39Ar / 40Ar dating of high-pressure rocks from the Ligurian Alps: Evidence for a continuous subduction exhumation cycle

Details 39Ar / 40Ar dating of high-pressure rocks from the Ligurian Alps: Evidence for a continuous subduction exhumation cycle 39Ar / 40Ar dating of high-pressure rocks from the Ligurian Alps: Evidence for a continuous subduction exhumation cycle

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005e%26psl.240..668f&link_type=abstract

Earth and Planetary Science Letters, Volume 240, Issue 3-4, p. 668-680.

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We present 39Ar 40Ar dating of phengite, muscovite and paragonite from a set of mafic and metasedimentary rocks sampled from the high-pressure (HP) metaophiolites of the Voltri Group (Western Alps) and from clasts in the basal layer conglomerates from the Tertiary molasse which overlie the high-pressure basement. The white mica-bearing rocks display peak eclogitic and blueschist-facies parageneses, locally showing complex greenschist-facies replacement textures. The internal discordance of age spectra is proportional to the chemical complexity of the micas. High-Si phengites from eclogite clasts record a 39Ar 40Ar age of ca. 49 Ma for the eclogite stage and ca. 43 Ma for the blueschist retrogression; phengites from a blueschist basement sample yield an age of ca. 40 Ma; low-Si muscovite from a metasediment dates the formation of the greenschist paragenesis at ca. 33 Ma. Our data indicate that the analyzed samples reached high-pressure conditions at different times over a time-span of c.a. 10 Ma. Subduction was continuing during exhumation and blueschist retrograde re-equilibration of higher-pressure, eclogite-facies rocks. This process kept the isotherms depressed, allowing the older HP-rocks to escape thermal re-equilibration. Our results, added to literature data, fit a tectonic model of a subduction exhumation cycle, with different tectonic slices subducted at different times from Early Eocene until the Eocene Oligocene boundary.

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