HR: 1400h
AN: V23A-03    [Abstracts]
TI: COMMENTS ON SOME PHANEROZOIC BATHOLITHS, IGNIMBRITES, F- RHYOLITES
AU: * Obenholzner, J H
EM: obenholzner@a1.net
AF: NHM/Volcanology, Postfach 417, Vienna, A-1014, Austria
AB: Mesozoic and Paleozoic ignimbrites and tuffs might support rejuvenation of batholiths as zircon dating caused in many cases a discrepancy between the inferred eruption age derived from zircon ages and the age of +/- time- equivalent fossils. Such age discrepancies exist in the range of several to more than 10 my i.e. tuffs within Permian beds (Königer et al.. 2002) and Blasseneck-Porphyroid ignimbrites (BPIs) - (Söllner et al. 1997). The BPIs (ca. Ord. VI) are exposed along ca. 300 km in the E Alps. Compacted thicknesses are max. 600 m at lower greenschist-facies. It is not determined yet if the BPIs represent a silicic LIP (1 pulse volcanism(?)). They are underlain by sediments and km-thick (flood?) basalts of lower Ord. age. The BPIs predate the Hirnantian glaciation. The end of zircon growth could also be interpreted as the age of the batholith emplacement. Rejuvenation did not cause further zircon growth. Zircons of Trias. ignimbrites (Obenholzner et al. 1999) showed a broad set of preservation from euhedral to highly corroded during diagenesis (see also Reddy et al. (2006)). Within the realm of Variscean batholiths of Cornwall and Erzgebirge F-rich rhyolites exist which are interpreted as late-stage fractionation or as products of an independent magmatic event. These rhyolites are ca. 20 m.y. younger than the associated granites and intruded or erupted at the very end of the subduction. The Cret. Lost River granite is characterized by distinctive greisenization events (Sainsbury, 1960) and intra-intrusive breccias (Dobson, 1982). In the Erzgebirge explosion breccias exist within the granite (Kempe et al., 2004). Does it represent an episode of rejuvenation including zircon growth? If the time gap of ca. 20 m.y. can be considered an average the F-rhyolites of the Sierra Madre Occidental (Orozco-Esquirel et al., 2002) might be related to batholith emplacement older than the ignimbrite-forming batholiths. The Variscean batholiths of the E Alps host many features indicative for a caldera-root system. Basalt dikes dissected tonalites and pillowed into aplites (Frasl et al., 1995). Permian ignimbrite sequences on Carb. volcanics are known from the S Alps. The F- and B-rich Plio./Quart. granite at Larderello (Italy) is a cooling intrusion with melt batches (Fulignati, pers. comm.). Thermal anomalies of the E Alps had been interpreted as surface manifestation of plutons. Atmospheric 210Po data exist from Bad Gastein (Austria; Wallner, 2001). Could such dormant "volcanoes" become sites of future F-rhyolite eruptions? Pure gas sparging cannot rejuvenate a non-solid batholith (Bachman et al., 2003). Heating of Na fluoride and Na borate in a glass vial (ca. 900°C) revealed surface modification accompanied by vesicle nucleation even inside the glass. Can F- and B- rich fluids operate as fluxing agents? Past super-eruptions need to be correlated with past mantle and core dynamics and maybe F- and B-rich fluid events. "3 magnetic superchrons precede the 4 largest Phanerozoic extinction events (K/T, G/T P/T doublet, end Ord.)".(Courtillot et al., 2006). This hypothesis needs to identify the Ord. LIPs (BPIs of the E Alps?). Bachman et al. 2003, Geology, 31, 789-792. Courtillot et al., 2006. EOS Trans. AGU, 87(52), Fall Meet. Suppl.. Frasl et al., 1995. Geol. Paläont. Mitt. Innsbruck, 20, 121-151. Kempe et al., 2004. Min. Depos., 39, 646-669. Königer et al., 2002. Int.J. Earth Sci., 91, 341-356. Reddy et al., 2006. Geology, 34, 4, 257-260. Obenholzner et al., 1999. Ann. Naturhist. Mus. Wien, 100A, 13-38. Vienna. Orozco-Esquirel et al., 2002. JVGR, 118, 37-56. Sainsbury, 1960. Econ. Geol., 55, 1478-1509. Söllner et al., 1997. Z. dt. geol. Ges., 148/3-4, 499-522. Wallner, G. (2001): Radiochim. acta, Bd. 89, 791-798. .
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
DE: 8428 Explosive volcanism
DE: 8434 Magma migration and fragmentation
DE: 8488 Volcanic hazards and risks
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly