S-type rims = magmatic
Granulite rims = metamc
50 Ma zircons in S-types
derived from metas. protolith
Mafic rocks
Mafic granulite
(MORB-like meta-basalt)
Magmatic zircons
2-px gabbro
(MORB-like)
 igneous crystallisation ages
Summary of U-Pb Ages
Two magmatic pulses in the Hidaka Belt
• ca. 37 Ma (late Eocene)
 emplacement of I-type magmas at all crustal levels
 mid-crustal anatexis/assimilation
• ca. 19 Ma (early Miocene)
 crystallisation of MORB-like gabbros
 granulite facies MM and partial melting
 generation of S-type magmas
Lu-Hf isotopes in zircon
Why?
- zircon has low Lu/Hf (< 0.001)
 preserve initial 176Hf/177Hf of magma
- robust, high Hf content (~1%)
 impervious to isotopic disturbance
- zoning in
176Hf/177Hf
 source rocks, magmatic evolution
(e.g. Griffin et al. 2002 Lithos 61, 237-239)
Lu-Hf isotopes in zircon
16
Depleted mantle +16
Mantle input
14
 Hf (t)
12
zrc crystallisation
10
8
6
crustal contamination
4
2
0
0.02
0.04
0.06
0.08 0.10
Yb / Hf
0.12
0.14
Lu-Hf isotopes in zircon
16
Depleted mantle +16
14
 Hf (t)
12
zrc crystallisation
10
8
6
4
2
crustal contamination
0
0.02
0.04
0.06
0.08 0.10
Yb / Hf
0.12
0.14
Lu-Hf isotopes in zircon
16
Depleted mantle +16
14
 Hf (t)
12
zrc crystallisation
10
8
6
4
2
crustal contamination
0
0.02
0.04
0.06
0.08 0.10
Yb / Hf
0.12
0.14
Hf isotope evolution
20
 Hf (t)
18
16
14
12
10
8
6
4
2
10
20
30
Age (Ma)
40
50
60
Hf isotope evolution
20
 Hf (t)
18
16
14
12
10
8
6
4
2
10
20
30
Age (Ma)
40
50
60
Conclusions
Hidaka magmatic arc was assembled episodically
• 37 Ma
- I-type (?arc) magmatism
- juvenile crustal growth
• 19 Ma
- S-type magmatism & granulite formation
- recycling of older crustal materials driven
by mafic magma under-plating
What were the tectonic controls?