U-Pb age of sphenes (titanites) from neoarchaean sanukitoids of the Karelian craton

Nesterova N.S.*, Kirnozova Т.I.**, Fugzan М.М.**

*IG, KarRC,RAS, Petrozavodsk, Russia; **GEOKHI, RAS, Moscow, Russia

Late- and post-tectonic sanukitoid massifs are characteristic of most cratons of the world, including the Karelian Craton located on the Fennoscandian Shield. There, massifs form the West and East Zones (Lobach-Zhuchenko et al., 2007; Bibikova et al., 2005; Lobach-Zhuchenko et al., 2008 and references ibid). The West Zone consists dominantly of single-phase intrusions dated at 2700 – 2720 Ma. Their composition varies from monzodiorites to quartz monzonites, gabbroids being scarce. Sanukitoids from the East Zone are older (2730-2750 Ma), and contain mafic to ultramafic rocks. Such intrusions most commonly exhibit a multi-phase structure. Most sanukitoid massifs of the Karelian Craton were dated using U-Pb zircon geochronology (Bibikova et al., 1997; Ovchinnikova et al., 1994; Sergeyev et al., 2007; Chekulaev et al., 2003; Bibikova et al., 2005 et al.), whereas datings of sphenes are scarce (Bibikova et al., 1997, 1999).

The goal of the present project was to estimate the U-Pb age of sphenes from the Panozero and Chalka massifs (new data) and the North Karelian massif. Sphenes were decomposed chemically and U and Pb were separated by T.Crow’s method (1973). Isotopic composition was measured on a TRITON multi-collector solid-phase mass-spectrometer at GEOHI, RAS, Moscow. Isotopic ages were calculated using Ludwig’s software (1991). All errors are presented at 2s level.

The Panozero massif is located near the boundary between the Central Karelian and Vodlozero terrains (Slabunov et al., 2006). It belongs to East Zone sanukitoids. The Panozero massif is a central-type triphase intrusion (Lobach-Zhuchenko et al., 2007; Chekulaev et al., 2003; Lobach-Zhuchenko et al., 2008). Phase I consists of a mafic-ultramafic complex and metapyroxenites. Phase II is made up of diorites, monzodiorites and monzonites. Phase III is composed of granodiorites and quartz monzonites. The zircon age of phase I of the Panozero intrusion is estimated at 2765±8 Ma (Sergeyev et al., 2007), that of phase II at 2741±12 Ma (Bibikova et al., 2005) and 2727.4±4.1 Ma (Chekulaev et al., 2003), and that of phase III at 2742±18 Ma (Bibikova et al., 2005) and 2726±14 Ma (Chekulaev et al., 2003). U-Pb dating of two sphene samples from phase-I monzogabbro (samples 201-Ар99 and E-PА-7/1) was done as part of the project. The sphenes are dark-brown and semi-transparent. They are rich in U and Pb. The U and Pb content of sample 201-Ар99 is much higher than that of sample E-PА-7/1. The age of the sphenes is discordant. As the samples were taken from rocks of one phase, they are expected to be similar in age. If so, they can be used to plot a discordia, whose upper intersection with a concordia is estimated at 2691.9±5.8 Ma (Fig. 1).

Fig. 1. Diagram with a concordia for sphenes and zircons from Panozero rocks

Fig. 2. Diagram with a concordia for sphenes and zircons from Chalka rocks

The Chalka sanukitoid massif, like the Panozero massif, belongs to the East Zone. The massif consists of granodiorites that cross-cut Mesoarchaean metavolcanics of the Hautavaara structure of the Vedlozero-Segozero greenstone belt. The U-Pb zircon age of the massif is estimated at 2744.5±4.7 Ma (Ovchinnikova et al., 1994). Sphenes, separated from diorites, are represented by brown transparent crystals. Their U and Pb content is low. The sphenes have a concordant age of 2695.4 ±2.3 Ma (Fig. 2).

The biggest sanukitoid massif in the Karelian Craton is the North Karelian massif. It is located in the northern Central Karelian terrain (Slabunov et al., 2006). The rock composition of the massif varies from gabbroids to leucogranites; quartz diorites and granodiorites dominate (Bibikova et al., 1997; Larionova et al., 2007). The U-Pb age of zircons from quartz diorites is 2724.4±8 Ma. Sphene from the same sample shows a younger concordant age of 2696±5 Ma. In addition, light-coloured sphene, dated at 2640.7 ±63 Ma, is present in the rock (Bibikova et al., 1997; Bibikova et al., 1999).

Thus, sphenes from all three sanukitoid massifs are similar in age (within the error) but differ in the U-Pb age of an igneous stage in the formation of the massifs estimated by zircon dating, i.e. no relationship between the ages of sphenes and zircons was revealed. The location of the sanukitoid massifs either within or at the boundary of the Central Karelian terrain suggests that the age of sphene shows the timing of the tectonic movement of terrain rocks into a temperature range below the closing temperature of sphene, for example, during Neoarchaean accretion processes (Slabunov et al., 2006).

The authors wish to thank Е.V. Bibikova (GEOHI, RAS, Moscow) and А.I. Slabunov (IG, KarRC, RAS, Petrozavodsk) for their contribution to the study and discussion of the results, and N.V. Gusyeva (IGGD, RAS, St.Petersburg), who kindly sent samples for analysis.

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