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Pb−Sr−Nd isotopic behavior of deeply subducted crustal rocks from the Dora Maira Massif, Western Alps, Italy-II: what is the age of the ultrahigh-pressure metamorphism?

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Abstract

Pb, Nd and Sr isotope data are reported from two localities on mineral separates from Mg-rich metapelites and associated rocks that have been subducted to depths of at least 100 km, for which metamorphic conditions are estimated at 28–33 kilobars pressure and 700°–800° C, and then returned to the surface. Initial isotope ratio data from the granitoid country rock are similar to those found in the metapelites. The initial ratios indicate predominantly recycled, aged granitic crustal materials for the sources of all of the samples. Five zircon samples, 4 from pyrope megacrysts and 1 from fine-grained pyrope quartzite lenses in the metapelites accurately define a chord yielding intercept ages of 304±10 and 38.0±1.4 Ma in a concordia diagram. Zircon from the country rock also plots along the chord. The zircon data, together with initial Nd and Sr data, indicate that the sedimentary sources of the rocks were derived mainly or entirely from sialic Hercynian rocks. Ellenbergerite from pyrope megacrysts and monazite from the fine-grained ground mass yield slightly younger ages of 30–34 Ma, apparently reflecting lower blocking temperatures than that of zircon. Sm−Nd data from a pyrope megacryst give an errorchron corresponding to an age of 38 Ma, in agreement with the zircon date. A major question concerns the timing of the ultrahigh-pressure metamorphism. Experimental data suggest that pyrope and quartz/coesite as well as ellenbergerite formed by various metamorphic reactions. If, as generally assumed, the ultrahigh-pressure metamorphism occurred ca. 100 Ma ago, our data require that the zircon did not experience measurable lead loss at that time, but lost major amounts of lead 38 Ma ago during late Alpine metamorphism. Estimates of diffusion rates for Nd in pyrope further suggest that the apparent Sm/Nd age of 38 Ma for the megacryst is not consistent with that model. Those problems are resolved if the ultrahigh-pressure metamorphism occurred 38–40 Ma ago, but problems remain from Ar/Ar dates of 100 Ma on phengite, an inferred 120 Ma age for zircon lead loss from another study, and possibly by the very rapid uplift required if the metamorphism is that young.

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Authors and Affiliations

  1. Abteilung Geochemie, Max-Planck-Institut für Chemie, W-6500, Mainz, FRG

    G. R. Tilton

  2. Institut für Mineralogie, Ruhr-Universität Bochum, W-4630, Bochum 1, FRG

    W. Schreyer & H. -P. Schertl

  3. Department of Geological Sciences, University of California, 93106, Santa Barbara, CA, USA

    G. R. Tilton

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  1. G. R. Tilton
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  2. W. Schreyer
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  3. H. -P. Schertl
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Dedicated to Professor Borwin Grauert on the occasion of his sixtieth birthday

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Tilton, G.R., Schreyer, W. & Schertl, H.P. Pb−Sr−Nd isotopic behavior of deeply subducted crustal rocks from the Dora Maira Massif, Western Alps, Italy-II: what is the age of the ultrahigh-pressure metamorphism?. Contr. Mineral. and Petrol. 108, 22–33 (1991). https://doi.org/10.1007/BF00307323

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  • DOI: https://doi.org/10.1007/BF00307323

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