Geodynamical Position of the Pre-Cretaceous Magmatism of the South-Western Edge of the Crimea

Shniukova K.Y.

M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of National Academy of Sciences of Ukraine, Kyiv, Ukraine

Outcrops of igneous rocks are revealed on the south-western edge of the Crimea both onshore and offshore. In the South-Western Crimea magmatic rocks are located in the southernmost part of the Geraclean plateau and exposed to the west from Balaklava along the sea shore for the space of about 7 km around cape Fiolent. This region is clear separated from the rest of Crimea by Georgievsky strike-slip fault that falls on the eastern ending of magmatic outcrops and goes on into the sea. Here, on the Black Sea continental slope magmatic rocks were dredged at three stations 44 km to the south from Balaklava during the expeditions of research vessels «Professor Vodjanitsky» and "Vladimir Parshin" from depths of 1240, 1606 and 1757 m within so-named Foros ledge. The last is formed by sharp bending of bottom contours to the south from Fiolent up to Foros and, in our opinion, is connected with above-mentioned Georgievsky fault. All this area is located in a triple junction of the Scythian Platform, Western Black Sea Depression and Mountainous Crimea and is characterized by continental basificated earth’s crust becoming thinner towards the center of depression (Geophysical parameters..., 1996).

Fiolentsky mass was thought to be a Middle-Jurassic volcano dissected by numerous dykes and underlaid by Tauric flysch (Shatalov et al., 1990), although flysch itself was found neither onshore nor offshore. In V.V.Yudin’s opinion (2003), this area is interpreted as a near-suture melange zone composed of root-free chaotically distributed blocks. After new sampling Fiolentsky mass (FM) has been divided into three sites, namely western, central and east, each of which reflects a certain phase of magmatism, the east being most similar to the offshore site. Acid magmatic rocks play an essential role only in the central site, in all other sites basic rocks being predominant. On the basis of study of petrogeochemical features of sampled rocks with the data on drilling in Geraclean plateau being taken into account two stages in the evolution of pre-Cretaceous magmatism of this area have been specified. Magmatic products bared in the sea and on the east FM site were formed during the first stage, named continental, while those bared on the western and central FM sites refer to the second one, named oceanic. Among the rocks of oceanic stage true andesites are absent, and among basalts there are neither calc-alkali nor tholeiitic rocks – they appear only among more abyssal varieties. Basic rocks of oceanic stage as compared with those of continental one are characterized by rather more high MgO content, little lower K₂O content and clear lower TiO₂, Y, Zr, Rb, Sr contents.

The first known magmatic event in studied area was represented by purely continental acid potassic magmatism of probably Carboniferous age. Since the end of Triassic for the Early Jurassic the area existed as a western elevated shoulder of Southern Crimea back-arc basin (400 million years..., 2005), that’s why Tauric flysch wasn’t accumulated. Compression-extension processes occuring in that basin owing to subduction movements were transferred thereto with delay. Probably in Late Triassic, when all region underwent an Andian-type compression (Early Cimmerian orogeny), revealed in the sea alkaline potassic-sodic and potassic trachybasalts then subalkaline sodic basalts were formed here under extentional conditions on a thick crust. After inversion and transition to an active continental margin in Early Jurassic compressional calc-alkali rocks were originated starting from potassic-sodic and sodic amygdaloidal basalts through andesibasalts (monzonites) founded both in the sea and on the east FM site to andesites (latites). Contitental stage was finished by offshore dacites, K-Ar whole-rock dating of which yields 197 Ma. By that time extension dominated to the east, in Southern Crimea trough. Throughout continental stage the crust became thinner, and after inversion toward the end of Early Jurassic an oceanic stage started with the formation of ophiolite-like thickness including exposed on the western FM site tholeiitic and subalkaline sodic gabbro-diabases, gabbro-norites interbedded with ultrabasic rocks (wehrlites, rare lherzolites, more rare cumulative dunites), as well as specific high-magnesium andesites and plagiorhyolites (the last are drilled in Geraclean plateau). Ultrabasics from the east FM site refer either to ultramafic interlayers of ophiolitic gabbroid complex or most probably to cumulose parts of ultrabasic volcanics of ophiolitic dike complex, to which exotic lamprophyric dikes belong, too. Later, under lasting extention a homogeneous thickness of subalkaline sodic (sometimes MOR-type) spilites occupying the central FM site was formed. Spilitic thickness is intersected by dikes of diabases of the same composition. During the oceanic stage subalkaline magnesium basalts and olivine-dolerites intruded, breaking through the formations of continental stage. Oceanic stage was completed by plagiorhyolites, which were formed, according to K-Ar whole-rock dating, 174 Ma ago, that is when the Southern Crimea trough had been underwent the Middle Cimmerian orogeny. In Late Jurassic magmatic formations had been broken by Georgievsky fault, east block with a part of products of continental stage having been lowered while western block uplifted and displaced.

Thus, from the end of Triassic to Early Jurassic studied area suffered the transition from a passive continental margin to an active one with the basification of crust, and from the end of Early Jurassic up to Middle Jurassic it underwent protracted extension.

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Geophysical Parameters of the Lithosphere of the Southern Sector of the Alpine Orogen / B.S.Volvovsky, V.I.Starostenko (Ed.). Kiev: Naukova Dumka, 1996. 213 p. (in Russian).

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