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“езисы международной конференции

–удный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

   

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

tsymbal@igmof.gov.ua

** Lunds Universitet, Lund, Sweden

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) bove mentioned minerals have been determined 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 Ц 30.7 Ц 39.5; TiO2 Ц 4.3 Ц 6.8; Al2O3 Ц 7.6 Ц 12.4; Cr2O3 Ц 0.10 Ц 0.14; FeO Ц 11.2 Ц 20.5; Fe2O3 Ц 5.5 Ц 12.4; MnO Ц 0.20 Ц 0.28; MgO Ц 5.7 Ц 12.6; CaO Ц 2.3 Ц 9.7; Na2O Ц 1.1 Ц 1.7; K2O Ц 0.9 Ц 1.4; P2O5 Ц 0.3 Ц 0.5; LOI Ц 2.8 Ц 12.4. These rocks are located in the subalkaline olivine basalts field on the classification SiO2 Ц (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 Ц 39.5 Ц 4.6; TiO2 Ц 3.0 Ц 3.5; Al2O3 Ц 9.5 Ц 12.3; Cr2O3 Ц 0.06 Ц 0.11; FeO Ц 7.8 Ц 10.3; Fe2O3 Ц 4.1 Ц 5.1; MnO Ц 0.1 Ц 0.3; MgO Ц 7.4 Ц 9.1; CaO Ц 7.4 Ц 11.4; Na2O Ц 0.9 Ц 2.4; K2O Ц 0.4 Ц 1.4; P2O5 Ц 0.25 Ц 0.42; CO2 Ц 2.8 Ц 4.2; LOI Ц 1.5 Ц 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 Ц alteration time of them by superimposed processes.

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

є grains

U, ppm

Th, ppm

Pb, ppm

Th/U

Isotope ratio

Age, Ma

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 Ц 7 Ц b.5851,  d. 54.0 Ц 57.0 m; grains 8 Ц 14 Ц b.729,  d. 855.5 Ц 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 EarthТs crust in the same time Ц 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.