Mineral chemistry and trace element composition of the lamprophyres recovered by deep drillings within southern Fennoscandian basement.

Krzemińska E., Krzemiński L.

Polish Geological Institute - National Research Institute, Warsaw, Poland

ewa.krzeminska@pgi.gov.pl

 

The southern part of the Fennoscandian basement was intruded by lamprophyres, which were intersected by several deep drill holes in the area of NE Poland. The veins and dyke swarms comprise mainly dark, unmetamorphosed lamprophyres, although some more evolved dykes also occur. The geological and petrogenetical relations of these dykes were poorly constrained, however all lamprophyric rocks were typically linked with emplacement of the Early Carboniferous cratonic Tajno and Ełk intrusions (Ryka, 1984). Indeed, several dyke swarms occur in close spatial relationship to the Ełk syenite or Tajno pyroxenite bodies and crosscut the host rocks in “satellite” Bargłów, Rydzewo, Drygały drill cores. In drillings the veins filled by of lamprophyric rocks have been observed at few regions of NE Poland, e.g. in Zambrów located in the area of southernmost part of the Late Paleoproterozoic Mazowsze Domain (MD), in Oziabły and Czyże connected with Paleoproterozoic Podlasie Belarus Granulite belt (BPG), as well as in Wigry and Sejny situated along Mesoproterozoic AMCG intrusions of the Mazury Complex (MC). Lamprophyres have been intruded into the basement rocks of different age.

Our primary effort was to study the petrogenetical relations of various lamprophyres by analyzing and comparing the mineral chemistry as well as whole-rock and trace element signatures. Lamprophyric rocks occur as veins, generally less than 0.5 m wide (Sejny), although larger dikes (>50 m wide, Zabłudów) were also identified. The veins crosscut the basement rocks, showing both sharp chilled margins, transitional, commonly irregular, aphanitic borders that grade into strongly porphyritic, locally flow-textured interiors. In terms of mineralogy there are more than three types of lamprophyres in the studied area: (1) a more abundant with essential olivine phenocrysts (always as pseudomorphs), (2) with phlogopite/biotite macrocrysts (3) with amphibole phenocrysts and (4) clinopyroxene phenocrysts or nodules. The mafic members are filled with alkaline groundmass including K-feldspar, albite or/and feldspathoid, Na-pyroxene, apatite.

 

Fig. 1. Various mineral assemblages of lamprophyres. Examples from one Wigry borehole, depth 1423 m and 849 m (BSE images). (a) Micaceous vein with zoned phlogopite-tetraferriannite phenocrysts with minor clinopyroxene grains, (b) Essentially clinopyroxene nodule, mostly fassaite composition, surrounded by groundmass composed of biotite, clinopyroxene, altered olivine microcrysts and alkali feldspar.

 

All the examined veins reveal great diversity in mineralogy and mineral chemistry, particularly with regard to mafic minerals, even within the same core section (Fig. 1). Mineral compositions were investigated using EMPA techniques on CAMECA electron microprobe. In the Wigry on depth 1423 m (vein from the bottom) the dark mica macrocrysts form zoned brown laths with strong pleochroism, commonly having red-orange phlogopite cores (ca. 17.46 wt.% of MgO, ca. 7.47 wt.% of TiO₂, and ca. 10.0 wt.% of FeO_T) and thin black extremely iron rich rims composed of tetraferriannite phase (ca. 0.96 wt.% of MgO, ca. 4.29 wt.% of TiO₂, ca. 41.4 wt.% of FeO_T). In contrast, within upper vein of Wigry drill core (depth 849 m) clinopyroxene nodules, ranging in composition from Ca-Al rich fassaite (ca. 25.8 wt.% of CaO and ca. 7. 25 wt.% of Al₂O₃) to diopside are dominated. Sub- to euhedral amphibole were noted in mineral assemblage of lamprophyres, related to the Tajno alkaline-ultramafic intrusion. Ca-amphiboles are mainly kaersutite and ferro-kaersutite (Bargłów), with X_Mg between 0.54–0.62. Values of Na₂O are somewhat higher than K₂O in all cases, ranging up to 2.86 wt.%. Amphiboles typically contain high amount of TiO₂ (4.85–7.8 wt. %).

Whole rock major and trace elements have been determined for 20 relatively fresh and unaltered samples. Major and trace elements were analyzed at the ACME Laboratory, Canada by XRF method. REE elements were determined by ICP-MS spectrometry. The compositional variability of lamprophyres from studied region is quite high. The comparison of major element analyses of SiO₂ accounts for 36.16–46.39 wt.%; Na₂O + K₂O, 4.9–8.5 wt.%; K₂O/Na₂O of 0.53–3.53. The data points of the lamprophyres mostly fall within the field of alkaline lamprophyres on Fe₂O₃/SiO₂ vs. K₂O/Al₂O₃ diagram (Rock, 1991). The contents of TiO₂ vary between 1.23 and 3.54 wt.% and those of P₂O₅ between 0.69 and 2.69 wt.%. The Mg-number [Mg/(Mg+Fe_T)] of lamprophyres in Wigry–Sejny region are relatively highest (70–67) compare to those from Ełk–Tajno region (ca. 56–50) and Czyże–Oziabły area (ca. 48–38).

Normative compositions, permit several subdivisions of the different groups, following the classification of Rock’s: alkaline lamprophyres, lacking normative larnite. Only two veins from Sejny with normative larnite could be classify as ultramafic lamprophyres. The alkaline lamprophyre group typically contain various amounts of normative olivine up to 13.3 wt.%. The mafic veins can also be subdivided into: lamprophyres with normative nepheline, up to 14.4 wt.% (e.g. Wigy, Sejny, and Oziabły), as well as with normative both nepheline and leucite and with lacking normative nepheline. In the other hand, they can be subdivided into hypersthene normative and hypersthene-free group.

Trace element compositions provide a greater insight into the variations among the studied rocks (Fig. 2). The few samples selected from Wigry and Sejny drill cores with Sc: 15 ~ 19 ppm, Cr: 233 ~ 294 ppm, Co: 25 ~ 80 ppm, and Ni: 218 ~ 234 ppm, falling within the range of the hypothetical primary magma responsible for lamprophyres (Sc: 15 ppm ~ 30 ppm, Cr: 200 ppm ~ 500 ppm, Co: 25 ppm ~80 ppm, and Ni: 90 ppm ~ 700 ppm), as shown by the statistical data provided by Rock (1991). This indicates that the lamprophyres of only Wigry region possess the characteristic features of primary magmas. The REE patterns of lamprophyres have a different appearance having (La/Lu)_N values from 9 to 89. Trace element diagram (normalized to primitive mantle) of the lamprophyres reveals three to four distinct groups (Fig. 2).

 

Fig. 2. Primitive mantle-normalized incompatible element patterns for representative samples of different lamprophyres from crystalline basement of NE Poland area. Normalizing values from Sun and McDonough (1987).

 

Relatively more “calc-alkaline” lamprophyres from Oziabły, Czyże and Zambrów boreholes, with specific negative “Ta-Nb” anomalies also show signatures of Nb vs Zr, which are typical for orogenic high-K plutonic and volcanic rocks (Leat et al., 1986). In the contrast, others plot as anorogenic high-K rocks.

All collected geochemical data do not point out previously postulated a cogenetic, coeval or comagmatic relationship between main groups of lamprophyres intersected within crystalline basement by drillings in the area of NE Poland.

 

References:

Leat P.T., Thompson R.N., Morrison M.A., Hendry G.L., Trayhorns S.C. Geodynamic significance of post-Variscan intrusive and extrusive potassic magmatism in SW England. // Royal Society of Edinburgh. 1986. Vol. 77, P. 349-360.

Rock N.M.S. Lamprophyres. Glasgow: Blackie and Son, 1991. 285 pp.

Ryka W. Precambrian evolution of the East European Platform in Poland. // Biul. Inst. Geol. 1984. Vol. 347. P. 17-28.