The 2 km × 5 m Newmains Dyke in the Scottish Southern Uplands consists mainly of primitive (mg number > 70), at least partly mantle-derived lamprophyre (vogesite spessartite, SiO₂ 50–60%), with hornblende-pyroxene-rich cumulates (SiO₂ < 50%) and small amounts of quartz syenitic to granite residua (60–73% SiO₂). The lamprophyres and cumulates yield an Rb-Sr whole-rock isochron age of 395±9 Ma (MSWD 2.0), and (⁸⁷Sr/⁸⁶Sr)_i 0.70514±5—identical values to the nearby Criffell granitic pluton. After some 65% hornblende + clinopyroxene fractionation of parent lamprophyre magma, a volatile phase separated and metasomatically enriched the country rock greywackes in K, Ba, and Rb. The remaining quartz syenitic liquids became contaminated by ⁸⁷Sr from these greywackes, which have a mean (⁸⁷Sr/⁸⁶Sr)₃₉₅ value of 0.70789±114. Further fractionation of the contaminated syenitic liquids gave the granitic rocks, which have (⁸⁷Sr/⁸⁶Sr)₃₉₅ values of 0.7056–64. The dyke appears to demonstrate in situ some of the hidden processes previously invoked to explain the evolution of the Criffell pluton itself.