Unravelling the protracted U-Pb zircon geochronological record of high to ultrahigh temperature metamorphic rocks:Implications for provenance investigations

The assessment of detrital zircon age records is a key method in basin analysis,but it is prone to several biases that may compromise accurate sedimentary provenance investigations.High to ultrahigh temper-ature(HT-UHT)metamorphism(especially if T＞850℃)is herein presented as a natural cause of bias in provenance studies based on U-Pb detrital zircon ages,since zircon from rocks submitted to these extreme and often prolonged conditions frequently yield protracted,apparently concordant,geochrono-logical records.Such age spreading can result from disturbance of the primary U-Pb zircon system,like-wise from(re)crystallization processes during multiple and/or prolonged metamorphic events.In this contribution,available geochronological data on Archean,Neoproterozoic and Palaeozoic HT-UHT meta-morphic rocks,acquired by different techniques(SIMS and LA-ICP-MS)and showing distinct composi-tions,are reassessed to demonstrate HT-UHT metamorphism may result in modes and age distributions of unclear geological meaning.As a consequence,it may induce misinterpretations on U-Pb detrital zircon provenance analyses,particularly in sedimentary rocks metamorphosed under such extreme temperature conditions.To evaluate the presence of HT-UHT metamorphism-related bias in the detrital zircon record,we suggest a workflow for data acquisition and interpretation,combining a multi-proxy approach with:(i)in situ U-Pb dating coupled with Hf analyses to retrieve the isotopic com-position of the sources,and(ii)the integration of a petrochronological investigation to typify fingerprints of the HT-UHT metamorphic event.The proposed workflow is validated in the investigation of one the-oretical and one natural example allowing a better characterization of the sedimentary sources,maxi-mum depositional ages,and the tectonic setting of the basin.Our workflow allows to the appraisal of biases imposed by HT-UHT metamorphism and resulting disturbances in the U-Pb detrital zircon record,particularly for sedimentary rocks that underwent HT-UHT metamorphism and,finally,suggests ways to overcome these issues.