Multimineral Rb/Sr internal isochrons from eclogite facies rocks of the Eclogite Zone (Tauern Window, Eastern Alps) consistently yield an Early Oligocene age of 31.5±0.7 Ma. This age has been obtained both for late-prograde, dehydration-related eclogitic veins, and for rocks variably deformed and recrystallized under eclogite facies conditions (2.0–2.5 GPa, 600°C). Initial Sr-isotopic equilibria among all phases indicate absence of significant post-eclogitic isotope redistribution processes, therefore the ages date eclogite facies assemblage crystallization. Equilibria also prove that no prolonged pre-eclogite facies history is recorded in the rocks. Instead, subduction, prograde mineral reactions, and eclogitization proceeded rapidly. Fast exhumation immediately after eclogitization, with minimum rates >36 mm/a is inferred from a 31.5±0.5 Ma internal mineral isochron age of a post-eclogitic greenschist facies vein assemblage. Such rates equal typical subduction rates. Late Eocene to Early Oligocene subduction of the European continental margin, with subsequent rapid exhumation of high-pressure nappe complexes has previously been recognized only in the Western Alps. The new data signify synchronous continental collision all along the Alpine belt. Our results demonstrate the unique potential of Rb/Sr assemblage ‘system analysis’ for precise dating of both eclogite facies and post-eclogitic events, thus for precisely constraining exhumation rates of deep-seated rocks, and for straightforward linkage of petrologic evidence with isotopic ages.
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This work was supported by the Deutsche Forschungsgemeinschaft (grant RI 538/25 to UR and JG). We thank J. Herwig and V. Kuntz for their help with mineral separation and figure drawing. J. Vervoort and an anonymous referee provided helpful comments on an earlier version of this manuscript. Careful and constructive reviews by M. Bröcker and I.M. Villa are gratefully acknowledged.
Communicated by J. Hoefs
EIS5 (47°04.560′N, 12°23.509′E, summit of Weißspitze mountain, 3,290 m). Eclogite facies mobilisate, very coarse-grained, dominated by kyanite+quartz+rutile+white mica. Assemblage: kyanite (cm-sized), phengite (up to several mm thick “books”), paragonite, omphacite (Fe-poor), garnet (few inclusions). Locally apatite megacrysts (up to 1 cm in length). No obvious post-eclogitic alteration phenomena.
352E3 (47°04.301′N, 12°27.071′E, Frosnitztal, 100 m SW of Steinsteg, 2,060 m). Mobilisate in matrix. Mobilisate: coarse-grained, dominated by quartz, kyanite, white mica (books up to 1 cm size), with minor amounts of carbonate, omphacite, garnet, epidote, rutile. Matrix: fine-grained eclogite, slightly foliated, with two different types of omphacite (inclusion-rich vs. inclusion-free). Assemblage: omphacite, garnet (with inclusions), with minor amounts of paragonite (rich in Sr), phengite, rutile, pyrite, zoisite, kyanite, calcite.
EIS1 (47°03.745′N, 12°23.159′E, ~100 m SE of Lake Eissee, 2,720 m). Strongly foliated eclogite, with up to 1 mm sized garnet. Locally (in pressure shadows): crystals of omphacite, epidote, and phengitic white mica up to several 100 μm in size, in an otherwise very fine-grained, omphacite+epidote-dominated matrix. Post-eclogitic alteration restricted to small domains around minute cracks (removed before analysis). Assemblage: garnet, omphacite, epidote, dolomite, apatite (Cl), phengite, paragonite, rutile, kyanite, quartz, zircon, pyrite.
AK5/03 (47°03.745′N, 12°23.159′E, ~100 m SE of Lake Eissee, 2,720 m). Eclogite, with mm-sized garnet. Few major omphacite, and phengitic white mica crystals in otherwise fine-grained, foliated, omphacite-dominated matrix. Assemblage: garnet (with inclusion-rich cores), omphacite (dominant); subordinate amounts of quartz, phengite, paragonite, talc, dolomite, epidote, rutile, ilmenite. apatite, pyrite, kyanite.
291 (47°04.799′N, 12°27.384′E, Frosnitztal, 1,150 m SW of Dabernitzkogel, 2,450 m). Fine-grained eclogite, with weakly developed foliation. Slightly weathered, with some intergranular stainings. Some small alterated domains along cracks (removed before analysis). Assemblage: garnet (with inclusion-rich cores), omphacite, clinozoisite, subordinate amounts of phengite, quartz, carbonate, rutile, epidote, apatite.
EIS9 (47°04.152′N, 12°22.603′E, 800 m SW of Wallhorntörl, 2,877 m). Vein-like, structurally discordant mobilisate, several m long and 20–50 cm wide in outcrop. Mobilisate shows 5–10 cm wide alteration aureole in wallrock, with greenschist facies overprint of eclogites. Vein assemblage: amphibole (actinolite with some Mg-hornblende; radial aggregates with crystals up to 15 cm in length), albitic feldspar megacrysts; minor amounts of muscovite (up to 5 mm diameter), titanite (poor in Fe), apatite, epidote, biotite, ilmenite. Rarity: relic of rutile, with titanite overgrowth.
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Glodny, J., Ring, U., Kühn, A. et al. Crystallization and very rapid exhumation of the youngest Alpine eclogites (Tauern Window, Eastern Alps) from Rb/Sr mineral assemblage analysis. Contrib Mineral Petrol 149, 699–712 (2005). https://doi.org/10.1007/s00410-005-0676-5
- Fluid-rock interaction
- Eastern Alps