Mineralogy and geochemistry of Palaeoproterozoic mafic dykes from the Central Karelian terrain of the Karelian Craton.

Yegorova S.V.

Petrozavodsk State University, Mining and Geology Department, Petrozavodsk, Russia;

sve5690@yandex.ru

 

Mafic dyke swarms are widespread on Precambrian shields and are common in Proterozoic units. They are understood as a constituent of a feeder system of large igneous provinces (LIPs), whose formation could have been triggered by the ascent of deep-seated mantle plumes (Coffin & Eldholm, 1994). The data, obtained by studying the geochemical and mineralogical characteristics of mafic dyke swarms, are expected to cast light on the formation and evolution of parent magma. Furthermore, dyke swarms can be used as regional time markers because they are widespread and relatively well-preserved.

A mafic dyke swarm, consisting of Fe-tholeiites, was discovered near Lake Tulos located in the western Karelian Craton, eastern Fennoscandian Shield. Ten bodies, composed dominantly of fine- to medium-grained dolerites, are exposed in the present erosion section. As the dykes are in cross-cutting contact with Archaean host complexes, their Palaeoproterozoic age is assumed. The bodies described are dominated by NW-trending (290-330°) dykes. Near-E-W-trending (10°) bodies, some of which have been traced along the strike over large distances, are less common. The bodies vary considerably in thickness, their average thickness being over 40 m. Structurally simple bodies prevail, but large dykes clearly show intrachamber differentiation: a chill zone, made up of fine-grained melanocratic rocks, is identified; grain size increases and percentages of dark-coloured minerals decrease away from the contact; as a result, the rocks in the central portion of the body attain a mesocratic, medium-grained, occasionally taxitic habit. Furthermore, lenticular leucogabbro schlieren were found to occur from the central portions of some bodies. The dyke rocks of the Tulos block are generally well-preserved and show minor secondary alterations.

The major rock-forming minerals of Tulos block dolerites are mafic plagioclase, which is composed of labrador and andesine and makes up 40-55% of rock volume on the average, and clinopyroxene, which is dominantly represented by augite and accounts for 45% of the rock volume. In some bodies, orthopyroxene, dominantly hypersthene (up to 8%) and late igneous hornblende (1-3%) are rock-forming minerals. Ore minerals, dominantly magnetite and ilmenite, make up 12% of some bodies, averaging 5-7%.  Quartz and biotite occur as minor minerals in the rocks; they were also revealed in the chill zone rocks of some bodies. Olivine, which constitutes about 4% of the rock volume, is encountered in some varieties.  

Data on the composition of rock-forming minerals, obtained on an INCA Energy 350 microanalyzer, based on a VEGA LSH scanning electron microscope at the Institute of Geology, Karelian Research Centre, were used to calculate the crystallization pressures and temperatures of the minerals equiponderous with the melt, using K. Putirka’s geothermobarometer (2008). Calculations have shown that the rocks were crystallized near the surface in the temperature range 1100-1200°С.  Furthermore, estimation of liquidus temperatures with Pele 7.0 and Comagmat 3.05 software suggests that magma began to crystallize over the temperature range 1150-1200°С.

In classification diagrams, the figurative points of Tulos dolerite compositions form compact fields, whose position is used to identify dolerites as tholeiite-series high-Fe rocks or Fe-tholeiites.

The percentages of petrogenetic elements in Tulos doleritesв vary slightly. High TiO₂ (1.66 to 3.08 wt.%) and Fe₂O₃* (13.91 to 19.72 wt.%) and low MgO (less than 6.5 wt. %) concentrations are characteristic. SiO₂ varies from 48.16 to 51.4 wt. %, averaging 49.6 wt. % ). Intrachamber differentiation processes increase the above values in the leucocratic portions of the bodies to 55.24 wt. %. A rise in the percentage of silicic acid in large bodies from the chill zone towards the central portions of the body, caused by differentiation, is accompanied by an expected decrease in MgO and a rise in Fe₂O₃*.

The concentrations of compatible trace elements in the rocks vary greatly, but are generally low: V:  64 - 576.51ppm, Cr: 9-89ppm and Ni: 11-102 ppm. The concentrations of incompatible trace elements are: Zr 55 –166 ppm, Nb 5.07 17.74 ppm, Hf – 1.62 to 4.61 ppm.

One thick body did not show any differentiation in petrogenetic elements: their percentages in the central and marginal portions of the body vary slightly, and the body typically shows the highest Cr concentrations, averaging ~130 ppm.

Chondrite-normalized REE distribution spectra (Fig. 1 А) in Tulos dolerite rocks show a poorly differentiated lanthanide distribution pattern ((Сe/Yb)_N varies from 2.2 to 4.3): with a slight enrichment in LREE (mean value (La/Sm)_N ~1.9), a flat distribution in the middle part of the spectrum and a slight depletion in HREE ((Gd/Yb)_N varies from 1.4 to 2.1). Depletion in HREE is most probably associated with the presence of garnet in the melting source.

The REE distribution spectra of one of the dykes intersect those of others in the medium lanthanide range, suggesting that it could not have been produced together with other bodies by one fractionation process from one source (Slabunov et al, 2007).

 

Fig.1 Distribution spectra of the rare-earth, chondrite-normalized (McD, 1995) (А) and rare, primitive mantle-normalized  (Wenderpohl & Hartman, 1994) (B) elements of Tulos block dolerites.

Negative Sr, Nb and Zr anomalies and a slight positive Ti peak in the REE distribution spectra (Fig.1 B) are noteworthy. Negative Nb anomaly could indicate the involvement of a crustal constituent in the magmatic process, and Sr anomaly is presumably associated with plagioclase fractionation. High percentages of LIL-elements could also be due to crustal magma contamination.

Correlation of Tulos block dolerites with Upper Jatulian basalts from Central Karelia (Malashin et al., 2003) in petrogenetic, rare and rare-earth elements has shown their considerable similarity – another argument in favour of Palaeoproterozoic age of Tulos block dolerites.

The study was carried out at the Institute of Geology, Karelian Research Centre, RAS.

 

References

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