Age and underlying cause of hot-spring activity at Rhynie, Aberdeenshire, Scotland
Rhynie ranks as the oldest known example of a subaerial hot-spring system. A new, robust radiometric age for the Rhynie Chert is herein presented, and a genetic model for the Rhynie hydrothermal system and its concomitant mineralization established. Consideration is also given to the Rhynie Outlier in the context of the Siluro-Devonian evolution of Scotland. The Milton of North (basaltic) andesite, a lava flow tentatively assigned to the Tillybrachty Sandstone Formation (i.e. lying stratigraphically beneath the Rhynie cherts), has yielded an ID-TIMS U-Pb zircon-titanite age of 409-6 ± 1.1 ma (2σ). This high-precision age is shown to date within error the Rhynie Cherts Unit (Dryden Flags and Shales Formation), and hence hot-spring activity at Rhynie. A published 40Ar- 39Ar isochron age of 396 ± 12 Ma does not represent the true age of the Rhynie Chert, as once believed. Given the sound palynological control on the age of the Dryden Flags and Shales Formation, the Rhynie Chert-bearing succession now assumes ‘stratigraphic tie-point’ status. Integration of the new absolute age for Rhynie with the (ID-TIMS) U-Pb K-bentonite age dataset of Tucker et al. (1998) tightly constrains the age of the Pragian-Emsian boundary at c. 409 Ma. Intermediate-to-acidic minor intrusions exposed in the Rhynie area belong to one of two temporal groups viz, c. 470 Ma (pegmatites and microgranites) and 425-430 Ma (porphyries and felsites). Although variably carrying all components of the Rhynie metal/metalloid assemblage, and in many cases having undoubtedly released a metal/metalloid-bearing fluid, these igneous bodies were clearly not directly involved in the Rhyme hot-spring activity/mineralization. Similarly, the ‘Cairngorm Suite’ plutons of NE Scotland, based upon a provisional U-Pb monazite age of 427.5 Ma for the Coilacriech Granite and unpublished data concerning the Lochnagar Complex, can have played no direct role in c. 410 Ma hydrothermal activity. the Rhynie hot-spring system must, therefore, have been driven by a subsurface manifestation of that volcanic event recorded in the Rhynie Outlier. It is suggested that the system-powering basaltic andesite magma represented a heavily contaminated upper mantle-derived partial melt, whose petrogenesis and ascent were inherently linked to the (dextral) transcurrent fault movement(s) responsible for the formation of the Rhynie basin. This magma ponded beneath the developing Rhynie basin in a dilatational void created at the intersection of the basin-governing strike-slip fault and the (syn-Grampian) shear zone which forms the northern margin of the ‘Younger basic’ Boganclogh Mass. In addition to heat, the sub-basinal basaltic andesite magma body supplied, by means of an exsolved fluid, Au and As to the Rhynie hydrothermal system. Mo and W were, by contrast, scavenged from ‘Younger basic’ syenitic fractionates and texturally-variable biotite microgranites (c. 470 Ma) occurring in the Rhynie Outlier basement. The atypical geochemical signature of the andesite-driven Rhynie system is a fortuitous consequence of the system’s location.