Polish Geological Institute - National Research Institute, Warsaw, Poland

ewa.krzeminska@pgi.gov.pl

The three cratonic ultramafic and alkaline bodies were discovered within the crystalline basement of NE Poland by regional gravity surveys. The Tajno pyroxenite-carbonatite complex, the Ełk alkaline syenite massif and the Pisz gabbro-syenite intrusion has been confirmed by several deep drillings. However, the prominent features of the geophysical map of such region is vast area of Y-shape gravity high, so called Mława anomaly. There is quite possible that part of them could be related to another alkaline body (Wybraniec and Cordell, 1994). Four drillings: Płońsk IG2/2, Ciechanów 1, Konopki Wielkie and Gradzanowo, all around and within the Mława anomaly, intersected a monotonous sequence of syenite (thickness ca. 178 m in Płońsk IG2/2) and microsyenite (ca. 32 m in Ciechanów 1, ca. 263 m Konopki Wielkie) or microsyenite vein (ca. 2 m Gradzanowo) crosscutting the Silurian/Permian sediments. The reconnaissance and unpublished whole rock K–Ar geochronological measurements (Ryka, 1979), documented mostly Carboniferous (295, 305, and 348 Ma) age of syenites.

The principal aim of this contribution is to present first geochemical (major and trace element, rare-earth element) data from Płońsk and Gradzanowo alkaline rocks as a part of hypothetical syenite massif and to discuss variation in their chemical composition as well as to establish possible relations to the much more known foid-bearing syenite Ełk intrusion.

The Mława alkaline body (Mława anomaly) is located in the central part of Svecofennian domain in NE Poland on the depth ca. 3302–3484.0 m (Płońsk), 2373–2510.3 m (Ciechanów), 2311–2732.0 m (Konopki Wielkie), and 3289.5–3401.1 m (Gradzanowo). The crystalline basement of this area consists mainly of metasedimentary and metavolcanic arc related rocks, which were metamorphosed at ca. 1.83 Ga as well as Late Paleoproterozoic granitoids (1.83–1.82 Ga) with VAG signature and mafic intrusive rocks (1.82–1.80 Ga). However all the syenitic rocks of Mława anomaly occur within unmetamorphosed Phanerozoic sediments cutting partially eroded Silurian strata (Płońsk) or laying before Permian strata (Gradzanowo).

The core samples of alkaline rocks were collected systematically from interval of ca. 3302–3484.0 m in Płońsk and 3400–3401.1 m in Gradzanowo boreholes. They are red or pinkish and partially weathered fine-grained rocks. They represent full sequence of alkaline rocks including thick veins of more mafic component (so-called “diabase”) from Płońsk borehole, depth 3799 – 3911 m.

Rock-forming minerals are dominantly K-feldspar (more than 50%) with lesser amounts of quartz and Na-plagioclase. Occasionally clinopyroxene and rare amphibole and biotite are found. Characteristic accessory minerals are titanite, magnetite, ilmenite, and apatite. Minor accessories include fine-grained REE-bearing fluorocarbonates and zircon.

Whole rock major and trace elements have been determined for 15 samples. Major and several trace elements were analysed at the Central Chemical Laboratory in PGI–NRI, Warsaw, by XRF method. REE elements were determined by ICP-MS spectrometry.

Most of analyses reveal higher values of LOI from 2.7 to 9.0 wt.% and approximately reflect the degree of weathering as well as presence of hydrous and calcic minerals. LOI free characteristics of alkaline rocks from Płońsk have medium SiO₂ content (61.0–68.5 wt. %), low TiO₂ (0.3–0.7 wt. %) content, with alkalies in range of 7.3–10.2 wt.%. The CaO contents are between 0.7 and 2.98 wt.%, whereas MgO contents are relatively low, ranging from 0.5 to 2.1 wt.% and giving low Mg-numbers of 5–34. The K₂O contents are consistently higher than Na₂O and reach up to 5.1–6.4 wt.%.

As the fine grain size of the groundmass and alterations preclude modal analysis for most rocks, the CIPW norms are used to give a rough idea of their mineral proportions. All the analysed Płońsk syenite are over-saturated, with normative quartz varying mostly from 7.9% to 17.7%, whilst Gradzanowo rocks are over-saturated up to 36.5 – 36.5 % of normative Q. The QAP triangle based on CIPW norms reveals a continuous trend within field of alkali-feldspar quartz syenite (Płońsk) but few plots within syeno–granite compositional field (Gradzanowo). From the other hand, a plot of total alkalies (Na₂O + K₂O) vs. silica (SiO₂) shows that the studied rocks are mostly syenite to quartz monzonite and reveal mildly alkaline nature. They have lower alkalies than the syenite from the Ełk intrusion (Ełk: Na₂O + K₂O on the level 12–16 wt.%). According to the Foley et al. (1987) criteria, they can be classified as potassic–ultrapotassic rocks with K₂O > 4.7 wt.%.

In terms of R₁–R₂ De la Roche’s et al. (1980) classification, the Płońsk rocks are mainly quartz syenite and syenite but Gradzanowo rocks are alkali granite. Diabase could be classified as monzogabbro. Whole-rock major elements in the Mława alkaline body with respect to SiO₂ in bivariate plots show coherent variations with linear trends, which are consistent sometimes with Ełk trends.

The agpaitic index between 0.68 and 0.97 and A/NK vs A/CNK diagram) points to the metaluminous character of the Płońsk syenite (Al₂O₃/[Na₂O+K₂O] =0.97–2.56; Al₂O₃/[CaO +Na₂O+K₂O] = 0.65–1.06). However, some of the samples plot close to the peralkaline and peraluminous fields (Maniar and Piccoli, 1989).

Q-syenite and syenite samples are quite similar with parallel slopes of REE patterns, with ΣREE from 584 to 1418 ppm, whereas sample of diabase, shows the lowest concentrations of rare earth elements (ΣREE ca. 183 ppm). Whole REE contents of syenite are highly fractionated, exhibiting moderate and variable LREE fractionation (La_N/Sm_N = 3.6–5.79) and higher LREE/HREE ratios (La_N/Yb_N = 19.4–50). Eu-anomalies are negative (Eu/Eu* = 0.22–0.82), except for diabase samples (Eu/Eu* =1.20).

Sr concentrations range from 22–56 ppm in the most samples to 780 ppm in the diabase one. Ba displays similar variation from 14 to 690 ppm. Primitive mantle-normalized patterns of selected trace elements show strong depletion in Ba, Sr, P and Ti (Fig. 1A).

Fig. 1. (A) Primitive mantle-normalized trace element concentrations in the Mława syenite massif . Data from Płońsk IG2/2 drillhole. PM values from Sun and McDonough (1989). (B) Backscattered electron image of Płońsk syenite mainly composed of coexisting K-feldspar (light grey) and Na-plagioclase (dark grey) and accessories: titanite (grey) and euhedral zircon grains (bright).

Syenites from the Mława massif have relatively high concentrations of Zr mainly about 1100 ppm (Zr 623–1134 ppm) and contain an important quantities of zircons, which crystallised from Zr-oversaturated magma. The dominant feature of zircon grains from the Płońsk syenite (as well as from the Ciechanów and Gradzanowo) is commonly their relatively large size (Fig. 1B). They typically have euhedral morphology and are dominated by internal zoning, typical for magmatically formed zircon.

This study presents, for the first time, petrographical and geochemical data on the mildly alkaline rocks of the large Mława magnetic and gravimetric anomaly. The rocks have relatively high concentrations of alkalies, Th, Zr, and REE, but lowest amounts of Ba, Sr, P, and Ti, and reveal in general very uniform geochemical signature.

Based on the low Mg-number (5–33), low Cr (5–37 ppm) and Ni (ca. 3 ppm) contents in the whole-rock samples, Q-syenites and syenites from Płońsk and Gradzanowo drillings appear to have crystallised from an evolved parent, rather than primary mantle-derived magma.

References:

Ryka W. Wyniki datowań skał krystalicznych metodą potasowo-argonową. In: Ryka W. (ed.). Objaśnienia do Mapy geologicznej podłoża krystalicznego platformy prekambryjskiej w skali 1:200 000. Arkusze Ostrołęka, Mława, Warszawa wschód, Warszawa zachód, Brodnica, Toruń, Płock. // Arch. CAG. 1979. P. 78-79.

Wybraniec S., Cordell L. Geophysical mapping of the Ełk syenite massif. In: Geology of the Ełk syenite massif, NE Poland. // Prace Państw. Inst. Geol. 1994. Vol. 144. P. 11-19.