Genesis of zoned granite plutons in the Iapetus Suture Zone: new constraints from high-precision micro-analysis of accessory minerals

Abstract

The Trans-Suture Suite (TSS) of granitic plutons located in Northern Britain span the Iapetus Suture and represent a particularly enigmatic stage of post-Caledonian Devonian magmatism. Despite calc-alkaline affinities, proximity to the Iapetus Suture precludes a direct relationship to active subduction. Furthermore, the absence of inherited zircons distinguishes the TSS from plutons of a similar age throughout the Scottish Highlands, and is not easily reconciled with the abundance of peraluminous and S-type granites. Micro-analytical techniques are employed to analyse accessory zircon and apatite from three TSS plutons (Criffell, Fleet and Shap) in order to investigate connections between magmatic and tectonic processes within a continental suture zone. Accessory minerals contain a robust and accessible record of magma evolution. However, their trace element compositions are shown to document the final stages of pluton emplacement only, and are determined primarily by competitive crystallisation of other accessory phases at shallow crustal levels. By contrast, whole-rock compositions record an earlier stage of magma evolution that occurred in deeper and open-system crustal hot zones. The absence of inherited zircon in the final crystal assemblage reflects resorption during rapid and potentially adiabatic ascent of super-liquidus and water-rich magmas from the crustal hot zone. The concentrations of REE in apatite inclusions decrease with increasing crystallisation of other accessory minerals and in some samples have been distinguished on the basis of their host phase. In metaluminous granodiorites, no preferred crystallisation sequence is observed between host phases. In peraluminous samples, zircon-hosted apatite compositions appear more primitive compared to those hosted by other phases and reflects earlier saturation of zircon in these magmas. The isotopic record of zircon is shown to be biased towards earlier stages of magma evolution in peraluminous magmas and provides a means of assessing zircon isotope compositions in the context of often protracted histories during silicic magma evolution. The Mn content of apatite varies independently to whole-rock composition, correlating positively with decreasing oxygen fugacity and indices of increasing reduction. Apatite is proposed as a robust and effective redox proxy with application to magmatic, detrital and lunar studies of redox conditions. Zircon O-HfU- Th-Pb isotope compositions have identified increasing degrees of compositional heterogeneity in more silicic components of the TSS, including the involvement of more mafic magmas in the generation of the large S-type pluton of Fleet. Compositional trends between zircons from plutons emplaced on both sides of the Iapetus Suture are used to identify a common Avalonian component beneath the Southern Uplands and English Lake District related to Avalonian underthrusting beneath the Laurentian margin during the late Caledonian. New geochronological dating indicates that pluton emplacement occurred during periods of both pre- and post-Acadian transtension. The distinguishing characteristics of these plutons relative to other Caledonian plutons reflect their unique emplacement into the hydrated lithosphere of the Iapetus Suture Zone. Oxygen isotope disequilibrium between the magmatic compositions recorded by zircon and those of the whole-rock and some quartz crystals reflect hydrothermal alteration of the latter two archives. Hydrothermal alteration and exchange caused by 18O-rich magmatic fluids has elevated the 18O compositions of the whole-rock and some quartz crystals, with most disequilibrium observed around the outer margins of the plutons due to further fluid interaction with local 18O-rich sediments. The hydrothermal history of the TSS is markedly different from that of the British Tertiary Igneous Province where hydrothermal alteration resulted from circulation of meteoric water, reflecting significantly different magmatic and emplacement histories.