Abstract
White mica has been widely used to date microstructures and tectonic events in faults, shear-zones and folds because of its suitability for radiogenic dating. However, complex (i) microstructural evolution, (ii) individual chemical evolution of the K-bearing phases, (iii) mixing of ‘detrital grains’ with newly formed and/or recrystallized or chemically reset grains as well as (iv) volume diffusion may result in apparent K-Ar ages. Here, specimens from a prograde sediment sequence of the exhumed fossil European Alpine accretionary wedge were used to investigate resetting processes of white mica by the type and intensity of deformation as well as peak metamorphic conditions. We combine the K-Ar system with mass and mineral quantities from grain size fractions to calculate the amount of recrystallized white mica in each grain size fraction along the metamorphic gradient. Increasing recrystallization with increasing metamorphic grade is related to thermally activated pressure solution and dissolution-precipitation creep, as seen by the formation of a spaced cleavage of recrystallized phyllosilicates documented through Synchrotron X-ray Fluorescence Microscopy and Scanning Electron Microscope imaging techniques. Increasing recrystallization by dissolution-precipitation processes induces chemical resetting of the isotopic system, resulting in a prograde decrease of apparent K-Ar ages. We demonstrate that Ar volume diffusion does not play a significant role for the low-temperature samples, promoting recrystallization as the important physico-chemical process for age resetting. However, white mica chemistry reveals that no simple relation between isotopic resetting and grain size exists along the prograde path. Reliable age information can therefore only be obtained in the case of (nearly) complete resetting, which accounts only for the smallest grain size fraction at the highest metamorphic temperature. These findings could shed new light on accurate dating of mica-rich fault rocks, where the time constraints depend not only on the temperature, but also on the amount and type of deformation.
Highlights
In subduction zones, the initially porous and fluid-saturated marine sediments dehydrate while being incorporated into the accretionary wedge and subducted under the upper plate (e.g., Dielforder et al.(2015) and references therein)
There are no major differences in the mineral assemblage between the different samples, and in all samples, white mica occurs as dioctahedral potassic white mica of the 2M poly type, as confirmed by the absence of the 2.58 Å peak in the obtained diffractometer diagrams, which is consistent with the data of Hunziker et al (1986)
The changing isotope and mineral chemistry data of white mica in slates during increasing metamorphism is related to a microstructural development of spaced cleavage formed by the precipitation of phyllo silicates controlled by dissolution-precipitation creep
Summary
The initially porous and fluid-saturated marine sediments dehydrate while being incorporated into the accretionary wedge and subducted under the upper plate (e.g., Dielforder et al.(2015) and references therein). The inter connectivity of phyllosilicates in layers and networks forms weak zones and results in an anisotropic rock texture along which deformation can localize (White and Knipe, 1978). This weakening enables slip along brittle faults, in the case of upper-crustal or rapidly deformed mid-. Berger et al (2017) investigated these processes in mica along the retrograde path (Aar massif, Central Alps, CH) Until now, it is not clear what the contribution of each single process to resetting is and how these are expressed on the prograde path in phyllosilicate-rich systems, as for example in sediments of active accretionary wedges
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