Późnoprekambryjski magmatyzm platformowy i karbonatyty w polskiej części platformy wschodnioeuropejskiej

Stanisław Kubicki, Wacław Ryka



In NE Poland, the origin of platform intrusion of the central type has been related to Late Proterozoic tectonic- igneous activation of the crystalline basement. The intrusions - alkaline-ultramafic Tajno, alkaline-gabbro Pisz, and alkaline Ełk, Mława and Olsztynek intrusions - have used deep, presumably Gothian tectonic fractures, responsible for partition of the fundament into blocks. This was accompanied by deposition of sandstones and mudstones of the Biebrza complex (an equivalent of the Jothnian) in tectonic troughs. The majority of isotopic datings indicate „Variscan", some - „Caledonian", and single ones - Late Precambrian age of the intrusive rocks.
Structure and composition of the platform intrusions appear determined by polyphase and polygenic development and alkaline magmatism is especially characteristic here. The best known intrusions include those from Ełk and Tajno, with top surface overlain by Meso-Cenozoic cover 800-900 and 600 m thick, respectively. The Pisz and Mława intrusions, situated at large depths, has been evidenced by single drillings whereas the Olsztynek intrusion remains to be known from geophysical data only.
The Ełk syenite intrusion, about 400 km2 in area, is known from 8 borehole columns. The drillings encountered nordmarkites, micropertite syenites with quartz, pyroxenes, biotite and arfvedsonite, micropertite sodalite syenites with either biotite or pyroxenes, microcline and albite syenites with pyroxenes, amphibolites and biotite, micropertite nepheline syenites, egirine nepheline syenites and nephelinites as well as microsyenites and lamprophyres. Evolution of that massif has proceeded from ring-like intrusions of granites, microcline granites, granodiorites and diorites, through quartz syenites and micropertite syenites to ring-like intrusions of sodalite syenites, nepheline syenites, nepheline syenites, vein deposits and zones of metasomatic-pneumo-hydrothermal transformations with traces of Nb, TR, U - Th and Zr mineralization, well- developed in rocks of the agpaite series and on smaller scale - in those of the miaskite series. Metal-bearing zones are varying from a few cm to some meters in thickness and are characterized by varying mineral composition, structure and texture. Well logs display gamma radiation anomalies. There may be differentiated nepheline-catapleiite metasomatites with rinkite and bastnaesite, nepheline-(albite)-egirine ones with loparite and ramsayite, and nepheline-microcline ones with zircon and pyrochlore, nepheline microclinites with lavenite, zircon, bastnaesite, cerite, allanite, eschinite and cerianite, egirine-albite microclinites with zircon and pyrochlore, and albitites with monacite and pyrochlore. Albitization and microclinization became overprinted by sulfidization, fluoritization, carbonatization, formation of quartz, hematization and zeolitization. The latter resulted in origin of complex parageneses such as silicate-carbonate paragenesis with fluorite, Fe, Cu, Zn, Pb and Mo sulfides, bastnaesite and calcinsite as well as veinlets of calcite-siderite-(hematite )-bastnaesite carbonatites with fluorite, barite, burbankite, calkinsite and rhabdophane.
The Tajno intrusion is about 5 km2 in area. It is built of pyroxenites, melasyenites, nepheline syenites and shonkites, microsyenites and lamprophyres and there are traces of carbonatites. Intrusive breccias, hybridal rocks and metasomatites are common here. Carbonatites has been found in a single borehole. They represent either cement of pyroxenite breccia or a component of complex intrusive breccia - a polyphase product of relatively low temperatures, forming vein-stockwork forms. Three stages in origin of carbonatites have been differentiated.
The early stage is represented by brecciated carbonatite syenites built of fragments of albite microsyenite or, sometimes, ijolite-urtite and cemented with pink calcite with admixture of manganocalcite and, on a smaller scale, burbankite with traces of parisite, fluorite and sometimes barite and sulfides. Albite and carbonates generally occur in comparable amounts. Early carbonatites are found to have interacted with pyroxenite breccia.
The major stage is represented by burbankite-calcite carbonatites built of pinkish calcite with manganocalcite, burbankite with pans1te (up to 20 wt.%) and fluorite, whereas albite and nepheline, sulfides, titanite, Nb-bearing ilmenite, flogopite, apatite, actinolite, chlorite, strontianite and barite are sporadical. Contacts with pyroxenite breccia are sharp, the reaction zones narrow, with characteristic vermiculitization of pyroxenites and fine penetrative carbonate veinlets. Decrepitometric identifications of calcite shows temperatures from 410 to 380°C, and of fluorite - the maximum in the range 320-290°C, and those for homogenization of fluorite - temperatures in the range 310-210°C.
The late stage is represented by calcite carbonatites built of milk-white calcite in paragenesis with fluorite, sporadically with alstonite, carbonates and TR. This stage also comprises sulfides which are accompanied by fine veinlets of quartz, barite, dolomite, or form along with them encrustations on fissure walls. The contact of calcite carbonatites and pyroxenites is sharp, without any zones of reaction. Temperatures of crystallization are estimated at below 160°C.
Identifications of isotopes δ18O (from -4.4 to -2.2‰) and δ13O (from -23.5 to -20.4‰) in carbonates from Tajno carbonatites indicate their affiliation to the alkaline-ultramafic formation and close affinities to kimberlinites.

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