Zircon U-Pb (& Lu-Hf) isotope geochronology of the Hidaka Metamorphic Belt, Hokkaido, NE Japan Tony KEMP, Toshiaki SHIMURA Department of Geology, Niigata University

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 Collaborators

Micro-chronology of zircon U-Pb isotopes Crystallisation age Lu-Hf isotopes Source age Ion microprobe Laser Ablation

Hidaka Metamorphic Belt ‘an exposed crustal slab derived from an immature island arc’ (Komatsu et al. 1986; Osanai et al. 1991)

Hidaka Metamorphic Belt Idealised succession of the metamorphic and igneous rocks in the ‘Hidaka Crust’ (modified from Komatsu et al. 1986) 1.What is the temporal framework? 2.Magmatic differentiation mechanisms?  Crustal evolutionary processes in arcs

Most K-Ar and Rb-Sr mineral ages cluster around Ma and 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 Ma (SHRIMP U-Pb on thin zircon rims) Maeda et al. 1990: Gabbro-diorite-granite K-Ar WR ages of Ma. Cooling/exhumation ages (??) Previous age studies

Pankenushi 2-Px Gabbro Niikappu Basal S-type tonalite Basal I-type tonalite Lower S-type tonalite Pankenushi Basal S-type tonalite & Enclave Satsunai Middle S-type tonalite Middle I-type tonalite Nissho Upper S-type granite Nupinai Upper I-type granite Horobetsu Grt-Opx granulite Opx-Crd-Ged granulite Opx mafic granulite Horoman Mafic layer in peridotite Pipairo Middle S-type tonalite Sample localities

Basal I-type tonalite Qtz + pl + opx + hbl + bt Basal S-type tonalite Qtz + pl + opx + grt + bt Mafic granuliteMetased. granulite Contrasting sources

Middle ‘I-type’ tonalite Centre of body Assimilation of metasedimentary rock at the periphery Margin of body

Middle hbl-bt tonalite Upper bt-hbl granite Basal opx-hbl tonalite I-type zircons

I-type samples #1 BASAL I-TYPE TONALITE (Niikappu) Data at 2  Concordia diagramAverage Pb/U age

I-type samples #2 MIDDLE I-TYPE TONALITE (Satsunai) Data at 2  Concordia diagramAverage Pb/U age

I-type samples #3 UPPER I-TYPE GRANITE (Nupinai) Data at 2  Concordia diagramAverage Pb/U age

Basal S-type zircons 53 Ma 42 Ma 19 Ma

Basal S-types Grt-opx tonalites - Pankenushi & Niikappu

Pipairo crd tonaliteSatsunai ms tonalite 71 Ma All ca. 37 Ma 53 Ma 37 Ma 38 Ma 52 Ma 46 Ma 37 Ma 38 Ma 112

Middle S-types Ms-bt-crd tonalites - Satsunai & Pipairo

Upper S-type Biotite granite - Nissho

Grt-opx granulite Grt-opx-crd granulite 21 Ma 19 Ma 18 Ma 54 Ma Metased. granulites 19 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) 2-px gabbro (MORB-like) Magmatic zircons  igneous crystallisation ages

Summary of U-Pb Ages Two magmatic pulses in the Hidaka Belt ca. 19 Ma (early Miocene) è crystallisation of MORB-like gabbros è granulite facies MM and partial melting è generation of S-type magmas è emplacement of I-type magmas at all crustal levels è mid-crustal anatexis/assimilation ca. 37 Ma (late Eocene)

Lu-Hf isotopes in zircon - zircon has low Lu/Hf (< 0.001) è preserve initial 176 Hf/ 177 Hf of magma - robust, high Hf content (~1%) è impervious to isotopic disturbance - zoning in 176 Hf/ 177 Hf è source rocks, magmatic evolution (e.g. Griffin et al Lithos 61, ) Why?

 Hf (t) Yb / Hf Lu-Hf isotopes in zircon zrc crystallisation Depleted mantle +16 crustal contamination Mantle input

 Hf (t) zrc crystallisation Depleted mantle +16 crustal contamination Lu-Hf isotopes in zircon Yb / Hf

 Hf (t) zrc crystallisation Depleted mantle +16 crustal contamination Lu-Hf isotopes in zircon Yb / Hf

4 6 8  Hf (t) Age (Ma) Hf isotope evolution 18 20

4 6 8  Hf (t) Age (Ma) Hf isotope evolution 18 20

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?