In this work we discuss the coexistence and possible interaction or two magma types in the evolution of the Early Miocene Borac Eruptive Complex (BEC), situated about I 00 km south of Belgrade. During the Early Miocene two phases of volcanic activity and caldera formation occurred. The first phase gave rise to widespread dacite lava flows (Ostrica caldera). After a short repose period, second stage Plinian to sub-Piinian eruptions occurred and rhyodacite/rhyolite pumice-fall deposits and unwe lded ignimbrites were fQrmed (Borac caldera). This was immediately followed by the emission of quartz latite high-aspect ratio lava flows, lava domes and finally andesite lava flows and hyaloclastic deposits. Lamprophyre sills and an autobrecciated lava flow also formed . Petrochemical characteristics imply the coexistence o f two different magmas during the second stage of the BEC evolution. The main suite, ranging from andesite to quartzlatite and rhyodacite/rhyolite is a high-K calc -alka line (HKCA) series. In contrast, the lamprophyres correspond to a potassic/ultrapotassic alkaline (P/UP) series (K2O/ Na20 ≈2). Major element mass balance calculations show that no simple fractional crystallisation process could have produced the Borac HKCA rocks from the coexisting lamprophyres. However, in a two-step model of crystal fractionation it is possible to obtain the most evolved calc-alkaline rock in the area from a pyroxene-hornblende andesite, using a quartzlatite as the intermediate member. Evolution of the HKCA suite occurred in a shallow magma chamber, approximately 7-8 km in diameter and around 5-6 km thick. Temperature estimates are 750-850°. Lamprophyric magma was injected into the chamber just as it had already become stratified with a rhyodacitic rhyolitic roof, a quartz latitic middle part and an andesitic base. This injection of hot, gas rich lamprophyric melt triggered a Plinian eruption thereby preventing the most evolved acid melts from being involved in the mixing.
Eruptive history and low-pressure evolution of the Early MioceneBorac Eruptive Complex (Central Serbia)
POLI, Giampiero;
2001
Abstract
In this work we discuss the coexistence and possible interaction or two magma types in the evolution of the Early Miocene Borac Eruptive Complex (BEC), situated about I 00 km south of Belgrade. During the Early Miocene two phases of volcanic activity and caldera formation occurred. The first phase gave rise to widespread dacite lava flows (Ostrica caldera). After a short repose period, second stage Plinian to sub-Piinian eruptions occurred and rhyodacite/rhyolite pumice-fall deposits and unwe lded ignimbrites were fQrmed (Borac caldera). This was immediately followed by the emission of quartz latite high-aspect ratio lava flows, lava domes and finally andesite lava flows and hyaloclastic deposits. Lamprophyre sills and an autobrecciated lava flow also formed . Petrochemical characteristics imply the coexistence o f two different magmas during the second stage of the BEC evolution. The main suite, ranging from andesite to quartzlatite and rhyodacite/rhyolite is a high-K calc -alka line (HKCA) series. In contrast, the lamprophyres correspond to a potassic/ultrapotassic alkaline (P/UP) series (K2O/ Na20 ≈2). Major element mass balance calculations show that no simple fractional crystallisation process could have produced the Borac HKCA rocks from the coexisting lamprophyres. However, in a two-step model of crystal fractionation it is possible to obtain the most evolved calc-alkaline rock in the area from a pyroxene-hornblende andesite, using a quartzlatite as the intermediate member. Evolution of the HKCA suite occurred in a shallow magma chamber, approximately 7-8 km in diameter and around 5-6 km thick. Temperature estimates are 750-850°. Lamprophyric magma was injected into the chamber just as it had already become stratified with a rhyodacitic rhyolitic roof, a quartz latitic middle part and an andesitic base. This injection of hot, gas rich lamprophyric melt triggered a Plinian eruption thereby preventing the most evolved acid melts from being involved in the mixing.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
