⁴⁰Ar/³⁹Ar geochronology of Carboniferous-Permian volcanism in the Midland Valley, Scotland

Twenty-one new ⁴⁰Ar/³⁹Ar step-heating experiments on mineral separates from intrusive and extrusive Carboniferous and Permian igneous rocks in the Midland Valley of Scotland yielded 17 concordant experiments with a relative age precision better than 1% (2σ). These ages resolve inconsistencies between existing K-Ar dates on the same samples and their stratigraphical constraints correlated to recently published timescales. The precise ⁴⁰Ar/³⁹Ar dates are stratigraphically constrained to stage level and can contribute to Carboniferous timescale tie points at the Tournaisian-Visean boundary, within the Visean and at the Carboniferous-Permian boundary. Situated in the extending Variscan foreland, two distinct phases of extension-related transitional-alkaline volcanism have been resolved in the Dinantian: the Garleton Hills Volcanic Formation in the eastern Midland Valley near the Tournaisian-Visean boundary, 342.1 ± 1.3 and 342.4 ± 1.1 Ma; and the Clyde Plateau Volcanic Formation in the western Midland Valley during the mid-Visean, 335 ± 2–329.2 ± 1.4 Ma. Alkaline basic sills near Edinburgh, previously thought to be Namurian, appear to be coeval with the Clyde Plateau Volcanic Formation at 331.8 ± 1.3–329.3 ± 1.5 Ma. The new ages allow correlation between these short-lived Dinantian magmatic pulses and extensional and magmatic phases in the Northumberland-Solway and Tweed basins to the south. After late Westphalian, end-Variscan, compression and a regionally important tholeiitic intrusive phase at <em>c.</em> 301–295 Ma, alkaline magmatism related to post-Variscan extension occurred in the central and western Midland Valley during the latest Carboniferous or Permian from 298.3 ± 1.3 to 292.1 ± 1.1 Ma. This correlates well with post-Varsican extension and magmatism observed across the NW European foreland from 300 to 280 Ma.

对苏格兰中部谷地（Midland Valley of Scotland）内石炭纪（Carboniferous）与二叠纪（Permian）火成侵入岩及喷出岩的矿物单矿物分离样（mineral separates），开展了21组新的⁴⁰Ar/³⁹Ar阶段升温实验（⁴⁰Ar/³⁹Ar step-heating experiments），其中17组获得了相对年龄精度优于1%（2σ）的和谐实验结果。
这些年龄数据解决了同批次样品现有钾氩法测年数据（K-Ar dates）与结合最新发布的地质时间标尺（timescales）得到的地层约束（stratigraphical constraints）之间的矛盾。
精准的⁴⁰Ar/³⁹Ar测年结果可落实至阶一级地层约束，可为杜内阶-维宪阶界线（Tournaisian-Visean boundary）、维宪阶（Visean）内部以及石炭纪-二叠纪界线（Carboniferous-Permian boundary）处的石炭纪时间标尺锚点提供支撑。
研究区位于伸展作用下的华力西前陆盆地（Variscan foreland），狄南统（Dinantian）中识别出两期与伸展作用相关的过渡碱性火山作用（transitional-alkaline volcanism）：其一为位于苏格兰中部谷地东部、对应杜内阶-维宪阶界线的加莱顿丘陵火山岩组（Garleton Hills Volcanic Formation），测年结果为342.1±1.3 Ma与342.4±1.1 Ma；其二为苏格兰中部谷地西部、形成于维宪阶中期的克莱德高原火山岩组（Clyde Plateau Volcanic Formation），测年区间为335±2–329.2±1.4 Ma。
此前被认为形成于纳缪尔阶（Namurian）的爱丁堡附近碱性基性岩床（alkaline basic sills），其测年结果为331.8±1.3–329.3±1.5 Ma，显示其与克莱德高原火山岩组形成时代同期。
新的测年数据可将这些短暂的狄南统岩浆活动脉冲与南部诺森伯兰-索尔韦盆地及特韦德盆地的伸展与岩浆活动阶段进行对比关联。
晚威斯特伐利亚阶（Westphalian）、华力西期末（end-Variscan）挤压作用以及约301–295 Ma的区域重要拉斑质侵入活动阶段（tholeiitic intrusive phase）之后，华力西期后伸展作用（post-Variscan extension）相关的碱性岩浆活动出现于苏格兰中部谷地中部与西部，时代为晚石炭世末或二叠纪（latest Carboniferous or Permian），测年区间为298.3±1.3至292.1±1.1 Ma。
该结果与西北欧前陆盆地（NW European foreland）300–280 Ma期间记录的华力西期后伸展与岩浆活动高度吻合。

搜集汇总

背景与挑战

背景概述

该数据集提供了苏格兰Midland Valley地区石炭纪-二叠纪火山活动的21个新40Ar/39Ar测年实验结果，包含17个高精度年龄数据，用于校正现有K-Ar测年与地层约束的不一致性。数据集揭示了两个主要的岩浆脉冲阶段：Dinantian期的Garleton Hills和Clyde Plateau火山岩层，以及晚石炭世-二叠纪的碱性岩浆活动，为区域地质时代标定和构造演化研究提供了关键数据支持。