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Lu-Hf and U-Pb isotope systematics of zircons from the Storgangen intrusion, Rogaland Intrusive Complex, SW Norway : implications for the composition and evolution of Precambrian lower crust in the Baltic Shield
The Storgangen orebody is a concordantly layered, sill-like body of ilmenite-rich norite, intruding anorthosites of the Rogaland Intrusive Complex (RIC), SW Norway. 17 zircon grains were separated from ca. 5 kg of sand-size flotation waste collected from the on-site repository from ilmenite mining. These zircons were analysed for major and trace elements by electron microprobe, and for U–Pb and Lu–Hf isotopes by laser ablation microprobe plasma source mass spectrometry. Eight of the zircons define a well-constrained (MSWD=0.37) concordant population with an age of 949±7 Ma, which is significantly older than the 920–930 Ma ages previously reported for zircon inclusions in orthopyroxene megacrysts from the RIC. The remaining zircons, interpreted as inherited grains, show a range of ²⁰⁷Pb/²⁰⁶Pb ages up to 1407±14 Ma, with an upper intercept age at ca. 1520 Ma. The concordant zircons have similar trace element patterns, and a mean initial Hf isotope composition of ¹⁷⁶Hf/¹⁷⁷Hf₉₄₉ Ma=0.28223±5 (εHf=+2±2). This is similar to the Hf-isotope composition of zircons in a range of post-tectonic Sveconorwegian granites from South Norway, and slightly more radiogenic than expected for mid-Proterozoic juvenile crust. The older, inherited zircons show Lu–Hf crustal residence ages in the range 1.85–2.04 Ga. One (undated) zircon plots well within the field of Hf isotope evolution of Paleoproterozoic rocks of the Baltic Shield. These findings indicate the presence of Paleoproterozoic components in the deep crust of the Rogaland area, but do not demonstrate that such rocks, or a Sveconorwegian mantle-derived component, contributed significantly to the petrogenesis of the RIC. If the parent magma was derived from a homogeneous, lower crustal mafic granulite source, the lower crustal protolith must be at least 1.5 Ga old, and it must have an elevated Rb/Sr ratio. This component would be indistinguishable in Sr, Nd and Hf isotopes from some intermediate mixtures between Sveconorwegian mantle and Paleoprotoerzoic felsic crust, but it cannot account for the initial ¹⁴³Nd/¹⁴⁴Nd of the most primitive, late Sveconorwegian granite in the region, without the addition of mantle-derived material.