Provenance and tectonic intricacies revealed by in situ Rb-Sr dating of detrital micas

Abstract

The use of detrital minerals to reconstruct sedimentary provenance is subject to a range of biases, which may reduce the value of the geological information they retain. With the advent of in situ Rb-Sr geochronology, rapid analyses of a representative (>100) number of grains can now be performed on radiogenic Sr-bearing minerals to provide more comprehensive knowledge of source-to-sink processes through more diversified mineral analysis. Here, we test the in situ Rb-Sr technique on detrital white mica and biotite from basin margin and axis samples in a simple rift basin (Perth Basin, Australia) for which previously published detrital zircon U-Pb and Hf isotopic data exist. For the basin margin adjacent to Archean basement, which has unimodal detrital zircon ages at c. 2600 Ma, two mica samples reveal a dominant age mode at c. 500 Ma, reflecting thermal resetting in the craton proximal to the Gondwanan orogenic front, and a subordinate portion of Archean mica detritus that can be interpreted as having been distally sourced (>100 km away). Similarly, a basin axis sample yielded minor c. 1200 Ma apparent mica ages, representing distal sources from the Albany-Fraser-Wilkes Orogen, and c. 500 Ma grains that are likely a resetting product. Whilst obscured in the detrital zircon record, mica samples allow quantification of the relative contributions of distal and proximal sources to the basin. Finally, detrital biotite that was (partially) altered to chlorite yielded partially to fully reset ages between c. 500 and 130 Ma, the latter linked to heating from the c. 137–130 Ma Bunbury Basalt. Ultimately, the use of in situ Rb-Sr geochronology from detrital micas reveal previously unrecognized provenance and tectonic information that is critical to understanding the true complexity of ancient geological histories, but which remains obscured in standard detrital zircon U-Pb geochronology.