Composition and ages of subalkaline gabbroids from dike complexe of Central part of Kirovograd megablock in the Ukrainian Shield

Tsymbal S.N.*, Kryvdik S.G.*, Shumlyanskyy L.V.*, Bogdanova S.V.**, Tsymbal Y.S.*

*N.P. Semenenko Institute of geochemistry, mineralogy and ore formation NAS of Ukraine, Kyiv, Ukraine

The broad distribution of subalkaline gabbrous dikes of hypabyssal and subvolcanic facies are established. The composition of these rocks and rockforming minerals are studied. Age of accessory zircon have been determined by U-Pb method. The central parts of its crystals from gabbrids have 2074-2041 and 2056-2046 Ma age in two dikes but in the rims age are 2023-1852 and 2026-2010 Ma respectively. Rejuvenation of the external zircon zones is caused by more late superimposed processes influence.

In the central part of Kirovograd megablock more then 200 dikes of different composition and age are known. They form Ustinivka-Znamenka fild elongated in the West-Eastern direction near 100 km and wide up to 30-35 km. These dikes have North-Western, North-Eastern, submeridian and sublatitudinal strikes. Frequently they crosscut each other that is the base of relative age determination. Unfortunately there is not enough pay attention to complex investigation and dating of dike complex rocks by modern methods.

Our investigation shown that dikes consist from rocks of normal, subalkaline and alkaline series of hypabyssal and subvolcanic facies. Among the rocks of subalkaline series crystalline gabbroids with variable contents of olivine, Ti-augite, kaersutite, Ti-biotite, plagioclase, potassic feldspar, ilmenite, magnetite and other minerals are mostly wide spread. In some dikes of subalkaline gabbroids accessory zircon is found. The chemical composition (microprobe analyses) of above mentioned minerals have been determined by means of JCXA 8200, and dating of separate zircon crystals (16 data) by means of ionic mass-spectrometer Cameca IMS 1270 in laboratory of Nature museum in Sweden (Stockholm) have been made.

One of studied dike is situated in 50 km to the East of Kirovograd city near Nova Praga town within the Western part of Ingul-Ingulec synclinorium mainly composed by gneisses and crystalline sheets of Ingulian series (2400-2100 Ma). The dike was uncovered by three boreholes. It has thickness of about 15 m and traced almost 5 km in submeridian direction. Composed rocks of dike were determined by A.I. Vorobey (1979)as gabbro-pyroxenites and refered to Mezoproterozoic gabbro-wehrlite complexe. Ours additional petrographic investigations testify that these rocks are meso- and melanocratic ones of subalkaline type. Their rock-forming minerals are represented by olivine, Ti-augite, plagioclase, kaersutite, Ti-biotite, ilmenite and Ti-magnetite. Accessory minerals are apatite, zircon, rutile.

Olivine forms coarse grains or their cumulates. Its amount is usually less 10% in the hanging dike wall and about 20% in the lying one. It is almost fully replaced by serpentine and talc. Titanaugite is observed as fine grains and glomeroporphyritic segregations up 2-6 mm length. Kaersutite is presented by coarse (up to 1-2 mm) prismatic-oblong grains with olivine, Ti-augite, apatite and zircon inclusions. Kaersutite is a postmagmatic mineral. The alkaline hornblende of green-blue color forms the rims around the kaersutite. Biotite is often associated with amphibole and ore mineral and sometimes contains fine zircon inclusions.

The chemical composition of gabboids of this dike varies in the range, %: SiO₂ – 30.7 – 39.5; TiO₂ – 4.3 – 6.8; Al₂O₃ – 7.6 – 12.4; Cr₂O₃ – 0.10 – 0.14; FeO – 11.2 – 20.5; Fe₂O₃ – 5.5 – 12.4; MnO – 0.20 – 0.28; MgO – 5.7 – 12.6; CaO – 2.3 – 9.7; Na₂O – 1.1 – 1.7; K₂O – 0.9 – 1.4; P₂O₅ – 0.3 – 0.5; LOI – 2.8 – 12.4. These rocks are located in the subalkaline olivine basalts field on the classification SiO₂ – (Na₂O + K₂O) diagram.

Zircon containing in these rocks was used for obtaining isotopic age data. It is represented by prismatic-oblong grains of 0.5 mm size with distinctly displayed magmatic zonality. The rim and central parts of crystal were analyzed (6 and 2 data correspondingly). The results are given in the tabl. Evidently, the rim parts of zircon crystal are usually richer in U (1005-2637 ppm) than central ones (147-584 ppm). The age of central parts of zircon crystal calculated by ratio ²⁰⁷Pb/²⁰⁶Pb is 2074-2041 Ma and age of rims is 2023-1852 Ma. Obtained data testify that zircon crystals have been influenced by superimpose processes after their formation and rejuvenation of the crystal rim caused by U introducing.

The second investigated dike of subalkaline gabbroids is located on Northern outskirts of Kirovograd near the Leleky micauos kimberlite dike aged 1800 Ma. Dikes are confined to Leleky deep fracture that is the branch of Kirovograd deep fracture of submeridian direction. The texture of these gabbroids is porphyraceous. Phecrysts make about 25-30% and are represented by olivine and Ti-augite of size up to 1.5 mm. Matrix consist (%) of plagioclase (45-50), Ti-augite (40-45), kaersutite (1-2), biotite (< 1) and ore minerals (about 5). The chemical composition of gabbroids is follow as (%): SiO₂ – 39.5 – 4.6; TiO₂ – 3.0 – 3.5; Al₂O₃ – 9.5 – 12.3; Cr₂O₃ – 0.06 – 0.11; FeO – 7.8 – 10.3; Fe₂O₃ – 4.1 – 5.1; MnO – 0.1 – 0.3; MgO – 7.4 – 9.1; CaO – 7.4 – 11.4; Na₂O – 0.9 – 2.4; K₂O – 0.4 – 1.4; P₂O₅ – 0.25 – 0.42; CO₂ – 2.8 – 4.2; LOI – 1.5 – 13.2.

Zircons selected from subalkaline gabbroids of subvolcanic facies are presented by prismatic crystals with distinctly developed prisms {100} and {110}. Crystal domes are represented by tetrahedral prism faces of bipyramid. On the crystal cut the magmatic zonality is distinctly displayed and sporadic inclusions of olivine microcrystal occur. As is shown in Tabl., zircons have increased U, Th and Pb contents. As exception is only one zircon crystal with negligible contents of mentioned elements. From eight determined values six are in the central crystal part and two in the rims. Age value is 2056-2046 and 2026-2010 Ma correspondingly. The first ones correspond to the time formation of gabbroids and the second ones – alteration time of them by superimposed processes.

U-Pb zircon age of subalkaline gabbroids from hypabyssal (grains 1 – 7) and subvolcanic (grains 8 – 14) facies in the central part of Kirovograd megablock

№ grains	U, ppm	Th, ppm	Pb, ppm	Th/U	Isotope ratio			Age, Ma
№ grains	U, ppm	Th, ppm	Pb, ppm	Th/U	²⁰⁶Pb/²⁰⁷Pb	²⁰⁶Pb/²³⁸U	²⁰⁷Pb/²³⁵U	²⁰⁶Pb/²³⁸U	²⁰⁷Pb/²³⁵U	²⁰⁷Pb/²⁰⁶Pb	²⁰⁸Pb/²³²Th
1 core	1168	279	461	0,203	0,12634	0,3313	5,77121	1844,7	1942,1	2047,6	1750,6
1 rim	2637	189	785	0,048	0,12465	0,2603	4,47357	1491,4	1726,1	2023,8	1367,4
2 core	184	121	81	0,623	0,12708	0,3329	5,83331	1852,5	1951,4	2058,0	1961,2
3 core	147	128	70	0,814	0,12826	0,3693	6,40991	1899,2	1984,4	2041,5	1900.2
4 core	584	90	251	0,143	0,12590	0,3426	6,05892	2025,9	2033,6	2074,3	1938,5
5 core	502	149	311	0,139	0,11324	0,2659	4,15099	1519,7	1664,4	1852,1	1738,1
6 rim	1005	144	195	0,239	0,12651	0,3213	5,60465	1796,1	1916,8	2050,0	1728,0
7 core	512	154	215	0,267	0,12671	0,3476	6,07258	1923,1	1986,3	2052,8	1831,2
8 core	495	227	226	0,459	0,12692	0,3618	6,33049	1990,5	2022,7	2055,7	2055,4
9 core	969	360	437	0,369	0,12691	0,3651	6,38926	2006,5	2030,8	2055,6	2039,6
10 core	357	125	163	0,354	0,12624	0,3716	6,46763	2036,9	2041,5	2046,2	2072,4
11 core	274	167	131	0,623	0,12669	0,3647	6,37093	2004,5	2028,3	2052,6	2103,3
12 core	607	487	254	0,670	0,12368	0,3107	5,29850	1744,2	1868,6	2010,0	1693,1
13 rim	1225	314	541	0,252	0,12695	0,3678	6,43747	2018,9	2037,4	2056,2	2019,2
14 core	113	49	53	0,439	0,12647	0,3725	6,49556	2041,2	2045,3	2049,5	2079,6
14 rim	603	174	246	0,276	0,12483	0,3372	5,80405	1873,3	1947,0	2026,3	1942,4

Grains 1 – 7 – b.5851, d. 54.0 – 57.0 m; grains 8 – 14 – b.729, d. 855.5 – 856.0 m.

Thus dikes of subalkaline gabbroids of hypabyssal facies distanced each from other in more then 50 km had been intruded in the Earth’s crust in the same time – about 2050 Ma ago. Obtained data unambiguously evidence that alkaline gabbroids have been formed some early than known here Leleky and Schors occurrences of dike facies kimberlites (1800 Ma). Presence of intrusive bodies of subalkaline gabbroids and basaltoids in the Ukrainian Shield allows to consider these rocks as an important petrological and search attribute for kimberlites. These rocks and areas of their distribution are to be purposefully studied by means of modern methods.