Nested incremental growth of zoned upper crustal plutons in the Southern Uplands Terrane, UK: Fractionating, mixing, and contaminated magma fingers

Abstract

The type and magnitude of the physical and chemical processes controlling magma ascent, storage and solidification in zoned plutons remain controversial, even though such plutons are widespread in arcs. Linking both plutonic and host rock characteristics is key towards determining quantitative models for the growth and evolution of these plutons. We report a synthesis of an integrated study of the Loch Doon pluton, an archetypal zoned pluton in the Southern Uplands Terrane of Scotland, including linked plutonic and host rock field mapping, structural analysis, LA-ICP-MS U-Pb zircon geochronology, whole-rock and isotope geochemistry and magnetic susceptibility analyses. Our results reveal that trapping of rising magma and construction of the pluton in the upper crust at ~397 Ma was dominated by vertical host rock displacement: likely a combination of downward host rock flow, stoping and floor subsidence, and to a lesser extent upward doming and faulting. Mixing of depleted mantle-derived and crustal melts, and fractionation and contamination of these at deeper crustal levels, was followed by continued in situ fractionation and mixing during crystallization that was extended by continual chamber rejuvenation and thermal buffering by recharge from magmas. Analysis of the pluton's ubiquitous magmatic foliations and their relationship to internal magmatic and external host rock structures suggests that later, typically more evolved magma batches 'nested' within the rheologically weakest portions of earlier batches and made space by largely displacing these earlier intrusive batches rather than the external host rock. The active parts of the nesting system decreased with time, consistent with a freezingmagma feeder system. Thismodel is applicable tomany other zoned late Caledonian Southern Uplands Terrane plutons of the United Kingdom. These Southern Uplands Terrane plutons appear to be magmatic 'fingers' rising from larger, deeper intrusive complexes that resided at depth. Our new data also allow us to present further constraints on the geodynamic model for this part of the Caledonian orogenic belt. Our interpretation of antecrystic and autocrystic zircon ages in Southern Uplands Terrane plutons indicates prolonged magmatism between ~430-382 Ma and analysis of whole-rock Sr/Y ratios suggests crustal thicknesses duringmagma generation may have varied in space and, or, time between ~30-45 km. Our data best fit a tectonic model in which magmatism was initiated at ~430 Ma in response to break-off of subducted Iapetan lithosphere and was extended by additional melt/heat input during regional transtension at ~415-400 Ma. Magmas evolved by complex open system fractionation, mixing, and contamination in a mid-lower crustal batholith before being emplaced as compositionally zoned nested plutons in the upper crust between ~414-382Ma.