In situ LA-ICP-MS U–Pb dating of metavolcanics of Norrbotten, Sweden: Records of extended geological histories in complex titanite grains

Abstract

In situ U–Pb dating of a variety of mineral phases is an important goal in petrology. This study reports data chiefly from titanite, but also from rutile and apatite, obtained using the laser ablation (LA)-ICP-MS methodology on polished thick sections in order to retain as much petrologic information as possible, and allowing trace element analyses from adjacent areas to the U–Pb analyses. The samples analysed come from Svecofennian intermediate to acid volcanic rocks of the Porphyry Group within the major iron ore province of Norrbotten, northern Sweden, where titanite is a common phase associated with the mineralisation. Using a laser beam of 30–45 μm in diameter, accurate and relatively precise (2–3% 2 σ on 206Pb/ 238U ratios) data can be obtained on titanite. Similar precision on Pb/U ratios can be obtained in rutile, but accuracy cannot be assessed because of the lack of well characterised standards. However, the data are a priori accurate as they are geologically reasonable. Apatite shows reverse discordance, which could be explained by a number of scenarios. However, the 207Pb/ 206Pb ages are fairly precise (1–3% 2 σ) and the calculated age is geologically reasonable, suggesting that the data may be accurate. The titanite grains studied show complex internal structures. Rare earth element (REE) analysis by LA-ICP-MS demonstrates that the core and rim zones are distinct and supports a model for two-stage evolution in two out of three samples, with the rims in these samples being enriched in total REE, and particularly in LREE in one sample, and displaying a positive Y-anomaly in contrast to the cores. In situ U–Pb analysis reveals that core zones from all samples retain distinct older ages of c.2050 Ma, whereas the reworked rims have either distinct younger or strongly reset ages. The older cores are interpreted as representing the first alteration/metamorphism of the volcanic pile. Titanite rims record a U–Pb age of 1870 ± 24 Ma (2 σ) at Luossavaara, and 1826 ± 15 Ma (2 σ) at the smaller Fe oxide–apatite body at Gruvberget. In both cases, the ages are consistent with previous age determinations from the Fe oxide–apatite deposits. A sample from Malmberget records a slightly different scenario, where U–Pb ages spread along concordia from c.1920 to 1708 Ma. We attribute this pattern to metamorphic reworking of primary metasomatic titanite. Rutile ages from inclusions within the core of Gruvberget titanite essentially mimic the ages obtained from the surrounding titanite and suggest that they have undergone the same history. Apatite inclusions within titanite at Malmberget record a younger age of 1584 ± 12 Ma (2 σ), which we attribute to later events known to have affected the area. These data indicate that the Porphyry Group volcanics must be at least c.2050 Ma in age, in contrast to previous age determinations from zircon which suggested a deposition age of c.1960 to 1880 Ma. The large grain size of the titanites studied, coupled with the ability for the first time to obtain in situ U–Pb ages from small areas, has allowed probing of this hitherto unrecognised event. Smaller zircon grain size, coupled with extremely saline hot, pressurised (high P– T) metasomatic fluids, relating to regional magnetite–apatite mineralisation in the period 1900 to 1800 Ma, may have resulted in resetting of zircon U–Pb systematics.