Geochemical features of xenoliths from the Paleozoic alkali igneous formation of Belarus

Mikhailov N.D., Laptsevich A.G.

Republican Unitary Enterprise «Belarusian Research Geological Exploration Institute», Minsk, Belarus

Abyssal xenoliths that were entrained, carried into the near-surface zones and “sealed” in igneous rocks have been studied in order to assess the depth of occurrence and evolution pattern of the primary magma, which generated the diversity of explosive and extrusive alkali rocks of the Zhlobin Saddle (Zhs), North-Pripyat zone of steps (NPzs) and Gomel structural dam (Gsd). Xenoliths of various crystalline rocks, the abyssal ones included, were distinguished among the studied igneous rocks. Such xenoliths are represented by wholly crystalline igneous and metamorphic rocks, among which mafic garnet granulites and basic ultramafites (hornblendites, more seldom, pyroxenites) are dominant. Xenoliths of ultramafic rocks represented by profoundly altered peridotites and characterized by the absence of garnet and almost total absence of ore minerals occur very seldom in isolated pipes of Zhs (Taran, Veretennikov, 2000).

Garnet granulites are most abundant in rocks of the revealed Zhs diatremes and their incurrent canals, as well as in the Gsd rocks. This type granulites involve a wide variety of petrographically similar ultrametamorphic rocks: from typically crustal garnet granulites relatively rich in plagioclase to plagioclase-deficient (5-10%) eclogitе-like granulites.

Xenoliths of hornblendites are mostly widespread among the NPzs rocks and are the second most abundant (after granulites) petrographic type of abyssal xenoliths in the Zhs and Gsd diatreme rocks. Single xenoliths of pyroxenite were found among the Zhs and Gsd rocks.

Besides, there are abundant xenoliths of amphibolites (these are mostly typical of the NPzs rocks), xenoliths of anorthosite (typical xenoliths in the NPzs magmatic rocks) and xenoliths of various acidic rocks – granites, granite-aplite, granitе-gneisses (rocks of the crystalline basement of Belarus) (Taran, Veretennikov, 2000).

Xenoliths of all the above rocks are usually fragmental inclusions clearly defined in their enclosing rock, irregular or rounded - angular in shape measuring from a few fractions to several centimeters, more seldom, 10 cm and over.

The Sr and Nd isotopic composition of the studied xenoliths from Paleozoic igneous rocks of Belarus is representative of the petrographic and petrochemical division of garnet granulites and hornblendites into groups (Markwick et al., 2001). The first group of xenoliths is described by the isotope ratios as follow: ⁸⁷Sr/⁸⁶Sr=0.70409-0.70465, ¹⁴³Nd/¹⁴⁴Nd=0.51142-0.51193. The second group is formed by hornblendites showing the less radiogenic strontium composition ⁸⁷Sr/⁸⁶Sr=0.70304-0.70341 and the more radiogenic niodymium composition ¹⁴³Nd/¹⁴⁴Nd=0.51237-0.51245. The abovesaid groups of xenoliths differ also in εSr values: εSr(+) in the first case, and εSr(-) in the second case. There are no such significant differences in εNd, though its values are higher (2.2 – -2.9) for hornblendites and smaller (-10.6 – -19.5) for granulites. The simulated Sm-Nd age of sources of the studied xenoliths corresponds to the age interval T_DM=0.98–2.47, T_DM2=0.97–2.75 Ga. The older age values were obtained for granulites – 2.03–2.75 Ga, the second age group includes hornblendites -0.97–1.39 Ga.

A diagram of distribution of the rare earth elements in xenoliths (see Figure) was constructed with the data available (Markwick et al., 2001) and new data obtained from the Gsd xenoliths. A comparison analysis revealed considerable differences in the slopes of the curves representing their distribution in various type xenoliths. The Gsd xenoliths are described by the higher REE amounts and the higher degree of fractionation of light rare earth elements. The other xenoliths are characterized by rather low REE concentrations, however, these are increased in garnet granulites, where the niodymium maximum is well pronounced and reflected in the isotope values too. The lowest REE concentrations were determined in hornblendites. The europium anomaly is noted in some types of xenoliths. Granulites and hornblendites show, as a rule, its positive value, which is apparently due to the crustal source, while the opposite sign is typical of amphibolites.

The petrographic variety of crystalline rock xenoliths revealed in Devonian igneous rocks of Belarus and their geochemical features suggest their different genetic nature, because their generating rocks could be formed only in different facies-depth zones of the Earth’s crust and upper mantle.

References:

Taran L.N., Veretennikov N.V. Abyssal xenoliths in volcanic pipe rocks of Belarus/Doklady NAS Belarusi. 2000. Vol.44, №6. P.63-65 (in Russian).

Markwick A.L.W., Downes M., Veretennikov M. The lower crust of SE Belarus: petrological, geophysical and geochemical constraints from xenoliths // Tectonophysics. 2001. № 339. P. 215-237.

Figure: Distribution of the rare earth elements in xenoliths from Paleozoic igneous rocks of Belarus.

Table. Isotope-geochemical and simulated age characteristics of alkali igneous rocks of Belarus

№	Sample	Rb, _ppm	Sr, _ppm	⁸⁷Sr/⁸⁶Sr ₍₃₇₀_Ma)	ε Sr	Sm,_ppm	Nd,_ppm	¹⁴⁷Sm/¹⁴⁴Nd	¹⁴³Nd/¹⁴⁴Nd _{(370 Ma)}	εNd	T (DM)	T (DM-2)
Zhlobin Saddle
1	Luch-13	48.2	887	0.703614		6.0	44.07	0.08195	0.512468	2.1	791	962
2	Bel-56	60.52	1223.8	0.704110		8.7	56.78	0.09223	0.512222	-3.2	1166	1401
3	By 14	36.7	1074	0.70420	1.61	8.34	54.5	0.09211	0.51232	-1.2	1041	1240
4	РК 501	45	858	0.70472	9.4	5.16	30.1	0.10318	0.51237	-0.8	1078	1205
5	РК1	61	346	0.71747	50.0	5.23	29.4	0.10707	0.51254	2.3	875	944
6	372	25.54	763.7	0.70427	0.2	5.239	30.78	0.10245	0.51229	-2.3	1180	1331
7	1	36.65	846.2	0.70404	-3.0	5.087	30.38	0.10079	0.51231	-1.9	1136	1292
8	34cpx	0.896	140.4	0.70325	-14.2	1.394	5.967	0.14062	0.51249	-0.2	1379	1157
9	34gm	41.1	1116.5	0.70391	-4.9	5.609	33.72	0.10012	0.51228	-2.4	1169	1338
10	74	31.8	452.3	0.70574	21.0	4.148	24.23	0.10304	0.51228	-2.6	1200	1350
11	122	65.5	733.3	0.70353	-10.3	2.618	20.91	0.07536	0.51256	4.2	652	787
12	163	66.2	773.7	0.70389	-5.1	6.3	35.1	0.10804	0.51253	2.1	897	964
13	550	46.6	614.1	0.70531	15.0	4.202	25.32	0.09989	0.51223	-3.4	1233	1418
North-Pripyat zone of steps
14	Bel-25	73.65	1553.5	0.704051		23.51	154.76	0.09144	0.512506	2.4	805	938
15	Bel -30	45.16	811.7	0.704706		8.55	49.56	0.10384	0.512336	-1.5	1131	1262
16	Bel -33	48.59	4523.2	0.706694		7.73	47.5	0.09795
Gomel structural dam
17	Bel -131	77.13	202.45	0.706133		2.37	10.56	0.13509	0,512257	-4.5	1731	1513
18	Bel -139	40.97	1332.4	0.704353		8.04	49.9	0.09700	0,512354	-0.8	1042	1206
19	Bel -132	103.61	339	0.705199		2.04	18.73	0.06555	0,512118	-3.9	1063	1465
Pripyat Graben
20	Bel -6	51.48	480.6	0.705559		9.84	58.44	0.10134	0.512302	-2.0	1152	1308
21	Bel -7	31.64	685.3	0.704436		3.69	16.25	0.13668	0.512759	5.2	777	705
22	Bel -16	14.49	525.7			6.93	43.9	0.09502	0.51249	1.9	853	981
23	ООР23	125	821	0.70328	-11.1	2.75	18.5	0.08947	0.51263	4.9	640	729
24	ООР1В	13	938	0.70382	-3.4	8.26	48	0.10358	0.51256	2.9	819	898
25	ООР2	56	1076	0.70373	-4.6	8.26	47.8	0.10401	0.51252	2.1	878	964
26	ООР40а	56	1076	0.70476	9.9	10.0	54.4	0.11065	0.51255	2.4	890	942

Note. Samples: Luch-13 – lamprophyre; Bel-56 – melaleucitite; By14, 372, 1 – melanephelinite; РК 501, РК 1, 74, 163, 550 – picrite; 34cpx – clinopyroxene; 34 gm – garnet; 122 – melilitite; Bel-25, Bel-33 – basaltoid; Bel-30 – ultramafic rock; ; Bel-131, Bel-139 – shonkinite, Del132 – syenite-porphyry; Bel-6 – lamprophyre; Bel-7, ООР23 – nephelinite; ООР1В, ООР2,ООР40а – picrite. Analyses 3-13 and 23-26 after Wilson, Lyashkevich, 1996, Markwick et al., 2001;Pervov et al., 2004.