Comment on \u201cUltrapotassic magmatism in the heyday of the Variscan Orogeny: the story of the T\u0159eb\xed\u010d Pluton, the largest durbachitic body in the Bohemian Massif\u201d by Janou\u0161ek et al.

U. Schaltegger,S. P. Gaynor,J. Kotková,P. Widmann

doi:10.1007/s00531-020-01975-w

U. Schaltegger, S. P. Gaynor + Show 2 more

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https://doi.org/10.1007/s00531-020-01975-w

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Abstract

This comment addresses the incorrect treatment and presentation of data from laser ablation ICP-MS U–Pb age determinations of two samples of the Třebíč Pluton. This results in inaccurate ages and error assessment, invalidating the age interpretations of the authors. To corroborate our arguments, new high-precision chemical abrasion ID-TIMS data are presented that unequivocally define the emplacement age of the Třebíč pluton.

Highlights

We argue that: (1) inappropriate consideration of decay-damage related Pb-loss in non-chemically abraded zircon, coupled with inheritance of old Pb leads to the previously observed age variations and (2) inaccurate calculation and reporting of laser-ablation, inductively coupled plasma mass spectrometry (LA-ICPMS) U–Pb ages and uncertainties in the recent paper of Janoušek et al (2020) have contributed to an inaccurate and problematic geochronological data set
The temporal dispersion observed in all of the published zircon geochronology data sets from ultrapotassic rocks in the Moldanubian domain indicates that zircon grains are suffering from the following two effects: (1) The radioactive decay damage-related loss of radiogenic Pb, due to elevated U concentrations, leading to normal discordance and lowered 206Pb/238U dates
(2) The presence of inherited components as a consequence of crustal contamination, which has been previously identified by whole-rock trace elements and Sr, Nd, Pb isotopes in numerous Variscan durbachitic rocks (e.g., Tabaud et al 2015) and by negative εHf values of − 4.3 to − 7.5 from zircon of the Třebíč intrusion (Kotková et al 2010), leading to normal discordance and elevated 206Pb/238U and 207Pb/235U dates

Summary

Introduction

We argue that: (1) inappropriate consideration of decay-damage related Pb-loss in non-chemically abraded zircon, coupled with inheritance of old Pb leads to the previously observed age variations and (2) inaccurate calculation and reporting of laser-ablation, inductively coupled plasma mass spectrometry (LA-ICPMS) U–Pb ages and uncertainties in the recent paper of Janoušek et al (2020) have contributed to an inaccurate and problematic geochronological data set. We explain our concerns on these published data, re-interpret the dates, and present new, high-precision chemical-abrasion, isotope-dilution, thermal ionization mass spectrometry (CA-ID-TIMS) U–Pb dates that unequivocally constrain the age of zircon crystallization in the Třebíč melagranite melt.

Results

Conclusion