Lamprophyric rocks of the Devonian alkali igneous complex of Belarus

Mikhailov N.D., Laptsevich A.G.

Republican Unitary Enterprise «Belarusian Research Geological Exploration Institute», Minsk, Belarus, mihailov@igig.org.by

 

Among petrographically various Devonian alkali igneous rocks found in the territory of Belarus lamprophyres described by the presence of biotite and phlogopite in their mineral composition along with considerable amounts of ore minerals are rare in occurrence. These rocks belong mainly to hypabissal formations and occur as small sills, dikes and stocks in the Pripyat Trough and North-Pripyat igneous province (Korzun V.P., Makhnach A.S., 1977; Veretennikov N.V. et al., 2007).

Within the Pripyat Trough lamprophyres are found among rocks of the Vasilievsk (Artukovskaya 3 borehole), Vetkhino (Vetkhinskaya 1 borehole), Dnieper (Dnieprovskaya 1 borehole) palaeovolcanoes. In the North-Pripyat igneous province lamprophyres are found in the southern part of the Luchin cluster of the Zhlobin diatreme field (anomaly named «Shliakh Selianina»).

Lamprophyres of the Pripyat graben are feldspathic. In the Vetkhinskaya 1 borehole lamprophyres (vogesites according to V.P. Korzun) were exposed in deposits of the Semiluki horizon (at a depth of 4.565-4.582m), where form a small dike about 20 m in thickness. The rocks are fine-granular with porphyries of hornblende substituted by calcite and chlorite (penninite). The bulk rock is composed of leistes of potassium-sodium feldspar sometimes showing trachytoid texture. The whole rock mass is abundant in biotite scales confined to feldspar leistes or occurring as microporphyries. There are numerous impregnations of ore minerals - titanomagnetite and magnetite, the first one being dominant. Lamprophyres from the Artukovskaya 3 borehole at a depth of 4.514 to 4.518m adjacent on the north to the Vetkhino palaeovolcano are in the similar geological position in a section, where these form a vein small in thickness (about 2 m) in the Semiluki deposits and are similar to lamprophyres from the Vetkhino palaeovolcano, but the porphyries are larger in size there. Lamprophyres have been also determined  in the northwestern area of igneous rock occurrence within the Pripyat graben (Dnieper palaeovolcano, Dnieprovskaya 1 borehole, depth of 3.564 m). They occur there as a dike about 5 m in thickness among quartzitic sandstones. The rock is a finely crystalline feldspar matrix, where coloured minerals form the lamprophyric texture, and hornblende, biotite and ore minerals comprise about 60% of the bulk rock. Pleochroic biotite from light yellow to dark-brown in colour together with porphyries and ore minerals form an abundant spot impregnation. The porphyries of strongly altered olivine and pyroxene are often found in the rock.

Lamprophyres from the second region (North-Pripyat igneous province) occur in two boreholes of the «Shliakh Selianina» anomaly: 761 (229-243m) and 763 (225-233m). In the borehole 761 lamprophyres were exposed in the lower part of the section. Rocks are grey, dark-grey to black in colour, mainly massive in structure, microcrystalline, often aphanitic in texture. Phlogopite (biotite), olivine, calcite, magnetite, monoclinal pyroxene are rock-forming minerals. Calcite, serpentine-serpophite, hydrophlogopite, chlorite are determined among secondary minerals. Apatite and spinel occur as accessories. The bulk rock is phlogopite which occurs as small scales or poikilocrystals with magnetite impregnation, carbonate and a mix of secondary minerals: serpentine-serpophite, calcite, montmorillonite-saponite with magnetite impregnation. Porphyric impregnations are represented by olivine pseudomorphs of two generations, phlogopite (biotite), magnetite, pyroxene replaced by calcite and chlorite is rarely observed in the depth range  from 231 to 232m. The calcite abundance increases in the contact zones. Lamprophyres from the borehole 763 are overlain by melilite-lamprophyric breccias. The bulk rock is carbonate with melilite, hydromica and chlorite pseudomorphs. Femic minerals are represented by phlogopite (biotite), magnetite. Lamprophyres in the bottom part consist of olivine of two generations, sodalite; pyroxene porphyries occur sometimes. Carbonate makes a considerable contribution to the bulk rock, which is typical of similar rocks (Williams X. et al., 1985).

Geochemical data obtained for the studied rocks suggest that lamprophyres from two regions show minor geochemical distinctions. These are generally described by rather high titanium, iron and alkali contents. These rocks are alkali varieties and in the TAS classification diagram occupy the bottom left part of all the ultrabasic rocks of the Palaeozoic igneous province determined in Belarus.  Lamprophyres are rich in potassium (Na/K=0.2-0.4), those from the North-Pripyat  province show an increase in the sodium content (Na/K=0.9). There is a difference in the coefficients of alumina content. Rocks from the Pripyat graben are described by moderate alumina contents, and lamprophyres from the North-Pripyat  province are low in alumina, however, with such an insignificant distinction both rocks from two regions are of miaskite nature (K_a<1), sometimes, insignificantly modified to the agpaitic one (Vetkhinskaya 1 borehole, K_a =0.7) (seeTable).

Geochemical peculiarities of lamprophyres are presented in a diagram showing the distribution of trace elements normalized to the primitive mantle (see Figure). With a rather similar pattern of the trace element distribution in lamprophyres of both regions, those from the Pripyat graben show increased amounts of K, Nd, Sm, Zr, Hf, Eu, Sn, Sb, Ti, Gd, Tb, Dy, Li, Y, Ho,Er, Tm, Yb, Lu and decreased amounts of U, Th, Pb as against their contents of rocks from the North-Pripyat  province. The maximum contents of Ba noted in lamprophyres of the North-Pripyat province, especially in their carbonatized varieties (where the Ba concentration in rock runs up to 1%), as well as of La and Pb should be particularly emphasized.

The research was performed under the financial backing of the BRFFI, grant X08P-087.

References:

Korzun V.P., Makhnach A.S. Upper-Devonian alkali volcanogenic formation of the Pripyat Trough. Minsk, Nauka i Tekhnika Publ. 1977. 164 p. (in Russian).

Sun S.S., McDonough W.F. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes // Magmatism in the Ocean Basins. Geol. Soc. London Spec. Publ.1989.Vol. 42. P. 313-346.

Veretennikov N.V., Laptsevich A.G.,Vladykin N.V., Mikhailov N.D. Association of alkali lamprophyres and carbonatites of the Luchin diatreme cluster as a positive magmatic criterion of forecast of and search for potentially diamondiferous kimberlites // Innovations in Geological Science - a Way of Efficient and Multipurpose Utilization of Mineral Resources. Minsk, 2007. P.55-58 (in Russian).

Williams H., Turner F., Gilbert Ch. Petrography. Vol.1. Moscow, 1985. 301 p. (in Russian).

 

Table

оxides	Feldspathic lamprophyres of the Pripyat graben				Lamprophyres of the North-Pripyat region
SiO2	39.23	38.69	39.6	44.53	37.03	36.7	35.76	35.16	34.58	28.54	35.72	33.65
TiO2	2.09	3.55	2.77	3.11	2.06	2.19	1.84	2.1	2.14	2.36	2.23	2.39
Al2O3	8.17	9.83	11.16	9.85	4.4	4.4	3.9	4.6	4.3	4.53	4.31	4.37
Fe2O3	10.12	16.12	13.99	10.97	15.46	16.12	14.4	15.15	14.49	15.00	13.92	15.13
MnO	0.12	0.13	0.09	0.08	0.17	0.17	0.2	0.24	0.23	0.20	0.24	0.23
MgO	12.65	9.16	15.18	10.73	22.87	23.51	26.52	21.7	21.74	16.46	18.55	19.35
CaO	12.37	10.67	2.94	5.74	1.63	2.13	2.44	6.0	7.52	12.45	9.45	8.99
SrO	0.05	0.09	0.04	-	0.08	0.1	0.11	0.1	0.12	0.08	0.09	0.08
BaO	0.25	0.09	0.13	-	0.05	0.05	0.05	0.05	0.05	0.09	0.10	0.10
Na2O	0.36	1.08	0.59	1.89	0.5	0.55	0.23	0.39	0.3	0.58	0.35	0.36
K2O	2.76	3.00	3.46	6.67	2.19	1.99	1.99	2.63	2.58	2.31	2.20	2.37
P2O5	0.45	0.74	0.61	0.61	0.6	0.6	0.55	0.61	0.6	0.57	0.58	0.59
H2O	4.65	3.76	6.76	0.00	11.35	9.53	9.59	6.43	6.17	-	-	-
CO2	6.13	2.83	2.55	-	1.02	1.14	1.61	4.04	4.85	-	-	-
F	0.34	0.26	0.23	0.07	0.31	0.32	0.35	0.34	0.32	-	-	-
LOI	10.48	6.11	8.83	-	12.37	10.67	11.20	10.47	11.02	16.55	12.11	12.22
Total	99.7	100.04	100.11	94.25	99.76	99.50	99.50	99.60	100.02	99.75	99.85	99.86
Na2O+K2O	3.12	4.08	4.05	8.56	2.69	2.54	2.22	3.02	2.88	2.86	2.55	2.73
Na2O/K2O	0.13	0.36	0.17	0.28	0.23	0.28	0.12	0.15	0.12	0.25	0.16	0.15
al'	0.36	0.39	0.38	0.45	0.11	0.11	0.10	0.12	0.12	0.15	0.13	0.13
Ka	0.38	0.42	0.36	0.87	0.61	0.58	0.57	0.66	0.67	0.63	0.59	0.62