A revised stratigraphy of the pre-Green Tuff ignimbrites at Pantelleria (Sicily Channel)

Abstract

Peralkaline silicic magmas were erupted at Pantelleria in a variety of eruptive typologies and magnitudes: pyroclastic flows, Plinian to strombolian pumice fallout and lava flows. The initial cycle (330- 180 ka) was characterized mainly by effusive activity, and was followed by an intermediate cycle (181 - 85 ka), characterized by a clear drift to explosive activity. This period, onto we focus, is bracketed by six ignimbrite-forming eruptions (older and intermediate of the two caldera collapses at 140 and 50 ka that characterize the volcanological history of the island, which drained silicic and variably peralkaline magma for a cumulative volume close to 6 km3 DRE. These ignimbrites lack of continuity among the few outcrops (commonly in vertical scars) and also suffer from strong welding and rheomorphism. First reconstructions and correlations [Mahood and Hildreth, 1986], although still valid for many aspects, suffer from the poor precision of K/Ar ages, especially when Ar excess and xenocrystic contamination are common, as is the case for Pantelleria lavas and pyroclastic rocks. Recently, new correlations based on paleomagnetic methods [Speranza et al., 2012] succesfully identified and correlated two welded pyroclastic breccias outcropping in the NE and SW sectors of the island with the old caldera collapse, and also demonstrated the equivalence of two other ignimbrite units previously considered as separate eruptions. Based on 14 new 40Ar/39Ar ages (laser heating on anorthoclase feldspar separates) ages and petrographic data, we propose an updated stratigraphy for the intermediate period and to provide accurate age brackets to the correlations. Recently, the application of 40Ar/39Ar methods on much younger (age < 20 ka) silicic lavas and tephra at Pantelleria [Scaillet et al., 2011], demonstrated highly precise and contributed to unravel the complex post-Green Tuff stratigraphy. Our results can be summarized as follows: - The pre-La Vecchia caldera eruptive history ended with the emplacement of two trachytic ignimbrites (at 181 ± 1.2 ka and 171 ± 1.7 ka), which were emplaced only in the south sector of the island. Both were truncated by the old (La Vecchia) caldera collapse. - The age of the old caldera collapse is now tightly constrained between 139-146 ka , i.e. the new age of the welded pyroclastic breccia (previously undated). that represents the caldera-forming eruption [Speranza et al., 2012]. - After the La Vecchia caldera collapse, the first ignimbrite eruption occurred at 123 ka with a trachy-comenditic and crystal-rich unit emplaced in the NE to NW sectors of the island. - At 107 ka a crystal-rich trachytic ignimbrite was erupted, together with some fines-rich lobes that (K/Ar ages) were previously considered as two different units, but with similar 40Ar/39Ar ages. - Higher up in the stratigraphic sequence, two previously considered different ignimbrites (for an inferred volume of erupted magma close to 1.5 km3 DRE) demonstrated coeval by 40Ar/39Ar dating, and together are part of an eruptive paroxysm at 85 ka comparable in magnitude to the Plinian eruption of the Green Tuff (age 50 ka), - The the recurrence time of the above mentioned eruptions, for a cumulative erupted magma volume close to 6 km3 DRE, suggest that at the end of the inter-eruptive period 85-50 ka the production and evolution rates of pantellerite magma peaked to reach a maximum peralkalinity.