K-Ar dating of Mesta volcanics (SW Bulgaria)

Pages: 
pp. 3-11
Institute of Nuclear Research, Hungarian Academy of Sciences (ATOMKI), Bem ter 18/c, H-4001, Debrecen, Hungary
Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
Abstract: 

The Mesta volcanic massif (SW Bulgaria) belongs to the Macedonian-Rhodope-North Aegean Magmatic Zone. It crops out along the middle course of Mesta River and covers about 200 km2 of the homonymous graben. The bаsеmеnt of Mesta graben and the neighbouring elevated blocks of Pirin and Western Rhodope mountains are composed of high-grade metamorphics, hosting granitoidic plutons of diverse ages, including the Paleogene Central Pirin and Teshevo plutons. Apart from the volcanic bodies, the graben-fill consists of complexly interfingered Paleogene stratified continental terrigenous sedimentary rocks, volcanogenic-sedimentary rocks and volcaniclastics (both руrо-, and epiclastics), overlain unconformably bу Neogene and Quaternary deposits.
   The Mesta volcanics are represented by two main types: high-K calc-alkaline to shoshonitic dacites (in some places with а transition to latites) and high-K calc-alkaline rhyodacites. The magmatic bodies of subvolcanic and extrusive facies are represented by а number of morphologic varieties, which belongs to two polygonal caldera structures (Кrеmеn and Banichan) and two linear magmato-tectonic zones (Gostoun and Dobrinishte).
   This paper presents 12 К-Аг ages obtained for 11 lауа bodies of different geological position and for one re-deposited volcanic clast from the middle part of the stratified Paleogene section. The ages are in support of the suggestion that the Mesta volcanics originated from а zoned magma chamber. More silicic (rhyodacitic) eruptions and subvolcanic intrusions are related to earlier volcanic events in the northern part of the massif - оn the territories of the Кrеmеn caldera, Gostoun and Dobrinishte zones. These events (approximately 33-31 Ма) are рге-, and sincollapse ones in respect to the Кrеmеn caldera formation. The later (sin- and post-collapse) rhyodacitic eruptions were accompanied and followed by eruptions and subvolcanic intrusions of less silicic, dacitic melts (31-28 Ма). The main portion of the dacites is related to the centres in the Banichan caldera (29-28 Ма).
   The time-span obtained covers the events of the paroxysmal volcanic activity in the Mesta graben only. Thus it is shorter than the real time-interval of the volcanism. This time-span is comparable with that of the voluminous Раlеоgene volcanism in the Smolyan region (Central Rhodopes). The Rb-Sr ages of neighbouring Teshevo and Central Pirin plutons plot in its boundaries as well.
   Some recommendations for more detailed and precise future radiometric studies are proposed to define the age of the initial and the final volcanic events in the Paleogene Mesta graben.

З. З. Печкай. Ал. В. Харковска. Ал. Хаджиев. K-Ar возраст вулканитов Местенского массива (Юго-Западная Болгария). Местенский вулканический массив сложен палеогеновыми кислыми (высококалиевыми и шошонитовыми) екструзивными, субвулканическими и эффузивными телами разнообразной морфологии и вулканокластическими (пиро- и эпикластическими) породами разнообразного генезиса, которые сложно сочленяются по латерали с терригенными континентальными отложениями Местенского грабена.
   В работе при водятся и обсуждаются результаты К-Ar анализа 11 вулканических тел, связанных с пароксизмальными извержениями в палеогеновом Местенском грабене и один анализ вулканической гальки из средних отделов разреза. Полученные данные подтверждают представления о том, что становление массива связано с развитием зонированной магматической камеры. Извержения более кислых вулканитов (риодацитов) в его северной части происходили на ранних этапах (33-31 Ма) магмато-тектонического развития, в ходе которых началось оформление Кременской полигональной кальдерной структуры и двух линейных структур (Гостунской и Добринишской). Их становление продолжалось и при непосредственно последовавших извержениях менее кислых, дацитовых расплавов (31-28 Ма). Основная масса дацитов приурочена к Баничанской кальдерной структуре в южной части массива (29-28 Ма).
   Полученные датировки сопоставимы с возрастом магмато-тектонических событий в области Центральных Родоп и с временем становления палеогеновых плутонов в соседнем Пиринском горсте.
   Приложен список обнажений, которые рекомендуются для будуших радиометрических исследований в целях установления возраста самых ранних и самых поздних вулканических проявлений в обхвате Местенского грабена.

Keywords: 

Paleogene, Mesta graben (SW Bulgaria), volcanics, K-Ar ages

VOLUME 30 (1-2)/June 2000

Institute of Nuclear Research, Hungarian Academy of Sciences (ATOMKI), Bem ter 18/c, H-4001, Debrecen, Hungary
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Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
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Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
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Institute of Nuclear Research, Hungarian Academy of Sciences, Debrecen, Hungary
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Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
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Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

Department of Geology and Palaeontology of the I. Javakhishvili Tbilisi State University, Chavchavadze Av. I, 380028, Tbilisi, Georgia
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Department of Geology and Palaeontology of the I. Javakhishvili Tbilisi State University, Chavchavadze Av. I, 380028, Tbilisi, Georgia
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Department of Geology and Palaeontology of the I. Javakhishvili Tbilisi State University, Chavchavadze Av. I, 380028, Tbilisi, Georgia

Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
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Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
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Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
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Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
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Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
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"Sofgeoprouchvane" Ltd.

Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

Natinal Museum of Natural History, Bulgarian Academy of Sciences, 1, Tsar Osvoboditel blvd., 1000 Sofia, Bulgaria
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University of Thessaloniki, Laboratory of Geology and Palaentology, 54006 Thessaloniki, Greece

Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria