International Geological Journal - Official Journal of the Carpathian-Balkan Geological Association

K/AR DATING OF NEOGENE CALC-ALKALINE VOLCANIC ROCKS FROM TRANSCARPATHIAN UKRAINE

Pages: 83 - 89

Authors: ZOLTAN PECSKAY, IOAN SEGHEDI, HILARY DOWNES, MICHAIL PRYCHODKO, BOGDAN MACKIV

Abstract: The Neogene Carpathian arc is a complex magmatic arc, extending from Slovakia into Romania. The Transcarpathian region in SW Ukraine comprises the central part of this arc and was active in the Middle-Late Miocene. The volcanic structures of the Transcarpathian region can be divided into three major areas: a—Outer Arc; b—Intermediate zone; c—Inner Arc. This division reflects the basic tectonic features of the Ukrainian Carpathians, but differs from other parts of the Carpathian arc. The Outer Arc consists of a number of overlapping stratovolcanic structures, generally composed of lava flows, domes, dykes/sills, volcanic necks and lahars of basaltic andesite, andesite and dacite composition. In the Inner Arc, tuffs, ignimbrites and ash deposits of dacite, rhyodacite and rhyolite are abundant. Lava flows of andesitic and basaltic-andesitic composition are present, together with domes of dacite and rhyolite. The Intermediate zone is composed of lava domes and small andesitic shield volcanoes. New K/Ar data obtained from 57 volcanic rock samples has yielded K/Ar ages of 13.4–9.1 Ma. This time interval (~4.5 million years) is similar to that of the neighbouring Carpathian volcanic regions to the west (Tokaj Mts., Hungary) and to the east (Călimani, Romania). Badenian rhyolitic tuffs buried within the Transcarpathian area represented the earlier phases of magmatism, but they have been dated only by biostratigraphic methods. Thus, there is no evidence in this area for any along-arc migration of volcanism, unlike in the Eastern Carpathians of Romania. Formation of volcanic structures started simultaneously in both Outer Arc and Inner Arc volcanic areas (13.4 Ma). Different peaks of volcanic activity were observed: (a) between 13.0–11.5 in the Inner Arc, interpreted as the major period of generation of a complex of resurgent domes related to formation of a caldera, probably situated toward the central part of the Transcarpathian depression, and (b) between 11.2–10.5 Ma in the Outer Arc, representing the main period of volcano generation. The end of the volcanic activity (9.5–9.1 Ma) was scattered and less voluminous.

Keywords: SW Ukraine, Transcarpathia, Neogene volcanic activity, calc-alkaline volcanic rocks, K/Ar dating, volcanology

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