In-situ zircon
Hf isotopic analysis provides additional information on the isotopic
composition of igneous rocks showing multiple sources, when compared to the
whole-rock isotope data (e.g., Griffin et al. 2002, Belousova et al. 2006, Kurhila
et al. 2010). We have analysed zircon Hf-isotope ratios of two rock samples from
southern Finland: the Muurla gabbro (MGB) and the Enklinge granodiorite (EG), which
show very different Hf isotope characteristics. The analyses were made at the
Finnish Geosciences Research Laboratory, Espoo, Finland.

            The MGB represents late-orogenic mantle
derived magmatism. Fourteen U-Pb analyses on 11 zircons yielded a concordia and
²⁰⁶Pb/²⁰⁷Pb age of c. 1.83 Ga (Kyllästinen
2014). Nine Lu-Hf analyses on zircons show variation in initial ε_Hf between
+1.4 to +5.3 with an average value of +3.4. The EG is related to early-orogenic
magmatism and shows continental arc affinities (Kara et al. 2016). U-Pb
analyses on 26 zircons, based on 28 measurements identified one magmatic
population and several inherited grains or domains. A concordia age of c. 1.88
Ga, which is regarded to represent the crystallisation age has been calculated
on 15 analyses. Thirteen analyses showed ²⁰⁶Pb/²⁰⁷Pb ages
between c. 2.25 and 1.95 Ga (Kara et al. 2017). Sixteen Lu-Hf analyses were
performed on the EG zircons. The grains representing the magmatic age showed initial
ε_Hf values between -3 to +4.4 with an average value of 0. The
inherited grains/domains yielded initial ε_Hf values from +3.5 to
+7.6 with increasing age between 1.95 to 2.25 Ga. The GJ-1 standard
reproducibility during the analyses was ±1.6 (1σ) ε-units.

            The MGB shows very coherent U-Pb and
Lu-Hf data. Variation in initial ε_Hf values is 3.9 ε-units, which
suggests a homogeneous magma source and evolution with crystallisation and/or
fractionation of a single magma without extensive crustal contamination or
magma mixing. This is also emphasised by uniform zircon morphology and
identical U-Pb ages in different zircons. The average initial ε_Hf
value of +3.4 indicates a rather juvenile mantle derived source. The EG zircons
representing the age of crystallisation show variation of 7.4 ε-units which
suggests, with an average ε_Hf value of 0, minor to moderate mixing
between a mildly depleted mantle source (ε_Hf>0) and crustal
sources (ε_Hf≤0). Crustal sources are obvious also from the inherited
zircons. In addition, the inherited grains show a clear trend towards the depleted
mantle with increasing age. We suggest that these grains represent the
“remnants” of a Svecofennian “proto-crust” (c.f., Lahtinen et al. 2005,
Andersen et al. 2009, Andersson et al. 2011) which has acted as a source for
the EG (Kara et al. 2017).