Effects of fluid-rock interaction on 40Ar/39Ar geochronology in high-pressure rocks (Sesia-Lanzo Zone, Western Alps)
Marschall, Horst R.
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Keyword40Ar/39 Ar geochronology; Fluid-rock interaction; Boron isotopes; Sesia-Lanzo Zone; Metasomatism
in situ UV laser spot 40Ar/39Ar analyses of distinct phengite types in eclogite-facies rocks from the Sesia-Lanzo Zone (Western Alps, Italy) were combined with SIMS boron isotope analyses as well as boron (B) and lithium (Li) concentration data to link geochronological information with constraints on fluid-rock interaction. In weakly deformed samples, apparent 40Ar/39Ar ages of phengite cores span a range of ∼20 Ma, but inverse isochrons define two distinct main high-pressure (HP) phengite core crystallization periods of 88-82 Ma and 77-74 Ma, respectively. The younger cores have on average lower B contents (∼36 mg/g) than the older ones (∼43-48 mg/g), suggesting that loss of B and resetting of the Ar isotopic system were related. Phengite cores have variable d11B values (-18 to -10 ‰), indicating the lack of km scale B homogenization during HP crystallization. Overprinted phengite rims in the weakly deformed samples generally yield younger apparent 40Ar/39Ar ages than the respective cores. They also show variable effects of heterogeneous excess 40Ar incorporation and Ar loss. One acceptable inverse isochron age of 77.1 ±1.1 Ma for rims surrounding older cores (82.6 ±0.6 Ma) overlaps with the second period of core crystallization. Compared to the phengite cores, all rims have lower B and Li abundances but similar d11B values (-15 to -9 ‰), reflecting internal redistribution of B and Li and internal fluid buffering of the B isotopic composition during rim growth. The combined observation of younger 40Ar/39Ar ages and boron loss, yielding comparable values of both parameters only in cores and rims of different samples, is best explained by a selective metasomatic overprint. In low permeability samples, this overprint caused recrystallization of phengite rims, whereas higher permeability in other samples led to complete recrystallization of phengite grains. Strongly deformed samples from a several km long, blueschist-facies shear zone contain mylonitic phengite that forms a tightly clustered group of relatively young apparent 40Ar/39Ar ages (64.7 to 68.8 Ma), yielding an inverse isochron age of 65.0 ±3.0 Ma. Almost complete B and Li removal in mylonitic phengite is due to leaching into a fluid. The B isotopic composition is significantly heavier than in phengites from the weakly deformed samples, indicating an external control by a high-d11B fluid (d11B = +7 ±4 ‰). We interpret this result as reflecting phengite recrystallization related to deformation and associated fluid flow in the shear zone. This event also caused partial resetting of the Ar isotope system and further B loss in more permeable rocks of the adjacent unit. We conclude that geochemical evidence for pervasive or limited fluid flow is crucial for the interpretation of 40Ar/39Ar data in partially metasomatized rocks.
Author Posting. © The Author(s), 2013. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 126 (2014):475-494, doi:10.1016/j.gca.2013.10.023.
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