Age constraints for the thermal evolution and erosional history of the central European Variscan belt: new data from the sediments and basement of the Carboniferous foreland basin in western Poland

Three Carboniferous-age detrital muscovites from the Variscan foreland basin of SW Poland and two muscovites from phyllites underlying the basement have been dated by the ⁴⁰Ar/³⁹Ar step-heating and single-crystal laser fusion method. ⁴⁰Ar/³⁹Ar analysis of the detrital micas defines step-heating preferred ages of 370.7 ± 1.4, 363.0 ± 1.9 and 355.0 ± 1.3 Ma. Single-crystal laser fusion data indicate little dispersion for the first of three samples, with an integrated age that closely matches the step-heating data, but the latter two describe inhomogeneous populations. The white mica concentrate from one phyllite yields a step-heating preferred age of 358.6 ± 1.8 Ma. The second phyllite sample displays an incremental discordant apparent age spectrum representing a mixture of white mica grains of varying ages. Our most important finding is that the Variscan foreland basin was supplied by source rocks that were exhumed and cooled in the Late Devonian, probably as a result of an early Variscan collisional event, previously largely undocumented. Although accessible exposures of the Variscan basement in SW Poland exhibit only a minor component of rocks exhumed before the Carboniferous, our work suggests that large tracts of rocks with a Devonian cooling signature are preserved at depth beneath the foreland basin.

采自波兰西南部华力西前陆盆地（Variscan foreland basin）的3件石炭纪碎屑白云母样品，以及采自基底之下千枚岩中的2件白云母样品，采用⁴⁰Ar/³⁹Ar阶梯加热法（step-heating）与单晶激光熔融法（single-crystal laser fusion）完成了同位素定年测试。对碎屑云母的⁴⁰Ar/³⁹Ar分析结果显示，三件样品的阶梯加热优选年龄分别为370.7 ± 1.4、363.0 ± 1.9及355.0 ± 1.3 Ma。三件样品中，第一件的单晶激光熔融数据年龄离散度极低，其综合年龄与阶梯加热法测得的结果高度吻合，但后两件样品的年龄群体呈现非均一性。来自其中一件千枚岩的白云母精矿的阶梯加热优选年龄为358.6 ± 1.8 Ma；第二件千枚岩样品则呈现出由不同时代白云母颗粒混合形成的增量不一致视年龄谱。本研究最重要的结论为，华力西前陆盆地的沉积物物源来自曾于晚泥盆世发生剥露与冷却的母岩，这一过程大概率由早期华力西碰撞事件引发，而该事件此前鲜有文献记录。尽管波兰西南部华力西基底的可出露露头仅存在少量石炭纪前剥露的岩石，但本研究表明，前陆盆地深部埋藏着大片具有晚泥盆世冷却特征的岩石。