Pleistocene eruptive chronology of the Gölcük volcano, Isparta Angle, Turkey. Chronologie des épisodes volcaniques pléistocènes du volcan Gölcük, Angle d’Isparta, Turquie

Abstract

In the Eastern Mediterranean region, the Isparta volcanic belongs to the post-collisional alkali-potassic to ultrapotassic magmatism active since the Miocene in this part of the Anatolian peninsula from Afyon to Isparta. In the so-called Isparta Angle (IA) the magmatism is contemporaneous with an extensional regime intiated during Late Miocene and active throughout the Pliocene and Quaternary. Previous K/Ar dating performed on lavas suggested that potassic-ultrapotassic magmatism occurred between 4.7 to 4 Ma. However, a more recent (Quaternary) activity of the Golcuk volcano is evidenced by the present-day morphology and field evidence although it remained undated and poorly studied so far. Field mapping and new radiometric data indicate that the main volcano-forming stages of the Golcuk volcano consist of three main eruptives cycles. (1) Cycle I, represented by more than 200m-thick pyroclastic flow deposits occasionally separated by paleosoils and corresponding to caldera-forming ignimbritic eruptions. (2) Cycle II, consisting of tephriphonolite lava dome-flows extruded throughout the caldera and currently found along the rim of the present crater. (3) Cycle III made up of tuff-ring deposits related to several phreatoplinian eruptions of a maar-type volcanic activity. This youngest cycle ends with trachytic domes protruding within the maar crater. Unspiked 40K/40Ar dating on mesostasis was performed on lavas (tephriphonolites and trachytic domes), and complemented by preliminary 40Ar/39Ar data on tephra deposits (sanidine). Our preliminary results show that the entire activity of Golcuk volcano took place during the Pleistocene and was disconnected from the older Pliocene volcanism. This volcanic activity can be considered as a new volcanic cycle, starting (Cycle I) around 200 ka with major explosive, regional-scale, events represented by at least six ignimbrites sheets. Cycle II occurred between 115 ± 3 ka to 62 ± 2 ka with probably some associated tephra deposits. Tuff-ring of Cycle III formed from 72.7 ± 4.7 ka to 24 ± 2 ka. The associated phreatoplinian eruptions have almost entirely destroyed the previously formed flow-dome. This latest activity corresponds to several volcanic crises as illustrated by the two domes protrusions separated by about 30 ka. The volcanic history of Golcuk ceased around 24 ka ± 2 ka, but the periodicity of eruptive events appears to be long and complex. Currently, the volcano is at rest, but there is no doubt that the Isparta town (more than 120 000 people) built on top of the most recent tephra falls is exposed to a major volcanic hazard in the future.