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（Carboniferous）碎屑白云母（detrital muscovites），以及采自基底（basement）之下千枚岩（phyllites）的2件白云母样品，通过氩-40/氩-39（⁴⁰Ar/³⁹Ar）阶段升温法（step-heating）与单晶体激光熔融法（single-crystal laser fusion）完成了定年测试。对碎屑云母的氩-40/氩-39分析结果显示，阶段升温法得到的优选年龄分别为370.7 ± 1.4、363.0 ± 1.9及355.0 ± 1.3 Ma。单晶体激光熔融数据表明，3个样品中的首个样品年龄离散度极低，其综合年龄（integrated age）与阶段升温法结果高度吻合，但后两个样品呈现非均一的云母颗粒群体。一件千枚岩的白云母精矿（white mica concentrate）经阶段升温法得到的优选年龄为358.6 ± 1.8 Ma。第二件千枚岩样品的增量不一致视年龄谱（incremental discordant apparent age spectrum）显示，其由不同年龄的白云母颗粒混合组成。本研究最核心的发现为：华力西前陆盆地的沉积物源区母岩曾于晚泥盆世（Late Devonian）发生剥露与冷却，该过程大概率由早期华力西碰撞事件引发，而此类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中，仅存在少量石炭纪之前剥露的岩石，但本研究表明，前陆盆地深部保存了大量带有晚泥盆世冷却信号的岩石。