New occurrences of potassium-rich (with affinities to lamproite) dykes and explosion breccia in the Northern Baltic Shield

Zozulya D.*, Kullerud K.**, Ravna E.K.**, Arzamastsev A.*, Hansen H.***

*Geological Institute, Kola Science Centre, Apatity, Russia; **University of Tromsø, Norway;

***Store Norske Gull AS, Tromsø, Norway. E-mail: zozulya@geoksc.apatity.ru

 

The previously known occurrences of potassium-rich dyke rocks in the north of the Baltic Shield were described from the Tuloma occurrence in the Kola region. They are represented by calc-alkaline lamprophyres of 1.75 Ga age, forming in a postcollisional geodynamic setting (Arzamastsev et al., 2009). From the mineralogy (diopside, ferrous phlogopite, Ba-bearing Na-K-feldspar, albite) and chemical composition (K/Na=0.9-1.1 (for one sample 3.1)), K/Al=0.3-0.4, K_agp=0.49-0.54, Mg#=73-81, La/Yb_n=45-86) the rock is defined as minette.

The recently discovered Kvalöya Island perpotassic dykes occur within the West Troms Basement Complex (WTBC) in Northern Norway. The WTBC is composed of Archean (TTG-gneisses, greenstone belt supracrustals) and Paleoproterozoic (mafic dykes, gabbro-diorite and granite intrusions) units. The complex occurs within a 120x20 km large window of the tectonostratigraphically overlying Caledonian allochthons. The Kvalöya dykes are 0.1-1.0 m in width and cross-cut a 1.8 Ga old granite. The dykes can be traced along a WSW-striking 5-6 km long lineament. The host granite is fractured and disintegrated up to 0.6-1.0m away from the contact, indicating that the dyke intruded under high dynamic conditions. The dyke rock is porphyritic and composed of phenocrystic phlogopite (up to 5 vol%) with rims of the tetraferriphlogopite. The fine-grained groundmass consists of K-magnesioarfvedsonite (~30 vol%), orthoclase (40-50 vol%); minor and accessory minerals are apatite (5-7 vol%), baotite Ba₄(Ti,Nb)₈Si₄O₂₈Cl (up to 3 vol%), rutile (1-3 vol%), barite, zircon and an unknown Na-Mg-Ba-P-O mineral species; chlorite (5-7 vol%) occurs as a secondary mineral. Locally, trace amounts of Ti-aegirine (1-5 wt% of TiO₂) and white Al-poor (1-2 wt% of Al₂O₃) and Mg-rich mica (taeniolite?) are found. From the exotic mineralogy and whole-rock composition the Kvaløya dyke belongs to the lamproite clan (according to Mitchell&Bergman (1991)), and more accurately the rock can be defined as a transitional type lamproite (according to Sheppard&Taylor (1992)) or an intermediate type lamproite (Fig. 1). Magnesioarfvedsonite is characterized by high contents of K₂O (4.5-6.0 wt%) and TiO₂ (0.7-3.5 wt%), orthoclase contains 1.6-3.6 wt% of FeO, phlogopite has low content of Al₂O₃ (9.2-10.7 wt%), but it is enriched in TiO₂ (2.1-2.6 wt%). Principal characteristics of the whole-rock geochemistry are K/Na=2.3-2.9, K/Al=1.0-1.2, K_agp=1.4-1.7, Mg#=76-84, La/Yb_n=100-140; SiO₂=54.8-56.8 wt%, TiO₂=3.2-4.0 wt%, BaO=0.55-1.47 wt%, P₂O₅=2.5-3.0 wt%, Zr=2650-3000 ppm, REE=900-1260 ppm, Sr=2300-2500 ppm. The distribution of normalized incompatible elements of the Kvalöya dyke is mostly similar to an average lamproite pattern rather than to lamprophyre (Fig. 2). The emplacement age of the Kvalöya lamproite is estimated at 300-320 Ma from the preliminary ⁴⁰Ar-³⁹Ar laser spot dating of phlogopite (personal communication of C. Davids, University of Tromsø). From this it is suggested that the rock has the initial Sr and Nd isotopic signatures similar to EM1 mantle source.

 

 

Fig. 1. SiO₂ versus Mg# classification diagram for potassic rocks (modified after Bogatikov et al. (1985)) with Kvalöya (circles) and Gremyakha (square) rocks shown. Lamproite occurrences: WK – West Kimberly; LH – Leucite Hills; MA – Murchia-Almeria. Dotted line outlines the minette field.

 

Rocks compositionally identical to the high-Si Kvalöya lamproite often accompany the ‘pure’ lamproitic suites with olivine- and leucite-bearing ultrabasic and basic rocks, e.g. in Leucite Hills (Wyoming), Smoky Butte (Montana), Mount Bayliss and Priestly Peak (Antarctica), Pendennis (UK), Murchia-Almeria province (SE Spain) [reviews in Bogatikov et al. (1991) and Mitchell&Bergman (1991)]. The rock types named as cancalite and cancarixite, composing of abundant sanidine and K-richterite and minor modal/normal olivine and diopside, have been described from the Murchia-Almeria, and they are most similar to the Kvalöya potassic rock in mineralogy and chemistry (Fig. 1).

The distinctive geochemical composition of the rock has resulted in a typical, for lamproite, Ti-Ba-P accessory mineralization, e.g. baotite, rutile, Sr-apatite, barite and an unknown Na-Mg-Ba-phosphate mineral (Zozulya et al., 2010). Baotite forms xenomorphic elongated and isometric grains of 10-500µ in size and belongs to the Nb-poor (0.03-0.05 apfu) isomorphic variety. Apatite forms mainly idiomorphic elongated grains with distinct zoning - their outer parts are extremely enriched in SrO (8-12 wt%) and REE₂O₃+Y₂O₃ (6-9 wt%). The unknown Na-Mg-Ba phosphate forms prismatic grains of 10-100µ in size. According to the atomic ratio of its main components (Na:Mg:Ba:P≈2:1:1:2), the composition of this mineral apparently corresponds to the conventional formula Na₂MgBa(PO₄)₂.

 

 

Fig. 2. Incompatible trace element variation diagrams for Kvalöya (shaded area) and Gremyakha (dotted line) rocks. Average data (from Mitchell&Bergman (1991)) for minette (squares), diamondiferrous lamproite (diamonds) and barren lamproite (circles) are shown for comparison.

 

The Gremyakha perpotassic explosion breccia occurs north of the Central Kola terrane and 15 km east of the Tuloma dykes. The breccia outcrops over an area of ca. 100 m² and has a very irregular form on the surface. The breccia itself represents an intrusion (?) in a 1.8 Ga old peralkaline granite, and is composed of numerous 0.1-20 cm thick veins. The rims of the veins are composed of a fine-grained orthoclase-amphibole groundmass with rare phenocrystic brown mica. The inner parts of the veins are medium-grained and composed mainly of mica and diopside. The breccia has the diagnostic mineral and whole-rock chemistries with affinities to the lamproite clan. Orthoclase has elevated contents of FeO (up to 1.1 wt%) and BaO (up to 1.7 wt%), mica is phlogopite with low content of Al₂O₃ (10.1-11.8 wt%) and high contents of TiO₂ (1.6-2.9 wt%) and Cr₂O₃ (up to 1.7 wt%), amphibole is K-eckermannite-richterite (K₂O is about 2 wt%); accessory minerals are Nb-rutile, Sr-apatite, zircon, barite and witherite (BaCO₃). Neither nepheline nor primary plagioclase is found in the rock. Principal characteristics of the whole-rock geochemistry are K/Na=1.4, K/Al=0.8, K_agp=1.4, Mg#=79, La/Yb_n=38; SiO₂=51.2 wt%, Ba=6520 ppm, Sr=1530 ppm, Zr=500 ppm, REE=1350 ppm, Cr=1290 ppm, Ni=280 ppm. The distribution of normalized incompatible elements in the Gremyakha breccia is similar both to average lamproite and to average minette (Fig. 2). At the same time the Gremyakha breccia is significantly different from Tuloma minette with higher contents of Rb, Ba, U, K, Zr, Hf, but relatively similar to the Kvalöya lamproite with respect to the incompatible element pattern.

The Kvalöya and Gremyakha perpotassic rocks display mineralogical and geochemical features of lamproites emplaced at destructive plate margins rather than lamproites of the intraplate tectonic settings and it is suggested these rocks might have originated in a post-orogenic setting due to the collapse of the Variscides and the following rifting in the Northern Europe.

The study was fulfilled under the frame of the Priority Programs 6&8 of the Earth Science Division RAS and Norwegian Barents Secretariat Project “Alkaline rocks within the Barents region”.

 

References:

Arzamastsev A.A., Fedotov J.A., Arzamastseva L.V. Dyke magmatism of the North-eastern Baltic Shield. Saint-Petersburg: Nauka, 2009. 383p. (in Russian).

Bogatikov O.A., Ryabchikov I.D., Kononova V.A. et al. Lamproites. Moscow: Nauka, 1991. 302p. (in Russian).

Mitchell R.H., Bergman S.C. Petrology of lamproites. New York: Plenum Press, 1991. 447p.

Sheppard S., Taylor W.R. Barium- and LREE-rich, olivine-mica-lamprophyres with affinities to lamproites, Mt. Bundey, Nortern Territory, Australia // Lithos. 1992. V. 28. P. 303-325.

Zozulya D.R., Savchenko Ye.E., Kullerud K. et al. Unique Ti-Ba-P accessory mineralization of an ultrapotassic dyke from Kvalöya, Northern Norway // Zapiski RMO. 2010. Pt. 139. No 2. P. 101-112 (in Russian).