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

Abstract

Twenty-one new 40 Ar/ 39 Ar 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% (2cr). These ages resolve inconsistencies between existing K-Ar dates on the same samples and their stratigraphical constraints correlated to recently published timescales. The precise 40 Ar/ 39 Ar 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.

Carboniferous and Permian magmatism in Scotland

“…the trachyte and riebeckite rhyolite of the Eildon Hills: McRobert 1914) and the quartz-trachyte and phonolitic trachyte of Skelfhill Pen (McRobert 1920), formerly thought to be related to the later (Visean) Glencartholm volcanism, could be Tournaisian, at least in part, on the basis of one new Ar-Ar date (Monaghan & Pringle 2004). …”

“…During the latest Tournaisian or earliest Visean, after some 10Ma of quiescence, volcanism resumed in south and central Scotland at 342-335Ma (Monaghan & Pringle 2004), across a zone c. 200 km wide, from Kintyre in the west to the Garleton Hills in the east. This episode saw the production of over 90% by volume of all the Scottish lavas produced in the Carboniferous and Permian, the original volume of which might have approached 6000km 3 (Tomkeieff 1937).…”

“…In contrast, alkali basalt magmas produced in the late Visean, Silesian and Permian rarely evolved beyond the hawaiitic stage. A revision of the time v. space diagram for the Scottish Carboniferous-Permian activity (from Stephenson et al 2003), first presented by Francis (1965), is given as Figure 2 (after Read et al 2002; see also Monaghan & Pringle 2004). …”

An improved chronology for the Arthur’s Seat volcano and Carboniferous magmatism of the Midland Valley of Scotland

“…A synthesis of Arthur's Seat dates with previous 40 Ar/ 39 Ar and U/Pb results and biostratigraphically constrained sedimentary units on recently published timescales is summarized in a definitive stratigraphic chart. All ages are given with 2σ errors and the previous 40 Ar/ 39 Ar results of Monaghan & Pringle (2004) and Monaghan & Browne (2010) have been recalculated to Renne et al (2011) non-LSC data, standard ages and decay constant values for direct comparison.…”

Garnet pyroxenite xenoliths and pyropic megacrysts in Scottish alkali basalts

“…(5) Elie Ness: Pyropic garnets have long been known from the late Carboniferous (or possibly earliest Permian; Monaghan & Pringle 2003) Elie Ness diatreme in Fife (Francis & Hopgood 1970) where they invariably occur as discrete crystals (Heddle 1901;Colvine 1968;Chapman 1976;Donaldson 1984). Sub-rounded pyrope-rich garnets, up to several centimetres diameter, are widespread throughout the Elie Ness tuffs and have also been reported from similar neighbouring vents at Ardross and Ruddon's Point (Heddle 1901;Cumming 1935).…”